Inhibition of Src family kinases enhances retinoic acid induced gene expression and myeloid differentiation.

Treatment of acute promyelocytic leukemia with retinoic acid (RA) results in differentiation of the leukemic cells and clinical remission. However, the cellular factors that regulate RA-induced myeloid differentiation are largely unknown, and other forms of acute myelogenous leukemia (AML) do not respond to this differentiation therapy. A greater understanding of the molecules that positively or negatively regulate RA-induced differentiation should facilitate the development of more effective differentiation therapies. In this study, we investigated the potential role of Src family kinases (SFK) in the regulation of RA-induced gene expression and myeloid differentiation. We report that inhibition of SFKs markedly enhanced RA-induced differentiation in myeloid cell lines and primary AML cells, as assessed by flow-cytometric analysis of cell surface markers, morphologic analysis, and nitroblue tetrazolium reduction. In addition, inhibition of SFKs enhanced expression from retinoic acid receptor (RAR) target genes encoding CCAAT/enhancer binding protein epsilon (C/EBPepsilon), PU.1, intercellular adhesion molecule-1 (ICAM-1), and cathepsin D. Moreover, a constitutively active Src inhibited RAR-dependent transcription, whereas a kinase-dead Src exerted little effect. These studies provide the first demonstration that SFKs act to negatively regulate RA-induced gene expression and myeloid differentiation and suggest that the combination of SFK inhibition and RA treatment may be therapeutically beneficial in AML.