Acute myelogenous leukemia (AML) is a disease characterized by dysregulated cell proliferation associated with impaired cell differentiation, and current treatment regimens rarely save the patient. Thus, new mechanism-based approaches are needed to improve prognosis of this disease. We have investigated in preclinical studies the potential anti-leukemia use of the plant-derived polyphenol Silibinin (SIL) in combination with 1,25-dihydroxyvitamin D3 (1,25D). Although most of the leukemic blasts ex vivo responded by differentiation to treatment with this combination, the reasons for the absence of SIL-1,25D synergy in some cases were unclear. Here we report that failure of SIL to enhance the action of 1,25D is likely due to the SIL-induced increase in the activity of differentiation-antagonizing cell components, such as ERK5. This kinase is under the control of Cot1/Tlp2, and inhibition of Cot1 activity by a specific pharmacological inhibitor 4-(3-chloro-4-fluorophenylamino)-6-(pyridin-3-yl-methylamino-3-cyano-[1-7]-naphthyridine, or by Cot1 siRNA, increases the differentiation by SIL/1,25D combinations. Conversely, over-expression of a Cot1 construct increases the cellular levels of P-ERK5, and SIL/1,25D-induced differentiation and cell cycle arrest are diminished. It appears that reduction in ERK5 activity by inhibition of Cot1 allows SIL to augment the expression of 1,25D-induced differentiation promoting factors and cell cycle regulators such as p27 (Kip1) , which leads to cell cycle arrest. This study shows that in some cell contexts SIL/1,25D can promote expression of both differentiation-promoting and differentiation-inhibiting genes, and that the latter can be neutralized by a highly specific pharmacological inhibitor, suggesting a potential for supplementing treatment of AML with this combination of agents.