CONSTITUTIVE EXPRESSION OF THE ORPHAN RECEPTOR, REV-ERBA-ALPHA, INHIBITS MUSCLE DIFFERENTIATION AND ABROGATES THE EXPRESSION OF THE MYOD GENE FAMILY

Rev-erbA alpha is an orphan steroid receptor that is expressed in skel etal muscle. Rev-erbA alpha binds to single/tandem copies of an AGGTCA motif, is transcribed on the noncoding strand of the c-erbA-alpha gen e locus, and is postulated to modulate the thyroid hormone (T-3) respo nse. T-3 induces terminal muscle differentiation and regulates fiber t ype composition via direct activation of the muscle-specific myoD gene family (e.g. myoD, myogenin). The myoD gene family can direct the fat e of mesodermal cell lineages and activate muscle differentiation. Hen ce we investigated the expression and physiological role of Rev-erbA a lpha during myogenesis. We observed abundant levels of Rev-erbA alpha mRNA in dividing C2C12 myoblasts, which were suppressed when the cells differentiated into postmitotic multinucleated myotubes. This decreas e in Rev-erbA alpha mRNA correlated with the appearance of muscle-spec ific mRNAs (e.g, myogenin and alpha-actin). Constitutive overexpressio n of full length Rev-erbA alpha cDNA in the myogenic cells completely abolished differentiation, suppressed myoD mRNA levels, and abrogated the induction of myogenin mRNA. We then demonstrated that 1) GAL4-Rev- erbA alpha chimeras that contain the 'AB' region and lack the 'E' regi on activated transcription of GAL4 response elements in the presence o f 8-Br-cAMP and 2) the ligand-binding domain (LED) contains an active transcriptional silencer. Overexpression of Rev-erbA alpha(Delta AB) i n myogenic cells had no impact on the ability of these cells to morpho logically or biochemically differentiate. Furthermore, this orphan rec eptor 1) down-regulated thyroid hormone receptor (TR)/T-3 mediated tra nsciptional activity from the myogenin promoter and thyroid hormone re sponse element (TRE) and 2) disrupted TR homodimer and TR/retinoid X r eceptor (RXR) heterodimer formation on a number of TREs found in the m yoD gene family. In conclusion, Rev-erbA alpha functions as a negative regulator of myogenesis by targeting the expression of the myoD gene family. The mechanism of action may involve inhibition of functional T R/RXR heterodimer formation on critical TREs and dominant trans-repres sion of gene expression.