Effects of intravenous and dietary lipid challenge on intramyocellular lipid content and the relation with insulin sensitivity in humans

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.

Retinoid X receptor expression in skeletal muscle of nondiabetic, obese and type 2 diabetic individuals

Retinoid X receptor (RXR) is a nuclear receptor that functions as an obligate heterodimeric partner of peroxisome proliferator-activator receptor (PPAR). Studies have shown that the alpha isoform of RXR and PPARgamma act synergistically to regulate gene expression and insulin action. The aim of the current study was to compare the expression and regulation of RXR in the primary insulin-sensitive tissue, skeletal muscle, of various degrees of insulin-resistant states including obese type 2 diabetic (T2D), obese nondiabetic (OND), and lean nondiabetic (LND) subjects. Insulin action/resistance was determined by a 3-hour hyperinsulinemic, euglycemic (5.0 to 5.5 mmol/L) clamp. Percutaneous biopsy of the vastus lateralis muscle was performed before and after the clamp. RXRalpha mRNA was measured using a quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assay, while protein was determined by Western blotting. All 3 isoforms of RXR, alpha, beta, and gamma, were present in skeletal muscle. Protein expression of RXR isoforms did not differ between groups; RXR alpha mRNA was also similar between groups. Neither RXR alpha mRNA, RXR -beta nor -gamma protein displayed significant relationships with any of the clinical or laboratory parameters measured, including insulin sensitivity. RXR alpha exhibited a negative correlation with free fatty acids levels (r, -.42, P <.05). There was also no relationship between RXR alpha and PPARgamma protein levels. RXR alpha mRNA was unaltered following insulin infusion. We conclude that RXR isoform (alpha, beta, gamma) expression is not tightly controlled by insulin, insulin resistance or type 2 diabetes. Instead, RXR isoforms are likely constitutive proteins or controlled by other factors.

Distribution of peroxisome proliferator-activated receptors (PPARs) in human skeletal muscle and adipose tissue: relation to insulin action

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 PPAR alpha, PPAR beta (delta) and PPAR gamma protein expression by western blot analysis. The PPAR gamma protein was also measured in muscle before and after 3-h hyperinsulinaemic (300 mU.m-2.min-1) euglycaemic clamps.

RESULTS

The PPAR alpha protein was expressed preferentially in muscle relative to fat (more than sevenfold). The PPAR beta protein was similar in fat and muscle. The amount of PPAR gamma 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 (preclamp) muscle PPAR (alpha, beta or gamma) protein expression. Subgroup analysis showed, however, significantly higher PPAR gamma 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/microgram protein, p < or = 0.01 in diabetic and vs 94 +/- 15, p < or = 0.04 in non-diabetic subjects). Muscle PPAR gamma protein and glucose disposal rates were inversely correlated in diabetic subjects (r = -0.47, p < or = 0.05).

CONCLUSION/INTERPRETATION

All PPARs (alpha, beta or gamma) are present in skeletal muscle and adipose tissue with different relative distributions. The PPAR gamma protein is abundant in skeletal muscle as well as adipose tissue. The altered expression of skeletal muscle PPAR gamma is consistent with a role for this nuclear protein in the impaired insulin action of Type II diabetes.