Short-term Acipimox decreases the ability of plasma from Type 2 diabetic patients and healthy subjects to stimulate cellular cholesterol efflux: a potentially adverse effect on reverse cholesterol transport

Niacin: a re-emerging pharmaceutical for the treatment of dyslipidaemia

Dyslipidaemias, particularly those characterized by the 'atherogenic profile' of high low-density lipoprotein-cholesterol and triglycerides and low high-density lipoprotein-cholesterol, are the major modifiable risk factor for atherosclerosis. The search for drugs to favourably alter such lipid profiles, reducing the associated morbidity and mortality, remains a major research focus. Niacin (nicotinic acid) is the most effective agent available for increasing high-density lipoprotein-cholesterol, but its use is associated with side effects that negatively affect patient compliance: these appear to arise largely as a result of production of prostaglandin D(2) and its subsequent activation of the DP(1) receptor. Desire to reduce the side effects (and improve pharmacokinetic parameters) has led to the development of a number of agonists that have differing effects, both in terms of clinical potency and the severity of adverse effects. The recent discovery of the niacin G-protein-coupled receptor HM74A (GPR109A) has clarified the distinction between the mechanism whereby niacin exerts its therapeutic effects and the mechanisms responsible for the generation of side effects. This has allowed the development of new drugs that show great potential for the treatment of dyslipidaemia. However, recent advances in understanding of the contribution of prostaglandin metabolism to vascular wall health suggest that some of the beneficial effects of niacin may well result from activation of the same pathways responsible for the adverse reactions. The purpose of this review is to emphasize that the search for agonists that show higher tolerability must take into account all aspects of signalling through this receptor.

The effect of sensitisation to insulin with pioglitazone on fasting and postprandial lipid metabolism, lipoprotein modification by lipases, and lipid transfer activities in type 2 diabetic patients

AIMS/HYPOTHESIS

Insulin resistance is thought to be central to the pathogenesis of diabetic dyslipidaemia. We hypothesised that improving insulin sensitivity would improve fasting and postprandial triglyceride metabolism in patients with type 2 diabetes. To this aim we studied fasting and postprandial lipaemia in type 2 diabetic patients before and after sensitisation to insulin with pioglitazone, compared with that observed in patients on an insulin-providing regime.

METHODS

In a double-blind placebo-controlled protocol, 22 patients with type 2 diabetes were randomly allocated to receive either pioglitazone (45 mg/day) or glibenclamide (5 mg/day), for a 20-week period. Fasting and postprandial lipid metabolism were investigated at baseline and at the end of the treatment period. A group of non-diabetic subjects was also studied.

RESULTS

Compared with glibenclamide treatment, pioglitazone treatment decreased fasting triglyceride, glucose and insulin levels and the homeostasis model assessment score of insulin resistance. Decreased fasting triglyceride after pioglitazone treatment was due to reduced VLDL triglyceride, particularly VLDL-2. Lipoprotein lipase activity was unchanged by pioglitazone treatment but hepatic lipase showed a significant decrease. Pioglitazone treatment lowered total postprandial triglyceride, as well as chylomicron- and chylomicron-remnant retinyl palmitate levels to normal. Glucose disposal improved but remained abnormal.

CONCLUSIONS/INTERPRETATION

Insulin sensitisation with pioglitazone has major effects in restoring postprandial lipaemia to normal, while also correcting fasting hypertriglyceridaemia; both factors may have consequences for atherogenic risk in diabetes.

Alterations in high-density lipoprotein metabolism and reverse cholesterol transport in insulin resistance and type 2 diabetes mellitus: role of lipolytic enzymes, lecithin:cholesterol acyltransferase and lipid transfer proteins

Insulin resistance and type 2 diabetes mellitus are generally accompanied by low HDL cholesterol and high plasma triglycerides, which are major cardiovascular risk factors. This review describes abnormalities in HDL metabolism and reverse cholesterol transport, i.e. the transport of cholesterol from peripheral cells back to the liver for metabolism and biliary excretion, in insulin resistance and type 2 diabetes mellitus. Several enzymes including lipoprotein lipase (LPL), hepatic lipase (HL) and lecithin: cholesterol acyltransferase (LCAT), as well as cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), participate in HDL metabolism and remodelling. Lipoprotein lipase hydrolyses lipoprotein triglycerides, thus providing lipids for HDL formation. Hepatic lipase reduces HDL particle size by hydrolysing its triglycerides and phospholipids. A decreased postheparin plasma LPL/HL ratio is a determinant of low HDL2 cholesterol in insulin resistance. The esterification of free cholesterol by LCAT increases HDL particle size. Plasma cholesterol esterification is unaltered or increased in type 2 diabetes mellitus, probably depending on the extent of triglyceride elevation. Subsequent CETP action results in transfer of cholesteryl esters from HDL towards triglyceride-rich lipoproteins, and is involved in decreasing HDL size. An increased plasma cholesteryl ester transfer is frequently observed in insulin-resistant conditions, and is considered to be a determinant of low HDL cholesterol. Phospholipid transfer protein generates small pre beta-HDL particles that are initial acceptors of cell-derived cholesterol. Its activity in plasma is elevated in insulin resistance and type 2 diabetes mellitus in association with high plasma triglycerides and obesity. In insulin resistance, the ability of plasma to promote cellular cholesterol efflux may be maintained consequent to increases in PLTP activity and pre beta-HDL. However, cellular cholesterol efflux to diabetic plasma is probably impaired. Besides, cellular abnormalities that are in part related to impaired actions of ATP binding cassette transporter 1 and scavenger receptor class B type I are likely to result in diminished cellular cholesterol efflux in the diabetic state. Whether hepatic metabolism of HDL-derived cholesterol and subsequent hepatobiliary transport is altered in insulin resistance and type 2 diabetes mellitus is unknown. Specific CETP inhibitors have been developed that exert major HDL cholesterol-raising effects in humans and retard atherosclerosis in animals. As an increased CETP-mediated cholesteryl ester transfer represents a plausible metabolic intermediate between high triglycerides and low HDL cholesterol, studies are warranted to evaluate the effects of these agents in insulin resistance- and diabetes-associated dyslipidaemia.

Phospholipid transfer protein

A role for phospholipid transfer protein (PLTP) in HDL remodelling and in the formation of pre-beta-HDL is now well established, both in vivo and in vitro. Over-expression of human PLTP in C57BL6 mice lowers plasma HDL levels, probably because of increased HDL catabolism. Despite these low HDL levels, plasma from these mice mitigates cholesterol accumulation in macrophages and has increased potential for pre-beta-HDL formation. Plasma HDL concentration is also decreased in PLTP knockout mice. These intriguing observations can be explained by recent studies that indicate that PLTP is not only involved in remodelling of HDL subfractions but also in VLDL turnover. The role of PLTP in atherogenesis and VLDL synthesis was demonstrated in transgenic mouse models with increased susceptibility for the development of atherosclerosis, bred into PLTP knockout mice. The data clearly show that PLTP can be proatherogenic. As mentioned above, however, PLTP may have antiatherogenic potential in wild-type C57BL6 mice. Information regarding the role and regulation of PLTP in human (patho)physiology is still relatively sparse but accumulating rapidly. PLTP activity is elevated in diabetes mellitus (both type 1 and type 2), obesity and insulin resistance.