Endothelial Progenitor Cell Status in Metabolic Syndrome

M etabolic Syndrome (MetS) is a very common disorder that afflicts at least 1 in 4 American adults. Also, it confers an increased risk for both diabetes and cardiovascular disease (CVD). An early event in atherosclerosis is endothelial cell dysfunction. Numerous groups have reported endothelial dysfunction in patients with MetS. Esposito et al. showed that compared with 60 control subjects matched for age and sex, patients with the MetS had decreased endothelial function. ¹ In the Framingham Offspring participants, Hamburg et al. showed in age and gender adjusted models, that MetS was associated with decreased flow-mediated dilation. ²

There was progressively lower vasodilator function with increasing number of MS components. Lteif et al., using leg blood flow measurements, showed that patients with MetS had worse endothelial function. ³ Also, in the Prospective Study of the Vasculature in Uppsala Seniors (PIVUS) study, using different techniques to assess vasodilation in conduit and resistance arteries in MetS the authors showed decreased flow-mediated vasodilation (FMD). ⁴ In the Northern Manhatten study (NOMA), Suzuki et al. reported that MetS was associated with decreased FMD and increased CVD over 81 months. ⁵ Thus, it is clear that MetS patients have impaired endothelial function. This has major implications with regards to subsequent CVD. Although, this has been reported in numerous studies the measurement of endothelial function by flow mediated dilation is far from optimum and there is much variability in the studies reported above such as the NOMA and Framingham studies in which the mean FMD in controls were 6.3% and 3.3%, respectively.

There has been much interest recently in a subtype of progenitor cells, isolated from bone marrow and peripheral blood of adults that have the capacity to circulate, proliferate and differentiate into mature endothelial cells, termed endothelial progenitor cells (EPCs). It needs to be stated at the outset that there is much controversy with respect to the correct definition of EPCs. ^{6 –8} Generally, it is accepted that EPCs are characterized by the assessment of surface markers such as CD34 and vascular endothelial growth factor-2, VEGF-2 (KDR). ⁸ Importantly, CD34⁺KDR⁺ combination is the only putative EPC phenotype that has been demonstrated repeatedly and convincingly to be an independent predictor of cardiovascular outcomes. ^9,10 In the larger study, Werner et al. have reported that CD34⁺KDR⁺ cell count predicted cardiovascular events and cardiovascular death during a 12-month follow-up in 519 patients with coronary artery disease (CAD). ¹⁰ Also, in a subset, colony forming units (CFUs) predicted cardiovascular events. Furthermore, Hill et al. reported a strong correlation between the number of circulating EPCs (measured as CFUs) and the subjects' combined Framingham risk factor score. ¹¹ Also, the measurement of flow-mediated brachial-artery reactivity revealed a significant relation between endothelial function and the number of progenitor cells. Indeed, levels of circulating EPC were a better predictor of vascular reactivity than was the presence or absence of conventional risk factors. In addition to, flow cytometric quantitation of CD34/KDR predicting CVD events, also functional assays such as CFU and EPC migration have been shown to correlate significantly with CAD risk factors, severity, and events. ^{10 –14} Thus, the measurement of EPCs may be a surrogate biologic marker for vascular function and cumulative cardiovascular risk, suggesting further that endothelial injury in the absence of sufficient circulating progenitor cells may unfavorably affect the progression of CVD. EPCs circulate in the blood and appear to home preferentially to sites of vascular or tissue injury, contributing significantly to both re-endothelialization and neoangiogenesis.

With regards to the MetS, there is sparse data on EPC number and functionality. There appears to be two studies that have directly looked at EPC number in patients and matched controls. In the study by Westerweel et al. they show that circulating CD34⁺KDR⁺ EPC levels were reduced by nearly 40% in obese men with MetS compared to non-obese men. ¹⁵ Although, this was a small study that included 19 patients with MetS, it is important to emphasize that in this study they excluded patients with overt clinical CVD or diabetes. They did not study EPC functionality.

In the other study the authors reported on EPC number and functionality in a larger sample size of subjects with MetS (n = 46) of which 77% were female and matched controls (n = 31). ¹⁶ In accord with the study in obese males, they showed a significant decrease in EPC number, numerated by CD34/KDR dual positivity. However, these investigators also looked at functionality of EPCs such as CFUs, migration and tubule formation. In addition to the reduction in numbers, they showed that there was significant impaired clonogenic capacity and also an impaired capacity to incorporate into tubule structures. While there was a decrease in migration of the EPCs in MetS this did not attain significance. However, it needs to be emphasized that none of the subjects were diabetic or has CVD in the above 2 studies and none were on medications that affect EPCs suggesting that the defect in EPCs manifest early in MetS prior to the development of Diabetes or CVD. Thus, these are the only 2 studies to date that have reported in patients with MetS without diabetes or CVD that indeed EPC number is decreased and functionality impaired. Fadini et al. have reported in a study decreased circulating EPCs and progenitor cells in diabetic patients with peripheral vascular disease. ¹⁷ In this article, they did a subgroup analysis of MetS patients vs. non-MetS patients. However, not much detail is provided with respect to co-existent diseases and morbidity such as diabetes and peripheral vascular disease in these 2 subgroups. Also, in this study, we are not provided much data with regards to the medications in the 2 groups of patients. Since, this was a study with the primary aim to look at EPC status in diabetic patients with peripheral vascular disease the data in patients in MetS is not as detailed as reported in the 2 studies that focused on MetS alone. In a subsequent report by Fadini et al., they showed that in patients with MetS, there was a decreased in progenitor cells (CD34⁺ cells). ¹⁸ Once again, they do not provide sufficient data with regards to the groups and it appears that many of these patients also could have diabetes, and be on medications such as statin, angiotensin converting enzyme Inhibitors (ACE-I), angiotensin receptor blockers (ARBs), and anti-diabetic therapy which could have influenced the data. ¹⁹ Interesting in this study, the usual recommended marker for EPC status (CD34/KDR) was not reported in the patients with or without MetS. However the reported decrease in progenitor cells in these 2 studies was confirmed in the study by Jialal et al., ¹⁶ but not in the study in obese males with MetS. ¹⁵ Thus, there is very limited data that specifically looked at EPC status either number and/or functionality in patients with MetS without the complications of diabetes and CVD. In the 2 studies which specifically address this, both have shown, decreased number of EPC however they are at variance with respect to a decrease in progenitor cells since levels were not significantly lower in the study in obese males. However if the data from the studies by Fadini are considered one could conclude that progentor cell exhaustion can be advanced as one mechanism resulting in decrease in EPC number.

Also, there is limited data with regards to mobilizing factors in patients with MetS. Some of the accepted mobilizing factors include VEGF, stromal derived factor-1 (SDF-1), and c-kit ligand. ²⁰ In the only study to date that has examined mobilizing factors in MetS, the investigators showed no significant differences in VEGF levels, but showed that progenitor cell mobilizing SCF and the soluble form of SCF receptor c-kit were both reduced in patients with MetS. ¹⁵ Since, this is a limited study in a small number of patients, these findings need to be confirmed in a larger study.

Thus, while there is much controversy with regards to the nomenclature and definition of EPCs, it needs to be emphasize that EPC number, EPC migration, and CFUs which appears to connote early EPCs have clearly been shown to correlate with risk factors, CVD severity and predict CVD events to date. However, while it is claimed that the late EPCs are more likely to become endothelial cells, it needs to be emphasize that to date no studies have reported that late EPCs predict CVD events in patients. ²¹

In conclusion, much further work is needed to confirm that indeed EPC numbers and functionality is impaired in MetS and that indeed this correlates with endothelial vaso-reactivity before it could serve at minimum as an additional biomarker of endothelial integrity and impaired neoangiogenesis in patients with MetS who clearly have manifest endothelial dysfunction. Strategies that have been shown to upregulate and enhance EPC number and functionality such as statins, ACE-I, ARBs, PPAR-gamma agonist and so on need to be studied more carefully with respect to both number and functionality of EPCs since this could inform us of their direct beneficial effects on the vulnerable vasculature of MetS.