Abstract 5451: Inhibition of the MEK5/ERK5 pathway can redirect 1,25-dihydroxyvitamin D3-treated human AML cells from monocytic to granulocytic lineage of differentiation.
Authors: Wang Xuening , Menon Ajay , Gocek Elzbieta , Danilenko Michael , Harrison Jonathan S. , Studzinski George P. 
Published: August 2014

Abstract
Induced differentiation of malignant cells presents a treatment alternative less toxic than the conventional therapeutic approaches. In particular, Acute Myeloid Leukemia (AML) cells have been shown to differentiate along the monocytic lineage when treated with the active form of vitamin D3 (1,25D), and along the granulocytic lineage after exposure to all-trans retinoic acid (ATRA). While induced differentiation of human AML cells is associated with cell cycle blocks and cessation of cell proliferation, whether treated with 1,25D or ATRA, 1,25D-induced monocytic differentiation is accompanied by somewhat increased cell survival during drug-elicited cellular stress, which counters the therapeutic objective of malignant cell elimination. In contrast, granulocytic differentiation leads to cell death, suggesting that this is the preferred mode of differentiation in a therapeutic setting. In the current study we have observed that inhibition of the MEK5/ERK5 MAPK pathway in 1,25D-treated human AML cells switches the phenotype from monocytic to granulocytic, as evidenced by decreased expression of monocytic markers CD14 and Monocyte Specific Esterase (MSE), concurrent with an increase in general myeloid/granulocytic markers CD11b and CD11c, as well as NBT positivity. On the molecular level, the switch from the monocytic to granulocytic phenotype was accompanied by reduced levels of phosphorylation of transcription factor (TF) C/EBPb but increased phosphorylation of C/EBPa. The requirement for activated ERK5 in monocytic differentiation was also shown by the use of Bix 02189, a specific inhibitor of MEK5 kinase activity, and by the transfection of siRNA to ERK5. Inhibition of ERK1/2 activity by PD098059 reduced monocytic differentiation, but did not alter the 1,25D-induced monocytic phenotype, showing that ERK1/2 and ERK5 serve different functions in differentiation signaling. Importantly, ERK5 appears to control the activation of the TF c-jun/AP1, previously shown to be required for 1, 25D- induced monocytic differentiation, and of the TF MEF2C, not previously linked to 1,25D action or to monocytic differentiation. Thus, our results suggest that ERK5 inhibitors should be evaluated as potentiators of the therapeutic effects of vitamin D analogs in the proposed clinical trials of AML therapy, and raise the possibility that ERK5 inhibitors may also augment the granulocytic differentiation induced by ATRA in the treatment of APL. (Supported by the NIH grant R01-CA-077422-22 from the National Cancer Institute).
Citation Format: Xuening Wang, Ajay Menon, Elzbieta Gocek, Michael Danilenko, Jonathan S. Harrison, George P. Studzinski. Inhibition of the MEK5/ERK5 pathway can redirect 1,25-dihydroxyvitamin D3-treated human AML cells from monocytic to granulocytic lineage of differentiation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5451. doi:10.1158/1538-7445.AM2013-5451