Treatment of HL60 cells with various combinations of retinoids and 1 alpha,25 dihydroxyvitamin D3 results in differentiation towards neutrophils or monocytes or a failure to differentiate and apoptosis.

It is well documented that treatment of serum-grown HL60 cells with 10(-7) M all-trans retinoic acid (all-trans RA) induces neutrophil differentiation, whereas treatment with 10(-7) M 1 alpha,25 dihydroxyvitamin D3(D3) induces differentiation towards monocytes. In recent investigations, using serum-free grown HL60 cells, we observed that all-trans RA, at 10(-7) M, did not induce neutrophil differentiation and that all-trans RA, at 10(-8) M, reduced the D3 concentration required for monocyte differentiation to 5 x 10(-9) M. In this study, co-operative interactions between all-trans and 9-cis RA and D3 which promote neutrophil and monocyte differentiation of HL60 cells have been analysed in detail. Treatment of serum-free grown HL60 cells with 5 x 10(-7) M all-trans RA or 9-cis RA resulted in sub-optimal neutrophil differentiation (up to 25% mature cells). As shown for all-trans RA, 9-cis RA cooperated with D3 to promote monocyte differentiation. Culture of HL60 cells in 5 x 10(-7) M 9-cis RA together with a wide range of concentrations of D3 resulted in promotion of neutrophil differentiation at 10(-15)-10(-12) D3, a failure to differentiate and apoptosis at 10(-11)-10(-10) M D3, followed by co-operativity between 9-cis RA and 5 x 10(-9) M D3 in inducing monocyte differentiation in the absence of neutrophil differentiation. Similar results were obtained when HL60 cells were treated with 5 x 10(-7) all-trans RA together with a wide range of concentrations of D3. Cross titration analyses of the effects of 9-cis RA and D3 on HL60 cell differentiation were undertaken to determine the boundaries of the concentrations of each agent, alone and in combination, that give rise to optimal neutrophil and monocyte differentiation of HL60 cells. The observed cooperativities between either 9-cis RA or all-trans RA and D3 have important implications for the use of combinations of these agents in differentiation therapy.