HUMAN BCL-2 EXPRESSION DELAYS ULTRAVIOLET-INDUCED APOPTOSIS IN MARSUPIAL CELLS

We have introduced the human bcl-2 gene under the control of the human metallothionein MTIIA promoter into the rat kangaroo PtK2 cell line. Two independent clones were obtained in which the levels of Bcl-2 prot ein expression can be controlled by the addition of metals in the cult ure medium. These cell lines were employed to investigate the effects of this protein in UV-induced apoptosis. Overexpression of Bcl-2 in Pt K2 cells resulted in a delay in the appearance of apoptosis markers, s uch as chromatin condensation and internucleosomal DNA fragmentation. However, colony survival after UV was not affected, suggesting that Bc l-2 did not impose a definitive block for cell death. The elimination of cyclobutane pyrimidine dimers through photoreactivation 24 h after irradiation in cells overexpressing Bcl-2 did not affect apoptosis. Th is indicates that irreversible events in the signaling pathway of apop tosis occur in the period between irradiation and photoreactivation ev en in the presence of high levels of Bcl-2 in the cell. Therefore, alt hough the human Bcl-2 protein can delay the onset of UV-induced apopto sis in these marsupial cells, early events triggered by the pyrimidine dimers, upstream from the Bcl-2 action, lead the cell to a state comm itted to die.