A retrospective review of the roles of multifunctional glucose-6-phosphatase in blood glucose homeostasis: Genesis of the tuning/retuning hypothesis

A detailed review on the phytochemical profiles and anti-diabetic mechanisms of Momordica charantia

Diabetes mellitus is the most well-known endocrine dilemma suffered by hundreds of million people globally, with an annual mortality of more than one million people. This high mortality rate highlights the need for in-depth study of anti-diabetic agents. This review explores the phytochemical contents and anti-diabetic mechanisms of M. charantia (cucurbitaceae). Studies show that M. charantia contains several phytochemicals that have hypoglycemic effects, thus, the plant may be effective in the treatment/management of diabetes mellitus. Also, the biochemical and physiological basis of M. charantia anti-diabetic actions is explained. M. charantia exhibits its anti-diabetic effects via the suppression of MAPKs and NF-κβin pancreatic cells, promoting glucose and fatty acids catabolism, stimulating fatty acids absorption, inducing insulin production, ameliorating insulin resistance, activating AMPK pathway, and inhibiting glucose metabolism enzymes (fructose-1,6-bisphosphate and glucose-6-phosphatase). Reviewed literature was obtained from credible sources such as PubMed, Scopus, and Web of Science.

Histidine Promotes the Glucose Synthesis through Activation of the Gluconeogenic Pathway in Bovine Hepatocytes

Simple Summary

This study evaluated the effect of histidine (His) on hepatic glucose output and the expression of genes related to the gluconeogenic pathway in vitro. The results demonstrate that the supplementation of HIS can significantly improve the mRNA expression of phosphoenolpyruvate carboxykinase 1 (PCK1), phosphoenolpyruvate carboxykinase 2 (PCK2), fructose-1,6-bisphosphatase 1 (FBP1), and glucose-6-phosphatase (G6PC). Moreover, the addition of His ameliorated bovine hepatocytes glucose output. This study demonstrated that bovine hepatocytes can efficiently convert His into glucose to provide the energy required.

Abstract

Histidine (His) is considered to be the first-limiting amino acid (AA) on grass silage-based diets in lactation cows, which correlate positively with lactose yield. The higher glucose requirements of lactating cows can be met through a combination of increased capacity for gluconeogenesis and increased supply of gluconeogenic precursors. However, the effect of His on the expression of gluconeogenic genes in the bovine hepatocytes is less known. Therefore, this study aimed to investigate the regulatory effect of His on the key gluconeogenic genes and glucose output in bovine hepatocytes. The addition of 0.15, 0.6, and 1.2 mM His in a medium significantly enhanced (p < 0.05) the viability of bovine hepatocytes. Remarkably, 1.2 mM His induced profound changes (p < 0.05) in the mRNA level of key genes involved in gluconeogenesis, including PCK1, PCK2, FBP1, and G6PC in vitro. Furthermore, the mRNA expression of PCK1 was significantly elevated (p < 0.05) by the addition of 1.2 mM His at 3, 6, 12, and 24 h of incubation. The hepatic glucose output increased (p < 0.05) linearly with increasing His concentration. These findings indicate that the addition of His may be efficiently converted into glucose via the upregulation of genes related to the gluconeogenic pathway.

Hesperidin prevents hyperglycemia in diabetic rats by activating the insulin receptor pathway

Diabetes, a disease with high prevalence in China, is a major risk factor of cardiovascular disease. Hesperidin is a flavanone glycoside with anti-hyperglycemic and anti-hyperlipidemic activities. Therefore, the present study aimed to investigate the potential preventive effect of hesperidin against type 2 diabetes mellitus (T2DM) using a rat model of alloxan and high fat diet (HFD)-induced insulin resistance. Male Sprague Dawley rats were orally administered with 100 mg/kg hesperidin or vehicle (sodium carboxy methyl cellulose) for 35 days. Insulin resistance was induced by feeding animals a HFD for 3 weeks (from day 7) and then with an alloxan injection on day 28. Results from the in vivo study demonstrated that hesperidin improved fasting serum glucose (from 19.8 to 10.6 mmol/l) without changing the fasting insulin level, suggesting that hesperidin prevented the development of insulin resistance and diabetes by improving insulin sensitivity. In the oral glucose tolerance test, the development of impaired glucose tolerance was also prevented by hesperidin treatment. Hesperidin was found to regulate glycolysis and gluconeogenesis by enhancing the activity of glucokinase, inducing the phosphorylation of insulin receptor (IR) and phosphoinositide-dependent kinase 1 (PDK1), while decreasing the activity of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase in the liver. In a cell-based assay, hesperidin increased glucose uptake in primary rat adipocytes. Collectively, the present study identified the potent preventive effect of hesperidin against HFD-induced insulin resistance by activating the IR/PDK1 pathway. The current results may provide a potential strategy lacking sides effects to improve metabolic health and reduce risks.

Hypothesis: A Novel Neuroprotective Role for Glucose-6-phosphatase (G6PC3) in Brain-To Maintain Energy-Dependent Functions Including Cognitive Processes

The isoform of glucose-6-phosphatase in liver, G6PC1, has a major role in whole-body glucose homeostasis, whereas G6PC3 is widely distributed among organs but has poorly-understood functions. A recent, elegant analysis of neutrophil dysfunction in G6PC3-deficient patients revealed G6PC3 is a neutrophil metabolite repair enzyme that hydrolyzes 1,5-anhydroglucitol-6-phosphate, a toxic metabolite derived from a glucose analog present in food. These patients exhibit a spectrum of phenotypic characteristics and some have learning disabilities, revealing a potential linkage between cognitive processes and G6PC3 activity. Previously-debated and discounted functions for brain G6PC3 include causing an ATP-consuming futile cycle that interferes with metabolic brain imaging assays and a nutritional role involving astrocyte-neuron glucose-lactate trafficking. Detailed analysis of the anhydroglucitol literature reveals that it competes with glucose for transport into brain, is present in human cerebrospinal fluid, and is phosphorylated by hexokinase. Anhydroglucitol-6-phosphate is present in rodent brain and other organs where its accumulation can inhibit hexokinase by competition with ATP. Calculated hexokinase inhibition indicates that energetics of brain and erythrocytes would be more adversely affected by anhydroglucitol-6-phosphate accumulation than heart. These findings strongly support the paradigm-shifting hypothesis that brain G6PC3 removes a toxic metabolite, thereby maintaining brain glucose metabolism- and ATP-dependent functions, including cognitive processes.

Synthesis of cationic branched tea polysaccharide derivatives for targeted delivery of siRNA to hepatocytes

The cationic branched tea polysaccharide (CTPSA) derivative bearing N-acylurea and 3-(dimethylamino)-1-propylamine residues was synthesized and characterized using FTIR and ¹H NMR spectroscopy. A nonspecific siRNA (NsiRNA) was used as a model molecule of functional siRNA that could downregulate over-expressed glycometabolism enzymes in the liver. The result from the agarose gel electrophoresis confirmed that the CTPSA and NsiRNA could form stable complexes when their weight ratio was larger than 18. The zeta potentials and sizes of the complexes were in the range of +8-+15 mv and 120-150 nm, respectively. The CTPSA/NsiRNA complex was observed as nanoparticles with a spherical shape of approximately 100 nm using scanning electron microscopy. The CTPSA derivative and the CTPSA/NsiRNA complexes exhibited lower cytotoxicity in HL-7702 cells when compared with the branched PEI (bPEI) and bPEI/NsiRNA complexes assessed by the Cell Counting Kit-8 assay. The results of flow cytometric analysis and laser confocal microscopy indicated that the CTPSA derivative could effectively target the transfer of the NsiRNA to HL-7702 cells. This work provides a potential approach to promote the CTPSA derivative as a nonviral vector for targeted delivery of functional siRNA to hepatocytes.

Pathway-selective insulin resistance and metabolic disease: the importance of nutrient flux

Hepatic glucose and lipid metabolism are altered in metabolic disease (e.g. obesity, metabolic syndrome, and Type 2 diabetes). Insulin-dependent regulation of glucose metabolism is impaired. In contrast, lipogenesis, hypertriglyceridemia, and hepatic steatosis are increased. Because insulin promotes lipogenesis and liver fat accumulation, to explain the elevation in plasma and tissue lipids, investigators have suggested the presence of pathway-selective insulin resistance. In this model, insulin signaling to glucose metabolism is impaired, but insulin signaling to lipid metabolism is intact. We discuss the evidence for the differential regulation of hepatic lipid and glucose metabolism. We suggest that the primary phenotypic driver is altered substrate delivery to the liver, as well as the repartitioning of hepatic nutrient handling. Specific alterations in insulin signaling serve to amplify the alterations in hepatic substrate metabolism. Thus, hyperinsulinemia and its resultant increased signaling may facilitate lipogenesis, but are not the major drivers of the phenotype of pathway-selective insulin resistance.

Metabolomic heterogeneity of pulmonary arterial hypertension

Although multiple gene and protein expression have been extensively profiled in human pulmonary arterial hypertension (PAH), the mechanism for the development and progression of pulmonary hypertension remains elusive. Analysis of the global metabolomic heterogeneity within the pulmonary vascular system leads to a better understanding of disease progression. Using a combination of high-throughput liquid-and-gas-chromatography-based mass spectrometry, we showed unbiased metabolomic profiles of disrupted glycolysis, increased TCA cycle, and fatty acid metabolites with altered oxidation pathways in the human PAH lung. The results suggest that PAH has specific metabolic pathways contributing to increased ATP synthesis for the vascular remodeling process in severe pulmonary hypertension. These identified metabolites may serve as potential biomarkers for the diagnosis of PAH. By profiling metabolomic alterations of the PAH lung, we reveal new pathogenic mechanisms of PAH, opening an avenue of exploration for therapeutics that target metabolic pathway alterations in the progression of PAH.

Elevated glucose represses liver glucokinase and induces its regulatory protein to safeguard hepatic phosphate homeostasis

OBJECTIVE

The induction of hepatic glucose 6-phosphatase (G6pc) by glucose presents a paradox of glucose-induced glucose intolerance. We tested whether glucose regulation of liver gene expression is geared toward intracellular homeostasis.

RESEARCH DESIGN AND METHODS

The effect of glucose-induced accumulation of phosphorylated intermediates on expression of glucokinase (Gck) and its regulator Gckr was determined in hepatocytes. Cell ATP and uric acid production were measured as indices of cell phosphate homeostasis.

RESULTS

Accumulation of phosphorylated intermediates in hepatocytes incubated at elevated glucose induced rapid and inverse changes in Gck (repression) and Gckr (induction) mRNA concomitantly with induction of G6pc, but had slower effects on the Gckr-to-Gck protein ratio. Dynamic metabolic labeling in mice and liver proteome analysis confirmed that Gckr and Gck are low-turnover proteins. Involvement of Max-like protein X in glucose-mediated Gck-repression was confirmed by chromatin immunoprecipitation analysis. Elevation of the Gck-to-Gckr ratio in hepatocytes was associated with glucose-dependent ATP depletion and elevated urate production confirming compromised phosphate homeostasis.

CONCLUSIONS

The lowering by glucose of the Gck-to-Gckr ratio provides a potential explanation for the impaired hepatic glucose uptake in diabetes. Elevated uric acid production at an elevated Gck-to-Gckr ratio supports a role for glucose regulation of gene expression in hepatic phosphate homeostasis.