Glucocorticoid-mediated attenuation of the hsp70 response in trout hepatocytes involves the proteasome

The physiological implication of elevated cortisol levels on cellular heat-shock protein 70 (hsp70) response was examined using primary cultures of rainbow trout (Oncorhynchus mykiss) hepatocytes. Trout hepatocytes treated with cortisol, the predominant glucocorticoid in teleosts, responded to the heat shock (+15degreesC for 1 h) with a significant drop in hsp70 accumulation over a 24-h recovery period. [S-35] methionine incorporation and pulse-chase studies confirmed that this cortisol impact was due to decreased hsp70 synthesis and not enhanced protein breakdown. Cortisol also significantly decreased glucocorticoid receptor (GR) expression in trout hepatocytes. This receptor downregulation was inhibited by the proteasomal inhibitors, lactacystin and MG-132, implying a role for the proteasome in GR downregulation by cortisol. Inhibiting the proteasome did not significantly modify heat-induced hsp70 accumulation in the absence of cortisol but significantly elevated hsp70 expression in the presence of cortisol in heat-shocked trout hepatocytes. Taken together, our results suggest proteasome-mediated GR degradation as a mechanism for the attenuation of hsp70 response by cortisol in heat-shocked hepatocytes.