Loss of GIPR in LEPR cells impairs glucose control by GIP and GIP:GLP-1 co-agonism without affecting body weight and food intake in mice

β-Hydroxy-β-methylbutyrate (HMB) increases muscle mass and diminishes weight gain in high-fat-fed mice

β-Hydroxy-β-methylbutyrate (HMB) is a catabolite of leucine, which promotes muscle growth. However, little is known about the effect of HMB on body composition in the context of a high-fat diet. Our aim was to study the circadian metabolic effect of HMB on body weight. C57BL male mice were fed ad libitum chow diet (C), chow diet supplemented with 500 mg Ca-HMB per 1 kg body weight (C+HMB), a high-fat diet (HFD) or HFD supplemented with 500 mg Ca-HMB per 1 kg body weight (HFD+HMB) for 7 weeks. HMB led to reduced fat weight (30%, P<.05) and body weight (7%, P<.05) and increased muscle weight (17%, P<.05) in the HFD+HMB group. HMB increased glucose oxidation (27%, P<.0001) and reduced fatty acid oxidation (30%, P<.0001) in the C group, but increased fatty acid oxidation in the HFD+HMB group (10%, P<.05). At the molecular level, HMB did not affect metabolic proteins in the liver, but lowered NF-κB levels in adipose tissue (24%, P<.05). In the muscle, HMB showed no activation of AKT and mTOR, but did show activation of P70S6K (67%, P<.01) and S6 (42%, P<.01). The activation of the P70S6K and S6 was independent of AKT and mTOR and was accompanied by increased activation of phospholipase D2 (PLD) (35%, P<.0001). In addition, HMB led to altered circadian rhythms. In conclusion, mice fed a HFD supplemented with HMB have increased muscle weight and reduced fat mass and body weight presumably due to increased locomotor activity. HMB induces myogenesis by activating P70S6K and S6 via PLD2.

Glucose-dependent insulinotropic polypeptide (GIP)

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) was the first incretin identified and plays an essential role in the maintenance of glucose tolerance in healthy humans. Until recently GIP had not been developed as a therapeutic and thus has been overshadowed by the other incretin, glucagon-like peptide 1 (GLP-1), which is the basis for several successful drugs to treat diabetes and obesity. However, there has been a rekindling of interest in GIP biology in recent years, in great part due to pharmacology demonstrating that both GIPR agonism and antagonism may be beneficial in treating obesity and diabetes. This apparent paradox has reinvigorated the field, led to new lines of investigation, and deeper understanding of GIP.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GIP biology and discuss the therapeutic implications of GIPR signal modification on various diseases.

MAJOR CONCLUSIONS

Following its classification as an incretin hormone, GIP has emerged as a pleiotropic hormone with a variety of metabolic effects outside the endocrine pancreas. The numerous beneficial effects of GIPR signal modification render the peptide an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, drug-induced nausea and both bone and neurodegenerative disorders.

GIPR agonism and antagonism decrease body weight and food intake via different mechanisms in male mice

Agonists and antagonists of the glucose-dependent insulinotropic polypeptide receptor (GIPR) enhance body weight loss induced by glucagon-like peptide-1 receptor (GLP-1R) agonism. However, while GIPR agonism decreases body weight and food intake in a GLP-1R-independent manner via GABAergic GIPR+ neurons, it remains unclear whether GIPR antagonism affects energy metabolism via a similar mechanism. Here we show that the body weight and food intake effects of GIPR antagonism are eliminated in mice with global loss of either Gipr or Glp-1r but are preserved in mice with loss of Gipr in either GABAergic neurons of the central nervous system or peripherin-expressing neurons of the peripheral nervous system. Single-nucleus RNA-sequencing shows opposing effects of GIPR agonism and antagonism in the dorsal vagal complex, with antagonism, but not agonism, closely resembling GLP-1R signalling. Additionally, GIPR antagonism and GLP-1R agonism both regulate genes implicated in synaptic plasticity. Collectively, we show that GIPR agonism and antagonism decrease body weight via different mechanisms, with GIPR antagonism, unlike agonism, depending on functional GLP-1R signalling.