PHOTODYNAMIC THERAPY RESULTS IN INDUCTION OF WAF1 CIP1/P21 LEADING TOCELL-CYCLE ARREST AND APOPTOSIS/

Photodynamic therapy (PDT) is a promising new modality that utilizes a combination of a photosensitizing chemical and visible light for the management of a variety of solid malignancies. The mechanism of PDT-me diated cell killing is not well defined. We investigated the involveme nt of cell cycle regulatory events during silicon phthalocyanine (Pc4) -PDT-mediated apoptosis in human epidermoid carcinoma cells A431, PDT resulted in apoptosis, inhibition of cell growth, and G(0)-G(1) phase arrest of the cell cycle, in a time-dependent fashion. Western blot an alysis revealed that PDT results in an induction of the cyclin kinase inhibitor WAF1/CIP1/p21, and a down-regulation of cyclin D1 and cyclin E, and their catalytic subunits cyclin-dependent kinase (cdk) 2 and c dk6, The treatment also resulted in a decrease in kinase activities as sociated with all the cdks and cyclins examined. PDT also resulted in (i) an increase in the binding of cyclin D1 and cdk6 toward WAF1/CIP1/ p21, and (ii) a decrease in the binding of cyclin D1 toward cdk2 and c dk6, The binding of cyclin E and cdk2 toward WAF1/CIP1/p21, and of cyc lin E toward cdk2 did not change by the treatment. These data suggest that PDT-mediated induction of WAF1/CIP1/p21 results in an imposition of artificial checkpoint at G(1) --> S transition thereby resulting in an arrest of cells in Go-GI phase of the cell cycle through inhibitio n in the cdk2, cdk6, cyclin D1, and cyclin E, We suggest that this arr est is an irreversible process and the cells, unable to repair the dam ages, ultimately undergo apoptosis.