The β₂-Subunit of Voltage-Gated Calcium Channels Regulates Cardiomyocyte Hypertrophy

L-type voltage-gated calcium channels (LTCCs) regulate crucial physiological processes in the heart. They are composed of the Ca_vα₁ pore-forming subunit and the accessory subunits Ca_vβ, Ca_vα₂δ, and Ca_vγ. Ca_vβ is a cytosolic protein that regulates channel trafficking and activity, but it also exerts other LTCC-independent functions. Cardiac hypertrophy, a relevant risk factor for the development of congestive heart failure, depends on the activation of calcium-dependent pro-hypertrophic signaling cascades. Here, by using shRNA-mediated Ca_vβ silencing, we demonstrate that Ca_vβ₂ downregulation enhances α1-adrenergic receptor agonist-induced cardiomyocyte hypertrophy. We report that a pool of Ca_vβ₂ is targeted to the nucleus in cardiomyocytes and that the expression of this nuclear fraction decreases during in vitro and in vivo induction of cardiac hypertrophy. Moreover, the overexpression of nucleus-targeted Ca_vβ₂ in cardiomyocytes inhibits in vitro-induced hypertrophy. Quantitative proteomic analyses showed that Ca_vβ₂ knockdown leads to changes in the expression of diverse myocyte proteins, including reduction of calpastatin, an endogenous inhibitor of the calcium-dependent protease calpain. Accordingly, Ca_vβ₂-downregulated cardiomyocytes had a 2-fold increase in calpain activity as compared to control cells. Furthermore, inhibition of calpain activity in Ca_vβ₂-downregulated cells abolished the enhanced α1-adrenergic receptor agonist-induced hypertrophy observed in these cells. Our findings indicate that in cardiomyocytes, a nuclear pool of Ca_vβ₂ participates in cellular functions that are independent of LTCC activity. They also indicate that a downregulation of nuclear Ca_vβ₂ during cardiomyocyte hypertrophy promotes the activation of calpain-dependent hypertrophic pathways.