Failure of efficient cardiac proteostatic adaptations to chronic cAMP-stress is associated with accelerated heart aging

Dysregulated proteostasis is a hallmark of aging. We investigated how efficiently proteostatic adaptations to chronic cardiac cyclic-adenosine-monophosphate (cAMP)-dependent stress change with aging in mice harboring marked cardiac-specific over-expression of adenylyl cyclase VIII (TG^AC8). We assessed protein quality control mechanisms (PQC) (ubiquitin proteasome system, autophagic flux via macroautophagy, and mitophagy) in left ventricles of TG^AC8 and wild-type littermates (WT) at 3-4 and 17-21 months of age. At 3-4 months, TG^AC8 exhibited markers of increased autophagic flux (microtubule-associated protein 1A/1B light chain 3B (LC3), p62, and their phospho-forms) and enhanced canonical mitophagy signaling (PARKIN, p62^S405 and p62^S349 receptors), confirming a more efficient proteostasis, vs WT. In aged TG^AC8, however, the PQC mechanisms were overwhelmed by proteotoxic stress, manifested in insufficient proteasome activity, slower autophagic flux, and increased mitochondrial dysfunction (network fragmentation). The accumulation of protein aggregates (increased ratio of insoluble/soluble protein fractions), of lipofuscin bodies and of desmin cardiac preamyloid oligomers, and of LC3⁺- and p62⁺-inclusions of aberrant sizes was increased in aged TG^AC8 compared to young TG^AC8. Thus, while increased proteostatic mechanisms maintain cardiac health in TG^AC8 in youth (3-4 months), long-term exposure to sustained activation of the AC/cAMP/PKA/Ca²⁺ signaling axis results in severe proteostasis insufficiency in aged TG^AC8, leading to cardiomyopathy and accelerated cardiac aging.