αB-crystallin gene induction and phosphorylation by MKK6-activated p38 -: A potential role for αB-crystallin as a target of the p38 branch of the cardiac stress response

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, alpha B-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 alpha B-crystallin mRNA, protein, and transcription. MKK6(Glu) also induced p3-dependent activation of the downstream MAPK-activated protein kinase, MAP-KAP-K2, and the phosphorylation of alpha B-crystallin on serine-59, Initially, exposure of cells to the hyperosmotic stressor, sorbitol, stimulated MKK6, p38, and MAP-KAP-K2 and increased phosphorylation of alpha B-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 alpha B-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 alpha B-crystallin on serine 59 and to the induction of alpha B-crystallin gene expression,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, alpha B-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 alpha B-crystallin mRNA, protein, and transcription. MKK6(Glu) also induced p38-dependent activation of the downstream MAPK-activated protein kinase, MAP-KAP-K2, and the phosphorylation of alpha B-crystallin on serine-59, Initially, exposure of cells to the hyperosmotic stressor, sorbitol, stimulated MKK6, p38, and MAP-KAP-K2 and increased phosphorylation of alpha B-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 alpha B-crystallin induction and phosphorylation, Thus, under these conditions, the activation of MKK6, p38, and MAP-KAP-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 alpha B-crystallin on serine 59 and to the induction of alpha B-crystallin gene expression.