Requirement of heat shock protein 90 in mesangial cell mitogenesis

Background. Hyperplasia of mesangial cells (MCs) is a frequent finding in g lomerulonephritis. Heat shock protein 90 (HSP90) is a major cellular chaper one that assists protein folding under physiological and stress conditions. Methods. To identify genes that are potentially involved in the pathogenesi s of glomerulonephritis, we analyzed glomerular gene expression in mesangio proliferative rat anti-Thy1.1 nephritis by representational difference anal ysis (RDA). Expression of HSP90 beta in anti-Thy1.1 nephritis was studied b y Northern and Western blot analyses and immunohistochemistry. In cultured rat MCs, the requirement of HSP90 for mitogenic signaling steps and MC repl ication was studied by incubation with the specific HSP90 inhibitor geldana mycin. Results. By RDA, a cDNA fragment homologous to HSP90 beta was identified. G lomerular mRNA and protein expression of HSP90 beta was markedly and transi ently up-regulated during the course of anti-Thy1.1 nephritis, with a maxim um at day 6, coinciding with the peak of MC proliferation. By immunohistoch emistry, HSP90 beta expression in normal glomeruli was detected in podocyte s. However, in anti-Thy1.1 nephritis, glomerular HSP90 beta protein express ion was strongly and transiently increased in mesangial localization. In vi tro, mitogenic stimulation of rat MCs led to the induction of HSP90 beta mR NA and protein. Incubation of MCs with geldanamycin dose-dependently inhibi ted DNA synthesis and replication. Moreover, geldanamycin interfered with m itogen-induced phosphorylation of extracellular signal-regulated kinase and transcription of c-fos and Egr-1, but not with transactivation of STAT1 tr anscription factor. Cell cycle analysis of serum-stimulated MCs revealed th at geldanamycin inhibited kinase activity of cyclin D1/CDK4 complexes and b locked progression in the G(0)/G(1) phase and at the S/G(2) phase transitio n. Conclusions. The up-regulation of HSP90 beta in anti-Thy1.1 nephritis may r eflect its functional involvement in phenotypical alterations of MCs in mes angioproliferative glomerulonephritis. Our in vitro studies indicate that H SP90 governs the capacity of MCs to respond to proliferative stimuli by reg ulating critical mitogenic signaling steps necessary for G(1) entry and S-p hase progression.