Downregulation of lipin-1 induces insulin resistance by increasing intracellular ceramide accumulation in C2C12 myotubes

Association of three single nucleotide polymorphisms in the LPIN1 gene with milk production traits in cows of the Yaroslavl breed

Lipin-1 is a member of the evolutionarily conserved family of proteins and is expressed predominantly in adipose tissue and skeletal muscle. On the one hand, lipin-1 is an enzyme that catalyzes the dephosphorylation of phosphatidic acid to diacylglycerol (DAG) and thus participates in the metabolic pathways of biosynthesis of storage lipids in the cell, membrane phospholipids, and intracellular signaling molecules. On the other hand, lipin-1 is able to be transported from the cytoplasm to the nucleus and is a coactivator of lipid metabolism gene transcription. It was shown, using the analysis of single nucleotide polymorphism (SNP) associations, that the lipin-1 coding gene (LPIN1) is a promising candidate gene for milk production traits in Holstein and Brown Swiss cows. However, it is unclear how much of its effect depends on the breed. The Yaroslavl dairy cattle breed was created in the 18-19 centuries in Russia by breeding northern Great Russian cattle, which were short and poor productive, but well adapted to local climatic conditions and bad food base. It was shown by whole genome genotyping and sequencing that the Yaroslavl breed has unique genetics compared to Russian and other cattle breeds. The aim of the study was to assess the frequency of alleles and genotypes of three SNPs in the LPIN1 gene and to study the association of these SNPs with milk production traits in Yaroslavl cows. Blood samples from 142 cows of the Yaroslavl breed were obtained from two farms in the Yaroslavl region. Genotyping of SNPs was carried out by polymerase chain reaction-restriction fragment length polymorphism method. Associations of SNPs with 305-day milk yield, fat yield, fat percentages, protein yield, and protein percentages were studied from the first to the fourth lactation. Statistical tests were carried out using a mixed linear model, taking into account the relationship between individuals. We identified three SNPs - rs110871255, rs207681322 and rs109039955 with a frequency of a rare allele of 0.042-0.261 in Yaroslavl cows. SNP rs110871255 was associated with fat yield during the third and fourth lactations. SNP rs207681322 was associated with milk yield for the second, third and fourth lactations, as well as protein yield for the third lactation. Thus, we identified significant associations of SNPs rs207681322 and rs110871255 in the LPIN1 gene with a number of milk production traits during several lactations in Yaroslavl cows.

m6 A mRNA methylation: Biological features, mechanisms, and therapeutic potentials in type 2 diabetes mellitus

As the most common internal post-transcriptional RNA modification in eukaryotic cells, N6-methyladenosine (m6 A) performs a dynamic and reversible role in a variety of biological processes mediated by methyltransferases (writers), demethylases (erasers), and m6 A binding proteins (readers). M6 A methylation enables transcriptome conversion in different signals that regulate various physiological activities and organ development. Over the past few years, emerging studies have identified that mRNA m6 A regulators defect in β-cell leads to abnormal regulation of the target mRNAs, thereby resulting in β-cell dysfunction and loss of β-cell identity and mass, which are strongly associated with type 2 diabetes mellitus (T2DM) pathogenesis. Also, mRNA m6 A modification has been implicated with insulin resistance in muscles, fat, and liver cells/tissues. In this review, we elaborate on the biological features of m6 A methylation; provide a comprehensive overview of the underlying mechanisms that how it controls β-cell function, identity, and mass as well as insulin resistance; highlight its connections to glucose metabolism and lipid metabolism linking to T2DM; and further discuss its role in diabetes complications and its therapeutic potentials for T2DM diagnosis and treatment.

Lipin-1 restrains macrophage lipid synthesis to promote inflammation resolution

Macrophages are critical to maintaining and restoring tissue homeostasis during inflammation. The lipid metabolic state of macrophages influences their function, but a deeper understanding of how lipid metabolism is regulated in pro-resolving macrophage responses is needed. Lipin-1 is a phosphatidic acid phosphatase with a transcriptional coregulatory activity (TC) that regulates lipid metabolism. We previously demonstrated that lipin-1 supports pro-resolving macrophage responses, and here, myeloid-associated lipin-1 is required for inflammation resolution, yet how lipin-1-regulated cellular mechanisms promote macrophage pro-resolution responses is unknown. We demonstrated that the loss of lipin-1 in macrophages led to increased free fatty acid, neutral lipid, and ceramide content and increased phosphorylation of acetyl-CoA carboxylase. The inhibition of the first step of lipid synthesis and transport of citrate from the mitochondria in macrophages reduced lipid content and restored efferocytosis and inflammation resolution in lipin-1mKO macrophages and mice. Our findings suggest macrophage-associated lipin-1 restrains lipid synthesis, promoting pro-resolving macrophage function in response to pro-resolving stimuli.

N6-methyladenosine RNA modification: an emerging molecule in type 2 diabetes metabolism

Type 2 diabetes (T2D) is a metabolic disease with an increasing rate of incidence worldwide. Despite the considerable progress in the prevention and intervention, T2D and its complications cannot be reversed easily after diagnosis, thereby necessitating an in-depth investigation of the pathophysiology. In recent years, the role of epigenetics has been increasingly demonstrated in the disease, of which N6-methyladenosine (m6A) is one of the most common post-transcriptional modifications. Interestingly, patients with T2D show a low m6A abundance. Thus, a comprehensive analysis and understanding of this phenomenon would improve our understanding of the pathophysiology, as well as the search for new biomarkers and therapeutic approaches for T2D. In this review, we systematically introduced the metabolic roles of m6A modification in organs, the metabolic signaling pathways involved, and the effects of clinical drugs on T2D.

Platycodin D ameliorates hyperglycaemia and liver metabolic disturbance in HFD/STZ-induced type 2 diabetic mice

In this work, we investigated the ameliorative effects of platycodin D (PD), a major active chemical ingredient isolated from the roots of Platycodon grandiflorum (PG), on high-fat diet (HFD)/streptozotocin (STZ)-induced type 2 diabetes (T2D) mice. PD treatment (2.5 and 5.0 mg kg^-1) improved HFD-induced body weight gain. PD administration also decreased the fasting blood glucose (FBG) level and improved glucose and insulin tolerance levels. These data collectively showed that PD could maintain glucose homeostasis. In addition, the diabetic mice with PD treatment also showed fewer pathological changes in liver tissues and improved hepatic functional indexes with respect to the levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and recovery of abnormal liver function caused by T2D. Except for these, PD decreased the decomposition of hepatic glycogen. The results from western blot analysis showed that PD treatment might regulate the hepatic gluconeogenesis pathway with the increased phosphorylation/expression of AMPK and decreased expressions of PCK1 and G6Pase. In the aspect of lipid metabolism, PD decreased the whole-body lipid levels, including total cholesterol (TC), triglycerides (TG), and high-density lipoprotein (HDL), and reduced the hepatic fat accumulation induced by T2D through the AMPK/ACC/CPT-1 fatty acid anabolism pathway. In addition, the results of molecular docking showed that PD may have a potential direct effect on AMPK and other key glycolipid metabolism proteins. To summarize, PD modulation of hepatic glycolipid metabolism abnormalities is promising for T2D therapy in the future.

Role of VEGFB in electrical pulse stimulation inhibits apoptosis in C2C12 myotubes

Skeletal muscle is the major effector organ for exercise. It has been proposed that VEGFB is significantly related to apoptosis in various cell types but not yet in skeletal muscle. We hypothesize that the decrease of VEGFB in skeletal muscle participates in the occurrence of skeletal muscle apoptosis and that exercise inhibits apoptosis by elevating the expression of VEGFB in skeletal muscle cells. Based on this hypothesis, we developed in vitro experiments to mimic the effect of exercise through electrical pulse stimulation (EPS) to observe the effect of EPS on apoptosis and the change in VEGFB expression in differentiated myotubes. In addition, we employed RNA interference to explore whether VEGFB is directly involved in the regulation of myotube apoptosis during EPS. Our results showed that exogenous VEGFB₁₆₇ significantly inhibited C2C12 myotube apoptosis induced by TNF-α treatment and that endogenous VEGFB in differentiated C2C12 myotubes was significantly upregulated by EPS. In addition, EPS significantly changed the expression of the apoptotic indicators Bax and Bcl-2 at the mRNA level and downregulated the protein expression of cleaved caspase-3. The antiapoptotic effect of EPS weakened substantially as VEGFB in C2C12 myotubes was inhibited. Taken together, these results indicate that exercise-like EPS inhibits apoptosis by increasing the expression of C2C12 myotube-derived VEGFB.

Emerging Role of Epitranscriptomics in Diabetes Mellitus and Its Complications

Diabetes mellitus (DM) and its related complications are among the leading causes of disability and mortality worldwide. Substantial studies have explored epigenetic regulation that is involved in the modifications of DNA and proteins, but RNA modifications in diabetes are still poorly investigated. In recent years, posttranscriptional epigenetic modification of RNA (the so-called 'epitranscriptome') has emerged as an interesting field of research. Numerous modifications, mainly N⁶ -methyladenosine (m⁶A), have been identified in nearly all types of RNAs and have been demonstrated to have an indispensable effect in a variety of human diseases, such as cancer, obesity, and diabetes. Therefore, it is particularly important to understand the molecular basis of RNA modifications, which might provide a new perspective for the pathogenesis of diabetes mellitus and the discovery of new therapeutic targets. In this review, we aim to summarize the recent progress in the epitranscriptomics involved in diabetes and diabetes-related complications. We hope to provide some insights for enriching the understanding of the epitranscriptomic regulatory mechanisms of this disease as well as the development of novel therapeutic targets for future clinical benefit.

Insulin Signal Transduction Perturbations in Insulin Resistance

Type 2 diabetes mellitus is a widespread medical condition, characterized by high blood glucose and inadequate insulin action, which leads to insulin resistance. Insulin resistance in insulin-responsive tissues precedes the onset of pancreatic β-cell dysfunction. Multiple molecular and pathophysiological mechanisms are involved in insulin resistance. Insulin resistance is a consequence of a complex combination of metabolic disorders, lipotoxicity, glucotoxicity, and inflammation. There is ample evidence linking different mechanistic approaches as the cause of insulin resistance, but no central mechanism is yet described as an underlying reason behind this condition. This review combines and interlinks the defects in the insulin signal transduction pathway of the insulin resistance state with special emphasis on the AGE-RAGE-NF-κB axis. Here, we describe important factors that play a crucial role in the pathogenesis of insulin resistance to provide directionality for the events. The interplay of inflammation and oxidative stress that leads to β-cell decline through the IAPP-RAGE induced β-cell toxicity is also addressed. Overall, by generating a comprehensive overview of the plethora of mechanisms involved in insulin resistance, we focus on the establishment of unifying mechanisms to provide new insights for the future interventions of type 2 diabetes mellitus.

Diverse roles of phosphatidate phosphatases in insect development and metabolism

The conversion of the glycerophospholipid phosphatidic acid (PA) into diacylglycerol (DAG) is essential for the biosynthesis of membrane phospholipids and storage fats. Importantly, both PA and DAG can also serve signaling functions in the cell. The dephosphorylation of PA that yields DAG can be executed by two different classes of enzymes, Mg²⁺-dependent lipins and Mg²⁺-independent lipid phosphate phosphatases. Here, I will discuss the current status of research directed at understanding the roles of these enzymes in insect development and metabolism. Special emphasis will be given to studies in the model organism Drosophila melanogaster.

Flavonoids from Sophora alopecuroides L. improve palmitate-induced insulin resistance by inhibiting PTP1B activity in vitro

Seventeen flavonoids (1-17) were isolated from Sophora alopecuroides L.. Compounds 1 and 2 were new compounds, and compounds 5, 8, 11, 12, and 17 were isolated from S. alopecuroides for the first time. The sources of compounds 1 and 2 were determined from the seeds of S. alopecuroides by UPLC-QE-Orbitrap-MS, and compounds 1, 2, 7, 13, 14, 15, 16, and 17 were proven to improve the insulin resistance of C2C12 myotubes and significantly increase glucose consumption levels. Among them, compounds 1, 2, 13, 14, 16, and 17 could bind to protein tyrosine phosphatase 1B (PTP1B), thereby significantly inhibiting the enzyme activity of PTP1B. Compound 2 had the strongest inhibitory effect, with an inhibition rate of 95.22% at 0.1 μg mL^-1.

m6A Regulates Liver Metabolic Disorders and Hepatogenous Diabetes

N⁶-methyladenosine (m⁶A) is one of the most abundant modifications on mRNAs and plays important roles in various biological processes. The formation of m⁶A is catalyzed by a methyltransferase complex (MTC) containing a key factor methyltransferase-like 3 (Mettl3). However, the functions of Mettl3 and m⁶A modification in hepatic lipid and glucose metabolism remain unclear. Here, we showed that both Mettl3 expression and m⁶A level increased in the livers of mice with high fat diet (HFD)-induced metabolic disorders. Overexpression of Mettl3 aggravated HFD-induced liver metabolic disorders and insulin resistance. In contrast, hepatocyte-specific knockout of Mettl3 significantly alleviated HFD-induced metabolic disorders by slowing weight gain, reducing lipid accumulation, and improving insulin sensitivity. Mechanistically, Mettl3 depletion-mediated m⁶A loss caused extended RNA half-lives of metabolism-related genes, which consequently protected mice against HFD-induced metabolic syndrome. Our findings reveal a critical role of Mettl3-mediated m⁶A in HFD-induced metabolic disorders and hepatogenous diabetes.

Distinct infrastructure of lipid networks in visceral and subcutaneous adipose tissues in overweight humans

BACKGROUND

Adipose tissue plays important roles in health and disease. Given the unique association of visceral adipose tissue with obesity-related metabolic diseases, the distribution of lipids between the major fat depots located in subcutaneous and visceral regions may shed new light on adipose tissue-specific roles in systemic metabolic perturbations.

OBJECTIVE

We sought to characterize the lipid networks and unveil differences in the metabolic infrastructure of the 2 adipose tissues that may have functional and nutritional implications.

METHODS

Paired visceral and subcutaneous adipose tissue samples were obtained from 17 overweight patients undergoing elective abdominal surgery. Ultra-performance LC-MS was used to measure 18,640 adipose-derived features; 520 were putatively identified. A stem cell model for adipogenesis was used to study the functional implications of the differences found.

RESULTS

Our analyses resulted in detailed lipid metabolic maps of the 2 major adipose tissues. They point to a higher accumulation of phosphatidylcholines, triacylglycerols, and diacylglycerols, although lower ceramide concentrations, in subcutaneous tissue. The degree of unsaturation was lower in visceral adipose tissue (VAT) phospholipids, indicating lower unsaturated fatty acid incorporation into adipose tissue. The differential abundance of phosphatidylcholines we found can be attributed at least partially to higher expression of phosphatidylethanolamine methyl transferase (PEMT). PEMT-deficient embryonic stem cells showed a dramatic decrease in adipogenesis, and the resulting adipocytes exhibited lower accumulation of lipid droplets, in line with the lower concentrations of glycerolipids in VAT. Ceramides may inhibit the expression of PEMT by increased insulin resistance, thus potentially suggesting a functional pathway that integrates ceramide, PEMT, and glycerolipid biosynthetic pathways.

CONCLUSIONS

Our work unveils differential infrastructure of the lipid networks in visceral and subcutaneous adipose tissues and suggests an integrative pathway, with a discriminative flux between adipose tissues.

Ceramides and diabetes mellitus: an update on the potential molecular relationships

Recent evidence suggests that ceramides can play an important pathophysiological role in the development of diabetes. Ceramides are primarily recognized as lipid bilayer building blocks, but recent work has shown that these endogenous molecules are important intracellular signalling mediators and may exert some diabetogenic effects via molecular pathways involved in insulin resistance, β-cell apoptosis and inflammation. In the present review, we consider the available evidence on the possible roles of ceramides in diabetes mellitus and introduce eight different molecular mechanisms mediating the diabetogenic action of ceramides, categorized into those predominantly related to insulin resistance vs those mainly implicated in β-cell dysfunction. Specifically, the mechanistic evidence involves β-cell apoptosis, pancreatic inflammation, mitochondrial stress, endoplasmic reticulum stress, adipokine release, insulin receptor substrate 1 phosphorylation, oxidative stress and insulin synthesis. Collectively, the evidence suggests that therapeutic agents aimed at reducing ceramide synthesis and lowering circulating levels may be beneficial in the prevention and/or treatment of diabetes and its related complications.

Transcriptomic Responses of Skeletal Muscle to Acute Exercise in Diabetic Goto-Kakizaki Rats

Physical activity exerts positive effects on glycemic control in type 2 diabetes (T2D), which is mediated in part by extensive metabolic and molecular remodeling of skeletal muscle in response to exercise, while many regulators of skeletal muscle remain unclear. In the present study, we investigated the effects of acute exercise on skeletal muscle transcriptomic responses in the Goto-Kakizaki (GK) rats which can spontaneously develop T2D. The transcriptomes of skeletal muscle from both 8-week-old GK and Wistar rats that underwent a single exercise session (60 min running using an animal treadmill at 15 m/min) or remained sedentary were analyzed by next-generation RNA sequencing. We identified 819 differentially expressed genes in the sedentary GK rats compared with those of the sedentary Wistar rats. After a single bout of running, we found 291 and 598 genes that were differentially expressed in the exercise GK and exercise Wistar rats when compared with the corresponding sedentary rats. By integrating our data and previous studies including RNA or protein expression patterns and transgenic experiments, the downregulated expression of Fasn and upregulated expression of Tbc1d1, Hk2, Lpin1, Ppargc1a, Sorbs1, and Hmox1 might enhance glucose uptake or improve insulin sensitivity to ameliorate hyperglycemia in the exercise GK rats. Our results provide mechanistic insight into the beneficial effects of exercise on hyperglycemia and insulin action in skeletal muscle of diabetic GK rats.

Approaches for probing and evaluating mammalian sphingolipid metabolism

Sphingolipid metabolism plays a critical role in regulating processes that control cellular fate. This dynamic pathway can generate and degrade the central players: ceramide, sphingosine and sphingosine-1-phosphate in almost any membrane in the cell, adding an unexpected level of complexity in deciphering signaling events. While in vitro assays have been developed for most enzymes in SL metabolism, these assays are setup for optimal activity conditions and can fail to take into account regulatory components such as compartmentalization, substrate limitations, and binding partners that can affect cellular enzymatic activity. Therefore, many in-cell assays have been developed to derive results that are authentic to the cellular situation which may give context to alteration in SL mass. This review will discuss approaches for utilizing probes for mammalian in-cell assays to interrogate most enzymatic steps central to SL metabolism. The use of inhibitors in conjunction with these probes can verify the specificity of cellular assays as well as provide valuable insight into flux in the SL network. The use of inhibitors specific to each of the central sphingolipid enzymes are also discussed to assist researchers in further interrogation of these pathways.

Sphingolipid Metabolism: New Insight into Ceramide-Induced Lipotoxicity in Muscle Cells

Insulin-resistance is a characteristic feature of type 2 diabetes (T2D) and plays a major role in the pathogenesis of this disease. Skeletal muscles are quantitatively the biggest glucose users in response to insulin and are considered as main targets in development of insulin-resistance. It is now clear that circulating fatty acids (FA), which are highly increased in T2D, play a major role in the development of muscle insulin-resistance. In healthy individuals, excess FA are stored as lipid droplets in adipocytes. In situations like obesity and T2D, FA from lipolysis and food are in excess and eventually accumulate in peripheral tissues. High plasma concentrations of FA are generally associated with increased risk of developing diabetes. Indeed, ectopic fat accumulation is associated with insulin-resistance; this is called lipotoxicity. However, FA themselves are not involved in insulin-resistance, but rather some of their metabolic derivatives, such as ceramides. Ceramides, which are synthetized de novo from saturated FA like palmitate, have been demonstrated to play a critical role in the deterioration of insulin sensitivity in muscle cells. This review describes the latest progress involving ceramides as major players in the development of muscle insulin-resistance through the targeting of selective actors of the insulin signaling pathway.