The role of Stat and C/EBP transcription factors in the synergistic activation of rat serine protease inhibitor-3 gene by interleukin-6 and dexamethasone

The rat serine proteinase inhibitor 3 gene is activated by interleukin 6 (IL-6) and glucocorticoids in hepatic cells. We report here that a 147 bp promoter is sufficient for both IL-6 stimulation and glucocorticoid enhancement of IL-6 induced transcription. Within this region we identified two functional elements binding transcription factors from the C/EBP (CCAAT/enhancer binding proteins) and Stat (signal transducers and activators of transcription) families. Mutations introduced into the Stat binding site resulted in a loss of responsiveness, showing that this element is indispensable for activation. In contrast, the promoter containing the mutated C/EBP binding site was still responsive to IL-6 and glucocorticoids; however, the magnitude of the induction was decreased by 50%. The Stat binding element is an enhancer capable of conferring both responsiveness to IL-6 and partial enhancement of glucocorticoids on to a heterologous promoter, In response to IL-6 this element rapidly binds acute-phase response factor (APRF/ Stat3) and, later, the protein(s) that require ongoing protein synthesis and is recognized by anti-Stat3 antibodies. In addition, long-term treatment with IL-6 results in sustained phosphorylation of APRF/Stat3.