Increased levels of apolipoprotein C3 (APOC3) in the blood predict cardiovascular disease risk in people with and without diabetes 1–4. Conversely, APOC3 loss-of-function variants are associated with cardioprotection in humans 5–7. The mechanisms whereby APOC3 promotes atherosclerotic cardiovascular disease risk have been the topic of intense studies. It is known that APOC3 slows the clearance of triglyceride (TG)-rich lipoproteins (TRLs) and their remnant lipoprotein particles (RLPs) by inhibiting the activity of lipoprotein lipase and by preventing hepatic uptake of TRLs and RLPs, leading to increased levels of TRLs and their remnants in circulation 8. The atherogenic effects of APOC3 are likely mediated, at least in part, by its effects on TRL metabolism, but recent findings that APOC3 predicts future coronary artery disease events in individuals with type 1 diabetes (T1D), who often have normal plasma lipid profiles 1, 2, suggest that other mechanisms might be at play. As cardiovascular disease is accompanied by heightened inflammation, and the recent Canakinumab Anti-inflammatory Thrombosis Outcome Study highlighted a causal role for the cytokine interleukin-1β (IL-1β) in incident cardiovascular disease 9, a possible proinflammatory role for APOC3 has generated much interest. Zewinger and colleagues elegantly demonstrated that delipidated APOC3 induces IL-1β release from human monocytes without requiring priming by lipopolysaccharide (LPS) 10. The mechanism involved APOC3-induced dimerization of toll-like receptor (TLR) 2 and TLR4, Ca2+-dependent superoxide production and activation of caspase 8 via an alternative NOD-, LRP-and pyrin domain-containing protein 3 (NLRP3) inflammasome activation pathway. The ability of native and delipidated very low-density lipoprotein (VLDL, a TRL) to mimic the effects of APOC3 on IL-1β release led to the conclusion that APOC3 bound to VLDL activates the NLRP3 inflammasome. However, we find that the ability of APOC3 to induce IL-1β release from human and mouse monocytes is dependent on the lipidation state of APOC3, and that only delipidated APOC3 is able to induce IL-1β release.

We show that both delipidated APOC3 (4 mg dl–1) and ultrapure LPS induce a striking increase in IL-1β release from human blood monocytes without prior priming of the NLRP3 inflammasome, consistent with the study by Zewinger and colleagues (Fig. 1a). However, this effect was not mimicked by human VLDL, which contained sufficient APOC3 (0.35 mg dl–1) to induce IL-1β release (Extended Data Fig. 1a). To investigate whether the APOC3-induced alternative inflammasome pathway is unique to human monocytes, we isolated mouse bone marrow monocytes, using an immunomagnetic negative selection method. As in the human monocytes, delipidated APOC3 (0.004–4 mg dl–1) induced increases in IL-1β release from mouse monocytes (Fig. 1b and Extended Data Fig. 1a, b). Again, VLDL was unable to induce IL-1β release from these cells.