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Facilitated pyruvate transport across the mitochondrial inner membrane is a critical step in carbohydrate, amino acid, and lipid metabolism. We report that clinically relevant concentrations of thiazolidinediones (TZDs), a widely used class of insulin sensitizers, acutely and specifically inhibit mitochondrial pyruvate carrier (MPC) activity in a variety of cell types. Respiratory inhibition was overcome with methyl pyruvate, localizing the effect to facilitated pyruvate transport, and knockdown of either paralog, MPC1 or MPC2, decreased the EC 50 for respiratory inhibition by TZDs. Acute MPC inhibition significantly enhanced glucose uptake in human skeletal muscle myocytes after 2 h. These data ( i ) report that clinically used TZDs inhibit the MPC, ( ii ) validate that MPC1 and MPC2 are obligatory components of facilitated pyruvate transport in mammalian cells, ( iii ) indicate that the acute effect of TZDs may be related to insulin sensitization, and ( iv ) establish mitochondrial pyruvate uptake as a potential therapeutic target for diseases rooted in metabolic dysfunction.

Type 2 diabetic subjects failing glyburide therapy were randomized to receive additional therapy with either metformin (2,550 mg/day) or troglitazone (600 mg/day) for 3–4 months. Biopsies of subcutaneous abdominal adipose tissue were obtained before and after therapy. Glycemic control was similar with both treatments. Metformin treatment increased insulin-stimulated whole-body glucose disposal rates by 20% (P < 0.05); the response to troglitazone was greater (44% increase, P < 0.01 vs. baseline, P < 0.05 vs. metformin). Troglitazone-treated subjects displayed a tendency toward weight gain (5 ± 2 kg, P < 0.05), increased adipocyte size, and increased serum leptin levels. Metformin-treated subjects were weight-stable, with unchanged leptin levels and reduced adipocyte size (to 84 ± 4% of control, P < 0.005). Glucose transport in isolated adipocytes from metformin-treated subjects was unaltered from pretreatment. Glucose transport in both the absence (321 ± 134% of pre-Rx, P < 0.05) and presence of insulin (418 ± 161%, P < 0.05) was elevated after troglitazone treatment. Metformin treatment had no effect on adipocyte content of GLUT1 or GLUT4 proteins. After troglitazone treatment, GLUT4 protein expression was increased twofold (202 ± 42%, P < 0.05). Insulin-stimulated serine phosphorylation of Akt was augmented after troglitazone (170 ± 34% of pre-Rx response, P < 0.05) treatment and unchanged by metformin. We conclude that the ability of troglitazone to upregulate adipocyte glucose transport, GLUT4 expression, and insulin signaling can contribute to its greater effect on whole-body glucose disposal.

An increased intramyocellular lipid (IMCL) content, as quantified by (1)H-magnetic resonance spectroscopy ((1)H-MRS), is associated with reduced insulin sensitivity. At present, it is unclear which factors determine IMCL formation and how rapidly IMCL accumulation can be induced. We therefore studied the impact of hyperinsulinemia and elevated circulating nonesterified fatty acid (NEFA) levels on IMCL formation and insulin sensitivity. We further evaluated the influence of a high-fat diet on IMCL storage. In the infusion protocol, 12 healthy male subjects underwent a 6-h hyperinsulinemic-euglycemic glucose clamp with concomitant infusion of Intralipid plus heparin. IMCL was quantified by (1)H-MRS in soleus (SOL) and tibialis anterior (TA) muscle at baseline and then every hour. IMCL levels started to increase significantly after 2 h, reaching a maximum of 120.8 +/- 3.4% (SOL) and 164.2 +/- 13.8% (TA) of baseline after 6 h (both P < 0.05). In parallel, the glucose infusion rate (GIR) decreased progressively, reaching a minimum of 60.4 +/- 5.4% of baseline after 6 h. Over time, the GIR was strongly correlated with IMCL in TA (r = -0.98, P < or = 0.003) and SOL muscle (r = -0.97, P < or = 0.005). In the diet protocol, 12 male subjects ingested both a high-fat and low-fat diet for 3 days each. Before and after completion of each diet, IMCL levels and insulin sensitivity were assessed. After the high-fat diet, IMCL levels increased significantly in TA muscle (to 148.0 +/- 16.9% of baseline; P = 0.005), but not in SOL muscle (to 114.4 +/- 8.2% of baseline; NS). Insulin sensitivity decreased to 83.3 +/- 5.6% of baseline (P = 0.033). There were no significant changes in insulin sensitivity or IMCL levels after the low-fat diet. The effects of the high-fat diet showed greater interindividual variation than those of the infusion protocol. The data from the lipid infusion protocol suggest a functional relationship between IMCL levels and insulin sensitivity. Similar effects could be induced by a high-fat diet, thereby underlining the physiological relevance of these observations.

Aims/hypothesis. To evaluate the tissue distribution and possible role of the peroxisome proliferator-activated receptors (PPARs) in insulin action in fat and muscle biopsy specimens from lean, obese and subjects with Type II (non-insulin-dependent) diabetes mellitus.¶Methods. We measured PPARα, PPARβ(δ) and PPARγ protein expression by western blot analysis. The PPARγ protein was also measured in muscle before and after 3-h hyperinsulinaemic (300 mU · m–2· min–1) euglycaemic clamps.¶Results. The PPARα protein was expressed preferentially in muscle relative to fat (more than sevenfold). The PPARβ protein was similar in fat and muscle. The amount of PPARγ protein found in muscle was, on average, two-thirds of that present in fat. There was no statistically significant difference between non-diabetic and diabetic subjects in baseline (pre-clamp) muscle PPAR (α, β or γ) protein expression. Subgroup analysis showed, however, significantly higher PPARγ protein in the most insulin resistant diabetic subjects with glucose disposal rates of 3–6 mg · kg–1· min–1 compared with their age and weight matched counterparts with glucose disposal rates of 6–9 (147 ± 23 vs 88 ± 10 AU/μg protein, p≤ 0.01 in diabetic and vs 94 ± 15, p≤ 0.04 in non-diabetic subjects). Muscle PPARγ protein and glucose disposal rates were inversely correlated in diabetic subjects (r = –0.47, p≤ 0.05).¶Conclusion/interpretation. All PPARs (α, β or γ) are present in skeletal muscle and adipose tissue with different relative distributions. The PPARγ protein is abundant in skeletal muscle as well as adipose tissue. The altered expression of skeletal muscle PPARγ is consistent with a role for this nuclear protein in the impaired insulin action of Type II diabetes. [Diabetologia (2000) 43: 304–311]