Abstract
Inhibins are ovarian dimeric glycoprotein hormones that suppress pituitary FSH production. They are synthesised by follicular granulosa cells as α plus βA/βB subunits (encoded by INHA, INHBA, INHBB, respectively). Inhibin concentrations are high in follicular fluid (FF) which is also abundant in ‘free’ α subunit, presumed to be of granulosal origin, but its role(s) remains obscure. Here, we report the unexpected finding that bovine theca cells show abundant INHA expression and ‘free’ inhibin α production. Thus, theca cells may contribute significantly to the inhibin α content of FF and peripheral blood. In vitro, knockdown of thecal INHA inhibited INSL3 and CYP17A1 expression and androgen production while INSL3 knockdown reduced INHA and inhibin α secretion. These findings suggest a positive role of thecal inhibin α on androgen production. However, exogenous inhibin α did not raise androgen production. We hypothesised that inhibin α may modulate the opposing effects of BMP and inhibin on androgen production. However, this was not supported experimentally. Furthermore, neither circulating nor intrafollicular androgen concentrations differed between control and inhibin α-immunized heifers, casting further doubt on thecal inhibin α subunit having a significant role in modulating androgen production. Role(s), if any, played by thecal inhibin α remain elusive.
Highlights
Inhibins are ovarian dimeric glycoprotein hormones that suppress pituitary FSH production
Since ovarian granulosa cells are recognised as the principle site of inhibin/activin subunit mRNA expression (INHA, INHBA and INHBB) in various mammalian species it has logically been deduced that the high concentrations of dimeric inhibin and activin proteins present in follicular fluid are primarily of granulosal origin
Appreciable amounts of INHA mRNA were detected in theca interna (TC) with levels similar to those in GC found in smaller antral follicles ranging from 1 to 8 mm in diameter (Fig. 1A)
Summary
Inhibins are ovarian dimeric glycoprotein hormones that suppress pituitary FSH production. Since ovarian granulosa cells are recognised as the principle site of inhibin/activin subunit mRNA expression (INHA, INHBA and INHBB) in various mammalian species it has logically been deduced that the high concentrations of dimeric inhibin (αβ dimer) and activin (ββ dimer) proteins present in follicular fluid are primarily of granulosal origin.
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