In the univariate analyses of HIV-infected subjects, a lower FMD was significantly associated with higher insulin resistance (by both HOMA-IR and Belfiore OGTT index), greater consumption of alcohol (≥1 drink/day), and higher baseline brachial artery diameter (P < 0.05 for all). In HIV seronegatives, higher insulin resistance, brachial artery diameter, and lower systolic blood pressure were predictive of lower FMD (P < 0.05 for all). Among HIV-infected and seronegative subjects, there was a significant correlation between CIMT and traditional cardiac risk factors including male sex, older age, history of hypertension, lower HDL cholesterol, higher glucose, higher insulin resistance, higher trunk/limb fat ratio (HIV-infected subjects with DEXA data), and the presence of an increasing number of metabolic syndrome criteria (P < 0.05 for all). In multivariate analyses (Table 3) among HIV-infected subjects, higher insulin resistance, greater consumption of alcohol, and higher resting brachial artery diameter remained significant independent predictors of FMD (adj. r2 = 0.40, P < 0.001 for model). History of hypertension, higher trunk/limb fat ratio, and insulin resistance remained significant predictors of CIMT (adj. r2 = 0.46, P < 0.001 for model). There was a strong correlation between the HOMA-IR and Belfiore OGTT indices (r = −0.82, P ≤ 0.001).
To further explore a potential confounding effect between ritonavir and insulin resistance, the relationship between current protease inhibitor use and brachial FMD or CIMT was also examined. All subjects on a protease-inhibitor-based regimen were on a ritonavir-boosted regimen (76% were on atazanavir/ritonavir). There was no significant association between either brachial FMD or CIMT and current use of a ritonavir-boosted protease inhibitor. Additionally, the cumulative exposure to HAART, as well as the cumulative exposure to individual drug classes (nucleosides, NNRTIs, protease inhibitors) revealed no significant association with either brachial FMD or CIMT. Studies have also found an association between thymidine analogue use and insulin resistance [20–22], but in the present study there was no significant correlation between cumulative exposure/use ever of zidovudine or stavudine and HOMA-IR/Belfiore index, brachial FMD, or CIMT. There have also been conflicting data with regard to effects of body fat changes (lipoatrophy) on FMD [13,23], but in our study we found no significant association between appendicular fat, trunk fat, or trunk/limb fat ratio and FMD. Finally, there was no significant correlation between HIV disease activity indicators (i.e., current/nadir CD4 cell count, HIV RNA, duration of HIV) and CIMT or brachial FMD. Overall, there were modest, significant correlations between FMD and CIMT (r = −0.40, P < 0.01) and between CIMT and 10-year Framingham score (r = 0.50, P < 0.01). There were no significant correlations between FMD and Framingham score, between FMD and hsCRP, and between CIMT and hsCRP. Traditional risk factors utilized in the Framingham risk score (age, smoking, HDL and total cholesterol, blood pressure) were more predictive of CIMT (r = 0.55, P = 0.01) than brachial FMD (r = 0.25, P = NS).
The results of this study of HIV-infected subjects on modern HAART regimens and with well controlled viremia demonstrate no significant differences in either brachial FMD or CIMT compared with seronegative controls. Higher insulin resistance was a significant predictor of lower brachial FMD and higher CIMT.
A recent study examined endothelial function in HIV-infected subjects vs. controls, and found that HIV-infected subjects had significantly impaired brachial FMD, and that current intravenous drug use was significantly associated with FMD . Although insulin resistance was not reported in that study, 63% of HIV-infected subjects had co-infection with hepatitis C, which has been associated with increased risk of insulin resistance in some [24,25] but not all studies . Injection drug users were excluded from the present study.
Recent studies of HIV-infected, therapy-naive, and uninfected persons exposed to modern drugs such as lopinavir/ritonavir or efavirenz have shown improvements in endothelial function within a short period [13,31]. In one study, endothelial function was improved 6 months after HAART initiation, although still below normal values, and was independently predicted by the decline in HIV RNA . Also, in-vitro studies of endothelial function have implicated HIV as potentially having direct and indirect effects on the endothelium, including direct infection and/or activation of endothelial cells by HIV, vascular injury as a result of chronic inflammation and immune activation, or dysregulation of the nitric oxide synthase system [32–35]. Both tat and gp120 proteins of the HIV virion may activate endothelial cells, resulting in increased expression of specific adhesion molecules (i.e., ICAM-1, E-selectin) that may contribute to endothelial dysfunction or a prothrombotic state [33–35]. Thus, it is possible that subjects in the current study had overall low cardiovascular risk due to good control of HIV viremia and adequate restoration of immunity. As only five subjects had detectable, low-level HIV viremia, we did not have statistical power to detect significant differences in CIMT/FMD due to differences in HIV viral load.
An important finding is the association of insulin resistance with both endothelial dysfunction (i.e., lower FMD) and higher CIMT in HIV-infected subjects (Table 3). Although we did not find a significant association between current protease inhibitor or thymidine analogue use and insulin resistance, brachial FMD, or CIMT, the use of specific antiretrovirals has been strongly associated with insulin resistance in other studies [3,4,20–22,27,28]. The development of insulin resistance in some patients on HAART may have important long-term implications in determining cardiovascular disease risk. The traditional Framingham risk score calculator does not adequately account for insulin resistance (it only accounts for established diabetes) and in this study we did not find a significant correlation between brachial FMD and Framingham risk score. The Framingham score has not been validated in HIV and thus determination of insulin resistance or use of endothelial function testing could be a useful adjunct in assessing long-term cardiovascular disease risk in this population. Unfortunately larger, longitudinal cohort studies rarely incorporate the use of sensitive metabolic testing such as oral glucose tolerance testing or insulin profile. We found that a simple index of insulin resistance (HOMA-IR) was a stronger predictor of brachial FMD and CIMT than more indirect assessments, such as the diagnosis of metabolic syndrome. HOMA-IR was also strongly correlated with the sensitive Belfiore OGTT index, which requires repeated measures of insulin and glucose levels during a 2-h OGTT.
There were minor limitations. This study was cross-sectional and the number of subjects was low, but still higher compared with several other endothelial function studies in HIV [11–13,36]. In addition, several HIV-infected subjects were on statin therapy, which may have a positive effect on brachial FMD and CIMT. Nevertheless, two recent studies of statin therapy in HIV found no more than a 0.7–1.2% improvement in brachial artery FMD with statin therapy [37,38], and the inclusion of statin-treated, HIV-infected subjects in the present study was felt to best reflect current standard of care for persons with HIV . When further controlling for use of lipid-lowering medication, there were still no significant differences in FMD or CIMT between HIV-infected and control groups.
In conclusion, in this HIV cohort on modern HAART and with well controlled HIV, we found no significant differences with regard to preclinical markers of cardiovascular disease risk such as endothelial function testing, CIMT, or Framingham risk score. Insulin resistance was a stronger predictor of brachial FMD and CIMT than lipid parameters, HAART regimen, indices of immune function, lifestyle factors, or body and limb adiposity. These findings suggest that insulin resistance is an important cardiovascular disease risk factor in the HIV-infected population, and that therapeutic strategies to improve insulin sensitivity may be warranted. Longitudinal studies of HIV-infected cohorts will be important to evaluate future changes in cardiovascular risk, especially as new HIV drug classes become available.
1 K23 AI065336-01 (K.E.M.), K12RR023249 (L.diF.), DK049393, DK059531, AT003083, DK056341, RR000036, RR000954, DK020579, AI025903.
We gratefully acknowledge the Barnes-Jewish Hospital Foundation for support to the Cardiovascular Imaging and Clinical Research Core Laboratory.
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