AbstractVitamin A metabolites are potent differentiation-inducing agents for myelomonocytic cell lines in vitro and are successfully used for the treatment of patients with acute promyelocytic leukemia. However, little is known about the effects of vitamin A on normal hematopoietic cells. Therefore, we investigated the effect of vitamin A on differentiation and activation of human blood monocytes (MO). Culturing MO for up to 4 days with 9-cis retinoic acid (RA) and all-trans RA but not retinol reduced MO survival, with the remaining cells being morphologically comparable to control cells. Because macrophage colony-stimulating factor (M-CSF) is a well-known survival factor for MO, we measured the M-CSF content of MO culture supernatants using enzyme-linked immunosorbent assay and found that RA suppressed the constitutive secretion of M-CSF. Northern analysis showed that the M-CSF mRNA expression was only slightly reduced by RA treatment, suggesting regulation on the posttranscriptional level. In contrast to MO, M-CSF secretion by MO-derived macrophages (MAC) was not altered by RA, suggesting a differentiation-dependent switch in the responsiveness of MO/MAC to RA. Because M-CSF is not only a survival-promoting but also a differentiation-promoting factor for myeloid cells, we analyzed the effect of RA on MO to MAC maturation. RA suppressed the expression of the maturation-associated antigen carboxypeptidase M (CPM)/MAX.1 at both the protein and mRNA levels and modulated the lipopolysaccharide-stimulated cytokine secretion of MO/MAC. The addition of exogenous M-CSF to RA-containing MO cultures fails to overcome the RA-induced inhibition of MO differentiation. However, the survival rate was improved by exogenous M-CSF. We conclude that RA acts via two different mechanisms on monocyte survival and differentiation: posttranscriptionally by controlling M-CSF secretion, which decreases MO survival, and transcriptionally regulating the expression of differentiation-associated genes. The regulation of M-CSF production may contribute to the antileukemic effect of RA in vivo by reducing autocrine M-CSF production by leukemic cells.