Activin Induction of Follicle Stimulating Hormone is Mediated by Transforming Growth Factor Beta Activated Kinase-1 (TAK-1) in Pituitary Gonadotropes.

Abstract

Follicle stimulating hormone (FSH) is an essential hormone for female folliculogenesis and plays an important role in male spermatogenesis. The hormone is secreted by pituitary gonadotropes in the anterior pituitary lobe, and its overall production is regulated by expression of the FSHβ subunit. The regulation of FSHβ subunit is achieved by combined actions of neurocrine, endocrine, and pituitary paracrine/autocrine factors. Activin shown to be produced locally within the pituitary is a potent stimulator of the FSHβ subunit.

This study used 4.7 kb of the ovine FSHβ promoter linked to luciferase (oFSHβLuc) plus a well characterized activins responsive construct, p3TPLuc, to investigate the hypothesis that Smad3, TAK1 (TGFβ activated kinase1), or both cause activin-mediated induction of FSH. Over-expression of either Smad3 or TAK1 induced oFSHβLuc in gonadotrope-derived LβT2 cells as much as activin itself. Induction of p3TPLuc by activin is known to require Smad3 activation in many cell types and this was true in LβT2 cells where 10-fold induction by activin (2-8 h after activin treatment) was blocked > 90% by two dominant negative (DN) inhibitors of Smad3 [DN-Smad3 (3SA) and DN-Smad3 (D407E)]. By contrast, 6.5-fold induction of oFSHβLuc by activin (10-24 h after activin treatment) was not blocked by either DN-Smad inhibitor, suggesting that activation of Smad3 did not trigger induction of oFSHβLuc.

By contrast, inhibition of TAK1 by a DN-TAK1 construct led to a 50% decrease in activin-mediated induction of oFSHβLuc, and a specific inhibitor of TAK1 (5Z-7-Oxozeanol) blocked induction by 100% indicating that TAK1 is necessary for activin induction of oFSHβLuc. Finally, inhibiting p38-mitogen activated protein kinase (p38-MAPK; often activated by TAK1) blocked induction of oFSHβLuc by 60%. In conclusion, the data presented here indicate that activation of TAK1 (and probably p38-MAPK), but not Smad3, is necessary for triggering induction of oFSHβ by activin.

In addition, a method was developed in our laboratory for purifying primary gonadotropes to study the solitary role of factors regulating FSHβ gene. We were able to isolate primary gonadotropes from pituitary with purities higher than 95%. The data show that gonadotropes are able to produce activin and/or activin-like molecule(s), however paracrine factors from pituitary non-gonadotropes play a major role in controlling FSHβ at the pituitary level. Overall, the study presented provides an understanding to the TAK1 signaling pathway mediating activin induction of FSHβ, and shows that primary gonadotropes rely on paracrine factors to produce activin.