Protective Role of p21^Waf1/Cip1 against Prostaglandin A₂-Mediated Apoptosis of Human Colorectal Carcinoma Cells

Abstract

Prostaglandin A₂ (PGA₂) suppresses tumor growth in vivo, is potently antiproliferative in vitro, and is a model drug for the study of the mammalian stress response. Our previous studies using breast carcinoma MCF-7 cells suggested that p21^Waf1/Cip1 induction enabled cells to survive PGA₂ exposure. Indeed, the marked sensitivity of human colorectal carcinoma RKO cells to the cytotoxicity of PGA₂ is known to be associated with a lack of a PGA₂-mediated increase in p21^Waf1/Cip1 expression, inhibition of cyclin-dependent kinase activity, and growth arrest. To determine if cell death following exposure to PGA₂ could be prevented by forcing the expression of p21^Waf1/Cip1 in RKO cells, we utilized an adenoviral vector-based expression system. We demonstrate that ectopic expression of p21^Waf1/Cip1 largely rescued RKO cells from PGA₂-induced apoptotic cell death, directly implicating p21^Waf1/Cip1 as a determinant of the cellular outcome (survival versus death) following exposure to PGA₂. To discern whether p21^Waf1/Cip1-mediated protection operates through the implementation of cellular growth arrest, other growth-inhibitory treatments were studied for the ability to attenuate PGA₂-induced cell death. Neither serum depletion nor suramin (a growth factor receptor antagonist) protected RKO cells against PGA₂ cytotoxicity, and neither induced p21^Waf1/Cip1 expression. Mimosine, however, enhanced p21^Waf1/Cip1 expression, completely inhibited RKO cell proliferation, and exerted marked protection against a subsequent PGA₂ challenge. Taken together, our results directly demonstrate a protective role for p21^Waf1/Cip1 during PGA₂ cellular stress and provide strong evidence that the implementation of cellular growth arrest contributes to this protective influence.