THE INDUCTION OF PARTIAL RESISTANCE TO PHOTODYNAMIC THERAPY BY THE PROTOONCOGENE BCL-2

Photodynamic therapy (PDT) is an efficient inducer of apoptosis, an ac tive form of cell death that can be inhibited by the BCL-2 oncoprotein . The ability of BCL-2 to modulate PDT-induced apoptosis and overall c ell killing has been studied in a pair of Chinese hamster ovary cell l ines that differ from one another by a transfected human BCL-2 gene in one of them (Bissonnette et al., Nature 359, 552-554, 1992). Cells we re exposed to the phthalocyanine photosensitizer Pc 4 and various flue nces of red light. Pc 4 uptake was identical in the two cell lines. Th e parental cells displayed a high incidence of apoptosis after PDT, wh ereas at each fluence there was a much lower incidence of apoptosis in the BCL-2-expressing cells. Apoptosis was monitored by (a) observatio n of 50 kbp and oligonucleosome-size DNA fragments by gel electrophore sis, (b) how cytometry of cells labeled with fluorescently tagged dUTP by terminal deoxynucleotidyl transferase and (c) fluorescence microsc opy of acridine orange-stained cells. The time course of apoptosis var ied with the PDT dose, suggesting that only after moderately high dose s (>99% loss of clonogenicity) was there a relatively synchronous and rapid entry of many cells into apoptosis. At PDT doses reducing cell s urvival by 90 or 99%, significant increases in apoptotic cells were fo und in the population after 6-12 h. Clonogenic assays showed that BCL- 2 protein inhibited not only apoptosis but overall cell killing as wel l, effecting a two-fold resistance at the 10% survival level. Thus, BC L-2-expressing cells may be relatively resistant to PDT.