Inhibition of hepatic gluconeogenesis in type 2 diabetes by metformin: complementary role of nitric oxide

Abstract

Metformin is the first-line treatment for type 2 diabetes mellitus. Type 2 diabetes mellitus is associated with decreased nitric oxide bioavailability, which has significant metabolic implications, including enhanced insulin secretion and peripheral glucose utilization. Similar to metformin, nitric oxide also inhibits hepatic glucose production, mainly by suppressing gluconeogenesis. This review explores the combined effects of metformin and nitric oxide on hepatic gluconeogenesis and proposes the potential of a hybrid metformin-nitric oxide drug for managing type 2 diabetes mellitus. Both metformin and nitric oxide inhibit gluconeogenesis through overlapping and distinct mechanisms. In hepatic gluconeogenesis, mitochondrial oxaloacetate is exported to the cytoplasm via various pathways, including the malate, direct, aspartate, and fumarate pathways. The effects of nitric oxide and metformin on the exportation of oxaloacetate are complementary; nitric oxide primarily inhibits the malate pathway, while metformin strongly inhibits the fumarate and aspartate pathways. Furthermore, metformin effectively blocks gluconeogenesis from lactate, glycerol, and glutamine, whereas nitric oxide mainly inhibits alanine-induced gluconeogenesis. Additionally, nitric oxide contributes to the adenosine monophosphate-activated protein kinase-dependent inhibition of gluconeogenesis induced by metformin. The combined use of metformin and nitric oxide offers the potential to mitigate common side effects. For example, lactic acidosis, a known side effect of metformin, is linked to nitric oxide deficiency, while the oxidative and nitrosative stress caused by nitric oxide could be counterbalanced by metformin's enhancement of glutathione. Metformin also amplifies nitric oxide -induced activation of adenosine monophosphate-activated protein kinase. In conclusion, a metformin-nitric oxide hybrid drug can benefit patients with type 2 diabetes mellitus by enhancing the inhibition of hepatic gluconeogenesis, decreasing the required dose of metformin for maintaining optimal glycemia, and lowering the incidence of metformin-associated lactic acidosis.

Keywords: glycemia; hepatocyte; hybrid; lactic acidosis; malate-aspartate shuttle; mitochondria; nitric oxide synthase; oxaloacetate; redox; tricarboxylic acid cycle.

Figure 1

An overview of hepatic gluconeogenesis. The malate (blue arrows), direct (orange arrows), fumarate (green arrows), and aspartate (red arrows) pathways for exporting oxaloacetate from the mitochondrion to the cytoplasm. Transparent green and red arrows show the general direction of glycolysis and gluconeogenesis, respectively. In hepatic gluconeogenesis, most reactions are the reverse of those in glycolysis, except three irreversible reactions of glycolysis, reactions 1 (glucose→G6P), 3 (F6p→F1,6BP), and 10 (PEP→ pyruvate). Created with BioRender.com. 1,3BPG: 1,3-Bisphosphoglycerate; 2PG: 2-phosphoglycerate; 3PG: 3-phosphoglycerate; ACON: aconitase; ADP: adenosine diphosphate; AGC2: aspartate-glutamate carrier 2; ALT: alanine aminotransferase; AMP: adenosine monophosphate; ARG1: arginase; ASL: argininosuccinate lyase; ASS: argininosuccinate synthase; AST: aspartate aminotransferase; ATP: adenosine triphosphate; CIC: citrate carrier; CS: citrate synthase; CP: carbamoyl phosphate; CPS: carbamoyl phosphate synthetase; cNOS: constitutive nitric oxide synthase; DHAP: dihydroxyacetone phosphate; DIC: dicarboxylate carrier; EX: anion transporter; FAD: flavin adenine dinucleotide; FADH: flavin adenine dinucleotide (reduced form); FH: fumarase; F1,6BP: fructose 1,6-bisphosphate; F1,6BPase: fructose 1,6-bisphosphatase; F2,6B: fructose 2,6-bisphosphate; F6P: fructose 6-phosphate; G3P: glycerol 3-phosphate; G6P: glucose-6-phosphate; G6Pase: glucose 6-phosphatase; GADP: glyceraldehyde 3-phosphate; GAPDH: glyceraldehyde 3-phosphate dehydrogenase; GCK: glucokinase; GDH: glutamate dehydrogenase; GDP: guanosine diphosphate; GK: glycerol kinase; GLAST: glutamate-aspartate antiporter; GLS: glutaminase; GPDH: glycerol 3-phosphate dehydrogenase; GTP: guanosine triphosphate; IDH: isocitrate dehydrogenase; KGDH: ketoglutarate dehydrogenase; LDH: lactate dehydrogenase; malate-α-KG antiporter: malate-α-ketoglutarate antiporter; MCT1: monocarboxylate transporter 1; MDH: malate dehydrogenase; MGT: mitochondrial glutamine transporter; MPC: mitochondrial pyruvate carrier; NAD⁺: nicotinamide adenine dinucleotide; NADH: nicotinamide adenine dinucleotide (reduced form); OGC: 2-oxoglutarate carrier; ORNT1: mitochondrial ornithine transporter; OTC: ornithine transcarbamylase; PC: pyruvate carboxylase; PEP: phosphoenolpyruvate; PEPCK: phosphoenolpyruvate carboxykinase; PFK1: phosphofructokinase 1; PGI: phosphoglucose isomerase; PGK: phosphoglycerate kinase; PGM: phosphoglycerate mutase; PK: pyruvate kinase; SCS: succinyl coenzyme-A synthetase; SDH: succinate dehydrogenase; SNAT: sodium-coupled neutral amino acid transporters; TCA: tricarboxylic acid; TPI1: triosephosphate isomerase 1.

Figure 2

Mechanisms underlying the inhibitory effect of metformin on hepatic gluconeogenesis. Metformin inhibits hepatic gluconeogenesis by transcriptional, allosteric, substrate-specific, and redox-dependent mechanisms. Created with BioRender.com. AC1: Adenylyl cyclase 1; AMP: adenosine monophosphate; AMPK: AMP-activated protein kinase; aPKC: atypical protein kinase C; ATP: adenosine triphosphate; CBP: CREB binding protein; CREB: cAMP-response element binding protein; CRTC2: CREB-regulated transcription coactivator 2; DHAP: dihydroxyacetone phosphate; ETC: electron transport chain; F1,6BPase: fructose 1,6-bisphosphatase; FAD: flavin adenine dinucleotide; FADH: flavin adenine dinucleotide (reduced form); FoxO1: forkhead box protein O1; G3P: glycerol 3-phosphate; G6PC: glucose-6-phosphatase catalytic subunit; GCN5: general control non-depressible 5; GPDH: glycerol-3-phosphate dehydrogenase; GPx: glutathione peroxidase; GR: glutathione reductase; GSH: glutathione; GSSG: glutathione disulfide; HNF4α: hepatocyte nuclear factor 4 alpha; Let-7: lethal microRNA; LDH: lactate dehydrogenase; NAD⁺: nicotinamide adenine dinucleotide; NADH: nicotinamide adenine dinucleotide hydrogen; OCT1: organic cation transporter; PEPCK1: cytoplasmic phosphoenolpyruvate carboxykinase; PGC-1α: peroxisome proliferator-activated receptor gamma (PPAR-γ) coactivator-1 alpha; PK: pyruvate kinase; PKA: protein kinase A; PP1: protein phosphatase 1; SHP: small heterodimer partner; SIRT1: sirtuin silent information regulator 1; TET3: tet methylcytosine dioxygenase 3.

Figure 3

Mechanisms underlying inhibitory effect of NO on hepatic gluconeogenesis. NO inhibits hepatic gluconeogenesis via cGMP-dependent pathways and S-nitrosylation. Created with BioRender.com. AMPK: AMP-activated protein kinase; cGMP: cyclic guanosine monophosphate; CREB: cAMP-response element binding protein; ETC: electron transport chain; G6PC: glucose-6-phosphatase catalytic subunit; GAPDH: glyceraldehyde 3-phosphate dehydrogenase; GCK: glucokinase; GTP: guanosine triphosphate; MDH2: mitochondrial malate dehydrogenase; NO: nitric oxide; PEPCK: phosphoenolpyruvate carboxykinase; PKG: cGMP-dependent protein kinase; sGC: soluble guanylate cyclase; TCA: tricarboxylic acid; VASP: vasodilator-stimulated phosphoprotein.

Figure 4

Effect of Met and NO on hepatic gluconeogenesis, indicating the site of combined actions. Created with BioRender.com. DHAP: Dihydroxyacetone phosphate; G6P: glucose-6-phosphate; GADP: glyceraldehyde 3-phosphate; Met: metformin; NO: nitric oxide; PEP: phosphoenolpyruvate.