Activity of the beta-myosin heavy chain antisense promoter responds to diabetes and hypothyroidism.

Two genes encoding cardiac myosin heavy chain (MHC) isoforms, beta and alpha, are arranged in tandem 4.5 kb apart. We examined pre-mRNA and mature mRNA levels of beta and alpha genes in control, diabetic (streptozotocin), hypothyroid (propylthiouracil), and hyperthyroid rat hearts and analyzed the naturally occurring antisense (AS) beta RNA species that starts in the middle of the 4.5-kb intergenic region and extends upstream to the beta-gene promoter. The beta and alpha genes are expressed antithetically in control, diabetic, hypothyroid, and hyperthyroid hearts. Expression of AS beta-RNA was positively correlated with alpha-mRNA and negatively correlated with sense beta mRNA. These results support the novel idea of common promoter-regulatory elements situated in the intergenic region that likely control transcription of both sense alpha and AS beta genes and that AS beta transcription negatively regulates beta-MHC gene expression. To test whether an intergenic promoter drives transcription of AS beta RNA, a 1340-bp sequence of the intergenic region 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 propylthiouracil and streptozotocin and increased in hyperthyroid rats, similar in pattern to the endogenous AS beta RNA. When a putative retinoic acid receptor (RAR) site (a known thyroid hormone receptor cofactor) in this promoter was mutated, the reporter activity was almost abolished in control, propylthiouracil, and streptozotocin hearts. We conclude that there is an intergenic promoter that is active in the AS direction and that the putative RAR element is a vital regulatory site.