The molecular mechanisms by which thiazolidinediones improve insulin sensitivity in type 2 diabetes are not fully understood. We hypothesised that pioglitazone would activate the adenosine 5′-monophosphate-activated protein kinase (AMPK) pathway and increase the expression of genes involved in adiponectin signalling, NEFA oxidation and mitochondrial function in human skeletal muscle.

A randomised, double-blind, parallel study was performed in 26 drug-naive type 2 diabetes patients treated with: (1) pioglitazone (n = 14) or (2) aggressive nutritional therapy (n = 12) to reduce HbA_1c to levels observed in the pioglitazone-treated group. Participants were assigned randomly to treatment using a table of random numbers. Before and after 6 months, patients reported to the Clinical Research Center of the Texas Diabetes Institute for a vastus lateralis muscle biopsy followed by a 180 min euglycaemic–hyperinsulinaemic (80 mU m⁻² min⁻¹) clamp.

All patients in the pioglitazone (n = 14) or nutritional therapy (n = 12) group were included in the analysis. Pioglitazone significantly increased plasma adiponectin concentration by 79% and reduced fasting plasma NEFA by 35% (both p < 0.01). Following pioglitazone, insulin-stimulated glucose disposal increased by 30% (p < 0.01), and muscle AMPK and acetyl-CoA carboxylase (ACC) phosphorylation increased by 38% and 53%, respectively (p < 0.05). Pioglitazone increased mRNA levels for adiponectin receptor 1 and 2 genes (ADIPOR1, ADIPOR2), peroxisome proliferator-activated receptor gamma, coactivator 1 gene (PPARGC1) and multiple genes involved in mitochondrial function and fat oxidation. Despite a similar reduction in HbA_1c and similar improvement in insulin sensitivity with nutritional therapy, there were no significant changes in muscle AMPK and ACC phosphorylation, or the expression of ADIPOR1, ADIPOR2, PPARGC1 and genes involved in mitochondrial function and fat oxidation. No adverse (or unexpected) effects or side effects were reported from the study.

Pioglitazone increases plasma adiponectin levels, stimulates muscle AMPK signalling and increases the expression of genes involved in adiponectin signalling, mitochondrial function and fat oxidation. These changes may represent an important cellular mechanism by which thiazolidinediones improve skeletal muscle insulin sensitivity.

Trial registration: NCT 00816218

Funding: This trial was funded by National Institutes of Health Grant DK24092, VA Merit Award, GCRC Grant RR01346, Executive Research Committee Research Award from the University of Texas Health Science Center at San Antonio, American Diabetes Association Junior Faculty Award, American Heart Association National Scientist Development Grant, Takeda Pharmaceuticals North America Grant and Canadian Institute of Health Research Grant.