ATP Secretion and Metabolism in Regulating Pancreatic Beta Cell Functions and Hepatic Glycolipid Metabolism

PANX1-mediated ATP release confers FAM3A's suppression effects on hepatic gluconeogenesis and lipogenesis

BACKGROUND

Extracellular adenosine triphosphate (ATP) is an important signal molecule. In previous studies, intensive research had revealed the crucial roles of family with sequence similarity 3 member A (FAM3A) in controlling hepatic glucolipid metabolism, islet β cell function, adipocyte differentiation, blood pressure, and other biological and pathophysiological processes. Although mitochondrial protein FAM3A plays crucial roles in the regulation of glucolipid metabolism via stimulating ATP release to activate P2 receptor pathways, its mechanism in promoting ATP release in hepatocytes remains unrevealed.

METHODS

db/db, high-fat diet (HFD)-fed, and global pannexin 1 (PANX1) knockout mice, as well as liver sections of individuals, were used in this study. Adenoviruses and adeno-associated viruses were utilized for in vivo gene overexpression or inhibition. To evaluate the metabolic status in mice, oral glucose tolerance test (OGTT), pyruvate tolerance test (PTT), insulin tolerance test (ITT), and magnetic resonance imaging (MRI) were conducted. Protein-protein interactions were determined by coimmunoprecipitation with mass spectrometry (MS) assays.

RESULTS

In livers of individuals and mice with steatosis, the expression of ATP-permeable channel PANX1 was increased (P < 0.01). Hepatic PANX1 overexpression ameliorated the dysregulated glucolipid metabolism in obese mice. Mice with hepatic PANX1 knockdown or global PANX1 knockout exhibited disturbed glucolipid metabolism. Restoration of hepatic PANX1 rescued the metabolic disorders of PANX1-deficient mice (P < 0.05). Mechanistically, ATP release is mediated by the PANX1-activated protein kinase B-forkhead box protein O1 (Akt-FOXO1) pathway to inhibit gluconeogenesis via P2Y receptors in hepatocytes. PANX1-mediated ATP release also activated calmodulin (CaM) (P < 0.01), which interacted with c-Jun N-terminal kinase (JNK) to inhibit its activity, thereby deactivating the transcription factor activator protein-1 (AP1) and repressing fatty acid synthase (FAS) expression and lipid synthesis (P < 0.05). FAM3A stimulated the expression of PANX1 via heat shock factor 1 (HSF1) in hepatocytes (P < 0.05). Notably, FAM3A overexpression failed to promote ATP release, inhibit the expression of gluconeogenic and lipogenic genes, and suppress gluconeogenesis and lipid deposition in PANX1-deficient hepatocytes and livers.

CONCLUSIONS

PANX1-mediated release of ATP plays a crucial role in maintaining hepatic glucolipid homeostasis, and it confers FAM3A's suppressive effects on hepatic gluconeogenesis and lipogenesis.

High Sucrose Diet-Induced Subunit I Tyrosine 304 Phosphorylation of Cytochrome c Oxidase Leads to Liver Mitochondrial Respiratory Dysfunction in the Cohen Diabetic Rat Model

The mitochondrial oxidative phosphorylation process generates most of the cellular energy and free radicals in mammalian tissues. Both factors play a critical role in numerous human diseases that could be affected by reversible phosphorylation events that regulate the function and activity of the oxidative phosphorylation complexes. In this study, we analyzed liver mitochondria of Cohen diabetes-sensitive (CDs) and Cohen diabetes-resistant (CDr) rats, using blue native gel electrophoresis (BN-PAGE) in combination with mitochondrial activity measurements and a site-specific tyrosine phosphorylation implicated in inflammation, a known driver of diabetes pathology. We uncovered the presence of a specific inhibitory phosphorylation on tyrosine 304 of catalytic subunit I of dimeric cytochrome c oxidase (CcO, complex IV). Driven by a high sucrose diet in both CDr and CDs rats, Y304 phosphorylation, which occurs close to the catalytic oxygen binding site, correlates with a decrease in CcO activity and respiratory dysfunction in rat liver tissue under hyperglycemic conditions. We propose that this phosphorylation, specifically seen in dimeric CcO and induced by high sucrose diet-mediated inflammatory signaling, triggers enzymatic activity decline of complex IV dimers and the assembly of supercomplexes in liver tissue as a molecular mechanism underlying a (pre-)diabetic phenotype.

Detrimental Actions of Chlorinated Nucleosides on the Function and Viability of Insulin-Producing Cells

Neutrophils are innate immune cells that play a key role in pathogen clearance. They contribute to inflammatory diseases, including diabetes, by releasing pro-inflammatory cytokines, reactive oxygen species, and extracellular traps (NETs). NETs contain a DNA backbone and catalytically active myeloperoxidase (MPO), which produces hypochlorous acid (HOCl). Chlorination of the DNA nucleoside 8-chloro-deoxyguanosine has been reported as an early marker of inflammation in diabetes. In this study, we examined the reactivity of different chlorinated nucleosides, including 5-chloro-(deoxy)cytidine (5ClC, 5CldC), 8-chloro-(deoxy)adenosine (8ClA, 8CldA) and 8-chloro-(deoxy)guanosine (8ClG, 8CldG), with the INS-1E β-cell line. Exposure of INS-1E cells to 5CldC, 8CldA, 8ClA, and 8CldG decreased metabolic activity and intracellular ATP, and, together with 8ClG, induced apoptotic cell death. Exposure to 8ClA, but not the other nucleosides, resulted in sustained endoplasmic reticulum stress, activation of the unfolded protein response, and increased expression of thioredoxin-interacting protein (TXNIP) and heme oxygenase 1 (HO-1). Exposure of INS-1E cells to 5CldC also increased TXNIP and NAD(P)H dehydrogenase quinone 1 (NQO1) expression. In addition, a significant increase in the mRNA expression of NQO1 and GPx4 was seen in INS-1E cells exposed to 8ClG and 8CldA, respectively. However, a significant decrease in intracellular thiols was only observed in INS-1E cells exposed to 8ClG and 8CldG. Finally, a significant decrease in the insulin stimulation index was observed in experiments with all the chlorinated nucleosides, except for 8ClA and 8ClG. Together, these results suggest that increased formation of chlorinated nucleosides during inflammation in diabetes could influence β-cell function and may contribute to disease progression.