Autophagy regulates cellular senescence by mediating the degradation of CDKN1A/p21 and CDKN2A/p16 through SQSTM1/p62-mediated selective autophagy in myxomatous mitral valve degeneration

Myxomatous mitral valve degeneration (MMVD) is one of the most important age-dependent degenerative heart valve disorders in both humans and dogs. It is characterized by the aberrant remodeling of extracellular matrix (ECM), regulated by senescent myofibroblasts (aVICs) transitioning from quiescent valve interstitial cells (qVICs), primarily under TGFB1/TGF-β1 control. In the present study, we found senescent aVICs exhibited impaired macroautophagy/autophagy as evidenced by compromised autophagy flux and immature autophagosomes. MTOR-dependent autophagy induced by rapamycin and torin-1 attenuated cell senescence and decreased the expression of cyclindependent kinase inhibitors (CDKIs) CDKN2A/p16INK4A and CDKN1A/p21CIP1. Furthermore, induction of autophagy in aVICs by ATG (autophagy related) gene overexpression restored autophagy flux, with a concomitant reduction in CDKN1A and CDKN2A expression and senescence-associated secretory phenotype (SASP). Conversely, autophagy deficiency induced CDKN1A and CDKN2A accumulation and SASP, whereas ATG re-expression alleviated senescent phenotypic transformation. Notably, CDKN1A and CDKN2A localized to autophagosomes and lysosomes following MTOR antagonism or MG132 treatment. SQSTM1/p62 was identified as the autophagy receptor to selectively sequester CDKN1A and CDKN2A cargoes for autophagic degradation. Our findings are the first demonstration that CDKN1A and CDKN2A are degraded through SQSTM1-mediated selective autophagy, independent of the ubiquitin-proteasome pathway. These data will inform development of therapeutic strategies for the treatment of canine and human MMVD, and for the treatment of Alzheimer disease, Parkinson disease and other age-related degenerative disorders.