The imprinted gene Delta like non-canonical Notch ligand 1 (Dlk1) is considered an inhibitor of adipogenesis, but its in vivo impact on fat mass indeed remains elusive and controversial. Fat deposits were assessed by MRI and DXA scanning in two cohorts of non-diabetic men, whereas glucose disposal rate (GDR) was determined during euglycemic hyperinsulinemic clamp. Blood analyte measurements were used for correlation and mediation analysis to investigate how age, BMI, and fat percentage affect the relation between DLK1 and GDR. Confirmatory animal studies performed in normal (NC) and high fat diet (HFD) fed Dlk1+/+ and Dlk1-/- mice included DXA scanning, glucose tolerance tests (GTTs), blood measurements, and skeletal muscle glucose uptake studies by positron emission tomography (PET), histology, qRT-PCR, and in vitro cell studies. Overall, DLK1 is positively correlated with fat amounts, which is consistent with a negative linear relationship between DLK1 and GDR. This relationship is not mediated by age, BMI, or fat percentage. In support, DLK1 also correlates positively with HOMA-IR and ADIPO-IR in these humans, but has no linear relationship with the early diabetic inflammation marker MCP-1. In Dlk1-/- mice, the increase in fat percentage and adipocyte size induced by HFD is attenuated, and these animals are protected against insulin resistance. These Dlk1 effects seem independent of gluconeogenesis, but at least partly relies on increased in vivo glucose uptake in skeletal muscles by Dlk1 regulating the major glucose transporter Glut4 in vivo as well as in two independent cell lines. Thus, instead of an adipogenic inhibitor, Dlk1 should be regarded as a factor causally linked to obesity and insulin resistance, and may be used to predict development of type 2 diabetes. FUND: The Danish Diabetes Academy supported by the Novo Nordisk Foundation, The Danish National Research Council (#09-073648), The Lundbeck Foundation, University of Southern Denmark, and Dep. Of Clinical Biochemistry and Pharmacology/Odense University Hospital, the Swedish Research Council, the Swedish Diabetes Foundation, the Strategic Research Program in Diabetes at Karolinska Institute and an EFSD/Lilly grant.

Objective: Recent studies have indicated that serum testosterone in aging men is associated with insulin sensitivity and expression of genes involved in oxidative phosphorylation (OxPhos), and that testosterone treatment increases lipid oxidation. Herein, we investigated the effect of testosterone therapy on regulators of mitochondrial biogenesis and markers of OxPhos and lipid metabolism in the skeletal muscle of aging men with subnormal bioavailable testosterone levels.Methods: Skeletal muscle biopsies were obtained before and after treatment with either testosterone gel (n=12) or placebo (n=13) for 6 months. Insulin sensitivity and substrate oxidation were assessed by euglycemic–hyperinsulinemic clamp and indirect calorimetry. Muscle mRNA levels and protein abundance and phosphorylation of enzymes involved in mitochondrial biogenesis, OxPhos, and lipid metabolism were examined by quantitative real-time PCR and western blotting.Results: Despite an increase in lipid oxidation (P<0.05), testosterone therapy had no effect on insulin sensitivity or mRNA levels of genes involved in mitochondrial biogenesis (PPARGC1A,PRKAA2, andPRKAG3), OxPhos (NDUFS1,ETFA,SDHA,UQCRC1, andCOX5B), or lipid metabolism (ACADVL,CD36,CPT1B,HADH, andPDK4). Consistently, protein abundance of OxPhos subunits encoded by both nuclear (SDHAandUQCRC1) and mitochondrial DNA (ND6) and protein abundance and phosphorylation of AMP-activated protein kinase and p38 MAPK were unaffected by testosterone therapy.Conclusion: The beneficial effect of testosterone treatment on lipid oxidation is not explained by increased abundance or phosphorylation-dependent activity of enzymes known to regulate mitochondrial biogenesis or markers of OxPhos and lipid metabolism in the skeletal muscle of aging men with subnormal bioavailable testosterone levels.

Background: Hypogonadism is prevalent during opioid treatment, but the effect of testosterone replacement treatment (TRT) on body composition, pain perception, and adrenal function is unclear. Purpose: To measure changes in body composition, pain perception, quality of life, and adrenal function after TRT or placebo in opioid-treated men with chronic non-malignant pain. Methods: Double-blind, placebo-controlled study in 41 men (>18 years) with total testosterone <12 nmol/L were randomized to 24 weeks TRT (Testosterone undecanoate injection three times/6 months, n = 20) or placebo (placebo-injections, n = 21). Outcomes: Body composition (lean body mass and fat mass assessed by DXA), clinical pain intensity (numerical rating scale), and experimental pain perception (quantitative sensory assessment), quality of life (SF36), and adrenocorticotrophic hormone (ACTH) test. Data were presented as median (quartiles). Mann–Whitney tests were performed on delta values (24–0 weeks) between TRT and placebo. Results: The median age was 55 years (46; 59) and total testosterone before intervention was 6.8 (5.0; 9.3) nmol/L. TRT was associated with change of testosterone levels: 12.3 (7.0; 19.9) nmol/L (P < 0.001 vs placebo), increased lean body mass: 3.6 (2.3; 5.0) kg vs 0.1 kg (−2.1; 1.5) during TRT vs placebo and decreased total fat mass: −1.2 (−3.1; 0.7) kg vs 1.2 kg (−0.9; 2.5) kg, both P < 0.003. Changed pain perception, SF36, and ACTH-stimulated cortisol levels were non-significantly changed during TRT compared with placebo. Conclusions: Six months of TRT improved body composition in men with opioid-induced hypogonadism without significant changes in outcomes of pain perception, quality of life, or adrenal function.