PLPP2: Potential therapeutic target of breast cancer in PLPP family

PLPPs (Phospholipid phosphatases) are widely expressed in different human tissues, regulate cell signal transduction, and are overexpressed in cancers such as gliomas, pancreatic adenocarcinoma, lung adenocarcinoma, and so on. As a member of the PLPP family, PLPP2 (phospholipid phosphatase 2) plays a vital role in the occurrence and development of breast cancer, but its mechanism is still unclear. Our research found that PLPP2 was overexpressed in breast cancer, and the higher expression level of PLPP2 showed a worse prognosis for breast cancer patients. Further analysis showed that overexpression of PLPP2 affected the expression of CDC34 (cell-division cycle 34), LSM7 (Like-Smith 7), and SGTA (small glutamine-rich tetratricopeptide repeat-containing protein alpha) through EMT (epigenetic-mesenchymal transition) related pathways to promote the occurrence and development of breast cancer. In vitro, silencing PLPP2 significantly reduced the proliferation, invasion, and migration abilities of human breast cancer cells MDA-MB-231. ER+ is a common subtype of breast cancer. Furthermore, we found that the overexpression of PLPP2 was significantly related to the poor prognosis of ER+ breast cancer. These results indicate that PLPP2 has value as a potential therapeutic target for breast cancer, especially for ER+ breast cancer.     

PKC isozymes and diacylglycerol-regulated proteins as effectors of growth factor receptors

Growth factors exert their cellular effects through signal transduction pathways that are initiated by the ligation of growth factors to their cell surface receptors. One of the well-established effectors of growth factor receptors is protein kinase C (PKC), a family of serine-threonine kinases that have been known for years as the main target of the phorbol ester tumor promoters. While there is abundant information regarding downstream PKC effectors and partners, how individual PKC isozymes become activated by growth factors and the regulation of receptor function by PKCs is only partially understood. Moreover, the identification of novel “non-kinase” DAG-binding proteins has added a new level of complexity to the field of DAG signaling.     

Genetic activation of pyruvate dehydrogenase alters oxidative substrate selection to induce skeletal muscle insulin resistance

The pyruvate dehydrogenase complex (PDH) has been hypothesized to link lipid exposure to skeletal muscle insulin resistance through a glucose-fatty acid cycle in which increased fatty acid oxidation increases acetyl-CoA concentrations, thereby inactivating PDH and decreasing glucose oxidation. However, whether fatty acids induce insulin resistance by decreasing PDH flux remains unknown. To genetically examine this hypothesis we assessed relative rates of pyruvate dehydrogenase flux/mitochondrial oxidative flux and insulin-stimulated rates of muscle glucose metabolism in awake mice lacking pyruvate dehydrogenase kinase 2 and 4 [double knockout (DKO)], which results in constitutively activated PDH. Surprisingly, increased glucose oxidation in DKO muscle was accompanied by reduced insulin-stimulated muscle glucose uptake. Preferential myocellular glucose utilization in DKO mice decreased fatty acid oxidation, resulting in increased reesterification of acyl-CoAs into diacylglycerol and triacylglycerol, with subsequent activation of PKC-θ and inhibition of insulin signaling in muscle. In contrast, other putative mediators of muscle insulin resistance, including muscle acylcarnitines, ceramides, reactive oxygen species production, and oxidative stress markers, were not increased. These findings demonstrate that modulation of oxidative substrate selection to increase muscle glucose utilization surprisingly results in muscle insulin resistance, offering genetic evidence against the glucose-fatty acid cycle hypothesis of muscle insulin resistance.Lipid-induced muscle insulin resistance plays a major role in the pathogenesis of type 2 diabetes (T2D), but the cellular mechanisms remain unknown (1, 2). More than 50 y ago Randle et al. (3) postulated the glucose-fatty acid cycle to explain the impairment of insulin-stimulated glucose disposal by fatty acids in muscle. In this model, fat oxidation increases mitochondrial acetyl-CoA/CoA and NADH/NAD⁺ ratios. Acetyl-CoA and NADH allosterically inhibit pyruvate dehydrogenase complex (PDH), the mitochondrial enzyme that links glycolysis to the TCA cycle by converting pyruvate to acetyl-CoA. Additionally, fatty acid-derived acetyl-CoA produces citrate, which inhibits phosphofructokinase. This in turn increases glucose-6-phosphate (G6P), a potent allosteric inhibitor of hexokinase. By these mechanisms, increased fatty acid oxidation was hypothesized to reduce glycolytic flux and prevent further muscle glucose uptake. However, in vivo studies of human skeletal muscle metabolism have challenged the Randle hypothesis. Five hours of a lipid infusion, combined with heparin to activate lipoprotein lipase, raised plasma fatty acids and induced muscle insulin resistance in healthy individuals, yet intramyocellular G6P and glucose concentrations were reduced compared with control glycerol infusion studies, implicating defects in insulin-stimulated glucose transport activity (4, 5). An alternative hypothesis to explain the muscle insulin resistance associated with lipid exposure posits that accumulation of bioactive lipid intermediates initiates signaling cascades that impair insulin action. Lipid species implicated include diacylglycerols (DAGs) (6–10), ceramides (11, 12), and long-chain acyl-CoAs (13). DAG activation of PKC-θ in skeletal muscle has been shown to impair canonical insulin signaling at the level of insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation through increased IRS-1 serine phosphorylation at the 1101 position (2, 6, 7, 14).More recently, incomplete fat oxidation and subsequent accumulation of mitochondrially derived acylcarnitines has been proposed to contribute to lipid-induced muscle insulin resistance (15–17). According to this model, insulin resistance stems from increased fat oxidation, leading to increased conversion of acyl-CoA to medium- and long-chain acylcarnitines, which may mediate insulin resistance via unknown mechanisms. In contrast, short-chain acylcarnitines have been suggested to promote metabolic flexibility. The shortest acylcarnitine, acetylcarnitine, is synthesized from acetyl-CoA and carnitine by carnitine acetyltransferase (CrAT), a mitochondrial matrix enzyme, and is responsible for buffering the mitochondrial acetyl-CoA pool and mitigating acetyl-CoA inhibition of PDH (18). Consistent with the notion that CrAT regulates substrate selection by modulating PDH flux, mice with muscle-specific deletion of CrAT exhibited reduced PDH activity during the fed-to-fasted transition, resulting in glucose intolerance and metabolic inflexibility, a term coined by Kelley and Mandarino (19) to explain the impairment in the ability to adjust fuel oxidation to fuel availability.Although these studies emphasize the importance of PDH in the promotion of metabolic inflexibility, the role of PDH and mitochondrial oxidative substrate selection in the regulation of basal and insulin-stimulated muscle glucose metabolism has not been directly assessed in vivo. To examine this question, we sought to determine whether modulation of oxidative substrate selection in a genetic mouse model with constitutively active PDH activity would affect insulin sensitivity in skeletal muscle.     

DGKα DNA vaccine relieves airway allergic inflammation in asthma model possibly via induction of T cell anergy

Induction of T cell immune tolerance is thought to be a good method for treatment of asthma. Diacylglycerol kinases alpha (DGKα), enzymes that catalyze phosphorylation of diacylglycerol to produce phosphatidic acid, could inhibit diacylglycerol (DAG)-mediated signaling following T-cell receptor engagement and prevent T cell hyperactivation, thus playing important roles in the induction of T cell anergy. In the present study, we aimed to investigate the effects of DNA vaccine encoding DGKα gene administration on allergen-induced airway allergic inflammation in the murine model of asthma. Animal models were created and plasmid containing DGKα were constructed. Cytokine production was detected after the administration of DGKα gene plasmid. Immunization of mice with alum-adsorbed ovalbumin (OVA) followed by challenged with inhalation of aerosolized OVA resulted in the development of airway allergic inflammation. Administration of DGKα gene before the aerosolized OVA challenge significantly decreased the allergic airway inflammation and eosinophil infiltration in bronchoalveolar lavage fluid (BALF). Immunization with DGKα DNA vaccine decreased OVA-specific IgE and interleukin 13 (IL-13) levels in sera, and increased the IFN-γ level in BALF. The results of the present study provide evidence for the potential utility of the administration of DGKα DNA vaccine as an approach to gene therapy for asthma.     

Somatostatin receptors: From signaling to clinical practice

Somatostatin is a peptide with a potent and broad antisecretory action, which makes it an invaluable drug target for the pharmacological management of pituitary adenomas and neuroendocrine tumors. Somatostatin receptors (SSTR1, 2A and B, 3, 4 and 5) belong to the G protein coupled receptor family and have a wide expression pattern in both normal tissues and solid tumors. Investigating the function of each SSTR in several tumor types has provided a wealth of information about the common but also distinct signaling cascades that suppress tumor cell proliferation, survival and angiogenesis. This provided the rationale for developing multireceptor-targeted somatostatin analogs and combination therapies with signaling-targeted agents such as inhibitors of the mammalian (or mechanistic) target of rapamycin (mTOR). The ability of SSTR to internalize and the development of rabiolabeled somatostatin analogs have improved the diagnosis and treatment of neuroendocrine tumors.     

Protein kinase C isoforms in atherosclerosis: Pro- or anti-inflammatory?

Atherosclerosis is a pathologic condition caused by chronic inflammation in response to lipid deposition in the arterial wall. There are many known contributing factors such as long-term abnormal glucose levels, smoking, hypertension, and hyperlipidemia. Under the influence of such factors, immune and non-immune effectors cells are activated and participate during the progression of atherosclerosis. Protein kinase C (PKC) family isoforms are key players in the signal transduction pathways of cellular activation and have been associated with several aspects of the atherosclerotic vascular disease. This review article summarizes the current knowledge of PKC isoforms functions during atherogenesis, and addresses differential roles and disputable observations of PKC isoforms. Among PKC isoforms, both PKCβ and PKCδ are the most attractive and potential therapeutic targets. This commentary discusses in detail the outcomes and current status of clinical trials on PKCβ and PKCδ inhibitors in atherosclerosis-associated disorders like diabetes and myocardial infarction. The risk and benefit of these inhibitors for clinical purposes will be also discussed. This review summarizes what is already being done and what else needs to be done in further targeting PKC isoforms, especially PKCβ and PKCδ, for therapy of atherosclerosis and atherosclerosis-associated vasculopathies in the future.     

High glucose-induced mesangial cell altered contractility: role of the polyol pathway

Summary Glomerular mesangial cells cultured in high glucose conditions display impaired contractile responsiveness. It was postulated that glucose metabolism through the polyol pathway leads to altered mesangial cell contractility involving protein kinase C. Rat mesangial cells were growth-arrested for 24 h with 0.5 % fetal bovine serum in either normal (5.6 mmol/l) or high (30 mmol/l) glucose concentrations or high glucose plus the aldose reductase inhibitor, ARI-509 (100 μmol/l). The reduction of cell planar surface area (contraction) in response to endothelin-1 (0.1 μmol/l), or to phorbol 12-myristate 13-acetate (50 pmol/l), was studied by videomicroscopy. In response to endothelin-1, mesangial cells in normal glucose contracted to 52 ± 3 % of initial planar area. In high glucose, the significantly (p < 0.05) smaller cell size and no contractile responsiveness to endothelin-1 were normalized with ARI-509. Membrane-associated diacylglycerol, measured by a kinase specific ³²P-phosphorylation assay, in high glucose was unchanged after 3 h, but significantly increased (p < 0.05) after 24 h which was normalized with ARI-509. Protein kinase C activity, measured by in situ ³²P-phosphorylation of the epidermal growth factor receptor substrate was: increased by 32 % at 3 h of high glucose, unchanged by ARI-509; and decreased significantly (p < 0.05) at 24 h compared to cells in normal glucose, normalized by ARI-509. Total cellular protein kinase C-alpha, -delta and -epsilon, analysed by immunoblotting, were unchanged in high glucose at 24 h. Only protein kinase C-epsilon content was reduced by ARI-509 in both normal and high glucose. Therefore, high glucose-induced loss of mesangial cell contractility, diacylglycerol accumulation and altered protein kinase C activity are mediated through activation of the polyol-pathway, although no specific relationship between elevated diacylglycerol and protein kinase C activity was observed. In high glucose, altered protein kinase C function, or another mechanism related to the polyol pathway, contribute to loss of mesangial cell contractile responsiveness. [Diabetologia (1998) 41: 507–515] Received: 18 July 1997 and in final revised form: 17 January 1998     

Diacylglycerol lipase disinhibits VTA dopamine neurons during chronic nicotine exposure

Chronic nicotine exposure (CNE) alters synaptic transmission in the ventral tegmental area (VTA) in a manner that enhances dopaminergic signaling and promotes nicotine use. The present experiments identify a correlation between enhanced production of the endogenous cannabinoid 2-arachidonoylglycerol (2-AG) and diminished release of the inhibitory neurotransmitter GABA in the VTA following CNE. To study the functional role of on-demand 2-AG signaling in GABAergic synapses, we used 1,2,3-triazole urea compounds to selectively inhibit 2-AG biosynthesis by diacylglycerol lipase (DAGL). The potency and selectivity of these inhibitors were established in rats in vitro (rat brain proteome), ex vivo (brain slices), and in vivo (intracerebroventricular administration) using activity-based protein profiling and targeted metabolomics analyses. Inhibition of DAGL (2-AG biosynthesis) rescues nicotine-induced VTA GABA signaling following CNE. Conversely, enhancement of 2-AG signaling in naïve rats by inhibiting 2-AG degradation recapitulates the loss of nicotine-induced GABA signaling evident following CNE. DAGL inhibition reduces nicotine self-administration without disrupting operant responding for a nondrug reinforcer or motor activity. Collectively, these findings provide a detailed characterization of selective inhibitors of rat brain DAGL and demonstrate that excessive 2-AG signaling contributes to a loss of inhibitory GABAergic constraint of VTA excitability following CNE.The mesocorticolimbic dopamine (DA) system provides a critical link between the brain regions that process cognitive information and those controlling motor behavior. Precise control of these ventral tegmental area (VTA) projections facilitates seeking rewarding stimuli, retreating from aversive stimuli, constraint of motivational state, and behavioral flexibility necessary for survival. GABAergic signaling provides robust inhibition that gates VTA DA cell excitability (1, 2), and loss of this inhibition leads to pathological dysregulation of mesocorticolimbic circuitry (3, 4).Endocannabinoids (eCBs) regulate DAergic activity through retrograde signaling from DA cell bodies onto presynaptic cannabinoid type 1 (CB₁) receptors expressed on both inhibitory and excitatory inputs. Although both 2-arachidonoylglycerol (2-AG) and anandamide (AEA) function as endogenous CB₁ agonists in the brain (5–7), these lipids exhibit distinct pharmacological profiles in vivo (8, 9) and mediate differential behavioral effects (10, 11). Endocannabinoids are produced and degraded on-demand, and the primary enzymes responsible for eCB degradation have been well-characterized using selective pharmacological tools that inactivate monoacylglycerol lipase (MAGL) or fatty acid amide hydrolase (FAAH) (11–13). However, a complete evaluation of the influence of eCB signaling in the brain has been hampered by the lack of appropriate corresponding tools for selectively inactivating on-demand eCB biosynthesis.Substantial evidence implicates eCB signaling in the etiology of nicotine addiction, and recent work demonstrates that chronic nicotine exposure (CNE) selectively enhances nicotine-induced increases in VTA 2-AG formation (14). The present study investigated the possible contribution of this effect to aberrant VTA DA cell excitation present following CNE (15). We find that sensitized nicotine-induced 2-AG release (14) strongly correlates with a loss of nicotine-induced GABA release, which may contribute to impaired inhibitory constraint of VTA DA cell excitation following CNE. To test this hypothesis, we characterized a series of selective inhibitors of 2-AG biosynthesis by diacylglycerol lipase α and β (DAGLα and DAGLβ; hereafter referred to as DAGL) (16–18) and 2-AG degradation by α/β-hydrolase domain 6 (ABHD6) and MAGL (11, 12, 19), and used these compounds to investigate the functional impact of enhanced 2-AG recruitment on GABAergic signaling at VTA synapses and nicotine self-administration.