Abstract
Novel functions of signaling molecules have been revealed in studies of cancer stem cells. Retinoic acid (RA) is an embryonic morphogen and stem cell regulator that controls the differentiation of a patient-derived leukemic cell line, HL-60, which is composed of progenitor cells with bipotent myelo-monocytic differentiation capability. RA treatment of HL-60 cells causes unusually long-lasting mitogen-activated protein kinase signaling, with the cells exhibiting the beginning of G0 cell cycle arrest and functional differentiation by 48 h after treatment with RA. This event coincides with the nuclear translocation of Raf-1, phosphorylated at serine 621. The present study shows how the novel localization of Raf-1 to the nucleus results in transcriptional changes that contribute to the differentiation of HL-60 cells induced by RA. We find that nuclear pS621 Raf-1 associates with NFATc3 near its cognate binding site in the promoter of CXCR5, a gene that must be up-regulated to drive RA-induced differentiation. NFATc3 becomes immunoprecipitable with anti-phosphoserine serum, and CXCR5 is transcriptionally up-regulated upon RA-induced differentiation. Inhibiting the pS621 Raf-1/NFATc3 association with PD98059 inhibits these processes and cripples RA-induced differentiation. In this novel paradigm for Raf-1 and RA function, Raf-1 has a role in driving the nuclear signaling of RA-induced differentiation of leukemic progenitor cells.