Clonal analysis of the response of HL-60 cells to retinoic acid and vitamin D in the presence or absence of leukocyte-conditioned medium

The capacity of retinoic acid (RA) and vitamin D to induce differentiation of HL-60 cells in an agar gel system that permits a high cloning efficiency was studied in the presence or absence of leukocyte-conditioned medium (LCM). Vitamin D was about 10 times more effective in inducing differentiation than RA. LCM significantly increased the differentiation-inductive capacity of both vitamins, although LCM did not have an effect of its own. During the first days of culture, the number of cells per clone increased with either vitamin or LCM alone as compared to control cultures. This suggests that cellular proliferation may be important in the initial phase of the differentiation induction. No synergy in this respect was found between RA and vitamin D in the presence or absence of LCM. Thus it is doubtful whether RA and vitamin D have a true synergistic effect on individual HL-60 cell differentiation as previously described in liquid cultures. Furthermore, the findings suggest that the process of differentiation induction regarding the growing cells in agar gel involves an all-or-none process. The data may also help elucide the observed relationship between the in vitro production of colony stimulating activity and favorable prognosis of AML.