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Nukes, Lactate, and Insulin Resistance
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The Relationship between Nuke Treatment Duration, Insulin Resistance, and Fasting Arterialized Lactate Level in Patients with HIV Infection
.... chronic low-grade elevations in lactate level as a consequence of NRTI exposure may contribute significantly to insulin resistance in HIV-infected patients.....
Clinical Infectious Diseases Nov 1, 2005;41:1335-1340
Joan C. Lo,a Mahmood R. Kazemi, Priscilla Y. Hsue, Jeffrey N. Martin, Steven G. Deeks, Morris Schambelan, and Kathleen Mulligan
Department of Medicine, University of California San Francisco, and San Francisco General Hospital, San Francisco, California
(See the editorial commentary by Hadigan on pages 1341-2)
ABSTRACT
Background. Treatment with nucleoside reverse-transcriptase inhibitors (NRTIs) is associated with hyperlactatemia, presumably as a result of NRTI-induced mitochondrial toxicity. We examined the association of NRTI treatment duration and lactate level in human immunodeficiency virus (HIV)-infected patients and assessed the relationship of treatment duration and lactate level with insulin resistance.
Methods. Fasting arterialized venous lactate levels, routine blood chemistry findings, insulin resistance (determined by homeostasis model assessment [HOMA-IR]), percentage of body fat (determined by dual-energy radiographic absorptiometry), and detailed histories of antiretroviral therapy were obtained for 95 HIV-infected individuals. The independent association of NRTI treatment duration and lactate level was examined using multivariable linear regression.
Results.
Among 95 subjects with a mean age (± standard deviation [SD]) of 44 ± 8 years), 95% had NRTI exposure, with current NRTI use in 83%. The mean (±SD) lactate level was 1.24 ± 0.46 mmol/L (6% had a lactate level >2 mmol/L). Longer duration of NRTI use was positively associated with lactate level (B=0.047; P < .01), as were age, duration of protease inhibitor treatment, and HOMA-IR. Female sex and percentage of body fat were negatively associated with lactate level. After adjustment for age, sex, diabetes, percentage of body fat, and duration of protease inhibitor therapy, an increased duration of NRTI therapy remained significantly associated with lactate level (B=0.035; P = .04). However, the addition of HOMA-IR to the adjusted model attenuated the relation between duration of NRTI therapy and lactate level (B=0.024; P = .14), whereas HOMA-IR was significantly associated with lactate level (B=0.206; P < .01). Furthermore, HOMA-IR was also associated with NRTI treatment duration in adjusted analyses.
There were 41 patients (43%) currently receiving stavudine and 83 patients (87%) who had current or previous stavudine use (median duration of stavudine treatment, 3.2 years; interquartile range, 1.7-4.6 years). A total of 81 subjects (85%) had current or past history of PI use (median duration of PI use, 4.5 years; interquartile range, 2.8-5.3 years).
Conclusion. NRTI treatment duration was independently associated with higher lactate level, but this relationship was attenuated after adjusting for HOMA-IR. These data raise the possibility that insulin resistance may be an additional mechanism through which NRTI therapy is related to lactate level.
Financial support. This study was conducted in the SFGH-GCRC with support from the National Center for Research Resources at the National Institutes of Health (RR00083), the National Institutes of Health (DK54615, DK45833, AI052745, P30 MH062246, P30 AI027763, RR00083-41S1/CAP), University of California, San Francisco (Academic Senate and Faculty Development Awards), the Doris Duke Charitable Foundation, and the Universitywide AIDS Research Program (CC99-SF-001).
Potential conflicts of interest. J.C.L. is a past recipient of the SmithKline Beecham-University of California, San Francisco, Junior Faculty Award. M.S. has consulted for or received honoraria from Bristol-Myers Squibb, Merck, and GlaxoSmithKline. K.M. has consulted for or received honoraria from GlaxoSmithKline, Vertex Pharmaceuticals, and Abbott Laboratories. J.N.M. receives grant support from GlaxoSmithKline. S.G.D. has received honoraria or grant support from Bristol-Myers Squibb, GlaxoSmithKline, and Gilead Sciences. P.Y.H. and M.R.K.: no conflicts.
EDITORIAL
CID Nov 1, 2005
Insulin Resistance among HIV-Infected Patients: Unraveling the Mechanism
Colleen Hadigan
Program in Nutritional Metabolism, Massachusetts General Hospital, Boston, Massachusetts
Insulin resistance and type 2 diabetes are increasingly recognized as a consequence of antiretroviral therapy for HIV infection [1]. In a recent study, HIV-infected men receiving antiretroviral therapy were found to have a >4-fold increased rate of type 2 diabetes, compared with healthy control subjects [1]. Diabetes was also identified as an independent risk factor associated with the development of cardiovascular disease in a large, multinational prospective cohort of HIV-infected patients [2]. Therefore, recognition and management of diabetes and insulin resistance among HIV-infected patients is crucial for long-term health maintenance in this patient population. Although much research has been conducted to identify the etiology of insulin resistance and diabetes in HIV-infected patients [3-6], and although evidence suggests that it is likely to be multifactorial in nature, further work is needed to better understand the mechanisms of, to treat, and to prevent insulin resistance and diabetes in patients with HIV infection.
The present study conducted by Lo et al. [7] provides useful new data on the relationship between cumulative exposure to nucleoside reverse-transcriptase inhibitors (NRTIs), insulin resistance, and plasma lactate levels. In this report involving 95 HIV-infected patients (90% of whom were men, and 96% of whom were antiretroviral experienced), duration of NRTI therapy was positively correlated with lactate levels and was also associated with insulin resistance, as evaluated by the homeostatic model for assessment of insulin resistance. Other factors noted to be associated with elevated lactate levels were a decreased percentage of body fat, age, and duration of protease inhibitor therapy.
Although the present study is cross-sectional and cannot attribute causality to the observed associations, it identifies an interesting and potentially important link between lactate levels and insulin resistance. Indeed, in a multivariate analysis controlling for potential confounders, insulin resistance (as evaluated by the homeostatic model for assessment of insulin resistance) and not duration of NRTI therapy was significantly associated with lactate levels. Lo et al. [7] postulate that elevated lactate levels may directly influence insulin sensitivity, and this may be one mechanism by which NRTI exposure leads to insulin resistance. Indeed, cross-sectional studies among obese, non-HIV-infected individuals have also shown similar correlations between insulin sensitivity and baseline lactate levels [8]. Furthermore, in animal studies, direct administration of lactate led to impaired insulin-stimulated glucose uptake into muscle as a result of acute suppression of glycolysis, as well as inhibition of downstream insulin receptor substrate signaling, without any effect on glucose transporter 4 (GLUT4) [9]. On the basis of these observations and the observations made in Lo et al. [7], chronic low-grade elevations in lactate level as a consequence of NRTI exposure may contribute significantly to insulin resistance in HIV-infected patients.
The possibility remains that the effects of NRTI therapy on adipose tissue may also be important in the etiology of insulin resistance in patients with HIV infection. Furthermore, adipose-derived lactate may also be playing a role. Peripheral lipoatrophy is a recognized complication of NRTI therapy [10, 11] that may be due to mitochondrial dysfunction effecting lipid metabolism in adipocytes [12]. Several studies have demonstrated a direct association between decreased limb fat and insulin resistance in HIV-infected patients with lipodystrophy [4, 13]. Lo et al. [7] do not provide estimates of limb fat per se, but total percentage of body fat (as a surrogate marker for peripheral lipoatrophy) was inversely associated with lactate levels and duration of NRTI exposure. Increased exposure to NRTI therapy may therefore lead to fat atrophy, as well as to increased lactate levels, which, independently or in combination, may directly contribute to impaired insulin sensitivity. Of note, oxidative stress can result in decreased adiponectin expression and increased lactate production in 3T3 L1 adipocytes [14]. Mitochandrial dysfunction associated with NRTI use may create oxidative stress and subsequent increases in lactate production from adipose tissue in patients with HIV infection and thereby contribute to insulin resistance. However, adiponectin has also been identified as a factor associated with insulin sensitivity in patients with HIV infection. In vitro exposure of adipocytes to NRTIs decreases adiponectin levels and alters lipid metabolism [15], and several studies have demonstrated a strong relationship between limb fat atrophy, decreased serum adiponectin levels, and insulin resistance in patients with HIV infection [16, 17]. Oxidative stress and/or mitochondrial insult from NRTI exposure may represent a common pathway for increased lactate levels and decreased adiponectin from adipocytes, which ultimately results in insulin resistance in patients with HIV infection who are receiving antiretroviral therapy.
Of interest, in Lo et al. [7], the cumulative duration of NRTI therapy was more predictive of lactate levels than was the presence or absence of current NRTI exposure. This observation may be limited by the relatively small number of subjects who were not currently receiving an NRTI (only 6 of 95 subjects were not currently receiving an NRTI). However, if this finding is reproducible in a larger cohort, it has important implications for the long-term consequences of NRTI exposure, and future studies assessing the possible reversibility of these effects and evaluating the differential effects of thymidine analogues and NRTI-sparing regimens on lactate levels and insulin resistance are needed.
Potential conflicts of interest. C.H. is a consultant for Gilead and has received a speaker honorarium from Bristol-Myers Squibb.
BACK TO STUDY ARTICLE
BACKGROUND
Treatment with nucleoside reverse-transcriptase inhibitors (NRTIs) has been associated with hyperlactatemia and lactic acidosis [1-6]. The postulated mechanism involves mitochondrial toxicity; this hypothesis is based on evidence that NRTIs inhibit human mtDNA replication, leading to mtDNA depletion [3, 7]. Increased lactate levels presumably result from a shift in glucose metabolism from the mitochondrion to the cytosol, where glucose is metabolized to lactate via glycolysis. Although acute lactic acidosis is rare, subclinical elevations in lactate level of as-yet-unknown clinical significance are associated with the use of some NRTIs [1, 2, 4-6].
In addition to NRTI therapy, other factors may be associated with altered lactate metabolism. For instance, diabetes mellitus, insulin resistance, and obesity have been associated with increased lactate levels in non-HIV-infected individuals [8-11]. These metabolic factors might be relevant to lactate metabolism in patients with HIV infection, given the association of NRTI treatment with lipoatrophy [12-14], the association of lipoatrophy or fat distribution abnormalities with insulin resistance [15, 16], and the metabolic effects attributable to protease inhibitor (PI) therapy [16, 17]. Hepatic dysfunction may also contribute to increased lactate level, because the liver plays a significant role in systemic lactate clearance [18].
In this study, we obtained fasting arterialized lactate levels in a cohort of HIV-infected patients who had carefully documented antiretroviral drug histories. We chose the arterialized sampling approach (in which samples are obtained from a heated hand vein) to obtain a better estimate of whole-body lactate concentration, which would be less affected by regional differences in lactate metabolism across a single muscle bed. We tested the hypothesis that duration of NRTI use was independently associated with higher lactate levels and assessed whether this association was related to insulin resistance.
RESULTS
Of the 130 individuals screened, 95 met our criterion for adequate venous arterialization (pO2 >55 mm Hg). Most (96%) of these individuals were antiretroviral-treatment experienced, with 83% currently using NRTIs. Among the 90 subjects who had a current or past history of NRTI use, the median cumulative NRTI treatment duration was 5.6 years (interquartile range, 4.3-9.6 years), with NRTI duration <1 year in only 5 subjects (3-6 months in 3 subjects and 9-10 months in 2 subjects). There were 41 patients (43%) currently receiving stavudine and 83 patients (87%) who had current or previous stavudine use (median duration of stavudine treatment, 3.2 years; interquartile range, 1.7-4.6 years). A total of 81 subjects (85%) had current or past history of PI use (median duration of PI use, 4.5 years; interquartile range, 2.8-5.3 years).
Seven subjects had diabetes mellitus: 1 was receiving insulin (and was excluded from HOMA-IR analyses), 4 were receiving single oral hypoglycemic drugs, and the remaining 2 had mild or borderline diabetes not requiring treatment. Only 2 patients (1 without diabetes) were treated with metformin, and neither had lactate levels that were above the upper limit of normal (2.0 mmol/L). Plasma lactate levels were 0.53-2.72 mmol/L and were normally distributed; 6 individuals had a lactate level >2.0 mmol/L. Mild abnormalities in liver function (i.e., ALT levels greater than the upper limit of normal [56 IU/L]) were observed in 21%; only 4 subjects had ALT >100 IU/L.
Figure 1 demonstrates the positive association between cumulative NRTI treatment duration and fasting arterialized lactate level. With use of linear regression, we similarly found that an increase in cumulative NRTI treatment duration was significantly associated with an increase in plasma lactate level (B=0.047; P < .001). This relationship was consistent among both individuals receiving and those not receiving NRTIs. Moreover, although current NRTI use was significantly associated with lactate level (B=0.256; P = .041), when both NRTI treatment duration and current NRTI use were included in the model, duration of NRTI treatment in years (B=0.043; P = .001) but not current NRTI use (B=0.089; P = .488) remained significantly associated with lactate level. Other variables associated with higher lactate level in unadjusted analyses included log-transformed HOMA-IR, duration of PI therapy, and older age. There was a trend toward higher lactate level in those individuals with diabetes, but the 95% CI was wide (95% CI, -0.049 to 0.659), reflecting the small number of subjects with reported diabetes. Percentage of body fat and female sex were associated with lower lactate level in this analysis. No relationship was seen between lactate level and ALT level or having an ALT level >1.5 times the upper limit of normal. Of note, duration of PI therapy, age, percentage of body fat, and HOMA-IR were also correlated with duration of NRTI therapy (r = 0.63, r = 0.34, r = -0.27, and r = 0.22, respectively; P < .05 for all).
After adjusting for age, sex, diabetes mellitus, percentage of body fat, and duration of PI therapy, increased duration of NRTI therapy remained significantly associated with lactate level (table 2), whereas the other variables were no longer significantly associated with lactate level.
Because we hypothesized that insulin resistance may partially mediate the relationship between NRTI therapy and lactate level, we then evaluated the association between NRTI use and lactate level after inclusion of log-transformed HOMA-IR. Notably, on addition of HOMA-IR, the association between NRTI treatment duration and lactate level was attenuated and was no longer significant, whereas HOMA-IR was significantly associated with lactate level. These findings raise the possibility that insulin resistance (as measured by HOMA-IR) may be a variable on a causal pathway relating cumulative NRTI treatment duration to fasting lactate level. This suggestion is supported by the finding that NRTI treatment duration was significantly associated with HOMA-IR (B=0.042; P = .032) after adjusting for age, sex, percentage of body fat, and diabetes status. It should be noted that neither current or cumulative PI use nor percentage of body fat were significantly associated with HOMA-IR (r < 0.2; P > .2) in our study.
We further explored whether the relationship between NRTI duration, lactate level, and HOMA-IR was specifically related to stavudine use. Lactate level was not associated with current stavudine use (B=0.060; P = .530). An increased duration of stavudine treatment was associated with a higher lactate level (B=0.050; P = .040) in unadjusted analyses, whereas this association was no longer significant after adjusting for age (B=0.037; P = .143), and it was further attenuated after adjustment for sex and diabetes status (B=0.027; P = .279). We also noted that substitution of NRTI treatment duration with stavudine treatment duration, current stavudine use, or current stavudine or didanosine use also did not indicate a significant association with lactate level in adjusted analyses.
DISCUSSION
Our data indicate that an increase in cumulative duration of NRTI therapy is associated with a statistically significant increase in fasting arterialized lactate level, independent of age, sex, duration of PI therapy, percentage of body fat, and diabetes status. Because 97% of subjects with NRTI exposure had been treated at for at least 6 months (and 94% for at least 1 year), these data do not address the early increase in lactate level associated with NRTI that has been reported by others [1, 4] Rather, our findings may provide additional insight into the cumulative effect of prolonged NRTI therapy on lactate level and suggest that there may be a subtle but significant increase in plasma lactate level with longer duration NRTI therapy. Others have also noted a gradual increment in plasma lactate level with increasing duration of nucleoside analogue therapy without a clear plateau effect in the first fourteen months [22]. These data are also unique in that we estimated whole-body lactate concentrations using arterialized venous sampling, which should inherently reduce the variability of venous lactate level related to phlebotomy conditions and differences in local tissue production of lactate.
The attenuation of the relationship between NRTI duration and lactate level after inclusion of HOMA-IR in the model suggests that insulin resistance may be an additional mechanistic pathway through which NRTI treatment affects lactate level. This speculation is supported by evidence that NRTI treatment duration is associated with insulin resistance in our analyses, and it is consistent with data from other investigators who have reported an association between duration of NRTI therapy and insulin resistance [23, 24] and with historical evidence that insulin resistance is associated with altered lactate metabolism [10]. Furthermore, there is evidence that prolonged NRTI therapy is associated with lipoatrophy [12, 13, 25], and lipoatrophy, in turn, is associated with insulin resistance [15], providing a further mechanistic link between NRTI treatment and insulin resistance. These results are also interesting in light of recent data indicating that the age-related decrease in mitochondrial function may explain the reduction in insulin sensitivity associated with aging [26] and that mitochondrial function is impaired in insulin-resistant offspring of individuals with type 2 diabetes [27].
The relationship between NRTI therapy and lactate level in our study was not primarily driven by stavudine use. This study population may differ from others in that there was a relatively low rate of hyperlactatemia among our subjects, and none had acute lactic acidemia at the time of investigation. If there are other pathways besides mitochondrial toxicity whereby NRTI therapy is associated with lactate metabolism, then stavudine might not be the only NRTI drug to have an effect. Unfortunately, for those individuals who were previously receiving stavudine, we do not know the reason for changes in the antiretroviral regimen. However, almost all subjects were studied during 2001-2002, an era in which the large majority of changes in stavudine-based regimens were likely to be the result of virologic failure rather than metabolic reasons. Nevertheless, some individuals may have discontinued stavudine therapy because of lipoatrophy or other concerns regarding mitochondrial toxicity. We also do not have information on dosing or changes in dosing for stavudine and did not separately quantify cumulative exposure to other NRTIs that have been associated with hyperlactatemia, including didanosine [2, 5] and zidovudine [1].
The fact that HOMA-IR is significantly associated with lactate level in unadjusted and adjusted analyses is interesting and deserves further investigation. Moyle et al. [2] reported that glucose levels <5.2 mmol/L (-94 mg/dL) were associated with lower odds of having hyperlactatemia, but, to our knowledge, the relationship between insulin resistance and lactate levels has not been explored in patients with HIV infection. The cross-sectional design of this study limits our ability to determine the direction or nature of the association between plasma lactate concentrations and insulin resistance. Data from animal studies have shown that infusion of lactate at high levels can induce insulin resistance [28]. Conversely, insulin resistance is believed to contribute to increased fasting lactate levels through a failure of insulin to suppress glucose production [10].
There is also preliminary evidence that coinfection with hepatitis C virus or hepatitis C virus-specific therapy may increase the risk of lipoatrophy, insulin resistance, and mitochondrial toxicity, as evidenced by studies in HIV-positive patients with chronic hepatitis C infection [29-32]. Unfortunately, we do not have systematic documentation of hepatitis C infection, hepatitis C viremia, and hepatitis C virus-associated treatment. However, only mild abnormalities in liver function test results were noted in our population, and only 4 subjects had ALT levels >100 IU/L.
In conclusion, we found that duration of NRTI therapy was associated with higher plasma lactate level but that this relationship was attenuated by adjustment for insulin resistance. These initial exploratory findings raise the possibility of a relationship between lactate metabolism and insulin resistance in HIV-infected individuals treated with NRTIs. Additional studies that involve larger HIV-infected populations are needed to investigate the association between NRTI treatment duration and plasma lactate level, the contribution of specific NRTI drugs or conditions such as hepatitis C virus coinfection, and the relationship of these variables to insulin resistance and other related factors.
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