TRANSCRIPTIONAL REGULATION OF THE JUNB PROMOTER - ANALYSIS OF STAT-MEDIATED SIGNAL-TRANSDUCTION

The product of the junB gene is a member of the AP-1 family of transcr iption factors that activate transcription by binding to TPA-responsiv e elements (TREs) within the promoters of target genes, Components of AP-1 are immediate-early genes whose expression is upregulated by a pl ethora of extracellular stimuli and are important in mediating cellula r proliferation and differentiation, Such stimuli include the pleiotro pic cytokine interleukin-6 (IL-6) which plays a role in immune and inf lammatory responses and ciliary neurotrophic factor (CNTF) which enhan ces survival and differentiation of neurons and glia. We have analysed expression from junB promoter-CAT reporter constructs in HepG2 cells and found that a region between -196 and -91 can mediate response to I L-6 and CNTF and was able to confer responsiveness to a heterologous p romoter, We further show by gel retardation analysis that distinct nuc lear factors induced by IL-6 specifically bind to this interleukin-6 r esponse element (IRE). This region contains both a putative ETS- and a STAT-transcription factor binding site, We show by mutational analysi s and supershift data that the IL-6 induced complex indeed contains th e transcription factor APRF/Stat3 that is both necessary and sufficien t for activation, Interestingly this site does not appear to bind Stat 1 itself, as shown by supershift analysis and a lack of response to IF N-gamma both at the DNA-binding and transcriptional level, Furthermore , we demonstrate that the junB IRE-binding activity induced by IL-6 re quires tyrosine kinase activity, whereas induced transactivation of IR E-constructs additionally occurs through an H7-sensitive pathway that is p21ras-independent, implicating serine/threonine kinases in the tra nsactivation of IRE-binding factors.