Ten-Eleven Translocation-2 (TET2) Is a Master Regulator of Smooth Muscle Cell Plasticity

Abstract

U nlike most mature cells, smooth muscle cells (SMCs) are remarkably plastic and can dedifferentiate in response to environmental cues, 1,2 adding a layer of complexity to the regulation of gene expression. Although several transcription factors have been identified, a global mechanism that coordinately regulates SMC phenotype has yet to be uncovered. How SMC genes become silenced and then reactivated is unknown and is an area of intense investigation. Recent demonstration that the ten-eleven-translocation (TET) family of proteins is involved in DNA demethylation [3][4][5] prompted us to evaluate the role of the TET proteins in the modulation of SMC phenotype. Editorial see p 2002 Clinical Perspective on p 2057The TET proteins (TET1-TET3) are a recently discovered family of DNA demethylases. TET proteins oxidize 5-methylcytosine (5-mC) to generate 5-hydroxymethylcytosine (5-hmC), frequently called the sixth DNA base, in mammalian cells. 4,5 Through the base excision repair pathway, 5-hmC is then converted to unmethylated cytosine, leading to DNA demethylation and gene activation. [6][7][8] Therefore, the 5-hmC modification and the TET enzymes have emerged as key activators of gene expression. Studies of TET proteins and 5-hmC function in embryonic stem cells (ESCs) demonstrate that they play a major role in maintaining cellular pluripotency through the regulation of lineage-specific genes. 4,[9][10][11] In contrast to this role in ESC pluripotency, the TET proteins (and their 5-hmC products) have an opposing role in adult stem cells and somatic tissues. TET2 mutations have been described in several types of hematopoietic disorders in which the loss of TET2 has been shown to promote hematopoietic stem cell self-renewal.12 TET2 and 5-hmC levels are increased during neurogenesis, 13 and more recently, loss of TET2 and 5-hmC was demonstrated to be a key epigenetic event associated with Background-Smooth muscle cells (SMCs) are remarkably plastic. Their reversible differentiation is required for growth and wound healing but also contributes to pathologies such as atherosclerosis and restenosis. Although key regulators of the SMC phenotype, including myocardin (MYOCD) and KLF4, have been identified, a unifying epigenetic mechanism that confers reversible SMC differentiation has not been reported. Methods and Results-Using human SMCs, human arterial tissue, and mouse models, we report that SMC plasticity is governed by the DNA-modifying enzyme ten-eleven translocation-2 (TET2). TET2 and its product, 5-hydroxymethylcytosine (5-hmC), are enriched in contractile SMCs but reduced in dedifferentiated SMCs. TET2 knockdown inhibits expression of key procontractile genes, including MYOCD and SRF, with concomitant transcriptional upregulation of KLF4. TET2 knockdown prevents rapamycin-induced SMC differentiation, whereas TET2 overexpression is sufficient to induce a contractile phenotype. TET2 overexpression also induces SMC gene expression in fibroblasts. Chromatin immunoprecipitation demonstrates that TET2 coordinately regul...

Vasa Vasorum in Normal and Diseased Arteries

“…Recent studies have demonstrated that a DNA demethylation protein, ten-eleven translocation-2 (TET2) enzyme, plays a significant role in reversible SMC differentiation in human SMCs, human arterial tissue, and mouse models of atherosclerosis and injury-induced intimal hyperplasia. 48 Mechanistic studies show that TET2 regulates SMC phenotype by converting 5-methylcytosine to unmethylated cytosine, which results in DNA demethylation and activation of key SMC differentiation genes; knockdown of TET2 increases expression of synthetic SMC phenotype markers. Furthermore, knockdown studies indicate that TET2 differentially regulates the chromatin accessibility of contractile and dedifferentiation genes.…”

“…The collective data suggest that TET2 is a master epigenetic regulator of SMC differentiation. 48 An alternative theory suggests that adult SMCs in the plaque originate from a variety of sources that include adventitial vessel wall progenitor cells 49 and can migrate from the adventitia into the media and intima after arterial injury. 4,50 The labeling of adventitial cells in rat common carotid arteries with β-galactosidase before a balloon catheter injury demonstrated their appearance in the media then the intima 7 and 14 days later.…”

Response to Letter Regarding Article, “Ten-Eleven Translocation-2 (TET2) Is a Master Regulator of Smooth Muscle Cell Plasticity”

“…Indeed, our work unexpectedly revealed the stable presence of 5hmC in differentiated SMC in vivo. 1 We reported that ectopic expression of TET2 is able to convert MRC5 fibroblasts into the smooth muscle lineage as demonstrated by the robust upregulation of SMC markers including SMA and MYH11. Interestingly, TET2 overexpression was only able to partially convert endothelial cells to the smooth muscle lineage.…”

“…We thank Drs Li and Yu for their enthusiastic comments on our recent work 1 identifying Ten-Eleven-Translocation 2 (TET2) as a novel epigenetic master regulator of smooth muscle cell (SMC) phenotype. We also appreciate their highlighting the implications of our study regarding a role for TET2 in cellular reprogramming.…”

“…Interestingly, TET2 overexpression was only able to partially convert endothelial cells to the smooth muscle lineage. 1 The mechanisms underlying the cell type-specific differences in TET2-mediated reprogramming efficiency remain to be explored. We speculate that the embryonic origin of MRC5, a cell line commonly used for reprogramming studies, may confer a greater plasticity that can therefore be more readily differentiated into multiple lineages compared with the more fully differentiated adult endothelial cells.…”