Signalling cross-talk between hepatocyte nuclear factor 4α and growth-hormone-activated STAT5b

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Abstract
In the present study, we have characterized signalling cross-talk between STAT5b (signal transducer and activator of transcription 5b) and HNF4α (hepatocyte nuclear factor 4α), two major regulators of sex-dependent gene expression in the liver. In a HepG2 liver cell model, HNF4α strongly inhibited β-casein and ntcp (Na+/taurocholate cotransporting polypeptide) promoter activity stimulated by GH (growth hormone)-activated STAT5b, but had no effect on interferon-γ-stimulated STAT1 transcriptional activity. By contrast, STAT5b synergistically enhanced the transcriptional activity of HNF4α towards the ApoCIII (apolipoprotein CIII) promoter. The inhibitory effect of HNF4α on STAT5b transcription was associated with the inhibition of GH-stimulated STAT5b tyrosine phosphorylation and nuclear translocation. The short-chain fatty acid, butyrate, reversed STAT5b transcriptional inhibition by HNF4α, but did not reverse the inhibition of STAT5b tyrosine phosphorylation. HNF4α inhibition of STAT5b tyrosine phosphorylation was not reversed by pervanadate or by dominant-negative phosphotyrosine phosphatase 1B, suggesting that it does not result from an increase in STAT5b dephosphorylation. Rather, HNF4α blocked GH-stimulated tyrosine phosphorylation of JAK2 (Janus kinase 2), a STAT5b tyrosine kinase. Thus STAT5b and HNF4α exhibit bi-directional cross-talk that may augment HNF4α-dependent gene transcription while inhibiting STAT5b transcriptional activity via the inhibitory effects of HNF4α on JAK2 phosphorylation, which leads to inhibition of STAT5b signalling initiated by the GH receptor at the cell surface.

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