Differentiation induction of HL60 cells by 1,25(OH)2D3, all trans retinoic acid, rTGF-beta2 and their combinations.

1,25 Dihydroxyvitamin D3 (D3), all trans retinoic acid (atRA) and the cytokine rTGF-beta2 are growth and differentiation modulators of promyelocytic leukemia. D3 gives rise to a functional monocytic cell population whereas single atRA therapy induces granulocytic cell features. rTGF-beta2 reduces HL60 cell proliferation but has no differentiating capacity. Combination treatment demonstrates additive effects between either D3 and atRA or D3 and rTGF-beta2, resulting in a cell population with mixed characteristics since individual cells exhibit both monocytic as granulocytic cell features. The capacity of single and combined treatments to induce a permanent differentiation was investigated. Therefore, cells were preincubated with the drugs during six days, test drugs were removed and cell number was monitored. The total cell count of populations treated with single agents remains constant for only a few days and then increases rapidly. rTGF-beta2 cooperated with D3 in inducing a long-lasting differentiation state (3 weeks). Addition of atRA to this combination did not significantly alter proliferation or differentiation, but some cells underwent apoptosis. Therefore, a total and permanent differentiation of leukemic cells may be achieved by repeated exposure to a combination of differentiation inducing agents.