Dietary restriction induces a sexually dimorphic type I interferon response in mice with gene-environment interactions

Effects of a protein kinase C epsilon inhibitor on insulin signalling in lipid-treated HepG2 hepatocytes and glucose tolerance in fat-fed mice

AIMS

Protein kinase C epsilon (PKCε) plays a causative role in the development of glucose intolerance, and is a potential target for the treatment of type 2 diabetes. Here, we examined the effects of the PKCε inhibitor CIDD-0150612 (CP612) on insulin action in palmitate-treated HepG2 hepatocytes in vitro and on glucose homeostasis in fat-fed mice in vivo.

METHODS

HepG2 cells were treated with palmitate and CP612 and stimulated with insulin. Insulin signalling was examined by immunoblotting and glucose incorporation into glycogen was measured using glucose tracer. Mice were fed a high-fat diet and treated with CP612 prior to glucose tolerance tests and tissue harvest. Proteomic analysis of liver was carried out by mass spectrometry.

RESULTS

CP612 promoted Akt phosphorylation in a highly insulin-dependent manner and reversed the inhibition of insulin-stimulated Akt phosphorylation and glucose incorporation into glycogen by palmitate. Fat-fed mice treated with CP612 had reduced fat mass, but not lean mass, compared with vehicle-treated littermates. Mice treated acutely with CP612 exhibited elevated fasting blood glucose. However, mice studied 24h after the last dose had lower fasting glucose and improved glucose tolerance with a lower insulin excursion. Proteomic analysis of liver from CP612-treated fat-fed mice indicated a reduction in gluconeogenic gene expression and decreased phosphorylation of the transcription factor Foxk1.

CONCLUSIONS

The PKCε inhibitor CP612 had beneficial effects on insulin action in hepatocytes and on fat mass and glucose homeostasis in mice. Because certain effects were not previously observed in genetically PKCε-deficient mice, off-target effects may be partly responsible.

Apolipoprotein A-IV is induced by high-fat diets and mediates positive effects on glucose and lipid metabolism

OBJECTIVE

Low-carbohydrate, high-fat diets under eucaloric conditions are associated with several health-beneficial metabolic effects in humans, particularly in the liver. We recently observed that apolipoprotein A-IV (apoA-IV), a highly abundant apolipoprotein, was among the most upregulated proteins in circulation after six weeks of consuming a high-fat diet in humans. However, the impact of dietary changes in regulating apoA-IV, and the potential effects of apoA-IV on regulation of glucose- and lipid metabolism remain to be fully established.

METHODS

We investigated the regulation of circulating fasting concentrations of apoA-IV in humans in response to diets enriched in either fat or carbohydrates. Moreover, to study the whole-body and tissue-specific glucose and lipid metabolic effects of apoA-IV, we administrered apoA-IV recombinant protein to mice and isolated pancreatic islets.

RESULTS

We demonstrate that in healthy human individuals high-fat intake increased fasting plasma apoA-IV concentrations by up to 54%, while high-carbohydrate intake suppressed plasma apoA-IV concentrations. In mice, administration of apoA-IV acutely lowered blood glucose levels both in lean and obese mice. Interestingly, this was related to a dual mechanism, involving both inhibition of hepatic glucose production and increased glucose uptake into white and brown adipose tissues. In addition to an effect on hepatic glucose production, the apoA-IV-induced liver proteome revealed increased capacity for lipoprotein clearance. The effects of apoA-IV in the liver and adipose tissues were concomitant with increased whole-body fatty acid oxidation. Upon glucose stimulation, an improvement in glucose tolerance by apoA-IV administration was related to potentiation of glucose-induced insulin secretion, while apoA-IV inhibited glucagon secretion ex vivo in islets.

CONCLUSIONS

We find that apoA-IV is potently increased by intake of fat in humans, and that several beneficial metabolic effects, previously associated with high fat intake in humans, are mimicked by administration of apoA-IV protein to mice.

FUT10 and FUT11 are protein O-fucosyltransferases that modify protein EMI domains

O-Fucosylation plays crucial roles in various essential biological events. Alongside the well-established O-fucosylation of epidermal growth factor-like repeats by protein O-fucosyltransferase 1 (POFUT1) and thrombospondin type 1 repeats by POFUT2, we recently identified a type of O-fucosylation on the elastin microfibril interface (EMI) domain of Multimerin-1 (MMRN1). Here, using AlphaFold2 screens, co-immunoprecipitation, enzymatic assays combined with mass spectrometric analysis and CRISPR-Cas9 knockouts, we demonstrate that FUT10 and FUT11, originally annotated in UniProt as α1,3-fucosyltransferases, are actually POFUTs responsible for modifying EMI domains; thus, we renamed them as POFUT3 and POFUT4, respectively. Like POFUT1/2, POFUT3/4 function in the endoplasmic reticulum, require folded domain structures for modification and participate in a non-canonical endoplasmic reticulum quality control pathway for EMI domain-containing protein secretion. This finding expands the O-fucosylation repertoire and provides an entry point for further exploration in this emerging field of O-fucosylation.

VPS41 deletion triggers progressive loss of insulin stores and downregulation of β-cell identity

Vacuolar protein sorting-associated protein 41 (VPS41) has been established as a requirement for normal insulin secretory function in pancreatic β cells. Genetic deletion of VPS41 in mouse pancreatic β cells results in diabetes, although the mechanisms are not understood. Presently, we show that VPS41 deletion results in rapid mature insulin degradation and downregulation of β-cell identity. This phenotype is observed in vivo, with VPS41KO mice displaying progressive loss of insulin content and β-cell function with age. In acute VPS41 depletion in vitro, the loss of insulin is associated with increased degradative pathway activity, increased Adapter Protein 3 complex colocalization with lysosomes, increased nuclear localization of transcription factor E3, and downregulation of PDX1 and INS mRNA expression. Inhibition of lysosomal degradation rescues the rapidly depleted insulin content. These data evidence a VPS41-dependent mechanism for both insulin content degradation and loss of β-cell identity in β cells.NEW & NOTEWORTHY In this study, we show that acute VPS41 deletion results in rapid degradation of insulin, whereas chronic VPS41 deletion results in downregulation of β-cell identity. In acute VPS41 depletion in vitro, the loss of insulin is associated with increased degradative pathway activity, increased Adapter Protein 3 complex colocalization with lysosomes, increased nuclear localization of transcription factor E3, and downregulation of PDX1 and INS mRNA expression. Inhibition of lysosomal degradation rescues the rapidly depleted insulin content.

Chronic intermittent fasting impairs β cell maturation and function in adolescent mice

Intermittent fasting (IF) is a nutritional lifestyle intervention with broad metabolic benefits, but whether the impact of IF depends on the individual's age is unclear. Here, we investigated the effects of IF on systemic metabolism and β cell function in old, middle-aged, and young mice. Short-term IF improves glucose homeostasis across all age groups without altering islet function and morphology. In contrast, while chronic IF is beneficial for adult mice, it results in impaired β cell function in the young. Using single-cell RNA sequencing (scRNA-seq), we delineate that the β cell maturation and function scores are reduced in young mice. In human islets, a similar pattern is observed in type 1 (T1D), but not type 2 (T2D), diabetes, suggesting that the impact of chronic IF in adolescence is linked to the development of β cell dysfunction. Our study suggests considering the duration of IF in younger persons, as it may worsen rather than reduce diabetes outcomes.

Cab45G trafficking through the insulin secretory pathway is altered in human type 2 diabetes

In type 2 diabetes (T2D), the rate of insulin secretory granule biogenesis can limit insulin secretion from pancreatic β-cells. Using rat insulinoma INS1 β-cells, we show that the soluble Ca2+-binding/trafficking protein, Cab45G, serves as a non-essential chaperone for insulin granule biogenesis. In β-cells, Cab45G is stored within a cis-Golgi reservoir. Cab45G deletion dysregulates Ca2+ homeostasis and leads to secretory abnormality, but insulin granule biogenesis remains intact. Increasing Cab45G biosynthesis leads to anterograde trafficking into insulin granules, stimulating their production. Using human donor islets, we identify increased anterograde Cab45G trafficking in obese humans with and without T2D, consistent with the heightened demand for granule biogenesis. However, humans with T2D demonstrate decreased Golgi Cab45G localization and increased granule Cab45G localization compared to those without T2D. Our study provides the first insight into Cab45G function in specialized secretory cells and opens avenues of investigation into mechanisms associated with β-cell compensation and failure.

Sex and organ specific proteomic responses to vitamin C deficiency in the brain, heart, liver, and spleen of Gulo-/- mice

Recent advances in mass spectrometry have indicated that the water-soluble antioxidant vitamin C differentially modulates the abundance of various proteins in the hepatic tissue of female and male mice. In this study, we performed LC-MS/MS to identify and quantify proteins that correlate with serum vitamin C concentrations in the whole brain, heart, liver, and spleen tissues in mice deficient for the enzyme L-Gulonolactone oxidase required for vitamin C synthesis in mammals. This work shows for the first time that various biological processes affected by a vitamin C deficiency are not only sex specific dependent but also tissue specific dependent even though many proteins have been identified and quantified in more than three organs. For example, the abundance of several complex III subunits of the mitochondrial electron transport chain correlated positively with the levels of serum vitamin C only in the liver and not in the other tissues examined in this study even though such proteins were identified in all the organs analyzed. Western blot analyses on the Uqcrc1 and Uqcrfs1 complex III subunits validated the mass spectrometry results. Interestingly, the ferritin subunits represented the few quantified protein complexes that correlated positively with serum vitamin C in all the organs examined. Concomitantly, serum ferritin light chain 1 was inversely correlated with vitamin C levels in the serum. Thus, our study provides an initial comprehensive atlas of proteins significantly correlating with vitamin C in four organs in mice that will be a useful resource to the scientific community.

Intermittent fasting influences immunity and metabolism

Intermittent fasting (IF) modifies cell- and tissue-specific immunometabolic responses that dictate metabolic flexibility and inflammation during obesity and type 2 diabetes (T2D). Fasting forces periods of metabolic flexibility and necessitates increased use of different substrates. IF can lower metabolic inflammation and improve glucose metabolism without lowering obesity and can influence time-dependent, compartmentalized changes in immunity. Liver, adipose tissue, skeletal muscle, and immune cells communicate to relay metabolic and immune signals during fasting. Here we review the connections between metabolic and immune cells to explain the divergent effects of IF compared with classic caloric restriction (CR) strategies. We also explore how the immunometabolism of metabolic diseases dictates certain IF outcomes, where the gut microbiota triggers changes in immunity and metabolism during fasting.

Deletion of miPEP in adipocytes protects against obesity and insulin resistance by boosting muscle metabolism

Mitochondria facilitate thousands of biochemical reactions, covering a broad spectrum of anabolic and catabolic processes. Here we demonstrate that the adipocyte mitochondrial proteome is markedly altered across multiple models of insulin resistance and reveal a consistent decrease in the level of the mitochondrial processing peptidase miPEP.

OBJECTIVE

To determine the role of miPEP in insulin resistance.

METHODS

To experimentally test this observation, we generated adipocyte-specific miPEP knockout mice to interrogate its role in the aetiology of insulin resistance.

RESULTS

We observed a strong phenotype characterised by enhanced insulin sensitivity and reduced adiposity, despite normal food intake and physical activity. Strikingly, these phenotypes vanished when mice were housed at thermoneutrality, suggesting that metabolic protection conferred by miPEP deletion hinges upon a thermoregulatory process. Tissue specific analysis of miPEP deficient mice revealed an increment in muscle metabolism, and upregulation of the protein FBP2 that is involved in ATP hydrolysis in the gluconeogenic pathway.

CONCLUSION

These findings suggest that miPEP deletion initiates a compensatory increase in skeletal muscle metabolism acting as a protective mechanism against diet-induced obesity and insulin resistance.