Combined staurosporine and retinoic acid induces differentiation in retinoic acid resistant acute promyelocytic leukemia cell lines.

All-trans retinoic acid (ATRA) resistance has been a critical problem in acute promyelocytic leukemia (APL) relapsed patients. In ATRA resistant APL cell lines NB4-R1 and NB4-R2, the combination of staurosporine and ATRA synergized to trigger differentiation accompanied by significantly enhanced protein level of CCAAT/enhancer binding protein ε (C/EBPε) and C/EBPβ as well as the phosphorylation of mitogen-activated protein (MEK) and extracellular signal-regulated kinase (ERK). Furthermore, attenuation of the MEK activation blocked not only the differentiation but also the increased protein level of C/EBPε and C/EBPβ. Taken together, we concluded that the combination of ATRA and staurosporine could overcome differentiation block via MEK/ERK signaling pathway in ATRA-resistant APL cell lines.