Bcl-XL is required for heme synthesis during the chemical induction of erythroid differentiation of murine erythroleukemia cells independently of its antiapoptotic function

Bcl-X-L is essential for the survival and normal maturation of erythroid cells, especially at the late stage of erythroid differentiation. It remains unclear whether Bcl-X-L serves only as a survival factor for erythroid cells or if it can induce a signal for differentiation. We have previously shown that dimethyl sulfoxide (DMSO) induction of erythroid differentiation in murine erythroleukemia (MEL) cells correlates with delay of apoptosis and specific induction of Bcl-X-L. In this study, we investigate the contribution of Bcl-2 and Bcl-X-L to survival and erythroid differentiation by generating stable MEL transfectants expressing these antiapoptotic regulators. Overexpression of Bcl-2 completely prevented apoptosis of MEL cells before and after DMSO induction, whereas overexpression of Bcl-X-L only delayed it. Overexpression of Bcl-2 or Bcl-X-L neither induced spontaneous erythroid differentiation nor accelerated DMSO-induced differentiation. Inhibition of Bcl-XL by antisense transcripts accelerated apoptosis in DMSO-treated MEL cells and blocked the synthesis of hemoglobin without altering the growth arrest associated with terminal erythroid differentiation. An antisense oligonucleotide to Bcl-X-L did not induce apoptosis in MEL cells overexpressing Bcl-2 but greatly decreased their hemoglobin synthesis when treated with DMSO, suggesting that Bcl-X-L is necessary for erythroid differentiation independently of its antiapoptotic function. Importantly, Bcl-X-L antisense transcripts prevented heme synthesis but not globin mRNA induction in DMSO-treated MEL cells. Furthermore, inhibition of hemoglobin synthesis by Bcl-X-L antisense was reversed by addition of exogenous hemin. Finally, Bcl-X-L localized to mitochondria during MEL erythroid differentiation, suggesting that it may mediate a critical mitochondrial transport function related to heme biosynthesis.