The inflammatory protein Pentraxin 3 in cardiovascular disease

Although cholesterol accumulation intimal layer is the major pathological feature of atherogenesis, growing evidence suggests that the inflammatory state represents the major detrimental factor for the progression of atherosclerosis both in young and in elderly subjects. In fact, the earliest stage of atherosclerotic damage is characterized by infiltration of macrophages and T-lymphocytes, which are progressively activated during the course of the damage [17, 18]. Based on the kind of cell phenotype being recruited (macrophages and vascular cells), which is known to produce PTX3, this protein was investigated as a potential modulator of atherosclerosis.

In fact, human sclerotic mammary arteries possess high levels of PTX3, which is mainly localized within endothelial cells and macrophages [19, 20]. In addition, smooth muscle cells, treated with inflammatory stimuli such as oxidized lipoproteins, increase their PTX3 mRNA level. This effect leads to a vascular acute-phase-response activating the classic pathway of complement [21], which represents one of the most important mechanisms leading to chemo tactic and pro-inflammatory effects. The link between PTX3 and complement system was confirmed by the occurrence of high PTX3 level within atherosclerosis-related coronary arterial thrombi that are mainly constituted by resident macrophages, neutrophils and foam cells [22].

Recently, epidemiological and clinical data candidate PTX3 as a valid biomarker for atherosclerosis [19, 22]. In particular, PTX3 plays a role in the regulation of innate resistance to inflammatory reactions, it is strongly expressed in atherosclerotic arteries [23] and its high plasma levels were found to be related with severity of coronary atherosclerosis [24]. Accordingly, PTX3 levels are significantly increased in patients with carotid stenosis [25], as well as in patients with acute coronary syndrome [26], in whom PTX3 is a candidate biomarker for plaque vulnerability [27]. Finally, PTX3 has been indicated also as a marker of neo-intimal thickening after vascular injury, since elevated levels of PTX3 have been found in patients with atherosclerosis, after 15 min from coronary stenting [28].

Although the presence of PTX3 in human atherosclerotic damage is well defined, its causal role in the onset and progression of atherosclerosis, remains unclear. Here, data obtained in experimental models are reviewed aimed at suggesting the mechanistic role of PTX3 in atherogenesis.

In particular, it has been reported that PTX3, by interacting with P-selectin, a cell-adhesion molecule involved in the tethering and rolling of leukocytes and platelets on activated endothelial cells, attenuates leukocytes recruitment at the site of inflammation [29]. These data clearly indicate a protective role of PTX3 in atherosclerosis. In keeping with this, in a double knockout mouse model for PTX3 and apolipoprotein E (ApoE), macrophages infiltration was enhanced, and a dramatic increase of atherosclerosis was reported. The molecular analysis indicates a dramatic rise in the pattern of inflammatory genes within vascular wall concomitant with up-regulation of pro-inflammatory mediators such as chemokines, cytokines, adhesion molecules, E-selectin and transcription factors, such as NF-kB [30].

In contrast, PTX3 was also reported to induce deleterious effects in the pathogenesis of atherothrombosis [31]. On this regard, it has been demonstrated that PTX3 increases the tissue factor (TF) expression in mononuclear and endothelial cells. The increased level of TF, the main orchestrator of the coagulation cascade, causes the thrombus formation, a feature of atherosclerosis [32]. PTX3 might also interfere with plaque stability by binding the fibroblast growth factor 2 (FGF2), that play a role in proliferation and migration of smooth muscle cells [33]. This evidence suggests a bad cop for PTX3, which promotes lesion progression through a stronger innate immune response. Recently, PTX3 has been proposed as a potential therapeutic target to limit the development of atherosclerosis. In fact, by using a cell model of atherosclerosis, it was shown that suppression of PTX3 reduces inflammation and apoptosis mediated by the IkB kinase (IKK)/IkB/nuclear factor-kB (NF-kB) pathway [34].

Actually, the role of PTX3 in the atherosclerosis is not well-understood. It will be interesting to examine the role of the protein in complement activation during atherogenesis to define the specific role of PTX3 in the atherosclerosis. Overall, PTX3 seems to exert detrimental effects on atherosclerotic process since it promotes plagues formation and leads to an amplification of vascular inflammation such as to be candidate it as a new possible biomarker for plaque vulnerability. However, only in those conditions in which it is able to reduce macrophages infiltration and the expression of adhesion molecules it exerts beneficial effects.

Endothelial dysfunction is a typical trait of several cardiovascular disorders including arterial hypertension. The impairment of the nitric oxide pathway and the enhanced smooth muscle vasoconstriction represent the main mechanisms leading to endothelial dysfunction in hypertension [35, 36].

A growing body of evidence attributes to local and systemic inflammation a key role in the development of endothelial dysfunction, thus suggesting a key role for acute phase proteins [37, 38].

On this regard, high plasma levels of the inflammatory molecule PTX3 were associated with endothelial dysfunction in different human diseases. Patients with chronic kidney diseases and with preeclampsia, a multisystemic disorder associated with hypertension [39‐41], show elevated PTX3 plasma levels which were correlated with the severity of endothelial dysfunction [39, 40, 42].

In addition, elevated plasma level of PTX3 have been found in patients with high systolic and diastolic blood pressure levels [16, 43] and in elderly hypertensive patients with high 24-h blood pressure levels [14]. In pulmonary arterial hypertension, PTX3 has been proposed as a more specific and sensitive biomarker compared with brain natriuretic peptide, which so far, was considered the gold standard marker for pulmonary hypertension [44, 45].

The evidence described so far in humans suggests a potential role of PTX3 in regulating vascular homeostasis. In experimental models, the absence of PTX3 is accompanied by massive suppression of tissue inflammation, cell injury and it determines decreased lethality after reperfusion of an ischemic superior mesenteric artery in mice, thus demonstrating that PTX3 is relevant in conditioning tissue damage [46]. In a recent study by Carrizzo et al. we investigated the molecular mechanisms recruited by PTX3 to induce damage of mesenteric artery. In particular, exogenous administration of PTX3 in mice impaired endothelial function in resistance vessels through a P-selectin/matrix metalloprotease1 pathway, thus producing morphological alterations of endothelial cells and disruption of nitric oxide signalling. These effects were associated with an increase in blood pressure in vivo. It is worth to be mentioned that PTX3, and its mediators of vascular damage are present at higher levels within plasma of hypertensive patients compared with normotensive subjects [47]. Based on these data, PTX3 could be considered as a novel biomarker for hypertension and a new target for future therapeutic strategy aimed to contain endothelial dysfunction and associated CVD.

Myocardial infarction (MI) continues to be a significant cause of mortality and morbidity worldwide [48]. Over the past 50 years, it has become clear that the cascade of thrombotic events following atherosclerotic plaque rupture causes occlusion of the coronary artery, interrupting blood supply and oxygen supply to myocardium, thus producing infarction. The significance of occurrence of PTX3 expression in the heart from normal and hypertrophy human cardiac cells still needs to be clarified for its physiological pathological role [49, 50].

In older adults it was reported an association between PTX3 plasma levels, CVD and all causes of death, independently from CVD risk factors [25]. In particular, plasma level of PTX3 increases in patients with acute myocardial infarction (AMI) after about 7 h from the onset of symptoms, with a decrease at baseline levels after three days [50, 51]. Based on these data, PTX3 seems to be at the same time an early indicator of AMI, and also a prognostic marker for the outcomes of heart diseases. PTX3 level also predicts cardiac events in patients with heart failure (HF), suggesting a stratification of HF patients based on PTX3 plasma level [52]. This is fully supported by the identification of coronary circulation as the main source of PTX3 in HF patients with normal ejection fraction [53]. Moreover, in patients with MI, ST tract elevation and with chronic HF, PTX3, but not other cardiac biomarkers, modulating complement components, predicted 3 months mortality, after adjustment for major risk factors [12, 28, 54].

Recently, PTX3 was suggested to be a prognostic marker also in patients with coronary artery diseases after drug stent implantation, and in patients with angina pectoris [55‐57], in whom adverse cardiac events were related with PTX3 plasma levels [58].

These studies in humans, do not allow to establish the specific action of PTX3 in the myocardium. Thus, to better characterize the cardiac action of PTX3, here we report data obtained in transgenic ptx3 deficient mice. After cardiac ischemia- reperfusion injury, ptx3 deficient mice develop increased myocardial damage, characterized by no-reflow area, increased neutrophil infiltration apoptotic cells and decreased number of capillaries. In addition, in the myocardium of these mice the C3 complement component increased focally being related with the area of damaged myocardium. The evidence that in PTX3-KO mice the administration of exogenous PTX3 reduces complement C3 deposition rescuing the phenotype, highlights the cardio-protective effect of PTX3, through the modulation of the complement cascade [59].

The discovery of PTX3 in the myocardial tissue and the characterization of its role lead to propose PTX3 as an early indicator of myocyte irreversible injury in ischemic cardiomyopathy. In addition, considering the local production and the rapid change in plasma concentration, PTX3 could be considered a novel potential biomarker of myocardial infarction.

Additional activities of PTX3 other than those related to innate immunity and inflammation have been described, such as extracellular matrix deposition (ECM), tissue remodeling, and angiogenesis. Several studies demonstrate that PTX3 is required for the secretion of extracellular matrix. This is critical for a number of functions such as maturation of the oophorus follicle, which explains why in PTX3-deficient mice sterility is observed. This condition seems to be related to a defective PTX3-dependent incorporation of the glycosaminoglycan hyaluronic acid into the matrix of the cumulus oophorus [60]. A similar molecular composition of the ECM, containing hyaluronan (HA), tumor necrosis factor-stimulated gene-6 (TSG-6), and inter-α-inhibitor (IαI), is observed in rheumatoid arthritis and other inflammatory infiltrates, suggesting a role of PTX3 also in these conditions [61].

Interestingly, PTX3 regulates angiogenesis via FGF2, thus interfering with a variety of conditions, including ontogenesis, growth, inflammation, tissue repair, atherosclerosis, and tumors [62]. The PTX3/FGF2 interaction prevents angiogenesis. Beyond the anti-angiogenetic activity, the inhibitory effects of PTX3 on FGF2 possess a therapeutic rationale in the treatment of re-stenosis, the progressive occlusion of the coronary artery which occurs often following angioplastic surgery, due to anomalous FGF2-dependent proliferation and accumulation of smooth muscle cells on the vessel wall [63]. On the other hand, angiogenesis involves remodeling of ECM. In this respect, FGF2 modulates degradation of the ECM by inducing expression of urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor (PAI)-1 on endothelial cell surface, with opposite effects on matrix metalloprotease (MMP) activity. Moreover, FGF2 regulates expression and distribution of several integrins and cadherins on the plasma membrane of endothelial cells, thus promoting cell scattering or adhesion in different stages of angiogenesis [64]. Finally, in PTX3 deficient mice models a dramatic reduction of vascular endothelial growth factor receptor 2 (VEGFR2)occurs. This contributes to worsening the outcome of cerebral ischemia associated with a reduction of vessels formation [65].

Recently, some studies have investigated on the role of the PTX3 on the modulation of retinal vasculature [66]. In particular, Moon Woo and colleagues have demonstrated that human retinal pigment epithelial cells are the major source of PTX3 following pro-inflammatory cytokines stimulation and that its release exerts an important role in retinal injury against inflammation and infection. Diabetic Retinopathy (DR) represents the well-known microvascular complication of diabetes [67]. A recent study has demonstrated that PTX3 levels were significantly elevated in diabetic patients with DR compared to diabetic patients without DR and normal subjects. This association is correlated with an important retinal microvascular dysfunction, characterized by capillary leakage or closure, leading to ischemia [67, 68]. This study suggests that plasma PTX3 may be a better predictor for DR than CRP in diabetic patients. Moreover, Noma et al. have highlighted the relationship between PTX3 and retinal diseases such as age-related macular degeneration (AMD) and retinal vascular occlusion [69]. On this regards, it has been reported an interaction between PTX3 and the complement regulator factor H (CFH), a soluble molecule of the alternative pathway of the complement system involved in AMD pathogenesis. The authors hypothesize that aberrant expression of PTX3 could be associated with pathophysiology of AMD [69] because there is a loss of control of CFH activity. PTX3 has been evaluated also in the retinal vein occlusion (RVO), the second most common retinal vascular disease after diabetic retinopathy. This study demonstrate that PTX3 could be used as a useful diagnostic biomarker since patients with RVO have high plasma level of PTX3 [70]. Finally, during 2016 Zhou and Hu suggest that the use of PTX3 could be used as an anti-angiogenic molecule because PTX3 interacts specifically with FGF2 factor reducing the proliferative diabetic retinopathy thus ameliorating DR condition [68]. Although there are not specific evaluation of molecular signaling recruited by PTX3 in retinal vasculature, it could be represent another important field in which is possible to act modulating PTX3 to obtain favorable vascular results.

Epigenetic modulation of PTX3 gene in cardiovascular diseases is increasingly emerging. A candidate modulator of PTX3 is TNFα. In fact, TNFα controls the expression of PTX3 by human adipose tissue, being potentially implicated in the development of atherosclerosis [71], and administration of antibodies against TNFα lowers the levels of PTX3 transcripts in Kawasaki disease (KD) patients with intravenous immunoglobulin resistance, thus reducing the risk to develop coronary artery aneurysms [72].

Another very recent study demonstrated that PTX3 gene is over expressed in the presence of Nogo-B, a member of the reticulon 4 protein family, which is critical in vessel regeneration [73, 74]. In rheumatoid synoviocytes transcription of the PTX3 messenger RNA (mRNA) was found to be directly promoted by serum amyloid A [75]. Moreover, plasma analysis and gene expression profile revealed that PTX3 might be involved in the remote cardioprotective effects observed in mice after subcutaneous transplantation of heme oxygenase-1-overexpressing mouse mesenchymal stem cells [76]. Interestingly, in experimental brain stroke PTX3 is found locally increased in response to the pro-inflammatory cytokine interleukin-1 and it contributes to brain recovery by promoting glial scar formation and resolution of edema [77]. It is worth to be mentioned that the epigenetic regulation of long pentraxin 3 was demonstrated to recruit the PI3K/Akt pathway [78]. Remarkably, it was shown that HDL may increase the expression of PTX3 via activation of the PI3K/Akt pathway [79], which discloses a further beneficial effects of PTX3 under the regulation of HDL.