Extracellular signals may be transmitted to nuclear or cytoplasmic effectors via the mitogen- activated protein kinases. In the passive Heymann nephritis (PHN) model of membranous nephropathy, complement C5b-9 induces glomerular epithelial cell (GEC) injury, proteinuria, and activation of phospholipases and protein kinases. This study addresses the complement-mediated activation of the extracellular signal-regulated kinase (ERK). C5b-9 induced ERK threonine²⁰²/tyrosine²⁰⁴ phosphorylation (which correlates with activation) in GEC in culture and PHN in vivo. Expression of a dominant-inhibitory mutant of Ras reduced complement-mediated activation of ERK, but activation was not affected significantly by downregulation of protein kinase C. Complement-induced ERK activation resulted in phosphorylation of cytosolic phospholipase A₂, and was, in part, responsible for phosphorylation of mitogen-activated protein kinase-associated protein kinase-2, but did not induce phosphorylation of the transcription factor, Elk-1. Activation of ERK was attenuated by drugs that disassemble the actin cytoskeleton (cytochalasin D, latrunculin B), and these compounds interfered with the activation of ERK by mitogen-activated protein kinase kinase (MEK). Overexpression of a constitutively active RhoA, as well as inhibition of Rho- associated kinase blocked complement-mediated ERK activation. Complement cytotoxicity was enhanced after disassembly of the actin cytoskeleton, but was unaffected after inhibition of complement-induced ERK activation. However, complement cytotoxicity was enhanced in GEC that stably express constitutively active MEK. Thus, complement-induced ERK activation depends on cytoskeletal remodelling, and affects the regulation of distinct downstream substrates, while chronic, constitutive ERK activation exacerbates complement-mediated GEC injury.