Regulation of expression of the rat SOCS-3 gene in hepatocytes by growth hormone, interleukin-6 and glucocorticoids - mRNA analysis and promoter characterization

Suppressors of cytokine signalling (SOCS) represent a newly discovered fami ly of molecules that seem to play an important role in the shutting off of cytokine and possibly peptide hormone action. Thus, understanding the mecha nisms controlling their expression is of cardinal importance. In the presen t study, we have cloned the rat SOCS-3 gene and analyzed its expression and the functioning of its promoter in hepatocytes. Expression of SOCS-3 mRNA, which is very weak in freshly isolated cells, tended to increase when hepa tocytes were incubated without hormones. Growth hormone (GH) and, to a much larger extent, interleukin-6 (IL-6) rapidly activated mRNA synthesis where as glucocorticoids (GC) strongly inhibited both basal and hormone-dependent expressions. A short promoter fragment (-137/+35) responded maximally to G H and IL-6 (a threefold stimulation for each effector) and to GC (a 70-80% inhibition), whereas longer promoter sequences supported higher basal activ ity and lower positive hormonal responses. Deletion and mutation analyses i ndicated that all hormonal responses were dependent on two cis-acting seque nces termed the G-rich and the A/T-rich elements. Only the A/T-rich element was active in a heterologous context, thus behaving as a typical enhancer. Unexpectedly, the two signal transducer and activator of transcription (ST AT) binding sites found immediately upstream of the G-rich motif didn't see m to participate in either GH or IL-6 effect, despite the fact that one of them strongly responded to IL-6 when placed in front of a heterologous prom oter. Finally, the negative regulation of SOCS-3 promoter by GC that may co ntribute to gene silencing in vivo, appeared to involve interactions of the GC receptor with other transcription factors and not direct binding to DNA , as no GC-response element was found in the sequence.