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Endocrinology Vol. 145, No. 12 5820-5831
Copyright © 2004 by The Endocrine Society

Interaction of Stress-Activated Protein Kinase-Interacting Protein-1 with the Interferon Receptor Subunit IFNAR2 in Uterine Endometrium

Shu-Zong Wang and R. Michael Roberts

Departments of Veterinary Pathobiology, Biochemistry, and Animal Sciences, University of Missouri, Columbia, Missouri 65211

Address all correspondence and requests for reprints to: Dr. R. Michael Roberts, 105F, Life Sciences Center, Rollins Road, Columbia, Missouri 65211-7310. E-mail: robertsrm{at}missouri.edu.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
During early pregnancy in ruminants, a type I interferon (IFN-{tau}) signals from the conceptus to the mother to ensure the functional survival of the corpus luteum. IFN-{tau} operates through binding to the type I IFN receptor (IFNR). Here we have explored the possibility that IFNAR2, one of the two subunits of the receptor, might interact with hitherto unknown signal transduction factors in the uterus that link IFN action to pathways other than the well established Janus kinase-signal transducer and activator of transcription pathways. A yeast two-hybrid screen of an ovine (ov) endometrial cDNA library with the carboxyl-terminal 185 amino acids of ovIFNAR2 as bait identified stress-activated protein kinase-interacting protein 1 (ovSin1) as a protein that bound constitutively through its own carboxyl terminus to the receptor. ovSin1 is a little studied, 522-amino acid-long polypeptide (molecular weight, 59,200) that is highly conserved across vertebrates, but has identifiable orthologs in Drosophila and yeast. It appears to be expressed ubiquitously in mammals, although in low abundance, in a wide range of mammalian tissues in addition to endometrium. Sin1 mRNA occurs in at least two alternatively spliced forms, the smaller of which lacks a 108-bp internal exon. ovSin1, although not exhibiting features of a membrane-spanning protein, such as IFNAR2, is concentrated predominantly in luminal and glandular epithelial cells of the uterine endometrium. When ovSin1 and ovIFNAR2 are coexpressed, the two proteins can be coimmunoprecipitated and colocalized to the plasma membrane and to perinuclear structures. Sin1 provides a possible link among type I IFN action, stress-activated signaling pathways, and control of prostaglandin production.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
TYPE I INTERFERON (IFN-{tau}) has a primary role in conferring antiviral responses on its target cells, but tends to be pleiotropic in its action, directing a broad range of activities and outcomes depending upon the cell type it targets. All of the functions of type I IFN appear to be mediated through a cell surface receptor, which is composed of two subunits, IFNAR1 and IFNAR2 (1). The first and still the best characterized of the downstream signal transduction pathways to be activated after IFN binding to the receptor is the one involving the tyrosine kinases, tyrosine kinase 2 and Janus kinase 1 (Jak1), and signal transducer and activator of transcription 1 (STAT1) and STAT2, which leads to up-regulation of key IFN-stimulated genes implicated in antiviral and other responses (2). Although the Jak/STAT pathway has been assumed to direct most of the activities of type I IFN, it is now clear that STATs in addition to STAT1 and -2 can become phosphorylated in response to IFN (3, 4, 5, 6), and that under some circumstances additional signal transduction pathways can be activated, including the phosphatidylinositol 3-kinase (7, 8, 9), the extracellular signal-regulated kinase/MAPK (10, 11), and the p38 MAPK pathways (12, 13, 14, 15, 16, 17). In addition, cells lacking STAT1 are still able to up-regulate the activities of several IFN-stimulated genes (18, 19, 20). These and other observations (21) suggest that type I IFN can signal through more than a single pathway.

Protein factors associated with the cytoplasmic domains of IFNAR1 and IFNAR2 are likely to provide insight into the type I IFN signaling pathways. Among the factors known to bind constitutively to the relatively short cytoplasmic domain of IFNAR1 are tyrosine kinase 2 (22, 23, 24) and arginine methyltransferase 1 (PRMT1) (25, 26). Some proteins may associate firmly with the cytoplasmic domain of IFNAR1 only after ligand binding; for example, STAT2 and STAT3 (27, 28, 29, 30, 31, 32), the T cell receptor signaling components Lck and ZAP-70 (33), and the protein phosphatase PTP1D (34). Many protein factors may have to be appropriately phosphorylated or covalently modified in some other manner before they can bind to IFNAR1.

IFNAR2, which was originally considered to be the ligand-binding subunit of the receptor because it is able to associate with type I IFN with relatively high affinity in the absence of IFNAR1 (1), has a much longer cytoplasmic domain than IFNAR1 (269 vs. 98 in sheep and 251 vs. 98 in humans) (35, 36, 37). Except for its ability to bind Jak1 (4) and STAT2 (31, 38, 39), little is known about the proteins that interact with this polypeptide. In particular, both highly conserved and potential specificity-conveying variable regions are found in the cytoplasmic domain of IFNAR2 (35, 36, 23, 40), even though their functions remain unknown. It seemed important to address the potential roles of these regions, particularly because type 1 IFN are able to target so many different cell types and have such broad pleiotropic activities.

This laboratory’s interest in signal transduction pathways emanating from the type I IFNR relates to the role of type I IFN in reproduction. In ruminant ungulate species, a type 1 IFN, IFN-{tau} with potent antiviral activity, is the signal from the preimplantation embryo that is responsible for preventing regression of the maternal corpus luteum during early pregnancy (41, 42). IFN-{tau} targets IFN receptors in the mucosal lining of the maternal uterus and ultimately interferes with the release and/or production of the luteolytic hormone, prostaglandin F2{alpha} (PGF). Suspecting that a nonclassical signal transduction pathway might be associated with this novel activity of IFN-{tau}, we conducted a yeast two-hybrid screen of an ovine (ov) endometrial cDNA library using regions of the cytoplasmic domain of ovIFNAR2 as bait.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animals
Cross-bred ewes, 2–5 yr of age, were injected with PGF (two 5-mg injections given 3 h apart), and estrus was detected with teaser rams. Ewes were either bred to intact rams or allowed to progress to selected days of their estrous cycles. Endometrium was removed from uteri of ewes after slaughter (43) according to a protocol approved by the University of Missouri institutional animal care and use committee.

Biological reagents
Amino acids (aa) used for preparing synthetic dropout (SD) medium in the yeast two-hybrid assay were purchased from Sigma-Aldrich Corp. (St. Louis, MO). T4 ligase and AMV reverse transcriptase were obtained from Promega Corp. (Madison, WI). Restriction enzymes were purchased from either New England Biolabs (Beverly, MA) or Promega Corp. KlenTaq DNA polymerase was obtained from Invitrogen Life Technologies, Inc. (Gaithersburg, MD). Pfu DNA polymerase was purchased from Stratagene (La Jolla, CA). Sequenase and [{alpha}-35S]deoxy-ATP were obtained from Amersham Biosciences (Arlington Heights, IL). Glutathione beads used for glutathione-S-transferase (GST) fusion protein purification were obtained from Amersham Biosciences (Uppsala, Sweden). Other reagents were purchased from Fisher Scientific (Pittsburgh, PA).

Oligonucleotides and their uses are listed in Table 1Go.


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  		TABLE 1. Oligonucleotides used as primers for either RT or PCRs

 
Plasmids
The bait, ovIFNAR2 medium fragment (ovIFNAR2MF), used to screen the cDNA library for potential IFNAR2-associating factors, was a 185-aa-long C-terminal peptide (T352-N536) that included the SLEDC tandem repeat, the most variable region, the conserved STAT2-binding subdomain, and the highly conserved C-terminal subdomain (36) (Fig. 1Go). DNA fragments were PCR-amplified from the full-length ovIFNAR2 cDNA with the primer pair R2 short fragment (R2SF) and R2LR (Table 1Go). The gel-purified PCR products were ligated into pGEM-T Easy plasmid and transformed into JM109 cells, and white colonies were randomly selected. Inserts were ligated into the pAS1 plasmid, amplified, and sequenced to confirm the identities of the junction regions and open reading frames.



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  		FIG. 1. Diagram of ovine IFNAR2 and the three fragments used for the yeast two-hybrid screening. TM, Transmembrane domain; JBD, Jak1-binding subdomain; SBD, STAT2-binding subdomain; SLEDC, tandem repeat; Box1, box 1-like motif; A6, conserved acidic region; ovIFNAR2, ovIFNAR2 full-length subunit.

 
Two other ovIFNAR2 fragments, ovIFNAR2SF, a 66-aa-long peptide (T352-Q417), containing the SLEDC repeat; and ovIFNAR2 long fragment (ovIFNAR2LF), the full-length 269-aa-long cytoplasmic domain (K268-N536), were constructed identically to pAS1:ovIFNAR2MF, except different primers were used for bait amplification (Table 1Go).

The positive control plasmids for the yeast two-hybrid screening or assay, pAS1:p53 and pAD-GAL4-2.1:simian virus 40 (SV40), were purchased from Stratagene (La Jolla, CA).

The GST-stress-activated protein kinase (SAPK)-interacting protein 1 (Sin1lf3) expression vector pGEX 4T-1:ovSin1lf3 was constructed as follows. Sin1lf3 was a cDNA fragment encoding the C-terminal 296-aa-long peptide (residues V227-Q522) of ovSin1. It was PCR-amplified using pGEM-T-Easy:Sin1lf3 as template, and the forward primer (pgadsinlf3fw) and the reverse primer (pgadsinlf3rv) to provide EcoRI and XhoI restriction sites (Table 1Go). The amplified fragment was digested with EcoRI and XhoI and ligated into the same double-enzyme-digested linear plasmid pGEX 4T-1 (Amersham Biosciences). The GST-ovIFNAR2MF expression vector pGEX 2T-1:ovIFNAR2MF was constructed in essentially the same manner from IFNAR2MF with a cDNA fragment encoding a 185-aa-long C-terminal peptide (residues T352-N536) of ovIFNAR2, and two primers, forward (R2MFFW) and reverse (R2T2RV), which provided BamHI and EcoRI restriction sites, respectively (Table 1Go).

To construct the mammalian expression vector pCMV-Myc:ovSin1 for producing epitope-tagged Myc-Sin1, the full-length ovSin1 cDNA was PCR amplified with primers pcmvsin1fw and pcmvsin1rv primers (Table 1Go). The amplified fragment was digested with EcoRI and KpnI, purified, and ligated into the plasmid pCMV-Myc (BD Clontech, Palo Alto, CA). The expression vector pFLAG-CMV-6b:ovIFNAR2CD was constructed from IFNAR2CD, the cDNA fragment encoding the cytoplasmic domain (residues K268-N536) of ovIFNAR2. The cDNA was PCR-amplified using pAS1:ovIFNAR2LF as template and two primers, pcmvr2cdfw and pcmvr2cdrv (Table 1Go). The amplified fragment was digested with EcoRI and KpnI and ligated into pFLAG-CMV-6b (Sigma-Aldrich Corp.).

All constructs were verified by the corresponding double-restriction enzyme digestion and DNA sequencing.

Construction of cDNA libraries
Endometrial tissues were collected and homogenized, and total RNA was extracted using RNA STAT-60 (Tel-Test, Friendswood, TX). The purity and amount of RNA were estimated by absorbance at 260 and 280 nm. The RNA used to construct the library was obtained from ewes at several stages of the estrous cycle (d 7, 300 µg; d 8, 300 µg; d 10, 280 µg; d 13, 300 µg; d 15, 300 µg; d 16, 300 µg) and pregnancy (d 13, 300 µg; d 15, 300 µg; d 16, 300 µg; d 21, 300 µg). These samples of RNA were pooled, and provided to Stratagene (La Jolla, CA) as a source of polyadenylated RNA for construction of the HybriZAP-2.1 {lambda} phage cDNA library. After first strand synthesis, an XhoI site was added at the 3' end of the cDNA. An EcoRI site was added at the 5' end of the cDNA by ligating an EcoRI adapter to the 5' end of the double-stranded cDNA. The cDNA was then ligated into {lambda} arms cut with EcoRI and XhoI.

To amplify the primary {lambda} phage library, XL1-Blue MRF' host cells were grown to the log phase and diluted to an OD600 of 0.5 in 10 mM MgSO4. Cells (600 µl) were then mixed with approximately 5 x 104 plaque-forming units (pfu) of {lambda} phage library and incubated for 15 min at 37 C. Infected bacteria were mixed with 6.5 ml melted casein hydrolysate/yeast extract (NZY) top agar (~48 C) and spread onto a bacterial lawn on a 150-mm NZY agar plate, which was incubated at 37 C for 6–8 h, at which time the plaques reached 1–2 mm in diameter. Suspension medium (SM) buffer (8 ml) was added to each plate and incubated at 4 C overnight. After removal of cell debris, the {lambda} phage suspension was either stored at 4 C with chloroform (0.3%, vol/vol) or at –80 C with dimethylsulfoxide (7%, vol/vol). This procedure was repeated 20 times to provide a large stable quantity of high titer stock representing 106 pfu of the primary library.

Freshly grown XL1-Blue MRF' cells (5.4 x 108 cells in 200 µl) were infected with the amplified {lambda} phage library (5.4 x 107 pfu in 20 µl, representing 106 independent clones) at a multiplicity of infection of 1:10 {lambda} phage to cell ratio. ExAssist helper phage (2 x 109 pfu in 200 µl) was then added at a 10:1 helper phage to cell ratio. Cells were incubated for 15 min at 37 C, and 20 ml Luria Bertoni medium were added. After incubation at 37 C for 3 h, cells were treated at 70 C for 20 min to release the phagemid. Cell debris was removed by centrifugation, and supernatant was collected and stored at 4 C. Freshly prepared XLOLR cells were used to determine the titer of excised phagemid. The excised phagemid library was amplified in XLOLR competent cells, and plasmids were prepared by using a Maxi-Prep kit from Promega Corp.

Yeast two-hybrid screening
The yeast cell line YRG2 (genotype Mat{alpha} ura3–52 his3–200 ade2–101 lys2–801 trp1–901 leu2–3 112 gal4–542 gal80–538 LYS2::USAGAL1-TATAGAL1HIS3URA::UASGAL4 17mers(x3)-TATACYC1-lacZ; Stratagene) was used for the yeast two-hybrid screen. The reporter genes were lacZ and HIS3, and the transformation markers were leu2 and trp1. Yeast cells were grown in rich medium YPD (yeast extract/peptone/dextrose) at 30 C according to the Clontech Matchmaker Manual (BD Clontech, Palo Alto, CA). After transformation, yeast cells were plated onto glucose-based SD medium without Trp (SD/–Trp), Leu (SD/–Leu), Trp/Leu (SD/–Trp-Leu), or Trp/Leu/His (SD/–Trp-Leu-His). To prevent basal activation, SD/–Trp-Leu-His media were supplemented with 15 mM 3-aminotriazole (3-AT).

Before the baits were used for the library screening, their basal activation and toxicity were determined. Basal activation was measured by estimating the growth rate of the yeast transformants with pAS1:ovIFNAR2MF (or the other two baits) on the synthetic dropout medium SD/–Trp-His and 15 mM 3-AT at 30 C. The ß-galactosidase (LacZ) activity of the yeast transformant was also measured using colony lift assay. The toxicity of the bait was determined by measuring and comparing the growth rate of the yeast transformant with pAS1:ovIFNAR2MF with that of the positive control YRG2/pAS1:p53 /pAD-GAL4-2.1:SV40 in the liquid SD/–Trp and SD/–Trp-Leu, respectively.

For library screening, yeast YRG2 cells were simultaneously cotransformed with the pAS1:ovIFNAR2 plasmid and the cDNA library plasmids by the lithium acetate method (BD Clontech). Positive clones that showed rapid growth on the medium SD/–Trp-Leu-His and 15 mM 3-AT were selected and then assayed for LacZ. Double-positive colonies, His3+LacZ+, were restreaked onto SD/–Trp-Leu-His and 15 mM 3-AT medium and retested for positive phenotypes.

To confirm a protein-protein interaction, two plasmids, pAS1:bait and pAD-GAL4-2.1:prey, recovered as described below, were simultaneously cotransformed into competent yeast cells by the lithium acetate method, and selection was performed as described above.

To recover plasmids from the yeast, cells from single colonies were grown on SD/–Trp-Leu-His liquid medium. After harvest, cells were lysed in 2% Triton X-100, 1% sodium dodecyl sulfate, 100 mM NaCl, 10 mM Tris-HCl (pH 8.0), 1 mM EDTA, and DNA was collected after phenol/chloroform/isoamyl alcohol (25:24:1; 0.2 ml) extraction. The plasmid DNA was resuspended in 20 µl 1x TE buffer (10 mM Tris-HCl and 1 mM EDTA).

Sequencing and computer-assisted sequence analysis
Inserts in the yeast plasmids were PCR-amplified with the pAD vector-specific primers (pgadf and pgadr; Table 1Go). PCR amplification of the bait fragments with the same templates and the pAS1 vector-specific primers (pmaf and pmar; Table 1Go) provided the positive control. The PCR products were gel-purified with a Qiagen kit (Qiagen, Hilden, Germany) and were used as templates for the subsequent sequencing analysis.

Sequencing was performed using the standard dideoxynucleotide chain termination method with activation domain (AD) vector-specific forward and the reverse primers (pgadf and pgadr; Table 1Go).

The beginning of the insert sequences was inferred from the preceding GAL4-AD sequence. Editing and formatting of DNA and protein sequences were performed by GCG sequence analysis software (Genetics Computing Group, Madison, WI). The identities of the inserts were determined by searching the databases at the National Center for Biotechnology Information website.

RT and nested PCR
RNA was extracted from various ovine tissues, and RT was performed on approximately 2 µg total RNA. Nested PCR used two rounds of PCR. In the first, the RT product was used as template with two oligonucleotides as outer primers (6502nstpcrf1and 6502nstpcrr; Table 1Go) under the following conditions: one pretreatment cycle of 95 C for 5 min; 35 cycles of 1 min at 95 C, 1 min at 56 C, and 1 min at 72 C; and one cycle of 72 C at 5 min. The PCR product from the first round of PCR, which was not visible on the 2% agarose gel, was used as template in the second round of PCR, with two other oligonucleotides as inner primers (6502nstpcrf2 and 65201pcdr; Table 1Go). The thermal cycles were one cycle of 95 C for 5 min for pretreatment; 35 cycles 1 min at 95 C, 1 min at 60 C, and 1 min at 72 C; and one cycle of 72 C for 5 min. The amount of DNA generated from the second PCR was sufficient for subsequent cloning and sequencing. Ovine ribosomal protein S25 was used as the internal control and was amplified for only one round of PCR (primers s25pet15bf and s25pet15br; Table 1Go). Water was used as the negative control.

Preparation and purification of rabbit anti-Sin1lf3 and anti-ovIFNAR2MF antisera
BL21 (DE3) pLyS/pGEX 4T-1:ovSin1lf3 cells were grown at 37 C until the OD600 was about 1.0 and were induced by addition of 5 mM isopropyl-ß-D-thiogalactopyranoside. After 4–6 h, cells were collected, washed, and lysed using a French pressure cell press (SLM-Aminco, Urbana, IL). GST fusion protein was purified from the supernatant solution by affinity chromatography on glutathione beads according to the manufacturer recommendation (Amersham Biosciences). Purity was assessed by SDS-PAGE gels, and the protein was sterilized by filtering through 0.22-µm pore size filters (Nalgene-Nunc, Rochester, NY). Antiserum was raised in New Zealand White rabbits (44). The first immunization involved 250 µg Sin1 in Freund’s complete adjuvant. Subsequent booster immunizations with 150 µg in incomplete adjuvant were performed at approximately monthly intervals for 6 months. Serum was collected from an ear vein. The antiserum was cleared of GST-reactive antibodies by passing through GST beads. A portion was then applied to a GST-Sin1lf3 affinity column to remove immunoglobulins that bound Sin1. The flow-through was used as a reagent for negative controls.

GST-ovIFNAR2MF was prepared as described above, and rabbits were immunized using an identical procedure. IgG was purified after adsorption (binding buffer: 3.2 M NaCl and 1.6 M glycine, pH 9.0) to an Affi-Prep protein A Matrix (Bio-Rad Laboratories, Hercules, CA) and eluting with 100 mM sodium citrate, pH 3.0. The solution was neutralized with 1 M Tris-HCl (pH 8.0). GST-reactive immunoglobulin was purified and processed as described above.

Mammalian cell lines
COS-1 cells were maintained in DMEM supplemented with 10% fetal bovine serum and 1% penicillin and streptomycin at 37 C under an atmosphere of 5% CO2 and air. The mouse fibroblast cell line L929, including the cells stably transfected with ovIFNAR2 (45), were maintained in RPMI 1640 supplemented with 10% fetal bovine serum and 1% penicillin and streptomycin at 37 C under 5% CO2 and air. Before transfection, the medium was changed to DMEM with the same supplements.

Immunohistochemistry
Immunohistochemistry was performed on 4.5-µm sections from d 14 pregnant sheep endometrium (44). Sections were exposed to rabbit anti-ovSin1lf3 antiserum in 1x Tris-buffered saline/Tween 20 (dilution, 1:1000) at 4 C overnight. After washing, the secondary antibody (biotinylated antimouse and antirabbit IgG, Dako LSAB2 system, Dako-Cytomation, Carpinteria, CA) was added without dilution at room temperature for 30 min. Slides were washed and exposed to undiluted streptavidin-coupled horseradish peroxidase (Dako LSAB2 system). Color development was performed by incubating the slides with diaminobenzidine substrate solution according to the instructions of the manufacturer. Sections were counterstained with hematoxylin for 30 sec. Preimmune and adsorbed antisera were used as negative controls. Sections were viewed under a Provis AX70 microscope (Olympus Corp., New Hyde Park, NY).

Transfection, coimmunoprecipitation, and immunoblotting
COS-1 cells (6.5 x 105/dish) were grown overnight to about 60% confluence in 60-mm culture dishes, then transiently transfected with 20 µg each of pCMV-Myc:Sin1 and pFLAG-CMV-6b:IFNAR2CD using the standard calcium phosphate precipitation method (46). Cells were washed 6 h after transfection and incubated in fresh DMEM with appropriate supplements. Whole cell lysates were prepared 48 h after transfection using 1x passive lysis buffer (Promega Corp.) supplemented with a protease inhibitor cocktail (1:200; Sigma-Aldrich Corp.). For immunoprecipitation, 600 µg cell lysate protein were incubated with 30 µg mouse anti-FLAG mAb in 1x immunoprecipitation buffer [50 mM Tris-HCl (pH 7.5), 15 mM EGTA, 100 mM NaCl, 0.1% (wt/vol) Triton X-100, 1x protease inhibitor mixture, 1 mM dithiothreitol, and 1 mM phenylmethylsulfonylfluoride] at 4 C for 4 h. The antigen-antibody complex was collected by adding 40 µl (bed volume) protein A/G Plus-Agarose (Santa Cruz Biotechnology, Inc., Santa Cruz, CA) and incubating at 4 C for 2 h. After three washes, the complex-bound resin was suspended in 1x sodium dodecyl sulfate buffer and boiled, and proteins were resolved on 12.5% SDS-PAGE gels. After Western blotting, proteins carrying the Myc epitope tag were detected with mouse anti-Myc mAb (BD Clontech) using chemiluminescence with the Western-Star kit (Tropix, Bedford, MA) according to the instructions of the manufacturer.

Immunofluorescence
To determine the distribution of Sin1 in COS-1 cells, cells were transiently transfected with the mammalian expression construct pCMV-Myc:ovSin1 using the calcium phosphate precipitation method. COS-1 cells were inoculated into six-well plates, each well containing a glass coverslip. Cells were grown for 24 h to 40–60% confluence (~7.5x 105 cells) and transfected with 10 µg plasmid. After 36 h, the coverslips were washed twice with 1x PBS and exposed to 2% (wt/vol) paraformaldehyde containing 0.1% Triton X-100 in 1x PBS for 30 min at 4 C. They were then incubated with 50% goat serum at room temperature for 2 h, washed with 1x PBST (1x PBS buffer with 0.1% Tween 20), and incubated with mouse anti-Myc mAb (dilution, 1:1000) at 4 C overnight. After washing three times with 1x PBST, the coverslips were incubated with Alexa Fluor 488 F(ab')2 of goat antimouse IgG (dilution, 1:1000) at room temperature for 2 h, followed by three washes with 1x PBST. Nuclei were stained with 4',6-diamido-2-phenylindole hydrochloride (DAPI; 1 µg/ml; Sigma-Aldrich Corp.) in 1x PBS at room temperature for 15 min. After an additional washes with 1x PBS, coverslips were mounted on glass slides with one drop of VectaShield (Vector Laboratories, Inc., Burlingame, CA) and sealed with nail polish. Fluorescence analysis was performed on an Eclipse E800 microscope (Nikon, Melville, NY). Mock-transfected cells were used as negative controls.

To colocalize transfected Myc-Sin1 in L929 cells stably expressing IFNAR2 (L929/ovIFNAR2 cells), cells were again grown in six-well plates on coverslips. When they reached 90–95% confluence (1 x 106 cells/well), the RPMI 1640 medium was replaced with DMEM, and cells were transfected with pCMV-Myc:ovSin1 by the Lipofectamine method (LipofectAmine2000 reagent, Invitrogen Life Technologies). A volume of 15 µl LF2000 and 15 µg plasmid were added to each well. Cells were treated as described above for COS-1 cells, except that they were incubated with mouse anti-Myc mAb (dilution, 1:1000). Bound mouse IgG was detected with Alexa Fluor 488 F(ab')2 of goat antimouse IgG (dilution, 1:1000). To detect ovIFNAR2, coverslips were incubated with purified rabbit anti-ovIFNAR2 polyclonal antiserum (1:200; 1 µg/ml) at 4 C overnight. After washing, coverslips were exposed to Alexa Fluor 568 F(ab')2 of goat antirabbit IgG (H+L) (dilution, 1:2000), at room temperature for 1 h, and nuclei were stained with DAPI. Cells were viewed on a Nikon Eclipse E800 microscope. Mock-transfected cells were used as a negative control. L929 cells not expressing ovIFNAR2 provided a second negative control. The third negative control employed the rabbit preimmune serum instead of IFNAR2 antiserum.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Identification of Sin1 as a protein that interacts with IFNAR2 in the yeast two-hybrid screen
To screen for proteins that bound to the cytoplasmic domain of IFNAR2, three baits were designed (Fig. 1Go). The first consisted of the entire cytoplasmic domain (ovIFNAR2LF). The second (ovIFNAR2MF) encoded the C-terminal region of the cytoplasmic domain ovIFNAR2, a 185-aa-long peptide that included SLEDC motifs, which are not present in either the murine (41) or human polypeptides (35, 36, 47), but are repeated three times in the ovine protein and twice in the bovine protein (36) (Fig. 1Go). The MF bait also contained the established STAT2 binding domain, five conserved acidic regions (A2-A6), and the conserved C-terminal domain. The third bait constructed was a short fragment (Thr352-Gln417; ovIFNAR2SF) that contained the SLEDC repeats at its amino terminus and represented the first third of the MF bait. Here we describe the outcome of a screen performed with ovIFNAR2MF.

Yeast colonies transformed with pAS1:ovIFNAR2MF and selected for growth on the SD/–Trp medium grew at only about one quarter the rate as controls on SD medium. To minimize this problem, yeast cells were transformed simultaneously with the bait vector and the library vector. To reduce the background growth on the selective SD/–Trp-Leu-His medium, it was necessary to add 15 mM 3-AT.

Approximately 6.5 x 106 yeast colonies were screened. Eighty His+LacZ+-positive clones were selected randomly for further analysis. Of them, 74 contained both the AD and DNA binding domain vectors and had inserts ranging in size from 20–1320 bp. Thirty of these clones had open reading frames and no frame shift, and could be placed into two groups (Table 2Go). The first group of 25 comprised five ribosomal proteins (RPS26, RPS23, RPL26, RPL12, and RPS25) and two ubiquitin-ribosomal protein fusion proteins (Ubq-L40 and Ubq-S27a). Because the bait was acidic (pH 4.0) and the proteins were basic, the interaction of these proteins with IFNAR2 was considered likely to be nonspecific, although a true interaction could not be ruled out. The second group consisted of five hypothetical polypeptides between 50 and 100 aa in length, which could not be matched functionally with other known proteins, and a 114-aa polypeptide corresponding to the C-terminus of a little studied protein previously named Sin1 (48, 49, 50).


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  		TABLE 2. Potential IFNAR2-interacting proteins obtained in the yeast two-hybrid screen with ovIFNAR2MF as bait

 
The interaction between ovSin1SF (in the AD vector used as the prey) and pAS1:ovIFNAR2MF (as the bait) was confirmed by cotransformation in a separate two-hybrid assay in virgin yeast cells (data not shown). No positive colonies were obtained when the bait vector was transformed with an empty prey vector, or when the prey vector was transformed with an empty bait vector. As expected, the positive control (p53 and SV40 T antigen) provided the His3+/LacZ+ phenotype.

The unique presence of the tandem SLEDC repeats in ovine and bovine IFNAR2 (36) suggested that they might play a unique role in IFN signal transduction in sheep and cattle, but not in mice or humans. The bait used for the yeast two-hybrid screening, ovIFNAR2MF, contained the SLEDC motifs at its N terminus. To determine whether this region of the bait mediated the interaction between ovIFNAR2MF and Sin1, the yeast-two hybrid assay was performed with the Sin1-AD vector and pAS1:ovIFNAR2SF (Fig. 1Go), which expressed a 65-aa polypeptide containing the three SLEDC repeats. The plasmid expressing the full-length cytoplasmic domain, pAS1:ovIFNAR2LF, was used as a positive control. As expected, ovIFNAR2LF provided the His3+/LacZ+ phenotype in presence of coexpressed Sin1, whereas ovIFNAR2SF did not (data not shown). Together these experiments suggest that the carboxyl end of ovine Sin1 binds within the 123-aa C-terminal region of IFNAR2 and that the SLEDC motifs are not involved in the interaction.

Cloning of full-length ovSin1 cDNA
The ovSin1 cDNA fragment identified above shared approximately 82% nucleotide sequence identity with chicken Sin1 cDNA (AF153127) and 94% with a human Sin1 cDNA fragment (BC002326) in their regions of overlap (data not shown). It was therefore assumed that a similar degree of similarity extended through the full-length transcripts. Two upstream forward primers were designed, one representing a sequence conserved within the 5'-untranslated region (5'UTR) of the human and chicken sequences, and the other a conserved sequence beginning at the start codon (Table 1Go). The two downstream primers were from within the 3'UTR of the ovSin1 cDNA obtained in the yeast two-hybrid screen. These pairs of primers were used as the inner and outer primers for the nested PCRs to clone an ovSin1 cDNA that included the entire open reading frame. The RT products from two randomly selected sheep tissues (kidney and pituitary) were used as template. A band of approximately 1500 bp was obtained in both reactions. Subsequent sequencing confirmed that the cloned cDNAs were identical and that both represented ovSin1.

ovSin1 is 522 aa in length (Fig. 2Go), of theoretical molecular weight 59,200, and encoded by a 1569-nucleotide long open reading frame (GenBank accession no. AF153127). The cDNA shares 94.3% and 81.7% sequence identity at the nucleotide level with the sequences from human and chicken, respectively (data not shown). The inferred aa sequences are even more highly conserved (98.6% and 88.3%, respectively) (Fig. 2Go), and all three proteins are of identical length. The majority of the substitutions are conservative. A search of the Drosophila genome sequence reveals a single intronless gene of 569 codons that corresponds to Sin1 (Fig. 2Go). Its inferred aa sequence could be aligned quite closely with that of the full-length ovine, human, and chicken proteins after introducing several gaps. It demonstrated approximately 30% identity with the ovine, human, and chicken Sin1 sequences. Apparent orthologs of Sin1 can be detected in the genome sequences of human, mouse, rat, zebrafish, Caenorhabditis, Neurospora, Saccharomyces cerevisiae, and Schizosaccharomyces pombe (data not shown). In each species only a single gene appears to be present (50). In addition established sequence tag (EST) sequences from cow and pig could be assembled to provide full-length cDNA (data not shown).



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  		FIG. 2. The deduced aa sequences of Sin1 from sheep, human, chicken, and fruit fly. Sin1 protein sequences from various species were aligned by using Pileup and Genedoc. Black shading indicates conserved aa. Oa, Ovis aries (GenBank accession no. AY547378). Hs, Homo sapiens (GenBank accession no. NM_024117 and BC002326); Gg, Gallus gallus (GenBank accession no. AF153127); Dm, Drosophila melanogaster (GenBank accession no. AE003814). The ovSin1 fragment identified in the yeast two-hybrid screen is underlined.

 
Expression of Sin1 transcripts across tissues and species
To determine the expression pattern of ovSin1 mRNA in sheep tissues, nested PCR was performed with RT products as templates. The first round of PCR with 35 thermal cycles did not yield products visible on the agarose gels, presumably reflecting the low abundance of Sin1 mRNA. In contrast, the internal control, ovRPS25, could be easily amplified by this procedure (Fig. 3Go, lower panel). After a second round of PCR with the nested primers, bands of the anticipated sizes were noted for all tissues (d 15 conceptus, kidney, pituitary, liver, and endometrium; Fig. 3Go, upper panel). ovSin1 mRNA appears to be widely expressed, but may be present in low concentrations.



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  		FIG. 3. The distribution of Sin1 mRNA in sheep. Reverse-transcribed RNA from tissues was used as the template. A Sin1 fragment (Sin1lf3, encoding residues V227-Q522; Fig. 2Go) was amplified using nested PCR. RPS25 was amplified using normal PCR. A, Upper panel, M, DNA marker; 1, water control; 2, conceptus; 3, endometrium; 4, kidney; 5, liver; 6, pituitary. Lower panel, ovRPS25 was used as the internal control. The pictures are negatives. Arrows show the positions of Sin1lf3 (891 bp) and RPS25 (378 bp). B, Alternatively spliced isoforms (Sin1a and Sin1b, denoted by arrowheads) of ovSin1 were detected in all tissues examined. Shown here is not the full-length ovSin1 cDNA but a fragment (Sin1a is 891 bp; Sin1b is 773 bp; determined by DNA sequencing.). Lane 1, endometrium; lane 2, kidney; lane 3, liver; lane 4, pituitary.

 
The widespread distribution of the Sin1 transcript is supported by evidence from other species. A search of a bovine ESTs indicated the presence of Sin1 transcripts in bovine kidney, skin, d 20–40 embryos, and a pooled library made from testis, thymus, semitendonosus muscle, longissimus muscle, pancreas, adrenal, and endometrium (see Table 3 published as supplemental data on The Endocrine Society’s Journals Online web site at http://endo.endojournals.org.). Similarly, Sin1 sequences have been entered into EST data bases from at least 30 different human and numerous different mouse sources, as well as from multiple tissues of rat, pig, chicken, zebrafish, frog, and fruit fly (see supplemental data, Table 3). Together, this information shows that Sin1 is not only highly conserved, particularly within vertebrates, but also widely distributed across different organs, tissues, and cell types.

Existence of alternatively spliced forms of ovSin1 mRNA
During cloning of a C-terminal fragment of Sin1, the PCR-amplified Sin1 transcript was always found as two bands (Fig. 3Go), each of which represented Sin1. The upper band, Sin1a, was 891 bp long, whereas the lower band, Sin1b, lacked a 108-bp fragment (encoding a 36-residue peptide, aa 321–356; Fig. 2Go) in the middle of the full-length transcript. Densitometric comparisons indicated that Sin1b generally provided only about 5% of the total cDNA in the amplified samples. Both isoforms were present in all ovine tissues examined (Fig. 3BGo).

To decide whether Sin1a and Sin1b represented different gene products or alternatively spliced forms of a single gene, alignment was performed between the Sin1 cDNA sequences and human genomic DNA sequences. Only a single human gene could be found, located on chromosome 9 (9q34.11–9q34.12; ~240 kb). The human Sin1 gene has 11 exons (Fig. 4Go). ovSin1b lacks exon 7. Schroder et al. (50) reached essentially identical conclusions in a recent independent study of the human Sin1 gene (50). All introns of the human Sin1 gene have typical boundaries, i.e. 5'-GT-3' at their 5' ends and 5'-AG-3' at their 3' ends (data not shown). These results strongly suggest that Sin1a and Sin1b result from alternative splicing of transcripts from a single Sin1 gene.



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  		FIG. 4. The gene structure of human Sin1. The human genome database was searched using BLASTn with the ovine Sin1 cDNA sequence including 5'- and 3'UTRs. Only a single Sin1 gene was found (on human chromosome 9; 9q34.11–9q34.12; ~240 kb). The synteny map between humans and sheep suggests that the ovSin1 gene is located on sheep chromosome 3. The green box (exon 7; 112 bp) is not present in the alternatively spliced form Sin1b (represented as a folded line in the lower panel). Exon 7 encodes aa 320–356 (see Fig. 2Go). The start codon ATG begins at the 72nd base in exon 1. The stop codon TAG begins at the123rd base of exon 11.

 
Identical alternative splicing for Sin1 appears to be conserved in the human. For example, the Sin1 cDNA cloned by Colicelli et al. (48) was Sin1a, whereas an EST encoding a hypothetical protein (MGC2754 mRNA, GenBank accession no. BC002326) corresponded to Sin1b and lacked exon 7.

Localization of Sin1 in ovine endometrium
To determine the expression pattern for Sin1 in endometrium, immunohistochemistry was performed on sections of d 14 pregnant sheep endometrium with rabbit anti-ovSin1 antiserum. The antigen was highly concentrated in the luminal epithelium and glandular epithelium, but was barely detectable in the stromal tissue (Fig. 5Go, A and B). An identical expression pattern was previously noted for ovIFNAR2 in ovine endometrium on d 14 of pregnancy (44). Accordingly, uterine epithelial cells are generally considered to be the target for conceptus-derived IFN-{tau}.



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  		FIG. 5. The localization of Sin1 on d 14 sheep endometrium. Rabbit anti-Sin1 antiserum was used as the first antibody (1:1000). Biotinylated goat antirabbit Ig was used as the secondary antibody and was detected by streptavidin-coupled horseradish peroxidase (HRP), with diaminobenzidine as the substrate. A, Preimmune antiserum (negative control); B, antiserum absorbed with GST-Sin1lf3 (negative control); C, antiserum against Sin1. Bars, 100 µm.

 
Cellular distribution of ovSin1 in transiently transfected COS1 cells
An in silico analysis (motif scan at http://hits.isb-sib.ch) (51) of Sin1 primary sequence was generally uninformative, but did reveal a weak match for a bipartite nuclear localization signal (R81RRSNTAQRLERLRKERQ98) of Sin1. However, fluorescence immunolocalization performed on nontransfected cells provided too equivocal a signal to judge whether Sin1 was, in fact, located in the nucleus. COS-1 cells were therefore transiently transfected with pCMV-Myc:ovSin1 and expressed antigen detected by the mouse monoclonal antibody {alpha}-Myc. Western blotting analysis revealed a single positive band of estimated molecular weight 70,000, which corresponded to the expected size of the epitope-tagged Sin1 fusion protein (Fig. 6Go, upper panel). The same antibody showed that the Myc-Sin1 antigen was present predominantly in the cytoplasm and not in the nucleus of transfected cells. Antigen appeared to be associated with the plasma membrane and internal cytoplasmic structures, rather than being distributed diffusely through the cytoplasm (Fig. 6Go, B and C). The same localization pattern, including lack of nuclear labeling, was observed whether transfected Myc-Sin1 was expressed at high or low levels (data not shown).



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  		FIG. 6. Expression and subcellular localization of Sin1 in transfected COS-1 cells. COS-1 cells were transiently transfected with Myc-Sin1. The upper panel shows the Western blot for the detection of Myc-Sin1 in lysates of transfected cells. Detection was by mouse anti-Myc monoclonal antibody and enhanced chemiluminescence (ECL). Lane 1, Nontransfected control cells; lane 2, empty plasmid transfection; lane 3, Myc-Sin1 transfection. The arrowhead denotes the position of Myc-Sin1. Bars, 15 µm. For the images, cells were fixed and stained with DAPI to visualize nuclei (A). Epitope-tagged Myc-Sin1 was detected with mouse anti-Myc monoclonal antibody (1:1000) and Alexa Fluor 488 F(ab')2 of goat antimouse IgG (dilution 1:1000; B). The bottom panel (C) shows the merged images.

 
Coimmunoprecipitation of ovSin1 and ovIFNAR2
To determine whether ovSin1 and ovIFNAR2 formed an association within cells, both proteins were expressed in COS-1 cells after transient transfection. Each protein was tagged with a different antigen (Myc and FLAG, respectively). Both Myc-Sin1 and FLAG-IFNAR2CD were readily detectable by Western blotting of the cell lysates (Fig. 7Go, B and C). When FLAG-IFNAR2CD from the cell extracts was collected as an immune complex using mouse anti-FLAG monoclonal antibody, Myc-Sin1 was found to be present in the complexes by Western blotting (Fig. 7DGo). The coassociation of Myc-Sin1 and FLAG-IFNAR2CD suggests that the two proteins interact with each other in vivo.



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  		FIG. 7. Interaction between ovIFNAR2CD and ovSin1after coexpression in COS-1 cells. COS-1 cells were cotransfected with expression vectors for ovIFNAR2CD (tagged with FLAG) and ovSin1 (tagged with Myc). Whole cell lysates were collected 48 h after transfection. Immunoprecipitation was then performed with the mouse monoclonal antibody anti-FLAG ({alpha}-FLAG). Immunoprecipitates were resolved by 12.5% SDS-PAGE and blotted, and antigen was detected with the mouse monoclonal antibody anti-Myc ({alpha}-Myc). A, Expression vector used for transfection; B, expression of FLAG-ovIFNAR2CD in transfected cells, detected with {alpha}-FLAG; C, expression of Myc-ovSin1 in transfected cells detected with {alpha}-Myc. D, Presence of Myc-ovIFNAR2 in the {alpha}-FLAG immunoprecipitates detected with {alpha}-Myc. IB, antiserum used for immunoblotting; IP, antiserum used for immunoprecipitation. Lane 1, Mock vector transfection; lane 2, cotransfection with expression vectors for IFNAR2CD and Sin1; lane 3, transfection with expression vector for Sin1; lane 4, transfection with expression vector for IFNAR2CD.

 
Colocalization of ovSin1 and ovIFNAR2 in transfected L929 cells
To determine whether Sin1 colocalized with ovIFNAR2, the mouse cell line L929/ovIFNAR2 (45), which stably expresses ovIFNAR2, was transiently transfected with pCMV-Myc:ovSin1. Dual color immunofluorescence was then employed to compare the localization of the two antigens (Fig. 8Go). IFNAR2 was strongly expressed in approximately 40% of these stably transfected cells (Fig. 8BGo). Staining was absent in the nucleus, but was associated with the plasma membrane and cytoplasmic structures, particularly in the perinuclear region of those cells where overall expression appeared to be high. Sin1 expression varied considerably between cells, presumably reflecting inconsistencies in the transfection efficiency of different cells. Only about 1% of the cells showed strong staining equivalent to that exhibited by IFNAR2 (Fig. 8CGo). In such cells, Sin1 staining generally mirrored, but was not exactly coincident with, that of IFNAR2, with a particularly high concentration of antigen present in the perinuclear region (Fig. 8DGo). These results are consistent with the view that Sin1 is predominantly associated with cellular membranes, but not exclusively with IFNAR2.



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  		FIG. 8. Colocalization of Sin1 and IFNAR2 in L929 cells. Stably transfected L929/ovIFNAR2 cells were transiently transfected with the expression vector for Sin1 (tagged with Myc). Cells were fixed, and immunofluorescence was per-formed after treating permeabilized, fixed cells with the rabbit polyclonal anti-ovIFNAR2 ({alpha}-R2), followed by goat antirabbit IgG fluorescent conjugate. A similar method was used to detect the location of Myc-Sin1 using mouse monoclonal antibody anti-Myc ({alpha}-Myc), followed by goat antimouse immunoglobulin G fluorescent conjugate. Nuclei were stained with DAPI. A, Nuclear staining (purple); B, ovIFNAR2 staining (red); C, Myc-ovSin1 staining (green); D, merged image of A–C. Bars, 15 µm. Arrows indicate the regions of apparent colocalization (yellow) in two cells that showed elevated expression of transfected ovSin1.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Sin1 was first identified in a search for human proteins that were capable of suppressing a RAS2 mutant (a constitutively active form of RAS) expressed in the budding yeast (S. cerevisae) (48). Among other abnormalities, the RAS2 mutant demonstrates high levels of activated adenylate cyclase, resulting in an elevated intracellular concentration of cAMP. Such cells are highly susceptible to a variety of environmental stresses, including heat shock. They also proliferate poorly. A transfected human Sin1 cDNA fragment was able to rescue this phenotype. Hence, the gene product was originally called a RAS suppressor (48). Some years later, a two-hybrid screen of a cDNA library from the fission yeast, S. pombe, in which the yeast SAPK, Sty1/Spc1, was used as the bait, led to the discovery of a gene product that interacted with Sty1/Sp1. As a result, the protein, which had sequence similarity to those identified previously in S. cerevisae and human, was called SAPK-interacting protein 1, hence the current name Sin1 (49). The absence of Sin1 in S. pombe provided a phenotype similar to that obtained when Sty1/Spc1 was deleted, namely sensitivity to environmental stressors, sterility, and slow proliferation (49). In yeast, Sin1 protein is phosphorylated constitutively, although phosphorylation becomes enhanced after stress, but not, curiously, as the result of Sty1/Spc1 kinase activity, nor is Sin1 required for Sty1/Spc1 activation. Rather, Sin1 is required to enhance the phosphorylation of Atf1 by Sty1/Spc1 and for effective transcription via the AP-1-like factor, Pap1, in response to environmental stress (49). Taken together, the data from yeast suggests that Sin1 is likely to be involved in either facilitating or altering stress responses through modulating the activity of the SAPK signaling pathway. Its exact placement in that pathway is unclear, nor are there motifs in the Sin1 aa sequence that give any clue to its functional activity. It carries no obvious signatures either of an enzyme activity or of docking sites for other proteins (51).

Type I IFNs have been known for several years to regulate the activity of the p38 SAPK pathway in some mammalian cells (12, 13, 14, 15, 16, 17, 52), but the relationship has been obscure. Given the interaction between ovIFNAR2 and ovSin1, the data suggest that Sin1 may provide a link between the two pathways. Conceivably, Sin1, constitutively bound to the carboxyl end of IFNAR2, becomes phosphorylated or otherwise activated after IFN binding to the receptor and in this form is able to bind p38 SAPK and direct downstream signaling events, including transcription of selected genes involved in stress responses.

Full-length vertebrate Sin1 is 522 aa in length (Fig. 2Go). Residues 341–522 of the chicken protein, when expressed in S. pombe, are sufficient to provide partial rescue of yeast Sin1 function in stress responses (49), but no other functional studies have been reported since then. The interaction with IFNAR2 also appears to reside close to the carboxyl end of the molecule and specifically within the terminal 114 residues. Accordingly, it is interesting to compare Sin1 sequences among species that have a functional IFN system (vertebrates) with species that appear to lack one, e.g. insects (Fig. 2Go). Conservation between vertebrates and Drosophila over this stretch of primary structure is relatively poor, but includes several vertebrate sequences, such as HDYKHLYFESDA (aa 458–469) and QKEKKSG (514–521), which have no obvious similarity with the aligned insect sequence. Additional experiments are needed to determine whether these motifs participate in binding to IFNAR2.

The results of the two-hybrid screen performed in yeast, which indicated a possible association between Sin1 and IFNAR2, were backed up by experiments performed on mammalian cells. For example, when IFNAR2 and Sin1 were overexpressed in COS-1 cells, the two could be collected together in immune complexes. Sin1 showed a similar distribution to IFNAR2 in uterine endometrium, with a high concentration of expression in the surface and glandular epithelium. Finally, the subcellular localizations of IFNAR2 and Sin1 were comparable when the two were coexpressed in cultured cells. In stably transfected 3T3 cells, ovIFNAR2, an integral membrane protein, was associated with the plasma membrane and with the perinuclear region of the cytoplasm, where membranes in transit to the surface are expected to reside. ovSin1, whose open reading frame lacks both a signal sequence and an obviously hydrophobic membrane-spanning domain, showed a very similar distribution when expressed in these cells (Fig. 8Go). In some instances the images portraying Sin1 and IFNAR2 localization converged, suggesting that Sin1 resides predominantly close to membrane surfaces, possibly in association with the cytoplasmic regions of receptors, which would include IFNAR2. The fact that Sin1 was identified in a yeast two-hybrid screen with IFNAR2 as bait suggests that the two proteins interact constitutively, most likely without a dependence on covalent modification.

The experiments described in this paper, which led to the discovery of Sin1 as a protein that associated with IFNAR2, were initiated on the premise that in sheep endometrium, where a type 1 IFN, IFN-{tau}, has an unusual function, namely, signaling from the conceptus to the mother during early pregnancy, a nonclassical signal transduction pathway for IFN might be operative. In addition, the epithelial lining of the ovine uterus is particularly rich in IFNRs and presumably in its ability to respond to IFN (43, 44). IFN-{tau} from the conceptus suppresses the oxytocin-induced secretion of PGF by the endometrium, thereby protecting the corpus luteum from regression and maintaining the pregnancy (53, 54, 55). In experiments performed with primary bovine endometrial epithelial cells and an established cell line (BEND), IFN-{tau} reduced the production of PGF, down-regulated the amount of cyclooxygenase-2 (COX-2; prostaglandin H synthase-2) protein (56, 57, 58, 59) and, through a transcription-dependent mechanism, rapidly destabilized COX-2 mRNA (60), although others have reported contrasting results (61, 62). Because IFN-{tau} stimulates tyrosine phosphorylation, homo- and heterodimer formation of nuclear translocation, and DNA binding of STAT1, -2, and -3 in such cells (63), it has generally been assumed that this pathway must be the one relevant to IFN-{tau} effects on prostaglandin metabolism. The p38 MAPK pathway might provide an alternative means for this cytokine to manifest its protective effects on the corpus luteum. Because Sin1 modulates the effects of activated Ras in yeast by acting downstream of the SAPK Sty1/Spc (49), there is a possibility that it has an analogous role in uterine endometrial cells. Conceivably, Sin1 is able to redirect or otherwise constrain the activated p38 MAPK pathway when IFN-{tau} targets the IFNR on the uterine epithelium, such that the half-life of COX-2 mRNA is not extended, PGF production is reduced, and inflammatory responses to the conceptus are minimized. Because the effect of IFN-{tau} on COX-2 expression and PGF production is concentration dependent, inhibiting at low concentrations and promoting at high concentrations (64), it will be interesting to determine whether these dose-dependent effects correlate with the activity of the p38 MAPK pathway as well as with the subcellular location and phosphorylation status of Sin1.


   Acknowledgments
 
We thank Dr. Chun-Sheng Han for providing the ovIFNAR2 cDNA and the L929/ovIFNAR2 cells. Mr. James Bixby assisted with application of the GCG software. Drs. Cheryl Rosenfeld and Peter Sutovsky were invaluable as instructors in immunocytochemistry and immunofluorescence experiments, respectively. We thank Wei Zhou for the COS-1 cells, Tina Parks and Anindita Chakrabarty for help in raising and purifying rabbit antibodies, and Dr. Wayne Schroder (Queensland Institute for Medical Research, Brisbane, Australia) for providing a copy of his manuscript (50 ) ahead of publication. Drs. Toshihiko Ezashi, Mark Hannink, and John Cannon provided critical input throughout the project.


   Footnotes
 
This work was supported by National Institutes of Health Grant HD-R01 21896.

Current address of S.-Z.W.: Center for Developmental Biology, University of Texas Southwestern Medical Center, 6000 Harry Hines Boulevard, Dallas, Texas 75390.

Abbreviations: aa, Amino acid; AD, activation domain; 3-AT, 3-aminotriazole; COX-2, cyclooxygenase-2; DAPI, 4',6-diamido-2-phenylindole hydrochloride; EST, expressed sequence tag; GST, glutathione-S-transferase; IFN, interferon; IFNR, interferon receptor; Jak, Janus kinase; ov, ovine; pfu, plaque-forming unit; PGF, prostaglandin F2{alpha}; SAPK, stress-activated protein kinase; SD, synthetic dropout; Sin1, stress-activated protein kinase-interacting protein 1; STAT, signal transducer and activator of transcription; SV40, simian virus 40; UTR, untranslated region.

Received July 30, 2004.

Accepted for publication August 23, 2004.


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 Introduction
 Materials and Methods
 Results
 Discussion
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