In the present study, we investigated the relationship between lipid variables and the prevalence of PAD at baseline and its incidence during follow-up in individuals with type 2 diabetes. We observed lower prevalent and incident PAD in participants in the highest, compared with those in the lowest, tertiles of HDL-cholesterol and ApoA-I. In addition, the prevalence of major PAD increased in the upper tertiles of total cholesterol/HDL-cholesterol ratio, compared with the lowest tertiles, and the incidence of major PAD increased in the upper tertiles of both total cholesterol/HDL-cholesterol ratio and non-HDL-cholesterol. These associations were independent on putative confounding variables including key cardiovascular risk factors and were mainly reliable after treating all-cause death and coronary events as competing risks. Comparable results were observed when we considered lower-limb amputation and requirement of revascularisation individually as secondary endpoints.
Few studies have examined the relationship between lipoproteins and PAD in people with type 2 diabetes and results have been contrasting. Reduced HDL-cholesterol concentration was an independent risk factor for PAD in the UK Prospective Diabetes Study (UKPDS) [16]. In contrast, no independent association was observed between lipid variables and PAD in the Action in Diabetes and Vascular Disease: PreterAx and DiamicroN Modified-Release Controlled Evaluation (ADVANCE) and the Bypass Angioplasty Revascularisation Investigation in Type 2 Diabetes (BARI-2D) studies [17, 18]. The lipoproteins’ profiles (decreased HDL-cholesterol and increased LDL-cholesterol and atherogenic lipids) seem to be more consistent in the general population with PAD than in people with diabetes [19,20,21]. The definitions of PAD were comparable in studies of diabetic and non-diabetic individuals, varying from abnormal ankle–brachial index to symptomatic PAD including intermittent claudication and requirement of revascularisation [16,17,18,19,20,21]. The principal difference between the two populations could be that amputations and microvascular disease are more common in individuals with diabetes [3, 17], albeit the involvement of microvascular disease in PAD was also reported in individuals without diabetes [22].
Overall, the inverse association between plasma HDL-cholesterol and the risk of CVD is among the most consistent and reproducible associations in epidemiological studies [23,24,25] but whether this association is causal remains unclear. The most recognised function of HDL lipoproteins is reverse cholesterol transport, leading to the removal of excess cholesterol from peripheral tissues to the liver. The uptake and transport of cholesterol by HDL for hepatic excretion prevents and potentially reverses its peripheral accumulation in arteries [26,27,28]. HDL particles have been also associated with some other valuable effects, including antioxidative, antithrombotic, anti-inflammatory and vasodilatory functions [29]. Normalisation of HDL-cholesterol has been described as an unmet need in the management of patients with high cardiovascular risk, including type 2 diabetes [30], but Mendelian randomisation studies have generated scepticism about the hypothetical HDL causality [31, 32]. Furthermore, randomised clinical trials showed that drugs increasing plasma HDL-cholesterol did not reduce the risk of atherosclerotic CVD. Cholesteryl ester transfer protein (CETP) inhibitors have either had negative, neutral or only minor beneficial effects on cardiovascular outcomes despite substantial increase in HDL-cholesterol levels [33,34,35,36]. The paradoxical inability of HDL-cholesterol-raising therapies to reduce cardiovascular adverse events may be explained by a highly complex and multifunctional biology of the HDL lipoprotein system. A recent study has shown significant associations between dysfunctional HDL particles and increased risk of acute coronary syndrome and its manifestations in individuals at high cardiovascular risk [37]. Treatments targeting HDL functions could be a potential therapeutic approach. Meanwhile, HDL-cholesterol measurement remains a key component of CVD risk stratification and is still recommended as such [38].
As far as we know, this is the first investigation of the relationship between plasma concentrations of apolipoproteins and the risk of major PAD in a prospective cohort of people with type 2 diabetes. Class A apolipoproteins are the major structural and functional protein components of HDL; they stabilise HDL lipoprotein structure, solubilise their lipid component and help in reverse cholesterol transport. They also act as ligands for cellular receptor binding and enzyme activators or inhibitors. ApoA-I accounts for approximately 70% of HDL structure while ApoA-II corresponds to about 20%. Our findings pointed out an independent and reliable association between high ApoA-I concentrations and reduced risk of major PAD. Plasma concentrations of ApoA-II were also inversely associated with a greater prevalence of PAD at baseline and increased risk of incident PAD during follow-up but the latter association was not consistent and was mainly dependent on confounding variables. We did not observe evidence for significant interaction between HLD-cholesterol and ApoA-I in their association with the risk of major PAD, suggesting that these lipid variables may interact differently on this condition, although our data cannot allow any mechanistic conclusion. ApoA-I has also been linked to CVD [25, 37]; a large meta-analysis emphasised not only an inverse association between ApoA-I and a reduction of major cardiovascular events but also showed that increase in ApoA-I concentrations led to decreased cardiovascular risk among statin-treated patients [25].
We did not observe significant association between triacylglycerol, ApoB-100 or Lp(a) and major PAD. Also, plasma concentrations of LDL-cholesterol were not significantly associated with increased risk of major PAD during follow-up (p = 0.05). The fact that LDL-cholesterol was not measured in our cohort but was only estimated using the Friedewald formula after excluding participants with high levels of triacylglycerol may have mitigated our results. Nevertheless, our findings are consistent with those of a recent prospective study reporting a significant association between low standard plasma concentration of HDL-cholesterol (but not LDL-cholesterol) and incident PAD events among women without known CVD at baseline [21]. However, this study showed strong associations between excess incident PAD and a series of atherogenic lipidomic features: reduced HDL; and elevated LDL particles, small LDL particles and medium and very large VLDL particles. Of note, we have observed an increased risk of incident major PAD in the top tertile of non-HDL-cholesterol, which estimates total concentrations of all atherogenic ApoB-containing lipoproteins including triacylglycerol-rich particles in VLDLs and their remnants. This association was independent of confounders without evidence for significant interaction with HDL-cholesterol, ApoA-I or use of statins. However, this association did not persist after treating all-cause death or coronary events as competing risks, although having a similar magnitude to the association observed in the primary analyses. In addition, plasma concentrations of non-HDL-cholesterol was not associated with prevalent PAD. This difference cannot be explained by the baseline characteristics of participants in the prevalent and the incident PAD groups, as these were roughly comparable. A potential explanation for this difference is that incident PAD seemed to be more likely related to large-vessel disease than prevalent PAD. Indeed, the requirement of revascularisation accounted for 72% of incident PAD while limb loss was more frequent at baseline (5.0% vs 3.9% at the time of endpoint), including mainly minor amputation (74% at baseline vs 45% at the time of endpoint). Additionally, arterial stenosis with significant haemodynamic effects was observed in 95% of amputees at the time of endpoint (vs 59% at baseline). Taken together, these findings suggest that increased non-HDL-cholesterol could mainly reflect a high risk of macrovascular disease in patients with PAD. Non-HDL-C has been suggested as a pragmatic and cost-effective cardiovascular biomarker, especially in people with type 2 diabetes [38, 39].
Our study also highlights the total cholesterol/HDL-cholesterol ratio as a strong and consistent lipid biomarker for the risk of major PAD. A high total cholesterol/HDL-cholesterol ratio was associated with both prevalent and incident PAD. This association was independent of relevant confounders, persisted when we dealt with all-cause death or coronary events as competing risk, and was also reliable when we considered incident lower-limb amputation and revascularisation individually. These findings are consistent with those of an earlier study reporting total cholesterol/HDL-cholesterol ratio as a strong and independent predictor of PAD in a nested case–control cohort from the Physicians’ Health Study [20].
The key strength of our work is the investigation of a prospective cohort of individuals with type 2 diabetes collecting a wide range of clinical and biological features at baseline with adjudicated outcomes during a median follow-up of 7 years. We have measured a set of lipid and apolipoprotein compounds, which may reflect at least partly a factual picture of lipoproteins’ profile in individuals with type 2 diabetes. However, our study may not be representative of all populations with type 2 diabetes as SURDIAGENE is a French mono-centre inpatient cohort. Also, 45% participants were on statin therapy at baseline, which may influence our findings as this treatment reduces the risk of PAD events [40]. However, all our analyses were adjusted for statin use and we did not observe significant interaction between statin use and relevant lipids biomarkers in their relationship with major PAD. On the other hand, we assessed only baseline use of statin, leaving some uncertainty about potential increase in statin use during follow-up that may possibly bias our results. The issue is that we cannot assess time-varying hazards as we do not have data regarding the use of statins over time. Our investigation may also omit potential association between lipid variables and early stages of PAD as we have evaluated only advanced PAD-related events. Finally, we did not have accurate and comprehensive data regarding peripheral neuropathy. Of note, all amputees had a strong evidence of PAD (abolition of peripheral pulses, intermittent claudication or lower-limb artery stenosis with haemodynamic effects). At the same time, peripheral diabetic neuropathy and foot infection were reported in 51–76% of amputees, supporting the notion that lower-limb amputation is a dramatic consequence of several concomitant complications including microvascular, macrovascular and infectious disease.
In conclusion, we have observed independent and reliable associations between plasma concentrations of HDL-cholesterol, total cholesterol/HDL-cholesterol ratio and ApoA-I and the prevalence at baseline and the incidence during follow-up of major PAD in individuals with type 2 diabetes. Increased non-HDL-cholesterol concentrations were also associated with increased incidence of major PAD. Our findings may help to identify a specific lipoprotein’s profile in individuals with type 2 diabetes at high risk of major PAD.