Cardiac myosin heavy chain gene regulation by thyroid hormone involves altered histone modifications

Abstract

The antithetical regulation of cardiac ␣-and ␤-myosin heavy chain (MHC) genes by thyroid hormone (T3) is not well understood but appears to involve thyroid hormone interaction with its nuclear receptor and MHC promoters as well as cis-acting noncoding regulatory RNA (ncRNA). Both of these phenomena involve epigenetic regulations. This study investigated the extent that altered thyroid state induces histone modifications in the chromatin associated with the cardiac MHC genes. We hypothesized that specific epigenetic events could be identified and linked to cardiac MHC gene switching in response to a hypothyroid or hyperthyroid state. A hypothyroid state was induced in rats by propylthiouracil treatment (PTU), whereas a hyperthyroid (T3) was induced by T3 treatment. The left ventricle was analyzed after 7 days for MHC pre-mRNA expression, and the chromatin was assessed for enrichment in specific histone modifications using chromatin immunoprecipitation quantitative PCR assays. At both the ␣-MHC promoter and the intergenic region, the enrichment in acetyl histone H3 at K9/14 (H3K9/14ac) and trimethyl histone H3 at K4 (H3K4me3) changed in a similar fashion. They were both decreased with PTU treatment but did not change under T3, except at a location situated 5= to the antisense intergenic transcription start site. These same marks varied differently on the ␤-MHC promoter. For example, H3K4me3 enrichment correlated with the ␤-promoter activity in PTU and T3 groups, whereas H3K9/14ac was repressed in the T3 group but did not change under PTU. Histone H3K9me was enriched in chromatin of both the intergenic and ␣-MHC promoters in the PTU group, whereas histone H4K20me1 was enriched in chromatin of ␤-MHC promoter in the normal control and T3 groups. Collectively, these findings provide evidence that specific epigenetic phenomena modulate MHC gene expression in altered thyroid states. gene transcription; epigenetic regulation; chromatin immunoprecipitation quantitative polymerase chain reaction; noncoding RNA; Sprague-Dawley rats TWO MYOSIN HEAVY CHAIN (MHC) isoforms are expressed in the myocardium, designated as the ␣-and ␤-MHC. Cardiac MHC phenotype is a major determinant of contractility, mechanical performance, and energy turnover of the heart (59). For example, hearts rich in ␣-MHC expression have high contractility, whereas those rich in ␤-MHC expression have slow speed of contraction but are more energy efficient (1). The normal control adult rodent heart expresses predominantly the ␣-MHC gene, but this can be readily altered by several pathophysiological conditions. For example, hypothyroidism, diabetes, pressure overload, and caloric deprivation enhance the ␤-MHC gene expression and repress the ␣-MHC expression, whereas thyroid hormone treatment produces the opposite effect (4, 48). During these MHC transformations, the MHC genes are regulated in an antithetical fashion; i.e., as one isoform's transcription increases, the other decreases in a coordinated fashion.

Activation of Foxo1 by Insulin Resistance Promotes Cardiac Dysfunction and β–Myosin Heavy Chain Gene Expression

“…29,34 During the fetal developmental period of rodent hearts, β-MHC is the principal isoform expressed in the ventricles when insulin is mostly absent; α-MHC, however, is the principal isoform expressed in adult animals when insulin dominates, to control energy homeostasis. Although a rise of circulating thyroid hormone shortly after birth is thought to suppress β-MHC and increase α-MHC gene expression, 35 rodent hearts continue to mature to adulthood where β-MHC is re-expressed, accounting for 10% to 15% of the total MHC pool, likely because of an increase in mechanical stress 34 and insulin resistance. Moreover, activation of fetal genes such as β-MHC was observed in the failing human hearts, in which overexpression of Foxo1 has been reported.…”

“…35,38 The β-MHC and α-MHC genes are 93% homologous and products of 2 distinct genes, situated in tandem, in a head-totail position, on mouse chromosome 2, providing an antithetical manner for expression of the genes. 35,39 The β-MHC gene is located 4 kb upstream from α-MHC and the 4 kb of intergenic space is transcriptionally active. 35,40 Myocardial deletion of Foxo1 reduced β-MHC and increased α-MHC gene expression, suggesting that Foxo1 has an important role in regulating cardiac MHC.…”

“…35,39 The β-MHC gene is located 4 kb upstream from α-MHC and the 4 kb of intergenic space is transcriptionally active. 35,40 Myocardial deletion of Foxo1 reduced β-MHC and increased α-MHC gene expression, suggesting that Foxo1 has an important role in regulating cardiac MHC. Binding of Foxo1 to the promoter region of β-MHC stimulates activity, possibly by recruiting p300/ CBP, which contains histone acetylase activity, as observed for insulin-like growth factor binding protein-1 (IGFBP-1).…”

Long Noncoding RNAs in Cardiac Development and Pathophysiology

“…However, the exact mechanism of repression used by MHC NATs is unclear. 91 The human heart uses 2 myosin light chain isoforms: ventricular-myosin light chain (VLC-1) and atrial myosin light chain (ALC-1). ALC-1 expression disappears from the ventricles soon after birth but remains in the atria for life.…”

Interplay of chromatin modifications and non-coding RNAs in the heart

“…61 Transcription of AS β-MHC progresses in the direction of the β-MHC gene and is thought to regulate the expression of MHC genes in response to pressure overload. 61,62 The regulation of MHC isoforms involves the coordinated actions of core machinery that include DNA-bound transcription factors, chromatin remodeling, and expression of antisense RNA transcripts. Perhaps the most interesting of recent experimental results highlights the complex regulation of the MHC genes includes both transcriptional and post-transcriptional changes.…”