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To better understand how leukemia inhibitory factor (LIF) activates proopiomelanocortin (POMC) gene transcription in pituitary corticotrophs, time-course studies of the induction of POMC promoter activity and specific tyrosine phosphorylation of signal transducer and activator of transcription 1 (STAT1) and STAT3 were performed. It was found that both phosphorylation of STAT1 and -3 and activation of the promoter activity rapidly and transiently take place within minutes and 2-6 h, respectively, in favor of a direct effect of the LIF pathway on POMC promoter. Activated STAT1 and -3 form homo-/heterodimers able to bind the Sis-inducible element. The most abundant Sis-inducible element binding dimers are STAT3/3 and STAT1/3. Degenerated STAT1/3-binding sites from the POMC promoter were tested for their ability to bind activated STAT1 and 3; only the -390/-379 site, partially overlapping the Nur response element, binds with low affinity activated STAT1 and -3. Analysis of the three domains and subregions of the POMC promoter showed that two subregions are specifically responsive to LIF. The response of the distal subregion requires the intact STAT1 and -3 DNA-binding site -390/-379, whereas the responsiveness of the proximal subregion takes place despite the absence of direct STAT1 and -3 DNA binding and may imply interaction of activated STAT with basal transcription factors.

Different Mechanisms for Leukemia Inhibitory Factor-Dependent Activation of Two Proopiomelanocortin Promoter Regions