SQSTM1/p62 and PPARGC1A/PGC-1alpha at the Interface of Autophagy and Vascular Senescence

Defective macroautophagy/autophagy and mitochondrial dysfunction are known to stimulate senescence. The mitochondrial regulator PPARGC1A (peroxisome proliferator activated receptor gamma, coactivator 1 alpha) regulates mitochondrial biogenesis, reducing senescence of vascular smooth muscle cells (VSMCs); however, it is unknown whether autophagy mediates PPARGC1A-protective effects on senescence. Using <i>ppargc1a^−/-</i> VSMCs, we identified the autophagy receptor SQSTM1/p62 (sequestosome 1) as a major regulator of autophagy and senescence of VSMCs. Abnormal autophagosomes were observed in VSMCs in aortas of <i>ppargc1a^−/-</i> mice. <i>ppargc1a^−/-</i> VSMCs in culture presented reductions in LC3-II levels; in autophagosome number; and in the expression of SQSTM1 (protein and mRNA), LAMP2 (lysosomal-associated membrane protein 2), CTSD (cathepsin D), and TFRC (transferrin receptor). Reduced SQSTM1 protein expression was also observed in aortas of <i>ppargc1a^−/-</i> mice and was upregulated by PPARGC1A overexpression, suggesting that SQSTM1 is a direct target of PPARGC1A. Inhibition of autophagy by 3-MA (3 methyladenine), spautin-1 or <i>Atg5</i> (autophagy related 5) siRNA stimulated senescence. Rapamycin rescued the effect of <i>Atg5</i> siRNA in <i>Ppargc1a^+/+</i>, but not in <i>ppargc1a^−/-</i> VSMCs, suggesting that other targets of MTOR (mechanistic target of rapamycin kinase), in addition to autophagy, also contribute to senescence. <i>Sqstm1</i> siRNA increased senescence basally and in response to AGT II (angiotensin II) and zinc overload, two known inducers of senescence. Furthermore, <i>Sqstm1</i> gene deficiency mimicked the phenotype of <i>Ppargc1a</i> depletion by presenting reduced autophagy and increased senescence <i>in vitro</i> and <i>in vivo</i>. Thus, PPARGC1A upregulates autophagy reducing senescence by a SQSTM1-dependent mechanism. We propose SQSTM1 as a novel target in therapeutic interventions reducing senescence.