P53, MDM-2, BAX AND BCL-2 AND DRUG-RESISTANCE IN CHRONIC LYMPHOCYTIC-LEUKEMIA

Most antitumor agents exert their cytotoxic effect through the inducti on of apoptosis, and this process may be mediated through an elevation in p53 protein, with a subsequent increase in bar and decrease in bcl -2. p53 also increases mdm-2 expression and mdm-2 may then bind and in activate p53. Cells from 31 patients with chronic lymphocytic leukemia (CLL) were treated in vitro with 2-chlorodeoxyadenosine (CdA), arabin osyl-2-fluoroadenine (F-ara-A), or chlorambucil (CLB) and drug sensiti vity measured using the MTT assay. The protein levels of bar and bcl-2 were measured in CLL cells from 25 patients, and were found to be hig her in leukemic cells than in normal B cells. The bcl-2 levels varied three-fold, the bar levels fifteenfold, and the bax:bcl-2 ratios range d from 0.44 to 2.91. The expression of mdm-2 mRNA was measured in CLL cells from 28 patients and was found to vary twenty-fold. However, no correlation was observed between drug sensitivity to CdA, F-ara-A, or CLB and the cellular levels of mdm-2 mRNA, or the protein levels of ba r or bcl-2, or the bax:bcl-2 ratio. Treatment of CLL cells having wild type p53 with CdA, F-ara-A or CLB produced an increase in p53, protei n and mdm-2 mRNA. This was not observed in cells having a p53 mutation , and these cells were highly resistant to both CLB and the nucleoside analogs. In contrast to the nucleoside analogs and CLB, dexamethasone and vincristine had no effect ox mdm-2 mRNA levels. Treatment of CLL cells containing a wild type p53 gene with CdA, F-ara-A, or CLB: did n ot produce any consistent changes in bar or bcl-2. Thus, CdA, F-ara-A and CLB appear to act in CLL cells through a p53-dependent pathway, wh ereas this does not occur with dexamethasone or vincristine. The cellu lar levels of mdm-2, bcl-2, bar or the bax:bcl-2 ratios are not predic tive indicators of clinical sensitivity in CLL, but an increase in mdm -2 levels after drug treatment is indicative of p53 function in these cells.