Targeting hepatocyte carbohydrate transport to mimic fasting and calorie restriction

Fasting as an Adjuvant Therapy for Cancer: Mechanism of Action and Clinical Practice

The fundamental biological characteristics of tumor cells are characterized by irregularities in signaling and metabolic pathways, which are evident through increased glucose uptake, altered mitochondrial function, and the ability to evade growth signals. Interventions such as fasting or fasting-mimicking diets represent a promising strategy that can elicit distinct responses in normal cells compared to tumor cells. These dietary strategies can alter the circulating levels of various hormones and metabolites, including blood glucose, insulin, glucagon, growth hormone, insulin-like growth factor, glucocorticoids, and epinephrine, thereby potentially exerting an anticancer effect. Additionally, elevated levels of insulin-like growth factor-binding proteins and ketone bodies may increase tumor cells' dependence on their own metabolites, ultimately leading to their apoptosis. The combination of fasting or fasting-mimicking diets with radiotherapy or chemotherapeutic agents has demonstrated enhanced anticancer efficacy. This paper aims to classify fasting, elucidate the mechanisms that underlie its effects, assess its impact on various cancer types, and discuss its clinical applications. We will underscore the differential effects of fasting on normal and cancer cells, the mechanisms responsible for these effects, and the imperative for clinical implementation.

Hepatocyte MMP14 mediates liver and inter-organ inflammatory responses to diet-induced liver injury

The matrix metalloproteinase MMP14 is a ubiquitously expressed, membrane-bound, secreted endopeptidase that proteolyzes substrates to regulate development, signaling, and metabolism. However, the spatial and contextual events inciting MMP14 activation and its metabolic sequelae are not fully understood. Here, we introduce an inducible, hepatocyte-specific MMP14-deficient model (MMP14LKO mice) to elucidate cell-intrinsic and systemic MMP14 function. We show that hepatocyte MMP14 mediates diet-induced body weight gain, peripheral adiposity, and impaired glucose homeostasis and drives diet-induced liver triglyceride accumulation and induction of hepatic inflammatory and fibrotic gene expression. Single-nucleus RNA sequencing revealed that hepatocyte MMP14 mediates Kupffer cell and T-cell accumulation and promotes diet-induced hepatocellular subpopulation shifts toward protection against lipid absorption. MMP14 co-immunoprecipitation and proteomic analyses revealed MMP14 substrate binding across both inflammatory and cytokine signaling, as well as metabolic pathways. Strikingly, hepatocyte MMP14 loss-of-function suppressed skeletal muscle and adipose inflammation in vivo, and in a reductionist adipose-hepatocyte co-culture model. Finally, we reveal that trehalose-type glucose transporter inhibitors decrease hepatocyte MMP14 gene expression and nominate these inhibitors as translatable therapeutic metabolic agents. We conclude that hepatocyte MMP14 drives liver and inter-organ inflammatory and metabolic sequelae of obesogenic dietary insult. Modulating MMP14 activation and blockade thus represents a targetable node in the pathogenesis of hepatic inflammation.

A Structure-function Analysis of Hepatocyte Arginase 2 Reveals Mitochondrial Ureahydrolysis as a Determinant of Glucose Oxidation

BACKGROUND & AIMS

Restoring hepatic and peripheral insulin sensitivity is critical to prevent or reverse metabolic syndrome and type 2 diabetes. Glucose homeostasis comprises in part the complex regulation of hepatic glucose production and insulin-mediated glucose uptake and oxidation in peripheral tissues. We previously identified hepatocyte arginase 2 (Arg2) as an inducible ureahydrolase that improves glucose homeostasis and enhances glucose oxidation in multiple obese, insulin-resistant models. We therefore examined structure-function determinants through which hepatocyte Arg2 governs systemic insulin action and glucose oxidation.

METHODS

To do this, we generated mice expressing wild-type murine Arg2, enzymatically inactive Arg2 (Arg2H160F) and Arg2 lacking its putative mitochondrial targeting sequence (Arg2Δ1-22). We expressed these hepatocyte-specific constructs in obese, diabetic (db/db) mice and performed genetic complementation analyses in hepatocyte-specific Arg2-deficent (Arg2LKO) mice.

RESULTS

We show that Arg2 attenuates hepatic steatosis, independent of mitochondrial localization or ureahydrolase activity, and that enzymatic arginase activity is dispensable for Arg2 to augment total body energy expenditure. In contrast, mitochondrial localization and ureahydrolase activity were required for Arg2-mediated reductions in fasting glucose and insulin resistance indices. Mechanistically, Arg2Δ1-22 and Arg2H160F failed to suppress glucose appearance during hyperinsulinemic-euglycemic clamping. Quantification of heavy-isotope-labeled glucose oxidation further revealed that mistargeting or ablating Arg2 enzymatic function abrogates Arg2-induced peripheral glucose oxidation.

CONCLUSION

We conclude that the metabolic effects of Arg2 extend beyond its enzymatic activity, yet hepatocyte mitochondrial ureahydrolysis drives hepatic and peripheral oxidative metabolism. The data define a structure-based mechanism mediating hepatocyte Arg2 function and nominate hepatocyte mitochondrial ureahydrolysis as a key determinant of glucose oxidative capacity in mammals.

The tetraspanin transmembrane protein CD53 mediates dyslipidemia and integrates inflammatory and metabolic signaling in hepatocytes

Tetraspanins are transmembrane signaling and proinflammatory proteins. Prior work demonstrates that the tetraspanin, CD53/TSPAN25/MOX44, mediates B-cell development and lymphocyte migration to lymph nodes and is implicated in various inflammatory diseases. However, CD53 is also expressed in highly metabolic tissues, including adipose and liver; yet its function outside the lymphoid compartment is not defined. Here, we show that CD53 demarcates the nutritional and inflammatory status of hepatocytes. High-fat exposure and inflammatory stimuli induced CD53 in vivo in liver and isolated primary hepatocytes. In contrast, restricting hepatocyte glucose flux through hepatocyte glucose transporter 8 deletion or through trehalose treatment blocked CD53 induction in fat- and fructose-exposed contexts. Furthermore, germline CD53 deletion in vivo blocked Western diet-induced dyslipidemia and hepatic inflammatory transcriptomic activation. Surprisingly, metabolic protection in CD53 KO mice was more pronounced in the presence of an inciting inflammatory event. CD53 deletion attenuated tumor necrosis factor alpha-induced and fatty acid + lipopolysaccharide-induced cytokine gene expression and hepatocyte triglyceride accumulation in isolated murine hepatocytes. In vivo, CD53 deletion in nonalcoholic steatohepatitis diet-fed mice blocked peripheral adipose accumulation and adipose inflammation, insulin tolerance, and liver lipid accumulation. We then defined a stabilized and trehalase-resistant trehalose polymer that blocks hepatocyte CD53 expression in basal and over-fed contexts. The data suggest that CD53 integrates inflammatory and metabolic signals in response to hepatocyte nutritional status and that CD53 blockade may provide a means by which to attenuate pathophysiology in diseases that integrate overnutrition and inflammation, such as nonalcoholic steatohepatitis and type 2 diabetes.

Hepatic Suppression of Mitochondrial Complex II Assembly Drives Systemic Metabolic Benefits

Alternate day fasting (ADF), the most popular form of caloric restriction, has shown to improve metabolic health in preclinical subjects, while intrinsic network underpinning the process remains unclear. Here, it is found that liver undergoes dramatic metabolic reprogramming during ADF, accompanied surprisingly with unique complex II dysfunction attributing to suspended complex II assembly via suppressing SDHAF4, a recently identified assembly factor. Despite moderate mitochondrial complex II dysfunction, hepatic Sdhaf4 knockout mice present intriguingly improved glucose tolerance and systemic insulin sensitivity, consistent with mice after ADF intervention. Mechanistically, it is found that hepatocytes activate arginine-nitric oxide (NO) biosynthesis axle in response to complex II and citric acid cycle dysfunction, the release of NO from liver can target muscle and adipocytes in addition to its autocrine action for enhanced insulin sensitivity. These results highlight the pivotal role of liver in ADF-associated systemic benefits, and suggest that targeting hepatic complex II assembly can be an intriguing strategy against metabolic disorders.

SIRT1 selectively exerts the metabolic protective effects of hepatocyte nicotinamide phosphoribosyltransferase

Calorie restriction abates aging and cardiometabolic disease by activating metabolic signaling pathways, including nicotinamide adenine dinucleotide (NAD⁺) biosynthesis and salvage. Nicotinamide phosphoribosyltransferase (NAMPT) is rate-limiting in NAD⁺ salvage, yet hepatocyte NAMPT actions during fasting and metabolic duress remain unclear. We demonstrate that hepatocyte NAMPT is upregulated in fasting mice, and in isolated hepatocytes subjected to nutrient withdrawal. Mice lacking hepatocyte NAMPT exhibit defective FGF21 activation and thermal regulation during fasting, and are sensitized to diet-induced glucose intolerance. Hepatocyte NAMPT overexpression induced FGF21 and adipose browning, improved glucose homeostasis, and attenuated dyslipidemia in obese mice. Hepatocyte SIRT1 deletion reversed hepatocyte NAMPT effects on dark-cycle thermogenesis, and hepatic FGF21 expression, but SIRT1 was dispensable for NAMPT insulin-sensitizing, anti-dyslipidemic, and light-cycle thermogenic effects. Hepatocyte NAMPT thus conveys key aspects of the fasting response, which selectively dissociate through hepatocyte SIRT1. Modulating hepatocyte NAD⁺ is thus a potential mechanism through which to attenuate fasting-responsive disease.

Pegylated arginine deiminase drives arginine turnover and systemic autophagy to dictate energy metabolism

Obesity is a multi-systemic disorder of energy balance. Despite intense investigation, the determinants of energy homeostasis remain incompletely understood, and efficacious treatments against obesity and its complications are lacking. Here, we demonstrate that conferred arginine iminohydrolysis by the bacterial virulence factor and arginine deiminase, arcA, promotes mammalian energy expenditure and insulin sensitivity and reverses dyslipidemia, hepatic steatosis, and inflammation in obese mice. Extending this, pharmacological arginine catabolism via pegylated arginine deiminase (ADI-PEG 20) recapitulates these metabolic effects in dietary and genetically obese models. These effects require hepatic and whole-body expression of the autophagy complex protein BECN1 and hepatocyte-specific FGF21 secretion. Single-cell ATAC sequencing further reveals BECN1-dependent hepatocyte chromatin accessibility changes in response to ADI-PEG 20. The data thus reveal an unexpected therapeutic utility for arginine catabolism in modulating energy metabolism by activating systemic autophagy, which is now exploitable through readily available pharmacotherapy.

Increased Beta-Hydroxybutyrate Level Is Not Sufficient for the Neuroprotective Effect of Long-Term Ketogenic Diet in an Animal Model of Early Parkinson's Disease. Exploration of Brain and Liver Energy Metabolism Markers

The benefits of a ketogenic diet in childhood epilepsy steered up hope for neuroprotective effects of hyperketonemia in Parkinson’s disease (PD). There are multiple theoretical reasons but very little actual experimental proof or clinical trials. We examined the long-term effects of the ketogenic diet in an animal model of early PD. A progressive, selective dopaminergic medium size lesion was induced by 6-OHDA injection into the medial forebrain bundle. Animals were kept on the stringent ketogenic diet (1% carbohydrates, 8% protein, 70% fat) for 3 weeks prior and 4 weeks after the brain operation. Locomotor activity, neuron count, dopaminergic terminal density, dopamine level, and turnover were analyzed at three time-points post-lesion, up to 4 weeks after the operation. Energy metabolism parameters (glycogen, mitochondrial complex I and IV, lactate, beta-hydroxybutyrate, glucose) were analyzed in the brain and liver or plasma. Protein expression of enzymes essential for gluconeogenesis (PEPCK, G6PC) and glucose utilization (GCK) was analyzed in the liver. Despite long-term hyperketonemia pre- and post-lesion, the ketogenic diet did not protect against 6-OHDA-induced dopaminergic neuron lesions. The ketogenic diet only tended to improve locomotor activity and normalize DA turnover in the striatum. Rats fed 7 weeks in total with a restrictive ketogenic diet maintained normoglycemia, and neither gluconeogenesis nor glycogenolysis in the liver was responsible for this effect. Therefore, potentially, the ketogenic diet could be therapeutically helpful to support the late compensatory mechanisms active via glial cells but does not necessarily act against the oxidative stress-induced parkinsonian neurodegeneration itself. A word of caution is required as the stringent ketogenic diet itself also carries the risk of unwanted side effects, so it is important to study the long-term effects of such treatments. More detailed metabolic long-term studies using unified diet parameters are required, and human vs. animal differences should be taken under consideration.