The unfolded protein response and its activation by insulin in muscle are not altered by obesity or type 2 diabetes

Insulin resistance in obesity and type 2 diabetes (T2D) is characterized by reduced insulin-stimulated glucose uptake, accumulation of triacylglycerol, mitochondrial dysfunction, and altered protein metabolism in skeletal muscle. This may involve disturbed endoplasmic reticulum (ER) homeostasis, leading to alterations in the unfolded protein response (UPR) and, hence, the protein folding capacity. Here, we investigated whether markers of UPR activity are elevated in skeletal muscle in obesity and T2D and to what extent insulin regulates these UPR markers. In a case-control design, we determined mRNA expression, protein abundance, and phosphorylation of key UPR markers in skeletal muscle biopsies obtained from patients with T2D, matched to glucose-tolerant individuals with obesity and lean individuals, before and after 4-h insulin infusion during a hyperinsulinemic-euglycemic clamp. The mRNA expression or protein abundance of GRP78, the canonical ER stress sensors (ATF6, PERK, and IRE-1α), several downstream UPR markers, and related markers of mitochondrial dynamics did not differ between groups. Insulin increased the mRNA expression of ATF6, ERN1 (encoding IRE-1α), XBP1, DDIT3 (encoding CHOP), and a marker of mitochondrial fission, DNM1l (encoding DRP1), as well as eIF2α Ser51 phosphorylation in skeletal muscle in all groups (all P <0.05), with no between-group differences. Our results demonstrate that markers of UPR activity are not elevated in skeletal muscle in obesity or T2D. Interestingly, insulin increases the expression of UPR markers and activates eIF2α, which is necessary for increasing the protein folding capacity of ER in muscle, and these responses are intact in obesity and T2D.