Rab1 GTPase regulates phenotypic modulation of pulmonary artery smooth muscle cells by mediating the transport of angiotensin II type 1 receptor under hypoxia

Fuyuan Xingnao Decoction Promotes Angiogenesis Through the Rab1/AT1R Pathway in Diabetes Mellitus Complicated With Cerebral Infarction

Fuyuan Xingnao decoction (FYXN), a traditional Chinese formula comprised of seven herbs, has been utilized to treat diabetes mellitus complicated with cerebral infarction (DMCI) for years. Yet, its protective and regulatory mechanism is poorly understood. The aim of the study is to investigate the effects of FYXN on DMCI in vitro and in vivo, as well as its mechanism in angiogenesis. For in vivo experiments, FYXN was administered to DMCI rats with streptozotocin (STZ) injection-induced diabetes. Then middle cerebral artery occlusion (MCAO) was conducted and the cerebral cortex sections of the rats were obtained. The ultrastructure of cerebral microvessels and new vessel density of ischemic penumbra were evaluated by the transmission electron microscopy (TEM) assay and immunohistochemistry, respectively. Protein and mRNA expression levels of Rab1/AT1R in cortex were assayed by Western blotting and real-time fluorescence quantitative real-time polymerase chain reaction (RT-qPCR). In vitro, FYXN serum was produced in rats on the fourth day 2 h after the last FYXN administration. Green fluorescence was observed after transfection with lentivirus packaged Rab1-WT or siRNA for 24 h. The activity of brain microvascular endothelial cells (BMECs) treated with sera from these rats was tested by MTT assay and Transwell assays, respectively. The expression of AT1R on the cell membrane and endoplasmic reticulum of BMECs was evaluated by immunofluorescence staining. Protein expression levels of signaling molecules in the Rab1/AT1R pathways were also detected. Results showed that in vivo, FYXN treatment significantly intensified CD31 staining in the cortical areas and enhanced the mRNA and protein levels of AT1R, Ang II, Rab1a, Rab1b and VEGF expression in ischemic cerebral cortex tissues. In vitro, the expression levels of AT1R, Ang II, Rab1a, Rab1b and VEGF in the cerebral infarction model group were significantly higher than those in the control group, with further increases after administration of FYXN drug serum. FYXN promoted the proliferation and migration of BMECs by activating the Rab1/AT1R signaling pathway. In conclusion, FYXN exerts a protective effect against DMCI by promoting angiogenesis via the Rab1/AT1R pathway, which provides strong evidence for the therapeutic effect of FYXN on DMCI.

Rab6A GTPase contributes to phenotypic modulation in pulmonary artery smooth muscle cells under hypoxia

Previous studies have demonstrated that hypoxia can induce phenotypic modulation of pulmonary smooth muscle cells; however, the mechanisms remain unclear. The present study aimed to investigate the effect of the GTPase Rab6A-mediated phenotypic modulation and other activities of rat pulmonary artery smooth muscle cells (RPASMCs). We revealed that Rab6A was induced by hypoxia (1% O₂ ) and was involved in a hypoxia-induced phenotypic switch and endoplasmic reticulum stress (ERS) in RPASMCs. After 48 hours of hypoxia, the expression of the phenotype marker protein smooth muscle actin was downregulated and vimentin (VIM) expression was upregulated. Rab6A was upregulated after 48 hours of hypoxia, and the level of glucose-regulated protein, 78 kDa (GRP78) after 12 hours of hypoxic stimulation was also increased. After transfection with a Rab6A short interfering RNA under hypoxic conditions, the expression levels of GRP78 and VIM in RPASMCs were downregulated. Overall, hypoxia-induced RPASMCs to undergo ERS followed by phenotypic transformation. Rab6A is involved in this hypoxia-induced phenotypic modulation and ERS in RPASMCs.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

MicroRNA-140-5p targeting tumor necrosis factor-α prevents pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is characterized by the apoptosis resistance and hyperproliferation of pulmonary artery smooth muscle cells (PASMCs). Its pathogenesis has not been revealed. Here, we carried out experiments to investigate the functions of miR-140-5p and tumor necrosis factor-α (TNF-α).

METHODS

We selected GSE703 from Gene Expression Omnibus (GEO) Database to conduct microarray analysis using R software and Gene Set Enrichment Analysis (GSEA). Combing bioinformatics results, the upregulation of miR-140-5p inhibited PAH progression through targeting TNF-α. RNA expression was measured by quantitative real-time polymerase chain reaction (RT-qPCR) and protein level was measured by western blot analysis and enzyme-linked immunosorbent assays (ELISA). We conducted monocrotaline (MCT) injection to rats to form PAH animal models. The lung tissues were observed by hematoxylin-eosin (HE) staining and Sirius red-picric acid staining. Right ventricular systolic pressure (RVSP) and the ratio of right ventricle (RV)-to-left ventricle (LV) plus septum (S) weight (RV/[LV + S]) were measured in MCT-induced animal models. Overexpression of miR-140-5p and TNF-α were utilized to research the proliferation, migration, and phenotypic variation of hypoxia-mediated PASMCs. The binding between miR-140-5p and TNF-α 3'-untranslated region (3'-UTR) was confirmed via luciferase reporter assay.

RESULTS

Downregulation of miR-140-5p and upregulation of TNF-α were observed in PAH rat model and hypoxia-mediated PASMCs. And we proved that overexpression of miR-140-5p could suppress the proliferation, migration, and phenotypic variation of PASMCs, therefore inhibiting PAH pathogenesis. Luciferase assay verified that miR-140-5p targeted TNF-α directly. A converse correlation was also shown between miR-140-5p and TNF-α in PASMCs.

CONCLUSIONS

miR-140-5p and TNF-α are important regulators in PAH pathology and may serve as a therapeutic target for PAH.

EndophilinA2 protects against angiotensin II-induced cardiac hypertrophy by inhibiting angiotensin II type 1 receptor trafficking in neonatal rat cardiomyocytes

Cardiac hypertrophy is one of the major risk factors for chronic heart failure. The role of endophilinA2 (EndoA2) in clathrin-mediated endocytosis and clathrin-independent endocytosis is well documented. In the present study, we tested the hypothesis that EndoA2 protects against angiotensin II (Ang II)-induced cardiac hypertrophy by mediating intracellular angiotensin II type 1 receptor (AT1-R) trafficking in neonatal rat cardiomyocytes (NRCMs). Cardiac hypertrophy was evaluated by using cell surface area and quantitative RT-PCR (qPCR) analyses. For the first time, we found that EndoA2 attenuated cardiac hypertrophy and fibrosis induced by Ang II. Moreover, EndoA2 inhibited apoptosis induced by excessive endoplasmic reticulum stress (ERS), which accounted for the beneficial effects of EndoA2 on cardiac hypertrophy. We further revealed that there was an interaction between EndoA2 and AT1-R.The expression levels of EndoA2, which inhibits AT1-R transport from the cytoplasm to the membrane, and the interaction between EndoA2 and AT1-R were obviously decreased after Ang II treatment. Furthermore, Ang II inhibited the co-localization of AT1-R with GRP-78, which was reversed by EndoA2 overexpression. In conclusion, our results suggested that EndoA2 plays a role in protecting against cardiac hypertrophy induced by Ang II, possibly by inhibiting AT1-R transport from the cytoplasm to the membrane to suppress signal transduction.

Role of Rab GTPases in the export trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) constitute a superfamily of cell surface receptors that regulate a variety of cell functions. As the cell surface is the functional destination for most GPCRs, the cell surface targeting process represents a crucial checkpoint in controlling the functionality of the receptors. However, the molecular mechanisms underlying the cell surface delivery of newly synthesized GPCRs remain poorly understood. In this review, we will highlight the role of Rab GTPases in GPCR cell surface transport, particularly post-Golgi traffic, and discuss the underlying molecular mechanisms.

Regulation on Toll-like Receptor 4 and Cell Barrier Function by Rab26 siRNA-loaded DNA Nanovector in Pulmonary Microvascular Endothelial Cells

The small GTPase Rab26 is involved in multiple processes, such as vesicle-mediated secretion and autophagy. However, the mechanisms and functions of Rab26 in the human pulmonary microvascular endothelial cells (HPMVECs) are not clear. In this study, we thoroughly investigated the role and novel mechanism of Rab26 in permeability and apoptosis of HPMVECs using a self-assembled Rab26 siRNA loaded DNA Y-motif nanoparticle (siRab26-DYM) and Rab26 adenovirus. We found that siRab26-DYM could be efficiently transfected into HPMVECs in a time- and dose-dependent manner. Importantly, the siRab26-DYM nanovector markedly aggravated the LPS-induced apoptosis and hyper-permeability of HPMVECs by promoting the nuclear translocation of Foxo1, and subsequent activation of Toll-like receptor 4 (TLR4) signal pathway. Overexpression of Rab26 by Rab26 adenoviruses partially inactivated LPS-induced TLR4 signaling pathway, suppressed the cell apoptosis and attenuated the hyperpermeability of HPMVECs. These results suggest that the permeability and apoptosis of HPMVECs can be modulated by manipulating Rab26 derived TLR4 signaling pathway, and that Rab26 can be potential therapeutic target for the treatment of vascular diseases related to endothelial barrier functions.

miR-26b-5p regulates hypoxia-induced phenotypic switching of vascular smooth muscle cells via the TGF-β/Smad4 signaling pathway

Hypoxia contributes to the phenotypic switch of vascular smooth muscle cells (VSMCs). Various microRNAs (miRNAs) participate in this process as post‑transcriptional regulators, however the mechanism remains unclear. In the present study, mouse VSMCs (mVSMCs) harvested from aortas were cultured in normoxic and hypoxic conditions, and the mRNA levels of miR-26b-5p, desmin, H‑caldesmon and smoothelin were quantified using reverse transcription‑quantitative polymerase chain reaction. Following treatment with a miR‑26b‑5p antagonist (agomir) or non‑targeting control (scramble), the cell areas of normoxic and hypoxic mVSMCs were analyzed by immunofluorescence staining. In addition, the protein expression levels of collagen Iα, Smad2/phosphorylated (p)‑Smad2, Smad3/p‑Smad3 and Smad4 were determined by western blotting. Potential miRNA26b‑5p binding sequences in the 3'‑untranslated region (UTR) of Smad4 were investigated, and the distribution of Smad4 in mVSMCs was visualized using immunofluorescence methods. Hypoxic mVSMCs exhibited a significant downregulation miR‑26b‑5p, upregulation of hypoxia inducible factor‑1α mRNA and suppression of desmin, H‑caldesmon and smoothelin mRNA levels. Additionally, miR‑26b‑5p agomir reduced the cell area and decreased collagen Iα expression levels in hypoxic mVSMCs compared with normoxic mVSMCs transfected with agomir, and the area was comparable with those of normoxic mVSMCs transfected with agomir or scramble. Furthermore, miR‑26b‑5p suppressed Smad4 expression in hypoxic mVSMCs, but did not change the expression levels of Smad2 and Smad3, p‑Smad2 and p‑Smad3, however p‑Smad2 and p‑Smad3 levels were upregulated in response to hypoxic stimuli. Additionally, the miR‑26b‑5p agomir caused weak immunoreactivity with Smad4 in hypoxic mVSMCs. The binding motif of miR‑26b‑5p in the Smad4 3'‑UTR was identified as UACUUGA at position 978-984. These findings suggest that miR‑26b‑5p regulates hypoxia‑induced phenotypic switching of VSMCs via the transforming growth factor β/Smad4 signaling pathway.

Regulation of α2B-Adrenergic Receptor Cell Surface Transport by GGA1 and GGA2

The molecular mechanisms that control the targeting of newly synthesized G protein-coupled receptors (GPCRs) to the functional destinations remain poorly elucidated. Here, we have determined the role of Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding proteins 1 and 2 (GGA1 and GGA2) in the cell surface transport of α_2B-adrenergic receptor (α_2B-AR), a prototypic GPCR, and studied the underlying mechanisms. We demonstrated that knockdown of GGA1 and GGA2 by shRNA and siRNA significantly reduced the cell surface expression of inducibly expressed α_2B-AR and arrested the receptor in the perinuclear region. Knockdown of each GGA markedly inhibited the dendritic expression of α_2B-AR in primary cortical neurons. Consistently, depleting GGA1 and GGA2 attenuated receptor-mediated signal transduction measured as ERK1/2 activation and cAMP inhibition. Although full length α_2B-AR associated with GGA2 but not GGA1, its third intracellular loop was found to directly interact with both GGA1 and GGA2. More interestingly, further mapping of interaction domains showed that the GGA1 hinge region and the GGA2 GAE domain bound to multiple subdomains of the loop. These studies have identified an important function and revealed novel mechanisms of the GGA family proteins in the forward trafficking of a cell surface GPCR.

A single mutation in helix 8 enhances the angiotensin II type 1a receptor transport and signaling

The amphipathic helix 8 in the membrane-proximal C-terminus is a structurally conserved feature of class A seven transmembrane-spanning G protein-coupled receptors (GPCRs). Mutations of this helical motif often cause receptor misfolding, defective cell surface transport and dysfunction. Surprisingly, we demonstrated here that a single point mutation at Lys308 in helix 8 markedly enhanced the steady-state surface density of the angiotensin II type 1a receptor (AT1aR). Consistent with the enhanced cell surface expression, Lys308 mutation significantly augmented AT1aR-mediated mitogen-activated protein kinase ERK1/2 activation, inositol phosphate production, and vascular smooth muscle cell migration. This mutation also increased the overall expression of AT1aR without altering receptor degradation. More interestingly, Lys308 mutation abolished AT1aR interaction with β-COP, a component of COPI transport vesicles, and impaired AT1aR responsiveness to the inhibition of Rab6 GTPase involved in the Golgi-to-ER retrograde pathway. Furthermore, these functions of Lys308 were largely dependent on its positively charged property. These data reveal previously unappreciated functions of helix 8 and novel mechanisms governing the cell surface transport and function of AT1aR.

Diversification of the duplicated Rab1a genes in a hypoxia-tolerant fish, common carp (Cyprinus carpio)

Common carp is a widely cultivated fish with longer than 2,000 years domestication history, due to its strong environmental adaptabilities, especially hypoxia tolerance. The common carp genome has experienced a very recent whole genome duplication (WGD) event. Among a large number of highly similar duplicated genes, a pair of Ras-associated binding-GTPase 1a (Rab1a) genes were found fast diverging. Four analogous Rab1a genes were identified in the common carp genome. Comparisons of gene structures and sequences indicated Rab1a-1 and Rab1a-2 was a pair of fast diverging duplicates, while Rab1a-3 and Rab1a-4 was a pair of less diverged duplicates. All putative Rab1a proteins shared conserved GTPase domain, which enabled the proteins serve as molecular switches for vesicular trafficking. Rab1a-1 and Rab1a-2 proteins varied in their C-terminal sequences, which were generally considered to encode the membrane localization signals. Differential expression patterns were observed between Rab1a-1 and Rab1a-2 genes. In blood, muscle, spleen, and heart, the mRNA level of Rab1a-1 was higher than that of Rab1a-2. In liver and intestine, the mRNA level of Rab1a-2 was higher. Expression of Rab1a-1 and Rab1a-2 showed distinct hypoxia responses. Under severe hypoxia, Rab1a-1 expression was down-regulated in blood, while Rab1a-2 expression was up-regulated in liver. Compared with the less diverged Rab1a-3/4 gene pair, common carp Rab1a-1/2 gene pair exhibited strong characteristics of sub-functionalization, which might contribute to a sophisticated and efficient Ras-dependent regulating network for the hypoxia-tolerant fish.

Regulation of α2B-Adrenerigc Receptor Export Trafficking by Specific Motifs

Intracellular trafficking and precise targeting to specific locations of G protein-coupled receptors (GPCRs) control the physiological functions of the receptors. Compared to the extensive efforts dedicated to understanding the events involved in the endocytic and recycling pathways, the molecular mechanisms underlying the transport of the GPCR superfamily from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane are relatively less well defined. Over the past years, we have used α(2B)-adrenergic receptor (α(2B)-AR) as a model to define the factors that control GPCR export trafficking. In this chapter, we will review specific motifs identified to mediate the export of nascent α(2B)-AR from the ER and the Golgi and discuss the possible underlying mechanisms. As these motifs are highly conserved among GPCRs, they may provide common mechanisms for export trafficking of these receptors.

Inhibition of hypoxia-induced proliferation of pulmonary arterial smooth muscle cells by a mTOR siRNA-loaded cyclodextrin nanovector

The proliferation of pulmonary arterial smooth muscle cells (PASMCs) is a key pathophysiological component of vascular remodeling in pulmonary arterial hypertension (PAH), an intractable disease, for which pharmacotherapy is limited and only slight improvement in survival outcomes have achieved over the past few decades. RNA interference provides a highly promising strategy to the treatment of this chronic lung disease, while efficient delivery of small interfering RNA (siRNA) remains a key challenge for the development of clinically acceptable siRNA therapeutics. With the aim to construct useful nanomedicines, the mammalian target of rapamycin (mTOR) siRNA was loaded into hybrid nanoparticles based on low molecular weight (Mw) polyethylenimine (PEI) and a pH-responsive cyclodextrin material (Ac-aCD) or poly(lactic-co-glycolic acid) (PLGA). This hybrid nanoplatform gave rise to desirable siRNA loading, and the payload release could be modulated by the hydrolysis characteristics of carrier materials. Fluorescence observation and flow cytometry quantification suggested that both Ac-aCD and PLGA nanovectors (NVs) may enter PASMCs under either normoxia or hypoxia conditions as well as in the presence of serum, with uptake and transfection efficiency significantly higher than those of cationic vectors such as PEI with Mw of 25 kDa (PEI25k) and Lipofectamine 2000 (Lipo 2k). Hybrid Ac-aCD or PLGA NV containing siRNA remarkably inhibited proliferation and activated apoptosis of hypoxic PASMCs, largely resulting from effective suppression of mTOR signaling as evidenced by significantly lowered expression of mTOR mRNA and phosphorylated protein. Moreover, these hybrid nanomedicines were more effective than commonly used cationic vectors like PEI25k and Lipo 2k, with respect to cell growth inhibition, apoptosis activation, and expression attenuation of mTOR mRNA and protein. Therefore, mTOR siRNA nanomedicines based on hybrid Ac-aCD or PLGA NV may be promising therapeutics for diseases related to hypoxic abnormal growth of PASMCs.

The angiotensin II type 1 receptor C-terminal Lys residues interact with tubulin and modulate receptor export trafficking

The physiological and pathological functions of angiotensin II are largely mediated through activating the cell surface angiotensin II type 1 receptor (AT1R). However, the molecular mechanisms underlying the transport of newly synthesized AT1R from the endoplasmic reticulum (ER) to the cell surface remain poorly defined. Here we demonstrated that the C-terminus (CT) of AT1R directly and strongly bound to tubulin and the binding domains were mapped to two consecutive Lys residues at positions 310 and 311 in the CT membrane-proximal region of AT1R and the acidic CT of tubulin, suggestive of essentially ionic interactions between AT1R and tubulin. Furthermore, mutation to disrupt tubulin binding dramatically inhibited the cell surface expression of AT1R, arrested AT1R in the ER, and attenuated AT1R-mediated signaling measured as ERK1/2 activation. These data demonstrate for the first time that specific Lys residues in the CT juxtamembrane region regulate the processing of AT1R through interacting with tubulin. These data also suggest an important role of the microtubule network in the cell surface transport of AT1R.

Contribution of increased VEGF receptors to hypoxic changes in fetal ovine carotid artery contractile proteins

Recent studies suggest that vascular endothelial growth factor (VEGF) can modulate smooth muscle phenotype and, consequently, the composition and function of arteries upstream from the microcirculation, where angiogenesis occurs. Given that hypoxia potently induces VEGF, the present study explores the hypothesis that, in fetal arteries, VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins. Pregnant ewes were acclimatized at sea level or at altitude (3,820 m) for the final 110 days of gestation. Endothelium-denuded carotid arteries from full-term fetuses were used fresh or after 24 h of organ culture in a physiological concentration (3 ng/ml) of VEGF. After 110 days, hypoxia had no effect on VEGF abundance but markedly increased abundance of the Flk-1 (171%) and Flt-1 (786%) VEGF receptors. Hypoxia had no effect on smooth muscle α-actin (SMαA), decreased myosin light chain (MLC) kinase (MLCK), and increased 20-kDa regulatory MLC (MLC(20)) abundances. Hypoxia also increased MLCK-SMαA, MLC(20)-SMαA, and MLCK-MLC(20) colocalization. Compared with hypoxia, organ culture with VEGF produced the same pattern of changes in contractile protein abundance and colocalization. Effects of VEGF on colocalization were blocked by the VEGF receptor antagonists vatalanib (240 nM) and dasatinib (6.3 nM). Thus, through increases in VEGF receptor density, hypoxia can recruit VEGF to help mediate remodeling of fetal arteries upstream from the microcirculation. The results support the hypothesis that VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins.

Structural insights into a unique Legionella pneumophila effector LidA recognizing both GDP and GTP bound Rab1 in their active state

The intracellular pathogen Legionella pneumophila hijacks the endoplasmic reticulum (ER)-derived vesicles to create an organelle designated Legionella-containing vacuole (LCV) required for bacterial replication. Maturation of the LCV involved acquisition of Rab1, which is mediated by the bacterial effector protein SidM/DrrA. SidM/DrrA is a bifunctional enzyme having the activity of both Rab1-specific GDP dissociation inhibitor (GDI) displacement factor (GDF) and guanine nucleotide exchange factor (GEF). LidA, another Rab1-interacting bacterial effector protein, was reported to promote SidM/DrrA-mediated recruitment of Rab1 to the LCV as well. Here we report the crystal structures of LidA complexes with GDP- and GTP-bound Rab1 respectively. Structural comparison revealed that GDP-Rab1 bound by LidA exhibits an active and nearly identical conformation with that of GTP-Rab1, suggesting that LidA can disrupt the switch function of Rab1 and render it persistently active. As with GTP, LidA maintains GDP-Rab1 in the active conformation through interaction with its two conserved switch regions. Consistent with the structural observations, biochemical assays showed that LidA binds to GDP- and GTP-Rab1 equally well with an affinity approximately 7.5 nM. We propose that the tight interaction with Rab1 allows LidA to facilitate SidM/DrrA-catalyzed release of Rab1 from GDIs. Taken together, our results support a unique mechanism by which a bacterial effector protein regulates Rab1 recycling.

Small GTPase regulation of GPCR anterograde trafficking

The physiological functions of heterotrimeric G protein-coupled receptors (GPCRs) are dictated by their intracellular trafficking and precise targeting to the functional destinations. Over the past decades, most studies on the trafficking of GPCRs have focused on the events involved in endocytosis and recycling. By contrast, the molecular mechanisms underlying anterograde transport of newly synthesized GPCRs from the endoplasmic reticulum (ER) to the cell surface have only now begun to be revealed. In this review we discuss current advances in understanding the role of Ras-like GTPases, specifically the Rab and Sar1/ARF subfamilies, in regulating cell-surface transport of GPCRs en route from the ER and the Golgi.