MODULATION OF THERMAL INDUCTION OF HSP70 EXPRESSION BY KU AUTOANTIGENOR ITS INDIVIDUAL SUBUNITS

Previously, we proposed a dual control mechanism for the regulation of the heat shock response in mammalian cells: a positive control mediat ed by the heat shock transcription factor HSF1 and a negative control mediated by the constitutive heat shock element-binding factor (CHEF). To study the physiological role of CHEF in the regulation of heat sho ck response, we purified CHEF to apparent homogeneity and showed it to be identical to the Ku autoantigen, a heterodimer consisting of 70-kD a (Ku-70) and 86-kDa (Ku-80) polypeptides. To study further the functi onal significance of Ku/CHBF in the cellular response to heat shock, w e established rodent cell lines that stably and constitutively overexp ressed one or both subunits of the human Ku protein, and examined the thermal induction of hsp70 and other heat shock proteins in these Ku-o verexpressing cells. We show that expression of the human Ku-70 and Ku -80 subunits jointly or of the Ku-70 subunit alone specifically inhibi ts heat-induced hsp70 expression. Conversely, expression of human Ku-8 0 alone does not have this effect. Thermal induction of other heat sho ck proteins in all of the Ku-overexpressing cell lines appears not to be significantly affected, nor is the state of phosphorylation or the DNA-binding ability of HSF1 affected. These findings support a model i n which hsp70 expression is controlled by a second regulatory factor i n addition to the positive activation of HSF1. The Ku protein, specifi cally the Ku-70 subunit, is involved in the regulation of hsp70 gene e xpression.