Attenuation of the induced differentiation of HL-60 leukemia cells by mitochondrial chaperone HSP70

The HSP70 family of heat shock proteins, which are involved in development and cellular differentiation, is elevated in various tumor cell lines. To examine the role of these proteins in neoplastic cell differentiation, four members of the HSP70 multiple gene family (i.e., HSP70, HSC70, GRP78, and mtHSP70) were examined during the induced differentiation of HL-60 promyelocytic leukemia cells. Western analyses showed that continuous exposure for 48 h of HL-60 cells to the differentiation-inducing agents, all-trans retinoic acid, 1,25-dihydroxyvitamin D3, or N-methylformamide, resulted in decreases in mitochondrial HSP70 (mtHSP70), with little change in the levels of HSP70, HSC70, and GRP78. To gain information on the role of mtHSP70 in the differentiation process, HL-60 cells were transfected with either murine mthsp70 cDNA or vector alone. Slightly greater than twofold increases in mtHSP70 protein levels occurred in cells transfected with the mthsp70 cDNA. In vector-transfected HL-60 cells, myeloid differentiation, measured as an increase in CD31 expression and nitroblue tetrazolium positivity, was observed following 3-6 days of treatment with each of the three inducing agents. In contrast, cell differentiation induced by each agent was markedly attenuated in mthsp70-transfected HL-60 cells. These findings suggest that a decrease in mtHSP70 is important for the induced differentiation of HL-60 cells.