Hepatocyte-specific PKCβ deficiency protects against high-fat diet-induced nonalcoholic hepatic steatosis

Fructose-induced progression of steatohepatitis involves disrupting aldolase B-AMPK signaling in methionine adenosyltransferase 1A deficient mice

OBJECTIVE

Aldolases (ALDO) are sensors that regulate AMPK via binding to fructose 1,6-biphosphate (FBP), an intermediate of glucose and fructose metabolism. Fructose consumption is linked to metabolic dysfunction-associated steatotic liver disease (MASLD) progression but whether ALDO-AMPK signaling is involved is unknown. Methionine adenosyltransferase alpha 1 (Mat1a) knockout (KO) mice have low hepatic S-adenosylmethionine (SAMe) level and spontaneously develop steatohepatitis. ALDOB methylation has not been reported and here we investigated whether SAMe level regulates ALDOB and ALDOB-AMPK signaling and whether fructose feeding accelerates MASLD progression by disrupting ALDOB-AMPK signaling.

METHODS

Mass spectrometry identified ALDOB methylation sites and recombinant in vitro approaches assessed how methylation at those sites affects ALDOB oligomerization and activity. Primary hepatocytes cultured with high/low glucose and/or fructose and wild type (WT) and Mat1a KO mice fed with a high-fructose diet examined AMPK-ALDOB signaling and MASLD progression.

RESULTS

In Mat1a KO livers ALDOB R173 is hypomethylated while ALDOB activity is enhanced. Recombinant ALDOB is methylated at R173 and R304 by protein arginine methyltransferase 1. Low hepatic SAMe level results in hypomethylated ALDOB, which favors the tetrameric form that has higher enzymatic activity, and higher capacity to signal to activate AMPK. Fructose, independently of glucose levels, inhibited AMPK activity and induced lipid accumulation in hepatocytes. Mat1a KO mice have hyperactivated AMPK and fructose feeding inhibits it, enhancing the accumulation of fat in the liver and the progression of MASLD.

CONCLUSION

Hepatic SAMe levels regulate ALDOB oligomeric state and enzymatic activity impacting on AMPK signaling and fructose-induced MASLD progression.

Effects of ultra-processed foods on the liver: insights from gut microbiome and metabolomics studies in rats

Purpose

High consumption of Ultra-processed foods (UPF) have been identified as a potential risk factor for Non-alcoholic fatty liver disease (NAFLD). Nevertheless, there is limited empirical evidence regarding the impact of UPF, which are typical combination of processed foods, on liver health through alterations in gut microbiota and metabolic processes. We aim to examine the potential impact of UPF on liver health and to explore the role of gut microbiota and metabolites.

Methods

This study used Sprague-Dawley rats to mimic modern UPF diets for 90 days. Some serum biochemical indices, inflammatory factors, oxidative stress markers, hematoxylin-eosin (HE) staining of the liver, 16S ribosomal RNA (rRNA) and Liquid chromatography-mass spectrometry (LC-MS) of rat feces were detected.

Results

The UPF diet-induced simple steatosis of the liver in rats without affecting the levels of IL-6, GSH, MDA, and SOD. Additionally, it modified the gut microbiota, increasing potentially harmful bacteria, such as norank_f__Desulfovibrionaceae and Staphylococcus, while also elevating the relative abundance of potentially beneficial bacteria, including Dubosiella and Allobaculum. Furthermore, the consumption of UPF led to a metabolomic disorder characterized by disruptions in the sphingolipid signaling pathway, sulfur relay system, and arachidonic acid metabolism.

Conclusion

In conclusion, the findings of this study indicate that the consumption of UPF influences the development of simple hepatic steatosis, potentially through alterations in gut microbiota and metabolomics.

Black rice bran extract exerts hepatoprotection via attenuating inflammation and apoptosis in obese-insulin-resistant rats

Global pandemic of obesity contributes to increasing the risk of diabetes and non-alcoholic fatty liver disease (NAFLD). To find an alternative approach to lower the risk caused by obesity.

Aims

We investigated the antidiabetic and hepatoprotective activity of black rice bran extract (BRE) in obese, insulin-resistant rats induced by a high-fat diet (HFD).

Main methods

After HFD feeding, the parameters related to glucose, lipid profiles, and liver injury were determined.

Key findings

Rats on a HFD exhibited significantly elevated plasma glucose and lipid levels, as well as increased liver enzyme activities (aspartate transaminase and alanine transaminase), relative to the control group. Interestingly, those parameters in the BRE-treated group were significantly decreased. We investigated the liver histological study, and the BRE-treated group showed to ameliorate the liver injury accompanied by lower inflammatory and apoptotic markers.

Significance

Our findings suggest that BRE has the potential to be used as a dietary supplement to lessen metabolic dysregulation and prevent liver impairment.

Disclosing the Novel Protective Mechanisms of Ocrelizumab in Multiple Sclerosis: The Role of PKC Beta and Its Down-Stream Targets

Ocrelizumab (OCR) is a humanized anti-CD20 monoclonal antibody approved for both Relapsing and Primary Progressive forms of Multiple Sclerosis (MS) treatment. OCR is postulated to act via rapid B cell depletion; however, by analogy with other anti-CD20 agents, additional effects can be envisaged, such as on Protein Kinase C (PKC). Hence, this work aims to explore novel potential mechanisms of action of OCR in peripheral blood mononuclear cells from MS patients before and after 12 months of OCR treatment. We first assessed, up-stream, PKCβII and subsequently explored two down-stream pathways: hypoxia-inducible factor 1 alpha (HIF-1α)/vascular endothelial growth factor (VEGF), and human antigen R (HuR)/manganese-dependent superoxide dismutase (MnSOD) and heat shock proteins 70 (HSP70). At baseline, higher levels of PKCβII, HIF-1α, and VEGF were found in MS patients compared to healthy controls (HC); interestingly, the overexpression of this inflammatory cascade was counteracted by OCR treatment. Conversely, at baseline, the content of HuR, MnSOD, and HSP70 was significantly lower in MS patients compared to HC, while OCR administration induced the up-regulation of these neuroprotective pathways. These results enable us to disclose the dual positive action of OCR: anti-inflammatory and neuroprotective. Therefore, in addition to B cell depletion, the effect of OCR on these molecular cascades can contribute to counteracting disease progression.

Consuming a Linoleate-Rich Diet Increases Concentrations of Tetralinoleoyl Cardiolipin in Mouse Liver and Alters Hepatic Mitochondrial Respiration

BACKGROUND

Hepatic mitochondrial dysfunction is a major cause of fat accumulation in the liver. Individuals with fatty liver conditions have hepatic mitochondrial structural abnormalities and a switch in the side chain composition of the mitochondrial phospholipid, cardiolipin, from poly- to monounsaturated fatty acids. Linoleic acid (LA), an essential dietary fatty acid, is required to remodel nascent cardiolipin (CL) to its tetralinoleoyl cardiolipin (L4CL, CL with 4 LA side chains) form, which is integral for mitochondrial membrane structure and function to promote fatty acid oxidation. It is unknown, however, whether increasing LA in the diet can increase hepatic L4CL concentrations and improve mitochondrial respiration in the liver compared with a diet rich in monounsaturated and saturated fatty acids.

OBJECTIVES

The main aim of this study was to test the ability of a diet fortified with LA-rich safflower oil (SO), compared with the one fortified with lard (LD), to increase concentrations of L4CL and improve mitochondrial respiration in the livers of mice.

METHODS

Twenty-four (9-wk-old) C57 BL/J6 male mice were fed either the SO or LD diets for ∼100 d, whereas food intake and body weight, fasting glucose, and glucose tolerance tests were performed to determine any changes in glycemic control.

RESULTS

Livers from mice fed SO diet had higher relative concentrations of hepatic L4CL species compared with LD diet-fed mice (P value = 0.004). Uncoupled mitochondria of mice fed the SO diet, compared with LD diet, had an increased baseline oxygen consumption rate (OCR) and succinate-driven respiration (P values = 0.03 and 0.01). SO diet-fed mice had increased LA content in all phospholipid classes compared with LD-fed mice (P < 0.05).

CONCLUSIONS

Our findings reveal that maintaining or increasing hepatic L4CL may result in increased OCR in uncoupled hepatic mitochondria in healthy mice whereas higher oleate content of CL reduced mitochondrial function shown by lower OCR in uncoupled mitochondria.

Hepatic protein kinase Cbeta deficiency mitigates late-onset obesity

Although aging is associated with progressive adiposity and a decline in liver function, the underlying molecular mechanisms and metabolic interplay are incompletely understood. Here, we demonstrate that aging induces hepatic protein kinase Cbeta (PKCβ) expression, while hepatocyte PKCβ deficiency (PKCβHep-/-) in mice significantly attenuates obesity in aged mice fed a high-fat diet. Compared with control PKCβfl/fl mice, PKCβHep-/- mice showed elevated energy expenditure with augmentation of oxygen consumption and carbon dioxide production which was dependent on β3-adrenergic receptor signaling, thereby favoring negative energy balance. This effect was accompanied by induction of thermogenic genes in brown adipose tissue (BAT) and increased BAT respiratory capacity, as well as a shift to oxidative muscle fiber type with an improved mitochondrial function, thereby enhancing oxidative capacity of thermogenic tissues. Furthermore, in PKCβHep-/- mice, we determined that PKCβ overexpression in the liver mitigated elevated expression of thermogenic genes in BAT. In conclusion, our study thus establishes hepatocyte PKCβ induction as a critical component of pathophysiological energy metabolism by promoting progressive hepatic and extrahepatic metabolic derangements in energy homeostasis, contributing to late-onset obesity. These findings have potential implications for augmenting thermogenesis as a means of combating aging-induced obesity.

Linoleic Acid-Enriched Diet Increases Mitochondrial Tetralinoleoyl Cardiolipin, OXPHOS Protein Levels, and Uncoupling in Interscapular Brown Adipose Tissue during Diet-Induced Weight Gain

Cardiolipin (CL) is a phospholipid unique to the inner mitochondrial membrane that supports respiratory chain structure and function and is demonstrated to be influenced by types of dietary fats. However, the influence of dietary fat on CL species and how this best supports mitochondrial function in brown adipose tissue (BAT), which exhibits an alternative method of energy utilization through the uncoupling of the mitochondrial proton gradient to generate heat, is not well understood. Therefore, the aim of our study was to evaluate metabolic parameters, interscapular BAT CL quantity, species, and mitochondrial function in mice consuming isocaloric moderate-fat diets with either lard (LD; similar fatty acid profile to western dietary patterns) or safflower oil high in linoleic acid (SO), shown to be metabolically favorable in large clinical meta-analyses. Mice fed the SO diet exhibited decreased adiposity, improved insulin sensitivity, and enrichment of LA-containing CL species in BAT CL. Furthermore, mice fed the SO diet exhibit higher levels of OXPHOS complex proteins and increased oxygen consumption in BAT. Our findings demonstrate that dietary consumption of LA-rich oil improves metabolic parameters, increases LA-containing CL species, and improves BAT function when compared to the consumption of lard in mice during diet-induced weight gain.

VEGF promotes diabetic retinopathy by upregulating the PKC/ET/NF-κB/ICAM-1 signaling pathway

Diabetic retinopathy (DR) is a common microvascular complication in patients with diabetes mellitus. DR is caused by chronic hyperglycemia and is characterized by progressive loss of vision because of damage to the retinal microvasculature. In this study, we investigated the regulatory role and clinical significance of the vascular endothelial growth factor (VEGF)/protein kinase C (PKC)/endothelin (ET)/nuclear factor-κB (NF-κB)/intercellular adhesion molecule 1 (ICAM-1) signaling pathway in DR using a rat model. Intraperitoneal injections of the VEGF agonist, streptozotocin (STZ) were used to generate the DR model rats. DR rats treated with the VEGF inhibitor (DR+VEGF inhibitor) were used to study the specific effects of VEGF on DR pathology and the underlying mechanisms. DR and DR+VEGF agonist rats were injected with the PKCβ2 inhibitor, GF109203X to determine the therapeutic potential of blocking the VEGF/PKC/ET/NF-κB/ICAM-1 signaling pathway. The body weights and blood glucose levels of the rats in all groups were evaluated at 16 weeks. DR-related retinal histopathology was analyzed by hematoxylin and eosin staining. ELISA assay was used to estimate the PKC activity in the retinal tissues. Western blotting and RT-qPCR assays were used to analyze the expression levels of PKC-β2, VEGF, ETs, NF-κB, and ICAM-1 in the retinal tissues. Immunohistochemistry was used to analyze VEGF and ICAM-1 expression in the rat retinal tissues. Our results showed that VEGF, ICAM-1, PKCβ2, ET, and NF-κB expression levels as well as PKC activity were significantly increased in the retinal tissues of the DR and DR+VEGF agonist rat groups compared to the control and DR+VEGF inhibitor rat groups. DR and DR+VEGF agonist rats showed significantly lower body weight and significantly higher retinal histopathology scores and blood glucose levels compared to the control and DR+VEGF inhibitor group rats. However, treatment of DR and DR+VEGF agonist rats with GF109203X partially alleviated DR pathology by inhibiting the VEGF/ PKC/ET/NF-κB/ICAM-1 signaling pathway. In summary, our data demonstrated that inhibition of the VEGF/ PKC/ET/NF-κB/ICAM-1 signaling pathway significantly alleviated DR-related pathology in the rat model. Therefore, VEGF/PKC/ET/NF-κB/ICAM-1 signaling axis is a promising therapeutic target for DR.

Role of hepatic PKCβ in nutritional regulation of hepatic glycogen synthesis

The signaling mechanisms by which dietary fat and cholesterol signals regulate central pathways of glucose homeostasis are not completely understood. By using a hepatocyte-specific PKCβ-deficient (PKCβHep-/-) mouse model, we demonstrated the role of hepatic PKCβ in slowing disposal of glucose overload by suppressing glycogenesis and increasing hepatic glucose output. PKCβHep-/- mice exhibited lower plasma glucose under the fed condition, modestly improved systemic glucose tolerance and mildly suppressed gluconeogenesis, increased hepatic glycogen accumulation and synthesis due to elevated glucokinase expression and activated glycogen synthase (GS), and suppressed glucose-6-phosphatase expression compared with controls. These events were independent of hepatic AKT/GSK-3α/β signaling and were accompanied by increased HNF-4α transactivation, reduced FoxO1 protein abundance, and elevated expression of GS targeting protein phosphatase 1 regulatory subunit 3C in the PKCβHep-/- liver compared with controls. The above data strongly imply that hepatic PKCβ deficiency causes hypoglycemia postprandially by promoting glucose phosphorylation via upregulating glucokinase and subsequently redirecting more glucose-6-phosphate to glycogen via activating GS. In summary, hepatic PKCβ has a unique and essential ability to induce a coordinated response that negatively affects glycogenesis at multiple levels under physiological postprandial conditions, thereby integrating nutritional fat intake with dysregulation of glucose homeostasis.