Department of Internal Medicine and Center for Human Nutrition, Washington University School of Medicine, St. Louis, Missouri 63110
Submitted 20 March 2003 ; accepted in final form 9 May 2003
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
stable isotopes; lipolysis; hypertriglyceridemia; metabolism; human immunodeficiency virus
The physiological mechanisms responsible for hypertriglyceridemia in patients with HIV infection are not completely clear but must be a result of increased hepatic secretion of VLDL-TG into plasma, reduced TG clearance from plasma, or a combination of both. Plasma fatty acid availability is a major regulator of hepatic VLDL-TG production (27). Therefore, circulating insulin helps control whole body TG flux during fed and fasted conditions by regulating lipolysis of adipose tissue TG (13) and plasma fatty acid availability for VLDL-TG production. A decrease in plasma insulin concentration during postabsorptive conditions increases lipolysis and plasma fatty acid availability, which increases VLDL-TG production. In contrast, the postprandial increase in plasma insulin concentration decreases lipolysis and fatty acid availability, which decreases endogenous VLDL-TG production and partially compensates for the delivery of dietary TG that are entering the systemic circulation as chylomicrons.
It is not clear whether fatty acid kinetics are altered in patients with HIV-dyslipidemia because of conflicting reports. Sekhar et al. (44) found that the postabsorptive fatty acid rate of appearance (Ra) in plasma was higher in subjects with HIV-dyslipidemia than in normal controls, whereas Hadigan et al. (15) found that the postabsorptive fatty acid Ra values were the same in patients with HIV-dyslipidemia and normal controls. The reason for the discrepancy between studies might be related to differences in insulin sensitivity in study subjects. Whole body lipoprotein kinetics have not been studied carefully in patients with HIV infection. We are aware of one study that found an increased rate of VLDL-apolipoprotein-B100 (apoB) secretion in patients with HIV-dyslipidemia (42) but are not aware of any studies that evaluated VLDL-TG production rates.
In the present study, we used stable-isotope-labeled tracer methods to evaluate fatty acid and VLDL-TG kinetics during basal conditions and during glucose infusion in men with HIV-dyslipidemia receiving HAART. All subjects had normal oral glucose tolerance to eliminate the confounding effect of severe insulin resistance on fatty acid metabolism. We hypothesized that HIV-dyslipidemia would be characterized by 1) increased fatty acid Ra and VLDL-TG secretion rates during postabsorptive conditions and 2) impaired suppression of fatty acid Ra and VLDL-TG secretion in response to hyperglycemia-hyperinsulinemia (achieved by glucose infusion).
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
A total of 12 men [6 men with HIV-dyslipidemia, age: 44 ± 4 yr; and 6 healthy men, matched on body mass index (BMI), age: 34 ± 5 yr] participated in this study (Table 1). All subjects completed a comprehensive medical examination, which included a history and physical examination, an electrocardiogram, standard blood tests, and an oral glucose tolerance test. All subjects had normal fasting blood glucose concentrations and normal oral glucose tolerance; they had been weight stable and sedentary (regular exercise <2 h/wk) for at least 3 mo before the study. Subjects who consumed more than three alcoholic drinks per week were excluded from the study. Subjects with HIV-dyslipidemia were included if they had a basal plasma TG concentration >250 mg/dl. Therefore, mean basal plasma TG concentration was greater in subjects with HIV-dyslipidemia than in healthy subjects (352 ± 24 and 91 ± 14 mg/dl, P < 0.01). All subjects with HIV infection had clinical evidence of a mixed lipodystrophy syndrome (16), determined by self-reported regional lipoatrophy (loss of facial or appendicular adipose tissue) and lipohypertrophy (accumulation of adipose tissue overlying the posterior cervical vertebrae, breasts, or trunk), which was confirmed by physical examination and by an increased truncal-to-appendicular adipose tissue mass ratio assessed by dual-energy X-ray absorptiometry (DEXA). The clinical characteristics of subjects with HIV-dyslipidemia, including duration of HIV infection and type of therapy, are shown in Table 2. No subject was taking other medications known to affect lipid or glucose metabolism. All subjects with HIV-dyslipidemia had stable CD4 cell counts, and low or undetectable viral loads (<500 copies/ml) for at least 6 mo before the study.
|
|
Written informed consent was obtained from all subjects before their participation in the study. The study was approved by the Human Studies Committee and the General Clinical Research Center (GCRC) Scientific Advisory Committee of Washington University School of Medicine (St. Louis, MO).
Experimental Protocol
Body composition analyses. Each subject's whole body and regional fat mass (FM) and fat-free mass (FFM) were determined by DEXA (QDR 4500/w; Hologic, Waltham, MA) 1 wk before the isotope infusion study. The truncal-to-appendicular FM ratio was calculated as trunk FM divided by the sum of the FM in the arms and legs, as previously described (17, 32).
Isotope infusion study. Each subject completed an isotope infusion
protocol to determine VLDL-TG kinetics on two separate occasions, in
randomized order, 2 wk apart. Basal and postabsorptive fatty acid
Ra and VLDL-TG secretion rate were measured after an overnight
fast; on a second occasion, fatty acid Ra and VLDL-TG secretion
were measured during glucose infusion.
On the evening before the isotope infusion study, subjects were admitted to the inpatient unit of the GCRC at Washington University School of Medicine. At 1900, subjects consumed a standardized meal containing a total of 12 kcal/kg body wt (55% of total energy from carbohydrates, 30% from fat, and 15% from protein). At 2000, subjects consumed a liquid formula (Ensure; Ross Laboratories, Columbus, OH) containing 250 kcal (40 g carbohydrates, 6.1 g fat, and 8.8 g protein) to ensure complete filling of hepatic glycogen stores and then fasted until the completion of the study. For the glucose infusion study, an infusion of dextrose (5.5 mg · kg FFM-1 · min-1) was initiated at 2100 and maintained until completion of the isotope infusion study the next day.
The next morning, at 0530, a catheter was inserted in an antecubital vein to administer stable-isotope-labeled tracers, and a second catheter was inserted in a hand vein of the opposite arm; the hand was heated to 55°C using a thermostatically controlled box to obtain arterialized blood samples (24). At 0700, a bolus of [1,1,2,3,3-2H5]glycerol (75 µmol/kg body wt), dissolved in 0.9% NaCl, was administered through the catheter in the antecubital vein, and a constant infusion of [2,2-2H2]palmitate (0.03 µmol · kg body wt-1 · min-1), bound to human albumin (Centeon, Kankakee, IL), was started and maintained for 4 h. At the completion of the study, all catheters were removed. All isotope-labeled tracers were obtained from Cambridge Isotope Laboratories (Andover, MA).
Blood sampling. Blood samples were obtained immediately before the administration of stable isotope-labeled tracers to determine plasma substrate (fatty acid, glucose, and TG) and insulin concentrations, the background palmitate tracer-to-tracee ratio (TTR) in plasma, and glycerol TTR in VLDL-TG. Blood samples were obtained again at 5, 15, 30, 60, 90, 120, 180, 240, 360, 480, 600, 660, and 720 min after the start of the isotope infusion to determine palmitate TTR in plasma and glycerol TTR in VLDL-TG. Blood samples to determine plasma glucose and VLDL-TG concentrations were obtained every 2 h during the entire study period, and blood samples for the determination of plasma insulin concentrations were collected at 4, 8, and 12 h. Subjects remained in bed for the entire duration of the study.
Sample collection. Blood samples to determine fatty acid and TG concentrations and TTRs were collected in chilled tubes containing EDTA; samples to determine plasma insulin concentrations were collected in chilled tubes containing EDTA and trasylol. Immediately after collection, blood samples were placed in ice, and plasma was separated by centrifugation within 30 min of collection. Aliquots of plasma (2 ml) were kept in the refrigerator for subsequent isolation of VLDL. The remaining plasma samples were stored at -70°C until final analyses were performed. Blood samples to determine plasma glucose concentration were collected in tubes containing heparin and analyzed immediately after collection.
Isolation of VLDL. Immediately after completion of the isotope
infusion study, the VLDL fraction in plasma was isolated by
ultracentrifugation as previously described
(11). Briefly, 1.5 ml of each
plasma sample was transferred to Optiseal tubes (Beckman Instruments, Palo
Alto, CA), overlaid with a saline solution (density = 1.006 kg/l), and
centrifuged for 16 h at 45,000 rpm and 10°C in a 50.4 Ti rotor (Beckman
Instruments). The top layer, containing VLDL, was removed by tube slicing
(Beckman Instruments). The exact volumes that were recovered (1.3 ml)
were recorded. Isolated VLDL fractions were stored at -70°C until final
analyses were performed.
Sample analyses. Plasma glucose concentration was determined by using an automated glucose analyzer (Yellow Springs Instruments, Yellow Springs, OH). Plasma insulin concentration was measured by using RIA (19). VLDL-TG concentration was measured enzymatically (Sigma Chemicals, St. Louis, MO). Plasma fatty acid concentrations were quantified by using gas chromatography (model 5890-II; Hewlett-Packard, Palo Alto, CA) after heptadecanoic acid was added to plasma as an internal standard (35).
All isotopic enrichments were measured by selected ion-monitoring electron impact ionization GC-MS using an MSD 5973 system (Hewlett-Packard). TTRs were determined as previously described (34, 35). Instrument response was calibrated by using standards of known isotopic enrichment.
Calculations
Palmitate Ra in plasma was calculated by dividing the palmitate tracer infusion rate by the average arterial palmitate TTR obtained between 1.5 and 4 h (Steele's equation for steady-state conditions; see Refs. 46 and 50). Because of technical difficulties, palmitate Ra could not be determined for one healthy subject.
The fractional catabolic rate (FCR) of VLDL-TG, which represents the
fraction of the VLDL-TG pool that disappears from plasma per unit of time, was
determined by monoexponential slope analysis, as previously described
(34), and expressed as pools
per hour. In all subjects, the VLDL-TG FCR was 0.4 pools/h, and we have
previously shown that the monoexponential slope analysis provides accurate FCR
values for VLDL-TG pools with a turnover rate in this range
(34). It was assumed that the
rate of VLDL-TG secretion was equal to the rate of VLDL-TG catabolism because
plasma VLDL-TG concentration remained constant throughout the entire 12-h
study period (data not shown). The absolute rate of VLDL-TG secretion (equal
to the absolute rate of VLDL-TG catabolism) was calculated as: 1)
total VLDL-TG secretion rate, which represents the amount of VLDL-TG produced
by the liver and secreted in the bloodstream, and 2) VLDL-TG
secretion rate per unit of plasma as follows
![]() |
![]() |
The plasma clearance of VLDL-TG (in ml/min) was calculated by dividing the rate of VLDL-TG disappearance (at steady state equal to the secretion rate; in µmol/min) by the VLDL-TG concentration in plasma (in µM).
Statistical Analysis
A Student's t-test for independent samples was used to evaluate
the significance of differences in age and body composition between men with
HIV-dyslipidemia and healthy men. A two-way (subject x treatment) ANOVA
with repeated measures on the last factor was used to assess the significance
of differences in plasma substrate and insulin concentrations and palmitate
and VLDL-TG kinetics between the two groups during basal conditions and
glucose infusion. A P value of 0.05 was considered statistically
significant. All data are expressed as means ± SE.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Basal conditions. Basal plasma glucose and insulin concentrations were not significantly different between men with HIV-dyslipidemia and healthy men. Basal plasma free fatty acid (FFA) and VLDL-TG concentrations were significantly higher (P < 0.05) in men with HIV-dyslipidemia than in healthy men (Table 3).
|
Glucose infusion. The total amount of glucose that was infused
during the study was 490 ± 21 g in healthy men and 474 ± 17 g in
men with HIV-dyslipidemia (P = not significant). Glucose infusion
raised plasma glucose concentrations by 45% and plasma insulin
concentration by
400% and decreased total plasma FFA concentration by
65% both in men with HIV-dyslipidemia and in healthy men (treatment
effect: P < 0.01 for all comparisons; no difference between
groups). Glucose infusion had no effect on plasma VLDL-TG concentration
(Table 3).
Fatty Acid Kinetics
Basal palmitate Ra was 60% greater (P < 0.01)
in men with HIV-dyslipidemia than in healthy men. Glucose infusion caused a
marked reduction in palmitate Ra in both groups (P <
0.01); however, the relative response to glucose infusion was greater in men
with HIV-dyslipidemia than in healthy men (P < 0.02 for treatment
x group interaction). Similarly, the absolute decline in palmitate
Ra was greater (P < 0.05) in men with HIV-dyslipidemia
(0.64 ± 0.07 µmol · kg body wt-1
· min-1) than in healthy men (0.34 ± 0.08
µmol · kg body wt-1 ·
min-1). Therefore, absolute palmitate Ra was
similar in the two groups during glucose infusion
(Fig. 1).
|
VLDL-TG Kinetics
Basal VLDL-TG FCR was 30% lower in men with HIV-dyslipidemia (0.23
± 0.02 pools/h) than in healthy men (0.33 ± 0.02 pools/h);
glucose infusion decreased VLDL-TG FCR to 0.26 ± 0.04 pools/h in
healthy men and to 0.15 ± 0.02 pools/h in men with HIV-dyslipidemia
(group effect: P < 0.05; treatment effect: P < 0.01;
no treatment x group interaction).
During basal conditions, the VLDL-TG secretion rate was two times as high
in men with HIV-dyslipidemia than in healthy men
(Fig. 2). Glucose infusion
caused an 30% reduction (P < 0.01) in VLDL-TG secretion rate
in both men with HIV-dyslipidemia and healthy men, but the rate of VLDL-TG
secretion remained higher (P < 0.01) in men with HIV-dyslipidemia
than in healthy men during glucose infusion.
|
Basal VLDL-TG plasma clearance was 30% lower in men with
HIV-dyslipidemia than in healthy men (13 ± 1 and 19 ± 2 ml/min);
glucose infusion significantly decreased VLDL-TG plasma clearance in both men
with HIV-dyslipidemia (to 9 ± 1 ml/min) and healthy men (to 15 ±
1 ml/min; group effect: P < 0.05; treatment effect: P
< 0.05; no treatment x group interaction).
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Basal plasma FFA concentrations and palmitate Ra were much greater in men with HIV-dyslipidemia than in healthy men. HIV infection itself rather than HAART was probably responsible for the alterations in fatty acid metabolism, because basal glycerol kinetics, an index of adipose lipolytic activity, have been reported to be similar in subjects with HIV-dyslipidemia and lipodystrophy either receiving HAART or not (49). The mechanism responsible for the increase in lipolytic rate in our subjects is not known. Even though insulin is a major regulator of basal lipolytic rate (21), and adipose tissue insulin resistance has been demonstrated in a previous study conducted in patients with HIV-lipodystrophy (48), it is unlikely that insulin resistance was responsible for increased lipolysis in our subjects. Glucose infusion and concomitant hyperinsulinemia caused a similar relative decrease and a greater absolute decrease in palmitate Ra in our men with HIV-dyslipidemia than in healthy men. Therefore, our data suggest that the sensitivity of adipose tissue lipolysis to insulin is not impaired in men with HIV-dyslipidemia who have normal oral glucose tolerance.
It is possible that the greater basal lipolytic rate in our men with
HIV-dyslipidemia was the result of increased -adrenergic stimulation of
adipose tissue. Other investigators have found increased plasma and urinary
catecholamine concentrations in subjects with HIV infection
(40,
49). In addition, increased
-adrenergic stimulation is responsible for increased basal lipolytic
rates in other inflammatory diseases, such as cancer
(25).
Our HIV-infected subjects had several important risk factors for
dyslipidemia. The presence of regional lipodystrophy is strongly associated
with hypertriglyceridemia
(16). All of our subjects had
a mixed lipodystrophy syndrome, which was characterized by the presence of
lipoatrophy of the face and limbs and lipohypertrophy of the posterior
cervical vertebrae and abdomen. In addition, treatment with PIs increases the
risk of developing hypertriglyceridemia
(5,
9). Four of our six
HIV-infected subjects were receiving PI therapy at the time of our study; the
other two subjects were not being actively treated with PIs but had received
PI therapy for 3-5 yr in the past. Although improvement of dyslipidemia has
been reported after switching from PIs to nonnucleoside RT inhibitors
(30), no improvement occurred
in body fat redistribution or blood lipids in our two subjects who had stopped
PI therapy 2 yr before our study. Despite the heterogeneity in
medications, fatty acid and lipoprotein kinetics were similar in all of our
subjects with HIV-dyslipidemia.
Although our men with HIV-dyslipidemia may have had normal insulin sensitivity in peripheral adipose tissue, the high rates of fatty acid release into plasma could cause resistance to insulin action in other tissues, such as the liver or skeletal muscles (6). In fact, it has recently been shown that administration of acipimox, which inhibits adipose tissue lipolysis, improved insulin sensitivity in men with HIV infection and fat redistribution (18).
Hepatic fatty acid availability is the major regulator of VLDL-TG secretion
(27). Therefore, increased
release of fatty acids from subcutaneous adipose tissue probably contributed
to the elevated VLDL-TG secretion rate in our men with HIV-dyslipidemia. Fatty
acid oxidation was found not to be increased in subjects with HIV-dyslipidemia
and lipodystrophy (44), which
suggested that excess fatty acids that are delivered to the liver are
reesterified and stored as intrahepatic TG and/or secreted as part of VLDL. It
is also likely that increased fatty acids from other sources also contributed
to VLDL-TG production in our men with HIV-dyslipidemia. Although we did not
measure intrahepatic fat content, it has previously been found that
intrahepatic TG content is greater in HIV-infected subjects with dyslipidemia
and lipodystrophy than in healthy subjects
(47). Our men with
HIV-dyslipidemia had 40% more trunk FM compared with the healthy
subjects; most of the excess trunk fat was probably located within the
intra-abdominal region (40).
Intraperitoneal fat cells are more lipolytically active than adipocytes in
subcutaneous adipose tissue (2,
41), and the inhibitory effect
of insulin on lipolysis is less pronounced in intra-abdominal than
subcutaneous adipose tissue
(31). Fatty acids derived from
de novo lipogenesis contribute <5% to total VLDL-TG secretion in healthy
men (1). However, increased
rates of hepatic de novo lipogenesis have been observed in patients with HIV
infection (20). Therefore, it
is likely that increased hepatic fatty acid availability, derived from
lipolysis of subcutaneous, intra-abdominal, and intrahepatic TG and from de
novo lipogenesis, may have contributed to the high rates of basal VLDL-TG
secretion in our men with HIV-dyslipidemia.
Although we did not measure the rate of VLDL-apoB secretion in the current study, our data are consistent with those from a previous study that found significantly higher rates of VLDL-apoB secretion in HIV-infected subjects receiving HAART than in a healthy control group (42). The increased rate of VLDL-apoB secretion was predominantly the result of secretion of large, buoyant, TG-rich VLDL particles (VLDL1) and was accompanied by a reduction in transfer of lipid from VLDL1 to denser, less TG-rich VLDL2 (42). Liang et. al. (29) found that PIs inhibit proteosomal degradation of apoB in hepatoma cells, which may be an important mechanism for the increase in VLDL-apoB production observed in vivo. Taken together, our findings and those from these previous studies (42) suggest that both the number and the size of VLDL particles secreted by the liver are greater in subjects with HIV-dyslipidemia than in healthy subjects.
The effect of glucose and/or insulin on VLDL-TG metabolism in subjects with HIV infection has not been studied previously. Hyperglycemia-hyperinsulinemia (1) and conditions that favor hepatic lipogenesis (22, 43) increase the relative contribution of de novo synthesized fatty acids to total VLDL-TG secretion. Furthermore, hyperglycemia-hyperinsulinemia inhibits fatty acid oxidation in the liver (45), which increases the availability of fatty acids delivered to the liver for VLDL-TG synthesis. However, the effect of insulin on adipose tissue lipolysis markedly reduces the delivery of plasma fatty acids to the liver and total hepatic fatty acid availability, which probably mediates a decrease in VLDL-TG production during hyperglycemia-hyperinsulinemia (28, 33). We found that the relative decrease of VLDL-TG secretion in response to glucose infusion was not different in men with HIV-dyslipidemia and healthy men. Nonetheless, the absolute rate of VLDL-TG secretion was greater in men with HIV-dyslipidemia than in healthy men during glucose infusion despite similar rates of fatty acid Ra and plasma fatty acid concentrations, suggesting that factors other than plasma fatty acid availability stimulate VLDL-TG production in men with HIV infection.
Plasma clearance of VLDL-TG was lower in men with HIV-dyslipidemia than in
healthy men during both basal conditions and glucose infusion. The reduced
rate of clearance may be related to alterations in lipoprotein lipase (LPL)
activity associated with HIV infection or HIV therapy. Baril et al.
(4) found that postheparin LPL
activity was 50% lower in men with HIV-dyslipidemia receiving HAART than
in age-matched healthy men, and Sekhar et al.
(44) reported impaired
disposal of chylomicrons and VLDL-TG in patients with HIV-lipodystrophy during
postprandial conditions. In addition, 2 wk of treatment with ritonavir
decreased hepatic lipase activity in healthy men
(37), and both saquinavir and
indinavir reduced LPL activity in vitro in cultured adipocytes
(38). Therefore, these data
suggest that medications used to treat HIV infection affect whole body LPL
activity, but it is not known if this effect is specific to certain
tissues.
In summary, the results of this study demonstrate the presence of considerable abnormalities in fatty acid and lipoprotein metabolism in men with HIV-dyslipidemia. Moreover, these alterations in lipid metabolism occurred before there was clinical evidence of insulin-resistant glucose metabolism, such as impaired oral glucose tolerance or type 2 diabetes. The rate of hepatic VLDL-TG secretion during fed and fasted conditions was much greater, and clearance of VLDL-TG from plasma was slightly lower, in men with HIV-dyslipidemia than in healthy men. Therefore, both increased VLDL-TG secretion into plasma and decreased plasma VLDL-TG clearance contribute to hypertriglyceridemia. Although it is likely that increased basal rates of fatty acid release into plasma stimulated basal VLDL-TG production in men with HIV-dyslipidemia, other factors must be involved because suppression of lipolysis by hyperinsulinemia did not normalize the VLDL-TG secretion rate.
![]() |
DISCLOSURES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
FOOTNOTES |
---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|