Zheng Ruifang, Wang Xuening, Studzinski George P
UH Cancer Center, Rutgers, New Jersey Medical School, 205 South Orange Ave., Newark, NJ 07103, USA.
Department of Pathology and Laboratory Medicine, Rutgers, New Jersey Medical School, 185 South Orange Ave., Newark, NJ 07103, USA.
J Steroid Biochem Mol Biol. 2015 Apr;148:132-7. doi: 10.1016/j.jsbmb.2014.11.016. Epub 2014 Nov 20.
Myogenic enhancer factor2 (Mef2) consists of a family of transcription factors involved in morphogenesis of skeletal, cardiac and smooth muscle cells. Among the four isoforms (Mef2A, 2B, 2C, and 2D), Mef2C was also found to play important roles in hematopoiesis. At myeloid progenitor level, Mef2C expression favors monocytic differentiation. Previous studies from our laboratory demonstrated that ERK5 was activated in 1,25-dihydroxyvitamin D3 (1,25D)-induced monocytic differentiation in AML cells and ERK5 activation was accompanied by increased Mef2C phosphorylation. We therefore examined the role of Mef2C in 1,25D-induced monocytic differentiation in AML cell lines (HL60, U937 and THP1) and found that knockdown of Mef2C with small interfering RNA (siRNA) significantly decreases the expression of the monocytic marker, CD14, without affecting the expression of the general myeloid marker, CD11b. CCAAT/enhancer-binding protein (C/EBP) β, which can bind to CD14 promoter and increase its transcription, has been shown to be the downstream effector of 1,25D-induced monocytic differentiation in AML cells. When Mef2C was knocked down, expression of C/EBPβ was reduced at both mRNA and protein levels. The protein expression levels of cell cycle regulators, p27(Kip1) and cyclin D1, were not affected by Mef2C knockdown, nor the monopoiesis related transcription factor, ATF2 (activating transcription factor 2). Thus, we conclude that 1,25D-induced monocytic differentiation, and CD14 expression in particular, are mediated through activation of ERK5-Mef2C-C/EBPβ signaling pathway, and that Mef2C does not seem to modulate cell cycle progression.
肌源性增强因子2（Mef2）由一族参与骨骼肌、心肌和平滑肌细胞形态发生的转录因子组成。在四种亚型（Mef2A、2B、2C和2D）中，Mef2C也被发现在造血过程中发挥重要作用。在髓系祖细胞水平，Mef2C的表达有利于单核细胞分化。我们实验室之前的研究表明，在急性髓系白血病（AML）细胞中，1,25-二羟基维生素D3（1,25D）诱导的单核细胞分化过程中ERK5被激活，并且ERK5的激活伴随着Mef2C磷酸化增加。因此，我们研究了Mef2C在AML细胞系（HL60、U937和THP1）中1,25D诱导的单核细胞分化中的作用，发现用小干扰RNA（siRNA）敲低Mef2C可显著降低单核细胞标志物CD14的表达，而不影响一般髓系标志物CD11b的表达。CCAAT/增强子结合蛋白（C/EBP）β可与CD14启动子结合并增加其转录，已被证明是AML细胞中1,25D诱导的单核细胞分化的下游效应因子。当Mef2C被敲低时，C/EBPβ的mRNA和蛋白水平均降低。细胞周期调节因子p27（Kip1）和细胞周期蛋白D1的蛋白表达水平不受Mef2C敲低的影响，与单核细胞生成相关的转录因子ATF2（激活转录因子2）也不受影响。因此，我们得出结论，1,25D诱导的单核细胞分化，尤其是CD14的表达，是通过ERK5-Mef2C-C/EBPβ信号通路的激活介导的，并且Mef2C似乎不调节细胞周期进程。
