The present study demonstrates that the adipokines, HMWA and resistin, further characterize the inflammatory phenotype of LEB patients and are associated with clinical outcomes. Levels of resistin were markedly elevated and strongly correlated with hsCRP, however resistin was independent of, and added predictive value to hsCRP with respect to AFS. Importantly, resistin was the only baseline variable that maintained its predictive ability for AFS within the DM-restricted cohort. By contrast, plasma HMWA concentrations were notably low in patients undergoing LEB for advanced PAD in comparison to patients with coronary artery disease(17
) suggesting the breadth of the insulin-resistant phenotype in this population. A striking finding was the association of lower levels of HMWA with increased risk for vein graft failure. To our knowledge, this represents the first report of a circulating biomarker that may be associated with a protective effect on vein graft primary patency.
Adiponectin exerts its biological action on the vascular wall through one of two receptors that have been found in the liver and on human endothelial and smooth muscle cells.(18
) Its biological activity is dependent, in part, on its plasma oligerimization state, with the HMWA species being the most biologically sensitive and relevant to clinical outcomes.(19
) Low levels of adiponectin have been associated with impaired vascular reactivity,(20
) symptomatic atherosclerotic peripheral arterial disease (PAD),(21
) and coronary artery disease (CAD). Similar to previous investigations, HMWA was significantly correlated with insulin resistance in our study (estimated by HOMA-IR).
The seemingly paradoxical finding that HMWA levels were higher in patients with CLI than in those with claudication merits discussion. Several studies have demonstrated that the beneficial effects of adiponectin are not a straight forward dose-response relationship, indicating that other factors are involved. For example, higher adiponectin values were associated with increased mortality in patients with congestive heart failure (CHF).(22
) This observation may be explained, in part, by a positive association of adiponectin and N-terminal pro-B-type natriuretic peptide (NT-proBNP) in CHF patients.(22
) It has recently been shown that NT-proBNP is associated with adverse cardiovascular events and mortality in patients undergoing vascular surgery.(23
) Another intriguing possibility is that higher HMWA in some patients might be indicative of a wasting process.(24
) It has been shown that circulating adiponectin levels are higher in patients undergoing weight loss.(25
) Therefore, patients with advanced CHF or or severe CLI have may have higher baseline energy expenditure.(26
) This is consistent with the present investigation, as patients with CLI had significantly lower total cholesterol, lower albumin, and lower body weight, suggesting a relative wasting state over claudication counterparts. A recent study by Komei et al, reported that total plasma adiponectin levels were associated with decreased survival following lower extremity bypass surgery.(27
) However, several noteworthy differences exist from the present study. While there was a weak univariate association of HMWA with decreased AFS in the present investigation, this result was attenuated after controlling for disease severity and inflammation. In addition, we elected to measure HMWA rather than total adiponectin as several previous investigations suggests that this is the more biologically active species and most closely correlates with clinical outcomes. Finally, there were only 12 deaths in the study by Komei et al, which significantly limits the ability to construct reliable event-driven models and therefore limits the conclusions.(27
Another noteworthy finding of the present investigation was that the low density lipoprotein (LDL) fraction of cholesterol was markedly low in this patient population, even among subjects not taking a statin, (mean LDL on statin, 60.9 mg/dL; mean LDL not on statin, 74.5 mg/dL). These values are well below the National Cholesterol Education Program Adult Treatment Plan III (NCEP ATP III) recommendation of targeting LDL to below 100 mg/dL with an option to decrease LDL to 70 mg/dL in patients with established cardiovascular disease.(28
) Despite this, the median hsCRP was 3.0 mg/L which places these patients in the American Heart Association’s highest risk category.(29
) Therefore, this combination of low LDL cholesterol and high levels of inflammatory biomarkers potentially makes this an ideal population to test the hypothesis that lowering inflammation per se will be efficacious in the secondary prevention of cardiovascular events as well as vein graft related outcomes.
In this investigation, resistin was independently associated with the clinical outcome, AFS, after controlling for hsCRP and other comorbidities. Further, in the diabetic-restricted cohort, resistin was the only significant predictor of AFS after the effect of high hsCRP and CLI were attenuated. Therefore, we believe that plasma resistin values represent a potentially useful marker for adverse outcomes following LEB surgery, particularly in patients with DM.
Another novel finding of the present study is that resistin was independently associated with CLI after controlling for other inflammatory markers and DM. How does resistin contribute to the inflammatory state of the PAD patient? Unlike rodents where high levels of resistin mRNA transcripts are found in adipose tissue, the majority of resistin in humans can be found in inflammatory and mononuclear cells.(30
) Thus, resistin appears to be produced in the periphery unlike the acute phase reactant, hsCRP, which is predominantly produced in the liver.(31
It should also be emphasized that the values of resistin observed in this patient population were extremely high. In the Study of Inherited Risk of Coronary Atherosclerosis (SIRCA), a study of apparently asymptomatic middle aged men and women, the median resistin values were about 5.6 ng/ml which compares to patients with DM, where the resistin levels were about 5.8 ng/ml.(32
)This is in stark contrast to the median values in the present study which were 12.9 ng/ml (15.1 ng/ml for CLI; 10.2 ng/ml for claudicants) and only patients in the lowest 10 percentile had values as low as those seen in the SIRCA study. It is not currently known why these patients had such high values but it may relate to resistin production by vascular wall resident macrophages within atherosclerotic plaques.(33
) Resistin, unlike hsCRP, has been shown to be correlated with coronary artery calcification, a well known marker of the burden of atherosclerosis.(32
) Hence, we surmise that resistin may reflect the total burden of atherosclerosis whereas CRP is most closely associated with the inflammatory activity of the plaque.
One limitation of the present study is that we elected to measure only one species of adiponectin, HMWA, rather than considering the complex distribution or ratios of specific fractions. However, recent studies have reported that the HMWA is more relevant to the pathogenesis of DM, obesity, metabolic syndrome, and vascular disease than total serum adiponectin concentrations.(34
) Others report a ratio of HMWA to total adiponectin as having a stronger predictive ability in some populations.(36
) Further investigation is required to determine the more accurate species with regards to vein graft related outcomes. In addition, the association between HMWA and vein graft outcomes is rather modest. HMWA was significant in the full multivariate model as well in models exhibiting more parsimony. Nevertheless, in a sample size of 225 with 78 graft-related events, the potential for erroneous association exists and therefore these results must be interpreted with caution. Clearly larger investigations are needed to corroborate the present findings. As with hsCRP and other biomarkers, a potential causal relationship between plasma adipokine levels and graft or other vascular events remains speculative at the present time.