INCREASING C-FMS (CSF-1 RECEPTOR) EXPRESSION DECREASES RETINOIC ACID CONCENTRATION NEEDED TO CAUSE CELL-DIFFERENTIATION AND RETINOBLASTOMA PROTEIN HYPOPHOSPHORYLATION

Increasing the expression of c-FMS (colony-stimulating factor 1 recept or) by introduction of a transgene reduced the concentration of retino ic acid or 1,25-dihydroxy vitamin D3 needed to cause myeloid or monocy tic cell differentiation and hypophosphorylation of the retinoblastoma tumor suppressor protein (RB) typically associated with cell cycle G( 0) arrest and differentiation of HL-60 human myelo-monoblastic precurs or cells. The data are consistent with a model in which signals origin ating with retinoic acid and c-FMS integrate to cause differentiation, RB hypophosphorylation, and G(0) arrest. Furthermore, these two signa ls can compensate for each other. Three HL-60 sublines described previ ously (A. Yen et al., Exp. Cell Res., 229: 111-125, 1996) expressing l ow (wild-type HL-60), intermediate, and high cell surface c-FMS were t reated with various concentrations of retinoic acid, The lowest concen tration tested, 10(-8) M, induced significant differentiation of only the high c-FMS-expressing cells, with no accompanying hypophosphorylat ed RB or G(0) arrest. The low and intermediate c-FMS expressing cells showed no induced differentiation, hypophosphorylation of RB, or G(0) arrest, A 10-fold higher retinoic acid concentration, 10(-7) M, induce d significant differentiation of both intermediate and high c-FMS-expr essing cells, It induced RB hypophosphorylation only in high c-FMS-exp ressing cells but with no accompanying G(0) arrest in any of the cells , The highest retinoic acid concentration, 10(-6) M, elicited differen tiation, hypophosphorylation of RB, and G(0) arrest in low, intermedia te, and high c-FMS-expressing cells. As the concentration of retinoic acid increased, cell differentiation, hypophosphorylation of RB, and G (0) arrest were progressively elicited within this ensemble of cells w ith different c-FMS expression levels. Thus, for example, at the lowes t concentration of retinoic acid, expression of high enough c-FMS stil l allowed differentiation. At higher concentrations, progressively les s c-FMS was needed for differentiation. The apparent threshold for the sum of the retinoic acid plus c-FMS originated signals to elicit diff erentiation, hypophosphorylation of RB, and G(0) arrest increased, in that order. Thus retinoic acid-induced cell differentiation, RB hypoph osphorylation, and G(0) arrest have different signal threshold require ments, 1,25-Dihydroxy vitamin D3, also a ligand for a member of the st eroid thyroid hormone receptor superfamily, caused monocytic different iation with a similar c-FMS dependency, indicating that these effects characterize both myeloid and monocytic differentiation.