| Abstrakt: |
In rodents, activins stimulate immediate-early increases in pituitary follicle-stimulating hormone {szligbeta} (Fshb) subunit transcription. Here, we investigated the underlying signaling mechanisms using the mouse gonadotrope cell line, L{szligbeta}T2. 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. |
