BCL-2 OVEREXPRESSION IS ASSOCIATED WITH RESISTANCE TO DEXAMETHASONE, BUT NOT MELPHALAN, IN MULTIPLE-MYELOMA CELLS

Multiple myeloma (MM) is an incurable disease despite an initial respo nse-rate of >60% with conventional or high-dose chemotherapy. Glucocor ticosteroids such as dexamethasone (DEX) and alkylating agents such as melphalan (MEL) are capable of inducing complete responses (CR) in >5 0% of MM patients, however, resistance to these drugs develop rapidly, in >90% of patients, within 2 years of treatment. The exact mechanism of resistance to these drugs is not known. We investigated the mechan ism of resistance to DEX and MEL. In particular, we investigated the r ole of bcl-2 in development of resistance to these two drugs. We teste d the role of bcl-2 by transfecting 2 low bcl-2-expressing myeloma cel l lines, ARP-1 and 8226, with a bcl-2 expression vector and compared t he effects of DEX and MEL on apoptosis, cell cycle distribution and th e levels of proapoptotic (bax) and antiapoptotic (bcl-2, bclx) protein s. The results indicate that the two drugs act by a different mechanis m with respect to all the parameters tested. While DEX-induced apoptos is was dependent on the level of bcl-2 expression, MEL-induced apoptos is was independent of bcl-2 levels. Treatment with DEX of the low bcl- 2-expressing cells (DEX-sensitive) resulted in a rapid apoptosis from all phases of the cell cycle. In contrast, treatment with MEL blocked the cells in late-S/G2 phase of the cell cycle and caused substantial apoptosis, regardless of bcl-2 expression. Major differences between D EX and MEL were also observed with respect to their effects on the lev els of bcl-2 and p53. Whereas DEX induced an early (day 1) downregulat ion of bcl-2 (only in the cells with low bcl-2), treatment with MEL di d not affect bcl-2 levels. The levels of bclx and bar remained unchang ed following treatment with either MEL or DEX. These results, taken to gether, suggest that the two drugs target different cellular component s and induce apoptosis by different pathways, and that resistance to D EX is associated with low levels of bcl-2, whereas resistance to MEL i s independent of bcl-2, and therefore, in vivo resistance to MEL, obse rved in MM patients, might involve other mechanisms rather than bcl-2.