Raf-1 signaling is required for the later stages of 1,25-dihydroxyvitamin D3-induced differentiation of HL60 cells but is not mediated by the MEK/ERK module.

We are interested in determining the signaling pathways for 1,25-dihydroxyvitamin D3 (1,25D)-induced differentiation of HL60 leukemic cells. One possible candidate is Raf-1, which is known to signal cell proliferation and neoplastic transformation through MEK, ERK, and downstream targets. It can also participate in the regulation of cell survival and various forms of cell differentiation, though the precise pathways are less well delineated. Here we report that Raf-1 has a role in monocytic differentiation of human myeloid leukemia HL60, which is not mediated by MEK and ERK, but likely by direct interaction with p90RSK. Specifically, we show that Raf-1 and p90RSK are increasingly activated in the later stages of differentiation of HL60 cells, at the same time as activation of MEK and ERK is decreasing. Transfection of a wild-type Raf-1 construct enhances 1,25D-induced differentiation, while antisense Raf-1 or short interfering (si) Raf-1 reduces 1,25D-induced differentiation. In contrast, antisense oligodeoxynucleotides (ODN) and siRNAs to MEK or ERK have no detectable effect on differentiation. In late stage differentiating cells Raf-1 and p90RSK are found as a complex, and inhibition of Raf-1, but not MEK or ERK expression reduces the levels of phosphorylated p90 RSK. These findings support the thesis that Raf-1 signals cell proliferation and cell differentiation through different intermediary proteins.