IN-VIVO GROWTH OF A MURINE LYMPHOMA CELL-LINE ALTERS REGULATION OF EXPRESSION OF HSP72

We have identified a murine B-cell lymphoma cell line, CH1, that has a much-diminished capacity to express increased levels of heat shock pr oteins in response to heat stress in vitro. In particular, these cells cannot synthesize the inducible 72-kDa heat shock protein (HSP72) whi ch if normally expressed at high levels in stressed cells. We show her e that CH1 fails to transcribe HSP72 mRNA after heat shock, even thoug h the heat shock transcription fatter, HSF, is activated correctly. Af ter heat shock, HSF from CH1 is found in the nucleus and is phosphoryl ated, trimerized, and capable of binding the heat shock element. We pr opose that additional signals which CH1 cells are unable to transduce are normally required to activate hsp72 transcription in vitro. Surpri singly, we have found that when the CH1 cells are heated in situ in a mouse, they show normal expression of HSP72 mRNA and protein. Therefor e, CH1 cells have a functional hsp72 gene which can be transcribed and translated when the cells are in an appropriate environment. A diffus ible factor present in ascites fluid is capable of restoring normal HS P72 induction in CH1 cells. We conclude that as-yet-undefined factors are required for regulation of the hsp72 gene or, alternatively, that heat shock in vivo causes activation of hsp70 through a novel pathway which the defect in CH1 has exposed and which is distinct from that op erating in vitro. This unique system offers an opportunity to study a physiologically relevant pathway of heat shock induction and to bioche mically define effectors involved in the mammalian stress response.