Our results show that TNFi produce a net beneficial effect in the lipoprotein profile of RA patients which may lead to a lower atherosclerotic risk. However, this biochemical effect is generally overlooked given that their effects seem to be centered in several aspects related with VLDL particles which are not routinely determined.
The VLDL particles are produced by the liver and are triglyceride rich. They contain various apolipoprotein and Apo B-100 is the core structural protein, each VLDL particle contains one Apo B-100 molecule. These nascent VLDL are in charge of transporting these lipids from the liver to the peripheral tissues where they suffer the lipolysis of the triglycerides(22, 23). The nascent VLDL particles do not portend a high atherogenic risk since they have a low content in cholesterol and a bigger molecular size than other lipoproteins, which makes difficult that these molecules can go through the endothelial cell barrier into the arterial walls. At the peripheral level, the triglyceride depletion of VLDL produces remnant VLDL molecules of smaller size and enriched in cholesterol concentration. Some of these particles are transformed into the LDL by the action of enzymes as hepatic lipase (HL) which totally depletes them from triglycerides and cholesteryl ester transfer protein (CEPT) that allows them to capture esterified cholesterol molecules(24, 25). On the other hand, part of the remnant VLDL remain in the circulation becoming highly atherogenic particles. A body of evidence exists pointing out an important role of Triglyceride-rich lipoproteins, and especially VLDL in cardiovascular risk (26, 27). Moreover, some recent studies have shown that this subfraction of lipoprotein is especially relevant for the cardiovascular risk in those individuals with LDL level below of 130 mg/dl, which is part of remnant cardiovascular risk(28).
According to the data of our study, RA patients on TNFi therapy present lower levels of atherogenic VLDL related parameters including the concentration of total VLDL-cholesterol, VLDL-proteins and specifically VLDL-Apo B, total VLDL mass, number of cholesterol molecules per VLDL particle and number of VLDL particles. Besides, regarding the last-mentioned parameter, it has been underscored that the number of particles is a more accurate measurement than the cholesterol lipoprotein concentration to assess the cardiovascular risk. A higher number of particles is usually associated with a smaller molecular size and the smaller the lipoproteins are the more atherogenic they become(29).
Regarding the rest of main lipoproteins our study showed less pronounced effects. HDL is a lipoprotein which consists of a core of hydrophobic lipids, including cholesteryl esters and triglycerides, and a surface monolayer containing phospholipids, free cholesterol, and apolipoproteins(30). Apo A is, largely, the most abundant protein, representing 70% of the total. In addition, HDL contain the enzyme PON 1 that is responsible for the antioxidant properties of the HDL. Thus, this lipoprotein exerts anti-inflammatory and athero-protective effects in healthy individuals due to its capacity of removing the excess cholesterol from macrophages of the arterial wall as well as preventing LDL particles of being oxidized(31). However, in several chronic inflammatory conditions such as RA, HDL may turn out into a dysfunctional state losing these protective functions and shifting to a more proinflammatory phenotype(13). In these situations, it has been observed that Apo A is displaced by SAA, that is elevated in chronic inflammatory states, and at the same time is largely oxidized, whereas there is also a decrease of PON activity. All these changes lead to a minor cholesterol efflux activity and a lower anti-oxidative capacity(32, 33). We did not assess functional HDL activity in our study and therefore we cannot know whether TNFi help to reverse the potential proinflammatory phenotype of HDL. Nevertheless, we did not observe major changes in the HDL composition. Our results only showed a significant decrease in HDL mass mainly due to a decrease in the protein content. We may speculate that this protein depletion might be due to a decrease in the HDL-SAA concentration given that we did not see significant differences in the HDL-Apo A levels (the other preponderant protein). Moreover, we observed that TNFi associate a borderline significant increase in the Apo A serum levels along with a no significant trend (by parametric analysis) to lower serum levels of SAA; in fact when a non-parametric analysis as Mann Whitney U test was performed, given the distribution skewness of this variable, the difference of serum SAA became significant (p = 0.001; data not showed). On the other hand, we did not observe differences in the serum concentration of PON (data not shown), although no anti-oxidative capacity specific tests were performed.
As it has been stated, LDL particles are produced from the VLDL. They are rich in cholesterol and Apo B is their main protein(34) and carry the highest risk for atherosclerosis. These particles form a relatively heterogeneous population of lipoproteins with different sizes and density being those smaller and denser more atherogenic. According our results, the only relevant change observed in the TNFi subset of patients in the LDL composition was a significant increase of triglycerides. Although, the triglyceride enrichment of theses particle might be associated to less penetration and deposit of particles under endothelium, and consequently a decrease in the cardiovascular risk, this fact has not yet clearly established (35). We did also observe a non-significant increase in the number of particles in patients treated with TNFi, which is difficult to interpret at this moment.
Finally, we measured serum levels of Lp (a) other cholesterol rich and atherogenic lipoprotein, but no significant differences were found. Lp (a) serum level are genetically determined and minor changes are expected through an individual's lifetime, except for acute inflammatory states. Treatment with statins do not modify circulating levels of Lp(a), and only recently some data have been published notifying a decrease in Lp(a) concentration in patients treated with PCSK9 inhibitors. Our results suggest that treatment with TNFi, despite their effect on the inflammatory state, do not seem to alter Lpa levels (36).
We recognize that our study has several limitations derived from its transversal design. However, we have tried to overcome in part such problems using multivariable models for adjusting by potential confounders as demographic factors, disease activity, or use of statins for example. The relatively limited sample size can also pose some concerns, especially when multiple comparisons are performed. Again, we think that the use of multivariate models may circumvent somehow this issue. Finally, as it has been mentioned functional analysis of HDL oxidative capacity are lacking but we focus our study in an in-depth analysis of the composition and structure of the main lipoprotein using determinations that to our knowledge has not been previously reported in these types of patients.
In conclusion, we are reporting that in RA patients treatment with TNFi produce beneficial effects at the lipoprotein level and from an atherogenic point of view with triglycerides enrichment of the LDL particles and, mainly, smaller remnant atherogenic risk with lower VLDL-cholesterol, VLDL-Apo B, total VLDL mass, number of cholesterol molecules per VLDL particle and number of VLDL particles. All this positive effect remains hidden using conventional lipid determinations and may provide some mechanistic explanations for the cardioprotective effects that these agents have shown in clinical registries.