Cardiac remodeling and heart failure are characterized by misaligned adaptations in protein synthesis and degradation, harboring the therapeutic potential for interventional harmonization. Partial inhibition of protein degradation via the ubiquitin-proteasome system was reported to reduce acute hypertrophic remodeling as well as recover systolic function. The studies are suggestive of an inadequate gain in proteasome activity, contributing to cardiac remodeling and heart failure.
Aim was to identify a common mechanism promoting proteasome activity during cardiac remodeling and heart failure and to test whether it contributes to the pathogenesis.
Incorporation of the inducible proteasome subunit Lmp2 has been described to increase proteasome activity. In fact, it is increasingly expressed and incorporated in proteasome complexes during isoproterenol-induced hypertrophic remodeling, which is associated with a gain in proteasome activity. We found that Lmp2 is also increasingly expressed upon transverse aortic constriction and in a murine model of familial hypertrophic cardiomyopathy. Knock-out (KO) of Lmp2 in mice increased the caspase-like proteasome activity by 19% (n=5, p<0.05), but had no apparent influence on cardiac morphology or function. As predicted, lack of Lmp2 confined the regulatory capacity of proteasomes to gain activity during hypertrophic remodeling induced by isoproterenol. Unexpectedly, the restriction in additional activity did not reduce cardiac remodeling as previously described for proteasome inhibition. In contrast, lack of Lmp2 exacerbated hypertrophic remodeling and caused loss of systolic function (fractional shortening: -32% vs. wildtype, n≥8, p<0.01). Cardiomyocyte-specific expression of Lmp2 via gene transfer into adult Lmp2 KO mice completely rescued the isoproterenol-induced deterioration of cardiac remodeling and function. Interestingly, the deprived gain in proteasome activity in Lmp2 KO hearts rather affected a specific subset than the general pool of ubiquitinated proteins.
In conclusion, a gain in proteasome activity during acute cardiac remodeling is not entirely maladaptive. At least in part, it is dependent on Lmp2, controlling remodeling and preserving cardiac function.