Expression of activated RAF accelerates cell differentiation and RB protein down-regulation but not hypophosphorylation.

Expression of an activated raf transgene accelerated the terminal myeloid differentiation of HL-60 human promyelocytic leukemia cells induced by retinoic acid. A similar result was obtained when 1,25-dihydroxyvitamin D3 was used to induce monocytic differentiation. The stable transfectants were derived by transfecting HL-60 cells with DNA encoding an N-terminal truncated raf-1 protein. In normal HL-60 cells retinoic acid is known to induce a colony-stimulating factor-1 (CSF-1)-dependent metabolic cascade culminating in G0 arrest and phenotypic conversion. Early in this cascade, expression of the RB tumor suppressor gene product is down-regulated. A progressive redistribution of the form of the protein from largely hyperphosphorylated protein to the hypophosphorylated form begins later with G0 arrest and differentiation. In the activated raf-transfected cells, RB down regulation occurred more rapidly, consistent with accelerated differentiation. But the conversion to the hypophosphorylated form was not accelerated and occurred after G0 arrest and phenotypic conversion to myeloid differentiated cells. Thus raf activation appears to be a component of the induced metabolic cascade culminating in terminal differentiation. In this cascade raf activation promotes RB down-regulation. The data are consistent with a model in which raf is an effector of the CSF-1-dependent metabolic cascade which culminates in terminal cell differentiation, and RB downregulation is one of the downstream consequences of RAF action. Furthermore, they indicate that RB down-regulation may be an essential component of the cellular processes causing G0 arrest and differentiation, but RB hypophosphorylation is more likely a consequence thereof and not a cause.