Maintenance of glucocorticoid receptor function following severe heat-shock of heat-conditioned cells.

The competence of the glucocorticoid receptor to regulate gene expression is thought to depend on Hsp70-driven continuous reactivation following spontaneous inactivation of its hormone-binding state. We show here that the glucocorticoid-binding capacity of HeLa cells fell with increasing temperature in the range 43-45 degrees C in a manner that closely paralleled the loss of soluble receptor protein. Receptor activity was maintained during moderate (43 degrees C) but not severe (45 degrees C) heat shock. Hsp70 was rapidly rendered insoluble and was replenished by soluble chaperone at 43 but not 45 degrees C. In heat-conditioned cells expressing different levels of Hsp70, we observed a positive correlation between the concentration of active receptor and the amount of Hsp70 rendered insoluble by heat shock. Much higher amounts of Hsp70 were rendered insoluble and receptor competence to regulate gene expression was preserved after severe heat shock of appropriately heat-conditioned cells. An excess of Hsp90 was found associated with resolubilized heat-inactivated receptor from severely heat-shocked cells. The data indicate that GR activity is maintained, provided that denaturation and/or aggregation of the receptor is prevented by Hsp70; and that the concentration of the chaperone is the limiting determinant of receptor activity in heat-shocked HeLa cells.