Activins regulate FSH synthesis by stimulating the phosphorylation and nuclear accumulation of SMAD2 and SMAD3, which bind to a consensus SMAD-binding element in the proximal murine FSHβ (Fshb) subunit gene to drive transcription. Previous over-expression and in vitro DNA binding analyses suggested that SMAD4 participates in complexes with SMAD2 and SMAD3 to regulate Fshb expression. Here, we have characterized the role of endogenous SMAD4 in activin A induction of Fshb transcription in immortalized murine gonadotropes (LβT2). We identified five murine Smad4 mRNA isoforms, of which, four are newly described; however, the canonical full-length form predominated at both the mRNA and protein levels. Depletion of endogenous SMAD4 by RNA interference (RNAi) abolished activin A-induced Fshb promoter-reporter activity and greatly attenuated constitutively active activin type IB receptor-stimulated Fshb mRNA levels. The activin A response was rescued with an RNAi-resistant form of wild-type SMAD4, but not with a DNA-binding-deficient (Lys88Arg) SMAD4, suggesting that DNA binding by SMAD4 is necessary for activin induction of the Fshb gene. Though SMAD2 and SMAD3 are generally thought to partner with SMAD4 prior to accumulation in the nucleus, treatment with leptomycin B, an inhibitor of SMAD4 nuclear export, reduced but did not prevent activin A induction of Fshb mRNA levels or promoter activity. In addition, a constitutively nuclear form of SMAD4 rescued the effect of endogenous SMAD4 depletion. Collectively, these data demonstrate a necessary role for SMAD4 in activin A induction of the murine Fshb gene in immortalized gonadotropes.
Ying Wang, Vanessa Libasci and Daniel J Bernard
Gauthier Schang, Chirine Toufaily and Daniel J Bernard
Fertility is dependent on follicle-stimulating hormone (FSH), a product of gonadotrope cells of the anterior pituitary gland. Hypothalamic gonadotropin-releasing hormone (GnRH) and intra-pituitary activins are regarded as the primary drivers of FSH synthesis and secretion. Both stimulate expression of the FSH beta subunit gene (Fshb), although the underlying mechanisms of GnRH action are poorly described relative to those of the activins. There is currently no consensus on how GnRH regulates Fshb transcription, as results vary across species and between in vivo and in vitro approaches. One of the more fully developed models suggests that the murine Fshb promoter is tonically repressed by histone deacetylases (HDACs) and that GnRH relieves this repression, at least in immortalized murine gonadotrope-like cells (LβT2 and αT3-1). In contrast, we observed that the class I/II HDAC inhibitor trichostatin A (TSA) robustly inhibited basal, activin A-, and GnRH-induced Fshb mRNA expression in LβT2 cells and in primary murine pituitary cultures. Similar results were obtained with the class I specific HDAC inhibitor, entinostat, whereas two class II-specific inhibitors, MC1568 and TMP269, had no effects on Fshb expression. Collectively, these data suggest that class I HDACs are positive, not negative, regulators of Fshb expression in vitro and that, contrary to earlier reports, GnRH may not stimulate Fshb by inhibiting HDAC-mediated repression of the gene.
Pankaj Lamba, Michelle M Santos, Daniel P Philips and Daniel J Bernard
In rodents, activins stimulate immediate-early increases in pituitary follicle-stimulating hormone β (Fshb) subunit transcription. Here, we investigated the underlying signaling mechanisms using the mouse gonadotrope cell line, LβT2. Activin A increased mouse Fshb-luciferase reporter activity within 4 h through a Smad-dependent signaling pathway. The ligand rapidly stimulated formation of SMAD2/3/4 complexes that could interact with a consensus palindromic Smad binding element (SBE) in the proximal Fshb promoter. SMAD over-expression potently stimulated transcription, with the combination of SMADs 2, 3 and 4 producing the greatest synergistic activation. A mutation in the SBE that abolished Smad binding greatly impaired the effects of acute (4 h) activin A treatment and SMAD over-expression on promoter activity, but did not abolish the effects of chronic (24 h) activin A exposure. Within activated SMAD complexes, SMADs 3 and 4 appeared to bind the SBE simultaneously and the binding of both was required for maximal transcriptional activation. Interestingly, the human FSHB promoter, which lacks the consensus SBE, was neither rapidly stimulated by activin A nor by over-expressed SMADs, but was activated by 24 h activin A. Addition of the SBE to the human promoter increased both SMAD2/3/4-sensitivity and acute regulation by activin A, though not to levels observed in mouse. We postulate that short reproductive cycles in female rodents, particularly the brief interval between the primary and secondary FSH surges of the estrous cycle, require the Fshb promoter in these animals to be particularly sensitive to the rapid, Smad-dependent actions of activins on transcription. The human FSHB promoter, in contrast, is chronically regulated by activins seemingly through a SMAD-independent pathway.
Katharine B Lee, Vishal Khivansara, Michelle M Santos, Pankaj Lamba, Tony Yuen, Stuart C Sealfon and Daniel J Bernard
Transforming growth factor β superfamily ligands regulate pituitary FSH production and secretion. The best-described examples are the activins and inhibins, which respectively stimulate and hinder Fshb subunit transcription in gonadotrope cells. More recently, members of the bone morphogenetic protein (BMP) sub-family were shown to regulate FSH production in a manner analogous to the activins. Here, we used the murine gonadotrope cell line, LβT2, to investigate mechanisms through which BMP2 regulates the Fshb gene. Although expressed at low levels in LβT2 cells, Bmp2 mRNA was readily detected in adult murine pituitary gland. Recombinant BMP2 stimulated Fshb promoter-reporter activity, although its effects were weaker than those of equimolar activin A or B. BMP4 stimulated transcription comparably with BMP2, but BMPs 6 and 7 were about tenfold less potent. Remarkably, BMP2 and activin A synergistically upregulated Fshb transcription and endogenous Fshb mRNA levels in LβT2 cells. Although functionally cooperative, the two ligands appeared to use distinct intracellular mechanisms to mediate their responses because neither ligand altered the timing or magnitude of the other’s effects. Receptor overexpression analyses suggested that BMP2 may preferentially signal through complexes of the type II receptor, BMPR2, and the type I receptor, activin receptor like kinase (ALK2; Acvr1), to stimulate Fshb transcription. BMP2 rapidly activated the Smad1/5/8 intracellular signaling cascade and Smad8 overexpression potentiated BMP2’s effects. In summary, BMPs regulate Fshb transcription in LβT2 cells and can amplify the already robust effects of the activins through a distinct signaling mechanism. Because BMP2 is expressed in the adult mouse pituitary, it may act as critical paracrine co-regulator of FSH synthesis by gonadotropes.