Dissecting the novel partners of nuclear c-Raf and its role in all-trans retinoic acid (ATRA)-induced myeloblastic leukemia cells differentiation

All-trans retinoic acid (ATRA) is an anti-cancer differentiation therapy agent effective for acute promyelocytic leukemia (APL) but not acute myeloid leukemia (AML) in general. Using the HL-60 human non-APL AML model where ATRA causes nuclear enrichment of c-Raf that drives differentiation and G1/G0 cell cycle arrest, we now observe that c-Raf in the nucleus showed novel interactions with several prominent regulators of the cell cycle and cell differentiation. One is cyclin-dependent kinase 2 (Cdk2). ATRA treatment caused c-Raf to dissociate from Cdk2. This was associated with enhanced binding of Cdk2 with retinoic acid receptor α (RARα). Consistent with this novel Raf/CDK2/RARα axis contributing to differentiation, CD38 expression per cell, which is transcriptionally regulated by a retinoic acid response element (RARE), is enhanced. The RB tumor suppressor, a fundamental regulator of G1 cell cycle progression or arrest, was also targeted by c-Raf in the nucleus. RB and specifically the S608 phosphorylated form (pS608RB) complexed with c-Raf. ATRA treatment induced S608RB-hypophosphorylation associated with G1/G0 cell cycle arrest and release of c-Raf from RB. We also found that nuclear c-Raf interacted with SMARCD1, a pioneering component of the SWI/SNF chromatin remodeling complex. ATRA treatment diminished the amount of this protein bound to c-Raf. The data suggest that ATRA treatment to HL-60 human cells re-directed c-Raf from its historically pro-proliferation functions in the cytoplasm to pro-differentiation functions in the nucleus.