Steroidogenic properties of a spontaneously established porcine granulosa cell line (PGC-2)

Establishment of A Reversibly Inducible Porcine Granulosa Cell Line

Granulosa cells (GCs) are the key components of ovarian follicles for regulating oocyte maturation. Previous established GC lines have allowed prolonged proliferation, but lost some physiological features owing to long-term immortalization. This study was to establish an induced immortal porcine GC line with reversible proliferation status by the tetracycline inducible (Tet-on) 3G system. Our conditional immortal porcine GCs (CIPGCs) line steadily propagated for at least six months and displayed primary GC morphology when cultured in the presence of 50 ng/mL doxycycline [Dox (+)]. Upon Dox withdrawal [Dox (–)], Large T-antigen expression, reflected by mCherry fluorescence, gradually became undetectable within 48 h, accompanied by less proliferation and size increase. The levels of estradiol and progesterone, and the expression of genes associated with steroid production, such as CYP11A1 (cytochrome P450 family 11), 3β-HSD (3β-hydroxysteroid dehydrogenase), StAR (steroidogenic acute regulatory protein), and CYP19A1 (cytochrome P450 family 19 subfamily a member 1), were all significantly higher in the Dox (–) group than Dox (+) group. The CIPGCs could switch into a proliferative state upon Dox induction. Interestingly, the expression of StAR and CYP19A1 in the CIPGCs (–Dox) was significantly increased by adding porcine follicular fluid (PFF) to mimic an ovary follicle environment. Moreover, PFF priming the CIPGCs in Dox (–) group resulted in similar estradiol production as that of primary GC, and enabled this cell line to respond to gonadotrophins in estradiol production. Collectively, we have established an inducible immortal porcine GC line, which offers a unique and valuable model for future research on the regulation of ovarian functions.

Increasing cell plating density mimics an early post-LH stage in cultured bovine granulosa cells

Cultured ovarian granulosa cells are essential models to study molecular mechanisms of gene regulation during folliculogenesis. Here, we characterize primary tissue culture models for bovine granulosa cells by morphological and physiological parameters and by novel molecular luteinization markers, as transcript abundance and DNA methylation levels. The data show that: (1) collagen substrate increased the number of attached, viable cells; (2) the expression of the key transcripts of estrogen synthesis, CYP19A1, could be induced and maintained in granulosa cells from small to medium but not from large follicles, whereas (3) only granulosa cells from large but not from smaller follicles were responsive to LH; (4) serum supplementation unfavorably transformed the cellular phenotype, induced proliferation and PCNA expression, reduced or abolished the transcript abundance of steroidogenic key genes and of gonadotropin receptor genes, CYP11A1, CYP19A1, FSHR and LHCGR but, however, did not increase the abundance of the luteinization-specific marker transcripts PTGS2, PTX3, RGS2 and VNN2; but (5) by increasing the plating density, estradiol production and the abundance of CYP19A1 transcripts, in particular those derived from the main ovarian promoter P2, were decreased concurrently leaving P2-specific DNA methylation levels unchanged, whereas progesterone secretion was stimulated and the expression of both luteinization-specific marker transcripts, RGS2 and VNN2, was significantly induced. From these data, we conclude that increasing the plating density induces a different, partly complementary, physiological and gene expression profile in cultured bovine granulosa cells and drives the cells towards an early post-LH stage of luteinization, even in the absence of luteinizing agents.

Characterization of Wnt2 overexpression in a rat granulosa cell line (DC3): effects on CTNNB1 activation

WNTs comprise a family of secreted glycoproteins that are essential for normal embryonic development of the female reproductive system. The functional role that WNTs play in the postnatal ovary is poorly defined. We have shown previously that Wnt2 and Fzd4 mRNAs are expressed in granulosa cells of the postnatal rat ovary. Here we examine the effects of Wnt2 overexpression in a rat granulosa cell line (DC3) that displays characteristics of granulosa cells at an early stage of follicular development. We show that DC3 cells express a 7.7-kb Fzd4 mRNA transcript similar in size to that detected in the rat and human ovary. Our results demonstrate that Wnt2 overexpression in DC3 promotes cytosolic and nuclear accumulation of beta-catenin (CTNNB1), but does not stimulate CTNNB1/TCF-dependent (pGL3-OT) transcriptional activity. We show that chibby (CBY1), a nuclear CTNNB1-associated antagonist of the WNT pathway, is expressed in DC3 cells and associates with CTNNB1 in the presence and absence of Wnt2 overexpression, suggesting that Cby1 contributes to suppression of CTNNB1/TCF-dependent transcription in these cells. Our results show that Wnt2 overexpression in DC3 cells increases follistatin (Fst) mRNA expression and promotes resistance to activin-induced cell deletion. Taken together, our results suggest that WNT2 opposes activin activity in granulosa cells by up-regulating expression of the activin antagonist Fst in a CTNNB1/TCF-independent manner, and that rat granulosa cells express factors, including Cby1, that suppress CTNNB1/TCF-dependent signal transduction in the presence of a WNT signal.

Cholesterol supply and SREBPs modulate transcription of the Niemann-Pick C-1 gene in steroidogenic tissues

We tested whether sterol-regulatory element binding proteins (SREBPs) mediate sterol-regulated transactivation of the Niemann-Pick C-1 (NPC-1) gene. Loading granulosa cells with 22- or 25-hydroxycholesterol decreased NPC-1 mRNA, whereas culturing in cholesterol-depleted medium or inhibition of cholesterol biosynthesis increased NPC-1 promoter activity and NPC-1 mRNA abundance. Cotransfection of SREBP1a, SREBP1c, and SREBP2 and the NPC-1 promoter-luciferase reporter into granulosa cell lines increased the transcriptional activity of porcine, human, and mouse NPC-1 promoters. Deletion analysis of the 5' flanking region of the pig NPC-1 gene demonstrated significant promoter activity between fragments -934 and -636 bp upstream from the transcription initiation site. Sequence analysis revealed three sterol-regulatory elements (SREs) clustered between -558 and -650 bp. Each site, along with E-box sequences, bound recombinant SREBP in electromobility shift assays. Mutation of all three sites attenuated the SREBP induction of promoter activity. Chromatin immunoprecipitation (ChIP) assays revealed that cholesterol depletion enriched the association of both SREBP and acetylated histone H3 with the NPC-1 promoter fragment containing the three SREs. ChIP analysis confirmed that SREBP's association with SRE and the E-box was enriched in cells cultured in cholesterol-depleted medium. We conclude that NPC-1 is sterol-regulated, achieved by SREBP acting via SRE and the E-box sequences.

FSH stimulates ovarian cancer cell growth by action on growth factor variant receptor

A number of FSH receptor (FSH-R) isoforms with distinct structural motifs and signaling paradigms have been described, including a single transmembrane domain variant that functions as a growth factor type receptor (FSH-R3). This study tested the hypothesis that FSH can stimulate ovarian cancer cell proliferation by acting on FSH-R3, using the tumorigenic mouse ovarian surface epithelial cell (MOSEC) line ID8. FSH enhanced ID8 proliferation in a concentration-dependent fashion. Moreover, FSH-treatment of ID8 elicited intracellular events consistent with activation of FSH-R3 and distinct from those associated with activation of the canonical G-protein coupled FSH-R isoform (FSH-R1). Specifically, the FSH-R3 signaling pathway included cAMP-independent activation of ERK downstream of an SNX-482 sensitive component likely to be the Cav2.3 calcium channel. Northern analysis using probes specific for exons 7 and 11 of FSH-R identified consistently only one 1.9kb transcript. Immunoblot analysis confirmed expression of FSH-R3 but not FSHR-1 in ID8. Together, these data suggest that FSH-R3 signaling promotes proliferation of ovarian cancer cells.

Establishment of an immortalized porcine granulosa cell line (PGV) and the study on the potential mechanisms of PGV cell proliferation

In order to establish an immortalized granulosa cell line and to investigate the potential mechanisms of immortalized cell proliferation, simian virus (SV) 40 was used to infect porcine granulosa cells from small follicles (1-2 mm in diameter), and one colony was selected after four weeks of culture. The colony was digested with trypsin and the cells were cultured for more than 300 days (named PGV). The SV40 large T antigen gene and its products were confirmed in immortalized cells by Southern blotting and immunohistochemistry. Progesterone production was not detected in the conditioned culture media with follicle-stimulating hormone (FSH) and forskolin, possibly due to the lack of P450scc gene transcription as examined by Northern blotting. PGV cells responded significantly to the stimulation of sera (fetal bovine and horse sera) and protein kinase C (PKC) stimulators (PMA and OAG), while PKC inhibitors (staurosporine and calphostin C) blocked both sera and PKC stimulation. Phospholipase C (PLC) and phosphatidic acid phosphatase (PAP) inhibitors (U73122 and propranolol) significantly reduced PGV cell proliferation, while PMA restored PLC and PAP inhibition. These data suggest that diacylglycerol (DAG) is produced in PGV cells by PLD as well as by PLC, and that DAG then activates PKC stimulating the PGV cell cycle through yet unknown mechanisms. Thus, an immortalized granulosa cell line is very useful to study granulosa cells in vitro, as the cells are homogeneous and are a functionally defined population.

Ovarian granulosa cell lines

The ovary is a complex endocrine gland responsible for production of sex steroids and is the source of fertilizable ova for reproduction. It also produces various growth factors, transcription factors and cytokines that assist in the complex signaling pathways of folliculogenesis. The ovary possesses two primary steroidogenic cell types. The theca cells (and to a lesser extent, the stroma) are responsible for androgen synthesis, and the granulosa cells are responsible for conversion of androgens to estrogens, as well as progesterone synthesis. These cells undergo a transformation in the luteal phase of the menstrual cycle, converting them from estrogen producing, to predominantly progesterone producing cells. Understanding the molecular mechanisms regulating these cells is essential in understanding the regulation of steroidogenesis and reproduction. Creation of appropriate in vitro cell model systems can provide important tools for the study of ovarian function. This has led to the development of ovarian steroidogenic cell lines in several laboratories. Developing theca cell lines has met with limited success. Conversely, numerous human and animal granulosa cell lines have been developed. This review will discuss the existing granulosa cell lines and their characteristics.

4-aminopyridine decreases progesterone production by porcine granulosa cells

BACKGROUND

Ion channels occur as large families of related genes with cell-specific expression patterns. Granulosa cells have been shown to express voltage-gated potassium channels from more than one family. The purpose of this study was to determine the effects of 4-aminopyridine (4-AP), an antagonist of KCNA but not KCNQ channels.

METHODS

Granulosa cells were isolated from pig follicles and cultured with 4-AP, alone or in combination with FSH, 8-CPT-cAMP, estradiol 17beta, and DIDS. Complimentary experiments determined the effects of 4-AP on the spontaneously established pig granulosa cell line PGC-2. Granulosa cell or PGC-2 function was assessed by radio-immunoassay of media progesterone accumulation. Cell viability was assessed by trypan blue exclusion. Drug-induced changes in cell membrane potential and intracellular potassium concentration were documented by spectrophotometric determination of DiBAC4(3) and PBFI fluorescence, respectively. Expression of proliferating cell nuclear antigen (PCNA) and steroidogenic acute regulatory protein (StAR) was assessed by immunoblotting. Flow cytometry was also used to examine granulosa cell viability and size.

RESULTS

4-AP (2 mM) decreased progesterone accumulation in the media of serum-supplemented and serum-free granulosa cultures, but inhibited cell proliferation only under serum-free conditions. 4-AP decreased the expression of StAR, the production of cAMP and the synthesis of estradiol by PGC-2. Addition of either 8-CPT-cAMP or estradiol 17beta to serum-supplemented primary cultures reduced the inhibitory effects of 4-AP. 4-AP treatment was also associated with increased cell size, increased intracellular potassium concentration, and hyperpolarization of resting membrane potential. The drug-induced hyperpolarization of resting membrane potential was prevented either by decreasing extracellular chloride or by adding DIDS to the media. DIDS also prevented 4-AP inhibition of progesterone production.

CONCLUSION

4-AP inhibits basal and FSH-stimulated progesterone production by pig granulosa cells via drug action at multiple interacting steps in the steroidogenic pathway. These inhibitory effects of 4-AP on steroidogenesis may reflect drug-induced changes in intracellular concentrations of K+and Cl- as well as granulosa cell resting membrane potential.

Are estrogens of import to primate/human ovarian folliculogenesis?

The notion that estrogens play a meaningful role in ovarian folliculogenesis stems from a large body of in vitro and in vivo experiments carried out in certain rodent models, (e.g., rats) wherein the stimulatory role of estrogen on granulosa cell growth and differentiation is undisputed. However, evidence derived from these polyovulatory species may not be readily generalizable to the monoovulatory subhuman primates, let alone the human. Only recently, significant observations on the ovarian role(s) of estrogen have been reported for the primate/human. It is thus the objective of this communication to review the evidence for and against a role for estrogens in primate/human ovarian follicular development with an emphasis toward the application of the concepts so developed to contemporary reproductive physiology and to the practice of reproductive medicine. The role(s) of estrogens will be examined not only by analyzing the physiological evidence to the effect that these hormones control ovarian function and follicular growth, but also by summarizing the molecular evidence for the existence and distribution of the cognate receptors.

Generation of stable cell lines by spontaneous immortalization of primary cultures of porcine granulosa cells

We report the generation of stable cell lines obtained by spontaneous immortalization of primary cultures of porcine granulosa cells. Three hundred stable cell lines were obtained from three independent immortalization trials. Two of these cell lines retained the steroidogenic capabilities characteristic of granulosa cells, such as de novo synthesis of progesterone and conversion of androstenedione into estradiol-17beta. All the stable cell lines expressed the P450arom and 3betaHSD genes, confirming their granulosa origin. Moreover, the steroidogenic stable granulosa cells also expressed StAR and P450scc genes. Stable cells were developed in cultures using Medium 199 supplemented with 5% newborn calf serum (NBCS). The surviving cells overcame the senescent phase and entered a stage of continuous growth for over one hundred generations. No stable colonies were obtained from cultures grown in MEM or DMEM or media supplemented with 10% NBCS or 5 and 10% fetal calf serum (FCS). Medium 199 is a formulation richer in nutrients compared to MEM or DMEM and the cell growth capability of NBCS is lower than that of FCS, probably due to deficiency of growth factors. We speculate that spontaneous immortalization of granulosa cells may be facilitated by using a rich culture formulation supplemented with low concentrations of serum deficient in growth factors. We have validated the stable cell lines for studying the effect of hormonal steroids on granulosa cell steroidogenesis and the expression of the steroidogenic genes. Therefore, we believe that they are useful models to study the molecular mechanism involved in granulosa cell differentiation and steroidogenesis.

Expression of granulosa cell-specific genes and induction of apoptosis in conditionally immortalized granulosa cell lines established from H-2Kb-tsA58 transgenic mice

Granulosa cell lines have been established from H-2Kb-tsA58 transgenic mice. Using immunocytochemistry, significant amounts of insulin-like growth factor-I (IGF-I) were found in all cell lines investigated, whereas estrogen and progesterone receptor expression could be detected in only some of the lines. All cell lines showed low basal production of the gonadal steroids estradiol and progesterone. The genes for the ovarian paracrine regulators IGF-I and basic fibroblast growth factor were expressed, as well as the genes for anti-Müllerian hormone and for the P450 side-chain cleavage enzyme (P450scc). Expression of P450scc could be shown to be up-regulated in the cell lines under conditions mimicking the hormonal environment of the luteinizing granulosa cells in vivo. Inactivation of the temperature-sensitive SV40 T antigen by a shift of the cell lines to the nonpermissive temperature of 39.5 degrees C led to massive induction of apoptosis in several cell lines. These cell lines will allow a detailed study of the mechanisms regulating the expression of granulosa cell-specific functions, as well as the induction of granulosa cell apoptosis.