Metformin increases the uptake of glucose into the gut from the circulation in high-fat diet-fed male mice, which is enhanced by a reduction in whole-body Slc2a2 expression

OBJECTIVES

Metformin is the first line therapy recommended for type 2 diabetes. However, the precise mechanism of action remains unclear and up to a quarter of patients show some degree of intolerance to the drug, with a similar number showing poor response to treatment, limiting its effectiveness. A better understanding of the mechanism of action of metformin may improve its clinical use. SLC2A2 (GLUT2) is a transmembrane facilitated glucose transporter, with important roles in the liver, gut and pancreas. Our group previously identified single nucleotide polymorphisms in the human SLC2A2 gene, which were associated with reduced transporter expression and an improved response to metformin treatment. The aims of this study were to model Slc2a2 deficiency and measure the impact on glucose homoeostasis and metformin response in mice.

METHODS

We performed extensive metabolic phenotyping and 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG)-positron emission tomography (PET) analysis of gut glucose uptake in high-fat diet-fed (HFD) mice with whole-body reduced Slc2a2 (Slc2a2+/-) and intestinal Slc2a2 KO, to assess the impact of metformin treatment.

RESULTS

Slc2a2 partial deficiency had no major impact on body weight and insulin sensitivity, however mice with whole-body reduced Slc2a2 expression (Slc2a2+/-) developed an age-related decline in glucose homoeostasis (as measured by glucose tolerance test) compared to wild-type (Slc2a2+/+) littermates. Glucose uptake into the gut from the circulation was enhanced by metformin exposure in Slc2a2+/+ animals fed HFD and this action of the drug was significantly higher in Slc2a2+/- animals. However, there was no effect of specifically knocking-out Slc2a2 in the mouse intestinal epithelial cells.

CONCLUSIONS

Overall, this work identifies a differential metformin response, dependent on expression of the SLC2A2 glucose transporter, and also adds to the growing evidence that metformin efficacy includes modifying glucose transport in the gut. We also describe a novel and important role for this transporter in maintaining efficient glucose homoeostasis during ageing.

L-leucine stimulation of glucose uptake and utilization involves modulation of glucose - lipid metabolic switch and improved bioenergetic homeostasis in isolated rat psoas muscle ex vivo

The antidiabetic effect of l-leucine has been attributed to its modulatory effect on glucose uptake and lipid metabolism in muscles. However, there is a dearth on its effect on glucose metabolism in muscles. Thus, the present study investigated the effect of l-leucine - stimulated glucose uptake on glucose metabolism, dysregulated lipid metabolic pathways, redox and bioenergetic homeostasis, and proteolysis in isolated psoas muscle from Sprague Dawley male rats. Isolated psoas muscles were incubated with l-leucine (30-240 μg/mL) in the presence of 11.1 mMol glucose at 37 ˚C for 2 h. Muscles incubated in only glucose served as the control, while muscles not incubated in l-leucine and/or glucose served as the normal control. Metformin (6.04 mM) was used as the standard antidiabetic drug. Incubation with l-leucine caused a significant increase in muscle glucose uptake, with an elevation of glutathione levels, superoxide dismutase, catalase, E-NTPDase and 5'nucleotidase activities. It also led to the depletion of malondialdehyde and nitric oxide levels, ATPase, chymotrypsin, acetylcholinesterase, glycogen phosphorylase, glucose-6-phosphatase, fructose-1,6-bisphosphatase and lipase activities. There was an alteration in lipid metabolites, with concomitant activation of glycerolipid metabolism, fatty acid metabolism, and fatty acid elongation in mitochondria in the glucose-incubated muscle (negative control). Incubation with l-leucine reversed these alterations, and concomitantly deactivated the pathways. These results indicate that l-leucine-enhanced muscle glucose uptake involves improved redox and bioenergetic homeostasis, with concomitant suppressed proteolytic, glycogenolytic and gluconeogenetic activities, while modulating glucose - lipid metabolic switch.

Identification of chebulinic acid as a dual targeting inhibitor of protein tyrosine phosphatases relevant to insulin resistance

Natural products as antidiabetic agents have been shown to stimulate insulin signaling via the inhibition of the protein tyrosine phosphatases relevant to insulin resistance. Previously, we have identified PTPN9 and DUSP9 as potential antidiabetic targets and a multi-targeting natural product thereof. In this study, knockdown of PTPN11 increased AMPK phosphorylation in differentiated C2C12 muscle cells by 3.8 fold, indicating that PTPN11 could be an antidiabetic target. Screening of a library of 658 natural products against PTPN9, DUSP9, or PTPN11 identified chebulinic acid (CA) as a strong allosteric inhibitor with a slow cooperative binding to PTPN9 (IC₅₀ = 34 nM) and PTPN11 (IC₅₀ = 37 nM), suggesting that it would be a potential antidiabetic candidate. Furthermore, CA stimulated glucose uptake and resulted in increased AMP-activated protein kinase (AMPK) phosphorylation. Taken together, we demonstrated that CA increased glucose uptake as a dual inhibitor of PTPN9 and PTPN11 through activation of the AMPK signaling pathway. These results strongly suggest that CA could be used as a potential therapeutic candidate for the treatment of type 2 diabetes.

Ginkgolic acid as a dual-targeting inhibitor for protein tyrosine phosphatases relevant to insulin resistance

Several protein tyrosine phosphatases (PTPs) that disrupt the insulin-signaling pathway were investigated by siRNAs to identify potential antidiabetic targets. Individual knockdown of PTPN9 and DUSP9 in 3T3-L1 preadipocytes increased AMPK phosphorylation, respectively, and furthermore, concurrent knockdown of both PTPN9 and DUSP9 synergistically increased AMPK phosphorylation. Next, 658 natural products were screened to identify dual inhibitors of both PTPN9 and DUSP9. Based on the selectivity and inhibition potency of the compounds, ginkgolic acid (GA) was selected for further study as a potential antidiabetic drug candidate. GA inhibited the enzymatic activity of PTPN9 (K_i = 53 µM) and DUSP9 (K_i = 2.5 µM) in vitro and resulted in a significant increase of glucose-uptake in differentiated C2C12 muscle cells and 3T3-L1 adipocytes. In addition, GA increased phosphorylation of AMPK in 3T3L1 adipocytes. In this study, GA as a dual targeting inhibitor of PTPN9 and DUSP9 increased glucose uptake in 3T3L1 and C2C12 cells by activating the AMPK signaling pathway. These results strongly suggest GA could be used as a therapeutic candidate for type 2 diabetes.