Subnuclear localization and differentiation-dependent increased expression of DGK-zeta in C2C12 mouse myoblasts

Molecular Regulation of Porcine Skeletal Muscle Development: Insights from Research on CDC23 Expression and Function

Cell division cycle 23 (CDC23) is a component of the tetratricopeptide repeat (TPR) subunit in the anaphase-promoting complex or cyclosome (APC/C) complex, which participates in the regulation of mitosis in eukaryotes. However, the regulatory model and mechanism by which the CDC23 gene regulates muscle production in pigs are largely unknown. In this study, we investigated the expression of CDC23 in pigs, and the results indicated that CDC23 is widely expressed in various tissues and organs. In vitro cell experiments have demonstrated that CDC23 promotes the proliferation of myoblasts, as well as significantly positively regulating the differentiation of skeletal muscle satellite cells. In addition, Gene Set Enrichment Analysis (GSEA) revealed a significant downregulation of the cell cycle pathway during the differentiation process of skeletal muscle satellite cells. The protein-protein interaction (PPI) network showed a high degree of interaction between genes related to the cell cycle pathway and CDC23. Subsequently, in differentiated myocytes induced after overexpression of CDC23, the level of CDC23 exhibited a significant negative correlation with the expression of key factors in the cell cycle pathway, suggesting that CDC23 may be involved in the inhibition of the cell cycle signaling pathway in order to promote the differentiation process. In summary, we preliminarily determined the function of CDC23 with the aim of providing new insights into molecular regulation during porcine skeletal muscle development.

Negative control of diacylglycerol kinase ζ-mediated inhibition of T cell receptor signaling by nuclear sequestration in mice

Diacylglycerol kinases (DGKs) play important roles in restraining diacylglycerol (DAG)-mediated signaling. Within the DGK family, the ζ isoform appears to be the most important isoform in T cells for controlling their development and function. DGKζ has been demonstrated to regulate T cell maturation, activation, anergy, effector/memory differentiation, defense against microbial infection, and antitumor immunity. Given its critical functions, DGKζ function should be tightly regulated to ensure proper signal transduction; however, mechanisms that control DGKζ function are still poorly understood. We report here that DGKζ dynamically translocates from the cytosol into the nuclei in T cells after TCR stimulation. In mice, DGKζ mutant defective in nuclear localization displayed enhanced ability to inhibit TCR-induced DAG-mediated signaling in primary T cells, maturation of conventional αβT and iNKT cells, and activation of peripheral T cells compared with WT DGKζ. Our study reveals for the first time nuclear sequestration of DGKζ as a negative control mechanism to spatially restrain it from terminating DAG mediated signaling in T cells. Our data suggest that manipulation of DGKζ nucleus-cytosol shuttling as a novel strategy to modulate DGKζ activity and immune responses for treatment of autoimmune diseases and cancer.

Role of Diacylglycerol Kinases in Glucose and Energy Homeostasis

Diacylglycerol kinases (DGKs) catalyze a reaction that converts diacylglycerol (DAG) to phosphatidic acid (PA). DAG and PA act as intermediates of de novo lipid synthesis, cellular membrane constituents, and signaling molecules. DGK isoforms regulate a variety of intracellular processes by terminating DAG signaling and activating PA-mediated pathways. The ten DGK isoforms are unique, not only structurally, but also in tissue-specific expression profiles, subcellular localization, regulatory mechanisms, and DAG preferences, suggesting isoform-specific functions. DAG accumulation has been associated with insulin resistance; however, this concept is challenged by opposing roles of DGK isoforms in the development of type 2 diabetes and obesity despite elevated DAG levels. This review focuses on the tissue- and isoform-specific role of DGK in glucose and energy homeostasis.

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 α deficiency alters inflammation markers in adipose tissue in response to a high-fat diet

Conversion of diacylglycerol to phosphatidic acid is mediated by diacylglycerol kinases (DGKs), with DGKα specifically linked to adaptive immune responses. We determined the role of DGKα in obesity and inflammatory responses to a high-fat diet (HFD). DGKα KO and WT littermates were either a) chow-fed, b) HFD-fed for 12 weeks (Long-Term HFD), or c) HFD-fed for 3 days (Acute HFD). Body weight/composition, oxygen consumption, food intake, and glucose tolerance was unaltered between chow-fed DGKα KO and WT mice. Insulin concentration during the intraperitoneal glucose tolerance (IPGT) test was elevated in chow-fed DGKα KO mice, suggesting mild insulin resistance. Insulin concentration during the IPGT test was reduced in Long-Term HFD-fed DGKα KO mice, suggesting a mild enhancement in insulin sensitivity. Acute HFD increased hormone sensitive lipase protein abundance and altered expression of interleukin 1β mRNA, an inflammatory marker in perigonadal adipose tissue of DGKα KO mice. In conclusion, DGKα ablation is associated with mild alterations in insulin sensitivity. However, DGKα is dispensable for whole body insulin-mediated glucose uptake, hepatic glucose production, and energy homeostasis. Our results suggest DGKα aids in modulating the early immune response of adipose tissue following an acute exposure to HFD, possibly through modulation of acute T-cell action.

DGKζ deficiency protects against peripheral insulin resistance and improves energy metabolism

Diacylglycerol kinases (DGKs) regulate the balance between diacylglycerol (DAG) and phosphatidic acid. DGKζ is highly abundant in skeletal muscle and induces fiber hypertrophy. We hypothesized that DGKζ influences functional and metabolic adaptations in skeletal muscle and whole-body fuel utilization. DAG content was increased in skeletal muscle and adipose tissue, but unaltered in liver of DGKζ KO mice. Linear growth, body weight, fat mass, and lean mass were reduced in DGKζ KO versus wild-type mice. Conversely, male DGKζ KO and wild-type mice displayed a similar robust increase in plantaris weight after functional overload, suggesting that DGKζ is dispensable for muscle hypertrophy. Although glucose tolerance was similar, insulin levels were reduced in high-fat diet (HFD)-fed DGKζ KO versus wild-type mice. Submaximal insulin-stimulated glucose transport and p-Akt Ser⁴⁷³ were increased, suggesting enhanced skeletal muscle insulin sensitivity. Energy homeostasis was altered in DGKζ KO mice, as evidenced by an elevated respiratory exchange ratio, independent of altered physical activity or food intake. In conclusion, DGKζ deficiency increases tissue DAG content and leads to modest growth retardation, reduced adiposity, and protection against insulin resistance. DGKζ plays a role in the control of growth and metabolic processes, further highlighting specialized functions of DGK isoforms in type 2 diabetes pathophysiology.

Decoding Gαq signaling

Gαq signals with phospholipase C-β (PLC-β) to modify behavior in response to an agonist-bound GPCR. While the fundamental steps which prime Gαq to interact with PLC-β have been identified, questions remain concerning signal strength with PLC-β and other effectors. Gαq is generally viewed to function as a simple ON and OFF switch for its effector, dependent on the binding of GTP or GDP. However, Gαq does not have a single effector, Gαq has many different effectors. Furthermore, select effectors also regulate Gαq activity. PLC-β is a lipase and a GTPase activating protein (GAP) selective for Gαq. The contribution of G protein regulating activity to signal amplitude remains unclear. The unique PLC-β coiled-coil domain is essential for maximum Gαq response, both lipase and GAP. Nonetheless, coiled-coil domain associations necessary to maximum response have not been revealed by the structural approach. This review discusses progress towards understanding the basis for signal strength with PLC-β and other effectors. Shared and effector-specific interactions have been identified. Finally, the evidence for allosteric regulation of lipase stimulation by protein kinase C, the membrane, phosphatidic acid, phosphatidylinositol-4, 5-bisphosphate and GPCR is explored. Endogenous allosteric regulators can suppress or enhance maximum lipase stimulation dependent on the PLC-β coiled-coil domain. A better understanding of allosteric modulation may therefore identify a wealth of new targets to regulate signal strength and behavior.

Opposing calcium-dependent signalling pathways control skeletal muscle differentiation by regulating a chromatin remodelling enzyme

Calcium signaling is important for differentiation-dependent gene expression, but is also involved in other cellular functions. Therefore mechanisms must exist to distinguish calcium signaling relevant to differentiation. Calcineurin is a calcium-regulated phosphatase that is required for myogenic gene expression and skeletal muscle differentiation. Here, we demonstrate that inhibition of calcineurin blocks chromatin remodeling and that the Brg1 ATPase of the SWI/SNF chromatin remodeling enzyme, which is required for the activation of myogenic gene expression, is a calcineurin substrate. Furthermore, we identify the calcium-regulated classical protein kinase C beta (PKCβ) as a repressor of myogenesis and as the enzyme that opposes calcineurin function. Replacement of endogenous Brg1 with a phosphomimetic mutant in primary myoblasts inhibits myogenesis, while replacement with a non-phosphorylatable mutant allows myogenesis despite inhibition of calcineurin signaling, demonstrating the functionality of calcineurin/PKC modified residues. Thus the Brg1 chromatin remodeling enzyme integrates two antagonistic calcium-dependent signaling pathways that control myogenic differentiation.

PIP4K and the role of nuclear phosphoinositides in tumour suppression

Phosphatidylinositol-5-phosphate (PtdIns5P)-4-kinases (PIP4Ks) are stress-regulated lipid kinases that phosphorylate PtdIns5P to generate PtdIns(4,5)P₂. There are three isoforms of PIP4Ks: PIP4K2A, 2B and 2C, which localise to different subcellular compartments with the PIP4K2B isoform being localised predominantly in the nucleus. Suppression of PIP4K expression selectively prevents tumour cell growth in vitro and prevents tumour development in mice that have lost the tumour suppressor p53. p53 is lost or mutated in over 70% of all human tumours. These studies suggest that inhibition of PIP4K signalling constitutes a novel anti-cancer therapeutic target. In this review we will discuss the role of PIP4K in tumour suppression and speculate on how PIP4K modulates nuclear phosphoinositides (PPIns) and how this might impact on nuclear functions to regulate cell growth. This article is part of a Special Issue entitled Phosphoinositides.

The role of diacylglycerol kinase ζ and phosphatidic acid in the mechanical activation of mammalian target of rapamycin (mTOR) signaling and skeletal muscle hypertrophy

The activation of mTOR signaling is essential for mechanically induced changes in skeletal muscle mass, and previous studies have suggested that mechanical stimuli activate mTOR (mammalian target of rapamycin) signaling through a phospholipase D (PLD)-dependent increase in the concentration of phosphatidic acid (PA). Consistent with this conclusion, we obtained evidence which further suggests that mechanical stimuli utilize PA as a direct upstream activator of mTOR signaling. Unexpectedly though, we found that the activation of PLD is not necessary for the mechanically induced increases in PA or mTOR signaling. Motivated by this observation, we performed experiments that were aimed at identifying the enzyme(s) that promotes the increase in PA. These experiments revealed that mechanical stimulation increases the concentration of diacylglycerol (DAG) and the activity of DAG kinases (DGKs) in membranous structures. Furthermore, using knock-out mice, we determined that the ζ isoform of DGK (DGKζ) is necessary for the mechanically induced increase in PA. We also determined that DGKζ significantly contributes to the mechanical activation of mTOR signaling, and this is likely driven by an enhanced binding of PA to mTOR. Last, we found that the overexpression of DGKζ is sufficient to induce muscle fiber hypertrophy through an mTOR-dependent mechanism, and this event requires DGKζ kinase activity (i.e. the synthesis of PA). Combined, these results indicate that DGKζ, but not PLD, plays an important role in mechanically induced increases in PA and mTOR signaling. Furthermore, this study suggests that DGKζ could be a fundamental component of the mechanism(s) through which mechanical stimuli regulate skeletal muscle mass.

Nuclear inositide specific phospholipase C signalling - interactions and activity

Evidence accumulated over the past 20 years has highlighted the presence of an autonomous nuclear inositol lipid metabolism, and suggests that lipid signalling molecules are important components of signalling pathways operating within the nucleus. Nuclear polyphosphoinositide (PI) signalling relies on the synthesis and metabolism of phosphatidylinositol 4,5-bisphosphate, which can modulate the activity of effector proteins and is a substrate of signalling enzymes. The regulation of the nuclear PI pool is totally independent from the plasma membrane counterpart, suggesting that the nucleus constitutes a functionally distinct compartment of inositol lipids metabolism. Among the nuclear enzymes involved in PI metabolism, inositide specific phospholipase C (PI-PLC) has been one of the most extensively studied. Several isoforms of PI-PLCs have been identified in the nucleus, namely PI-PLC-β1, γ1, δ1 and ζ; however, the β1 isozyme is the best characterized. In the present review, we focus on the signal transduction-related metabolism of nuclear PI-PLC and review the most convincing evidence for PI-PLC expression and activity being involved in differentiation and proliferation programmes in several cell systems. Moreover, nuclear PI-PLC is an intermediate effector and interactor for nuclear inositide signalling. The inositide cycle exists and shows a biological role inside the nucleus. It is an autonomous lipid-dependent signalling system, independently regulated with respect to the one at the plasma membrane counterpart, and is involved in cell cycle progression and differentiation.

Cluster analysis reveals differential transcript profiles associated with resistance training-induced human skeletal muscle hypertrophy

Using genomic microarray analysis, we sought to identify and annotate differences in the pretraining skeletal muscle transcriptomes among human subjects clustered as nonresponders (Non), modest responders (Mod), and extreme responders (Xtr) based on differential magnitudes of myofiber hypertrophy in response to progressive resistance training (RT) (Non-6 μm², Mod 1,111 μm², or Xtr 2,475 μm²). In prior work, we noted differences among clusters in the prevalence of myogenic stem cells prior to and during RT (35), and in the translational signaling responses to the first bout of resistance exercise (30). Here we identified remarkable differences in the pretraining transcript profiles among clusters (8,026 gene transcripts differentially expressed between Xtr and Non, 2,463 between Xtr and Mod, and 1,294 between Mod and Non). Annotated functions and networks of differentially expressed genes suggest Xtr were "primed" to respond to RT through transcriptional regulation, along with a uniquely expressed network of genes involved in skeletal muscle development, while the failed response in Non may have been driven by excessive proinflammatory signaling. Protein follow-up analysis revealed higher basal levels of acetylated histone H3 (K36) in the two responder clusters (Mod, Xtr) compared with Non, and only the responders experienced alterations in the muscle content of select proteins (e.g., α-tubulin, p27(kip)) in response to the first resistance exercise stimulus. Overall, the widely disparate transcriptomes identified prior to RT among the three clusters support the notion that at least some of the interindividual heterogeneity in propensity for RT-induced myofiber hypertrophy is likely predetermined.

cAMP-stimulated transcription of DGKθ requires steroidogenic factor 1 and sterol regulatory element binding protein 1

Diacylglycerol kinase (DGK)θ is a lipid kinase that phosphorylates diacylglycerol to form phosphatidic acid (PA). We have previously shown that PA is a ligand for the nuclear receptor steroidogenic factor 1 (SF1) and that cAMP-stimulated expression of SF1 target genes requires DGKθ. In this study, we sought to investigate the role of cAMP signaling in regulating DGKθ gene expression. Real time RT-PCR and Western blot analysis revealed that dibutyryl cAMP (Bt2cAMP) increased the mRNA and protein expression, respectively, of DGKθ in H295R human adrenocortical cells. SF1 and sterol regulatory element binding protein 1 (SREBP1) increased the transcriptional activity of a reporter plasmid containing 1.5 kb of the DGKθ promoter fused to the luciferase gene. Mutation of putative cAMP responsive sequences abolished SF1- and SREBP-dependent DGKθ reporter gene activation. Consistent with this finding, chromatin immunoprecipitation assay demonstrated that Bt2cAMP signaling increased the recruitment of SF1 and SREBP1 to the DGKθ promoter. Coimmunoprecipitation assay revealed that SF1 and SREBP1 interact, suggesting that the two transcription factors form a complex on the DGKθ promoter. Finally, silencing SF1 and SREBP1 abolished cAMP-stimulated DGKθ expression. Taken together, we demonstrate that SF1 and SREBP1 activate DGKθ transcription in a cAMP-dependent manner in human adrenocortical cells.

Nuclear phosphoinositides: location, regulation and function

Lipid signalling in human disease is an important field of investigation and stems from the fact that phosphoinositide signalling has been implicated in the control of nearly all the important cellular pathways including metabolism, cell cycle control, membrane trafficking, apoptosis and neuronal conduction. A distinct nuclear inositide signalling metabolism has been identified, thus defining a new role for inositides in the nucleus, which are now considered essential co-factors for several nuclear processes, including DNA repair, transcription regulation, and RNA dynamics. Deregulation of phoshoinositide metabolism within the nuclear compartment may contribute to disease progression in several disorders, such as chronic inflammation, cancer, metabolic, and degenerative syndromes. In order to utilize these very druggable pathways for human benefit there is a need to identify how nuclear inositides are regulated specifically within this compartment and what downstream nuclear effectors process and integrate inositide signalling cascades in order to specifically control nuclear function. Here we describe some of the facets of nuclear inositide metabolism with a focus on their relationship to cell cycle control and differentiation.

Physiology and pathology of nuclear phospholipase C β1

The existence and function of inositide signaling in the nucleus is well documented and we know that the existence of the inositide cycle inside the nucleus has a biological role. An autonomous lipid-dependent signaling system, independently regulated from its plasma membrane counterpart, acts in the nucleus and modulates cell cycle progression and differentiation.We and others focused on PLCβ1, which is the most extensively investigated PLC isoform in the nuclear compartment. PLCβ1 is a key player in the regulation of nuclear inositol lipid signaling, and, as discussed above, its function could also be involved in nuclear structure because it hydrolyses PtdIns(4,5)P2, a well accepted regulator of chromatin remodelling. The evidence, in a number of patients with myelodysplastic syndromes, that the mono-allelic deletion of PLCβ1 is associated with an increased risk of developing acute myeloid leukemia paves the way for an entirely new field of investigation. Indeed the genetic defect evidenced, in addition to being a useful prognostic tool, also suggests that altered expression of this enzyme could have a role in the pathogenesis of this disease, by causing an imbalance between proliferation and apoptosis. The epigenetics of PLCβ1 expression in MDS has been reviewed as well.

Diacylglycerol kinase zeta is associated with chromatin, but dissociates from condensed chromatin during mitotic phase in NIH3T3 cells

Diacylglycerol kinase (DGK) converts diacylglycerol (DG) to phosphatidic acid, both of which act as second messengers to mediate a variety of cellular mechanisms. Therefore, DGK contributes to the regulation of these messengers in cellular signal transduction. Of DGK isozymes cloned, DGKzeta is characterized by a nuclear localization signal that overlaps with a sequence similar to the myristoylated alanine-rich C-kinase substrate. Previous studies showed that nuclear DG is differentially regulated from plasma membrane DG and that the nuclear DG levels fluctuate in correlation with cell cycle progression, suggesting the importance of nuclear DG in cell cycle control. In this connection, DGKzeta has been shown to localize to the nucleus in fully differentiated cells, such as neurons and lung cells, although it remains elusive how DGK behaves during the cell cycle in proliferating cells. Here we demonstrate that DGKzeta localizes to the nucleus during interphase including G1, S, and G2 phases and is associated with chromatin although it dissociates from condensed chromatin during mitotic phase in NIH3T3 cells. Furthermore, this localization pattern is also observed in proliferating spermatogonia in the testis. These results suggest a reversible association of DGKzeta with histone or its related proteins in cell cycle, plausibly dependent on their post-translational modifications.

Cyclic AMP-stimulated interaction between steroidogenic factor 1 and diacylglycerol kinase theta facilitates induction of CYP17

In the human adrenal cortex, adrenocorticotropin (ACTH) activates CYP17 transcription by promoting the binding of the nuclear receptor steroidogenic factor 1 (SF1) (Ad4BP, NR5A1) to the promoter. We recently found that sphingosine is an antagonist for SF1 and inhibits cyclic AMP (cAMP)-dependent CYP17 gene transcription. The aim of the current study was to identify phospholipids that bind to SF1 and to characterize the mechanism by which ACTH/cAMP regulates the biosynthesis of this molecule(s). Using tandem mass spectrometry, we show that in H295R human adrenocortical cells, SF1 is bound to phosphatidic acid (PA). Activation of the ACTH/cAMP signal transduction cascade rapidly increases nuclear diacylglycerol kinase (DGK) activity and PA production. PA stimulates SF1-dependent transcription of CYP17 reporter plasmids, promotes coactivator recruitment, and induces the mRNA expression of CYP17 and several other steroidogenic genes. Inhibition of DGK activity attenuates the binding of SF1 to the CYP17 promoter, and silencing of DGK-theta expression inhibits cAMP-dependent CYP17 transcription. LXXLL motifs in DGK-theta mediate a direct interaction of SF1 with the kinase and may facilitate binding of PA to the receptor. We conclude that ACTH/cAMP stimulates PA production in the nucleus of H295R cells and that this increase in PA concentrations facilitates CYP17 induction.

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

Diacylglycerol kinase (DGK) is one of lipid-regulating enzymes, catalyzes phosphorylation of diacylglycerol to phosphatidic acid. Because skeletal muscle, a major insulin-target organ for glucose disposal, expresses DGK, we investigated in the present study a role of DGK on glucose transport in skeletal muscle cells. PCR study showed that C2C12 myotubes expressed DGKalpha, delta, epsilon, zeta, or theta isoform mRNA. R59022, a specific inhibitor of DGK, significantly increased glucose transport, p38 and MKK3/6 activation in C2C12 myotubes. The R59022-induced glucose transport was blocked by SB203580, a specific p38 inhibitor. In contrast, R59022 failed to stimulate both possible known mechanisms to enhance glucose transport, an IRS1-PI3K-Akt pathway, muscle contraction signaling or GLUT1 and 4 expression. All these results suggest that DGK may play a role in glucose transport in the skeletal muscle cells through modulating a MKK3/6-p38 signaling pathway.

Nuclear diacylglycerol kinase‐ζ is a negative regulator of cell cycle progression in C2C12 mouse myoblasts

The nucleus contains diacylglycerol kinases (DGKs), i.e., the enzymes that, by converting diacylglycerol (DG) into phosphatidic acid, terminate DG-dependent events. It has been demonstrated that nuclear DGK-zeta interferes with cell cycle progression. We previously reported that nuclear DGK-zeta expression increased during myogenic differentiation, whereas its down-regulation impaired differentiation. Here, we evaluated the possible involvement of nuclear DGK-zeta in cell cycle progression of C2C12 myoblasts. Overexpression of a wild-type DGK-zeta, which mainly localized to the nucleus (but not of a kinase dead mutant or of a mutant that did not enter the nucleus), blocked the cells in the G1 phase of the cell cycle, as demonstrated by in situ analysis of biotinylated-16-dUTP incorporated into newly synthesized DNA and by flow cytometry. In contrast, down-regulation of endogenous DGK-zeta by short interfering RNA (siRNA) increased the number of cells in both the S and G2/M phases of the cell cycle. Cell cycle arrest of cells overexpressing wild-type DGK-zeta was accompanied by decreased levels of retinoblastoma protein phosphorylated on Ser-807/811. Down-regulation of endogenous DGK-zeta, using siRNA, prevented the cell cycle block characterizing C2C12 cell myogenic differentiation. Overall, our results identify nuclear DGK-zeta as a key determinant of cell cycle progression and differentiation of C2C12 cells.