Induction of Hsp72 and transient nuclear localization of Hsp73 and Hsp72 correlate with the acquisition and loss of thermotolerance in postimplantation rat embryos.

A number of cell culture studies have indicated that there is a positive correlation between the induction and decay of thermotolerance and the kinetics of Hsp72 expression. In this study, we have demonstrated that, in gestational day 10 embryos, induction and decay of thermotolerance occur over an 8 hr period. To test the hypothesis that there is a correlation between loss of thermotolerance and the decline of Hsp72 or Hsp73 gene products over time, expression levels of both Hsp72 and constitutively expressed Hsp73 mRNAs and proteins were examined at several time points following exposure to a thermotolerance-inducing exposure of 42 degrees C. Our results indicated that Hsp72 mRNA was strongly induced 1 hr after exposure but no longer detectable by 8 hr. Although our Western blot results indicated that Hsp72 protein was present beyond 8 hr after exposure, Northern blot analysis showed that Hsp72 mRNA was no longer present 5 hr after exposure to 42 degrees C. The latter finding indicates that no new Hsp72 can be synthesized at this time point and beyond. Although there was very little or no induction of Hsp73, immunohistochemical analysis revealed a dramatic, transient shift in intracellular localization of Hsp73 protein, as well as Hsp72. Under non-stress conditions, Hsp73 was cytoplasmically localized but localization was largely nuclear 1 hr after exposure, when thermotolerance was demonstrable. Hsp73 and Hsp72 proteins were no longer localized in the nucleus by 8 hr, when thermotolerance was no longer detectable. Thus, the induction of Hsp72 and the transient nuclear localization of both Hsp72 and Hsp73 correlate with the kinetics of thermotolerance in the postimplantation rat embryo.