Ezrin turnover and cell shape changes catalyzed by proteasome in oxidatively stressed cells

We find that ezrin, a cytoskeletal protein involved in anchoring actin to the cell membrane, is preferentially degraded and resynthesized after oxidative stress. Ezrin was identified using 2-dimensional gels and amino-terminal microsequencing as one of a select few [35S]methionine prelabeled proteins degraded in clone 9 rat liver cells exposed to hydrogen peroxide (H2O2). Metabolic labeling of cellular proteins with [35S]methionine after oxidative stress showed that resynthesis of ezrin rose dramatically but carboxyl terminus anti-ezrin monoclonal antibodies revealed constant intracellular ezrin levels; in other words, degradation and resynthesis were exactly matched. Ezrin degradation was blocked by selective inhibitors of the proteasome (lactacystin, NLVS, and epoxomycin) and by an antisense oligonucleotide directed against the proteasome C2 subunit. H2O2 also caused major changes in cell shape, including significant increases in cell diameter, which must require substantial cytoskeletal rearrangement. Peroxide-induced increases in cell diameter were, however, blocked by the selective proteasome inhibitor lactacystin. The degradation and resynthesis of ezrin may therefore be an underlying mechanism for overall cell shape changes observed during oxidative stress. Oxidative stress induces extensive protein oxidation and degradation and significant increases in cell blebbing, rounding-up, and overall size. Our results indicate that all these oxidant-induced changes may actually be catalyzed by the proteasome.