Retinoids induce U937 cells to express macrophage phenotype

The effects of two retinoids, all-trans-retinoic acid (t-RA) and 13-cis-retinoic acid (c-RA) were studied in a model of osteoclast precursors. The model employs the U937 cell line induced to differentiate when incubated with 1 alpha,25-dihydroxyvitamin D3 and conditioned medium from stimulated human lymphocytes. t-RA and c-RA (10(-8) to 10(-6) M) inhibited cellular growth rates and increased surface adherence. However, t-RA and c-RA both partially blocked the differentiation induced by 1,25(OH)2D3 and lymphokines. Thus, retinoids alone promoted the differentiation of U937 cells but partially blocked the differentiation induced by a vitamin D metabolite and lymphokines. These results suggest that vitamins A and D may exert antagonistic effects on the recruitment of osteoclast precursors.