Evading apoptosis by calcitriol-differentiated human leukemic HL-60 cells is not mediated by changes in CD95 receptor system but by increased sensitivity of these cells to insulin

Previous studies revealed that 1,25-dihydroxyvitamin D-3 (calcitriol)-induc ed differentiation of human promyelocytic leukemia cells leads to an increa sed resistance of the cells to apoptosis-inducing agents. However many atte mpts were made to explain it, the mechanism underlying this effect still re mains unclear. Our results suggest that the acquired resistance to apoptosi s-inducing agents in HL-60 cells is not mediated by the CD95 receptor/ligan d system. The expression of CD95 on the surface of HL-60 cells is very low and does not change during the calcitriol-induced differentiation of HL-60 cells. Studies presented here provide a strong indication that this recepto r is unable to transmit the death signal in either differentiated or undiff erentiated HL-60 cells. We therefore asked if evading apoptosis by differen tiated human leukemia HL-60 cells may be caused by their increased sensitiv ity to growth factors contained in fetal calf serum. This study demonstrate s that HL-60 promyelocytic leukemia cells, differentiated by exposure to ca lcitriol, undergo apoptosis in serum-free conditions. As low as 1% of fetal calf serum is enough to prevent cell death of differentiated HL-60 cells. The ability of 1% fetal calf serum to prevent apoptosis can be blocked by t he specific inhibitor of phosphatidylinositol 3-kinase, LY294002. We then t ried to find out which component of fetal calf serum may be able to prevent serum-free cell death of differentiated cells. It appeared that serum-free cell death of differentiated HL-60 cells is reversed by addition of 10 muM insulin to the culture medium. The antiapoptotic activity of insulin can b e inhibited by LY294002. Moreover, insulin increases the viability of diffe rentiated, but not of undifferentiated, HL-60 cells. (C) 2001 Academic Pres s.