Narciclasine attenuates diet-induced obesity by promoting oxidative metabolism in skeletal muscle.

Obesity develops when caloric intake exceeds metabolic needs. Promoting energy expenditure represents an attractive approach in the prevention of this fast-spreading epidemic. Here, we report a novel pharmacological strategy in which a natural compound, narciclasine (ncls), attenuates diet-induced obesity (DIO) in mice by promoting energy expenditure. Moreover, ncls promotes fat clearance from peripheral metabolic tissues, improves blood metabolic parameters in DIO mice, and protects these mice from the loss of voluntary physical activity. Further investigation suggested that ncls achieves these beneficial effects by promoting a shift from glycolytic to oxidative muscle fibers in the DIO mice thereby enhancing mitochondrial respiration and fatty acid oxidation (FAO) in the skeletal muscle. Moreover, ncls strongly activates AMPK signaling specifically in the skeletal muscle. The beneficial effects of ncls treatment in fat clearance and AMPK activation were faithfully reproduced in vitro in cultured murine and human primary myotubes. Mechanistically, ncls increases cellular cAMP concentration and ADP/ATP ratio, which further lead to the activation of AMPK signaling. Blocking AMPK signaling through a specific inhibitor significantly reduces FAO in myotubes. Finally, ncls also enhances mitochondrial membrane potential and reduces the formation of reactive oxygen species in cultured myotubes.

Molecular regulation of insulin granule biogenesis and exocytosis.

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by hyperglycemia, insulin resistance and hyperinsulinemia in early disease stages but a relative insulin insufficiency in later stages. Insulin, a peptide hormone, is produced in and secreted from pancreatic ß-cells following elevated blood glucose levels. Upon its release, insulin induces the removal of excessive exogenous glucose from the bloodstream primarily by stimulating glucose uptake into insulin-dependent tissues as well as promoting hepatic glycogenesis. Given the increasing prevalence of T2DM worldwide, elucidating the underlying mechanisms and identifying the various players involved in the synthesis and exocytosis of insulin from ß-cells is of utmost importance. This review summarizes our current understanding of the route insulin takes through the cell after its synthesis in the endoplasmic reticulum as well as our knowledge of the highly elaborate network that controls insulin release from the ß-cell. This network harbors potential targets for anti-diabetic drugs and is regulated by signaling cascades from several endocrine systems.

Nrf2 regulates the expression of the peptide transporter PEPT1 in the human colon carcinoma cell line Caco-2.

BACKGROUND: PEPT1 is a rheogenic transport protein in the apical membrane of intestinal epithelial cells capable of transporting essentially all possible di- and tripeptides that are generated from the luminal protein breakdown. In addition, several anticancer, antimicrobial and antiviral drugs are taken up from the intestinal lumen via PEPT1 and therefore PEPT1 is a target for efficient drug delivery via prodrug approaches. Thus, understanding PEPT1 gene regulation is not only of importance for dietary adaptation but also for drug treatment. METHODS: In silico analysis of the Pept1 promoter was performed using MatInspector. Pept1 promoter constructs were generated and cotransfected with an Nrf2 expression plasmid. Caco-2 cells were stimulated with Nrf2 inducers followed by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP). Biological relevance was investigated using western blot analysis and transport activity assays. RESULTS: Reporter gene assays showed transcriptional activation of the Pept1 promoter in response to Nrf2 overexpression. EMSA as well as ChIP analysis validated Nrf2 binding to the ARE located closest to the start codon (Pept1-ARE1). Induction of the Nrf2 pathway resulted in increased endogenous PEPT1 protein abundance as well as transport activity. Moreover, we demonstrate that also the induction of autophagy by MG132 resulted in elevated Nrf2 binding to Pept1-ARE1 and increased PEPT1 protein expression. CONCLUSION: In summary, we identified a biologically active Nrf2 binding site within the Pept1 promoter which links Pept1 to the cellular defense program activated by Nrf2. GENERAL SIGNIFICANCE: This study identifies Pept1 as an inducible target gene of the Nrf2 pathway.

Pancreatic regulation of glucose homeostasis.

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

The role of SGLT1 and GLUT2 in intestinal glucose transport and sensing.

Intestinal glucose absorption is mediated by SGLT1 whereas GLUT2 is considered to provide basolateral exit. Recently, it was proposed that GLUT2 can be recruited into the apical membrane after a high luminal glucose bolus allowing bulk absorption of glucose by facilitated diffusion. Moreover, SGLT1 and GLUT2 are suggested to play an important role in intestinal glucose sensing and incretin secretion. In mice that lack either SGLT1 or GLUT2 we re-assessed the role of these transporters in intestinal glucose uptake after radiotracer glucose gavage and performed Western blot analysis for transporter abundance in apical membrane fractions in a comparative approach. Moreover, we examined the contribution of these transporters to glucose-induced changes in plasma GIP, GLP-1 and insulin levels. In mice lacking SGLT1, tissue retention of tracer glucose was drastically reduced throughout the entire small intestine whereas GLUT2-deficient animals exhibited higher tracer contents in tissue samples than wild type animals. Deletion of SGLT1 resulted also in reduced blood glucose elevations and abolished GIP and GLP-1 secretion in response to glucose. In mice lacking GLUT2, glucose-induced insulin but not incretin secretion was impaired. Western blot analysis revealed unchanged protein levels of SGLT1 after glucose gavage. GLUT2 detected in apical membrane fractions mainly resulted from contamination with basolateral membranes but did not change in density after glucose administration. SGLT1 is unequivocally the prime intestinal glucose transporter even at high luminal glucose concentrations. Moreover, SGLT1 mediates glucose-induced incretin secretion. Our studies do not provide evidence for GLUT2 playing any role in either apical glucose influx or incretin secretion.