Activation of DNA synthesis and AP-1 by profilin, an actin-binding protein, via binding to a cell surface receptor in cultured rat mesangial cells

Profilin is known to bind to actin monomers (to regulate actin polymerizati on) and to phosphatidylinositol-4,5-bisphosphate (to inhibit hydrolysis by unphosphorylated phospholipase C-gamma 1). It was recently reported that pr ofilin is overexpressed in glomerular mesangial cells (MC) of rats with ant i-Thy-1.1-induced glomerulonephritis and is accumulated in the extracellula r space around MC. In this study, the biologic activities of extracellular profilin were examined. Scatchard analysis indicated the existence of a sin gle class of cell surface binding sites, with similar equilibrium dissociat ion constants for purified splenic profilin and recombinant profilin, in cu ltured rat MC. Profilin increased [H-3]thymidine incorporation in a dose-de pendent manner and produced additive effects on platelet-derived growth fac tor-induced [H-3]thymidine incorporation. Profilin increased AP-1 DNA-bindi ng activity in a concentration-dependent (ED50 = 30 nM) and time-dependent manner after transient c-jun gene expression, as measured using gel-shift a ssays and competitive reverse transcription-PCR. Pretreatment of profilin w ith an anti-profilin inhibitory antibody suppressed profilin-induced AP-1 a ctivation and [H-3]thymidine incorporation Furthermore, profilin induced ra pid transient activation of protein kinase C, and staurosporine and H-7 red uced the profilin-induced activation of AP-1, suggesting protein kinase C-d ependent activation of AP-1. These findings indicate that profilin in the e xtracellular space can bind to cell surface receptors of MC and act as an i nducer of signal transduction. These results suggest that extracellular pro filin may be involved in the progression of glomerular diseases, by affecti ng cell growth.