Mechanistic Insights Into Lipoprotein(a): From Infamous to 'Inflammous'

Stefan Coassin; Florian Kronenberg


Eur Heart J. 2020;41(24):2272-2274. 

The ups and downs of lipoprotein(a) [Lp(a)] in the last 30 years of atherosclerosis research are considerable. They reach from a first enthusiasm in the 1980s to an almost infamous end in the early 1990s caused by flawed epidemiological studies, to a resurrection by the first Mendelian randomization studies.[1] Now, on the eve of the introduction of potent Lp(a)-lowering drugs, the interest in Lp(a) has been growing dramatically, although we still do not understand many aspects of the role of Lp(a) in the development of atherosclerosis. In a series of publications during the last years, the group around Eric Stroes and colleagues has elegantly investigated the influence of Lp(a) on arterial wall inflammation and monocyte trafficking to the arterial wall.

Indeed, the hypothesis that monocytes play a role in Lp(a) pathophysiology has a long history. Lp(a) has been shown early on to be a potent chemoattractant for monocytes,[2,3] and the first work in single to a few patients already proposed an impact of Lp(a) on the gene expression profiles of monocytes.[4] The connection between the innate immune system and Lp(a) has been further strengthened by the identification of Lp(a) as the major plasma carrier of oxidized phospholipids (OxPLs). These oxidized lipid species are recognized by pattern recognition receptors of innate immune cells and trigger the whole cascade of inflammatory processes that can finally lead to plaque destabilization.[5]

Four years ago in the first work of a series of publications investigating the impact of Lp(a) and LDL cholesterol on arterial wall inflammation, the group around Eric Stroes showed that individuals with elevated Lp(a) concentrations have increased arterial inflammation and enhanced peripheral blood mononuclear cell trafficking to the arterial wall compared with subjects with normal Lp(a). Monocytes isolated from subjects with high Lp(a) concentrations showed an increased capacity to transmigrate the endothelium and produce proinflammatory cytokines which was mediated by OxPLs (Figure 1).[6] Accordingly, blocking of the OxPLs using a specific antibody also reduced the proinflammatory responsiveness of the monocytes.[6] These findings proposed a further mechanism as to how Lp(a) might mediate cardiovascular disease and added a new layer to our still shallow understanding of the exact pathophysiological mechanisms by which high Lp(a) causes cardiovascular disease.

Figure 1.

Schematic illustration how Lp(a) and a marked therapeutic reduction of Lp(a) might be involved in the arterial wall inflammation: Individuals with high Lp(a) have significantly activated monocytes, which results in an increased transendothelial migration of the monocytes, an increased cytokine release and finally inflammation of the arterial wall.6 The study by Stiekema and colleagues9 demonstrates that high Lp(a) is associated with a pro-inflammatory transcriptome in monocytes. Treatment with different regimes of apo(a) antisense therapy against the apolipoprotein(a) component of Lp(a) resulted in a pronounced reduction of Lp(a) concentrations, a reduced pro-inflammatory signature in the transcriptome, a reduced transendothelial monocyte migration capacity as well as a reduced expression of chemokines and toll-like receptors on the monocytes' surface.9 This is expected to result in an improved vascular endothelial function and less cardiovascular events.

In a further randomized, double-blind, placebo-controlled short-term trial, the same group showed that 14 weeks of treatment with the PCSK9 (proprotein convertase subtilisin/kexin type 9) inhibitor alirocumab in statin-intolerant patients with high cardiovascular risk resulted in a robust LDL cholesterol-lowering response with a concomitant reduction in arterial wall inflammation (–8.2% compared with placebo, P = 0.05) but no changes in the inflammatory plasma markers.[7]

Concomitantly, in another double-blind, placebo-controlled study, 129 patients with elevated Lp(a) concentrations (median 80 mg/dL) and mean LDL cholesterol concentrations of 144 mg/dL were randomized to monthly evolocumab or placebo. This therapy, lasting 16 weeks, reduced LDL cholesterol by >60% to a mean of 60 mg/dL and Lp(a) slightly by 14% to a median of 75.2 mg/dL. However, in contrast to the effect observed in the previous study with alirocumab,[7] and despite the pronounced LDL cholesterol reduction, the arterial wall inflammation was not significantly altered when compared with the placebo group (–3.0%, P = 0.18). The authors suggested that the persistent elevated Lp(a) levels may have contributed to the unaltered arterial wall inflammation.[8]

In the newest investigation by Stiekema et al.[9] presented in this issue of the European Heart Journal, the same group now adds a further piece to the puzzle: they examined whether patients with cardiovascular disease and elevated Lp(a) concentrations (median 82 mg/dL) experience anti-inflammatory effects following large reductions of Lp(a) by apolipoprotein(a) [apo(a)] antisense therapy. They showed in a first step that circulating monocytes of healthy individuals and patients, both having high Lp(a) concentrations, are characterized by a markedly proinflammatory gene expression profile with several pathways of the innate immune system being up-regulated. This resembles recent observations of a more proatherogenic monocyte subset distribution in individuals with high Lp(a) concentrations.[10] Most interestingly, in an intervention, the authors investigated the effect of Lp(a) lowering on gene expression and monocyte function in 14 patients with cardiovascular disease who received apo(a) antisense therapy, as well as in 18 patients with PCSK9 antibody therapy. They showed that the antisense therapy with a 47% lowering of Lp(a) concentrations was indeed capable of reversing the proinflammatory gene expression signature to levels close to that of controls with normal Lp(a) concentrations (median 7 mg/dL). This was accompanied by a 22% functional reduction in transendothelial migration capacity of monocytes ex vivo (Figure 1). The PCSK9 antibody therapy reduced Lp(a) only by 16% and, most importantly, did not alter the transcriptome or the functional properties of monocytes, despite an additional reduction of LDL cholesterol by 65%.[9] These data suggest that a pronounced and not only a moderate Lp(a) lowering is required to reduce the proinflammatory state of circulating monocytes in patients with elevated Lp(a) concentrations and so provide a further piece for the puzzle of by how much Lp(a) needs to be lowered. Unfortunately, from the mechanistic side, one further puzzle piece is still missing to date. This would be a direct comparison of the effects of PCSK9 inhibition in individuals with high and low Lp(a) concentrations matched by LDL cholesterol concentrations in order to disentangle whether the absence of arterial wall inflammation reduction by PCSK9 inhibitors is truly caused only by high Lp(a) concentrations.

But what is 'a pronounced Lp(a) reduction' and what is 'a moderate Lp(a) reduction'? This has recently created a major discussion in the planning phase of specific Lp(a)-lowering therapies (reviewed in Kronenberg[11]). Mendelian randomization studies using genetic data from association studies with Lp(a) and LDL cholesterol concentrations as well as association studies with coronary heart disease were used to estimate the required Lp(a)-lowering effect size that might result in the same 22% lowering of coronary heart disease risk as a 38.67 mg/dL (1 mmol/L) therapeutic reduction in LDL cholesterol. The first study revealed the very high number of a 101.5 mg/dL Lp(a) lowering required.[12] This surprisingly high estimate was probably caused by the fact that the main study on which these results are based had median Lp(a) concentrations two- to three-fold higher compared with other studies of the same ethnicity. Indeed, subsequent studies using the same genetic approach proposed that a much smaller lowering of Lp(a) of ~65 mg/dL might be sufficient.[13,14] This was independent of whether data from population-based[13] or secondary prevention studies[14] were analysed. Post-hoc analysis of PCSK9 inhibitor trials also aimed to calculate whether the additional slight lowering of Lp(a) besides this drug's main target of LDL cholesterol has a beneficial effect on outcomes. Although these attempts have their own limitations, they suggest that a markedly less pronounced Lp(a) lowering is required to result in a clinical benefit.[11] In this context, it is interesting that the lower estimates of ~65 mg/dL are similar to the average reduction observed by Stiekema et al. in the apo(a) antisense-treated group (mean reduction of 51 mg/dL), suggesting that a reduction of such a magnitude indeed already produces a biological correlate. Whether this is sufficient to also produce an effect on the clinical endpoints remains to be seen and will hopefully be revealed by the ongoing clinical trials.

A second interesting aspect is that the authors had shown in previous work that monocytes exposed to plasma of individuals with high Lp(a) concentrations (50–195 md/dL, n = 30) remained in an activated state for at least 7 days and produced a stronger cytokine release than monocytes previously exposed to plasma with low Lp(a) concentrations. However, it has not been investigated how long this increased reactivity would persist. The present work shows that an isolated Lp(a) reduction is indeed capable of attenuating the proinflammatory gene expression signature of monocytes from high Lp(a) individuals within a relatively short period of time. Albeit no complete normalization was achieved within the observation period, the proinflammatory transcriptome profiles of monocytes derived from high Lp(a) individuals became closer to those with low Lp(a) concentrations within the relatively short period of 26 weeks. While this might be reassuring, it remains to be addressed how these effects will relate to the clinical endpoints.

Finally, a major issue still remaining in all Lp(a) research is the tremendous heterogeneity of the Lp(a) particle caused by the KIV-2 copy number polymorphism which can encompass up to 60–70% of the coding sequence. The various isoforms are associated with distinctive Lp(a) concentrations,[1] have been proposed to have different atherogenic potency,[5,15] and possibly show different interaction with receptors. Albeit most of the high Lp(a) individuals in the apo(a) antisense-treated group are likely to carry a small isoform, it may be of interest to determine whether the observed effects are purely mediated by the Lp(a) concentrations or whether the apo(a) isoform modifies the effect of Lp(a) reduction on vascular inflammation. The most likely causal factor for the monocyte activation by Lp(a)[6] are the OxPLs that are transported by Lp(a). It has been previously shown that the OxPL concentrations correlate much better with Lp(a) concentrations in carriers of small isoforms than in carriers of large isoforms.[16] This suggests that the reported higher atherogeneity of small isoforms stems at least partially from a higher load with OxPLs and might imply that the response of vascular inflammation to Lp(a) reduction could be modified by the apo(a) isoform. Future larger studies may be able to shed some light on this question and may add a new layer to the complex interplay of Lp(a) concentrations, apo (a) isoforms, and OxPL load, respectively the 'inflammous' response.