The ovarian insulin-like growth factors, a local amplification mechanism for steroidogenesis and hormone action

Physiological alterations associated with intrauterine growth restriction in fetal pigs: Causes and insights for nutritional optimization

Intrauterine growth restriction (IUGR) remains a major problem in swine production since the associated low birth weight leads to high rates of pre-weaning morbidity and mortality plus permanent retardation of growth and development. Complex biological events-including genetics, epigenetics, maternal maturity, maternal nutrition, placenta efficiency, uterine capacity, and other environmental factors-can affect fetal growth and development during late gestation, as well as maturity of oocytes, duration of estrus, and both implantation and placentation of conceptuses in uteri of sows. Understanding the physiological changes related to initiation and progress of IUGR are, therefore, of great importance to formulate nutritional strategies that can mitigate IUGR in gilts and sows. Altering the nutritional status of sows prior to mating and during early-, mid-, and late-gestation may be effective at increasing the uniformity of oocytes and conceptuses, decreasing variation among conceptuses during elongation and implantation, and preventing increases in within-litter variation in fetal weights during late gestation. This review summarizes current progress on physiological alterations responsible for IUGR fetuses, as well as possible nutritional interventions to prevent the initiation and continuation of IUGR in gilts and sows.

Membrane receptor cross talk in gonadotropin-, IGF-I-, and insulin-mediated steroidogenesis in fish ovary: An overview

Gonadal steroidogenesis is critical for survival and reproduction of all animals. The pathways that regulate gonadal steroidogenesis are therefore conserved among animals from the steroidogenic enzymes to the intracellular signaling molecules and G protein-coupled receptors (GPCRs) that mediate the activity of these enzymes. Regulation of fish ovarian steroidogenesis in vitro by gonadotropin (GtH) and GPCRs revealed interaction between adenylate cyclase and calcium/calmodulin-dependent protein kinases (CaMKs) and also MAP kinase pathway. Recent studies revealed another important pathway in GtH-induced fish ovarian steroidogenesis: cross talk between GPCRs and membrane receptor tyrosine kinases. Gonadotropin binding to Gαs-coupled membrane receptor in fish ovary leads to production of cAMP which in turn trans-activate the membrane-bound epidermal growth factor receptor (EGFR). This is followed by activation of ERK1/2 signaling that promotes steroid production. Interestingly, GtH-induced trans-activation of EGFR in the fish ovary uniquely requires matrix-metalloproteinase-mediated release of EGF. Inhibition of these proteases blocks GtH-induced steroidogenesis. Increased cAMP production in fish ovarian follicle upregulate follicular cyp19a1a mRNA expression and aromatase activity leading to increased biosynthesis of 17β-estradiol (E2). Evidence for involvement of SF-1 protein in inducing cyp19a1a mRNA and aromatase activity has also been demonstrated. In addition to GtH, insulin-like growth factor (IGF-I) and bovine insulin can alone induced steroidogenesis in fish ovary. In intact follicles and isolated theca cells, IGF-I and insulin had no effect on GtH-induced testosterone and 17a,hydroxysprogeaterone production. GtH-stimulated E2 and 17,20bdihydroxy-4-pregnane 3-one production in granulosa cells however, was significantly increased by IGF-I and insulin. Both IGF-I and insulin mediates their signaling via receptor tyrosine kinases leading to activation of PI3 kinase/Akt and MAP kinase. These kinase signals then activates steroidogenic enzymes which promotes steroid production. PI3 kinase, therefore considered to be an initial component of the signal transduction pathways which precedes MAP kinase in IGF-1 and insulininduced steroidogenesis in fish ovary. Thus, investigation on the mechanism of signal transduction regulating fish ovarian steroidogenesis have shown that multiple, apparently independent signal transduction pathways are needed to convey the message of single hormone or growth factor.

Within-litter variation in birth weight: impact of nutritional status in the sow

Accompanying the beneficial improvement in litter size from genetic selection for high-prolificacy sows, within-litter variation in birth weight has increased with detrimental effects on post-natal growth and survival due to an increase in the proportion of piglets with low birth-weight. Causes of within-litter variation in birth weight include breed characteristics that affect uterine space, ovulation rate, degree of maturation of oocytes, duration of time required for ovulation, interval between ovulation and fertilization, uterine capacity for implantation and placentation, size and efficiency of placental transport of nutrients, communication between conceptus/fetus and maternal systems, as well as nutritional status and environmental influences during gestation. Because these factors contribute to within-litter variation in birth weight, nutritional status of the sow to improve fetal-placental development must focus on the following three important stages in the reproductive cycle: pre-mating or weaning to estrus, early gestation and late gestation. The goal is to increase the homogeneity of development of oocytes and conceptuses, decrease variations in conceptus development during implantation and placentation, and improve birth weights of newborn piglets. Though some progress has been made in nutritional regulation of within-litter variation in the birth weight of piglets, additional studies, with a focus on and insights into molecular mechanisms of reproductive physiology from the aspects of maternal growth and offspring development, as well as their regulation by nutrients provided to the sow, are urgently needed.

Acute dietary restriction in heifers alters expression of genes regulating exposure and response to gonadotrophins and IGF in dominant follicles

Dietary restriction in growing cattle and severe negative energy balance in lactating cows have been associated with altered gonadotropin secretion, reduced follicle diameter, reduced circulating oestradiol concentrations and anovulation. Therefore, we hypothesised that acute dietary restriction would influence the fate and function of the dominant follicle by altering the expression for genes regulating gonadotrophin and IGF response in ovarian follicles. Newly selected dominant follicles were collected 7-8 days after prostaglandin F(2α) (PGF) administration from heifers (n=25) that were individually fed a diet supplying 1.2 maintenance (M; control, n=8) or 0.4 M (restricted, n=17) for a total duration of 18-19 days. Heifers within 0.4 M were ovulatory (n=11) or anovulatory (n=6) depending on whether the dominant follicle present at PGF ovulated or became atretic following luteolysis. Control animals were all ovulatory. Acute dietary restriction decreased IGF-I (P<0.001) and insulin (P<0.05) in circulation; oestradiol (P<0.01) and IGF-I (P<0.01) in follicular fluid; and mRNA for FSHR (P<0.01) in granulosa cells but increased mRNA for IGFBP2 (P<0.05) in theca cells of the newly selected dominant follicle. However, this only led to anovulation when dietary restriction also decreased mRNA for CYP19A1 (P<0.05), IGF2 (P<0.01) and IGF1R (P<0.05) in granulosa cells and LHCGR (P<0.05) in theca cells of follicles collected from heifers fed 0.4 M. These results suggest that the catabolic environment induced by dietary restriction may ultimately cause anovulation by reducing oestradiol synthesis, FSH-responsiveness and IGF signaling in granulosa, and LH-responsiveness in theca cells of dominant follicles.

Molecular cloning of IGF-1 and IGF-1 receptor and their expression pattern in the Chilean flounder (Paralichthys adspersus)

Insulin-like growth factor-1 and insulin-like growth factor-1 receptor (IGF-1 and IGF-1R) play main roles in vertebrate growth and development. In fish, besides contributing to somatic growth, both molecules exhibit pleiotropic functions. We isolated complete cDNAs sequences encoding for both IGF-1 and IGF-1R in the Chilean flounder by using RT-PCR and rapid amplification of cDNAs ends (RACE) techniques. We analyzed gene expression in pre-metamorphic larvae and different organs of juvenile fish through whole mount in situ hybridization and RT-PCR, respectively. In addition, we studied the presence of calcified skeletal structures in pre-metamorphic larvae through the fluorescent chromophore calcein. The IGF-1 cDNA sequence displays an open reading frame of 558 nucleotides, encoding a 185 amino acid preproIGF-1. Moreover, IGF-1R contains an open reading frame spanning 4239 nucleotides, rendering a 702 amino acid subunit alpha and a 676 amino acid subunit beta. The deduced mature IGF-1 and IGF-1R exhibited high sequence identities with their corresponding orthologs in fishes, especially those domains involved in biological activity. RT-PCR showed expression of IGF-1 and IGF-1R transcripts in all studied tissues, consistent with their pleiotropic functions. Furthermore, we observed IGF-1 expression in notochord and IGF-1R expression in notochord, somites and head in larvae of 8 and 9 days post fertilization. Complementarily, we detected in larvae of 8 days post fertilization the presence of calcified skeletal structures in notochord and head. Interestingly, both mRNAs and calcified structures were found in territories such as notochord, an embryonic midline structure essential for the pattern of surrounding tissues as nervous system and mesoderm. Our results suggest that IGF-1 and its receptor play an important role in the development of the nervous system, muscle and bone-related structures during larval stages.

Regulation of ovarian steroidogenesis in vitro by IGF-I and insulin in common carp, Cyprinus carpio: stimulation of aromatase activity and P450arom gene expression

Regulation of ovarian steroidogenesis in vitro by recombinant human insulin-like growth factor-I (IGF-I) and bovine insulin (b-insulin) was investigated in intact follicles and isolated follicular cells of carp, Cyprinus carpio at vitellogenic stage of oocyte maturation. In intact follicles, IGF-I and b-insulin stimulated testosterone and 17beta-estradiol production in vitro. In isolated theca cells, IGF-I and b-insulin stimulated testosterone production, whereas in granulosa cells, they stimulated 17beta-estradiol production when testosterone was added in the incubation medium as precursor substrate. In intact follicles and in theca cells, IGF-I and b-insulin had no effect on HCG-stimulated testosterone production. HCG-stimulated 17beta-estradiol production, however, was significantly increased by IGF-I and b-insulin. To clarify the mechanism of 17beta-estradiol production by the ovarian follicles during vitellogenic stage of carp, effects of IGF-I and b-insulin either alone or in combination with HCG on aromatase activity (conversion of testosterone to 17beta-estradiol) and cytochrome P450 aromatase (P450arom) gene expression were investigated in vitro. IGF-I and b-insulin alone stimulated aromatase activity and P450arom gene expression and significantly enhanced HCG-induced enzyme activity and P450arom gene expression. Our results thus indicate that IGF-I and b-insulin alone can stimulate testosterone and 17beta-estradiol production in vitellogenic follicles of C. carpio by stimulating aromatase activity and P450arom gene expression. Evidence also provided for the modulation of HCG-induced aromatase activity and P450arom gene expression by IGF-I and b-insulin in such follicles.

Modulating peripheral gonadotrophin levels affects follicular expression of mRNAs encoding insulin-like growth factor binding proteins in sheep

Evidence suggests that the insulin-like growth factor binding proteins (IGFBPs) are involved in modulating the role that IGF-I and -II play in regulating follicular growth and development in sheep. However, little information exists as to the role that key peripheral factors play in regulating the expression of IGFBP components within the follicle. The present study investigated the regulatory effects of FSH and LH on gene expression for IGFBP-2 to -6 in ovine follicles, using bovine follicular fluid (bFF) and gonadotrophin-releasing hormone antagonist (GnRHa) model systems to perturb endogenous gonadotrophin secretion. Gene expression studies were carried out using in situ hybridisation with sheep-specific ribonucleotide probes. Treatment of ewes with bFF had few consistent effects on IGFBP-3 and -4 mRNA expression in follicles, whereas bFF treatment resulted in significant decreases in IGFBP-2 and IGFBP-6 mRNA expression in only medium follicles 60 h after onset of bFF, and in follicles 12h after the onset of bFF, respectively. Treatment of ewes with GnRHa resulted in transient significantly increased IGFBP-2 mRNA in healthy follicles in comparison to early atretic or atretic follicles 12h post-GnRHa (P<0.05), in IGFBP-3 mRNA in early atretic and atretic follicles 36 h post-GnRHa (P<0.001), and IGFBP-5 mRNA in follicles 12h post-GnRHa (P<0.001). In contrast, GnRHa treatment resulted in significant decreases in IGFBP-4 (P<0.001) and IGFBP-6 (P<0.01) mRNA expression in large follicles 36 h post-GnRHa, and atretic follicles by 60 h post-GnRHa, respectively. These data highlight that FSH and LH are involved, at least in part, in mediating the proliferative and differentiative changes in intrafollicular IGFBP levels that are observed during follicular growth and atresia in the sheep.

Insulin-like growth factor I in sera, ovarian follicles and follicular fluid of cows with spontaneous or induced cystic ovarian disease

The objective of this research was to determine changes in IGF-I levels in serum and follicular fluid, and immunoreactivity of the follicle wall of cows with spontaneous (slaughter specimens) or ACTH-induced follicular cysts, and to compare results to normal cycling (control) cows after selection of the ovulatory follicle. Concentrations of IGF-I in serum did not differ between control and cystic animals (p=0.76). Fluid from the ovulatory follicle in control cows had 41% higher concentrations of IGF-I than that from cystic follicles collected at slaughter (spontaneous cysts; p<0.05) and 70% higher than that in induced follicular cysts (p<0.05). An intense positive immunostaining with anti-IGF-I was observed in granulosa cells (p<0.05) and in the theca interna (p<0.05) of secondary and tertiary follicles in all three groups of animals, but staining was less intense in cystic (p<0.05) and atretic follicles (p<0.05). This study provides evidence to suggest that cystic ovarian disease in cattle is associated with decreased concentrations of IGF-I in follicular fluid, but not in serum, and decreased production of IGF-I in the follicular wall. These data support the notion that IGF-I plays a role in the regulation of folliculogenesis, and may participate in the pathogenesis of cystic ovarian disease in cattle.

LH and IGF-1 release during oestrus and early luteal phase in lactating and non-lactating horse mares

The aim of the present study was to determine effects of lactation on basal LH and IGF-1 concentrations and on the LH response to a GnRH-analogue at different stages of the oestrous cycle in mares. A total of 17 cyclic Haflinger mares were included in the study. Experiments were performed on lactating mares in first postpartum oestrus, the subsequent early luteal phase, and second postpartum oestrus. Non-lactating mares were used in oestrus and early luteal phase. Blood samples were taken for 1 h at 15 min intervals. Mares were then injected with the GnRH-analogue buserelin (GnRHa; 5 microg i.v.) and blood samples were drawn every 15 min for further 2 h. LH in all samples and basal IGF-1-concentrations were determined by RIA. In lactating mares, basal LH concentrations during the early luteal phase tended to be lower (p = 0.07) and the LH response to GnRHa, calculated as area under the curve, was significantly less pronounced compared to non-lactating mares (p < 0.01). As well in lactating mares, the basal LH concentration between first early luteal phase and second oestrus differed significantly (p < 0.05) and the net response to GnRHa was significantly lower between first oestrus as well as second oestrus and first early luteal phase (p < 0.05) but not between first and second oestrous postpartum. Within the group of non-lactating mares, the LH response to GnRHa was as well significantly lower during oestrus than during early luteal phase (p < 0.01). IGF-1 concentrations differed neither between groups nor stages of the cycle within groups. In conclusion, basal and GnRHa-stimulated LH release in lactating mares is lower than in non-lactating mares. This difference, however, occurs only in the early luteal phase. In lactating mares, concentrations of LH appear adequate to allow ovulation to occur.

Molecular cloning and mRNA profile of insulin-like growth factor type 1 receptor in orange-spotted grouper, Epinephelus coioides

The insulin-like growth factor type 1 receptor (IGF-IR) belongs to the tyrosine kinase (TK) receptor family. Besides being mitogenic, IGF-IR plays a crucial role in cell survival, transformation and maintenance of the malignant phenotype. In this study, we cloned the cDNA from the hypothalamus of the orange-spotted grouper (Epinephelus coioides) using reverse transcription PCR (RT-PCR) and the rapid amplification of cDNA ends (RACE) method. The deduced amino acid sequence showed that the receptor comprises 1413 amino acid residues. It contains cysteine-rich domains in its alpha-subunit, and a conserved transmembrane domain and TK domains in its beta-subunit. Comparison of the amino acid sequence with those of other species showed that the grouper IGF-IR shares 90.2%, 89.6%, 71.9% and 72% similarity with the IGF-IR of the Japanese flounder, turbot, zebrafish-a and zebrafish-b, respectively. When compared with its mammalian homologue, grouper IGF-IR contains a large insertion at its C-terminus. Phylogenetic analysis has revealed that the grouper IGF-IR belongs to the b-type IGF-IRs and has a higher similarity with flounder and turbot IGF-IR, and a lower similarity (<70%) with human, mouse and avian IGF-IR. Grouper IGF-IR transcripts were detected in the brain, peripheral tissues, embryos and early development larvae by semi-quantitative RT-PCR assay. It was observed that IGF-IR mRNA expression was greater in the brain than in peripheral tissues. The level of IGF-IR mRNA expression was much higher in retina, gonad, skeletal muscle and gill tissues than in liver, heart and thymus tissues. The expression of IGF-IR can be visualized as a ubiquitous signal in unfertilized eggs, embryos and early development larvae. The distribution pattern of IGF-IR mRNA in grouper development suggests that IGF-IR plays an important role in the embryo and early larval development stages.

Insulin-like growth factor-binding protein-3 in porcine ovarian granulosa cells: gene cloning, promoter mapping, and follicle-stimulating hormone regulation

The role and regulation of IGF-binding protein-3 (IGFBP-3) in the ovary is not fully understood. We cloned and determined the sequence of 12,257 bp of the pig IGFBP-3 gene that includes 4,296 bp of the flanking promoter sequence. The porcine IGFBP-3 promoter sequence shares two highly conserved regions with the human and bovine IGFBP-3 promoters and a mouse DNA clone. The first is a 38 bp region between -1095 and -1058, whereas the second is a 73-bp region between -63 and +10 of the pig sequence. Projected translation of the open reading frame of our sequence gave a peptide sequence identical to that determined by peptide sequencing, but with 27 additional amino acids upstream of this sequence and is highly similar to the human, bovine, rat, and mouse IGFBP-3 peptides. Using RT-PCR we demonstrated that FSH regulates IGFBP-3 mRNA expression in a biphasic manner, with an early induction (maximal at 3 h) and an inhibition at 24 h after FSH treatment. The inhibition at 24 h was not due to changes in IGFBP-3 mRNA stability. A similar pattern of FSH modulation of the IGFBP-3 gene transcription was demonstrated by the reporter activity of granulosa cells transiently transfected with IGFBP-3 promoter constructs. The site for FSH stimulation of the IGFBP-3 gene was localized to the sequence between -61 and -48 relative to the transcription start site. Regulation of IGFBP-3 transcription by FSH suggests a role for IGFBP-3 in follicular development that may be independent of IGF-I.

Effect of follicle size on in vitro production of steroids and insulin-like growth factor (IGF)-I, IGF-II, and the IGF-binding proteins by equine ovarian granulosa cells

Little is known regarding the hormonal regulation of granulosa cell steroidogenesis and the ovarian insulin-like growth factor (IGF) system in the mare. The objectives of this study were to determine, first, if estradiol, insulin, and/or FSH affect steroid production by equine granulosa cells (experiment 1) and, second, if the components of the IGF system are produced by equine granulosa cells in culture as well as whether estradiol, insulin, and/or FSH affects IGF and/or IGF-binding protein (IGFBP) production by equine granulosa cells (experiment 2). Granulosa cells from small (6-15 mm), medium (16-25 mm), and large (25-48 mm) follicles were collected from cyclic mares (n = 14), cultured for 2 days in medium containing 10% fetal calf serum, washed, and then treated for an additional 2 days in serum-free medium with or without added hormones. In experiment 1, large-follicle granulosa cells produced less progesterone and more estradiol than did medium- and/or small-follicle granulosa cells (P < 0.05). Progesterone production was inhibited (P < 0.05) by FSH and insulin in small- and medium- but not in large-follicle granulosa cells; estradiol was without effect. Insulin increased (P < 0.05) estradiol production in small- and medium-follicle granulosa cells but had no effect in large-follicle granulosa cells. In experiment 2, IGF-I production was inhibited (P < 0.05) by insulin across all follicle sizes but was not affected by estradiol or FSH. Granulosa cells of medium and large follicles produced more IGF-II than did granulosa cells of small follicles (P < 0.05). Insulin and FSH inhibited (P < 0.05) IGF-II production by granulosa cells of large and medium but not of small follicles; estradiol was without effect. Only IGFBP-2 and -5 were produced by equine granulosa cells. Production of IGFBP-2 was less (P < 0.10) in granulosa cells of large versus those of small and medium follicles, whereas medium-follicle granulosa cells produced more (P < 0.05) IGFBP-5 than did small- or large-follicle granulosa cells. Averaged across follicle sizes, estradiol increased (P < 0.05) IGFBP-2 production, FSH increased (P < 0.10) IGFBP-2 and -5 production, and insulin was without effect. These results indicate that IGF-I, IGF-II, IGFBP-2, and IGFBP-5 are produced by equine granulosa cells and that insulin, FSH, and estradiol play a role in the regulation of steroidogenesis and the IGF system of equine granulosa cells.

Insulin-like growth factor-I (IGF-I) stimulates the development of cultured rat pre-antral follicles

The aim of the present study is to investigate the effect of insulin-like growth factor (IGF-I) on the development of cultured rat pre-antral follicles. Pre-antral follicles with a diameter between 140 and 160 microm are mechanically isolated from ovaries of 10-day-old rats and cultured in groups for 6 days in FSH and insulin-containing serum-free medium in the absence or presence of IGF-I at concentrations of 1, 10, and 100 ng/ml, respectively. DNA content of the follicles before and after culture is measured to evaluate whether a possible growth is due to proliferation of follicular cells and the ultrastructure of the cultured follicles is studied with transmission electron microscopy to evaluate the quality of follicles cultured under different conditions. Furthermore, reverse transcriptase polymerase chain reaction (RT-PCR) is used to assess the gene expression of IGF-I and type I IGF receptor in pre-antral follicles. When follicles were cultured in medium containing IGF-I at a concentration of 1 and 10 ng/ml, the increase in follicle diameter after a 6-day culture differs significantly from that in the absence of IGF-I. The ultrastructure of follicles cultured in IGF-I-containing medium is better sustained when compared to that of follicles cultured in the absence of IGF-I. Especially, theca cells, which are undergoing degeneration under controlled culture condition, demonstrate normal cellular ultrastructure with some characteristics of steroid-secreting cells in the presence of IGF-I. Moreover, in oocytes of follicles cultured in the presence of IGF-I, cortical granules are observed distributed along the ooplasma membrane whereas cortical granules are hardly present in follicles cultured without IGF-I. With RT-PCR, the presence of the mRNA of IGF-I and type I IGF receptor is demonstrated in both the somatic follicular cells and the oocytes. Taken together, these results suggest that IGF-I, in the presence of follicle-stimulating hormone (FSH), enhances rat pre-antral follicle development in vitro and supports the morphology of cultured pre-antral follicles. The stimulatory effect of exogenous IGF-I on the development of pre-antral follicle together with the gene expression of both IGF-I and type I IGF receptor in pre-antral follicles suggests the involvement of IGF-I in early folliculogenesis and its actions are likely mediated by the putative type I IGF receptor, via both endocrine and paracrine/autocrine pathways.

Secretion of IGF-1 by ovine granulosa cells: effects of growth hormone and follicle stimulating hormone

Insulin-like growth factor-1 (IGF-1) is implicated in follicle development and is considered to mediate the actions of growth hormone (GH) and gonadotrophins at the ovarian level. However, the expression and secretion of IGF-1 by the ovary are controversial, partly because of species and cell-type specificity. The present study investigated whether IGF-1 is produced by ovine granulosa cells and whether its production is regulated by GH and follicle stimulating hormone (FSH). Follicles (>/=4.0 mm) were obtained from ewes during seasonal anoestrus. Granulosa cells were cultured for a total period of 96 h in Dulbecco's modified Eagle's medium (DMEM)/Ham's F-12 medium supplemented with BSA (0.1%, w:v), transferrin (0.5 microg/ml) and testosterone (100 ng/ml). In the first set of experiments, cells were incubated in the presence of bovine calf serum (BCS) (2.5%) for the initial 48 h of culture. The cells were then cultured for the next 48 h in medium without BCS, but containing either GH (0, 2, 20, and 200 ng/ml) or FSH (0, 20, 200, and 2000 ng/ml). The medium was assayed for oestradiol (E), progesterone (P) and IGF-1. There were six wells per treatment and the experiment was carried out four times. Control granulosa cells maintained both IGF-1 and E secretion, with only low levels of progesterone output. In all experiments, both GH and FSH produced significant (P<0.001) dose-related increases in E, IGF-1 and P secretion into the medium. The maximum responses to GH (20 or 200 ng/ml) were 402% for E and 528% for IGF-1 compared with controls. The maximum responses to FSH (200 or 2000 ng/ml) were 460% for E and 514% for IGF-1. The objective of the second set of experiments was to determine the effect of the progestogenic status of cells on IGF-1 production. Granulosa cells were cultured both in the presence and absence of BCS (2.5% in the medium) during the initial 48 h of culture. For the next 48 h, cells were cultured in serum-free medium. Addition of BCS to the medium during the initial 48 h of culture stimulated progesterone production. However, it did not affect either IGF-1 or oestradiol secretion between 49 and 96 h of culture, or the cell numbers at the end of culture. In conclusion, (1) IGF-1 is secreted by granulosa cells irrespective of their progestogenic status and (2) concomitant increases in E and IGF-1 production by granulosa cells as a result of GH and/or FSH treatment suggest a role for GH and FSH in the regulation of ovarian function.

Insulin-like growth factor binding protein-3: its biological effect on bovine granulosa cells

This study was aimed at testing the hypothesis that insulin-like growth factor binding protein (IGFBP)-3 can modulate hormone-dependent differentiation of granulosa cells in vitro. Granulosa cells from small (1 to 5 mm) follicles were collected from cattle, cultured for 2 d in medium containing 10% fetal calf serum, washed, and then treated for an additional 2 d in serum-free medium with follicle-stimulating hormone (FSH) (50 ng/ml), recombinant human IGF-I (0, 1.3, 4.0, or 13.3 nM), or recombinant human IGFBP-3 (0 to 4.26 nM). In one series of experiments, IGFBP-3 (0.53 and 2.13 nM) inhibited (51% to 92% decreases; P < 0.05) progesterone and estradiol production induced by 1.3 nM of IGF-I, but did not influence (P > 0.10) granulosa cell numbers or steroidogenesis in the absence of IGF-I. Only 4.26 nM of IGFBP-3 inhibited (by 35%) the increase in granulosa cell numbers induced by 1.3 nM of IGF-I. In another series of experiments, 13.3 nM of IGF-I, but not 4.0 nM of IGF-I, was able to completely overcome the inhibitory effect of 4.26 nM of IGFBP-3 on estradiol production. The increase in cell numbers induced by 4.0 and 13.3 nM of IGF-I was attenuated (P < 0.001) by 4.26 nM of IGFBP-3. In a third series of experiments, IGFBP-3 inhibited 125I-IGF-I binding to granulosa cells. These results indicate that IGFBP-3 has a pronounced inhibitory effect on IGF-I action in cultured bovine granulosa cells, and that this inhibitory effect is likely attributable to IGFBP-3 binding/sequestering IGF-I. Thus, IGFBP-3 may play a significant role in regulating granulosa cell proliferation and steroidogenesis during follicular development in cattle.

Expression of mouse ovarian insulin growth factor system components during follicular development and atresia

Insulin growth factor I (IGF-I) appears necessary for the completion of follicular development in mice. However, little is known about changes in the IGF system components during follicular development and luteinization. This study determined the relation between gene expression of specific IGF system components and follicular growth, survival, or atresia in mice. Immature mice from three different strains (129, C57, and MF1), with or without gonadotropin treatment (2.5 IU PMSG/2.5 IU human CG (hCG)], were used. The strains were similar in all parameters measured. Apoptosis, as detected by in situ labeling of nicked DNA, preceded the appearance of morphological signs of atresia. In healthy follicles, IGF-I transcripts were low during the primary follicular stage but increased to a maximum in the late preantral and early antral stages (P < 0.001) irrespective of hormone treatment. Occasionally, IGF-I transcripts were also detected in apoptotic follicles but decreased (P < 0.05) as a function of atresia as assessed by morphological criteria. IGF binding protein-4 (IGFBP-4) messenger RNA (mRNA) expression in granulosa cells was restricted to apoptotic and atretic follicles (P < 0.001). IGFBP-5 transcript levels, on the other hand, were elevated in granulosa cells of healthy primary and secondary follicles but decreased in subsequent follicular stages and in atretic follicles (P < 0.001). Conversely, IGFBP-2 mRNA was constitutively expressed in granulosa cells. PMSG/hCG treatment induced the appearance of IGFBP-2 transcripts in the ovarian interstitium. Following PMSG/hCG-induced ovulation, IGFBP-2 and -4 and IGF type-I receptor mRNAs were strongly expressed in virtually all luteal cells, whereas IGFBP-3 and -5 transcripts were selectively localized to some cell types in the corpus luteum. Conversely, IGF-I mRNA was essentially undetectable in the corpus luteum. This study represents the most comprehensive and detailed analysis of the physiology and anatomy of the mouse ovarian IGF system, and shows that 1) IGFBP-5-is linked to the survival of the slow growing and immature preantral follicles; 2) IGF-I is associated with the growth and survival of the rapidly growing large preantral and antral follicles; 3) IGFBP-4 is an atretogenic candidate for mouse ovarian follicles; 4) ovulatory doses of PMSG/hCG up-regulate IGFBP-2 mRNA expression in the ovarian interstitium; and 5) transcripts of IGF type-I receptor and IGFBP-2 through -5, but not those of IGF-I are highly expressed in the mouse corpus luteum.

Paracrine mechanisms of ovarian follicle apoptosis

It has become evident that the physiological removal of cells through apoptosis during embryonic and postnatal development of multicellular organisms is a mandatory process to maintain a homeostatic state of the individual. In the ovary, massive cell death occurs during neonatal and postnatal life as an integral part of the normal ovarian development. At birth, mammalian ovaries are endowed with a fixed number of non-growing follicles that will be gradually recruited into a growing pool during reproductive life. Once follicles start growth they are either selected for ovulation or, for the majority of them, removed by apoptosis. Thus, removal of excess ovarian cells by apoptosis is necessary for normal development of the ovary. Despite the important role of follicle atresia in the maintenance of normal follicle development, studies on the hormonal control of follicle cell demise during follicle growth have not been possible until the recent development of apoptosis detection methods. Recent biochemical analysis has revealed the occurrence of internucleosomal DNA fragmentation, a hallmark of apoptosis, in atretic follicles and has facilitated the investigation into the intra-ovarian hormonal regulation of follicle atresia. This review summarizes the recent advances in the intra-ovarian hormonal mechanisms that control follicle apoptosis.

Follicular dominance in cattle is associated with divergent patterns of ovarian gene expression for insulin-like growth factor (IGF)-I, IGF-II, and IGF binding protein-2 in dominant and subordinate follicles

A decrease in insulin-like growth factor (IGF) binding protein (BP) amount occurs within the follicular fluid of dominant ovarian follicles. At the same time, concentrations of follicular fluid IGF-I do not change. The mRNA for IGF-I, IGF-II, IGFBP-2, and IGFBP-3, in dominant and subordinate follicles were measured to determine if changes in IGF or IGFBP gene expression are associated with follicular dominance. Heifers were ovariectomized during a follicular wave, either during early-dominance (emerging dominant follicle, 9 mm diameter) or mid-dominance (established dominant follicle, 14-16 mm diameter). Follicles were classified as either dominant (DF), subordinate (SF), or not-recruited (NRF; small antral follicles). mRNA was localized by in situ hybridization and measured by image analyses. The IGF-I mRNA (granulosa cells) was greatest in DF and increased in DF, SF, and NRF from early- to mid-dominance. Likewise, IGF-II mRNA (theca cells) was greatest in DF compared with SF or NRF. The IGFBP-2 mRNA (granulosa cells), however, was nearly undetectable in DF, whereas adjacent SF expressed abundant IGFBP-2 mRNA. The NRF were not uniform in their IGFBP-2 expression because only 5 of 13 NRF had IGFBP-2 mRNA. The IGFBP-3 mRNA (granulosa cells) was found only in two NRF, suggesting that local synthesis is not a predominant source of follicular fluid IGFBP-3. These data show that changes in gene expression for IGFBP-2 are opposite to those for IGF-I or IGF-II. Increased IGF-I and IGF-II mRNA and decreased IGFBP-2 mRNA within the DF may be one mechanism leading to follicular dominance. The opposite pattern of IGFBP-2 gene expression in SF and some NRF may lead to follicular atresia.

Insulin-like growth factor I regulates gonadotropin responsiveness in the murine ovary

The present study shows that insulin-like growth factor I (IGF-I) and FSH receptor (FSHR) mRNAs are selectively coexpressed in a subset of healthy-appearing follicles in murine ovaries, irrespective of cycle stage. Aromatase gene expression, a prime marker for FSH effect, is found only in IGF-I/FSHR-positive follicles, showing that these are healthy, gonadotropin-responsive follicles. Given the striking coexpression of FSHR and IGF-I, we hypothesized that FSH was responsible for follicular IGF-I expression. We found, however, that granulosa cell IGF-I mRNA levels are not reduced in hypophysectomized (+/-PMSG) or FSH knockout mice, indicating that FSH does not have a major role in regulation of granulosa cell IGF-I gene expression. To test the alternative hypothesis that IGF-I regulates FSHR gene expression, we studied ovaries from IGF-I knockout mice. FSHR mRNA was significantly reduced in ovaries from homozygous IGF-I knockout compared with wild type mice and was restored to control values by exogenous IGF-I treatment. The functional significance of the reduced FSHR gene expression in IGF-I knockout ovaries is suggested by reduced aromatase expression and by the failure of their follicles to develop normally beyond the early antral stage. In fact, IGF-I knockout and FSH knockout ovaries appear very similar in terms of arrested follicular development. In summary, we have shown that IGF-I and FSHR are selectively coexpressed in healthy, growing murine follicles and that FSH does not affect IGF-I expression but that IGF-I augments granulosa cell FSHR expression. These data suggest that ovarian IGF-I expression serves to enhance granulosa cell FSH responsiveness by augmenting FSHR expression.

The mouse intraovarian insulin-like growth factor I system: departures from the rat paradigm

Although the rat intraovarian insulin-like growth factor I (IGF-I) system is well documented, the increasing availability of null mouse mutants for components of the IGF system necessitates characterization of the mouse model as well. Therefore, we undertook to define the components of the mouse intraovarian IGF-I system and to examine its operational characteristics. The cellular pattern of ovarian gene expression was comparable in the immature rat and mouse for IGF-I and the type I IGF receptor. In both species, IGF-I messenger RNA (mRNA) is selectively expressed by granulosa cells in growing, healthy appearing follicles. Type I IGF receptor mRNA was also concentrated in granulosa cells, but was uniformly expressed in all follicles large and small, healthy and atretic appearing alike. Cellular patterns of IGF-binding protein (IGFBP) gene expression were similar in mouse and rat, except in the case of IGFBP-2. IGFBP-2 mRNA was localized to the mouse granulosa cell, in contrast to its concentration in the rat thecal-interstitial compartment. This difference in IGFBP expression pattern was also noted in cultured mouse and rat granulosa cells. Although immunoreactive IGFBP-4 (24 and 28 kDa) and IGFBP-5 (29 kDa) were shared by both species, the cultured mouse granulosa cell also featured immunoreactive IGFBP-2 (30 kDa). The mouse paradigm further differed from its rat counterpart in that a maximal dose of FSH, previously shown to suppress the elaboration of rat granulosa cell-derived IGFBPs, was without effect. The addition of IGF-I proved stimulatory to the accumulation of the 28- to 29-kDa IGFBPs, as previously reported for the rat. However, IGF-I proved inhibitory to the accumulation of the 24-kDa IGFBP (presumptive nonglycosylated IGFBP-4); no consistent effect was reported for the rat model. Functional comparisons of mouse and rat ovarian cell cultures revealed qualitatively comparable FSH-stimulated steroidogenesis, disposition of radiolabeled pregnenolone, IGF-I-amplified FSH action, and IGFBP-mediated antigonadotropic activity. These findings indicate that the mouse intrafollicular IGF-I system differs from the rat paradigm in both the makeup and regulation of granulosa cell-derived IGFBPs as well as in the intensity and character of the steroidogenic process. Studies employing the mouse model must take into account these important distinctions relative to the more established rat paradigm.

Ovarian receptors for insulin and insulin-like growth factor I (IGF-I) and effects of IGF-I on steroid production by isolated follicular layers of the preovulatory coho salmon ovarian follicle

In this study, receptors for insulin and insulin-like growth factor I (IGF-I) in isolated theca-interstitial layers and granulosa cells of the coho salmon preovulatory ovary were characterized, and the effects of IGF-I on ovarian steroidogenesis were examined. Specific receptors for insulin and IGF-I were found in granulosa and theca-interstitial layers. In both follicular layers, IGF-I receptors were greater in number and higher in affinity than insulin receptors. The effects of IGF-I on in vitro production of testosterone (T) and 17 alpha-hydroxyprogesterone (17OH-P) by theca-interstitial layers and of 17 beta-estradiol (E2) and 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) by granulosa cell layers were evaluated during the preovulatory period. Both human and salmon recombinant IGF-I inhibited the basal and GTH II-stimulated T and 17OH-P production by theca-interstitial layers throughout the preovulatory period. In contrast, IGF-I stimulated the production of both E2 and 17,20 beta-P by granulosa cell layers prior to germinal vesicle breakdown (GVBD) but only stimulated the production of 17,20 beta-P by granulosa cell layers after GVBD. The inhibitory effects of IGF-I on steroid production by the theca-interstitial layer and the opposite stimulatory effects on steroid production by the granulosa cell layer, coupled by the presence of specific IGF-I receptors in both follicular layers, suggest that IGF-I may play a role in the regulation of steroidogenesis in the preovulatory coho salmon ovary.

Insulin receptor-like tyrosine kinase in the tobacco hornworm, Manduca sexta

Phosphotyrosine-containing proteins are present in the prothoracic glands, muscle, and fat body of Manduca sexta, as determined by immunoprecipitation followed by kinase assay, and by Western blotting. One such protein (M(r) 178,000) can also be immunoprecipitated using antibodies directed against the human insulin receptor and insulin receptor substrate. The 178 kD protein appears to be expressed more strongly in prothoracic glands removed just prior to wandering (days 3-4) and prior to pupation (days 7-9), and phosphorylation of the protein is enhanced by an M. sexta brain factor. The results suggest that a tyrosine-kinase-linked molecule similar to the insulin receptor may play a regulatory role in M. sexta.

Intra-ovarian regulation of oocyte developmental competence in cattle

Each oestrous cycle the ovary produces one or more oocytes, depending on the species considered, which are fully competent to sustain the development of a new individual and are defined as matured. Ovulation is the terminal phase of a lengthy and complex selection process which enables only a minute proportion of the oocytes present in the ovary to be ovulated and possibly fertilised. The use of exogenous gonadotrophins for the pharmacological stimulation of the ovaries has clearly indicated that oocytes naturally excluded by the selection process can also be matured and, if fertilisedin vivoorin vitro, can generate normal offspring.

The ovarian insulin and insulin-like growth factor system with an emphasis on domestic animals

Insulin and insulin-like growth factors (IGFs) have direct effects on cultured ovarian cells. These effects include stimulation of granulosa cell mitogenesis, granulosa and luteal cell progesterone production, and thecal cell androgen production and appear similar among species. However, species differences exist with regard to insulin and IGF-I effects on granulosa cell estradiol production. In addition to endocrine effects of insulin and IGFs, IGFs are produced by granulosa, thecal, and luteal cells, allowing for an intraovarian autocrine and paracrine system. Granulosa, thecal, and luteal cells contain receptors for insulin and IGFs, and these receptors appear to mediate the effects of insulin and IGFs. Adding to the complexity of the regulatory role of IGFs is the presence of IGF-binding proteins (IGFBPs) within the ovary. These IGFBPs are produced by granulosa, thecal, and luteal cells, and their production is hormonally regulated. Evidence for a coherent mechanism by which insulin, IGFs, and IGFBPs interact and regulate ovarian function in vivo has yet to be found.

Effects of somatotropin on the conceptus, uterus, and ovary during maternal recognition of pregnancy in cattle

Effects of recombinant bovine somatotropin (rbST) on ovarian and uterine function and the production of components of the insulin-like growth factor (IGF) system were examined during the period of maternal recognition of pregnancy in cattle. Lactating dairy cows were treated with 25 mg/d rbST (n = 8) or saline (n = 8) for 16 d after estrus. Ovaries, uteri, and conceptuses were collected on Day 17 after estrus. The length (millimeters) of the conceptus was recorded. The concentration of IGF-I and the content of IGF-binding proteins (BP) in uterine flushings were determined. Corpora lutea (CL) were weighed, and the number of follicles (> or = 2 mm in diameter) were counted. Follicular fluid from the largest and second-largest follicles was assayed for the concentration of IGF-I, IGFBP, progesterone, and estradiol. The length of the conceptus and the total amount of IGF-I in uterine fluid were similar for rbST and control. Recombinant bST increased 1) the weight of the CL, 2) the number of largest follicles (10 to 15 mm in diameter), 3) the concentration of IGF-I in the follicular fluid, 4) the follicular fluid content of IGFBP of the largest estrogenic follicle, and 5) the quantity of IGFBP in uterine flushings. The concentration of progesterone in the follicular fluid tended to be increased in rbST-treated cows, whereas the concentration of estradiol was similar to that of control cows. The concentration of progesterone in plasma was similar for rbST compared with control. In conclusion, the administration of rbST in lactating dairy cows for 16 d after estrus did not alter the growth of the conceptus collected on Day 17. The greatest responses to rbST were found within the ovary, where rbST increased the weight of the CL and altered the amount of IGF-I and IGFBP in the follicular fluid.

Insulin-like growth factor-I receptors in ovarian granulosa cells: effect of follicle size and hormones

The objectives of the present studies were to determine if the numbers of IGF-I receptors in bovine granulosa cells differed with size of follicle and to determine if growth factors and hormones affected the number of IGF-I receptors in granulosa cells. Granulosa cells from small (1-5 mm) and large (> or = 8 mm) follicles were cultured for 2-4 days in 10% FCS and then assessed for levels of IGF-I receptors. Numbers of IGF-I receptors were 15-fold greater in granulosa cells from large than small follicles. In addition, bFGF (5 and 50 ng/ml) decreased whereas estradiol (1 microgram/ml), FSH (10 ng/ml) and EGF (10 and 100 ng/ml) increased the number of IGF-I receptors in granulosa cells from small follicles. These hormones had no effect on the number of IGF-I receptors in granulosa cells from large follicles. In conclusion, granulosa cells from large follicles have a greater number of IGF-I receptors than cells from small follicles, and thus, it appears that granulosa cells acquire a greater number of IGF-I receptors during the process of differentiation.

Extended function of the corpus luteum and earlier development of the second follicular wave in heifers treated with bovine somatotropin

Effects of recombinant bovine somatotropin (bST) on growth of the corpus luteum (CL) and development of ovarian follicles were tested. Starting at estrus (Day=0), the following treatments were administered: control (saline injected Days 0 to 19, n=5); bST[0-9] (25 mg bST injected Days 0 to 9, saline injected Days 10 to 19, n=5); bST[10-19] (saline injected Days 0 to 9, 25 mg bST injected Days 10 to 19, n=5); and bST[0-19] (25 mg bST injected Days 0 to 19, n=6). Blood was collected daily for progesterone analysis, and ultrasound examinations were performed daily for measurement of follicles and CL. Compared with the heifers treated with saline, those treated with bST had larger CL and more progesterone during the early (</= Day 10) and late (Days 15 to 21) phases of the estrous cycle. At mid-cycle (Days 10 to 15), luteal tissue area and progesterone levels were similar for bST- and saline-treated heifers. Ovarian follicular growth was similar for bST- and saline-treated heifers during the first follicular wave. However, during recruitment for the second follicular wave (Days 10 to 14), second largest follicles emerged sooner (P<0.001), the number of Class-3 (>/=10 mm) follicles was greater (P<0.01) and largest follicles were smaller (P<0.001) in bST than in saline-treated heifers. Estrous cycle length and ovulation rate were similar for each group. In conclusion, bST increased initial development of the CL and extended its function. Furthermore, the second follicular wave was earlier with bST.

Mitogenic factors of corpora lutea

The mammalian corpus luteum (CL), which plays a central role in the reproductive process because of its production of hormones such as progesterone, appears to be an exceptionally dynamic organ. Its rate of growth and development are extremely rapid and, even when the CL is functionally mature, its rate of cell turnover remains relatively high. Associated with this high rate of cell turnover, the mature CL receives the greatest blood supply per unit tissue of any organ, and also exhibits a relatively high metabolic rate. Although numerous growth factors have been identified in luteal tissue, their role in growth and differentiation of this dynamic organ remains unclear. Recently, while attempting to identify mitogenic factors of ovine and bovine CL, we have found that they produce several mitogens during the estrous cycle as well as pregnancy. The majority of these luteal-derived mitogenic factors are heparin-binding, and although some may represent previously identified factors, several appear to be novel heparin-binding growth factors. Isolation and purification of mitogenic factors produced by the CL will enable us to determine their roles in luteal growth, development and differentiated function, which will contribute to our understanding not only of the regulation of fertility but also of tissue growth and development in general.

Effect of feed restriction on serum somatotropin, insulin-like growth factor-I-(IGF-I) and IGF binding proteins in cyclic heifers actively immunized against growth hormone releasing factor

Feed restriction often increases serum somatotropin (ST) and decreases insulin-like growth factor-I (IGF-I) in ruminants; however, the mechanisms responsible for this change in ST and IGF-I are not well defined. We investigated the effects of feed restriction on serum ST, IGF-I, IGF binding proteins (IGFBP), insulin and nonesterified fatty acids (NEFA) in cyclic Angus and Charolais heifers (n = 15) previously immunized against growth hormone releasing factor (GRFi) or human serum albumin (HSAi). Cows were fed a concentrate diet ad libitum (AL) or were restricted to 2 kg cotton seed hulls (R) for 4 d. Each heifer received each dietary treatment in a single reversal design. As anticipated, GRFi decreased ST, IGF-I and insulin (P < .05). In addition, GRFi decreased serum IGFBP-3 (P < .01), but increased IGFBP-2 (P < .01). Feed restriction resulted in an increase in serum ST in HSAi, but not in GRFi heifers. Regardless of immunization treatment, feed restriction decreased serum IGF-I and insulin, and increased NEFA (P < .01). In conclusion, the increase in serum ST levels observed during feed restriction was blocked by active immunization against GRF. However, feed restriction resulted in decreased serum IGF-I in GRFi heifers in spite of initial low levels of IGF-I (due to GRFi). Although GRFi decreased levels of IGFBP-3 and increased levels of IGFBP-2, feed restriction for 4 d did not alter serum IGFBP.

Growth hormone (GH) and reproduction: a review

Interaction between growth and reproduction occurs in many vertebrates and is particularly obvious at certain stages of the life cycle in fish. Endocrine interactions between the gonadotropic axis and the somatotropic axis are described, the potential role of GH being emphasised. A comparative analysis of these phenomena in mammals, amphibians and fish, suggests a specific role of GH in the physiology of puberty, gametogenesis and fertility. It also shows the original contribution made by studies on the fish model in this field of investigations.

Insulin and IGF-I receptors and tyrosine kinase activity in carp ovaries: changes with reproductive cycle

Insulin and insulin-like growth factor I (IGF-I) receptors from carp ovaries were semipurified with wheat germ agglutinin at different moments of the reproductive cycle and their binding characteristics and tyrosine kinase activity were studied. Specific receptors for insulin and IGF-I were found. IGF-I receptors presented higher binding (23.8 ± 1.5%), number of receptors (965 ± 20fm/mg) and affinity (KD 0.24 ± 0.03nM) than those shown for insulin receptors (4.1 ± 1%, 530 ± 85fm/mg and 0.85 ± 0.1nM, respectively). Insulin and IGF-I receptors have a tyrosine kinase activity which is not different from that found in muscle of the same species. Seasonal changes were found in binding, with maximum values for insulin and IGF-I reached at the end of pre-spawning period (June). However, while IGF-I binding was observed in all stages, insulin binding decreased in autumn and disappeared in winter, which suggests a different role for the two peptides in ovarian physiology.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.

Characterization of insulin-like growth factor binding proteins secreted by cultured bovine theca and granulosa cells

Insulin-like growth factor binding proteins (IGFBPs) secreted by bovine granulosa and theca interna cells cultured in the presence of different luteinizing factors--insulin (2 micrograms/ml), forskolin (10 microM), or a combination of the two were examined and characterized. Direct binding of [125I]IGF to the conditioned media was compared to progesterone production under these different treatments. In theca cells, maximal secretion of IGFBPs was achieved using forskolin alone, whereas maximal progesterone production was induced by the insulin+forskolin treatment. In contrast, maximal secretion of both IGFBPs and progesterone in granulosa cells was achieved using forskolin alone. IGFBP species secreted by the two cell types under the different treatments were detected by ligand blotting. Conditioned media from theca cells in serum-free medium collected on the seventh day of culture exhibited three bands of 34, 40 and 44 kDa when treated with insulin or forskolin. The intensity of the 40-44 kDa complex was enhanced and a 21 kDa band appeared when cells were treated with a combination of insulin plus forskolin. Conditioned media of granulosa cells stimulated with insulin or forskolin exhibited 21, 27, 29, 34 and 40-44 kDa bands. Treatment with insulin+forskolin greatly increased the intensity of a 40-44 kDa complex. A similar shift towards high molecular weight binding proteins was observed when these media were analyzed by high-performance liquid chromatography gel filtration. These findings substantiate the secretion of IGFBPs by bovine theca and granulosa cells and show it to be dependent on culture treatment.(ABSTRACT TRUNCATED AT 250 WORDS)