BAX IS AN IMPORTANT DETERMINANT OF CHEMOSENSITIVITY IN PEDIATRIC TUMOR-CELL LINES INDEPENDENT OF BCL-2 EXPRESSION AND P53 STATUS

Susceptibility of a tumor cell to undergo chemotherapy-induced apoptos is appears to be dependent upon the balance of proapoptotic and surviv al factors that are expressed within any given cell. We have chosen to evaluate how expression of several of these proteins influences chemo sensitivity of a panel of 10 pediatric tumor cell lines chosen from th ree tumor histiotypes: neuroblastoma, rhabdomyosarcoma, and pediatric glial tumors. The proteins evaluated were p53 and six members of the B ax/Bcl-2 family: three proapoptotic proteins (Bax, Bak, and Bcl-xS) an d three survival factors (Bcl-2, Bcl-xL, and Mcl-1). We investigated w hether there was any relationship between endogenous expression of the se proteins and chemosensitivity (or resistance) to three chemotherape utic agents that directly damage DNA (doxorubicin, actinomycin D, and topotecan) and a mitotic spindle poison (vincristine). Even though exo genous overexpression of wild-type p53 has been associated with a chem osensitive phenotype in several model systems we demonstrated no such relationship in these studies. In addition, expression levels of Bcl-2 , Bcl-xL, Bcl-xS, Bak, or Mcl-1 did not correlate with sensitivity or resistance to the four drugs. However, there was a statistically signi ficant correlation between endogenous levels of Bar protein and sensit ivity to both doxorubicin and actinomycin D. We conclude that even tho ugh many proteins such as p53 and Bcl-2 have been shown to influence d rug response when exogenously overexpressed in model systems, in unmod ified cell lines endogenous protein levels may not generate the same r esults. We have demonstrated that endogenous Bar expression was the on ly protein found to be associated with chemosensitivity across the thr ee different tumor histiotypes and propose that analysis of Bar may be a more useful prognostic indicator for tumor response to therapy than either p53 or Bcl-2.