Obesity and Type 2 diabetes are associated with high fat diet (HFD), reduced mitochondrial mass and function and insulin resistance as characterized by glucose disposal and relative to body fatness.
We hypothesized that (a) HFD affects expression of genes involved in mitochondrial biogenesis and function, (b) carbohydrate metabolism and storage is under transcriptional control and (c) both overall fatness and characteristics of adipose tissue influence the interplay between free fatty acids (FFAs) and insulin-stimulated glucose disposal. To test hypotheses “a” and “b”, we fed 10 insulin-sensitive males an isoenergetic HFD for 3 days with muscle biopsies before and after intervention. Oligonucleotide microarrays revealed 370 genes differentially regulated in response to HFD (Bonferonni adjusted p < 0.001). Expression of six genes involved in oxidative phosphorylation decreased. PGC1α and PGC1β mRNAs decreased by ~22%. Seven genes in the carbohydrate metabolism pathway changed in response to HFD, and three genes confirmed by qRT-PCR: PFKFB3, PDK4 and GYS1. In a separate experiment, C57Bl/6J mice were fed HFD for three weeks and the same OXPHOS and PGC1 mRNAs decreased by ~90%, Cytochrome C and PGC1α protein by ~40%, while the same glucose metabolism genes changed by ~70%. These results suggest a mechanism whereby HFD downregulates genes necessary for oxidative phosphorylation and mitochondrial biogenesis, as well as glucose utilization and storage. These changes mimic those observed in diabetes and insulin resistance.
To test hypothesis “c”, we measured changes in respiratory quotient (ΔRQ; metabolic flexibility) before and during euglycemic-hyperinsulinemic clamps in healthy young males. Anthropometric, laboratory measurements, fat biopsies and fat cell size (FCS) were measured after overnight fast. Adipose tissue gene expression (qRT-PCR) was measured. Metabolic inflexibility (lower ΔRQ) was associated with higher body fat, larger FCS and higher insulin-suppressed FFAs. ΔRQ was not related to fasting FFAs, but lower ΔRQ was associated with lower serum adiponectin levels. Higher adipose tissue inflammatory gene expression was associated with higher insulin-suppressed FFAs and lower ΔRQ. These results indicate fatness, adipocyte hypertrophy, blunted insulin suppression of FFAs, decreased adiponectin levels and inflammation, are associated with decreased insulin-stimulated glucose uptake and oxidation, an important component of metabolic inflexibility.