Silencing alanine transaminase 2 in diabetic liver attenuates hyperglycemia by reducing gluconeogenesis from amino acids

Increased hepatic gluconeogenesis and type 2 diabetes mellitus

Abnormally increased hepatic gluconeogenesis is a significant contributor to hyperglycemia in the fasting state in patients with type 2 diabetes mellitus (T2DM) due to insulin resistance. Metformin, the most prescribed drug for the treatment of T2DM, is believed to exert its effect mainly by reducing hepatic gluconeogenesis. Here, we discuss how increased hepatic gluconeogenesis contributes to T2DM and we review newly revealed mechanisms underlying the attenuation of gluconeogenesis by metformin. In addition, we analyze the recent findings on new determinants involved in the regulation of gluconeogenesis, which might ultimately lead to the identification of novel and targeted treatment strategies for T2DM.

Disruption of hepatic mitochondrial pyruvate and amino acid metabolism impairs gluconeogenesis and endurance exercise capacity in mice

Exercise robustly increases the glucose demands of skeletal muscle. This demand is met by not only muscle glycogenolysis but also accelerated liver glucose production from hepatic glycogenolysis and gluconeogenesis to fuel mechanical work and prevent hypoglycemia during exercise. Hepatic gluconeogenesis during exercise is dependent on highly coordinated responses within and between muscle and liver. Specifically, exercise increases the rate at which gluconeogenic precursors such as pyruvate/lactate or amino acids are delivered from muscle to the liver, extracted by the liver, and channeled into glucose. Herein, we examined the effects of interrupting hepatic gluconeogenic efficiency and capacity on exercise performance by deleting mitochondrial pyruvate carrier 2 (MPC2) and/or alanine transaminase 2 (ALT2) in the liver of mice. We found that deletion of MPC2 or ALT2 alone did not significantly affect time to exhaustion or postexercise glucose concentrations in treadmill exercise tests, but mice lacking both MPC2 and ALT2 in hepatocytes (double knockout, DKO) reached exhaustion faster and exhibited lower circulating glucose during and after exercise. Use of 2H/1³C metabolic flux analyses demonstrated that DKO mice exhibited lower endogenous glucose production owing to decreased glycogenolysis and gluconeogenesis at rest and during exercise. Decreased gluconeogenesis was accompanied by lower anaplerotic, cataplerotic, and TCA cycle fluxes. Collectively, these findings demonstrate that the transition of the liver to the gluconeogenic mode is critical for preventing hypoglycemia and sustaining performance during exercise. The results also illustrate the need for interorgan cross talk during exercise as described by the Cahill and Cori cycles.NEW & NOTEWORTHY Martino and colleagues examined the effects of inhibiting hepatic gluconeogenesis on exercise performance and systemic metabolism during treadmill exercise in mice. Combined inhibition of gluconeogenesis from lactate/pyruvate and alanine impaired exercise endurance and led to hypoglycemia during and after exercise. In contrast, suppressing either pyruvate-mediated or alanine-mediated gluconeogenesis alone had no effect on these parameters. These findings provide new insight into the molecular nodes that coordinate the metabolic responses of muscle and liver during exercise.

Alanine to glycine ratio is a novel predictive biomarker for type 2 diabetes mellitus

AIM

We aimed to evaluate the metabolite ratios that could predict the clinical incidence or remission of type 2 diabetes mellitus (T2D).

METHODS

The Cox proportional hazards regression model was used to assess 1813 individuals without T2D to test the predictive value of metabolite ratios for T2D incidence and 451 newly diagnosed T2D for remission. The receiver operating characteristic curve analysis was performed to determine the best cut-off values for the metabolite ratios. Survival analyses were performed to compare the four subgroups defined by baseline metabolite ratios and clinical status of obesity.

RESULTS

The alanine/glycine was the most significant marker for T2D incidence (hazard ratio per SD: 1.24; p < .001). On the other hand, metabolite hydroxy sphingomyelin C22:2 was most specific for T2D remission (hazard ratio per SD: 1.32; p = .029). Survival analysis of T2D incidence among the subgroups defined by the combination of alanine/glycine and obesity showed the group with a high alanine/glycine and obesity had the highest risk of T2D incidence (p < .001). The alanine/glycine as a T2D risk marker was also validated in the independent external data.

CONCLUSIONS

The combination of obesity and the alanine/glycine ratio can be used to evaluate the diabetes risk.

Screening for new inhibitors of the human Mitochondrial Pyruvate Carrier and their effects on hepatic glucose production and diabetes

BACKGROUND

The mitochondrial pyruvate carrier (MPC) is a protein complex composed of two subunits, MPC1 and MPC2. This carrier is at the interface between glycolysis and mitochondrial metabolism and plays an essential role in hepatic glucose production.

METHODS

Here we describe an in vitro screen for small molecule inhibitors of the MPC using a strain of Lactococcus lactis that has been engineered to co-express the two subunits of the human MPC and is able to import exogenous 14C-pyruvate. We then tested the top candidates for potential antidiabetic effects through the repression of gluconeogenesis.

RESULTS

By screening the Prestwick compound library of 1'200 drugs approved by the Food and Drug Administration for inhibitors of pyruvate uptake, twelve hit molecules were identified. In a secondary screen, the most potent inhibitors were found to inhibit pyruvate-driven oxygen consumption in mouse C2C12 muscle cells. Assessment of gluconeogenesis showed that Zaprinast, as well as the established MPC inhibitor UK5099, inhibited in vitro and in vivo hepatic glucose production. However, when tested acutely in mice without the administration of gluconeogenic substrates, MPC inhibitors raised blood glucose levels, pointing to liver-independent effects. Furthermore, chronic treatment with Zaprinast failed to correct hyperglycemia in both lean and obese diabetic mouse models.

CONCLUSIONS

New MPC inhibitors have been identified, showing inhibitory effects on hepatic glucose production.

GENERAL SIGNIFICANCE

For potential antidiabetic applications, MPC inhibitors should target the liver without undesired inhibition of mitochondrial pyruvate metabolism in the skeletal muscles or pancreatic beta-cells in order to avoid dual effects on glycemia.

Effects of hepatic mitochondrial pyruvate carrier deficiency on de novo lipogenesis and gluconeogenesis in mice

The liver coordinates the systemic response to nutrient deprivation and availability by producing glucose from gluconeogenesis during fasting and synthesizing lipids via de novo lipogenesis (DNL) when carbohydrates are abundant. Mitochondrial pyruvate metabolism is thought to play important roles in both gluconeogenesis and DNL. We examined the effects of hepatocyte-specific mitochondrial pyruvate carrier (MPC) deletion on the fasting-refeeding response. Rates of DNL during refeeding were impaired by hepatocyte MPC deletion, but this did not reduce intrahepatic lipid content. During fasting, glycerol is converted to glucose by two pathways; a direct cytosolic pathway and an indirect mitochondrial pathway requiring the MPC. Hepatocyte MPC deletion reduced the incorporation of 13C-glycerol into TCA cycle metabolites, but not into new glucose. Furthermore, suppression of glycerol and alanine metabolism did not affect glucose concentrations in fasted hepatocyte-specific MPC-deficient mice, suggesting multiple layers of redundancy in glycemic control in mice.

Hepatic pyruvate and alanine metabolism are critical and complementary for maintenance of antioxidant capacity and resistance to oxidative insult

OBJECTIVE

Mitochondrial pyruvate is a critical intermediary metabolite in gluconeogenesis, lipogenesis, and NADH production. As a result, the mitochondrial pyruvate carrier (MPC) complex has emerged as a promising therapeutic target in metabolic diseases. Clinical trials are currently underway. However, recent in vitro data indicate that MPC inhibition diverts glutamine/glutamate away from glutathione synthesis and toward glutaminolysis to compensate for loss of pyruvate oxidation, possibly sensitizing cells to oxidative insult. Here, we explored this in vivo using the clinically relevant acetaminophen (APAP) overdose model of acute liver injury, which is driven by oxidative stress.

METHODS

We used pharmacological and genetic approaches to inhibit MPC2 and alanine aminotransferase 2 (ALT2), individually and concomitantly, in mice and cell culture models and determined the effects on APAP hepatotoxicity.

RESULTS

We found that MPC inhibition sensitizes the liver to APAP-induced injury in vivo only with concomitant loss of alanine aminotransferase 2 (ALT2). Pharmacological and genetic manipulation of neither MPC2 nor ALT2 alone affected APAP toxicity, but liver-specific double knockout (DKO) significantly worsened APAP-induced liver damage. Further investigation indicated that DKO impaired glutathione synthesis and increased urea cycle flux, consistent with increased glutaminolysis, and these results were reproducible in vitro. Finally, induction of ALT2 and post-treatment with dichloroacetate both reduced APAP-induced liver injury, suggesting new therapeutic avenues.

CONCLUSIONS

Increased susceptibility to APAP toxicity requires loss of both the MPC and ALT2 in vivo, indicating that MPC inhibition alone is insufficient to disrupt redox balance. Furthermore, the results from ALT2 induction and dichloroacetate in the APAP model suggest new metabolic approaches to the treatment of liver damage.

Disruption of Hepatic Mitochondrial Pyruvate and Amino Acid Metabolism Impairs Gluconeogenesis and Endurance Exercise Capacity in Mice

Exercise robustly increases the glucose demands of skeletal muscle. This demand is met not only by muscle glycogenolysis, but also by accelerated liver glucose production from hepatic glycogenolysis and gluconeogenesis to fuel mechanical work and prevent hypoglycemia during exercise. Hepatic gluconeogenesis during exercise is dependent on highly coordinated responses within and between muscle and liver. Specifically, exercise increases the rate at which gluconeogenic precursors such as pyruvate/lactate or amino acids are delivered from muscle to the liver, extracted by the liver, and channeled into glucose. Herein, we examined the effects of interrupting gluconeogenic efficiency and capacity on exercise performance by deleting hepatic mitochondrial pyruvate carrier 2 (MPC2) and/or alanine transaminase 2 (ALT2) in mice. We found that deletion of MPC2 or ALT2 alone did not significantly affect time to exhaustion or post-exercise glucose concentrations in treadmill exercise tests, but mice lacking both MPC2 and ALT2 in liver (DKO) reached exhaustion faster and exhibited lower circulating glucose during and after exercise. Use of ²H/¹³C metabolic flux analyses demonstrated that DKO mice exhibited lower endogenous glucose production owing to decreased glycogenolysis and gluconeogenesis at rest and during exercise. The decreased gluconeogenesis was accompanied by lower anaplerotic, cataplerotic, and TCA cycle fluxes. Collectively, these findings demonstrate that the transition of the liver to the gluconeogenic mode is critical for preventing hypoglycemia and sustaining performance during exercise. The results also illustrate the need for interorgan crosstalk during exercise as described by the Cahill and Cori cycles.

Signaling pathways and intervention for therapy of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) represents one of the fastest growing epidemic metabolic disorders worldwide and is a strong contributor for a broad range of comorbidities, including vascular, visual, neurological, kidney, and liver diseases. Moreover, recent data suggest a mutual interplay between T2DM and Corona Virus Disease 2019 (COVID-19). T2DM is characterized by insulin resistance (IR) and pancreatic β cell dysfunction. Pioneering discoveries throughout the past few decades have established notable links between signaling pathways and T2DM pathogenesis and therapy. Importantly, a number of signaling pathways substantially control the advancement of core pathological changes in T2DM, including IR and β cell dysfunction, as well as additional pathogenic disturbances. Accordingly, an improved understanding of these signaling pathways sheds light on tractable targets and strategies for developing and repurposing critical therapies to treat T2DM and its complications. In this review, we provide a brief overview of the history of T2DM and signaling pathways, and offer a systematic update on the role and mechanism of key signaling pathways underlying the onset, development, and progression of T2DM. In this content, we also summarize current therapeutic drugs/agents associated with signaling pathways for the treatment of T2DM and its complications, and discuss some implications and directions to the future of this field.

Mitochondrial pyruvate carrier inhibition initiates metabolic crosstalk to stimulate branched chain amino acid catabolism

OBJECTIVE

The mitochondrial pyruvate carrier (MPC) has emerged as a therapeutic target for treating insulin resistance, type 2 diabetes, and nonalcoholic steatohepatitis (NASH). We evaluated whether MPC inhibitors (MPCi) might correct impairments in branched chain amino acid (BCAA) catabolism, which are predictive of developing diabetes and NASH.

METHODS

Circulating BCAA concentrations were measured in people with NASH and type 2 diabetes, who participated in a recent randomized, placebo-controlled Phase IIB clinical trial to test the efficacy and safety of the MPCi MSDC-0602K (EMMINENCE; NCT02784444). In this 52-week trial, patients were randomly assigned to placebo (n = 94) or 250 mg MSDC-0602K (n = 101). Human hepatoma cell lines and mouse primary hepatocytes were used to test the direct effects of various MPCi on BCAA catabolism in vitro. Lastly, we investigated how hepatocyte-specific deletion of MPC2 affects BCAA metabolism in the liver of obese mice and MSDC-0602K treatment of Zucker diabetic fatty (ZDF) rats.

RESULTS

In patients with NASH, MSDC-0602K treatment, which led to marked improvements in insulin sensitivity and diabetes, had decreased plasma concentrations of BCAAs compared to baseline while placebo had no effect. The rate-limiting enzyme in BCAA catabolism is the mitochondrial branched chain ketoacid dehydrogenase (BCKDH), which is deactivated by phosphorylation. In multiple human hepatoma cell lines, MPCi markedly reduced BCKDH phosphorylation and stimulated branched chain keto acid catabolism; an effect that required the BCKDH phosphatase PPM1K. Mechanistically, the effects of MPCi were linked to activation of the energy sensing AMP-dependent protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) kinase signaling cascades in vitro. BCKDH phosphorylation was reduced in liver of obese, hepatocyte-specific MPC2 knockout (LS-Mpc2-/-) mice compared to wild-type controls concomitant with activation of mTOR signaling in vivo. Finally, while MSDC-0602K treatment improved glucose homeostasis and increased the concentrations of some BCAA metabolites in ZDF rats, it did not lower plasma BCAA concentrations.

CONCLUSIONS

These data demonstrate novel cross talk between mitochondrial pyruvate and BCAA metabolism and suggest that MPC inhibition leads to lower plasma BCAA concentrations and BCKDH phosphorylation by activating the mTOR axis. However, the effects of MPCi on glucose homeostasis may be separable from its effects on BCAA concentrations.

Untargeted Metabolomics Based Prediction of Therapeutic Potential for Apigenin and Chrysin

The prominent flavonoids apigenin and chrysin have been demonstrated to have systemic benefits. Our previous work was first to establish the impact of apigenin and chrysin on cellular transcriptome. In the current study, we have revealed the ability of apigenin and chrysin to alter the cellular metabolome based on our untargeted metabolomics. Based on our metabolomics data, both these structurally related flavonoids demonstrate diverging and converging properties. Apigenin demonstrated the potential to possess anti-inflammatory and vasorelaxant properties through the upregulation of intermediate metabolites of alpha-linolenic acid and linoleic acid pathways. Chrysin, on the other hand, exhibited abilities to inhibit protein and pyrimidine synthesis along with downregulation of gluconeogenesis pathways based on the altered metabolites detected. Chrysin-mediated metabolite changes are mostly due to its ability to modulate L-alanine metabolism and the urea cycle. On the other hand, both the flavonoids also demonstrated converging properties. Apigenin and chrysin were able to downregulate metabolites involved in cholesterol biosynthesis and uric acid synthesis, namely 7-dehydrocholesterol and xanthosine, respectively. This work will provide understanding regarding the diverse therapeutic potential of these naturally occurring flavonoids and help us in curbing an array of metabolic complications.

Identification of ferroptosis-related genes and predicted overall survival in patients with burns

Introduction

Burns are a common trauma associated with considerable mortality and morbidity. Although a lot is known regarding burns' pathogenesis, the involvement of ferroptosis is uncertain. Here, we aimed to explore vital ferroptosis-related genes and molecules in burns, through bioinformatics analysis, to uncover new effective therapeutic targets.

Methods

The FerrDb database was used to acquire ferroptosis-related genes and GSE19743 was downloaded from Gene Expression Omnibus (GEO), a dataset with analysis of control and burned individuals. Hub genes were selected with Cytoscape software, and Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were conducted. Cox proportional hazard function and Kaplan-Meier survival analyses were implemented to screen prognosis-related genes. Additionally, the miRWalk database was used to acquire the miRNAs relevant to our hub genes function and analyzed for enrichment.

Result

We identified 64 differentially expressed genes and through the intersection with ferroptosis-related genes, 10 were selected as hub genes. GO analysis revealed that the hub genes' most enriched activities were response to oxidative stress, pyridine-containing compound metabolic processes, and reactive oxygen species metabolic processes. KEGG pathways' analysis showed that these overlapped genes were enriched in several pathways, namely, in VEGF signaling. Furthermore, the molecular miRNA functions significantly enriched were signal transduction and cell communication, namely, the biological pathways of the glypican pathway and the ErbB receptor signaling network. SLC40A1 and GPT2 genes were found to be associated with overall survival, suggesting an important role in burn prognosis.

Discussion

This study may improve our understanding of the underlying burn mechanisms and provide a new direction for the prevention of poor outcomes, advances in burns treatment, and drug development.

Role of liver parameters in diabetes mellitus - a narrative review

Diabetes mellitus is characterized by hyperglycemia and abnormalities in insulin secretion and function. This review article focuses on various liver parameters, including albumin, alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), alpha fetoprotein (AFP), alpha 1 antitrypsin (AAT), ammonia, bilirubin, bile acid, gamma-glutamyl transferase (GGT), immunoglobulin, lactate dehydrogenase (LDH), and total protein. These parameters play significant roles in the development of different types of diabetes such as type 1 diabetes (T1DM), type 2 diabetes (T2DM) and gestational diabetes (GDM). The article highlights that low albumin levels may indicate inflammation, while increased ALT and AST levels are associated with liver inflammation or injury, particularly in non-alcoholic fatty liver disease (NAFLD). Elevated ALP levels can be influenced by liver inflammation, biliary dysfunction, or bone metabolism changes. High bilirubin levels are independently linked to albuminuria in T1DM and an increased risk of T2DM. Elevated GGT levels are proposed as markers of oxidative stress and liver dysfunction in T2DM. In GDM, decreased serum AFP levels may indicate impaired embryo growth. Decreased AFP levels in T2DM can hinder the detection of hepatocellular carcinoma. Hyperammonemia can cause encephalopathy in diabetic ketoacidosis, and children with T1DM and attention deficit hyperactivity disorder often exhibit higher ammonia levels. T2DM disrupts the regulation of nitrogen-related metabolites, leading to increased blood ammonia levels. Bile acids affect glucose regulation by activating receptors on cell surfaces and nuclei, and changes in bile acid metabolism are observed in T2DM. Increased LDH activity reflects metabolic disturbances in glucose utilization and lactate production, contributing to diabetic complications. Poor glycemic management may be associated with elevated levels of IgA and IgG serum antibodies, and increased immunoglobulin levels are also associated with T2DM.

Proportion of Concentrate in the Diet of Early Lactation Dairy Cows Has Contrasting Effects on Circulating Leukocyte Global Transcriptomic Profiles, Health and Fertility According to Parity

The functionality of circulating leukocytes in dairy cows is suppressed after calving, with negative energy balance as a risk factor. Leukocyte transcriptomic profiles were compared separately in 44 multiparous (MP) and 18 primiparous (PP) Holstein-Friesian cows receiving diets differing in concentrate proportion to test whether immune dysfunction could be mitigated by appropriate nutrition. After calving, cows were offered either (1) low concentrate (LC); (2) medium concentrate (MC) or (3) high concentrate (HC) diets with proportions of concentrate to grass silage of 30%:70%, 50%:50% and 70%:30%, respectively. Cow phenotype data collected included circulating metabolites, milk yield and health and fertility records. RNA sequencing of circulating leukocytes at 14 days in milk was performed. The HC diet improved energy balance in both age groups. There were more differentially expressed genes in PP than MP cows (460 vs. 173, HC vs. LC comparison) with few overlaps. The MP cows on the LC diet showed upregulation of the complement and coagulation cascade and innate immune defence mechanisms against pathogens and had a trend of more cases of mastitis and poorer fertility. In contrast, the PP cows on the HC diet showed greater immune responses based on both gene expression and phenotypic data and longer interval of calving to conception. The leukocytes of MP and PP cows therefore responded differentially to the diets between age, nutrient supply and immunity affecting their health and subsequent fertility.

Metabolomics of various samples advancing biomarker discovery and pathogenesis elucidation for diabetic retinopathy

Diabetic retinopathy (DR) is a universal microvascular complication of diabetes mellitus (DM), which is the main reason for global sight damage/loss in middle-aged and/or older people. Current clinical analyses, like hemoglobin A1c, possess some importance as prognostic indicators for DR severity, but no effective circulating biomarkers are used for DR in the clinic currently, and studies on the latent pathophysiology remain lacking. Recent developments in omics, especially metabolomics, continue to disclose novel potential biomarkers in several fields, including but not limited to DR. Therefore, based on the overview of metabolomics, we reviewed progress in analytical technology of metabolomics, the prominent roles and the current status of biomarkers in DR, and the update of potential biomarkers in various DR-related samples via metabolomics, including tear as well as vitreous humor, aqueous humor, retina, plasma, serum, cerebrospinal fluid, urine, and feces. In this review, we underscored the in-depth analysis and elucidation of the common biomarkers in different biological samples based on integrated results, namely, alanine, lactate, and glutamine. Alanine may participate in and regulate glucose metabolism through stimulating N-methyl-D-aspartate receptors and subsequently suppressing insulin secretion, which is the potential pathogenesis of DR. Abnormal lactate could cause extensive oxidative stress and neuroinflammation, eventually leading to retinal hypoxia and metabolic dysfunction; on the other hand, high-level lactate may damage the structure and function of the retinal endothelial cell barrier via the G protein-coupled receptor 81. Abnormal glutamine indicates a disturbance of glutamate recycling, which may affect the activation of Müller cells and proliferation via the PPP1CA-YAP-GS-Gln-mTORC1 pathway.