Particular combinations of signals, by retinoic acid and 1 alpha, 25 dihydroxyvitamin D3, promote apoptosis of HL60 cells.

The promyeloid cell line HL60, when grown in serum-free medium, is induced to differentiate towards either neutrophils or monocytes by treatment with particular concentrations of 9-cis retinoic acid (9-cis RA) and 1 alpha, 25 dihydroxyvitamin D3 (D3). We have investigated whether treatment of HL60 cells with 9-cis RA and D3 can lead to growth arrest and a failure to undergo cell differentiation. This occurred in two circumstances and HL60 cells died rapidly by apoptosis. First, treatment with 5 x 10(-7) M 9-cis RA and 1.25 x 10(-9)-3.1 x 10(-10) M D3 promoted growth arrest and apoptosis of HL60 cells. The amount of 9-cis RA alone promoted significant neutrophil differentiation of HL60 cells. The amounts of D3 alone promoted a very low level of monocyte differentiation. Treatment with each agent alone did not result in increased levels of apoptosis. Second, HL60 cells were treated with concentrations of 9-cis RA (5 x 10(-7) M) and D3 (3.9 x 10(-14) M) that were appropriate for induction of neutrophil differentiation. At the time when they were undergoing commitment to the neutrophil pathway of differentiation (days 1-2), an amount of D3 (1 x 10(-7) M) that promotes monocyte differentiation was added to the cultures. HL60 cells failed to differentiate and died by apoptosis. Hence, certain combinations of signals, elicited by 9-cis RA and D3, promote apoptosis of HL60 cells. This finding has important implications for the use of retinoids and D3 in differentiation therapy.