Oxidative stress and calpain inhibition induce alpha B-crystallin phosphorylation via p38-MAPK and calcium signalling pathways in H9c2 cells

We investigated the response of αB-crystallin to oxidative stress and calpain inhibition in an attempt to elucidate the signalling pathways mediating its phosphorylation. Given the high expression levels of αB-crystallin in cardiac muscle one can evaluate the significance of its participation in preservation of homeostasis under adverse conditions. H9c2 cardiac myoblasts were used as our experimental model since their response reflects the signal transduction pathways activated by stress conditions in the myocardium. Thus, in H9c2 cells treated with H2O2 the mechanism regulating αB-crystallin phosphorylation was found to involve p38-MAPK/MSK1 as well as intracellular free calcium levels. Our immunocytochemical experiments demonstrated phosphorylated αB-crystallin to be co-localized with tubulin, potentially preserving cytoskeletal architecture under these interventions. In H9c2 cells treated with calpain inhibitors (ALLN, ALLM) αB-crystallin exhibited a p38-MAPK- and [Ca 2+]i-dependent phosphorylation pattern since the latter was ablated in the presence of the selective p38-MAPK inhibitor SB203580 and calcium chelator BAPTA-AM. Calpain activity repression ultimately led to apoptosis confirmed by PARP fragmentation and chromatin condensation. However, the apoptotic pathway activated by ALLM and ALLN differed, underlying the diverse transduction mechanisms stimulated. In addition to this, an anti-apoptotic role for phospho-αB-crystallin was verified by confirmation of its interaction with pro-caspase 3, hindering its cleavage and subsequent activation. Collectively, our findings underline αB-crystallin crucial role as a participant of cardiac cells early response to stressful stimuli compromising their survival.