HEAT-INDUCED PROTEOLYSIS OF HSF CAUSES PREMATURE DEACTIVATION OF THE HEAT-SHOCK RESPONSE IN NB2 LYMPHOMA-CELLS

Nb2-11 cells, a prolactin (PRL)-dependent T-lymphoma cell line, displa y an unusual response to heat stress characterized by the lack of expr ession of inducible hsp70 mRNA transcripts and a reduction in the leve ls of constitutively expressed heat shock protein (HSP) genes. This ab errant heat shock response appears to result from heat-induced proteol ytic 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 condit ions. PRL protected Nb2-11 cells from heat-induced fragmentation where as sodium butyrate (NaBT) rendered a fragmentation-resistant cell line (Nb2-SFJCD1 cells) sensitive to HSF proteolysis. Heat-induced HSF fra gmentation in Nb2-11 cells was not affected by pretreating cultures wi th several serine protease inhibitors. However, a dose-dependent decre ase in HSF fragmentation was achieved by pretreating cultures with iod oacetamide, a cysteine protease inhibitor that is active in apoptosis. Apparently, the heat shock response in Nb2 cells is attenuated by a m echanism that involves the premature deactivation of HSF by its select ive proteolysis. Attenuation of this critical cellular stress response may be an important contributor to the progression of hormone-depende nt tumors possibly by influencing apoptotic processes known to regulat e the activity of these cells.