Mice lacking DGKε show increased beige adipogenesis in visceral white adipose tissue after long-term high fat diet in a COX-2- dependent manner

Ablation of diacylglycerol kinase ε promotes whitening of brown adipose tissue under high fat diet feeding

Adipose tissue (AT) comprises distinct fat depots such as white AT and brown AT. White and brown adipocytes exhibit different morphological and physiological properties. White adipocytes containing large single lipid droplet (LD) provide energy on demand whereas brown adipocytes loaded with multilocular LDs consume energy to generate heat or dissipate excess energy. Recent studies have shown that multilocular brown-like cells emerge in white AT under certain conditions. These cells termed beige adipocytes participate in energy expenditure and heat generation. In the process of lipolysis, TG is broken down into free fatty acid and diacylglycerol (DG). In this regard, DG also serves as a signaling molecule activating some proteins such as protein kinase C. Therefore, DG kinase (DGK), an enzyme which phosphorylates DG into phosphatidic acid (PA), plays a pivotal role in integrating energy homeostasis and intracellular signaling. Recently, we described that DGKε-KO mice exhibit increased adiposity in visceral white AT accompanied with impaired glucose tolerance early (40 days) in the course of high fat diet (HFD) feeding, although these mice exhibit "browning or beiging" in visceral white AT associated with improved glucose tolerance after longer term HFD feeding (180 days). This study was conducted to understand the overall features of adipose tissues and investigate changes in subcutaneous (inguinal) white AT and interscapular brown AT of DGKε-KO mice during the course of HFD feeding. Results demonstrated that fat accumulation is promoted in all fat depots under 40 days of HFD feeding conditions. Remarkably, "whitening" of brown adipocytes was identified in DGKε-deficient brown AT during the course of HFD feeding, suggesting brown adipocyte dysfunction. In addition, insulin levels were considerably elevated in DGKε-KO mice under 180 days of HFD feeding conditions. Collectively, these findings suggest that brown adipocytes are dysfunctional in DGKε-KO mice, which promotes browning or beiging in visceral white AT. Beige adipocytes may take over energy disposal and contribute to improving glucose tolerance with the aid of high levels of insulin in DGKε-KO mice upon excess feeding.

Multi-Omics Analysis Reveals Sphingomyelin Accumulation, Glycerolipids Loss, and Disorders of Lipid Metabolism Regulated by Leucine Deprivation in the Liver of Mice

SCOPE

Branched-chain amino acids, especially leucine, have been reported to play a role in regulating lipid metabolism. This study aims to examine the effects of leucine deprivation on hepatic lipid metabolism.

METHODS AND RESULTS

C57BL/6 mice are fed with a chow diet (control group, n = 8) or a leucine-free diet (-Leu group, n = 8) for 7 days. Histology, lipidomics, targeted metabolomics, and transcriptomics are performed to analyze the liver tissue. Compared to control group, -Leu group exhibits a notably reduced liver weight, accompanied by hepatic injury, and disorders of lipid metabolism. The level of sphingomyelin (SM) is significantly increased in the liver of -Leu group, while the glycerolipids (GL) level is significantly decreased. The expression of sphingomyelin synthase 1 (SGMS1) is upregulated by leucine deprivation in a time-dependent manner, leading to hepatic SM accumulation. Moreover, leucine deprivation results in hepatic GL loss via suppressing fatty acid synthase (FASN) and acetyl-CoA carboxylase 1 (ACC1) expression.

CONCLUSION

The findings demonstrate that leucine deprivation results in abnormal lipid metabolism in the liver, mainly manifested as SM accumulation and GL loss. These results provide insights into the role of leucine in regulating lipid metabolism.

Crosstalk between diacylglycerol kinase and protein kinase A in the regulation of airway smooth muscle cell proliferation

BACKGROUND

Diacylglycerol kinase (DGK) regulates intracellular signaling and functions by converting diacylglycerol (DAG) into phosphatidic acid. We previously demonstrated that DGK inhibition attenuates airway smooth muscle (ASM) cell proliferation, however, the mechanisms mediating this effect are not well established. Given the capacity of protein kinase A (PKA) to effect inhibition of ASM cells growth in response to mitogens, we employed multiple molecular and pharmacological approaches to examine the putative role of PKA in the inhibition of mitogen-induced ASM cell proliferation by the small molecular DGK inhibitor I (DGK I).

METHODS

We assayed cell proliferation using CyQUANT™ NF assay, protein expression and phosphorylation using immunoblotting, and prostaglandin E2 (PGE2) secretion by ELISA. ASM cells stably expressing GFP or PKI-GFP (PKA inhibitory peptide-GFP chimera) were stimulated with platelet-derived growth factor (PDGF), or PDGF + DGK I, and cell proliferation was assessed.

RESULTS

DGK inhibition reduced ASM cell proliferation in cells expressing GFP, but not in cells expressing PKI-GFP. DGK inhibition increased cyclooxygenase II (COXII) expression and PGE2 secretion over time to promote PKA activation as demonstrated by increased phosphorylation of (PKA substrates) VASP and CREB. COXII expression and PKA activation were significantly decreased in cells pre-treated with pan-PKC (Bis I), MEK (U0126), or ERK2 (Vx11e) inhibitors suggesting a role for PKC and ERK in the COXII-PGE2-mediated activation of PKA signaling by DGK inhibition.

CONCLUSIONS

Our study provides insight into the molecular pathway (DAG-PKC/ERK-COXII-PGE2-PKA) regulated by DGK in ASM cells and identifies DGK as a potential therapeutic target for mitigating ASM cell proliferation that contributes to airway remodeling in asthma.

Diacylglycerol Kinase ε in Adipose Tissues: A Crosstalk Between Signal Transduction and Energy Metabolism

Diacylglycerol (DG) is unique in lipid metabolism because it serves not only as an intermediate product for triglyceride synthesis, but also as a signaling molecule that activates proteins containing DG-responsive elements, such as protein kinase C. Consequently, DG acts as a hub between energy metabolism and intracellular signaling. Of DG metabolizing pathways, DG kinase (DGK) phosphorylates DG to produce phosphatidic acid, which also serves as a second messenger. Several lines of evidence suggest that DGK is deeply involved in metabolic diseases such as obesity and insulin resistance. Of DGK isozymes, DGKε is simplest in terms of structure, but it is characterized by substrate specificity toward arachidonoyl-DG. Recently, we have reported that DGKε deficiency promotes adipose tissue remodeling in mice during the course of high fat diet (HFD) feeding regimen including obesity, insulin resistance, and beige adipogenesis. DGKε ablation engenders altered expression of other lipid metabolizing enzymes, including adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL), and diacylglycerol acyltransferase (DGAT). Subcellular localization of DGKε in the endoplasmic reticulum suggests involvement of this isozyme in lipid energy homeostasis. This review presents current findings of DGKε in lipid-orchestrated pathophysiology, especially unique phenotypes of DGKε-knockout mice in the early and late stages of obesogenic conditions.

Autocrine regulation of airway smooth muscle contraction by diacylglycerol kinase

Diacylglycerol kinase (DGK), a lipid kinase, catalyzes the conversion of diacylglycerol (DAG) to phosphatidic acid, thereby terminating DAG-mediated signaling by Gq-coupled receptors that regulate contraction of airway smooth muscle (ASM). A previous study from our laboratory demonstrated that DGK inhibition or genetic ablation leads to reduced ASM contraction and provides protection for allergen-induced airway hyperresponsiveness. However, the mechanism by which DGK regulates contractile signaling in ASM is not well established. Herein, we investigated the role of prorelaxant cAMP-protein kinase A (PKA) signaling in DGK-mediated regulation of ASM contraction. Pretreatment of human ASM cells with DGK inhibitor I activated PKA as demonstrated by the phosphorylation of PKA substrates, VASP, Hsp20, and CREB, which was abrogated when PKA was inhibited pharmacologically or molecularly using overexpression of the PKA inhibitor peptide, PKI. Furthermore, inhibition of DGK resulted in induction of cyclooxygenase (COX) and generation of prostaglandin E₂ (PGE₂ ) with concomitant activation of Gs-cAMP-PKA signaling in ASM cells in an autocrine/paracrine fashion. Inhibition of protein kinase C (PKC) or extracellular-signal-regulated kinase (ERK) attenuated DGK-mediated production of PGE₂ and activation of cAMP-PKA signaling in human ASM cells, suggesting that inhibition of DGK activates the COX-PGE₂ pathway in a PKC-ERK-dependent manner. Finally, DGK inhibition-mediated attenuation of contractile agonist-induced phosphorylation of myosin light chain 20 (MLC-20), a marker of ASM contraction, involves COX-mediated cAMP production and PKA activation in ASM cells. Collectively these findings establish a novel mechanism by which DGK regulates ASM contraction and further advances DGK as a potential therapeutic target to provide effective bronchoprotection in asthma.