Mechanism of cytotoxicity induced by the cigarette smoke extract (CSE) of heated tobacco products in vascular smooth muscle cells: A comparative study of the cytotoxic effects of CSE and the ferroptosis inducer, erastin.

Heated tobacco products (HTPs) are marketed worldwide as less harmful alternatives to combustible cigarettes; however, their cytotoxic mechanisms in vascular smooth muscle cells are poorly understood. Ferroptosis is defined as iron-dependent cell death caused by the accumulation of lipid peroxidation products. In this study, the cytotoxic effects of nicotine- and tar-free cigarette smoke extracts (CSE) derived from three types of HTPs and the ferroptosis inducer, erastin, on vascular smooth muscle A7r5 cells were compared. Cigarette smoke from all HTPs was generated according to the following puffing regime: 55 mL, puff volume; 30 s, puff interval; 2 s, puff duration; bell-shaped, puff profile; and no blocking of the ventilation holes. Erastin and CSE decreased mitochondrial metabolic activity and increased lactate dehydrogenase leakage. The cytotoxic effects of erastin were almost completely inhibited by the radical-trapping antioxidant, UAMC-3203; iron chelator, deferoxamine mesylate (DFO); 12/15-lipoxygenase (12/15-LOX) inhibitor, baicalein; and selective 15-LOX inhibitor, ML351. In contrast, CSE-induced cell damage was partially attenuated by UAMC-3203, baicalein, and ML351 but not by DFO. These results suggest that erastin induces ferroptosis via 15-LOX-mediated iron-dependent lipid peroxidation, whereas CSE causes iron-independent cell damage via 15-LOX-mediated lipid peroxidation-dependent and -independent mechanisms.

LRRK2 is involved in the chemotaxis of neutrophils and differentiated HL-60 cells, and the inhibition of LRRK2 kinase activity increases fMLP-induced chemotactic activity.

BACKGROUND: Neutrophils depend heavily on glycolysis for energy production under normal conditions. In contrast, neutrophils require energy supplied by mitochondrial oxidative phosphorylation (OXPHOS) during chemotaxis. However, the mechanism by which the energy supply changes from glycolysis to OXPHOS remains unknown. Leucine-rich repeat kinase 2 (LRRK2) is partially present in the outer mitochondrial membrane fraction. Lrrk2-deficient cells show mitochondrial fragmentation and reduced OXPHOS activity. We have previously reported that mitofusin (MFN) 2 is involved in chemotaxis and OXPHOS activation upon chemoattractant N-formyl-Met-Leu-Phe (fMLP) stimulation in differentiated HL-60 (dHL-60) cells. It has been previously reported that LRRK2 binds to MFN2 and partially colocalizes with MFN2 at the mitochondrial membranes. This study investigated the involvement of LRRK2 in chemotaxis and MFN2 activation in neutrophils and dHL-60 cells. METHODS: Lrrk2 knockout neutrophils and Lrrk2 knockdown dHL-60 cells were used to examine the possible involvement of LRRK2 in chemotaxis. Lrrk2 knockdown dHL-60 cells were used a tetracycline-inducible small hairpin RNA (shRNA) system to minimize the effects of LRRK2 knockdown during cell culture. The relationship between LRRK2 and MFN2 was investigated by measuring the GTP-binding activity of MFN2 in Lrrk2 knockdown dHL-60 cells. The effects of LRRK2 kinase activity on chemotaxis were examined using the LRRK2 kinase inhibitor MLi-2. RESULTS: fMLP-induced chemotactic activity was reduced in Lrrk2 knockout neutrophils in vitro and in vivo. Lrrk2 knockdown in dHL-60 cells expressing Lrrk2 shRNA also reduced fMLP-induced chemotactic activity. Lrrk2 knockdown dHL-60 cells showed reduced OXPHOS activity and suppressed mitochondrial morphological change, similar to Mfn2 knockdown dHL-60 cells. The amount of LRRK2 in the mitochondrial fraction and the GTP-binding activity of MFN2 increased upon fMLP stimulation, and the MFN2 GTP-binding activity was suppressed in Lrrk2 knockdown dHL-60 cells. Furthermore, the kinase activity of LRRK2 and Ser935 phosphorylation of LRRK2 were reduced upon fMLP stimulation, and LRRK2 kinase inhibition by MLi-2 increased the migration to fMLP. CONCLUSIONS: LRRK2 is involved in neutrophil chemotaxis and the GTP-binding activity of MFN2 upon fMLP stimulation. On the other hand, the kinase activity of LRRK2 shows a negative regulatory effect on fMLP-induced chemotactic activity in dHL-60 cells. Video Abstract.

Effects of cigarette smoke extract derived from heated tobacco products on the proliferation of lung cancer stem cells.

Epidemiological studies have suggested that cigarette smoking can increase a person's risk of developing several types of cancer, including lung cancer. Lung cancer originates from cancer stem cells (CSCs), which constitute a minor cell population in tumors, and contribute to drug resistance and recurrence. Heated tobacco products (HTPs) produce aerosols that contain nicotine and toxic chemicals. Current evidence, however, is insufficient to accurately determine if HTPs are less harmful than burned cigarettes. This study has investigated the effects of cigarette smoke extract (CSE) from HTPs on lung CSCs in lung cancer cell lines. We found that CSEs induced the proliferation of lung CSCs and increased the expression levels of stem cell markers. In addition, CSE induced epithelial-mesenchymal transition (EMT) expression and cytokine production. These results suggest that HTPs can induce lung CSCs in vitro.

Comparison of cytotoxicity of cigarette smoke extract derived from heat-not-burn and combustion cigarettes in human vascular endothelial cells.

The present study compared the properties of mainstream smoke generated from heat-not-burn (HNB) cigarettes and a combustion cigarette (hi-lite™ brand). Three types of cigarette heating devices were used to generate cigarette smoke at different heating temperatures [Ploom S™ (200 °C), glo™ (240 °C), and IQOS™ (300-350 °C)]. Mainstream smoke was generated using the following puffing regimen: volume, 55 mL; duration, 3 s; and interval, 30 s. The rank order of particulate phase (nicotine and tar) amounts trapped on a Cambridge filter was Ploom S < glo < IQOS < hi-lite. Heated cigarette-derived smoke extract (hCSE) from the devices except for Ploom S, and burned CSE (bCSE) decreased mitochondrial metabolic activity (glo < IQOS < hi-lite) in human vascular endothelial cells. Furthermore, the cytotoxicity was reduced by removing the particulate phase from the mainstream smoke. Endothelial nitric oxide synthase activity was reduced by nicotine- and tar-free CSE of IQOS and hi-lite (IQOS < hi-lite), but not Ploom S and glo. These inhibitory effects were diminished by removing the carbonyl compounds from the mainstream smoke. These results indicated that the cytotoxicity of hCSE was lower than that of bCSE in vascular endothelial cells.

Fibrinolysis-resistant carbonylated fibrin detected in thrombi attached to the vascular wall of abdominal aortic aneurysms.

In this study, we investigated how carbonylation of fibrinogen by acrolein modified its indispensable function to enhance fibrinolysis after being converted to fibrin and contributed to generating a fibrinolysis-resistant fibrin clot. Acrolein-treated fibrinogen was subjected to tissue plasminogen activator-induced fibrinolysis assay and the effect of lysine residue carbonylation in fibrinogen on fibrinolysis was analyzed. The acrolein-treated fibrinogen-derived fibrin clot appeared more resistant to fibrinolysis and the N-acetyl 3-formyl-3,4-dehydropiperidino (FDP)-Lysine levels in the lysed solution were positively correlated with the duration of clot lysis. The lysine analog 6-amino hexanoic acid (6AHA), which mimics the C-terminal lysine of fibrin, was carbonylated and its enhancing effect on Glu1-plasminogen activation was evaluated. After incubation with acrolein, 6AHA was converted to N-acetyl FDP-6AHA, losing its ability to enhance Glu1-plasminogen activation. These results suggest that fibrinogen carbonylation by acrolein to generate N-acetyl FDP-Lysine resulted in the generation of fibrinolysis-resistant fibrin by attenuating the C-terminal lysine-dependent activation of the Glu1-plasminogen. In abdominal aortic aneurysms, fibrin(ogen) containing the acrolein adduct N-acetyl FDP-Lysine was detected in the vascular wall-attached thrombi. These results suggest that this mechanism is likely involved in the modification of fibrinolysis-resistant thrombi and to their persistence for a long period.

Cigarette Smoke Extract and Its Cytotoxic Factor Acrolein Inhibit Nitric Oxide Production in Human Vascular Endothelial Cells.

Acrolein (ACR), a highly reactive α,ß-unsaturated aldehyde, is a major cytotoxic factor in nicotine- and tar-free cigarette smoke extract (CSE). There are conflicting results regarding endothelial functions despite the fact that both CSE and ACR cause cellular damage. Several lines of evidence indicate that CSE impairs endothelium-derived nitric oxide (NO)-dependent vasodilation by reducing the activity and protein expression of endothelial NO synthase (eNOS), whereas ACR elicits endothelium-dependent vasorelaxation by increasing the production of NO and expression of eNOS. To clarify whether CSE and its cytotoxic factor ACR cause endothelial dysfunction, this study examined the effects of CSE and ACR on human vascular endothelial EA.hy926 cells. CSE and ACR reduced the phosphorylation of eNOS at serine (Ser)1177 and total expression of eNOS. The CSE- and ACR-induced decrease in the phosphorylation and expression of eNOS was counteracted by glutathione (reduced form), an antioxidant. Basal NO production was inhibited by CSE, ACR, NG-nitro-L-arginine methyl ester (a competitive eNOS inhibitor), and nominally Ca2+-free solution supplemented with BAPTA-AM (a membrane permeable Ca2+ chelator). These results indicate that CSE and ACR increase oxidative stress, and reduce NO production by reducing the activity and total protein level of eNOS.

Extracellular Ca2+ promotes nitric oxide production via Ca2+-sensing receptor-Gq/11 protein-endothelial nitric oxide synthase signaling in human vascular endothelial cells.

This study examined the possible involvement of Ca2+-sensing receptor (CaSR) in nitric oxide (NO) production in human vascular endothelial cells. Extracellular Ca2+ elevated the intracellular Ca2+ concentration, the endothelial NO synthase (eNOS) phosphorylation level, and NO release from the cells. These responses were inhibited by a CaSR antagonist and a Gq/11 protein inhibitor. Application of an endothelial cell suspension induced vasorelaxation in isolated rat thoracic aorta precontracted by phenylephrine. Adding an NO scavenger to the organ bath abolished this vasorelaxation response. These results suggest that extracellular Ca2+ promotes NO generation via CaSR- and Gq/11 protein-mediated eNOS activation.

Ca2+ signal is involved in endothelin-1-induced internalization of endothelin type A receptor expressed in Chinese hamster ovary cells.

Endothelin type A receptor (ETAR) is internalized upon agonist stimulation; however, the mechanism thereof remains controversial. In this study, we characterized the endothelin-1 (ET-1)-induced internalization of ETAR expressed in Chinese hamster ovary cells. ET-1 elicited ETAR internalization and increase in intracellular Ca2+ concentration. ET-1-induced ETAR internalization was completely inhibited by a reduction in intracellular and extracellular Ca2+ levels and partially suppressed by inhibitors of protein kinase C (PKC) and extracellular signal-regulated kinases 1/2 (ERK1/2), both of which are downstream molecules in ETAR signaling. These results suggest that Ca2+ mobilization, PKC, and ERK1/2 are involved in ET-1-induced ETAR internalization.

Chinese herbal medicine Qing-Dai-induced pulmonary arterial hypertension in a patient with ulcerative colitis: A case report and experimental investigation.

A recent case report described a case of pulmonary arterial hypertension (PAH) associated with use of the Chinese herbal medicine Qing-Dai; however, the clinical course and possible mechanisms have not been characterized. We present the case of a man with ulcerative colitis who was diagnosed with idiopathic PAH. After initiating oral beraprost therapy, the patient showed significant hemodynamic improvements and an unusual course of clinical recovery. In 2016, the Japanese Ministry of Health, Labour, and Welfare issued a warning regarding the possible side effects of Qing-Dai. We learned that our patient had been taking self-purchased Qing-Dai for 2 years. Therefore, we performed an experimental study and determined that Qing-Dai may cause PAH through a mechanism involving nitric oxide synthase inhibition and pulmonary artery endothelial dysfunction.

Annexin A2 is involved in activation of extracellular signal-regulated kinase upon endothelin-1 stimulation.

The overexpression of endothelin (ET)-1 or ET receptors (ETRs) is related to initiation and progression of tumor. In cancer cells, ET-1 activates various signaling pathways, including mitogen-activated protein kinase, phosphatidylinositol 3-kinase, protein kinase C through ETRs, although the mechanisms by which ET-1 activates these signaling pathways remain uncertain. Here, we found that ETRs interacted with annexin A2, which is overexpressed in various cancers. Annexin A2 bound to ET type A receptor and ET type B receptor. Upon ET-1 stimulation, serine phosphorylation of annexin A2 increased, while there is no change in tyrosine phosphorylation of annexin A2. On the other hand, annexin A2 silencing suppressed activation of ERK upon ET-1 stimulation. These results suggest that interaction of ETRs and annexin A2 may play important roles in activation of extracellular signal-regulated kinase upon ET-1 stimulation.

Endothelin type B receptor interacts with the 78-kDa glucose-regulated protein.

Endothelin (ET)-1 is involved in the vascular system, cell proliferation and apoptosis. ET receptors consist of ET type A receptor (ETA R) and ET type B receptor (ETB R). ETA R and ETB R generally exhibit opposite responses, although many exceptions exist. In the present study, we attempted to identify ETA R- or ETB R-specific binding proteins to understand the differences in ETA R- and ETB R-mediated responses after ET-1 stimulation. The 78-kDa glucose-regulated protein (GRP78) showed a stronger binding affinity towards ETB R than towards ETA R. Moreover, GRP78 overexpression promoted ETB R-mediated ERK activation and GRP78 silencing suppressed ETB R-mediated ERK activation. Furthermore, ETB R can localize GRP78 to the cell periphery. These results suggest that the interaction of ETB R with GRP78 affects ERK activation and GRP78 localization.

[Molecular mechanism for ET-1-induced insulin resistance in skeletal muscle cells].

Insulin resistance is a condition where the sensitivity to insulin of the tissues expressing insulin receptor (InsR) is decreased due to a functional disturbance of InsR-mediated intracellular signaling. Insulin promotes the entry of glucose into the tissues and skeletal muscle is the most important tissue responsible for the insulin's action of decreasing blood glucose levels. Endothelin-1 (ET-1), a potent vasoconstrictor and pro-inflammatory peptide, induces insulin resistance through a direct action on skeletal muscle. However, the signaling pathways of ET-1-induced insulin resistance in skeletal muscle remain unclear. Here we show molecular mechanism underlying the inhibitory effect of ET-1 on insulin-stimulated Akt phosphorylation and glucose uptake in myotubes of rat L6 skeletal muscle cell line. mRNA expression levels of differentiation marker genes, MyoD and myogenin, were increased during L6 myoblasts differentiation into myotubes. Some of myotubes possessed the ability to spontaneously contract. In myotubes, insulin promoted Akt phosphorylation at Thr308 and Ser473, and [3H]-labelled 2-deoxy-D-glucose ([3H]2-DG) uptake. The insulin-facilitated Akt phosphorylation and [3H]2-DG uptake were inhibited by ET-1. The inhibitory effect of ET-1 was counteracted by blockade of ET type A receptor (ETAR), inhibition of Gq/11 protein, and siRNA knockdown of G protein-coupled receptor kinase 2 (GRK2). The exogenously overexpressed GRK2 directly bound to endogenous Akt and their association was facilitated by ET-1. In summary, activation of ETAR with ET-1 inhibits insulin-induced Akt phosphorylation and [3H]2-DG uptake in a Gq/11 protein- and GRK2-dependent manner in skeletal muscle. These findings indicate that ETAR and GRK2 are potential targets for insulin resistance.

ARF1 recruits RAC1 to leading edge in neutrophil chemotaxis.

BACKGROUND: The small GTPase ARF1 mediates membrane trafficking mostly from the Golgi, and is essential for the G protein-coupled receptor (GPCR)-mediated chemotaxis of neutrophils. In this process, ARF1 is activated by the guanine nucleotide exchanger GBF1, and is inactivated by the GTPase-activating protein GIT2. Neutrophils generate the Gßγ-PAK1-αPIX-GIT2 linear complex during GPCR-induced chemotaxis, in which αPIX activates RAC1/CDC42, which then employs PAK1. However, it has remained unclear as to why GIT2 is included in this complex. RESULTS: We investigated the association between ARF1 and RAC1/CDC42 during the fMLP-stimulated chemotaxis of HL60 cells. We found that the silencing of GBF1 significantly impaired the recruitment of RAC1 to the leading edges, but not PAK1, αPIX, RAC2, or CDC42. A significant population of RAC1 colocalized with ARF1 at the leading edges in stimulated cells, whereas fMLP activated both ARF1 and ARF5. Consistently, the silencing of ARF1, but not ARF5, impaired the recruitment of RAC1, whereas the silencing of RAC1 did not affect the recruitment of ARF1 to the leading edges. CONCLUSIONS: Our results indicated that the activation of ARF1 triggers the plasma membrane recruitment of RAC1 in GPCR-mediated chemotaxis, which is essential for cortical actin remodeling. Thus, membrane remodeling at the leading edges appears to precede actin remodeling in chemotaxis. Together with the fact that GIT2, which inactivates ARF1, is an integral component of the machinery activating RAC1, we proposed a model in which the ARF1-RAC1 linkage enables the regulation of ARF1 by repetitive on/off cycles during GPCR-mediated neutrophil chemotaxis.

Cigarette Smoke Extract Inhibits Platelet Aggregation by Suppressing Cyclooxygenase Activity.

The results of studies that were performed to determine whether cigarette smoking affects platelet function have been controversial, and the effects of nicotine- and tar-free cigarette smoke extract (CSE) on platelet function remain to be determined. The aim of this study was to determine the effect of CSE on platelet aggregation and to clarify the mechanism by which CSE affects platelet function. CSE inhibited murine platelet aggregation induced by 9,11-dideoxy-9α,11α-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U-46619), a thromboxane (TX) A 2 receptor agonist, and that induced by collagen with respective IC 50 values of 1.05 ± 0.14% and 1.34 ± 0.19%. A similar inhibitory action of CSE was also observed in human platelets. CSE inhibited arachidonic acid-induced TXA 2 production in murine platelets with an IC 50 value of 7.32 ± 2.00%. Accordingly, the inhibitory effect of CSE on collagen-induced aggregation was significantly blunted in platelets lacking the TXA 2 receptor compared with the inhibitory effect in control platelets. In contrast, the antiplatelet effects of CSE in platelets lacking each inhibitory prostanoid receptor, prostaglandin (PG) I 2 receptor and PGE 2 receptor subtypes EP 2 and EP 4 , were not significantly different from the effects in respective control platelets. Among the enzymes responsible for TXA 2 production in platelets, the activity of cyclooxygenase (COX)-1 was inhibited by CSE with an IC 50 value of 1.07 ± 0.15% in an uncompetitive manner. In contrast, the activity of TX synthase was enhanced by CSE. The results indicate that CSE inhibits COX-1 activity and thereby decreases TXA 2 production in platelets, leading to inhibition of platelet aggregation.

The dynamics of mucosal-associated invariant T cells in multiple sclerosis.

BACKGROUND: Multiple sclerosis (MS) is an autoimmune disease characterized by inflammatory demyelination, gliosis and axonal loss in the Central Nervous System. Although the etiology of the disease has remained enigmatic, recent studies have suggested a role of the innate-like T cells, called Mucosal Associated Invariant T cells (MAITs) in the pathophysiology. In the present study, we have analyzed the relative frequency of MAITs and the expression of the cell surface antigens in MAITs to seek a possible link to the disease. RESULTS: There was little difference in the frequency of total MAITs between healthy donors (HDs) and untreated MS patients, whereas the latter harbored more CD8(lo/neg) (DN) MAITs concomitant with a decrease in CD8(high) MAITs and in CD4 MAITs compared with those in HDs. While the expression of CCR5, CCR6, CD95, CD127, and CD150 has increased in untreated subjects compared with that in HDs, CD45RO has declined in untreated subjects in both DN MAITs and CD8(hi) MAITs. FTY720 therapy has increased the relative frequency of total MAITs in a time-dependent fashion up to 2 years. Intriguingly, FTY720 therapy for 3 years reversed the above phenotype, engendering more CD8(high) MAITs accompanied with decreased DN MAITs. FTY720 therapy affected the cytokine production from CD4 T cells and also enhanced the relative frequency of cells producing both TNF-α and IFN-γ from MAITs, CD8 T cells, and CD4 T cells compared with that in untreated subjects. CONCLUSIONS: FTY 720 therapy enhanced the relative frequency of MAITs in MS patients in a time-dependent manner. Although the expression of CD8 in MAITs has been affected early by FTY720, longer treatment has reversed the phenotypic change. These data demonstrated that FTY720 induced dynamic change in the relative frequency and in the phenotype of MAITs in MS.

A Standardized Method for the Preparation of a Gas Phase Extract of Cigarette Smoke.

The gas phase of cigarette smoke is important from the viewpoint of human health, because it can pass through alveolar epithelium and enter the circulation. There is no standard method for the preparation of a gas phase extract of cigarette smoke (CSE), although CSE is widely used for research instead of whole cigarette smoke. We have established a standard method for the preparation of CSE. One cigarette per trial is continuously combusted under a reduced pressure generated by an aspiration pump with a velocity of 1.050 L/min: the main stream of the smoke is passed through a Cambridge filter to remove tar, and subsequently, bubbled through a glass ball filter (pore size, 20-30 µm) into 15 mL of phosphate-buffered saline (PBS). To express the concentration of CSE, a virtual tar concentration is introduced, which is calculated assuming that tar trapped on the Cambridge filter is dissolved in the PBS. CSEs prepared from smaller numbers of cigarettes (original virtual tar concentration≤15 mg/mL) show similar concentration-response curves for cytotoxicity versus virtual tar concentrations. CSEs prepared from various brands of cigarettes and by different smoking regimes (continuous and puff smoking) show similar cytotoxic potency if the virtual tar concentrations are the same. In conclusion, using the standardized method for CSE preparation in combination with the virtual tar concentration, it becomes possible to simply and rapidly prepare standard CSEs with defined concentrations from any brand of cigarettes, which are toxicologically equivalent to CSE prepared by puff smoking.

Carbonyl Compounds in the Gas Phase of Cigarette Mainstream Smoke and Their Pharmacological Properties.

Cigarette mainstream smoke is composed of gas and tar phases and contains >4000 chemical constituents, including nicotine and tar. The substances in the gas phase but not in the tar phase can pass through the airway epithelial barrier, enter the systemic circulation via the pulmonary circulation, and increase systemic oxidative damage, leading to the development of cigarette smoking-related diseases such as atherosclerosis. Recently, we identified some stable carbonyl compounds, including acrolein (ACR) and methyl vinyl ketone (MVK), as major cytotoxic factors in nicotine- and tar-free cigarette smoke extract (CSE) of the gas phase. CSE, ACR, and MVK induce protein kinase C (PKC)-dependent activation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) and subsequent generation of reactive oxygen species (ROS) via NOX, causing plasma membrane damage and cell apoptosis. CSE, ACR, and MVK also trigger carbonylation of PKC, which is an irreversible oxidative modification. Cell damage and PKC carbonylation in response to treatment with CSE, ACR, or MVK are abolished by thiol-containing antioxidants such as N-acetyl-L-cysteine and reduced glutathione. Thus pharmacological modulation of PKC and NOX activities and the trapping of ROS are potential strategies for the prevention of diseases related to cigarette smoking.

Using Phos-Tag in Western Blotting Analysis to Evaluate Protein Phosphorylation.

Protein phosphorylation has traditionally been detected by radioisotope phosphate labeling of proteins with radioactive ATP. Several nonradioactive assays with phosphorylation site-specific antibodies are now available for the analysis of phosphorylation status at target sites. However, due to their high specificity, these antibodies they cannot be used to detect unidentified phosphorylation sites. Recently, Phos-tag technology has been developed to overcome the disadvantages and limitations of phosphospecific antibodies. Phos-tag and its derivatives conjugated to biotin, acrylamide, or agarose, form alkoxide-bridged dinuclear metal complexes, which can capture phosphate monoester dianions bound to serine, threonine, and tyrosine residues, in an amino acid sequence-independent manner. Here, we describe our method, which is based on in vitro kinase assay and Western blotting analysis using biotinylated Phos-tag and horseradish peroxidase-conjugated streptavidin, to determine the sites of TRPC6 (transient receptor potential canonical 6) channel phosphorylated by protein kinase A.

Endothelin-1 suppresses insulin-stimulated Akt phosphorylation and glucose uptake via GPCR kinase 2 in skeletal muscle cells.

BACKGROUND AND PURPOSE: Endothelin-1 (ET-1) reduces insulin-stimulated glucose uptake in skeletal muscle, inducing insulin resistance. Here, we have determined the molecular mechanisms underlying negative regulation by ET-1 of insulin signalling. EXPERIMENTAL APPROACH: We used the rat L6 skeletal muscle cells fully differentiated into myotubes. Changes in the phosphorylation of Akt was assessed by Western blotting. Effects of ET-1 on insulin-stimulated glucose uptake was assessed with [(3) H]-2-deoxy-d-glucose ([(3) H]2-DG). The C-terminus region of GPCR kinase 2 (GRK2-ct), a dominant negative GRK2, was overexpressed in L6 cells using adenovirus-mediated gene transfer. GRK2 expression was suppressed by transfection of the corresponding short-interfering RNA (siRNA). KEY RESULTS: In L6 myotubes, insulin elicited sustained Akt phosphorylation at Thr(308) and Ser(473) , which was suppressed by ET-1. The inhibitory effects of ET-1 were prevented by treatment with a selective ETA receptor antagonist and a Gq protein inhibitor, overexpression of GRK2-ct and knockdown of GRK2. Insulin increased [(3) H]2-DG uptake rate in a concentration-dependent manner. ET-1 noncompetitively antagonized insulin-stimulated [(3) H]2-DG uptake. Blockade of ETA receptors, overexpression of GRK2-ct and knockdown of GRK2 prevented the ET-1-induced suppression of insulin-stimulated [(3) H]2-DG uptake. In L6 myotubes overexpressing FLAG-tagged GRK2, ET-1 facilitated the interaction of endogenous Akt with FLAG-GRK2. CONCLUSIONS AND IMPLICATIONS: Activation of ETA receptors with ET-1 suppressed insulin-induced Akt phosphorylation at Thr(308) and Ser(473) and [(3) H]2-DG uptake in a GRK2-dependent manner in skeletal muscle cells. These findings suggest that ETA receptors and GRK2 are potential targets for overcoming insulin resistance.