Differential regulation of HSP70 expression by the JNK kinases SEK1 and MKK7 in mouse embryonic stem cells treated with cadmium

JNK, a member of the mitogen-activated protein kinases (MAPKs), is activated by the MAPK kinases SEK1 and MKK7 in response to environmental stresses. In the present study, the effects Of CdCl2 treatment on MAPK phosphorylation and HSP70 expression were examined in mouse embryonic stem (ES) cells lacking the sek1 gene, the mkk7 gene, or both. Following CdCl2 exposure, the phosphorylation of JNK, p38, and ERK was suppressed in sek1(-/-) mkk7(-/-) cells. When sek1(-/-) or mkk7(-/-) cells were treated with CdCl2, JNK phosphorylation, but not the phosphorylation of either p38 or FRK, was markedly reduced, while a weak reduction in p38 phosphorylation was observed in sek1(-/-) cells. Thus, both SEK1 and MKK7 are required for JNK phosphorylation, whereas their role in p38 and ERK phosphorylation could overlap with that of another kinase. We also observed that CdCl2-induced HSP70 expression was abolished in sek1(-/-) mkk7(-/-) cells, was reduced in sek1(-/-) cells, and was enhanced in mkk7(-/-) cells. Similarly, the phosphorylation of heat shock factor 1 (HSF1) was decreased in sek1(-/-) mkk7(-/-) and sek1(-/-) cells, but was increased in mkk7(-/-) cells. Transfection with siRNA specific for JNK1, JNK2, p38 alpha, ERK1, or ERK2 suppressed CdCl2-induced HSP70 expression. In contrast, silencing of p38 beta or p38 gamma resulted in further accumulation of HSP70 protein. These results suggest that HSP70 expression is up-regulated by SEK1 and down-regulated by MKK7 through distinct MAPK isoforms in mouse ES cells treated with CdCl2. J. Cell. Biochem. 104: 1771 -1780, 2008. (C) 2008 Wiley-Liss, Inc.