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in Fetal Lung: Developmental Regulation and Effects of Cyclic Adenosine 3',5'-Monophosphate and Glucocorticoids1
Departments of Biochemistry (D.R.B., J.L.A., C.R.M.), Pediatrics (D.R.B.), Pathology (L.R.M.), and Obstetrics-Gynecology (C.R.M.) and Cecil H. and Ida Green Center for Reproductive Biology Sciences (C.R.M.), University of Texas Southwestern Medical Center, Dallas, Texas 75235
Address all correspondence and requests for reprints to: Dr. Carole R. Mendelson, Department of Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75235-9038. E-mail: cmende{at}biochem.swmed.edu
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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,
which is induced by glucocorticoids during the early phases of
adipocyte differentiation, is expressed at relatively high levels in
lung compared with other tissues. In the present study we have analyzed
developmental changes in C/EBP messenger RNA levels in fetal rabbit
lung as well as changes in the levels of immunoreactive C/EBP in
human fetal lung during differentiation in organ culture and after
treatment with cAMP and glucocorticoids. We observed that C/EBP
messenger RNA is detectable in fetal rabbit lung on day 19 of gestation
and is increased 
3.7-fold to maximum levels on day 28 of gestation,
the time when SP-A gene transcription increases to maximum levels.
Immunohistochemical analysis of C/EBP in midgestation human fetal
lung before culture revealed trace nuclear staining in epithelial and
occasional stromal cells. After 12 h of organ culture in
serum-free medium, nuclear staining of C/EBP was markedly increased
in epithelial cells lining the prealveolar ducts of the human fetal
lung tissue. By immunoblot analysis, it was found that C/EBP levels
were induced rapidly during organ culture in control medium and were
increased further by treatment with dexamethasone and
(Bt)2cAMP. C/EBP levels were maximally induced during
the first 24 h of culture and declined thereafter; after 72 h
of incubation in control or cAMP-containing medium, C/EBP was
reduced markedly. By contrast, in fetal lung tissues incubated in
medium containing dexamethasone or dexamethasone plus
(Bt)2cAMP, the decline in C/EBP was more modest, so that
levels remained elevated throughout the 96-h culture period. Our
findings that C/EBP is localized primarily to alveolar epithelial
cells, rapidly induced during differentiation of human fetal lung in
culture, and increased by cAMP and glucocorticoids suggest a possible
role in the regulation of type II cell differentiation and in the
synthesis of surfactant phospholipids and proteins. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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75%
of gestation is completed (1). In prematurely born infants, inadequate
surfactant synthesis can result in respiratory distress syndrome, a
major cause of neonatal morbidity and mortality (2). The major and most
surface-active glycerophospholipid in surfactant is
dipalmitoylphosphatidylcholine (3). Surfactant also contains a number
of proteins, surfactant protein-A (SP-A), SP-B, SP-C, and SP-D, that
are lung specific and developmentally regulated in fetal lung tissue
(4). Whereas expression of the genes encoding SP-B and SP-C are
initiated in human fetal during midgestation (5), SP-A gene expression
is initiated during the third trimester, in concert with augmented
glycerophospholipid synthesis (6, 7). SP-D gene expression is initiated
in the fetal lung just before birth (8). In previous studies we found that lung explants from midtrimester human abortuses differentiate spontaneously when maintained in organ culture in serum-free defined medium (9). Within several days of culture, there is enlargement of the prealveolar ducts, appearance of differentiated type II cells containing lamellar bodies and induction of synthesis of surfactant glycerophospholipids and SP-A (9, 10). The rate of enlargement of the prealveolar ducts, the appearance of differentiated type II cells, and the induction of SP-A gene transcription are enhanced by cAMP treatment (11). There are two SP-A genes in the human, SP-A1 and SP-A2 (12, 13). The SP-A2 gene is more highly regulated by cAMP and during development than that encoding SP-A1 (14). Within the 5'-flanking region of the human SP-A2 gene, we have identified enhancer elements that are functionally required for elevated basal and cAMP induction of SP-A2 promoter activity in type II cells (15, 16).
Glucocorticoids in combination with insulin or insulin plus PRL increase the rate of surfactant glycerophospholipid synthesis in midgestation human fetal lung in organ culture (17). Glucocorticoids also increase SP-A gene transcription in human fetal lung explants when added alone and act synergistically with cAMP (18, 19); however, the molecular basis for this regulation is not understood, as we have been unable to identify a functional glucocorticoid-responsive element (GRE) within the 5'-flanking sequence of the human SP-A2 gene (Young, P. P., and C. R. Mendelson, unpublished observations). Paradoxically, glucocorticoids cause a dose-dependent decrease in the steady state levels of SP-A messenger RNA (mRNA) in human fetal lung in culture (20) due to a dominant inhibitory effect on SP-A mRNA stability (19).
The CCAAT/enhancer-binding protein (C/EBP) family of transcription
factors has been postulated to serve a role in adipocyte
differentiation (21, 22). Furthermore, members of this family are
expressed at relatively high levels in tissues that have the capacity
to synthesize, store, and metabolize lipids at exceptionally high
rates, including adipose tissue, liver, placenta, small intestine, and
lung (23). There are three major isoforms of C/EBP: 
, ß, and ;
each is encoded by a distinct intronless gene (24, 25). Whereas
C/EBP is expressed at highest levels in liver and adipose tissues,
C/EBPß expression is more ubiquitous, although it is detected at the
highest levels in intestine, liver, lung, and adipose tissues (25). On
the other hand, C/EBP is expressed at highest levels in lung, with
lower levels in intestine and adipose tissues (23, 25). It has been
suggested by McKnight and colleagues that C/EBPs may modulate the
transcription of genes involved in energy metabolism (23, 26). In light
of their expression in lung, it was suggested that these factors may
act to regulate genes involved in surfactant glycerophospholipid
metabolism (23).
Recently, it was reported that C/EBP mRNA and protein are present in
rat pulmonary type II cells, and that C/EBP expression in fetal rat
lung is developmentally induced on day 20 of gestation (27) subsequent
to the appearance of differentiated type II cells and the initiation of
synthesis of surfactant glycerophospholipids and of surfactant proteins
SP-A, SP-B, and SP-C (28). Interestingly, mice carrying a homozygous
mutation of the C/EBP gene die at birth from severe hypoglycemia,
not from respiratory distress (29). Their lungs are described as being
morphologically immature due to the presence of multicellular layers of
epithelial cells lining the alveolar ducts (30). Based on these
findings, it is likely that C/EBP serves to maintain a quiescent
state and restrict type II cell proliferation rather than to regulate
the expression of enzymes involved in surfactant glycerophospholipid
synthesis.
In consideration of the relatively high levels of expression of
C/EBP in lung compared with those in other tissues and its hormonal
induction during the early stages of adipogenesis (25), in the present
study we have analyzed developmental changes in C/EBP mRNA levels in
fetal rabbit lung as well as changes in the levels of immunoreactive
C/EBP in human fetal lung during differentiation in organ culture
and after treatment with cAMP and glucocorticoids.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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14–18 weeks gestation
were obtained in accordance with the Donors Anatomical Gift Act of the
State of Texas. Consent forms and protocols were approved by the human
research review committee at the University of Texas Southwestern
Medical Center (Dallas, TX). Fetal lung explants were maintained in
serum-free Waymouth’s MB752/1 medium (changed daily) as previously
described and incubated in the absence or presence of
(Bt)2cAMP (1 mM), dexamethasone
(10-10–10-7 M), or
(Bt)2cAMP plus dexamethasone for periods up to 5 days (9).
Each experiment was performed using pooled lung tissues from one to
three human abortuses. For the data shown in Fig. 2B
, each experiment
was repeated using five independent human fetal lung tissues or pools
of lung tissues. The scanned data shown in Fig. 3B are from three
independent experiments using tissues from three different human
abortuses or pools of lung tissue from several fetuses.
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mRNA (25) (provided by Dr. Steve
McKnight, University of Texas Southwestern Medical Center) that was
radiolabeled with phosphorus-32 using a random primed DNA labeling kit
(Boehringer Mannheim, Indianapolis, IN). The relative levels of
C/EBP were assessed by autoradiography using intensifying screens,
quantitated using a computing densitometer (Molecular Dynamics,
Sunnyvale, CA) and corrected for loading and transfer after reprobing
and scanning the blots for 18S ribosomal RNA. Protocols for research
using pregnant rabbits were approved by the institutional animal care
and research advisory committee, University of Texas Southwestern
Medical Center.
Immunoblot analysis
Lung explants that were maintained in culture for various
periods of time were homogenized in water (0–2 C) containing
phenylmethylsulfonylfluoride (1 mM). Crude nuclear and
supernatant fractions were obtained after centrifugation of the
homogenates at 600 x g. We observed by both immunoblotting
(unpublished observations) and immunocytochemistry (Fig. 5) that the
majority of C/EBP immunoreactivity was present in the nucleus.
Proteins (40 µg) from the nuclear pellet were, therefore, separated
on 11% SDS-polyacrylamide gels and transferred to nitrocellulose
membranes as described previously (11). The membranes were then
analyzed for C/EBP using specific rabbit polyclonal antibodies
raised against the full-length protein (provided by Dr. Steven
McKnight, University of Texas Southwestern Medical Center) and an
Enhanced Chemiluminescence System (ECL) according to manufacturer’s
recommendations (Amersham, Arlington Heights, IL). This was followed by
autoradiography and analysis via computing densitometry, as noted
above. On all immunoblots performed, the polyclonal antiserum to
C/EBP, which was raised against the full-length C/EBP protein,
reacted only with a single protein species of 32 kDa, the predicted
size of C/EBP. Furthermore, when this antiserum was used to probe
immunoblots of equivalent amounts of peptides corresponding to
sequences at or near the carboxy-termini of C/EBP, -ß, and -,
the antiserum reacted only with the C/EBP peptide.
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(1:500 dilution). Immunoreactivity
was detected by streptavidin-biotin methodology, using biotinylated
antirabbit IgG and diaminobenzidine as chromagen (Dako, Carpenteria,
CA). Adjacent sections were analyzed concurrently using nonimmune
rabbit serum in place of C/EBP antiserum as negative controls. |
Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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mRNA levels are increased in fetal rabbit lung tissue
during development expression, we analyzed
C/EBP mRNA levels in lung tissues from fetal rabbits from 19–30
days gestational age (term = 31 days) and from adult rabbits by
Northern blotting. The rabbit provides a suitable model for such
studies because of its relatively long gestation period (term = 31
days). We have found that SP-A gene transcription is increased in fetal
rabbit lung tissue by day 24 of gestation and reaches maximal levels by
day 28 (34). We elected to use Northern analysis rather than
immunoblotting for these developmental studies because the C/EBP
polyclonal antiserum used for immunoblot analysis was raised in
rabbits, and the second antibody used (antirabbit IgG) cross-reacts
with IgG in rabbit tissues. A representative Northern blot and
quantitation of the densitometric scans of four different Northern
blots corrected for loading and transfer of RNA are shown in Fig. 1, A and B, respectively. As can be seen,
C/EBP mRNA was detectable in lung tissues of 19-day gestation fetal
rabbits, increased 3-fold by day 21, and reached maximal levels by
day 28 of gestation. The levels of C/EBP mRNA in adult rabbit lung
were comparable to those in the 28-day gestation fetal lung tissue. The
maximum induction of C/EBP mRNA in fetal rabbit lung on day 28 of
gestation is coincident with the time that increased numbers of type II
cells and increased surfactant phosphatidylcholine synthesis are
observed (1) and when SP-A gene transcription reaches its maximum level
(34).
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expression is induced during differentiation of human
fetal lung in culture and is stimulated by glucocorticoids and cAMP expression before and after differentiation of midgestation
human fetal lung in organ culture as well as the effects of
glucocorticoids and cAMP analogs. In Fig. 2A
is a representative immunoblot of
C/EBP in midgestation human fetal lung tissue before culture (0) and
after 1, 3, and 5 days of organ culture in the absence (control) or
presence of (Bt)2cAMP (1 mM) and dexamethasone
(10-7 M), added either alone or in
combination. In Fig. 2B are the combined data from five independent
experiments. The scanned values are the mean ± SEM of
three to five data points for each treatment group at each time point
from each of these independent experiments [the one exception,
(Bt)2cAMP-treated tissue after 1 day of culture, includes
only two data points, which are presented as two dots]. The scanned
values for each independent experiment were normalized to those of
(Bt)2cAMP-treated tissues on day 5 of culture, which were
given an arbitrary value of 1500. In fetal lung tissues incubated in
control medium for 1 day, the levels of a single immunoreactive protein
of approximately 32,000 Mr, consistent with that of
C/EBP, were comparable to those in the fetal lung tissues before
culture (0). After 1 day of incubation, (Bt)2cAMP and
dexamethasone each had a modest effect to increase C/EBP levels
compared with those in control tissues; in fetal lung tissues incubated
with both agents in combination, C/EBP rose to levels 2-fold
greater than control values. After 3–5 days of culture, the levels of
C/EBP were decreased in control and (Bt)2cAMP-treated
tissues compared with those on day 1. By contrast, in
dexamethasone-treated fetal lung explants and in lung tissues incubated
with (Bt)2cAMP plus dexamethasone, the decline in C/EBP
as a function of incubation time was less pronounced. As a consequence,
after 5 days of incubation, C/EBP levels were increased more than
2-fold in dexamethasone-treated fetal lung tissues and 3-fold in
tissues incubated with dexamethasone plus (Bt)2cAMP
compared with levels in control tissues at the corresponding time
points (Fig. 2B). It should be noted that the values obtained by
densitometric scanning of autoradiograms may not strictly be linear and
could result in an underestimation of the inductive effects of
dexamethasone and (Bt)2cAMP; however, the differences
observed among treatment groups were consistent from one experiment to
another.
To further investigate the glucocorticoid induction of C/EBP
expression, we analyzed the effects of dexamethasone added in
concentrations of 10-10-10-7 M in
the absence or presence of (Bt)2cAMP after 72 h of
incubation. In Fig. 3A is shown a
representative immunoblot of such an experiment. The scanned data shown
in Fig. 3B are from three independent experiments using tissues from
three different human abortuses or pools of lung tissue from several
fetuses. The values shown for all treatments are the mean ±
SEM of three data points from the three independent
experiments. The scanned values for each independent experiment were
normalized to C/EBP levels in the human fetal lung after 3 days of
culture in the absence of glucocorticoids or (Bt)2cAMP (0),
which were assigned an arbitrary value of 100. As can be seen,
dexamethasone caused a dose-dependent induction of C/EBP protein
levels in human fetal lung explants. This dose-dependent effect was
further augmented in the presence of Bt2cAMP. Similar
dose-dependent inductive effects were observed after 1 and 5 days in
culture (data not shown).
As the highest levels of immunoreactive C/EBP were observed after 1
day of organ culture, it was of interest to analyze changes in C/EBP
expression at various time points within the first 24 h of
incubation. A densitometric scan of the levels of C/EBP in human
fetal lung explants before culture and after incubation in the absence
or presence of dexamethasone (10-8 M) for
1–24 h are shown in Fig. 4. The data
shown are from one experiment using lung tissues from a midgestation
human abortus. The levels of immunoreactive C/EBP in control tissues
reached maximal levels after 6 h of incubation and subsequently
declined. An inductive effect of dexamethasone was evident as early as
1 h of incubation; levels of C/EBP in the dexamethasone-treated
tissues continued to increase, reaching maximal levels after 18 h
of incubation.
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is primarily localized to nuclei of
epithelial cells lining the prealveolar ducts protein in human fetal lung before
culture and after 12 h of incubation in the absence or presence of
dexamethasone (10-7 M; Fig. 5). Before culture, the midgestation
human fetal lung, which is in the early canalicular phase of
development, showed trace nuclear staining in occasional stromal and
epithelial cells (Fig. 5A). After 12 h of organ culture in control
medium, there was a marked induction of nuclear staining for C/EBP,
predominantly within epithelial cells lining the prealveolar ducts and
in many of the stromal cells (Fig. 5B); however, endothelium and smooth
muscle showed no staining. Similar patterns and levels of
immunoreactivity were observed in the dexamethasone-treated tissue at
this time point (Fig. 5C) and in control and dexamethasone-treated
fetal lung explants after 18 and 24 h of incubation (data not
shown); however, diminished staining was apparent in epithelia and
stroma after 30 h (data not shown). Persistent immunoreactivity
was detected for up to 3 days of culture; however, fewer epithelial and
stromal cells were immunoreactive (data not shown). All concurrently
processed negative controls were nonreactive (Fig. 5D). The induction
of C/EBP in the cultured human fetal lung tissue preceded
enlargement of the prealveolar ducts and the appearance of
differentiated type II cells containing lamellar bodies (9). Although
2-fold differences in the levels of immunoreactive C/EBP were
evident in control vs. dexamethasone-treated tissues by
immunoblot analysis, these differences were not readily discerned by
immunocytochemistry. |
Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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gene expression is
developmentally regulated in fetal rabbit lung. C/EBP mRNA was
detected in the fetal lung tissue as early as 19 days gestation,
reaching peak levels by day 28, a time when augmented surfactant
glycerophospholipid synthesis is evident (1, 9), SP-A gene
transcription reaches maximum levels (17, 32, 34), and type II cells
containing increased numbers of lamellar bodies are present within the
alveolar epithelium (35). Furthermore, we found that immunoreactive
C/EBP, which is present at low levels in the midgestation human
fetal lung before culture, was rapidly induced during spontaneous
differentiation in organ culture in serum-free medium and was further
increased by treatment with dexamethasone and (Bt)2cAMP.
The stimulatory effects of dexamethasone were rapid and dose dependent;
a stimulatory effect of dexamethasone was evident within 2 h of
incubation.
The temporal changes in C/EBP levels in the cultured human fetal
lung explants were biphasic. In explants incubated in control medium or
in medium containing (Bt)2cAMP or dexamethasone, levels of
immunoreactive C/EBP were increased to a maximum within 24 h
of incubation. Thereafter, in control and (Bt)2cAMP-treated
tissues, immunoreactive C/EBP declined markedly, so that after
72 h of incubation, levels were less than those in the human fetal
lung before culture. By contrast, in fetal lung explants incubated with
dexamethasone with or without (Bt)2cAMP, the decline in
C/EBP protein levels as a function of time in culture was more
modest. After 24 h of incubation with dexamethasone and
(Bt)2cAMP, in combination, an additive stimulatory effect
on C/EBP levels was observed compared with that in fetal lung
explants incubated with either factor alone; however, after 3 and 5
days of incubation, the stimulatory effects of these agents appeared to
be synergistic. This apparent synergism is due to the loss of an
inductive effect of cAMP when added to the medium alone despite its
continued ability to enhance the stimulatory effect of dexamethasone on
C/EBP expression. Immunohistochemical analysis revealed that
C/EBP was barely detectable in midgestation human fetal lung before
culture. After 12 h of organ culture there was intense nuclear
C/EBP immunostaining, localized primarily to epithelial cells lining
the prealveolar ducts. These epithelial cells differentiate into type
II cells containing numerous lamellar bodies after 2–4 days of culture
(9, 11).
Previously, we observed that treatment of fetal lung explants with agents that increase intracellular cAMP accelerated the rate of type II cell differentiation, induced transcriptional activity of the SP-A gene, and increased the accumulation of SP-A mRNA and protein (6, 11, 18). Glucocorticoid treatment of rabbit and human fetal lung explants increased surfactant glycerophospholipid synthesis and the rate of SP-A gene transcription (18, 34). When human fetal lung explants were incubated with dexamethasone and (Bt)2cAMP in combination, transcriptional activity of the SP-A gene was synergistically increased (18, 19).
In recent studies using transfected type II cells to functionally map genetic elements that regulate the expression of the human and rabbit SP-A genes, we identified a number of critical regulatory elements within a 400-bp region upstream of the transcription initiation site. An element with sequence similarity to a cAMP-response element (CRESP-A, TGACCTC/TA) (15, 36, 37), a GT box (GGGGTGGG) (16), as well as several E box motifs (38) have been identified that are critical for basal and cAMP-induced expression in transfected type II cells. We observed that the proteins that bind to the CRESP-A appear to change during differentiation of midgestation fetal lung in culture (15) and that neither CRE-binding protein nor a related bZIP transcription factor binds to this sequence; rather, a member of the nuclear receptor superfamily likely binds to this site (37). We also have found that Sp1 together with an unidentified 55-kDa protein are components of the complex that binds to the GT box (16), and that the helix-loop-helix-leucine zipper proteins upstream stimulatory factors, USF1 (39) and USF2 (Gao, E., Y. Wang, J. L. Alcom, and C. R. Mendelson, unpublished observations), bind to the E box sequences. Additionally, within the -400 bp region, there are several binding sites for the homeodomain transcription factor thyroid transcription factor-1 (TTF-1) (Li, J., E. Gao, and C. R. Mendelson, unpublished observations), which is essential for development of thyroid, lung, and anterior pituitary (40). TTF-1 has been reported to mediate expression of the murine SP-A gene in transfected lung cell lines (41); in preliminary studies we observed that the TTF-1-binding elements also are critical for cAMP induction of SP-A promoter activity (Li, J., E. Gao, and C. R. Mendelson, unpublished observations). cAMP induction of SP-A promoter activity, therefore, is dependent upon the concerted actions of members of several different transcription factor families bound to response elements within the 5'-flanking region of the SP-A gene.
The mechanisms for the glucocorticoid induction of SP-A gene transcription have not been elucidated. We have been unable to find a GRE consensus sequence within or surrounding the rabbit (36) and human (Young, P. P., S. M. McCormick, and C. R. Mendelson, unpublished observations) SP-A genes. Furthermore, in cell transfection studies, we have been unable to identify a functional stimulatory GRE within 2.0 and 1.2 kilobases of DNA flanking the 5'-ends of the rabbit (36) and human (Young, P. P., and C. R. Mendelson, unpublished observations) SP-A genes, respectively, or within the first exon and intron of the rabbit SP-A gene (36). Based on these findings, it is likely that the stimulatory effects of glucocorticoids on transcription of the endogenous SP-A gene are mediated either directly by a GRE(s) that lies outside the confines of the fusion gene constructs tested to date or indirectly through the induction of other transcription factors.
The rapid induction of C/EBP in cultured human fetal lung and its
augmentation by glucocorticoids are reminiscent of the time course of
C/EBP expression during differentiation of 3T3-L1 preadipocytes to
adipocytes. When 3T3-L1 cells were cultured in medium containing the
phosphodiesterase inhibitor isobutylmethylxanthine, dexamethasone, and
insulin, there was a rapid induction of C/EBP and C/EBPß to peak
levels on days 2 and 4 of incubation, respectively (25, 42).
Thereafter, C/EBP and -ß levels declined rapidly, and C/EBP
markedly increased in association with the induction of
adipocyte-specific mRNAs, lipid accumulation, and cessation of mitotic
growth (25). Isobutylmethylxanthine acted to increase C/EBPß gene
expression, whereas dexamethasone stimulated C/EBP (25, 42). It is
suggested that C/EBPß and/or - act to increase the expression of
C/EBP, which, in turn, activates adipocyte-specific genes and
promotes terminal differentiation. Early during adipocyte
differentiation, there also is induction of the adipose-specific
nuclear receptor, peroxisome proliferator-activated receptor-
2
(PPAR2); adipogenesis was enhanced by addition of the PPAR
activator 5,8,11,14-elcosatetraynoic acid (ETYA) (43). Interestingly,
we observed that PPAR1 mRNA and protein levels are induced in human
fetal lung explants in association with type II cell differentiation,
and that cAMP increases PPAR1 mRNA and de novo synthesis
of PPAR1 protein in type II cells isolated from human and rabbit
fetal lung explants (44).
Recently, it was reported that C/EBP mRNA, which was undetectable in
fetal rat lung on day 18 of gestation, was markedly induced by day 20
(27). It also was found that immunoreactive C/EBP was present in
nuclei of type II cells of adult rat lung tissue and declined to
undetectable levels within 24 h of culture on plastic dishes, a
condition that fails to support the maintenance of type II cell
differentiation (27). SP-C mRNA transcripts are readily detectable in
fetal rat lung on day 17 of gestation, SP-A and SP-B mRNA transcripts
are first detectable on day 18, and type II cells containing lamellar
bodies are first detected on day 19 of gestation (28). The finding that
C/EBP mRNA was detected relatively late in gestation, coordinate
with or even after the appearance of differentiated type II cells,
suggests that C/EBP may not play a regulatory role in type II cell
differentiation and the developmental induction of surfactant
glycerophospholipid synthesis.
On the other hand, the findings of the present study indicate
that the level of immunoreactive C/EBP increases rapidly during
differentiation of human fetal lung in culture, and that C/EBP is
predominantly localized to nuclei of epithelial cells lining the
prealveolar ducts. These findings together with the observed induction
of C/EBP by glucocorticoids and cAMP, agents known to enhance the
rate of type II cell differentiation and synthesis of surfactant
glycerophospholipids and proteins, suggest that C/EBP may serve as a
mediator of the hormonal regulation of surfactant lipoprotein synthesis
and type II pneumonocyte differentiation.
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Acknowledgments |
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Footnotes |
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Received May 6, 1997.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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stimulates adipogenesis. Genes Dev 9:2350–23631
expression is induced during cyclic adenosine
3',5'-monophosphate-stimulated differentiation of alveolar type II
pneumonocytes. Endocrinology 138: 3695–3703
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 K. V. Reddy, K. J. Serio, C. R. Hodulik, and T. D. Bigby 5-Lipoxygenase-activating Protein Gene Expression. KEY ROLE OF CCAAT/ENHANCER-BINDING PROTEINS (C/EBP) IN CONSTITUTIVE AND TUMOR NECROSIS FACTOR (TNF) alpha -INDUCED EXPRESSION IN THP-1 CELLS J. Biol. Chem., April 11, 2003; 278(16): 13810 - 13818. [Abstract] [Full Text] [PDF]
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Y. He and E. Crouch Surfactant Protein D Gene Regulation. INTERACTIONS AMONG THE CONSERVED CCAAT/ENHANCER-BINDING PROTEIN ELEMENTS J. Biol. Chem., May 24, 2002; 277(22): 19530 - 19537. [Abstract] [Full Text] [PDF]
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T. N. Cassel, L. Nordlund-Möller, O. Andersson, J.-A. Gustafsson, and M. Nord C/EBPalpha and C/EBPdelta Activate the Clara Cell Secretory Protein Gene through Interaction with Two Adjacent C/EBP-Binding Sites Am. J. Respir. Cell Mol. Biol., April 1, 2000; 22(4): 469 - 480. [Abstract] [Full Text]
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 T. L. McCarthy, C. Ji, Y. Chen, K. Kim, and M. Centrella Time- and Dose-Related Interactions between Glucocorticoid and Cyclic Adenosine 3',5'-Monophosphate on CCAAT/Enhancer-Binding Protein-Dependent Insulin-Like Growth Factor I Expression by Osteoblasts Endocrinology, January 1, 2000; 141(1): 127 - 137. [Abstract] [Full Text] [PDF]
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C. Ji, Y. Chen, T. L. McCarthy, and M. Centrella Cloning the Promoter for Transforming Growth Factor-beta Type III Receptor. BASAL AND CONDITIONAL EXPRESSION IN FETAL RAT OSTEOBLASTS J. Biol. Chem., October 22, 1999; 274(43): 30487 - 30494. [Abstract] [Full Text] [PDF]
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J. P. O'Rourke, G. C. Newbound, J. A. Hutt, and J. DeWille CCAAT/Enhancer-binding Protein delta Regulates Mammary Epithelial Cell G0 Growth Arrest and Apoptosis J. Biol. Chem., June 4, 1999; 274(23): 16582 - 16589. [Abstract] [Full Text] [PDF]
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L. M. Grønning, M. K. Dahle, K. A. Taskén, S. Enerbäck, L. Hedin, K. Taskén, and H. K. Knutsen Isoform-Specific Regulation of the CCAAT/Enhancer-Binding Protein Family of Transcription Factors by 3',5'-Cyclic Adenosine Monophosphate in Sertoli Cells Endocrinology, February 1, 1999; 140(2): 835 - 843. [Abstract] [Full Text]
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