Growth hormone does not increase the expression of insulin-like growth factors and their receptor genes in the pre-menopausal human ovary

Molecular crosstalk between insulin-like growth factors and follicle-stimulating hormone in the regulation of granulosa cell function

Background

The last phase of folliculogenesis is driven by follicle-stimulating hormone (FSH) and locally produced insulin-like growth factors (IGFs), both essential for forming preovulatory follicles.

Methods

This review discusses the molecular crosstalk of the FSH and IGF signaling pathways in regulating follicular granulosa cells (GCs) during the antral-to-preovulatory phase.

Main findings

IGFs were considered co-gonadotropins since they amplify FSH actions in GCs. However, this view is not compatible with data showing that FSH requires IGFs to stimulate GCs, that FSH renders GCs sensitive to IGFs, and that FSH signaling interacts with factors downstream of AKT to stimulate GCs. New evidence suggests that FSH and IGF signaling pathways intersect at several levels to regulate gene expression and GC function.

Conclusion

FSH and locally produced IGFs form a positive feedback loop essential for preovulatory follicle formation in all species. Understanding the mechanisms by which FSH and IGFs interact to control GC function will help design new interventions to optimize follicle maturation, perfect treatment of ovulatory defects, improve in vitro fertilization, and develop new contraceptive approaches.

The Role of Growth Hormone on Ovarian Functioning and Ovarian Angiogenesis

Although not yet well-understood, today it is clear that Growth Hormone (GH) exerts a relevant role in the regulation of ovulation and fertility; in fact, fertility is lower in women with GH deficiency (GHD), and GH receptors (GHR) and GH mRNA have been found in the ovary since the onset of follicular development in humans. However, despite the strong evidence of GH in the regulation of fertility, many aspects of GH actions at this level are still not well-established, and it is likely that some controversial data depend on the species analyzed, the dose of the hormone and the duration of use of GH. Folliculogenesis, ovulation, and corpus luteum formation and maintenance are processes that are critically dependent on angiogenesis. In the ovary, new blood vessel formation facilitates oxygen, nutrients, and hormone substrate delivery, and also secures transfer of different hormones to targeted cells. Some growth factors and hormones overlap their actions in order to control the angiogenic process for fertility. However, we still know very little about the factors that play a critical role in the vascular changes that occur during folliculogenesis or luteal regression. To promote and maintain the production of VEGF-A in granulosa cells, the effects of local factors such as IGF-I and steroids are needed; that VEGF-A-inducing effect cannot be induced by luteinizing hormone (LH) or chorionic gonadotropin (CG) alone. As a result of the influences that GH exerts on the hypothalamic-pituitary-gonadal axis, facilitating the release of gonadotropins, and given the relationship between GH and local ovarian factors such as VEGF-A, FGF-2, IGF-1, or production of sex steroids, we assume that GH has to be a necessary factor in ovarian angiogenesis, as it happens in other vascular beds. In this review we will discuss the actions of GH in the ovary, most of them likely due to the local production of the hormone and its mediators.

FSH Regulates IGF-2 Expression in Human Granulosa Cells in an AKT-Dependent Manner

CONTEXT

IGF-2 is highly expressed in the granulosa cells of human dominant ovarian follicles; however, little is known about the regulation of the IGF-2 gene or the interaction of IGF-2 and FSH during follicle development.

OBJECTIVE

To examine the mechanisms involved in the regulation of the IGF-2 gene by FSH and the interplay between FSH and IGF-2 during granulosa cell differentiation. Design, Setting, Patients, and Interventions: Cumulus granulosa cells were separated from cumulus-oocyte complexes isolated from the follicular aspirates of in vitro fertilization patients and cultured for in vitro studies.

MAIN OUTCOME

Protein and mRNA levels of IGF-2 and CYP19A1 (aromatase) were quantified using Western blot and quantitative real-time PCR. IGF-2 promoter-specific activation was determined by the amplification of alternative exons by PCR. Cell proliferation was assessed after treatment with FSH and/or IGF-2.

RESULTS

FSH significantly enhanced IGF-2 expression after 8 hours of treatment and at low doses (1 ng/mL). Reciprocally, IGF-2 synergized with FSH to increase cell proliferation and the expression of CYP19A1. When IGF-2 activity was blocked, FSH was no longer able to stimulate CYP19A1 expression. Determination of IGF-2 promoter usage in human cumulus cells showed that the IGF-2 gene is driven by promoters P3 and P4. However, FSH exclusively increased P3 promoter-derived transcripts. Moreover, the FSH-induced stimulation of P3-driven IGF-2 transcripts was blocked by cotreatment with inhibitors of AKT or IGF-1 receptor (IGF-1R). The inhibitory effect of the IGF-1R inhibitor on FSH-induced IGF-2 mRNA accumulation was reversed by overexpression of a constitutively active AKT construct.

CONCLUSIONS

FSH is a potent enhancer of IGF-2 expression in human granulosa cells. In return, IGF-2 activation of the IGF-1R and AKT is required for FSH to stimulate CYP19A1 expression and proliferation of granulosa cells. These findings suggest a positive loop interaction between FSH and IGF-2 that is critical for human granulosa cell proliferation and differentiation.