Antisense Promoter of Beta Myosin Heavy Chain Gene Responds to Diabetes and Hypothyroidism

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Myosin heavy chain (MHC) is an integral contractile component of the muscle sarcomere, and is fundamental in determining the rate of cross-bridge cycling, and thus the speed of muscle contraction. Heart muscle expresses two genes encoding MHC isoforms, α and β, which are arranged in tandem 4.5 Kb apart. Rat hearts typically express a predominance of the α MHC isoform and have high intrinsic contractility. Hypothyroid (PTU), hypertensive, and diabetic (STZ) rats express a significantly higher proportion of the β isoform and thus have a lower rate of tension development, but a greater energy economy of force production. The normal human heart predominantly expresses the β isoform, yet there is still an increase in β and a decrease in α MHC isoforms observed in the failing human heart. This antithetical expression of the α and β genes is highly coordinated, but the mechanism underlying this regulation is poorly understood. We discovered an antisense (AS) β RNA transcript that starts in the middle of the 4.5Kb (intergenic spacer, IGS) and extends upstream to the βMHC gene promoter region, which makes the AS transcript fully complementary to the βMHC gene. The expression of the AS transcript was positively correlated with levels of α MHC gene sense mRNA and negatively correlated with the β MHC gene sense mRNA.. We hypothesize that the IGS sequence contains a bidirectional promoter that regulates the expression of the two genes. We propose that the IGS bidirectional promoter in the 3′ direction drives the transcription of the AS transcript which may serve to interfere with the sense βMHC processing. A promoter corresponding to the −2285/−945 region of the IGS was inserted into a luciferase plasmid in the 3′ to 5′ AS direction, and was injected into rat ventricle. This promoter was activated in control heart and decreased greatly in response to PTU and STZ and increased 9× in hyperthyroid rats. The promoter activity responded in a manner similar to the endogenous antisense β RNA. When the retinoic acid receptor (RAR) site (a known thyroid hormone receptor cofactor) was mutated within this promoter, the reporter activity was almost abolished in Cont, PTU, and STZ hearts, and greatly diminished T3 responsiveness, indicating that RAR is an important regulatory site.