RESISTANCE TO ETOPOSIDE-INDUCED APOPTOSIS IN A BURKITTS-LYMPHOMA CELL-LINE

Burkitt's lymphoma cells that vary in their phenotypic characteristics show significantly different degrees of susceptibility to radiation-i nduced apoptosis. Propensity to undergo apoptosis is reflected in the degradation of substrates such as DNA-dependent protein kinase but the status of bcl-2, c-myc and p53 has been uninformative. In this study, we have focused on 2 Epstein-Barr virus (EBV)-associated Burkitt's ce ll lines, one (WW2) susceptible and the other (BL29) resistant to etop oside-induced apoptosis. Differences in expression of BHRF1, an EBV ge ne that is homologous to the Bcl-2 proto-oncogene and known to inhibit apoptosis, or changes in apoptosis inhibitory proteins (IAPs), did no t appear to account for the difference in susceptibility in the 2 cell lines. Cytoplasmic extracts from etoposide-treated WW2 cells caused a poptotic changes in nuclei isolated from either BL29 or WW2 cells, whe reas extracts from BL29 cells failed to do so. In addition, extracts f rom etoposide-treated WW2 cells degraded the catalytic subunit of DNA- dependent protein kinase (DNA-PKcs), an important indicator of apoptos is, but this protein was resistant to degradation by BL29 extracts. It appears likely that caspase 3 (CPP32) is involved in this degradation since it was activated only in the apoptosis susceptible cells and th e pattern of cleavage of DNA-PKcs was similar to that reported previou sly with recombinant caspase 3. As observed previously, addition of ca spase 3 to nuclei failed to induce morphological changes indicative of apoptosis, but addition of caspase 3 to nuclei in the presence of ext ract from the resistant cells led to apoptotic changes, We conclude th at resistance to apoptosis in BL29 cells is due to a failure of etopos ide to activate upstream effecters of caspase activity, (C) 1998 Wiley -Liss, Inc.