There is growing interest in the role of triglyceride-rich lipoproteins, namely chylomicrons and very low-density lipoproteins (VLDL), as mediators of residual cardiovascular risk [1,2]. Lipoprotein lipase (LPL) plays a central role in the clearance of these atherogenic lipoproteins from the circulation. More precisely, LPL is the primary enzyme that hydrolyzes lipoprotein triglycerides (TG), releasing fatty acids for utilization by tissues [[3], [4], [5]]. LPL undergoes tissue-specific and calorically sensitive regulation by multiple proteins, including the angiopoietin like proteins 3, 4, and 8 (ANGPTL3/4/8) [[3], [4], [5]]. These proteins are crucial for the tissue-specific utilization of VLDL and chylomicron TG in either adipose or oxidative tissues, depending on the nutritional state.

According to current thinking, in the postprandial state, ANGPTL8 forms a circulating complex with ANGPTL3 (the ANGPTL3/8 complex) that greatly increases the ability of ANGPTL3 to inhibit LPL activity in oxidative tissues so that TG can be primarily used for storage in adipose tissue [[3], [4], [5]]. In contrast, ANGPTL4 potently inhibits LPL in adipose tissue in the fasting state so that TG are routed to oxidative tissues for energy use. After feeding, ANGPTL8 forms a localized complex with ANGPTL4 (the ANGPTL4/8 complex) in the fat that decreases the LPL-inhibitory effect of ANGPTL4 to preserve maximal LPL activity so that TG can be efficiently stored in adipose tissue [[3], [4], [5]].

The mechanism by which ANGPTL8 protects LPL from inhibition by ANGPTL4 and other LPL inhibitors involves the conversion of ANGPTL4, which is a furin substrate, to ANGPTL4/8, which becomes a plasmin substrate [6]. This enables the binding of tissue plasminogen activator (tPA) and plasminogen to ANGPTL4/8, which stimulates the generation of plasmin [6]. The plasmin generated then cleaves the ANGPTL4/8 complex itself, as well as other LPL inhibitors including ANGPTL3/8 and apolipoprotein C3 to ensure that LPL is fully active in the fat after feeding [7].

Cleavage of the ANGPTL4/8 complex by plasmin, as well as cleavage of ANGPTL4 by furin, generate a 29 kD C-terminal domain-containing ANGPTL4 fragment (CD-ANGPTL4) that is the predominate form of ANGPTL4 found in human serum [6]. In the previously described LURIC and getABI studies, we observed that circulating levels of the ANGPTL4/8 complex, and especially CD-ANGPTL4, were positively associated with cardiovascular mortality and fatal myocardial infarctions [8]. The mechanisms accounting for this association, which may involve diabetes and inflammation, remained incompletely understood.

Interestingly, serum levels of the ANGPTL3/8 complex and ANGPTL3 were not consistently associated with cardiovascular mortality in the LURIC and getABI studies, despite positive associations that were especially observed between the ANGPTL3/8 complex and TG, low-density lipoprotein cholesterol (LDL-C), and remnant cholesterol [8]. This was somewhat surprising since ANGPTL3/8 was noted to be a slightly more potent inhibitor of LPL enzymatic activity than N-terminally intact ANGPTL4 and circulates at much higher levels [9].

Significant knowledge is accumulating regarding human genetics of the ANGPTL3/4/8 protein family. There is solid evidence that genetic inactivation of ANGPTL3, ANGPTL4, and ANGPTL8 is associated with reduced cardiovascular risk [[10], [11], [12]]. In addition, Mendelian randomization studies have suggested that ANGPTL4 is associated with aortic valve calcification and have provided further evidence that therapies to reduce ANGPTL3 and ANGPTL4 may decrease the risk of coronary artery disease and diabetes [[13], [14], [15]]. Genetic mimicry studies have suggested that ANGPTL4 inhibition might be associated with a decreased risk of heart disease and diabetes through mechanisms involving LPL, while ANGPTL3/8 and ANGPTL3 may impact cardiac risk via LPL and endothelial lipase (EL), with ANGPTL3 variants being associated with decreased levels of LDL-C and apolipoprotein B (ApoB) [[16], [17], [18], [19]].

To better elucidate the potential pathophysiology of the positive associations of ANGPTL4/8 and CD-ANGPTL4 with cardiovascular mortality, we aimed to study the impact of ANGPTL4/8 and CD-ANGPTL4 (as well as ANGPTL3/8 and ANGPTL3) on the progression of coronary atherosclerosis. Moreover, we sought to further study the associations of each of these ANGPTL proteins and complexes with coronary events. To do this, we measured ANGPTL3, ANGPTL3/8, CD-ANGPTL4, and ANGPTL4/8 in the Heinz-Nixdorf Recall (HNR) study. In this study, 4814 participants without a history of coronary artery disease (CAD) at baseline were followed over a 5-year period for coronary artery atherosclerosis progression, with subsequent, continued follow-up for occurrence of coronary events [[20], [21], [22], [23], [24], [25]].