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Abstract
The aim of the present article is to update our understanding of the expression of the insulin-like growth factor binding proteins (IGFBPs), IGFBP proteases and their implication in the different processes of ovarian folliculogenesis in mammals. In the studied species, IGFs and several small-molecular weight IGFBPs (in particular IGFBP-2 and IGFBP-4) are considered, respectively, as stimulators and inhibitors of follicular growth and maturation. IGFs play a key role in sensitizing ovarian granulosa cells to FSH action during terminal follicular growth. Concentrations of IGFBP-2 and IGFBP-4 in follicular fluid strongly decrease during follicular growth, leading to an increase in IGF bioavailability. Inversely, atresia is characterized by an increase of IGFBP-2 and IGFBP-4 levels, leading to a decrease in IGF bioavailability. Changes in intrafollicular IGFBPs content are due to variations in mRNA expression and/or proteolytic degradation by the pregnancy-associated plasma protein-A (PAPP-A), and likely participates in the selection of dominant follicles. The identification of PAPP-A2, as an IGFBP-3 and -5 protease, and stanniocalcins (STCs) as inhibitors of PAPP-A activity extends the IGF system. Studies on their implication in folliculogenesis in mammals are still in the early stages.
Highlights
Specialty section: This article was submitted to Experimental Endocrinology, a section of the journal Frontiers in Endocrinology
Rivera and Fortune have reported that insulin-like growth factor binding proteins (IGFBPs)-5 is degraded by plasma protein-A (PAPP-A) in bovine preovulatory follicles but not in subordinate follicles of the same cohort [64]. These results suggest that degradation of IGFBP-2, IGFBP-4, and IGFBP-5 by PAPP-A in preovulatory follicles is a well-conserved mechanism in mammalian species
In the last two decades, studies on IGFBPs and folliculogenesis have been revisited with the identification of IGFBP proteases, PAPP-A and Plasma Protein-A2 (PAPP-A2), and more recently by the PAPP-A inhibitors STCs
Summary
The IGF system is composed of different elements [2,3,4]:. – Two ligands, IGF-I and IGF-II. – Two receptors: the type I receptor mediates most of the somatomedin-like actions of both IGF-I and -II. The IGF system is composed of different elements [2,3,4]:. – Two receptors: the type I receptor mediates most of the somatomedin-like actions of both IGF-I and -II. The type II receptor, or IGF-II/Mannose-6-Phosphate (IGF-II/M6P) receptor, binds
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