ENFORCED EXPRESSION OF BCL-X(S) INDUCES DIFFERENTIATION AND SENSITIZES CHRONIC MYELOGENOUS LEUKEMIA-BLAST CRISIS K562 CELLS TO 1-BETA-D-ARABINOFURANOSYLCYTOSINE-MEDIATED DIFFERENTIATION AND APOPTOSIS

Human chronic myelogenous leukemia-blast crisis K562 cells have been d emonstrated to be relatively resistant to antileukemic drug-induced ap optosis. This has been attributed to the activity of p210(bcr-abl) tyr osine kinase present in the K562 cells, which is known to suppress dru g-induced apoptosis. Recently, K562 cells have been shown to express t he antiapoptosis Bcl-x(L) but not Bcl-2 proteins. To investigate the c ontribution of Bcl-x(L) toward resistance to drug-induced apoptosis, w e created K562/Bcl-x(S) and K562/neo cells by electroporating the expr ession plasmids pSFFVneo-Bcl-x(S) and pSFFVneo, containing the bcl-x(S ) and neomycin resistance genes, respectively, into K562 cells, K562/B cl-x(S) but not K562/neo cells expressed the bcl-x(S) mRNA and p19Bcl- x(S) protein, in contrast, both cell types expressed equivalent levels of Bcl-x(L), Bax, Bcl-2, Myc, retinoblastoma, p210(bcr-abl), and p145 (abl) proteins. A significant increase in the hemoglobin levels was ob served in the K562/Bcl-x(S) compared with the K562/neo cells (P < 0.05 ). In addition, K562/Bcl-x(S), cells were significantly more sensitive than K562/neo cells to undergoing erythroid differentiation induced b y lewd close 1-beta-D-arabinofuranosylcytosine (ara-C) and hexamethyl bisacetamide (P < 0.05), but not by all-trans-retinoic acid. Low-dose ara-C- or hexamethyl bisacetamide-induced differentiation was not asso ciated with apoptosis of K562/Bcl-x(S) or K562/neo cells. Low-dose ara -C-induced erythroid differentiation was accompanied by conversion of the retinoblastoma protein to predominantly its underphosphorylated is oform as well as by down-regulation of Myc levels in K562/Bcl-x(S) and K562/neo cells. Importantly, exposure to high-dose ara-C (HIDAC; 100 mu M ara-C for 4 h) caused internucleosomal DNA fragmentation and the morphological features of apoptosis in K562/Bcl-x(S) cells. These effe cts were modestly enhanced by cotreatment with HIDAC plus herbimycin A . In contrast, K562/neo cells were completely resistant to HIDAC- and herbimycin A-induced apoptosis. These results indicate that the expres sion of Bcl-x(S) induces erythroid differentiation and partially sensi tizes chronic myelogenous leukemia-blast crisis-derived K562 cells to ara-C-induced differentiation and apoptosis.