PREVENTION OF MYELOMA CELL APOPTOSIS BY ECTOPIC BCL-2 EXPRESSION OR INTERLEUKIN 6-MEDIATED UP-REGULATION OF BCL-X(L)

Upon cytokine withdrawal, interleukin (IL) 6-dependent murine plasmacy toma/hybridoma (myeloma) cells die in a way characteristic of apoptosi s, Although gene transfer-mediated elevation in Bcl-2 protein levels h as been demonstrated to repress a number of apoptotic death programs, it has been reported that ectopic bcl-2 expression is unable to prolon g the survival of IL-6-deprived myeloma cells, In view of the recent i dentification of Bar as a protein that antagonizes the anti-apoptotic function of Bcl-2, we sought to determine whether the inability of tra nsfected bcl-2 to protect against myeloma cell apoptosis might simply be due to insufficient levels of Bcl-2 protein produced to counteract this inhibitor, We show here that high-level expression of an exogenou s bcl-2 gene, introduced into IL-6-dependent B9 myeloma cells via retr oviral or bovine papilloma virus-based vectors, is indeed able to supp ress apoptotic death following cytokine deprivation, with the extent o f protection provided correlating with the amount of Bcl-2 protein syn thesized in relation to the amount of endogenous Bar protein present i n the cells, Of note, however, we found that IL-6-mediated suppression of B9 apoptosis does not involve induction of endogenous bcl-2 expres sion but is associated instead with the upregulation of cellular bcl-x mRNA and Bcl-x(L) protein. These results thus extend the apoptotic de ath mechanisms that are inhibitable by both bcl-2 and bcl-x(L) to incl ude that operative in IL-6-dependent cells and suggest that apoptosis in other cell types using the gp130 subunit of the IL-6 receptor might also be bcl-2 regulable or bcl-x(L) dependent.