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Nb2-11 cells, a prolactin (PRL)-dependent T-lymphoma cell line, display an unusual response to heat stress characterized by the lack of expression of inducible hsp70 mRNA transcripts and a reduction in the levels of constitutively expressed heat shock protein (HSP) genes. This aberrant heat shock response appears to result from heat-induced proteolytic fragmentation of heat shock factor (HSF). In this report, we have investigated processes that promote HSF fragmentation and identified characteristics of a protease that may be responsible for this effect. Cycloheximide did not affect HSF fragmentation of heat-shocked Nb2-11 cells suggesting that proteases responsible for this proteolysis are constitutively expressed and become activated by the heat shock conditions. PRL protected Nb2-11 cells from heat-induced fragmentation whereas sodium butyrate (NaBT) rendered a fragmentation-resistant cell line (Nb2-SFJCD1 cells) sensitive to HSF proteolysis. Heat-induced HSF fragmentation in Nb2-11 cells was not affected by pretreating cultures with several serine protease inhibitors. However, a dose-dependent decrease in HSF fragmentation was achieved by pretreating cultures with iodoacetamide, a cysteine protease inhibitor that is active in apoptosis. Apparently, the heat shock response in Nb2 cells is attenuated by a mechanism that involves the premature deactivation of HSF by its selective proteolysis. Attenuation of this critical cellular stress response may be an important contributor to the progression of hormone-dependent tumors possibly by influencing apoptotic processes known to regulate the activity of these cells.

Heat-induced proteolysis of HSF causes premature deactivation of the heat shock response in Nb2 lymphoma cells