αB-crystallin Gene Induction and Phosphorylation by MKK6-activated p38

The MAPK kinase MKK6 selectively stimulates p38 MAPK and confers protection against stress-induced apoptosis in cardiac myocytes. However, the events lying downstream of p38 that mediate this protection are unknown. The small heat shock protein, alphaB-crystallin, which is expressed in only a few cell types, including cardiac myocytes, may participate in MKK6-mediated cytoprotection. In the present study, we showed that, in cultured cardiac myocytes, expression of MKK6(Glu), an active form of MKK6, led to p38-dependent increases in alphaB-crystallin mRNA, protein, and transcription. MKK6(Glu) also induced p38-dependent activation of the downstream MAPK-activated protein kinase, MAPKAP-K2, and the phosphorylation of alphaB-crystallin on serine-59. Initially, exposure of cells to the hyperosmotic stressor, sorbitol, stimulated MKK6, p38, and MAPKAP-K2 and increased phosphorylation of alphaB-crystallin on serine 59. However, after longer times of exposure to sorbitol, the cells began to undergo apoptosis. This sorbitol-induced apoptosis was increased when p38 was inhibited in a manner that would block alphaB-crystallin induction and phosphorylation. Thus, under these conditions, the activation of MKK6, p38, and MAPKAP-K2 by sorbitol can provide a degree of protection against stress-induced apoptosis. Supporting this view was the finding that sorbitol-induced apoptosis was nearly completely blocked in cells expressing MKK6(Glu). Therefore, the cytoprotective effects of MKK6 in cardiac myocytes are due, in part, to phosphorylation of alphaB-crystallin on serine 59 and to the induction of alphaB-crystallin gene expression.