Shear stress-induced Ang II AT1 receptor activation: G-protein dependent and independent mechanisms

Membrane mediated mechanical stimuli produces distinct active-like states in the AT1 receptor

The Angiotensin II Type 1 (AT1) receptor is one of the most widely studied GPCRs within the context of biased signaling. While the AT1 receptor is activated by agonists such as the peptide AngII, it can also be activated by mechanical stimuli such as membrane stretch or shear in the absence of a ligand. Despite the importance of mechanical activation of the AT1 receptor in biological processes such as vasoconstriction, little is known about the structural changes induced by external physical stimuli mediated by the surrounding lipid membrane. Here, we present a systematic simulation study that characterizes the activation of the AT1 receptor under various membrane environments and mechanical stimuli. We show that stability of the active state is highly sensitive to membrane thickness and tension. Structural comparison of membrane-mediated vs. agonist-induced activation shows that the AT1 receptor has distinct active conformations. This is supported by multi-microsecond free energy calculations that show unique landscapes for the inactive and various active states. Our modeling results provide structural insights into the mechanical activation of the AT1 receptor and how it may produce different functional outcomes within the framework of biased agonism.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Effectiveness of losartan on infrapatellar fat pad/synovial fibrosis and pain behavior in the monoiodoacetate-induced rat model of osteoarthritis pain

Infrapatellar fat pad (IFP)/ synovial fibrosis is closely associated with the clinical symptoms of joint pain and stiffness, which contribute to locomotor restriction in osteoarthritis (OA) patients. Hence, this study was designed to gain insight on whether losartan, a selective angiotensin II type 1 receptor (AT1R) antagonist, has therapeutic benefit to reverse IFP/synovial fibrosis and secondarily to attenuate pain behavior. In male Wistar rats with monoiodoacetic acid (MIA)-induced IFP/synovial fibrosis, a possible role for increased AT1R expression in the pathogenesis of IFP/synovial fibrosis was assessed over an 8-week period. Pain behavior comprised static weight bearing and von Frey paw withdrawal thresholds (PWTs), which were assessed once or twice weekly, respectively. Groups of MIA-rats received oral losartan (30-mg/kg; n = 8 or 100-mg/kg; n = 9) or vehicle (n = 9) for 28-days according to a prevention protocol. Animals were euthanized on day 28 and various tissues (IFP/synovium, cartilage and lumbar dorsal root ganglia (DRGs)) were collected for histological, immunohistochemical and western blot analyses. Administration of once-daily losartan for 28-days dose-dependently attenuated the development of static weight bearing. This was accompanied by reduced IFP/synovial fibrosis and suppression of TGF-β1 expression. Chronic treatment of MIA-rats with losartan had an anti-fibrotic effect and it attenuated pain behavior in this animal model.

Gαq Is the Specific Mediator of PAR-1 Transactivation of Kinase Receptors in Vascular Smooth Muscle Cells

AIMS

G protein-coupled receptor (GPCR) transactivation of kinase receptors greatly expands the actions attributable to GPCRs. Thrombin, via its cognate GPCR, protease-activated receptor (PAR)-1, transactivates tyrosine and serine/threonine kinase receptors, specifically the epidermal growth factor receptor and transforming growth factor-β receptor, respectively. PAR-1 transactivation-dependent signalling leads to the modification of lipid-binding proteoglycans involved in the retention of lipids and the development of atherosclerosis. The mechanisms of GPCR transactivation of kinase receptors are distinct. We aimed to investigate the role of proximal G proteins in transactivation-dependent signalling.

MAIN METHODS

Using pharmacological and molecular approaches, we studied the role of the G⍺ subunits, G⍺q and G⍺11, in the context of PAR-1 transactivation-dependent signalling leading to proteoglycan modifications.

KEY FINDINGS

Pan G⍺q subunit inhibitor UBO-QIC/FR900359 inhibited PAR-1 transactivation of kinase receptors and proteoglycans modification. The G⍺q/11 inhibitor YM254890 did not affect PAR-1 transactivation pathways. Molecular approaches revealed that of the two highly homogenous G⍺q members, G⍺q and G⍺11, only the G⍺q was involved in regulating PAR-1 mediated proteoglycan modification. Although G⍺q and G⍺11 share approximately 90% homology at the protein level, we show that the two isoforms exhibit different functional roles.

SIGNIFICANCE

Our findings may be extrapolated to other GPCRs involved in vascular pathology and highlight the need for novel pharmacological tools to assess the role of G proteins in GPCR signalling to expand the preeminent position of GPCRs in human therapeutics.

Translating the Force - mechano-sensing GPCRs

Incorporating mechanical cues into cellular responses allows us to experience our direct environment. Specialized cells can perceive and discriminate between different physical properties such as level of vibration, temperature, or pressure. Mechanical forces are abundant signals that also shape general cellular responses such as cytoskeletal rearrangement, differentiation, or migration and contribute to tissue development and function. The molecular structures that perceive and transduce mechanical forces are specialized cytoskeletal proteins, cell junction molecules, and membrane proteins such as ion channels and metabotropic receptors. G protein-coupled receptors (GPCRs) have attracted attention as metabotropic force receptors as they are among the most important drug targets. This review summarizes the function of mechano-sensitive GPCRs, specifically, the angiotensin II type 1 receptor and adrenergic, apelin, histamine, parathyroid hormone 1, and orphan receptors, focusing particularly on the advanced knowledge gained from adhesion-type GPCRs. We distinguish between shear stress and cell swelling/stretch as the two major types of mechano-activation of these receptors and contemplate the potential contribution of the force-from-lipid and force-from-tether models that have previously been suggested for ion channels.

Effects of Statins on Renin-Angiotensin System

Statins, a class of drugs for lowering serum LDL-cholesterol, have attracted attention because of their wide range of pleiotropic effects. An important but often neglected effect of statins is their role in the renin–angiotensin system (RAS) pathway. This pathway plays an integral role in the progression of several diseases including hypertension, heart failure, and renal disease. In this paper, the role of statins in the blockade of different components of this pathway and the underlying mechanisms are reviewed and new therapeutic possibilities of statins are suggested.

Angiotensin II type 1 receptor is involved in flow-induced vasomotor responses of isolated middle cerebral arteries: role of oxidative stress

This study aimed to determine the mechanosensing role of angiotensin II type 1 receptor (AT1R) in flow-induced dilation (FID) and oxidative stress production in middle cerebral arteries (MCA) of Sprague-Dawley rats. Eleven-week old, healthy male Sprague-Dawley rats on a standard diet were given the AT1R blocker losartan (1 mg/mL) in drinking water (losartan group) or tap water (control group) ad libitum for 7 days. Blockade of AT1R attenuated FID and acetylcholine-induced dilation was compared with control group. Nitric oxide (NO) synthase inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) and cyclooxygenase inhibitor indomethacin (Indo) significantly reduced FID in control group. The attenuated FID in losartan group was further reduced by Indo only at Δ100 mmHg, whereas l-NAME had no effect. In losartan group, Tempol (a superoxide scavenger) restored dilatation, whereas Tempol + l-NAME together significantly reduced FID compared with restored dilatation with Tempol alone. Direct fluorescence measurements of NO and reactive oxygen species (ROS) production in MCA, in no-flow conditions revealed significantly reduced vascular NO levels with AT1R blockade compared with control group, whereas in flow condition increased the NO and ROS production in losartan group and had no effect in the control group. In losartan group, Tempol decreased ROS production in both no-flow and flow conditions. AT1R blockade elicited increased serum concentrations of ANG II, 8-iso-PGF2α, and TBARS, and decreased antioxidant enzyme activity (SOD and CAT). These results suggest that in small isolated cerebral arteries: 1) AT1 receptor maintains dilations in physiological conditions; 2) AT1R blockade leads to increased vascular and systemic oxidative stress, which underlies impaired FID.NEW & NOTEWORTHY The AT1R blockade impaired the endothelium-dependent, both flow- and acetylcholine-induced dilations of MCA by decreasing vascular NO production and increasing the level of vascular and systemic oxidative stress, whereas it mildly influenced the vascular wall inflammatory phenotype, but had no effect on the systemic inflammatory response. Our data provide functional and molecular evidence for an important role of AT1 receptor activation in physiological conditions, suggesting that AT1 receptors have multiple biological functions.

Vascular Metabolic Mechanisms of Pulmonary Hypertension

Pulmonary hypertension (PH) is a severe and progressive disease characterized by increased pulmonary vascular resistance leading to right heart failure and death. In PH, the cellular metabolisms including those of the three major nutrients (carbohydrate, lipid and protein) are aberrant in pulmonary vascular cells. Glucose uptake, glycolysis, insulin resistance, sphingolipid S1P, PGE₂, TXA₂, leukotrienes and glutaminolysis are upregulated, and phospholipid-prostacyclin and L-arginine-nitric oxide pathway are compromised in lung vascular cells. Fatty acid metabolism is disordered in lung endothelial cells and smooth muscle cells. These molecular mechanisms are integrated to promote PH-specific abnormal vascular cell proliferation and vascular remodeling. This review summarizes the recent advances in the metabolic reprogramming of glucose, fatty acid, and amino acid metabolism in pulmonary vascular remodeling in PH and the mechanisms for how these alterations affect vascular cell fate and impact the course of PH.

Physiological and Biochemical Vascular Reactivity Parameters of Angiotensin II and the Action of Biased Agonist TRV023

Vascular reactivity experiments using isolated aortic rings have been widely used as a model for physiological and pharmacological studies since the early sixties. Here, we suggest several parameters that the researcher should pay attention to when investigating angiotensin II in their experimental models. Angiotensin II is one of the active peptides of the renin-angiotensin system and exerts its effect through the AT1 and AT2 receptors. Some studies seek to understand the effects of angiotensin II receptors at the vascular level by using vascular reactivity experiments. However, because of the large number of variations, there are only a handful of reactivity studies that seek to use this method. Thus, the objective of this study was to standardize experimental methods with angiotensin II, through vascular reactivity protocols. For this, variables such as basal tension, concentration interval, single concentration, curve concentration response, and multiple experiments using the same aortic ring were developed using the technique of vascular reactivity in an organ bath. This is the first study that has standardized the vascular reactivity protocol. In addition, we demonstrated the effects of TRV023-biased ligand of the AT1R at vascular sites.

Integrated proteomics and metabolomics analysis reveals differential lipid metabolism in human umbilical vein endothelial cells under high and low shear stress

Atherosclerotic plaque development is closely associated with the hemodynamic forces applied to endothelial cells (ECs). Among these, shear stress (SS) plays a key role in disease development since changes in flow intensity and direction could stimulate an atheroprone or atheroprotective phenotype. ECs under low or oscillatory SS (LSS) show upregulation of inflammatory, adhesion, and cellular permeability molecules. On the contrary, cells under high or laminar SS (HSS) increase their expression of protective and anti-inflammatory factors. The mechanism behind SS regulation of an atheroprotective phenotype is not completely elucidated. Here we used proteomics and metabolomics to better understand the changes in endothelial cells (human umbilical vein endothelial cells) under in vitro LSS and HSS that promote an atheroprone or atheroprotective profile and how these modifications can be connected to atherosclerosis development. Our data showed that lipid metabolism, in special cholesterol metabolism, was downregulated in cells under LSS. The low-density lipoprotein receptor (LDLR) showed significant alterations both at the quantitative expression level as well as regarding posttranslational modifications. Under LSS, LDLR was seen at lower concentrations and with a different glycosylation profile. Finally, modulating LDLR with atorvastatin led to the recapitulation of a HSS metabolic phenotype in EC under LSS. Altogether, our data suggest that there is significant modulation of lipid metabolism in endothelial cells under different SS intensities and that this could contribute to the atheroprone phenotype of LSS. Statin treatment was able to partially recover the protective profile of these cells.

Cyclic compressive loading activates angiotensin II type 1 receptor in articular chondrocytes and stimulates hypertrophic differentiation through a G-protein-dependent pathway

Angiotensin II type 1 receptor (AT1R) appears to have a mechanosensing function in a number of cell types. The purpose of this study was to examine whether AT1R expressed in articular chondrocytes is involved in osteoarthritis (OA) progression in vivo and whether cyclic compressive loading activates the AT1R and stimulates hypertrophic differentiation of chondrocytes in vitro. The relationships between the modified Mankin score for cartilage degeneration and the expression of AT1R and type X collagen (Col X) were studied in mouse knees with OA induced using the destabilization-of-medial-meniscus model. Cyclic compressive loads were applied to cultured bovine articular chondrocytes in three-dimensional agarose scaffolds. Expression of Col X and runt-related transcription factor 2 (Runx2) was analyzed using RT-PCR and western blotting. We dissected the downstream pathway for intracellular signal transductions of AT1R including G-protein-dependent and G-protein-independent pathways. Positive significant correlations between the Mankin score and the rate of AT1R-immunopositive cells and between the rates of AT1R and Col X expression were noted. The expression of Col X and Runx2 was increased by compressive loading but suppressed by addition of olmesartan, an Ang II receptor blocker, to the agarose scaffolds. Compressive loading upregulated the phosphorylation of c-Jun N-terminal kinase (JNK), Src, and STAT1, but olmesartan significantly suppressed only JNK phosphorylation. We conclude that AT1R expressed by articular chondrocytes may be involved in OA progression in vivo. Mechanical stress can activate AT1R and stimulate hypertrophic differentiation of chondrocytes through the G-protein-dependent pathway. AT1R has a mechanosensing function in chondrocytes and may be a new therapeutic target in OA.

Cardiac AT(1) receptor-dependent and IGF1 receptor-independent signaling is activated by a single bout of resistance exercise

AT(1) receptor (AT1R) blockade prevents physiological cardiac hypertrophy induced by resistance training. Also, our group showed that a single bout of resistance exercise (RE) activates the AKT/mTOR which was also inhibited by AT1R blocker. Here, we investigated whether IGF1-receptor (IGF1-R) and MAPKs were also activated after a single bout of RE. Wistar rats were divided into Sedentary (Sed), Sedentary treated with losartan (Sed+LOS), Exercise (EX), and Exercise treated with losartan (EX+LOS). Cardiac tissue was obtained 5 and 30 min after 4 sets of 12 repetitions of squat exercise (80 % 1RM). We demonstrated that a single bout of RE did not induce IGF1-R tyrosine phosphorylation. ERK1/2 and P38 phosphorylation levels were elevated in the EX 5min and EX 30min groups however, only ERK1/2 was inhibited by losartan treatment (AT1R blocker). Next, we showed that beta-arrestin-2 expression increased 28 % in trained animals compared to sedentary group. Altogether, our results demonstrate that AT1R, but not IGF1-R, may exert the hypertrophic cardiac stimulus RE-induced. Also, activation of AKT/mTOR and ERK1/2 pathways may occur through the beta-arrestin-dependent pathway.

Cellular diversity of the somatosensory cortical map plasticity

Sensory maps are representations of the sensory epithelia in the brain. Despite the intuitive explanatory power behind sensory maps as being neuronal precursors to sensory perception, and sensory cortical plasticity as a neural correlate of perceptual learning, molecular mechanisms that regulate map plasticity are not well understood. Here we perform a meta-analysis of transcriptional and translational changes during altered whisker use to nominate the major molecular correlates of experience-dependent map plasticity in the barrel cortex. We argue that brain plasticity is a systems level response, involving all cell classes, from neuron and glia to non-neuronal cells including endothelia. Using molecular pathway analysis, we further propose a gene regulatory network that could couple activity dependent changes in neurons to adaptive changes in neurovasculature, and finally we show that transcriptional regulations observed in major brain disorders target genes that are modulated by altered sensory experience. Thus, understanding the molecular mechanisms of experience-dependent plasticity of sensory maps might help to unravel the cellular events that shape brain plasticity in health and disease.

Elevated Wall Tension Initiates Interleukin-6 Expression and Abdominal Aortic Dilation

BACKGROUND

Hypertension (HTN) has long been associated with abdominal aortic aneurysm (AAA) development, and these cardiovascular pathologies are biochemically characterized by elevated plasma levels of angiotensin II (AngII) as well as interleukin-6 (IL-6). A biologic relationship between HTN and AAA has not been established, however. Accordingly, the objective of this study was to evaluate whether elevated tension may initiate IL-6 production to accumulate monocyte/macrophages and promote dilation of the abdominal aorta (AA).

METHODS

An IL-6 infusion model (4.36 μg/kg/day) was created utilizing an osmotic infusion pump, and after 4 weeks, AA diameter was measured by digital microscopy. The AA was then excised for CD68 immunostaining and flow cytometric analysis with CD11b and F4/80 to identify macrophages. Aortic segments from wild-type mice were suspended on parallel wires in an ex vivo tissue myograph at experimentally derived optimal tension (1.2 g) and in the presence of elevated tension (ET, 1.7 g) for 3 hr, and expression of IL-6 and monocyte chemoattractant protein-1 (MCP-1) was evaluated by quantitative polymerase chain reaction (QPCR). Isolated aortic vascular smooth muscle cells (VSMCs) were subjected to 12% biaxial cyclic stretch or held static (control) for 3 hr (n = 7), and IL-6 and MCP-1 expressions were evaluated by QPCR.

RESULTS

Four-week IL-6 infusion resulted in an AA outer diameter that was 72.5 ± 5.6% (P < 0.05) greater than that of control mice, and aortic dilation was accompanied by an accumulation of macrophages in the AA medial layer as defined by an increase in CD68 + staining as well as an increase by flow cytometric quantification of CD11b+/F4/80+ cells. Wild-type AA segments did not respond to ex vivo application of ET but cyclic stretch of isolated VSMCs increased IL-6 (2.03 ± 0.3 fold) and MCP-1 (1.51 ± 0.11 fold) expression compared to static control (P < 0.05). Pretreatment with the selective STAT3 inhibitor WP1066 blunted the response in both cases. Interestingly, AngII did not stimulate expression of IL-6 and MCP-1 above that initiated by tension and again, the response was inhibited by WP1066, supporting an integral role of STAT3 in this pathway.

CONCLUSIONS

An IL-6 infusion model can initiate macrophage accumulation as well as aortic dilation, and under conditions of elevated tension, this proinflammatory cytokine can be produced by aortic VSMCs. By activation of STAT3, MCP-1 is expressed to increase media macrophage abundance and create an environment susceptible to dilation. This biomechanical association between HTN and aortic dilation may allow for the identification of novel therapeutic strategies.

Inhibition of angiotension II type 1 receptor reduced human endothelial inflammation induced by low shear stress

Low shear stress (LSS)-induced endothelial inflammation is the basis for the development of atherosclerosis. However, the mechanism underlying LSS-induced inflammation is not well understood. The angiotensin II type 1 receptor (AT1R), a component of the renin-angiotensin system, participates in atherosclerotic plaque progression. The aim of this study was to investigate the role of AT1R in LSS-induced endothelial activation. Using immunohistochemistry, we noted significant increases in AT1R, vascular endothelial adhesion cell-1 (VCAM1), and intercellular adhesion molecule-1 (ICAM1) expression in the inner curvature of the aortic arch in C57BL/6 mice compared to the descending aorta in these mice. Moreover, western blotting revealed that these LSS-induced increases in AT1R, ICAM1 and VCAM1 expression were time dependent. However, the expression of these proteins was significantly abolished by treatment with the AT1R antagonist Losartan (1μM) or AT1R small interfering RNA (siRNA). AT1R inhibition significantly suppressed extracellular signal-regulated kinase 1/2 (ERK) upregulation, which also resulted in decreases in ICAM1 and VCAM1 protein expression. These findings demonstrate that LSS induces endothelial inflammation via AT1R/ERK signaling and that Losartan has beneficial effects on endothelial inflammation.

Low shear stress induces endothelial reactive oxygen species via the AT1R/eNOS/NO pathway

Reactive oxygen species (ROS) contribute to many aspects of physiological and pathological cardiovascular processes. However, the underlying mechanism of ROS induction by low shear stress (LSS) remains unclear. Accumulating evidence has shown that the angiotensin II type 1 receptor (AT1R) is involved in inflammation, apoptosis, and ROS production. Our aim was to explore the role of AT1R in LSS-mediated ROS induction. We exposed human umbilical vein endothelial cells (HUVECs) to LSS (3 dyn/cm² ) for different periods of time. Western blotting and immunofluorescence showed that LSS significantly induced AT1R expression in a time-dependent manner. Using immunohistochemistry, we also noted a similar increase in AT1R expression in the inner curvature of the aortic arch compared to the descending aorta in C57BL/6 mice. Additionally, HUVECs were cultured with a fluorescent probe, either DCFH, DHE or DAF, after being subjected to LSS. Cell chemiluminescence and flow cytometry results revealed that LSS stimulated ROS levels and suppressed nitric oxide (NO) generation in a time-dependent manner, which was reversed by the AT1R antagonist Losartan. We also found that Losartan markedly increased endothelial NO synthase (eNOS) phosphorylation at Ser(633,1177) and dephosphorylation at Thr(495), which involved AKT and ERK. Moreover, the ROS level was significantly reduced by endogenous and exogenous NO donors (L-arginine, SNP) and increased by the eNOS inhibitor L-NAME. Overall, we conclude that LSS induces ROS via AT1R/eNOS/NO.

LIPUS suppressed LPS-induced IL-1α through the inhibition of NF-κB nuclear translocation via AT1-PLCβ pathway in MC3T3-E1 cells

Inflammatory cytokines, interleukin (IL)-1, IL-6, and TNF-α, are involved in inflammatory bone diseases such as rheumatoid osteoarthritis and periodontal disease. Particularly, periodontal disease, which destroys alveolar bone, is stimulated by lipopolysaccharide (LPS). Low-intensity pulsed ultrasound (LIPUS) is used for bone healing in orthopedics and dental treatments. However, the mechanism underlying effects of LIPUS on LPS-induced inflammatory cytokine are not well understood. We therefore aimed to investigate the role of LIPUS on LPS-induced IL-1α production. Mouse calvaria osteoblast-like cells MC3T3-E1 were incubated in the presence or absence of LPS (Porphyromonas gingivalis), and then stimulated with LIPUS for 30 min/day. To investigate the role of LIPUS, we determined the expression of IL-1α stimulated with LIPUS and treated with an angiotensin II receptor type 1 (AT1) antagonist, Losartan. We also investigate to clarify the pathway of LIPUS, we transfected siRNA silencing AT1 (siAT1) in MC3T3-E1. LIPUS inhibited mRNA and protein expression of LPS-induced IL-1α. LIPUS also reduced the nuclear translocation of NF-κB by LPS-induced IL-1α. Losartan and siAT1 blocked all the stimulatory effects of LIPUS on IL-1α production and IL-1α-mediated NF-κB translocation induced by LPS. Furthermore, PLCβ inhibitor U73122 recovered NF-κB translocation. These results suggest that LIPUS inhibits LPS-induced IL-1α via AT1-PLCβ in osteoblasts. We exhibit that these findings are in part of the signaling pathway of LIPUS on the anti-inflammatory effects of IL-1α expression.

Shear stress induces Gαq/11 activation independently of G protein-coupled receptor activation in endothelial cells

Mechanochemical signal transduction occurs when mechanical forces, such as fluid shear stress, are converted into biochemical responses within the cell. The molecular mechanisms by which endothelial cells (ECs) sense/transduce shear stress into biological signals, including the nature of the mechanosensor, are still unclear. G proteins and G protein-coupled receptors (GPCRs) have been postulated independently to mediate mechanotransduction. In this study, we used in situ proximity ligation assay (PLA) to investigate the role of a specific GPCR/Gα_q/11 pair in EC shear stress-induced mechanotransduction. We demonstrated that sphingosine 1-phosphate (S1P) stimulation causes a rapid dissociation at 0.5 min of Gα_q/11 from its receptor S1P₃, followed by an increased association within 2 min of GPCR kinase-2 (GRK2) and β-arrestin-1/2 with S1P₃ in human coronary artery ECs, which are consistent with GPCR/Gα_q/11 activation and receptor desensitization/internalization. The G protein activator AlF₄ resulted in increased dissociation of Gα_q/11 from S1P₃, but no increase in association between S1P₃ and either GRK2 or β-arrestin-1/2. The G protein inhibitor guanosine 5'-(β-thio) diphosphate (GDP-β-S) and the S1P₃ antagonist VPC23019 both prevented S1P-induced activation. Shear stress also caused the rapid activation within 7 s of S1P₃/Gα_q/11 There were no increased associations between S1P₃ and GRK2 or S1P₃ and β-arrestin-1/2 until 5 min. GDP-β-S, but not VPC23019, prevented dissociation of Gα_q/11 from S1P₃ in response to shear stress. Shear stress did not induce rapid dephosphorylation of β-arrestin-1 or rapid internalization of S1P₃, indicating no GPCR activation. These findings suggest that Gα_q/11 participates in the sensing/transducing of shear stress independently of GPCR activation in ECs.

β-arrestin is critical for early shear stress-induced Akt/eNOS activation in human vascular endothelial cells

Recent evidence suggests that β-arrestins, which are involved in G protein-coupled receptors desensitization, may influence mechanotransduction. Here, we observed that nitric oxide (NO) production was abrogated in human saphenous vein endothelial cells (SVECs) transfected with siRNA against β-arrestin 1 and 2 subjected to shear stress (SS, 15 dynes/cm², 10 min). The downregulation of β-arrestins 1/2 in SVECs cells also prevented the SS-induced rise in levels of phosphorylation of Akt and endothelial nitric oxide synthase (eNOS, Serine 1177). Interestingly, immunoprecipitation revealed that β-arrestin interacts with Akt, eNOS and caveolin-1 and these interactions are not influenced by SS. Our data indicate that β-arrestins and Akt/eNOS downstream signaling are required for early SS-induced NO production in SVECs, which is consistent with the idea that β-arrestins and caveolin-1 are part of a pre-assembled complex associated with the cellular mechanotransduction machinery.

G Protein-coupled Receptor Biased Agonism

G protein-coupled receptors are the largest family of targets for current therapeutics. The classic model of their activation was binary, where agonist binding induced an active conformation and subsequent downstream signaling. Subsequently, the revised concept of biased agonism emerged, where different ligands at the same G protein-coupled receptor selectively activate one downstream pathway versus another. Advances in understanding the mechanism of biased agonism have led to the development of novel ligands, which have the potential for improved therapeutic and safety profiles. In this review, we summarize the theory and most recent breakthroughs in understanding biased signaling, examine recent laboratory investigations concerning biased ligands across different organ systems, and discuss the promising clinical applications of biased agonism.

Ion Channels in Endothelial Responses to Fluid Shear Stress

Fluid shear stress is an important environmental cue that governs vascular physiology and pathology, but the molecular mechanisms that mediate endothelial responses to flow are only partially understood. Gating of ion channels by flow is one mechanism that may underlie many of the known responses. Here, we review the literature on endothelial ion channels whose activity is modulated by flow with an eye toward identifying important questions for future research.

Regulators and effectors of bone morphogenetic protein signalling in the cardiovascular system

Bone morphogenetic proteins (BMPs) play key roles in the regulation of cell proliferation, differentiation and apoptosis in various tissues and organs, including the cardiovascular system. BMPs signal through both Smad-dependent and -independent cascades to exert a wide spectrum of biological activities. Cardiovascular disorders such as abnormal angiogenesis, atherosclerosis, pulmonary hypertension and cardiac hypertrophy have been linked to aberrant BMP signalling. To correct the dysregulated BMP signalling in cardiovascular pathogenesis, it is essential to get a better understanding of how the regulators and effectors of BMP signalling control cardiovascular function and how the dysregulated BMP signalling contributes to cardiovascular dysfunction. We hence highlight several key regulators of BMP signalling such as extracellular regulators of ligands, mechanical forces, microRNAs and small molecule drugs as well as typical BMP effectors like direct downstream target genes, mitogen-activated protein kinases, reactive oxygen species and microRNAs. The insights into these molecular processes will help target both the regulators and important effectors to reverse BMP-associated cardiovascular pathogenesis.

Minireview: Role of intracellular scaffolding proteins in the regulation of endocrine G protein-coupled receptor signaling

The majority of hormones stimulates and mediates their signal transduction via G protein-coupled receptors (GPCRs). The signal is transmitted into the cell due to the association of the GPCRs with heterotrimeric G proteins, which in turn activates an extensive array of signaling pathways to regulate cell physiology. However, GPCRs also function as scaffolds for the recruitment of a variety of cytoplasmic protein-interacting proteins that bind to both the intracellular face and protein interaction motifs encoded by GPCRs. The structural scaffolding of these proteins allows GPCRs to recruit large functional complexes that serve to modulate both G protein-dependent and -independent cellular signaling pathways and modulate GPCR intracellular trafficking. This review focuses on GPCR interacting PSD95-disc large-zona occludens domain containing scaffolds in the regulation of endocrine receptor signaling as well as their potential role as therapeutic targets for the treatment of endocrinopathies.

Allosteric modulation of β-arrestin-biased angiotensin II type 1 receptor signaling by membrane stretch

It has recently been appreciated that the angiotensin II type 1 receptor (AT1R), a prototypic member of the G protein-coupled receptor superfamily, also functions as a mechanosensor. Specifically, mechanical stretch activates the AT1R to promote downstream signaling mediated exclusively by the multifunctional scaffold protein, β-arrestin, in a manner consistent with previously identified β-arrestin-biased ligands. However, the ligand-independent mechanism by which mechanical stretch promotes β-arrestin-biased signaling remains unknown. Implicit in the concept of biased agonism (i.e. the ability of an agonist to activate a subset of receptor-mediated signaling pathways) is the notion that distinct active conformations of the receptor mediate differential activation of signaling pathways. Here we determined whether mechanical stretch stabilizes distinct β-arrestin-activating conformations of the AT1R by using β-arrestin2-biased agonists as conformational probes in pharmacological and biophysical assays. When tested at cells expressing the AT1R fused to β-arrestin (AT1R-β-arrestin2), we found that osmotic stretch increased the binding affinity and potency of the β-arrestin-biased agonist TRV120023, with no effect on the balanced agonist AngII. In addition, the effect of osmotic stretch on ERK activation was markedly augmented in cells expressing the AT1R-β-arrestin2 fusion compared with the wild type AT1R and completely blocked in cells expressing the AT1R-Gq fusion. Biophysical experiments with an intramolecular BRET β-arrestin2 biosensor revealed that osmotic stretch and TRV120023 activate AT1Rs to stabilize β-arrestin2 active conformations that differ from those stabilized by the AT1R activated by angiotensin II. Together, these data support a novel ligand-independent mechanism whereby mechanical stretch allosterically stabilizes specific β-arrestin-biased active conformations of the AT1R and has important implications for understanding pathophysiological AT1R signaling.

Hemodynamic regulation of reactive oxygen species: implications for vascular diseases

SIGNIFICANCE

Arterial blood vessels functionally and structurally adapt to altering hemodynamic forces in order to accommodate changing needs and to provide stress homeostasis. This ability is achieved at the cellular level by converting mechanical stimulation into biochemical signals (i.e., mechanotransduction). Physiological mechanical stress helps maintain vascular structure and function, whereas pathologic or aberrant stress may impair cellular mechano-signaling, and initiate or augment cellular processes that drive disease.

RECENT ADVANCES

Reactive oxygen species (ROS) may represent an intriguing class of mechanically regulated second messengers. Chronically enhanced ROS generation may be induced by adverse mechanical stresses, and is associated with a multitude of vascular diseases. Although a causal relationship has clearly been demonstrated in large numbers of animal studies, an effective ROS-modulating therapy still remains to be established by clinical studies.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This review article focuses on the role of various mechanical forces (in the form of laminar shear stress, oscillatory shear stress, or cyclic stretch) as modulators of ROS-driven signaling, and their subsequent effects on vascular biology and homeostasis, as well as on specific diseases such as arteriosclerosis, hypertension, and abdominal aortic aneurysms. Specifically, it highlights the significance of the various NADPH oxidase (NOX) isoforms as critical ROS generators in the vasculature. Directed targeting of defined components in the complex network of ROS (mechano-)signaling may represent a key for successful translation of experimental findings into clinical practice.

Activating types 1 and 2 angiotensin II receptors modulate the hypertrophic differentiation of chondrocytes

A local tissue-specific renin–angiotensin system (local RAS) has been identified in many organs. However, no report has described the role of a local RAS in the hypertrophic differentiation of chondrocytes. To examine the role of a local RAS in the hypertrophic differentiation, we activated angiotensin II type 1 receptor (AT1R) and angiotensin II type 2 receptor (AT2R) separately in the cell line ATDC5, which involves differentiation from mesenchymal stem cells to hypertrophic chondrocytes. Activation of AT1R suppressed and activation of AT2R enhanced the expression of markers of hypertrophic differentiation, including type X collagen, matrix metalloproteinase 13 and runt-related transcription factor 2.

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Renin–angiotensin system components are expressed during hypertrophic differentiation.

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AT1R is expressed intensely during chondrocyte proliferation.

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AT2R is expressed intensely in the hypertrophic phase.

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The expression of Col.X, MMP13 and Runx2 are repressed by activating AT1R.

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The expression of Col.X, MMP13 and Runx2 are enhanced by activating AT2R.