p38-dependent enhancement of cytokine-induced nitric-oxide synthase gene expression by heat shock protein 70

Heat shock protein (hsp) 70 protects cells against stress by means of its a bility to chaperone denatured proteins and to modulate stress-activated sig naling pathways. Because inflammatory processes are often accompanied by hs p expression and because stress and cytokines share several signaling pathw ays, we investigated the possibility that hsp70 might modulate the cellular response to cytokines. We found that stable cell clones overexpressing hsp 70, or cells shortly after transfection with hsp70, produced 2 times more n itric oxide and inducible nitric-oxide synthase (iNOS) protein and mRNA in response to cytokines than control cells expressing undetectable amounts of hsp70. Since mitogen-activated protein kinases participate in the activati on of iNOS by cytokines, we investigated whether hsp70 affected the activat ion of these signaling pathways. hsp70 overexpression led to a specific enh ancement of the activation of the p38 pathway by cytokines, producing littl e or no effect on the activation of extracellular signal-regulated kinase o r Jun N-terminal kinase. Blocking p38 activity with SB203580 totally abolis hed the enhancing effect of hsp70 on cytokine-induced endogenous iNOS mRNA accumulation or transcription of an iNOS promoter-driven luciferase gene, w hile having little effect on the cytokine response observed in control cell s. We conclude that the p38 pathway acts as an enhancing factor in the acti vation of iNOS by cytokines and that, hsp70 can modulate the cellular respo nse to cytokines by acting on signaling elements upstream of p38.