Effects of peroxisome proliferator-activated receptor activation on gonadotropin transcription and cell mitosis induced by bone morphogenetic proteins in mouse gonadotrope LβT2 cells

Involvement of peroxisome proliferator-activated receptor-γ (PPAR-γ) activation and bone morphogenetic protein (BMP) signaling in regulating cell proliferation and hormonal production of pituitary tumors has been reported, although the underlying mechanism remains poorly understood. Here, we investigated regulatory roles of PPARα and PPARγ in gonadotropin transcription and cell mitosis modulated by pituitary activin/BMP systems using a mouse gonadotropinoma cell line LβT2, which expresses activin/BMP receptors, transcription factor Smads, PPARα, and PPAKγ. In LβT2 cells, BMP signaling shown by Smad1/5/8 phosphorylation and Id-1 transcription was readily activated by BMPs. A PPARγ agonist, pioglitazone significantly reduced BMP-induced DNA synthesis by LβT2; whereas the PPARα agonist, fenofibric acid, did not. In accordance with the effects on cell mitosis, pioglitazone but not fenofibric acid significantly decreased BMP-induced Id-1-Luc activation. Neither fenofibric acid nor pioglitazone affected activin signaling detected by (CAGA)⁹-Luc activity. Both PPARα and PPARγ ligands directly suppressed transcriptional activities of FSHβ, LHβ, and GnRHR. Activation of PPARα and PPARAγ increased mRNA levels of follistatin, but did not affect the expression of follistatin-related gene. Thus, PPAR agonists not only directly suppress gonadotropin transcription and BMP signaling, but also inhibit the biological actions of activins which facilitate gonadotropin transcription through upregulating follistatin expression. In addition, pioglitazone increased BMP ligands mRNA, but decreased activin-βB mR-NA in LβT2 cells. Collectively, PPAR activation differentially regulates gonadotrope cell proliferation and gonadotropin transcription in a ligand-dependent manner.