Hyperosmotic stress activates the insulin receptor in CHO cells

Stress factors, such as osmotic stress and genotoxic agents, activate stres s kinases, whereas growth factors preferentially stimulate the structurally homologous mitogen-activated protein kinases, ERK1/2. Hyperosmolarity also has insulin-mimicking action as reflected by ERK1/2 activation and by the stimulation of glucose uptake in adipocytes. We examined to what extent hyp erosmolarity activates components of the insulin receptor (IR) signalling p athway. CHO cells expressing the human IR were treated with 500 mM NaCl or 700 mM sorbitol and the activation of insulin signalling intermediates was studied. Hyperosmolarity induced tyrosine phosphorylation of the IR beta -s ubunit, and the adaptor proteins p52-Shc. p66-Shc, and IRS1. Furthermore, t he stress kinases JNK and p38 were activated. When CHO cells were transfect ed with a kinase-dead IR (K1030R) mutant, hyperosmolarity did not induce ty rosine phosphorylation of the IR, indicating that hyperosmolarity induced I R autophosphorylation directly, rather than inducing phosphorylation by an exogenous tyrosine kinase. A partially purified and detergent-solubilized I R was not phosphorylated in response to hyperosmolarity, suggesting that hy perosmolarity activates the receptor only when present in the plasma membra ne. In cells stably expressing the kinase-dead IR, IRS I and Shc Tyr phosph orylation was abrogated, indicating that the hyperosmolarity signalling was dependent on an active IR tyrosine kinase. In contrast, the stress kinases p38 and JNK were normally activated by hyperosmolarity in the IR-K1030R mu tant. We conclude that, at least in CHO cells, hyperosmolarity signals part ially through IR autophosphorylation and subsequent activation of the IR do wnstream targets. This may be responsible for some of the insulin-mimicking effects of hyperosmolarity. The activation of stress kinases by hyperosmol arity occurs independent of the IR. (C) 2001 Elsevier Science B.V. All righ ts reserved.