Pioglitazone stimulates apolipoprotein A-I production without affecting HDL removal in HepG2 cells: involvement of PPAR-alpha

OBJECTIVE

Pioglitazone, an antihyperglycemic drug, increases plasma high-density lipoprotein (HDL)-cholesterol in patients with type 2 diabetes. The mechanisms by which pioglitazone regulate HDL levels are not clear. This study examined the effect of pioglitazone on hepatocyte apolipoprotein AI (apoA-I) and apoA-II production and HDL-protein/cholesterol ester uptake.

METHODS AND RESULTS

In human hepatoblastoma (HepG2) cells, pioglitazone, dose-dependently (0.5 to 10 micromol/L), increased the de novo synthesis (up to 45%), secretion (up to 44%), and mRNA expression (up to 59%) of apoA-I. Pioglitazone also increased apoA-II de novo synthesis (up to 73%) and mRNA expression (up to 129%). Pioglitazone did not affect the uptake of HDL3-protein or HDL3-cholesterol ester in HepG2 cells. The pioglitazone-induced apoA-I lipoprotein particles increased cholesterol efflux from THP-1 macrophages. The pioglitazone-induced apoA-I secretion or mRNA expression by the HepG2 cells was abrogated with the suppression of PPAR-alpha by small interfering RNA or a specific inhibitor of PPAR-alpha, MK886.

CONCLUSIONS

The data indicate that pioglitazone increases HDL by stimulating the de novo hepatic synthesis of apoA-I without affecting hepatic HDL-protein or HDL-cholesterol removal. We suggest that pioglitazone-mediated hepatic activation of PPAR-alpha may be one of the mechanisms of action of pioglitazone to raise hepatic apoA-I and HDL.

Effectiveness and tolerability of a new lipid-altering agent, gemcabene, in patients with low levels of high-density lipoprotein cholesterol

This study evaluated the efficacy and tolerability of gemcabene, a new lipid-altering agent, in a double-blind, randomized, dose-response study of 161 patients with high-density lipoprotein (HDL) cholesterol of <35 mg/dl and serum triglyceride (TG) levels of either >/=200 (n = 94) or <200 mg/dl (n = 67). After a 6-week, placebo, dietary lead-in period, patients were administered either 150, 300, 600, or 900 mg of gemcabene or placebo once daily for 12 weeks. In the TG >/=200 mg/dl stratum, gemcabene significantly increased serum HDL cholesterol by 18% with corresponding significant increases of 6% in both apolipoprotein A-I and A-II levels at the 150-mg dose. HDL cholesterol levels also increased 12% at the 300-mg dose; however, this did not reach statistical significance. Also, in the TG >/=200 mg/dl stratum, serum TG levels were significantly reduced by 27% and 39% at the 150- and 300-mg doses of gemcabene, respectively. No significant differences were found in serum HDL cholesterol or TG levels in the TG >/=200 mg/dl groups that received 600 or 900 mg of gemcabene, or in TG <200 mg/dl groups administered any dose of gemcabene. However, at these higher 600- and 900-mg doses, gemcabene significantly reduced serum low-density lipoprotein (LDL) cholesterol levels by 15% to 25%, respectively, in both TG strata, with proportionate decreases in the levels of apolipoprotein B. Gemcabene was well tolerated with a frequency of adverse events similar to that of placebo. In conclusion, at the lower doses, gemcabene significantly increased HDL cholesterol and reduced TG serum levels in patients with low HDL cholesterol and TG >/=200 mg/dl. At the higher doses, gemcabene significantly reduced LDL cholesterol levels in all patients with low HDL cholesterol.

Dietary fat modulation of apoA-II metabolism and prevention of senile amyloidosis in the senescence- accelerated mouse

Senescence-accelerated mouse-prone (SAMP1; SAMP1@Umz) is an animal model of senile amyloidosis with apolipoprotein A-II (apoA-II) amyloid fibril (AApoAII) deposits. This study was undertaken to investigate the effects of dietary fats on AApoAII deposits in SAMP1 mice when purified diets containing 4% fat as butter, safflower oil, or fish oil were fed to male mice for 26 weeks. The serum HDL cholesterol was significantly lower (P < 0.01) in mice on the diet containing fish oil (7.4 +/- 3.0 mg/dl) than in mice on the butter diet (38.7 +/- 12.5 mg/dl), which in turn had significantly lower (P < 0.01) HDL levels than mice on the safflower oil diet (51.9 +/- 5.6 mg/dl). ApoA-II was also significantly lower (P < 0.01) in mice on the fish oil diet (7.6 +/- 2.7 mg/dl) than on the butter (26.9 +/- 7.3 mg/dl) or safflower oil (21.6 +/- 3.7 mg/dl) diets. The mice fed fish oil had a significantly greater ratio (P < 0.01) of apoA-I to apoA-II, and a smaller HDL particle size than those fed butter and safflower oil. Severe AApoAII deposits in the spleen, heart, skin, liver, and stomach were shown in the fish oil group compared with those in the butter and safflower oil groups (fish oil > butter > safflower oil group, P < 0.05). These findings suggest that dietary fats differ in their effects on serum lipoprotein metabolism, and that dietary lipids may modulate amyloid deposition in SAMP1 mice.

Dynamic changes in mouse lipoproteins induced by transiently expressed human phospholipid transfer protein (PLTP): importance of PLTP in prebeta-HDL generation

The plasma phospholipid transfer protein (PLTP) plays an important role in the regulation of plasma high density lipoprotein (HDL) levels and governs the distribution of HDL sub-populations. In the present study, adenovirus mediated overexpression of human PLTP in mice was employed to investigate the distribution of PLTP in serum and its effect on plasma lipoproteins. Gel filtration experiments showed that the distributions of PLTP activity and mass in serum are different, suggesting that human PLTP circulated in mouse plasma as two distinct forms, one with high and the other with low specific activity. Our study further demonstrates that overexpression of PLTP leads to depletion of HDL and that, as PLTP activity declines, replenishment of the HDL fraction occurs. During this process, the lipoprotein profile displays transient particle populations, including apoA-IV and apoE-rich particles in the LDL size range and small particles containing apoA-II only. The possible role of these particles in HDL reassembly is discussed. The increased PLTP activity enhanced the ability of mouse sera to produce pre(beta)-HDL. The present results provide novel evidence that PLTP is an important regulator of HDL metabolism and plays a central role in the reverse cholesterol transport (RCT) process.

Albumin inhibits apolipoprotein AI and AII production in human hepatoblastoma cell line (Hep G2): additive effects of oleate-albumin complex

Although the role of multiple humoral agents (such as plasma albumin, glucose, hormones etc.) are implicated in lipoprotein metabolism, the mechanism of action of these agents on various steps of the synthesis and secretion of lipoproteins and apolipoproteins (protein moieties of lipoproteins) are not completely understood. Specifically, the hepatocellular mechanisms of the effect of albumin and fatty acids on apolipoprotein (apo) AI and AII [major proteins of high density lipoproteins (HDL)] synthesis and secretion are not known. Using human hepatoblastoma cells (Hep G2) as an in vitro model system, this study examined the effect of albumin and fatty acids on the synthesis, secretion, and the steady-state mRNA expression of apo AI and AII. The data indicated that the incubation of Hep G2 cells with albumin, dose-dependently, inhibited apo AI and AII accumulation (secretion) in the media, de novo synthesis, and the steady-state mRNA expression. Albumin did not alter total protein synthesis; thus the effect of albumin appeared to be specific for the synthesis and secretion of apo AI and apo AII. Free fatty acids (FFA) are transported by albumin and diseases characterized by enhanced FFA mobilization (e.g. diabetes mellitus) are associated with low HDL levels. Studies were therefore performed to examine the effect of albumin-bound-oleic acid on apo AI and apo AII production. The results showed that the albumin-oleate complex further increased the inhibitory effects of albumin on apo AI and apo AII production. These data suggest how HDL metabolism may be affected at the hepatocellular level by alterations in plasma albumin concentrations and/or fatty acid mobilization in clinical situations characterized by altered HDL levels.

A controlled trial of the effects of pattern of alcohol intake on serum lipid levels in regular drinkers

To determine whether the effects of drinking pattern (predominantly weekend versus daily drinking) have differential effects on serum lipids, 55 healthy male drinkers were recruited on the basis of a regular alcohol intake, 210-500 ml absolute alcohol/week (approximately 3-6 standard drinks/day), with more than 60% consumed as beer. Fourteen subjects were categorised as predominantly weekend drinkers, while 41 subjects regularly drank on a daily basis. After maintenance of their drinking pattern during a 4-week familiarisation, subjects were randomised to either consume low alcohol beer (0.9%, v/v) only, or to maintain their usual drinking habit consuming full-strength beer (5%, v/v) for the next 4 weeks. They then switched to full-strength or low alcohol beer, respectively, for a further 4 weeks. Their drinking pattern remained constant during the study. In both weekend and daily drinkers, a reduction in alcohol intake (i.e. from 387 ml/week to 88 ml/week for weekend drinkers and from 418 ml/week to 95 ml/week for daily drinkers, respectively, P < 0.001) resulted in a similar 0.12 mmol/l fall in HDL-C (P < 0.01) with a concomitant significant fall in both apolipoproteins A-I and A-II. In daily drinkers total cholesterol fell by 0.28 mmol/l (P < 0.001) and triglyceride by 0.22 mmol/l (P < 0.01) with a reduction in alcohol intake, but no change in LDL-C was seen. In contrast, weekend drinkers total cholesterol was unchanged while triglyceride decreased by 0.26 mmol/l (P < 0.05) and LDL-C increased by 0.25 mmol/l (P < 0.01). Lp(a) increased with a reduction in alcohol intake in both daily (9.1 U/l, P < 0.05) and weekend drinkers (27.6 U/l, P = 0.07). Previous reports of a more atherogenic lipid profile with episodic versus regular daily drinking were not confirmed in this study and potentially favourable effects of alcohol to increase HDL-C and decrease Lp(a) were shown to be independent of drinking pattern in these moderate to heavy drinkers.

Apolipoprotein A-II levels and coronary artery disease in subjects with and without diabetes: a study with use of a specific radioimmunoassay for apolipoprotein A-II

High-density lipoprotein cholesterol (HDL-C), apolipoprotein (apo) A-I, and apo A-II levels were measured in 1,219 normal subjects with no clinical evidence of coronary artery disease, 81 subjects without diabetes but with "significant" coronary artery disease determined by coronary arteriography, and 151 subjects with non-insulin-dependent diabetes mellitus (48 with clinical coronary artery disease and 103 without such disease). In the normal subjects, apo A-II levels were less influenced by age, gender, and use of medications than were apo A-I or HDL-C levels. HDL-C, apo A-I, and apo A-II levels were significantly lower in subjects who had coronary artery disease with or without diabetes than in control subjects. After adjustments were made for age and sex, however, apo A-II levels were no longer significantly different between subjects with diabetes who had and those who did not have coronary artery disease. In subjects without diabetes, apo A-II may provide some advantages over apo A-I and HDL-C in the assessment of risk of coronary artery disease because it is influenced less by age, gender, and medications. In subjects with diabetes, however, apo A-II levels are similar in the presence or absence of coronary artery disease.