Differential activation of p38 mitogen-activated protein kinase and extracellular signal-regulated protein kinases confers cadmium-induced HSP70 expression in 9L rat brain tumor cells

We have reported that treatment with CdCl2 at 40-100 microM induces the heat shock proteins (HSPs) in 9L rat brain tumor cells, during which the activation of heat shock factor (HSF) is essentially involved. By exploiting protein kinase inhibitors, we further analyzed the possible participation of specific protein kinases in the above processes. It was found that induction of HSP70 in cells treated with a high concentration of cadmium (i.e. 100 microM) is preceded by the phosphorylation and activation of p38 mitogen-activated protein kinase (p38(MAPK)), while that in cells treated with a low concentration (60 microM) is accompanied by the phosphorylation and activation of extracellular-regulated protein kinases 1 and 2 (ERK1/2). In 100 microM cadmium-treated cells, both HSP70 induction and HSF1 activation are eliminated in the presence of SB203580, a specific inhibitor of p38(MAPK). By contrast, in 60 microM cadmium-treated cells, the processes are not affected by SB203580 but are significantly suppressed by PD98059, which indirectly inhibits ERK1/2 by acting on MAPK-ERK kinase. Taken together, we demonstrate that p38(MAPK) and ERK1/2 can be simultaneously or independently activated under different concentrations of cadmium and that the signaling pathways participate in the induction of HSP70 by acting on the inducible phosphorylation of HSF1. We thus provide the first evidence that both p38(MAPK) and ERK signaling pathways can differentially participate in the activation of HSF1, which leads to the induction of HSP70 by cadmium.     

Reduced induction of HSP70 in PC12 cells during neuronal differentiation

Rat pheochromocytoma PC12 cells differentiate into nonreplicating neuronal cells with neurite extensions in response to nerve growth factor (NGF). To gain better understanding of the regulation of stress responses in neuronal cells, we examined the induction of HSP70, HSP70 mRNA, and heat shock factor 1 (HSF1) DNA-binding activity following treatment by heat shock or with sodium arsenite or amino acid analogue in PC12 cells treated with or without NGF. The induction of HSP70 and HSP70 mRNA following these stresses was diminished in the differentiated PC12 cells compared to the undifferentiated cells, whereas the HSF1 DNA-binding activity was enhanced in the differentiated PC12 cells. This phenomenon was characteristic of the differentiated neuronal cells rather than growth-arrested cells. Thus, neuronal cells appear to show an altered stress response depending on their differentiation state, and the diminished HSP70 expression in the differentiated neuronal cells may explain the sensitivity of neuronal cells to pathophysiological stressors.