ROLE OF BCL-X(L) IN THE CONTROL OF APOPTOSIS IN MURINE MYELOMA CELLS

The development of an increasing number of tumors has been shown to in volve the deregulation of not only cell proliferation but also normal cell death by apoptosis. Expression of the bcl-2 proto-oncogene has be en shown to inhibit the apoptotic cell death of many types of cells. R ecent work also has revealed the existence of several bcl-2-related ge nes that also can inhibit (e.g., bcl-X(L) and Mcl-1) or activate (e.g. , bax, bcl-X(S), bag, and bad) apoptosis in several systems. Myelomas are antibody-secreting tumor cells derived from terminally differentia ted B lymphocytes, and previous work from our laboratory showed that m urine SP2/0 myeloma cells and derived B-cell hybridomas were highly se nsitive to apoptosis induction by a block of gene expression (cyclohex imide). Additional work revealed that a related murine myeloma cell li ne, P3X63Ag8.653, was resistant to apoptosis induction in similar cond itions. To understand the genetic basis of this differential susceptib ility, we examined the expression of apoptosis-related genes in these cell lines. Northern blot experiments showed no significant difference in the expression of myc and bar apoptosis-promoting genes in suscept ible (SP2/0 and D5) and resistant (P3X63) cell lines. Also, no signifi cant expression of the bcl-2 gene could be detected in these cell line s. However, a much higher expression level of bcl-X(L) mRNA was observ ed in apoptosis-resistant P3X63Ag8.653 cells. The role of bcl-X(L), wa s supported by the finding that expression of bcl-X(L) cDNA in transfe cted, apoptosis-sensitive D5 cells increased the viability of these ce lls greatly and reduced DNA fragmentation following apoptosis inductio n. Significant bcl-X(L) but not bcl-2 expression was also detected in three other murine myeloma cell lines (MOPC 315, RPC 5.4, and J558) de rived from different plasmacytoma tumors. These results indicate a pre dominant role of bcl-X(L) in preventing apoptosis in myeloma cells and suggest that the expression of bcl-2 or bcl-X(L) genes in B-cell tumo rs depends on the differentiation stage of the precursor normal cell.