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Endocrinology Vol. 140, No. 12 5469-5477
Copyright © 1999 by The Endocrine Society


ARTICLES

Characterization of the Antiapoptotic Bcl-2 Family Member Myeloid Cell Leukemia-1 (Mcl-1) and the Stimulation of Its Message by Gonadotropins in the Rat Ovary1

Chandra P. Leo2, Sheau Yu Hsu, Sang-Young Chun, Hyun-Wook Bae and Aaron J. W. Hsueh

Division of Reproductive Biology, Department of Gynecology and Obstetrics, Stanford University Medical Center (C.P.L., S.Y.H., A.J.W.H.), Stanford, California 94305-5317; and the Hormone Research Center, Chonnam National University (S.Y.C., H.W.B.), Kwangju 500–757, Republic of Korea

Address all correspondence and requests for reprints to: Dr. Aaron J. W. Hsueh, Division of Reproductive Biology, Department of Gynecology and Obstetrics, Stanford University Medical Center, Stanford, California 94305-5317. E-mail: aaron.hsueh{at}stanford.edu


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The majority of ovarian follicles undergo atresia mediated by apoptosis. Bcl-2-related proteins act as regulators of apoptosis via the formation of dimers with proteins inside and outside the Bcl-2 family. Previous studies have identified BAD as a proapoptotic Bcl-2 family member expressed in the ovary. It is known that BAD phosphorylation induced by survival factors leads to its preferential binding to 14–3-3 and suppression of the death-inducing function of BAD. To identify ovarian binding partners for hypophosphorylated BAD, we performed a yeast two-hybrid screening of a rat ovary complementary DNA library using as bait a mutant BAD incapable of binding to 14–3-3. Screening of yeast transformants yielded positive clones encoding the rat ortholog of Mcl-1 (myeloid cell leukemia-1), an antiapoptotic Bcl-2 protein. Amino acid sequence analysis revealed that rat and human Mcl-1 showed a complete conservation of the Bcl-2 homology domains BH1, BH2, and BH3. In the yeast two-hybrid system, Mcl-1 binds to the hypophosphorylated mutant of BAD and interacts preferentially with different proapoptotic (Bax, Bak, Bok, Bik, and BOD) compared with antiapoptotic Bcl-2 family members (Bcl-2, Bcl-xL, Bcl-w, Bfl-1, CED-9, and BHRF-1). Northern blot hybridization demonstrated expression of Mcl-1 transcripts of 2.3 and 3.7 kb in the ovary and diverse other rat tissues. In immature rats, PMSG treatment led to a transient increase in the 2.3-kb Mcl-1 transcript, peaking at 6 h after injection and returning to baseline levels after 24 h. Moreover, the same transcript was induced in the PMSG-primed preovulatory rat ovary 6 h after the administration of ovulatory doses of either hCG or FSH. In situ hybridization studies revealed that the gonadotropin stimulation of ovarian Mcl-1 message occurs in both granulosa and thecal cells. In conclusion, rat Mcl-1 was identified as an ovarian BAD-interacting protein and the message for the antiapoptotic Mcl-1 protein was induced after treatment with gonadotropins in granulosa and thecal cells of growing follicles.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
IN THE MAMMALIAN ovary, less than 1% of all follicles reach ovulation, whereas more than 99% undergo atresia during reproductive life. Recent studies have established that apoptosis (programmed cell death) is the molecular mechanism underlying follicle atresia (1, 2). Moreover, gonadotropins (as well as estrogens, various growth factors, and cytokines) were found to act as extracellular survival factors for early antral and preovulatory follicles by suppressing apoptosis and thereby rescuing follicles from atresia (3, 4, 5, 6, 7, 8, 9, 10, 11). However, the exact intracellular mechanisms by which gonadotropins suppress apoptosis remain largely unknown.

One key group of intracellular factors regulating apoptosis is the Bcl-2 family of proteins (12). The members of this family can be subdivided into antiapoptotic proteins (such as Bcl-2 and Bcl-xL) and proapoptotic proteins (such as Bax and BAD). It has been proposed that anti- and proapoptotic proteins regulate cell death by binding to each other and forming heterodimers (13, 14). According to this model, a delicate balance between anti- and proapoptotic Bcl-2 family members exists in each cell, and the relative concentrations of these two groups of proteins determine whether the cell survives or undergoes apoptosis.

Of the more than 15 known Bcl-2 family members, each tissue or cell type expresses a specific subset. This laboratory has previously shown that BAD is a proapoptotic Bcl-2 family member expressed in the rat ovary (15). The function of BAD is controlled through its phosphorylation by survival factor-dependent kinases (16, 17). Only the hypophosphorylated form of BAD can heterodimerize with the antiapoptotic proteins Bcl-xL and Bcl-2, thereby leading to cell death. By contrast, the hyperphosphorylated BAD preferentially binds to 14–3-3 proteins, resulting in diminished cell killing (16, 18).

To identify the ovarian binding partners for hypophosphorylated BAD, we screened an ovarian fusion complementary DNA (cDNA) library using the yeast two-hybrid system. As bait, we used a modified rat BAD molecule in which the critical phosphorylation site for 14–3-3 interaction was mutated (serine 137 to alanine). This mutation abolishes the interaction of BAD with 14–3-3 while allowing its interaction with Bcl-xL and thus mimics the hypophosphorylated state of BAD (16, 18). The library screening yielded several positive clones encoding the full-length rat ortholog of human Mcl-1 (myeloid cell leukemia-1).3 Mcl-1 was first discovered as an early induction gene during the differentiation of a human myeloblastic leukemia cell line (19). Subsequent studies established Mcl-1 as an antiapoptotic Bcl-2 family protein with an expression pattern differing from that of Bcl-2 and capable of suppressing cell death induced by various stimuli (20, 21, 22, 23). The expression of Mcl-1 in hemopoietic cells can be induced by various survival factors, leading to enhanced cell viability (24, 25, 26). Here, we describe the cloning of the rat Mcl-1 cDNA, dimerization properties of Mcl-1 with diverse pro- and antiapoptotic Bcl-2 family members, as well as the cellular localization of Mcl-1 messenger RNAs (mRNAs) in the rat ovary and their induction by gonadotropins, which are known follicle survival factors.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Yeast two-hybrid screening of an ovarian cDNA library
The full-length open reading frame (ORF) of the rat BADS137A cDNA (18) was fused in-frame with the GAL4-binding domain into the pGBT9 yeast shuttle vector (CLONTECH Laboratories, Inc., Palo Alto, CA). This vector was used to identify proteins interacting with the mutant BAD molecule by screening 1.5 x 106 transformants from a GAL4-activation domain-tagged rat ovarian fusion cDNA library. Positive transformants were isolated as described previously (18).

Protein interaction assays in the yeast two-hybrid system
Interactions between rat Mcl-1 and diverse other Bcl-2 members were assessed in the yeast two-hybrid system using a pGADGH/Mcl-1 construct and pGBT9 vectors containing the cDNAs of various pro- and antiapoptotic Bcl-2 family proteins (27). Specific binding of different protein pairs was evaluated based on the activation of the GAL1-HIS3 reporter gene in the presence of 30 mM 3-aminotriazole (18).

Animal treatment
For time-course analyses of mRNA expression, female Sprague Dawley rats (Simonsen Laboratories, Inc., Gilroy, CA) were injected sc with 10 IU PMSG (Calbiochem, La Jolla, CA) at 26 days of age and received an ip injection of 10 IU hCG (Schein Pharmaceuticals, Florham Park, NJ) 48 h later. Rats were killed at different time points, and the ovaries were collected for total RNA extraction or were fixed for in situ hybridization analysis. For analysis of Mcl-1 message in enriched granulosa cells, total RNA was extracted from granulosa cells isolated by needle puncture (28). All animal protocols were approved by the administrative panel on laboratory animal care at Stanford University.

For quantification of Mcl-1 message in preovulatory follicles after hCG or FSH administration, 25-day-old female rats received a sc injection of 10 IU PMSG. In addition, sc injections of the GnRH antagonist Org30850 (40 µg/kg BW; Organon, Oss, The Netherlands) were administered on days 25 and 26 to suppress the endogenous secretion of pituitary gonadotropins (29). On day 27, a single ip injection of 10 IU hCG or 30 IU recombinant FSH (Org32489E, Organon) was administered. These doses were chosen because they were found to reliably induce ovulation in this model system (30). Rats were killed before or 6 h after gonadotropin administration, and ovaries were collected for RNA extraction.

RNA extraction and Northern blot analysis
Ovaries were dissected free of adherent tissue, snap-frozen in a dry ice/ethanol bath, and stored at -70 C. Total RNA was extracted using the TRIzol reagent (Life Technologies, Inc., Gaithersburg, MD) according to the manufacturer’s instructions. Twenty micrograms of total RNA per lane were run on agarose-formaldehyde gels before transfer to nitrocellulose membranes and UV cross-linking. Blots containing 2 µg/lane polyadenylated [poly(A)+] RNA extracted from various rat tissues were obtained from CLONTECH Laboratories, Inc. cDNA probes for rat Mcl-1, tissue plasminogen activator (tPA), ß-actin, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were 32P radiolabeled by random priming using a commercial kit (Life Technologies, Inc.). Northern blots were prehybridized and hybridized using ExpressHyb solution (CLONTECH Laboratories, Inc.) following the manufacturer’s instructions. After several washes in 0.1 x SSC-0.5% SDS at 60 C, membranes were exposed to film (at -70 C) or to phosphorimager screens.

Quantitation of Mcl-1 mRNA levels
After exposure of membranes to phosphorimager screens, the signal intensities for the Mcl-1 transcripts (as well as for the GAPDH transcript from a subsequent hybridization) were quantified for each RNA sample using the Storm 860 PhosphorImager and ImageQuant image analysis software (Molecular Dynamics, Inc., Sunnyvale, CA). Alternatively, films were scanned on a GS-710 Imaging Densitometer and analyzed using the Quantity One Software package (Bio-Rad Laboratories, Inc., Hercules, CA). The values for the 2.3- and 3.7-kb Mcl-1 transcripts in RNA samples extracted from rat ovaries before and 6 h after injection of hCG or FSH were then normalized to the respective GAPDH signal. The results are expressed as fold induction compared with the 0 h control, which was arbitrarily set at 1.

In situ hybridization studies
Ovaries were fixed at 4 C for 6 h in 4% paraformaldehyde in PBS, followed by immersion in 0.5 M sucrose in PBS overnight. Cryostat sections (14 µm thick) were mounted on microscope slides coated with poly-L-lysine (Sigma Chemical Co., St. Louis, MO), fixed in 4% paraformaldehyde in PBS, and stored at -80 C until analyzed. To allow for direct comparison between ovarian sections from different experimental groups, all slides were processed simultaneously under identical conditions. The hybridization procedure was essentially the same as that previously described (31). In brief, sections were pretreated serially with 0.2 M HCl, 2 x SSC, pronase (0.125 mg/ml), 4% paraformaldehyde, and acetic anhydride in triethanolamine. Hybridization was carried out at 52–55 C overnight in a mixture containing 35S-labeled rat Mcl-1 complementary RNA probe (108 cpm/ml), 50% formamide, 0.3 M NaCl, 10 mM Tris-HCl, 5 mM EDTA, 1 x Denhardt’s solution, 10% dextran sulfate, 1 µg/ml carrier transfer RNA, and 10 mM dithiothreitol. Posthybridization washing was performed under stringent conditions that included ribonuclease A (25 µg/ml) treatment at 37 C for 30 min and a final stringency of 0.1 x SSC. Slides were dipped into NTB-2 emulsion (Eastman Kodak Co., Rochester, NY) and exposed at 4 C for 3–4 weeks before development. The slides were stained with hematoxylin and eosin and examined under a light microscope with bright- and darkfield illumination.

Statistical analysis
Results are presented as the mean ± SEM. For the ovulation induction experiments using hCG or FSH, differences in normalized levels of Mcl-1 transcript expression among groups were assessed by ANOVA followed by Fisher’s protected least significant differences post-hoc test. Differences in Mcl-1 transcript expression before and after hCG treatment in granulosa cells were analyzed using the unpaired t test. Statistical significance was inferred at P < 0.05.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Isolation of rat Mcl-1 in a yeast two-hybrid screening of an ovarian cDNA library
To identify ovarian binding partners for the proapoptotic protein BAD, a cDNA encoding the rat BADS137A mutant was used as a bait to screen 1.5 x 106 yeast transformants from a rat ovary fusion cDNA library. The serine to alanine substitution in BAD eliminates the critical phosphorylation site of BAD, thus mimicking the hypophosphorylated, apoptosis-inducing form of BAD (16, 18). Based on activation of the GAL1-HIS3 reporter gene, several positive clones encoding BAD-interacting proteins were identified. In addition to multiple clones representing P11 (18), three clones were found to encode a full-length protein with extensive homology to the human Mcl-1 protein (see Fig. 1Go).



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Figure 1. Comparison of the deduced amino acid sequences for rat, human, and chicken Mcl-1 and a zebrafish EST (AI544581). The PEST sequence, Bcl-2 homology domains (BH1, BH2, and BH3), and transmembrane region (TM) are marked. As the chicken and zebrafish sequences contain only partial ORFs, the amino acid numbers (asterisks) refer to the respective start of the known coding sequence fragments.

 
Sequence analysis of rat Mcl-1
Comparison between the deduced amino acid sequences showed that rat and human Mcl-1 share 78% identity with a complete conservation of the Bcl-2 homology domains BH1, BH2, and BH3 as well as the transmembrane region (Fig. 1Go). In addition, rat Mcl-1 retains the long N-terminal sequence containing a proline-glutamic acid-serine-threonine-rich (PEST) domain that has been implicated in targeting the protein for rapid intracellular turnover. Furthermore, use of the rat Mcl-1 amino acid sequence as a query to search the GenBank expressed sequence tag (EST) division identified a zebrafish EST (AI544581) that encodes a protein fragment with closest homology to the mammalian Mcl-1 proteins. Figure 1Go shows an alignment of the deduced amino acid sequences for rat and human Mcl-1 as well as for the recently published partial sequence of chicken Mcl-1 (32) and the zebrafish EST.

Interaction of Mcl-1 with diverse pro- and antiapoptotic Bcl-2 family proteins in the yeast two-hybrid system
The dimerization properties of rat Mcl-1 with different pro- and antiapoptotic Bcl-2 members were assessed in the yeast two-hybrid system (Fig. 2Go). In agreement with its identification in the yeast two-hybrid screening, Mcl-1 interacted strongly with the phosphorylation site mutant of BAD. However, under the conditions employed, Mcl-1 did not bind to wild-type BAD, which appears to be constitutively hyperphosphorylated in yeast cells as suggested by its strong interaction with the {theta}-isoform of 14–3-3. In addition, Mcl-1 also interacted with the proapoptotic Bcl-2 family members Bax, Bak, Bok/Mtd, Bik, and BOD/Bim. By contrast, Mcl-1 dimerized only weakly (Bcl-w, Bfl-1, CED-9, and BHRF-1) or not at all (Mcl-1, Bcl-2, and Bcl-xL) with the antiapoptotic Bcl-2 family members tested in this assay.



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Figure 2. Interaction of Mcl-1 with different Bcl-2 family proteins in the yeast two-hybrid system. The interactions between Mcl-1 and different pro- and antiapoptotic Bcl-2 family proteins are rated as strong (3+), moderate (2+), weak (+), or absent (-), based on the activation of the GAL1-HIS3 reporter gene. pGADGH and pGBT9 denote the empty activation domain and binding domain vectors, respectively, which were included as negative controls. The specific interactions of wild-type BAD and BADS137A with 14–3-3 {theta} and Mcl-1, respectively, illustrate the importance of serine phosphorylation at position 137 in determining the binding partners for BAD (see text). All Bcl-2 family proteins tested in this assay are of mammalian origin, except for CED-9, an antiapoptotic Bcl-2 family protein found in the nematode Caenorhabditis elegans, and BHRF-1, a Bcl-2-homolog encoded by the Epstein-Barr virus.

 
Northern blot analysis of Mcl-1 expression in different rat tissues and developmental expression in the ovary
To investigate the tissue expression pattern of Mcl-1, we hybridized Northern blots containing RNA extracted from various rat tissues with a radiolabeled rat Mcl-1 cDNA probe. As shown in Fig. 3Go, two Mcl-1 transcripts with sizes of 2.3 and 3.7 kb, respectively, were expressed at varying levels in all rat tissues examined. The highest levels of expression were found in spleen, lung, heart, and kidney [Fig. 3AGo, poly(A)+ RNA analyzed], whereas lower expression levels were observed in reproductive tissues, including ovary, oviduct, uterus, and testis (Fig. 3BGo, total RNA analyzed). To further investigate developmental expression of Mcl-1 in the female gonad, total RNA extracted from ovaries at different points of postnatal development was hybridized with the same probe. As shown in Fig. 4AGo, the Mcl-1 message is expressed from day 3 after birth and increased after day 12 of age.



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Figure 3. Expression of the Mcl-1 mRNA in different rat tissues. Northern blots containing RNA extracted from different nonreproductive [A; poly(A)+-selected RNA] and reproductive rat tissues (B; total RNA) were hybridized with a radiolabeled Mcl-1 cDNA probe. Arrowheads indicate the two major Mcl-1 transcripts (2.3 and 3.7 kb). Sk. muscle, Skeletal muscle. The ß-actin message is shown as a control for RNA loading.

 


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Figure 4. Regulation of Mcl-1 message in the rat ovary during development and after gonadotropin treatment. A, Developmental regulation of Mcl-1 message in the prepubertal rat ovary. B, Effect of PMSG treatment on ovarian Mcl-1 message in the 26-day-old rat. C, Effect of hCG treatment (48 h after PMSG priming) on ovarian Mcl-1 message. The GAPDH message is shown as a control for RNA loading. The results shown are representative of two (A and B) or three separate experiments (C).

 
Gonadotropin induction of Mcl-1 message in the ovary
As it had previously been shown that the Mcl-1 message can be rapidly induced by survival factors in hemopoietic cells (24, 25, 33), we investigated whether gonadotropins, which are known to act as follicle survival factors, could increase Mcl-1 expression in the ovary. Immature rats received an injection of 10 IU PMSG, followed 48 h later by 10 IU hCG to induce ovulation. Using Northern blot analysis, the expression of Mcl-1 message in rat ovaries collected at different time points was assessed. As shown in Fig. 4Go, B and C, the two Mcl-1 transcripts were found to be expressed throughout the treatment time course. After PMSG injection, expression of the 2.3-kb transcript increased, peaking at 3–6 h and then returning to baseline levels by 24 h. Two days after PMSG priming, ovulation induction with hCG resulted in an 8.5-fold induction (P < 0.05) of the 2.3-kb transcript (normalized to GAPDH expression; n = 3) at the 6 h point, whereas expression of this transcript at 12 h was not significantly different from the 0 h expression level (P > 0.10). There were no statistically significant differences among the expression levels of the larger 3.7-kb transcript at 0, 6, and 12 h after hCG administration (P > 0.05).

Both hCG and FSH cause a preovulatory induction of Mcl-1 message in the rat ovary
As both hCG and FSH can act as follicle survival factors in preovulatory follicles (7) as well as induce follicle rupture in PMSG-primed immature rats (30), we further compared the effects of these two gonadotropins on the induction of Mcl-1 mRNA in the ovary. Immature rats were primed with PMSG, but additionally received two injections of the GnRH antagonist Org30850 to suppress endogenous pituitary gonadotropin secretion. At 48 h after PMSG administration, the rats were injected with an ovulatory dose of either hCG or FSH. As the earlier time-course studies had demonstrated a maximal induction of Mcl-1 message after 6 h, expression at this time point was compared with levels before the injection of hCG or FSH.

Northern blots of total RNA isolated from rat ovaries before treatment or at 6 h after hCG or FSH administration were hybridized with a radiolabeled Mcl-1 cDNA probe. Subsequently, the same blots were also hybridized with control probes for tPA and GAPDH (Fig. 5AGo). The message for tPA had previously been shown to be induced by both hCG and FSH (30) and therefore served as a positive control, whereas the housekeeping gene GAPDH was used as a control for RNA loading. The expression of each of the two Mcl-1 transcripts was quantified separately and normalized to the GAPDH signal. As shown in Fig. 5BGo, expression of the 2.3-kb Mcl-1 transcript in the ovaries of GnRH antagonist-pretreated rats was induced 3.3- and 2.4-fold (P < 0.01) by hCG and FSH, respectively, whereas expression of the 3.7-kb transcript remained essentially unchanged (P > 0.05).



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Figure 5. Induction of Mcl-1 message in the preovulatory ovary by hCG or FSH. A, Representative Northern blot of total ovarian RNA extracted from PMSG-primed ovaries before or at 6 h after the injection of ovulatory doses of either hCG or FSH. The blot was successively hybridized with probes for rat Mcl-1, tPA, and GAPDH. B, Quantification of ovarian Mcl-1 mRNA induction by hCG or FSH. The phosphorimager signal for each of the two Mcl-1 transcripts was quantified and normalized to the GAPDH signal. The results are expressed as the fold induction over the 0 h control, which was arbitrarily set at 1. Data shown are the mean ± SEM for five independent samples per group. Asterisks, Significantly different from the 0 h control group, P < 0.01.

 
Localization of ovarian Mcl-1 message by in situ hybridization and gonadotropin stimulation of Mcl-1 expression in isolated granulosa cells
To further examine the expression pattern of Mcl-1 in the preovulatory ovary, we performed in situ hybridization analysis. As shown in Fig. 6AGo, ovaries from immature rats contained multiple early antral follicles, preantral follicles, and some atretic follicles. Mcl-1 mRNA was mainly detected in the thecal cells of early and preantral follicles (Fig. 6BGo). Although treatment with PMSG for 6 h did not cause obvious changes in ovarian morphology (Fig. 6CGo), a major increase in Mcl-1 message could be detected in both granulosa and thecal cells (Fig. 6Go, D and E). The specificity of the signal obtained with the antisense Mcl-1 probe was demonstrated by the lack of hybridization in sections treated with the sense Mcl-1 probe (Fig. 6FGo). After PMSG priming for 48 h, the rat ovaries contained multiple large antral follicles, and the Mcl-1 message was again mainly expressed in the thecal cells of these follicles (Fig. 6Go, G and H). Six hours after the hCG injection, hybridization with the Mcl-1 antisense probe demonstrated an increase in Mcl-1 mRNA in both thecal and granulosa cells of these follicles (Fig. 6Go, I and J).



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Figure 6. In situ localization of Mcl-1 mRNA in the immature rat ovary before and after PMSG and hCG treatment. Sections of ovaries collected from 26-day-old rats were hybridized with a 35S-labeled rat Mcl-1 antisense probe and processed for liquid emulsion autoradiography. Ovaries from rats before (A and B) and 6 h after (C and D) PMSG injection were analyzed. Brightfield (A and C) and corresponding darkfield (B and D) photomicrographs are shown (x50). E, Higher magnification view of D (x125). F, Darkfield view of an ovarian section hybridized with an Mcl-1 sense probe as a control (x50). In addition, ovarian sections from rats primed with PMSG for 48 h were analyzed before (G and H) and 6 h after hCG injection (I and J). Brightfield (G and I) and corresponding darkfield (H and J) photomicrographs are shown (x50). PAF, Preantral follicle; EAF, early antral follicle; AtF, atretic follicle; LAF, large antral follicle; Oo, oocyte; Tc, thecal cells; Gc, granulosa cells.

 
As the Mcl-1 signal observed in granulosa cells was weaker than that in thecal cells, we further substantiated the induction of the Mcl-1 message in granulosa cells, the cell type undergoing apoptosis during follicle atresia in the rat (1). Therefore, enriched granulosa cells were isolated from the follicles of PMSG-primed rat ovaries by needle puncture. Northern blots containing total RNA extracted from granulosa cells at 0 and 6 h after hCG injection were hybridized with the Mcl-1 cDNA probe. Although treatment with hCG did not significantly change the expression of the 3.7-kb transcript (P > 0.10), it caused an almost 3-fold induction (2.8 ± 0.64 vs. 1.0 ± 0.13; P < 0.05) of the 2.3-kb Mcl-1 message in enriched granulosa cells, similar to the induction seen in the preovulatory ovary as a whole.


    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Follicle atresia is a hormonally controlled apoptotic process (1, 2). In particular, interference with gonadotropin secretion or receptor binding causes atresia of preovulatory follicles in vivo (34, 35, 36). Conversely, early atretic follicles can be rescued by administration of exogenous gonadotropins (3). However, the exact intracellular mechanisms mediating the antiatretic and antiapoptotic effects of follicle survival factors such as gonadotropins have remained unclear. Here, we demonstrate that the rat ortholog of the antiapoptotic human Mcl-1 molecule is capable of dimerization with different proapoptotic Bcl-2 family proteins and that it is expressed in thecal and granulosa cells of the ovary, where it is induced by the survival-promoting gonadotropins PMSG, hCG, and FSH.

Among the intracellular regulators of apoptosis, the Bcl-2 family of proteins occupies a central position by integrating diverse survival and death signals and linking them to downstream apoptotic events such as mitochondrial cytochrome c release and caspase activation (12). The importance of Bcl-2-mediated pathways to the regulation of follicle survival and atresia is illustrated by the phenotype of mice overexpressing Bcl-2 under the control of the inhibin-{alpha} promoter/enhancer. The targeted overexpression of this antiapoptotic gene in somatic ovarian cells leads to diminished follicular cell apoptosis and increased ovulation rates and litter sizes in transgenic animals (37). This phenotype is somewhat reminiscent of the effects seen in wild-type animals hyperstimulated with gonadotropins, raising the possibility that gonadotropins might induce an antiapoptotic Bcl-2 family protein in follicular cells.

Although the mRNAs for the antiapoptotic molecules Bcl-2 and Bcl-xL have been found in the ovary, their expression appears to be unaffected by PMSG treatment in the immature rat model (38). To identify additional antiapoptotic Bcl-2 family members in the ovary, we performed a yeast two-hybrid screening of a rat ovary cDNA library using a mutant BAD molecule as bait. The proapoptotic BAD is constitutively expressed in the rat ovary (15). Although presumably hyperphosphorylated BAD preferentially binds different isoforms of 14–3-3, a BAD mutant mimicking the hypophosphorylated form of BAD was found to interact with P11, a survival gene induced by nerve growth factor in the PC12 pheochromocytoma cell line (18). Here, we describe the identification of Mcl-1 as an additional interaction partner for this phosphorylation site mutant of BAD.

In living eukaryotic cells, Mcl-1 is capable of interacting with the wild-type forms of several other proapoptotic Bcl-2 family proteins (Bax, Bak, Bok/Mtd, Bik, and BOD/Bim), as demonstrated by yeast two-hybrid assays. Both Bok/Mtd (27, 39) and BOD/Bim (40, 41) were previously shown to be expressed in the rat ovary. In a recently performed yeast two-hybrid screening using BOD/Bim as bait, several of the clones with the strongest interaction were also found to encode Mcl-1 (data not shown). The dimer formation between pro- and antiapoptotic Bcl-2 family proteins, mediated by their highly conserved Bcl-2 homology domains, is thought to be a critical determinant of their mutual functional antagonism (12, 13, 14). In this respect, the observation that Mcl-1 can interact promiscuously with different proapoptotic Bcl-2-related proteins in yeast cells complements previous findings showing that Mcl-1 coimmunoprecipitates with and suppresses apoptosis induced by diverse proapoptotic Bcl-2 members (22, 27).

In addition to its heterodimerization with proapoptotic Bcl-2 family members, Mcl-1 could inhibit apoptosis by at least two other mechanisms. Similar to Bcl-xL and Bcl-2, Mcl-1 could form pores in the outer mitochondrial membrane to regulate cytochrome c release (42, 43) and/or inhibit the activation of caspases by retaining the adaptor molecule Apaf-1 in an inactive conformation (12, 44). Although its exact mechanism of action remains to be examined, Mcl-1 has been shown to increase cell viability under different apoptosis-inducing conditions, including growth factor deprivation and exposure to chemotherapeutic agents or UV irradiation (21, 22, 23).

The complete conservation of the Bcl-2 homology domains BH1, BH2, and BH3 (located in the C-terminal portion of the protein) between rat and human is consistent with their suggested importance in providing the structural basis for dimerizations between Bcl-2 family proteins (12, 45). The long N-terminal portion of Mcl-1, which bears no homology to other Bcl-2-related proteins, is also conserved between rat and human, although to a lesser degree. The functional significance of this domain is unclear, except for its PEST sequence, which is thought to account for the short half-life of the protein (see below). The partial amino acid sequences, deduced from a published chicken nucleotide sequence (32) and a recently released zebrafish EST, offer further insights into the conservation of Mcl-1-related sequences in nonmammalian species. The chicken sequence and zebrafish EST encode protein fragments with 61% and 53% identity to rat Mcl-1, respectively. The zebrafish sequence described here, therefore, most likely represents the first Bcl-2 family gene to be found in teleosts.

Several antiapoptotic Bcl-2 family genes (Bcl-2, Bcl-xL, A1/Bfl-1, and Mcl-1) are transcriptionally induced by specific cytokines in hemopoietic cell types (12). Prosurvival factors inducing Mcl-1 in hemopoietic cells include granulocyte-macrophage colony-stimulating factor, interleukin-1ß, and vascular endothelial growth factor (24, 25, 46). The stimulation of Mcl-1 expression by gonadotropins described here represents the first evidence of a Bcl-2 family gene being induced by follicle survival factors in the ovary. The rapid and transient increase in the ovarian Mcl-1 message after gonadotropin treatment mirrors the pattern of induction observed in hemopoietic cells (24, 47). This pattern was shown to reflect a transient transcriptional activation of the Mcl-1 gene, generating a message with a short half-life of less than 2 h (47). In hemopoietic cells, these changes in Mcl-1 mRNA levels closely correlate to similar alterations in the levels of the Mcl-1 protein, which is also rapidly turned over, probably due to its N-terminal PEST sequence (24, 47, 48). It has been suggested that the transient induction of Mcl-1 message and protein by survival factors serves to protect cells during specific stages of differentiation when they are particularly vulnerable to apoptosis (23).

Because of its induction by PMSG, FSH, or hCG, the gonadotropin-dependent expression of Mcl-1 in ovarian cells is probably mediated by a mechanism in the common downstream signaling pathways of the LH/hCG and FSH receptors. The murine Mcl-1 promoter contains a DNA sequence similar to the consensus site recognized by the CRE-2-binding protein, which is involved in transcriptional induction by cAMP (24, 49). However, a detailed molecular analysis of the Mcl-1 gene promoter will be necessary to delineate potential cis-responsive elements mediating the gonadotropin induction of Mcl-1. Interestingly, the induction of the 2.3-kb Mcl-1 transcript at 6 h after hCG injection was lower in Org30850-pretreated rats (3.3 ± 0.30-fold) than in rats not pretreated with the GnRH antagonist (8.5 ± 0.17-fold). This observation could be due to the different age of animals in these two series of experiments (27 vs. 28 days) and/or differences in their endogenous gonadotropin levels.

The two Mcl-1 transcripts observed in rat tissues (2.3 and 3.7 kb) correspond in size to the two known human Mcl-1 messages (2.5 and 3.8 kb) (19). Both Mcl-1 transcripts in the rat are long enough to contain the full-length coding sequence. Although one cannot exclude the possibility that the rat mRNAs may encode different ORFs, the two human Mcl-1 transcripts probably result from the use of alternative polyadenylation sites in the 3'-untranslated region of the gene (19). Although the significance of selective induction of the 2.3-kb Mcl-1 transcript by gonadotropins remains unknown, it is interesting to note that a similar differential induction of two splicing variants by gonadotropins in granulosa cells has recently been observed for the kit ligand gene (50).

During follicle atresia in the rodent ovary, apoptotic cell death is confined to granulosa cells (9, 11). The basal expression and gonadotropin induction of Mcl-1 in granulosa cells are therefore consistent with its potential role in regulating follicle atresia. Whether the levels of Mcl-1 expression in individual follicles correlate with granulosa cell apoptosis remains to be determined. Interestingly, levels of Mcl-1 mRNA expression in thecal cells, a cell type resistant to apoptosis in this model, were higher than those in granulosa cells. A similar differential ovarian expression pattern has been observed for the inhibitor of apoptosis proteins, Hiap-2 and Xiap, which are also induced by gonadotropins (51). It is likely that the high expression of antiapoptotic genes in thecal cells confers a protection from apoptosis.

From the data presented, we propose the following model. Mcl-1 is an antiapoptotic Bcl-2 member expressed in ovarian follicles that can dimerize with BAD and other proapoptotic ovarian Bcl-2 family members (e.g. Bax, BOD, and Bok). Gonadotropins induce a transient increase in the expression of Mcl-1 and may thereby shift the balance of ovarian anti- and proapoptotic Bcl-2 family proteins to favor survival and rescue follicles from atresia. The induction of Mcl-1 along with other antiapoptotic genes such as inhibitor of apoptosis proteins may therefore represent a molecular basis for the suppression of follicular apoptosis and atresia by gonadotropins.


    Acknowledgments
 
We are grateful to Connie Schindler in the laboratory of Amato Giaccia (Stanford, CA) for help with the phosphorimager analysis, and to Sameena Beguwala for assistance with the yeast two-hybrid assays. We thank the following individuals for the provision of cDNAs for different proteins of the Bcl-2 family: M. Cleary (Stanford, CA; Bcl-2), S. Cory (Victoria, Australia; Bcl-w), G. Chinnadurai (St. Louis, MO; Bfl-1/A1 and Bik), A. Rickinson (Birmingham, UK; BHRF1), R. Horvitz (Cambridge, MA; CED-9), T. Chittenden (Cambridge, MA; Bak), and C. Thompson (Chicago, IL; Bcl-xL). We also thank Dr. Lenus Kloosterboar from Organon (Oss, The Netherlands) for providing the GnRH antagonist Org 30850 and recombinant human FSH.


    Footnotes
 
1 This work was supported by NIH Grant HD-31566 (to A.J.W.H.) and Korean Science and Engineering Foundation Grants 97–04-01–06-01–3 and HRC-98k1–0405 (to S.Y.C.). Back

2 Supported by a postdoctoral fellowship from the German Academic Exchange Service. Present address: Department of Obstetrics and Gynecology, University of Leipzig, Leipzig, Germany 04207. Back

3 The GenBank accession number for rat Mcl-1 is AF115380. Back

Received May 6, 1999.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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