Activator protein-1 (AP-1) plays an important role in the regulation of gene expression in mesangial cells (MC) during the pathogenesis of glomerular inflammatory disease. The precise regulation of the AP-1 family by agents that are known to activate MC is, however, poorly understood. The action of platelet-derived growth factor (PDGF) and, for the first time, lipopolysaccharide (LPS), interleukin-6 (IL-6), interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) on AP-1 gene expression in MC was therefore studied. Whilst the expression of JunD was not affected by any of the mediators, the mRNA levels of c-fos and JunB were induced by LPS, IL-6, IFN-gamma, PDGF and TNF-alpha, and that of c-jun by LPS, IFN-gamma, PDGF and TNF-alpha. Electrophoretic mobility shift assays showed a time-dependent increase in AP-1 DNA binding activity with JunB representing the major mediator-inducible member involved in DNA-protein interactions. However, stimulus-specific changes in the kinetics and magnitude of AP-1 mRNA expression and DNA binding activity were identified and, additionally, the results showed the potential existence of cell-type-specific mechanisms in the regulation of the AP-1 family. These studies provide novel insights into the mediator-specific modulation of AP-1-regulated gene expression and the activation of MC in renal diseases.