Research ArticleVASCULAR BIOLOGY

Inclusion bodies enriched for p62 and polyubiquitinated proteins in macrophages protect against atherosclerosis

Sci. Signal.  05 Jan 2016:
Vol. 9, Issue 409, pp. ra2
DOI: 10.1126/scisignal.aad5614

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Clearing proteins to limit atherosclerosis

The release of proinflammatory cytokines, such as IL-1β, by macrophages increases the size and number of atherosclerotic plaques. Macrophages in atherosclerotic plaques have a defect in autophagy, a process that eliminates dysfunctional proteins, and Sergin et al. showed that p62, a chaperone protein involved in autophagy, sequestered polyubiquitinated proteins in cytoplasmic inclusion bodies in macrophages. Macrophages lacking p62 released more IL-1β, and one of the proteins required for the production of IL-1β partially colocalized with these inclusion bodies. In a mouse model of atherosclerosis, p62 deficiency increased macrophage infiltration in atherosclerotic plaques and exacerbated atherosclerosis. Thus, enhancing the function of p62 to promote the sequestration of polyubiquitinated proteins could prevent macrophages from exacerbating atherosclerosis.

Abstract

Autophagy is a catabolic cellular mechanism that degrades dysfunctional proteins and organelles. Atherosclerotic plaque formation is enhanced in mice with macrophages deficient for the critical autophagy protein ATG5. We showed that exposure of macrophages to lipids that promote atherosclerosis increased the abundance of the autophagy chaperone p62 and that p62 colocalized with polyubiquitinated proteins in cytoplasmic inclusions, which are characterized by insoluble protein aggregates. ATG5-null macrophages developed further p62 accumulation at the sites of large cytoplasmic ubiquitin-positive inclusion bodies. Aortas from atherosclerotic mice and plaques from human endarterectomy samples showed increased abundance of p62 and polyubiquitinated proteins that colocalized with plaque macrophages, suggesting that p62-enriched protein aggregates were characteristic of atherosclerosis. The formation of the cytoplasmic inclusions depended on p62 because lipid-loaded p62-null macrophages accumulated polyubiquitinated proteins in a diffuse cytoplasmic pattern. Lipid-loaded p62-null macrophages also exhibited increased secretion of interleukin-1β (IL-1β) and had an increased tendency to undergo apoptosis, which depended on the p62 ubiquitin-binding domain and at least partly involved p62-mediated clearance of NLRP3 inflammasomes. Consistent with our in vitro observations, p62-deficient mice formed greater numbers of more complex atherosclerotic plaques, and p62 deficiency further increased atherosclerotic plaque burden in mice with a macrophage-specific ablation of ATG5. Together, these data suggested that sequestration of cytotoxic ubiquitinated proteins by p62 protects against atherogenesis, a condition in which the clearance of protein aggregates is disrupted.

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