Enforced expression of Bcl-x(S) induces differentiation and sensitizes chronic myelogenous leukemia-blast crisis K562 cells to 1-β-D- arabinofuranosylcytosine-mediated differentiation and apoptosis

Human chronic myelogenous leukemia-blast crisis K562 cells have been demonstrated to be relatively resistant to antileukemic drug-induced apoptosis. This has been attributed to the activity of p210(bcr-abl) tyrosine kinase present in the K562 cells, which is known to suppress drug-induced apoptosis. Recently, K562 cells have been shown to express the antiapoptosis Bcl-x(L) but not Bcl-2 proteins. To investigate the contribution of Bcl-x(L) toward resistance to drug-induced apoptosis, we created K562/Bcl-x(s) and K562/neo cells by electroporating the expression plasmids pSFFVneo-Bcl-x(s) and pSFFVneo, containing the bcl-x(s) and neomycin resistance genes, respectively, into K562 cells. K562/Bcl-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 observed 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 by low-dose 1-β-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 associated 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 isoform 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 μM ara-C for 4 h) caused internucleosomal DNA fragmentation and the morphological features of apoptosis in K562/Bcl-x(s) cells. These effects 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 expression of Bcl-x(s) induces erythroid differentiation and partially sensitizes chronic myelogenous leukemia-blast crisis-derived K562 cells to ara-C-induced differentiation and apoptosis.