β2-Adrenergic receptor overexpression driven by α-MHC promoter is downregulated in hypertrophied and failing myocardium

Objective: The α-myosin heavy chain (α-MHC) promoter is frequently used to direct cardiac specific transgene expression. We studied whether transgene expression controlled by this promoter was altered under conditions of cardiac hypertrophy and failure. Methods: Transgenic (TG) mice overexpressing human β2-adrenergic receptors (β2AR) and wild type (WT) controls were subjected to thoracic aortic constriction (TAC) or sham operation and studied at 1, 3 and 8 weeks after surgery. Results: Sham operated TG mice had higher heart rates and left ventricular (LV) contractility than WT (all P <0.01), demonstrating enhanced βAR activation. TAC at 1, 3 and 8 weeks produced progressive LV hypertrophy which was similar between WT and TG mice. Evidence of heart failure was more marked in TG mice with a greater increase in weights of the right ventricle and lungs and a higher prevalence of atrial thrombus ( P <0.05 in each case). In hypertrophied TG hearts, endogenous α-MHC mRNA transcripts in LV were maintained at 1 and 3 weeks, but were reduced by approximately 40% relative to the sham-operated group at 8 weeks after TAC. Transgene expression, measured as human β2AR mRNA, was reduced by 45% at 1 and 3 weeks and by 70% at 8 weeks after TAC. β2AR binding sites were reduced by 35, 47 and 65%, respectively, at 1, 3 and 8 weeks. Conclusion: Cardiac hypertrophy and failure cause downregulation of the endogenous α-MHC as well as cardiac specific overexpression of the transgene directed by an α-MHC promoter.