Effects of Pioglitazone on Reverse Cholesterol Transport and HDL Function in Persons With Diabetes

Thiazolidinediones (TZDs) are pharmacological ligands for the nuclear receptor peroxisome-proliferator-activated receptor gamma (PPAR-γ). When activated, the receptor binds with response elements on DNA, altering transcription of a variety of genes that regulate carbohydrate and lipid metabolism1. The hypoglycemic and insulin sensitizing effects of PIO and other TZD compounds are well established2-4. The most prominent effect is increased insulin-stimulated glucose uptake by skeletal muscle cells5,6. The receptor is most highly expressed in adipocytes, while expression in myocytes is comparatively minor. Therefore, the increase in glucose uptake by muscle may largely be an indirect effect mediated through TZD interaction with adipocytes7-9. Candidates for the intermediary signal between fat and muscle include leptin, free fatty acids, tumor necrosis factor-α, adiponectin, and resistin.

T2D is associated with a cluster of lipid and lipoprotein abnormalities including reduced HDL, elevated triglycerides and a predominance of small dense LDL particles10. Altered metabolism of triglyceride rich lipoproteins is crucial in the pathophysiology of diabetic dyslipidemia. Alterations include increased hepatic production and delayed clearance from plasma of large very low density lipoproteins (VLDL) and intestinal chylomicrons. Increased levels of these particles also results in increased production of small dense low density lipoprotein (LDL). The reduction in high density lipoprotein (HDL) associated with T2D appears related to CETP-mediated transfer of cholesterol from HDL to triglyceride rich particles in exchange for triglyceride. The triglyceride rich HDL are hydrolyzed by hepatic lipase, reducing particles size, then more rapidly cleared from the circulation11. Reduced HDL is due to mostly a decrease in HDL2, however, there are increased levels of small HDL3 12.

In addition to their ability to induce insulin sensitivity in T2D subjects, TZDs also have certain lipid benefits. HDL cholesterol concentrations are often increased with TZD therapy and triglyceride concentrations frequently fall13. A nonrandomized clinical comparison of potential differences in lipid effects among TZDs14 demonstrated the beneficial effect on lipids was most with pioglitazone (PIO) and least with rosiglitazone (ROSI)15. These observations were confirmed in a study investigating the lipid-lowering effects of TZDs showing that PIO was associated with significantly greater improvements in triglycerides, HDL cholesterol, non-HDL cholesterol, and LDL particle size compared with ROSI 16. The mechanism(s) by which these agents exert differential effects on the lipid profile are not clearly understood. Whether these differences in lipid effects translate into differences for the risk of CVD is not clear. Trials to determine the effects of pioglitazone and rosiglitazone on CVD outcomes are underway and should identify any cardiovascular benefits of the two drugs.

Lipid metabolism plays a central role in the development of atherosclerosis. Elevated LDL and decreased HDL cholesterol are important risk factors for the development of coronary artery disease (CAD). The major cholesterol-carrying lipoprotein in the blood is LDL and many studies have shown the independent relationship between LDL cholesterol and atherosclerosis in both non-diabetic and diabetic subjects17. The metabolism of HDL, which are inversely related to risk of atherosclerotic cardiovascular disease, involves a complex interplay of factors regulating HDL synthesis, intravascular remodeling, and catabolism18. The anti-atherogenic property of HDL has been attributed, at least in part, to the ability of HDL to promote cholesterol removal (efflux) from cells, the first step in the reverse cholesterol transport pathway 19.

Reduced HDL in T2D results from increased clearance of small HDL particles20, and PIO treatment of these subjects raises HDL levels by 10-15% through as yet poorly defined mechanisms. Studies by Ginsberg and colleagues21, in an elegant study, examined the effects PIO treatment in patients with T2D on various aspects of lipoprotein metabolism. PIO raised HDL cholesterol levels 14%, but no change in apoA-I production rates, or fall in apoA-I synthetic rates were observed during PIO therapy22. ApoA-I synthesis is regulated by several transcription factors, including PPAR-α; there is no evidence that PPAR-α plays a role in apoA-I synthesis in vivo, although both PIO and ROSI have been reported to stimulate apoA-I secretion from HepG2 cells23. The authors suggest that the rise in HDL may have resulted from reduced CETP-mediated exchange of VLDL triglycerides for HDL cholesterol, concomitant with the PIO-associated fall in VLDL levels or a reduced the mass or activity of HL thus increasing HDL levels. There are no published data regarding PPAR-γ agonists on HL activity, but the authors found no change in HL mass in preheparin serum by PIO treatment. A final possibility proposed by these authors was PPAR-γ signaling may play a role in stimulating expression of the gene encoding ABCA1 which could increase the flux of cholesterol from cells onto nascent apoA-I.