Abstract 2198: ERK5 inhibition potentiates the effects of leukemia cell differentiation inducers by augmenting the function of master transcription factors
Authors: Zheng Ruifang , Studzinski George 
Published: August 2015

Abstract
ERK5, a member of MAPK family, is activated by multiple AML (Acute Myeloid Leukemia) cell differentiation inducers including 1,25D (1, 25-dihydroxyvitamin D3), TPA (12-O-tetradecanoylphorbol-13-acetate) and ATRA (All-trans retinoic acid). Inhibiting ERK5 with XMD8-92 which targets its auto phosphorylation, or BIX02188 which inhibits MEK5, a kinase that activates ERK5, leads to more advanced cell maturation compared to these inducers alone. Specifically, cells treated with 1,25D and TPA combined with MEK5/ERK5 inhibitors exhibit a macrophage-like phenotype, whereas, ATRA with MEK5/ERK5 inhibitors cause the cells to differentiate further in the granulocytic direction. An increased expression of the general myeloid marker CD11b was observed following treatments with all three inducers when MEK5/ERK5 were inhibited. In this study, we compared the underlying mechanisms of the potentiation effects of MEK5/ERK5 inhibition by these inducers. PU1 is considered to be a master transcription factor (TF) directing the cells to the myeloid lineage, and sustained PU1 expression over C/EBPα results in monocytic differentiation. The role of C/EBPα is controversial: it is generally believed to be a TF favoring granulocyte differentiation, however, in some studies, overexpression of C/EBPα, together with activated estrogen receptor, can lead to the expression of macrophage-related genes. Thus, we measured the expression of these two TFs in HL60 and U937 cells treated with differentiation inducers alone or together with MEK/ERK5 inhibitors. 1,25D alone was able to increase PU1 protein expression in both cell lines; when combined with BIX02188 or XMD8-92, PU.1 expression further increased. Interestingly, increased C/EBPα expression was also observed with XMD8-92 + 1,25D. TPA induced similar changes in PU.1 expression, as well as decreased C/EBPα protein levels in the presence of the inhibitors, which was not seen with 1,25D and the inhibitors. Although ATRA alone or with BIX02188 increased PU1 expression, ATRA with XMD8-92 completely abolished PU1 expression which resulted in dramatically increased C/EBPα over PU1 ratio. From the above data, we conclude that 1) ERK5 activation acts as a brake preventing the cells from further differentiation by all inducers studied here 2) Terminal differentiation which results from the addition of ERK5 inhibitors can be due to the combined effects of two master transcription factors. 3) The same TF can function differently in different settings. (Supported by T32 CA134268 training grant and R01 CA044722-25, both from the National Cancer Institute, NIH, and by the American Institute for Cancer Research, grant 10A049).
Citation Format: Ruifang Zheng, George Studzinski. ERK5 inhibition potentiates the effects of leukemia cell differentiation inducers by augmenting the function of master transcription factors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2198. doi:10.1158/1538-7445.AM2015-2198