Induction of p53-Dependent Apoptosis by Prostaglandin A2

Prostaglandin (PG) A₂, one of cyclopentenone PGs, is known to induce activation of Apoptosis in various Cancer cells. Although PGA₂ has been reported to cause activation of Apoptosis by altering the expression of apoptosis-related genes, the role of p53, one of the most critical pro-apoptotic genes, on PGA₂-induced Apoptosis has not been clarified yet. To address this issue, we compared the Apoptosis in HCT116 p53 null cells (HCT116 p53-/-) to that in HCT116 cells containing the wild type p53 gene. Cell death induced by PGA₂ was associated with phosphorylation of histone H2A variant H2AX (H2AX), activation of Caspase-3 and cleavage of poly(ADP-ribose) polymerase 1 in HCT116 cells. Induction of Apoptosis in PGA₂-treated cells was almost completely prevented by pretreatment with a pan-caspase inhibitor, z-VAD-Fmk, or an inhibitor of protein synthesis, cycloheximide. While PGA₂ induced Apoptosis in HCT116 cells, phosphorylation of p53 and transcriptional induction of p53-target genes such as p21^WAF1, PUMA, Bax, NOXA, and DR5 occurred. Besides, pretreatment of pifithrin-α (PFT-α), a chemical inhibitor of p53's transcriptional activity, interfered with the induction of Apoptosis in PGA₂-treated HCT116 cells. Pretreatment of NU7441, a small molecule inhibitor of DNA-activated protein kinase (DNA-PK) suppressed PGA₂-induced phosphorylation of p53 and Apoptosis as well. Moreover, among target genes of p53, knockdown of DR5 expression by RNA interference, suppressed PGA₂-induced Apoptosis. In the meanwhile, in HCT116 p53-/- cells, PGA₂ induced Apoptosis in delayed time points and with less potency. Delayed Apoptosis by PGA₂ in HCT116 p53-/- cells was also associated with phosphorylation of H2AX but was not inhibited by either PFT- or NU7441. Collectively, these results suggest the following. PGA₂ may induce p53-dependent Apoptosis in which DNA-PK activates p53, and DR5, a transcriptional target of p53, plays a pivotal role in HCT116 cells. In contrast to Apoptosis in HCT116 cells, PGA₂ may induce Apoptosis in a fashion of less potency, which is independent of p53 and DNA-PK in HCT116 p53-/- cells.

DNA-activated protein kinase; apoptosis; death receptor; prostaglandin A2; tumor suppressor Protein p53.