Biology Department Seminar

Understanding the regulation of apoptotic signaling pathways in cancer cells should eventually lead to breakthroughs in treatment and diagnosis of human cancers. Our research is currently focused on elucidating the role and regulation of the p53 tumor suppressor and its siblings p63 and p73 in initiating cell death in human tumor cells.
The checkpoint kinases Chk1 and Chk2 have been shown to block cell cycle progression after DNA damage through their ability to phosphorylate and inactivate the Cdc25 phosphatases. Chk2 has been shown to regulate p53 and apoptosis in mouse models and so we tested whether Chk2 is activated to phosphorylate p53 after cells are subjected to DNA damage. Surprisingly, while Chk2 purified from cells treated with DNA damaging agents has dramatically increased ability to phosphorylate Cdc25C when compared to untreated cells, its ability to phosphorylate p53 is weak before treatment and there is no increase in its activity towards p53 after DNA damage. Furthermore, introduction of Chk2 siRNA into different human tumor cell lines leads to marked reduction of Chk2 protein but p53 is still stabilized and active after DNA damage. Results with Chk1 siRNA indicate as well that Chk1 does not play a role in human p53 stabilization after DNA damage. Thus, Chk1 and Chk2 are unlikely to be regulators of p53 in at least some human tumor cells. There is nevertheless compelling evidence that Chk2 function is critical for apoptosis in mice. We have therefore questioned how this kinase might function to regulate the induction of apoptosis in cells lacking p53. We discovered that in several human tumor cell lines Chk1 and Chk2 control the induction of the p53 related transcription factor p73 in response to DNA damage. Using different approaches we showed that interference with or augmentation of Chk1 or Chk2 signaling strongly impacts p73 accumulation. Furthermore, Chk1 and Chk2 control p73 mRNA accumulation after DNA damage. We found as well that E2F1 d