A diacylglycerol kinase inhibitor, R59022, stimulates glucose transport through a MKK3/6-p38 signaling pathway in skeletal muscle cells

Dreh, a long noncoding RNA repressed by metformin, regulates glucose transport in C2C12 skeletal muscle cells

AIMS

The anti-hyperglycemic action of metformin on skeletal muscles is presently unclear. Long noncoding RNAs (lncRNAs) are implicated in multiple cellular functions. This study aims to explore the role of lncRNAs in the glucometabolic action of metformin on skeletal muscle cells.

MAIN METHODS

Metformin accumulation was assessed using [¹⁴C]-metformin. A lncRNA array was used to investigate metformin-regulated lncRNAs in C2C12 skeletal muscle cells. Knockdown studies were applied to evaluate the function of lncRNA Dreh. A colorimetric assay was used for the measurement of medium glucose concentration; glucose transport was assessed using [³H]-2-deoxyglucose; real-time PCR was used for RNA expression analysis, and western blotting was used to assess protein expression in myotubes. A Dreh overexpression plasmid was transfected into the cells.

KEY FINDINGS

Metformin accumulated in C2C12 myotubes. Metformin reduced medium glucose concentration and repressed lncRNA Dreh expression in the myotubes. Knockdown of Dreh in the myotubes resulted in reduced glucose concentration in the culture medium, increased glucose transport, and increased levels of GLUT4 protein in the plasma membrane. Overexpression of Dreh attenuated the glucose-lowering effect of metformin in myotubes.

SIGNIFICANCE

The glucoregulatory actions of metformin are mediated in part by a lncRNA, Dreh, in the skeletal muscle cells. Dreh is a novel regulator for glucose transport and could be a therapeutic target for diabetes.

A new hypothesis for Parkinson's disease pathogenesis: GTPase-p38 MAPK signaling and autophagy as convergence points of etiology and genomics

The combination of genetics and genomics in Parkinson´s disease has recently begun to unveil molecular mechanisms possibly underlying disease onset and progression. In particular, catabolic processes such as autophagy have been increasingly gaining relevance as post-mortem evidence and experimental models suggested a participation in neurodegeneration and alpha-synuclein Lewy body pathology. In addition, familial Parkinson´s disease linked to LRRK2 and alpha-synuclein provided stronger correlation between etiology and alterations in autophagy. More detailed cellular pathways are proposed and genetic risk factors that associate with idiopathic Parkinson´s disease provide further clues in dissecting contributions of single players. Nevertheless, the fine-tuning of these processes remains elusive, as the initial stages of the pathways are not yet clarified.In this review, we collect literature evidence pointing to autophagy as the common, downstream target of Parkinsonian dysfunctions and augment current knowledge on the factors that direct the subsequent steps. Cell and molecular biology evidence indicate that p38 signaling underlies neurodegeneration and autoptic observations suggest a participation in neuropathology. Moreover, alpha-synuclein and LRRK2 also appear involved in the p38 pathway with additional roles in the regulation of GTPase signaling. Small GTPases are critical modulators of p38 activation and thus, their functional interaction with aSyn and LRRK2 could explain much of the detailed mechanics of autophagy in Parkinson´s disease.We propose a novel hypothesis for a more comprehensive working model where autophagy is controlled by upstream pathways, such as GTPase-p38, that have been so far underexplored in this context. In addition, etiological factors (LRRK2, alpha-synuclein) and risk loci might also combine in this common mechanism, providing a powerful experimental setting to dissect the cause of both familial and idiopathic disease.

Diacylglycerol kinase δ controls down-regulation of cyclin D1 for C2C12 myogenic differentiation

Diacylglycerol kinase (DGK) is a lipid-metabolizing enzyme that phosphorylates diacylglycerol (DG) to produce phosphatidic acid (PA). DGKδ is highly expressed in the skeletal muscle, and a decrease in DGKδ expression increases the severity of type 2 diabetes. However, the role of DGKδ in myogenic differentiation is still unknown. The present study demonstrated that DGKδ expression was down-regulated in the early stage of C2C12 myogenic differentiation almost concurrently with a decrease in cyclin D1 expression. The knockdown of DGKδ by DGKδ-specific siRNAs significantly increased the levels of cyclin D1 expression at 48 h after C2C12 myogenic differentiation. In contrast, at the same time, the knockdown of DGKδ decreased the levels of myogenin expression and the number of myosin heavy chain (MHC)-positive cells. These results indicate that DGKδ regulates the early differentiation of C2C12 myoblasts via controlling the down-regulation of cyclin D1 expression. Moreover, the suppression of DGKδ expression increased the phosphorylation levels of conventional and novel protein kinase Cs (cnPKCs). Furthermore, DGKδ suppression increased the levels of cyclin D1 and phospho-cnPKCs even at the first 24 h of myogenic differentiation. These results suggest that DGKδ controls the down-regulation of cyclin D1 expression by attenuating the PKC signaling pathway for C2C12 myogenic differentiation.

Diacylglycerol kinase ζ generates dipalmitoyl-phosphatidic acid species during neuroblastoma cell differentiation

Phosphatidic acid (PA) is one of the phospholipids composing the plasma membrane and acts as a second messenger to regulate a wide variety of important cellular events, including mitogenesis, migration and differentiation. PA consists of various molecular species with different acyl chains at the sn-1 and sn-2 positions. However, it has been poorly understood what PA molecular species are produced during such cellular events. Here we identified the PA molecular species generated during retinoic acid (RA)-induced neuroblastoma cell differentiation using a newly established liquid chromatography/mass spectrometry (LC/MS) method. Intriguingly, the amount of 32:0-PA species was dramatically and transiently increased in Neuro-2a neuroblastoma cells 24-48 h after RA-treatment. In addition, 30:0- and 34:0-PA species were also moderately increased. Moreover, similar results were obtained when Neuro-2a cells were differentiated for 24 h by serum starvation. MS/MS analysis revealed that 32:0-PA species contains two palmitic acids (16:0 s). RT-PCR analysis showed that diacylglycerol kinase (DGK) δ and DGKζ were highly expressed in Neuro-2a cells. The silencing of DGKζ expression significantly decreased the production of 32:0-PA species, whereas DGKδ-siRNA did not. Moreover, neurite outgrowth was also markedly attenuated by the deficiency of DGKζ. Taken together, these results indicate that DGKζ exclusively generates very restricted PA species, 16:0/16:0-PA, and up-regulates neurite outgrowth during the initial/early stage of neuroblastoma cell differentiation.

Regulation of diacylglycerol kinase δ2 expression in C2C12 skeletal muscle cells by free fatty acids

Decreased expression of diacylglycerol kinase (DGK) δ in skeletal muscles is closely related to the pathogenesis of type 2 diabetes. However, the regulation of DGKδ expression is not well understood. In this study, we found that myristic acid (14:0) significantly increased DGKδ2 protein expression in a dose-dependent manner (EC(50) = 0.16 mM) in mouse C2C12 myotubes. In contrast, oleic [18:1(n-9)], eicosenoic [20:1(n-9)] and erucic [22:1(n-9)] acids markedly decreased DGKδ2 expression. Myristic acid slowly enhanced DGKδ2 expression at the transcription level. Therefore, DGKδ2 expression is positively regulated by the relatively short-chain saturated fatty acid myristic acid but attenuated by n-9 monounsaturated fatty acids.

Suppression of lipin-1 expression increases monocyte chemoattractant protein-1 expression in 3T3-L1 adipocytes

Lipin-1 plays a crucial role in the regulation of lipid metabolism and cell differentiation in adipocytes. Expression of adipose lipin-1 is reduced in obesity, and metabolic syndrome. However, the significance of this reduction remains unclear. This study investigated if and how reduced lipin-1 expression affected metabolism. We assessed mRNA expression levels of various genes related to adipocyte metabolism in lipin-1-depleted 3T3-L1 adipocytes by introducing its specific small interfering RNA. In lipin-1-depleted adipocytes, mRNA and protein expression levels of monocyte chemoattractant protein-1 (MCP-1) were significantly increased, although the other genes tested were not altered. The conditioned media from the cells promoted monocyte chemotaxis. The increase in MCP-1 expression was prevented by treatment with quinazoline or salicylate, inhibitors of nuclear factor-κB activation. Because MCP-1 is related to adipose inflammation and systemic insulin resistance, these results suggest that a reduction in adipose lipin-1 in obesity may exacerbate adipose inflammation and metabolism.

Mechanotransduction and the regulation of mTORC1 signaling in skeletal muscle

Mechanical stimuli play a major role in the regulation of skeletal muscle mass, and the maintenance of muscle mass contributes significantly to disease prevention and issues associated with the quality of life. Although the link between mechanical signals and the regulation of muscle mass has been recognized for decades, the mechanisms involved in converting mechanical information into the molecular events that control this process remain poorly defined. Nevertheless, our knowledge of these mechanisms is advancing and recent studies have revealed that signaling through a protein kinase called the mammalian target of rapamycin (mTOR) plays a central role in this event. In this review we will, (1) discuss the evidence which implicates mTOR in the mechanical regulation of skeletal muscle mass, (2) provide an overview of the mechanisms through which signaling by mTOR can be regulated, and (3) summarize our current knowledge of the potential mechanisms involved in the mechanical activation of mTOR signaling.

Antidiabetic Activities of Abutilon indicum (L.) Sweet Are Mediated by Enhancement of Adipocyte Differentiation and Activation of the GLUT1 Promoter

Abutilon indicum (L.) Sweet is an Asian phytomedicine traditionally used to treat several disorders, including diabetes mellitus. However, molecular mechanisms supporting the antidiabetic effect of A. indicum L. remain unknown. The aim of this study was to evaluate whether extract of A. indicum L. improves insulin sensitivity. First, we observed the antidiabetic activity of aqueous extract of the entire plant (leaves, twigs and roots) of A. indicum L. on postprandial plasma glucose in diabetic rats. The subsequent experiments revealed that butanol fractions of the extract bind to PPARγ and activate 3T3-L1 differentiation. To measure glucose uptake enhanced by insulin-like activity, we used rat diaphragm incubated with various concentrations of the crude extract and found that the extract enhances glucose consumption in the incubated solution. Our data also indicate that the crude extract and the fractions (water and butanol) did not affect the activity of kinases involved in Akt and GSK-3β pathways; however, the reporter assay showed that the crude extract could activate glucose transporter 1 (GLUT1) promoter activity. These results suggest that the extract from A. indicum L. may be beneficial for reducing insulin resistance through its potency in regulating adipocyte differentiation through PPARγ agonist activity, and increasing glucose utilization via GLUT1.

A Jak2 inhibitor, AG490, reverses lipin-1 suppression by TNF-alpha in 3T3-L1 adipocytes

Lipin-1 is a multifunctional metabolic regulator, involving in triacylglycerol and bioactive glycerolipids synthesis as an enzyme, transcriptional regulation as a coactivator, and adipogenesis. In obesity, adipose lipin-1 expression is decreased. Although lipin-1 is implicated in the pathogenesis of obesity, the mechanism is still not clear. Since TNF-alpha is deeply involved in the pathogenesis of obesity, insulin resistance, and diabetes, here we investigated the role of TNF-alpha on lipin-1 expression in adipocytes. Quantitative PCR studies showed that TNF-alpha suppressed both lipin-1A and -1B isoform expression in time- and dose-dependent manners in mature 3T3-L1 adpocytes. A Jak2 inhibitor, AG490, reversed the suppressive effect of TNF-alpha on both lipin-1A and -1B. In contrast, NF-kappaB, MAPKs, ceramide, and beta-catenin pathway tested were not involved in the mechanism. These results suggest that TNF-alpha could be involved in obesity-induced lipin-1 suppression in adipocytes and Jak2 may play an important role in the mechanism.

Glucose regulates diacylglycerol intracellular levels and protein kinase C activity by modulating diacylglycerol kinase subcellular localization

Although chronic hyperglycemia reduces insulin sensitivity and leads to impaired glucose utilization, short term exposure to high glucose causes cellular responses positively regulating its own metabolism. We show that exposure of L6 myotubes overexpressing human insulin receptors to 25 mm glucose for 5 min decreased the intracellular levels of diacylglycerol (DAG). This was paralleled by transient activation of diacylglycerol kinase (DGK) and of insulin receptor signaling. Following 30-min exposure, however, both DAG levels and DGK activity returned close to basal levels. Moreover, the acute effect of glucose on DAG removal was inhibited by >85% by the DGK inhibitor R59949. DGK inhibition was also accompanied by increased protein kinase C-alpha (PKCalpha) activity, reduced glucose-induced insulin receptor activation, and GLUT4 translocation. Glucose exposure transiently redistributed DGK isoforms alpha and delta, from the prevalent cytosolic localization to the plasma membrane fraction. However, antisense silencing of DGKdelta, but not of DGKalpha expression, was sufficient to prevent the effect of high glucose on PKCalpha activity, insulin receptor signaling, and glucose uptake. Thus, the short term exposure of skeletal muscle cells to glucose causes a rapid induction of DGK, followed by a reduction of PKCalpha activity and transactivation of the insulin receptor signaling. The latter may mediate, at least in part, glucose induction of its own metabolism.