DNA-dependent protein kinase protects against heat-induced apoptosis

Purified heat shock transcription factor 1 (HSF1) binds to both the regulat ory and catalytic components of the DNA-dependent protein kinase (DNA-PK). This observation suggests that DNA-PK may have a physiological role in the heat shock response. To investigate this possibility, we performed a compar ison of cell lines that were deficient in either the Ku protein or the DNA- PK catalytic subunit versus the same cell lines that had been rescued by th e introduction of a functional gene. DNA-PK-negative cell lines were up to 10-fold more sensitive to heat-induced apoptosis than matched DNA-PK-positi ve cell lines. There may be a regulatory interaction between DNA-PK and HSF 1 in vivo, because constitutive overexpression of HSF1 sensitized the DNA-P K-positive cells to heat but had no effect in DNA-PK-negative cells. The in itial burst of hsp70 mRNA expression was similar in DNA-PK-negative and -po sitive cell lines, but the DNA-PK-negative cells showed an attenuated rate of mRNA synthesis at later times and, in some cases, lower heat shock prote in expression. These findings provide evidence for an antiapoptotic functio n of DNA-PK that is experimentally separable from its mechanical role in DN A double strand break repair.