THERMAL SENSITIVITY IN NIH 3T3 FIBROBLASTS TRANSFORMED BY THE V-FOS ONCOGENE - CORRELATION WITH REDUCED ACCUMULATION OF 68-KDA AND 25-KDA STRESS PROTEINS AFTER HEAT-SHOCK

The effect of v-fos transformation on the cellular response to heat sh ock has been investigated. NIH 3T3 fibroblasts were transfected with t he FBR p75(gag-fos) gene fusion under the control of the long terminal repeat (LTR) promoter of Finkel-Biskin-Reilly (FBR) murine sarcoma vi rus and with the gene encoding hygromycin resistance. Several hygromyc in-resistant clone isolates, that expressed various levels of p75(gag- fos) oncoprotein, were analyzed as they displayed properties of transf ormed cells, such as altered morphology, shorter doubling time, serum- independent growth and foci formation in soft agar. The thermal respon se of these clones was compared to that of the control cells expressin g the hygromycin-resistance gene only. Here, we report that the v-fos- transformed clones displayed an enhanced thermosensitivity which resul ted in a reduced tolerance to thermal stress. Heat-treated v-fos-trans formed cells displayed a decreased expression and accumulation of the major stress proteins Hsp68 (68-kDa heat-shock protein) and Hsp25 whic h probably resulted of a reduced accumulation of the corresponding mRN As. This effect was particularly intense at the level of Hsp25. These alterations in cell survival and stress-protein expression appeared co rrelated to the level of p75(gag-fos). At least for Hsp68, the transcr iption of this gene was not found altered by v-fos expression suggesti ng that this oncogene increases the turn-over of Hsp68 mRNA. After the heat-shock treatment, v-fos transformation also reduced the time peri od during which the constitutively expressed stress protein Hsc70 redi stributes inside the nucleus. Since Hsp68 and Hsp25 are molecular chap erones that in vivo protect cells against the deleterious effects of h eat shock, it is conceivable that their reduced accumulation and alter ed cellular distribution following heat shock may contribute, at least in part, to the thermosensitivity of v-fos-transformed NIH 3T3 fibrob lasts.