Activation of Sp1 and its functional co-operation with serum amyloid a-activating sequence binding factor in synoviocyte cells trigger synergistic action of interleukin-1 and interleukin-6 in serum amyloid A gene expression

The serum amyloid A (SAA) protein has been implicated in the progression and pathogenesis of rheumatoid arthritis through induction of collagenase activity in synovial fibroblast cells that line the joint tissues. We demonstrate that SAA is synergistically induced in synovial cells by interleukin (IL)-1 and IES that are present at significantly high level in the synovial fluid of arthritis patients. These cytokines induced phenotypic changes in synovial cells, promoting protrusion and increased cellular contact. Induction of SAA under this condition is mediated by promoter elements located between -254 and -226, which contains binding sites for transcription factors Sp1 and SAA activating sequence binding factor (SAF). Mutation of these sequences abolishes SAA promoter response to IL-1 and IL-6. The role of Spl in SAA induction was demonstrated by increased DNA binding activity, phosphorylation, and increased protein content of Spl during cytokine treatment. Spl interacts with the SAA promoter in association with SAF as an SAF Spl heteromeric complex. Furthermore, using a phosphatase inhibitor, we demonstrated increased transactivation potential of both Spl and SAF as a consequence of a phosphorylation event. These results provide first evidence for cytokine-mediated activation of Spl in synovial fibroblast cells and its participation in regulating SAA expression by acting in conjunction with SAF.