Constitutive and induced synthesis of heat shock proteins in transplantable hepatomas

The synthesis of heat shock proteins (HSP) was studied in rat liver and in a series of transplantable Morris hepatomas with different growth rates, subjected to heat shock in vivo and in vitro. Different from the liver, hepatomas synthesized HSP constitutively, i.e., also before exposure to heat. This constitutive synthesis was low and limited to one HSP in the slowest-growing tumor, more marked and involving other HSP in the intermediate- and fast-growing hepatomas. In tumor that synthesized HSP constitutively, the induction of HSP in response to heat was proportionately reduced. These patterns of reaction were essentially similar in vivo ad in vitro. The amount of HSP 68 was well correlated to the levels of its mRNA in liver and in all hepatomas, whereas the increase in HSP 89 was accompanied by a corresponding increase in the related mRNA in liver and in slow-growing hepatoma, not in the other tumors, thus suggesting a different mechanism of control of HSP 89 synthesis in the more malignant hepatomas.