Scaffolds direct Src-specific signaling in response to angiotensin II: new roles for Cas and GIT1

β-PIX cooperates with GIT1 to regulate endothelial nitric oxide synthase in sinusoidal endothelial cells

Previous studies have demonstrated that G protein-coupled receptor kinase interacting-1 protein (GIT1) associates with endothelial nitric oxide synthase (eNOS) to regulate nitric oxide production in sinusoidal endothelial cells (SECs). Here, we hypothesized that GIT1's tightly associated binding partner, β-PIX (p21-activated kinase-interacting exchange factor β, ARHGEF7) is specifically important in the regulation of eNOS activity. We examined β-PIX expression in normal rat liver by immunohistochemistry and explored β-PIX protein-protein interactions using immunoprecipitation and immunoblotting. The role of β-PIX in regulating eNOS enzymatic activity was studied in GIT1-deficient SECs. Finally, structural analysis of interaction sites in GIT1 and β-PIX required to regulate eNOS activity were mapped. β-PIX was expressed primarily in SECs in normal liver and was either absent or expressed at extremely low levels in other liver cells (stellate cells, Kupffer cells, and hepatocytes). β-PIX interacted with GIT1 and eNOS to form a trimolecular signaling module in normal SECs and was important in stimulating eNOS activity. Of note, GIT1-β-PIX interaction led to synergistic enhancement of eNOS activity, and β-PIX-driven increase in eNOS activity was GIT1 dependent. Disruption of β-PIX or GIT1 in normal SECs using β-PIX siRNA or GIT1-deficient SECs led to reduced eNOS activity. Finally, specific GIT1 domains [Spa2 homology domain (SHD) and synaptic localization domain (SLD), aa 331-596] and the β-PIX COOH terminal (aa 496-555) appeared to be critical in the regulation eNOS activity. The data indicate that β-PIX regulates eNOS phosphorylation and function in normal SECs and highlight the importance of the GIT1/β-PIX/eNOS trimolecular complex in normal liver SEC function.NEW & NOTEWORTHY β-PIX is a multidomain protein known to be a GIT1 binding partner. We report here that in the normal liver, the distribution and cellular localization of β-PIX are restricted largely to sinusoidal endothelial cells. Furthermore, β-PIX interacts with eNOS and GIT1 promotes eNOS activity and NO production and therefore exerts a novel posttranslational regulatory function on eNOS activity in sinusoidal endothelial cells. We also have identified specific molecular domains important in GIT1 and β-PIX's interaction with eNOS, which may represent novel therapeutic targets in the control of sinusoidal blood flow and intrahepatic resistance.

Extracellular matrix and its therapeutic potential for cancer treatment

The extracellular matrix (ECM) is one of the major components of tumors that plays multiple crucial roles, including mechanical support, modulation of the microenvironment, and a source of signaling molecules. The quantity and cross-linking status of ECM components are major factors determining tissue stiffness. During tumorigenesis, the interplay between cancer cells and the tumor microenvironment (TME) often results in the stiffness of the ECM, leading to aberrant mechanotransduction and further malignant transformation. Therefore, a comprehensive understanding of ECM dysregulation in the TME would contribute to the discovery of promising therapeutic targets for cancer treatment. Herein, we summarized the knowledge concerning the following: (1) major ECM constituents and their functions in both normal and malignant conditions; (2) the interplay between cancer cells and the ECM in the TME; (3) key receptors for mechanotransduction and their alteration during carcinogenesis; and (4) the current therapeutic strategies targeting aberrant ECM for cancer treatment.

Evaluating the benefits of renin-angiotensin system inhibitors as cancer treatments

G-protein-coupled receptors (GPCRs) are the largest and most diverse group of cellular membrane receptors identified and characterized. It is estimated that 30 to 50% of marketed drugs target these receptors. The angiotensin II receptor type 1 (AT1R) is a GPCR which signals in response to systemic alterations of the peptide hormone angiotensin II (AngII) in circulation. The enzyme responsible for converting AngI to AngII is the angiotensin-converting enzyme (ACE). Specific inhibitors for the AT1R (more commonly known as AT1R blockers or antagonists) and ACE are well characterized for their effects on the cardiovascular system. Combined with the extensive clinical data available on patient tolerance of AT1R blockers (ARBs) and ACE inhibitors (ACEIs), as well as their non-classical roles in cancer, the notion of repurposing this class of medications as cancer treatment(s) is explored in the current review. Given that AngII-dependent AT1R activity directly regulates angiogenesis, remodeling of vasculature, pro-inflammatory responses, stem cell programming and hematopoiesis, and electrolyte balance; the modulation of these processes with pharmacologically well characterized medications could present a valuable complementary treatment option for cancer patients.

GIT1 is critical for formation of the CD31hiEmcnhi vessel subtype in coupling osteogenesis with angiogenesis via modulating preosteoclasts secretion of PDGF-BB

G protein-coupled receptor kinase 2 interacting protein-1 (GIT1) is a scaffold protein that plays a vital role in bone modeling and remodeling during osteogenesis coupled with angiogenesis. Recent studies have shown that a specialized subset of vascular endothelium strongly positive for CD31 and Endomucin (CD31^hiEmcn^hi) is coupled with anabolic bone formation. Based on our previous finding that GIT1 knockout (GIT1 KO) mice have impaired angiogenesis and bone formation, we hypothesized that GIT1 affects formation of the CD31^hiEmcn^hi vessel subtype. In the current study, GIT1 knockout (GIT1 KO) mice displayed a significant decrease in trabecular bone mass and CD31^hiEmcn^hi vessel number, compared to their wild-type counterparts. In the fracture healing mouse model, GIT1 KO mice contained a lower number of CD31^hiEmcn^hi vessels in fracture callus at days 7 and 14. However, no significant differences in the number of preosteoclasts in bone marrow, trabecular bone and callus in GIT1 KO mice were observed, compared with wild-type mice. Notably, concentrations of serum platelet-derived growth factor-BB(PDGF-BB) secreted by preosteoclasts associated with CD31^hiEmcn^hi vessel formation were lower in GIT1 KO mice. In addition, PDGF-BB-associated expression of phosphorylated extracellular signal-regulated kinase- 1/2 (ERK1/2) and specificity protein 1 (SP1) was significantly decreased in preosteoclasts of GIT1 KO mice. These results collectively suggest that GIT1 is a critical participant in formation of the CD31^hiEmcn^hi vessel subtype, highlighting a novel biologic function of this scaffold protein in preosteoclasts.

Angiotensin II receptors and peritoneal dialysis-induced peritoneal fibrosis

The vasoactive hormone angiotensin II initiates its major hemodynamic effects through interaction with AT1 receptors, a member of the class of G protein-coupled receptors. Acting through its AT1R, angiotensin II regulates blood pressure and renal salt and water balance. Recent evidence points to additional pathological influences of activation of AT1R, in particular inflammation, fibrosis and atherosclerosis. The transcription factor nuclear factor κB, a key mediator in inflammation and atherosclerosis, can be activated by angiotensin II through a mechanism that may involve arrestin-dependent AT1 receptor internalization. Peritoneal dialysis is a therapeutic modality for treating patients with end-stage kidney disease. The effectiveness of peritoneal dialysis at removing waste from the circulation is compromised over time as a consequence of peritoneal dialysis-induced peritoneal fibrosis. The non-physiological dialysis solution used in peritoneal dialysis, i.e. highly concentrated, hyperosmotic glucose, acidic pH as well as large volumes infused into the peritoneal cavity, contributes to the development of fibrosis. Numerous trials have been conducted altering certain components of the peritoneal dialysis fluid in hopes of preventing or delaying the fibrotic response with limited success. We hypothesize that structural activation of AT1R by hyperosmotic peritoneal dialysis fluid activates the internalization process and subsequent signaling through the transcription factor nuclear factor κB, resulting in the generation of pro-fibrotic/pro-inflammatory mediators producing peritoneal fibrosis.

Impaired angiogenesis during fracture healing in GPCR kinase 2 interacting protein-1 (GIT1) knock out mice

G protein coupled receptor kinase 2 (GRK2) interacting protein-1 (GIT1), is a scaffold protein that plays an important role in angiogenesis and osteoclast activity. We have previously demonstrated that GIT1 knockout (GIT1 KO) mice have impaired angiogenesis and dysregulated osteoclast podosome formation leading to a reduction in the bone resorbing ability of these cells. Since both angiogenesis and osteoclast-mediated bone remodeling are involved in the fracture healing process, we hypothesized that GIT1 participates in the normal progression of repair following bone injury. In the present study, comparison of fracture healing in wild type (WT) and GIT1 KO mice revealed altered healing in mice with loss of GIT1 function. Alcian blue staining of fracture callus indicated a persistence of cartilagenous matrix in day 21 callus samples from GIT1 KO mice which was temporally correlated with increased type 2 collagen immunostaining. GIT1 KO mice also showed a decrease in chondrocyte proliferation and apoptosis at days 7 and 14, as determined by PCNA and TUNEL staining. Vascular microcomputed tomography analysis of callus samples at days 7, 14 and 21 revealed decreased blood vessel volume, number, and connection density in GIT1 KO mice compared to WT controls. Correlating with this, VEGF-A, phospho-VEGFR2 and PECAM1 (CD31) were decreased in GIT1 KO mice, indicating reduced angiogenesis with loss of GIT1. Finally, calluses from GIT1 KO mice displayed a reduced number of tartrate resistant acid phosphatase-positive osteoclasts at days 14 and 21. Collectively, these results indicate that GIT1 is an important signaling participant in fracture healing, with gene ablation leading to reduced callus vascularity and reduced osteoclast number in the healing callus.

Phosphorylation of G protein-coupled receptor kinase 2-interacting protein 1 tyrosine 392 is required for phospholipase C-gamma activation and podosome formation in vascular smooth muscle cells

OBJECTIVE

Podosomes, which are actin-rich structures, contribute to cell motility, matrix remodeling, and tissue remodeling. We have shown that G protein-coupled receptor kinase 2-interacting protein 1 (GIT1) colocalizes with podosomes and is important in podosome formation in endothelial cells. Src stimulates GIT1 tyrosine phosphorylation, which is critical for phospholipase C-γ (PLCγ) activation. In this study, we identified specific GIT1 tyrosines required for PLCγ activation and podosome formation in vascular smooth muscle cells (VSMC).

METHODS AND RESULTS

We used phorbol 12,13-dibutyrate (PDBU) to induce podosomes in A7r5 VSMC. GIT1 colocalized with podosomes and GIT1 knockdown using short interfering RNA significantly reduced podosome formation. PDBU stimulated GIT1 tyrosine phosphorylation. GIT1 tyrosine phosphorylation was dramatically decreased in SYF-/- cells, and it was also reduced by pretreatment with the protein kinase C (PKC) and Src inhibitors, suggesting that GIT1 phosphorylation was dependent on PKC and Src. By mutation analysis of multiple tyrosines, we found that PDBU specifically increased GIT1-Y392 phosphorylation. Overexpression of GIT1 (Y392F) but not of GIT1 (Y321F) decreased PDBU-mediated PLCγ activation and podosome formation without effect on extracellular signal-regulated kinase 1/2 activation. Additionally, we provide evidence that GIT1 knockout VSMC have markedly fewer podosomes on PDBU treatment compared with wild-type VSMC. These data show that GIT1 is a key regulator of podosome formation in VSMC.

CONCLUSIONS

In conclusion, our data suggest that GIT1-Y392 phosphorylation is critical for PDBU-induced podosome formation by regulating PLCγ activation. We propose that specific signaling modules are assembled in a GIT1 phosphotyrosine-dependent manner as exemplified by PLCγ activation versus extracellular signal-regulated kinase 1/2 activation.

IGF-1 receptor transactivation mediates Src-dependent cortactin phosphorylation in response to angiotensin IIThis article is one of a selection of papers published in a special issue celebrating the 125th anniversary of the Faculty of Medicine at the University of Manitoba

Release of angiotensin II (Ang II) after vascular injury promotes tissue repair by stimulating phenotypic modulation of smooth muscle cells, which enables cell proliferation and migration. This process requires cytoskeleton remodeling, which involves cortactin, a scaffold protein that is phosphorylated by Src kinase in response to Ang II. Since insulin-like growth factor (IGF)-1 receptor transactivation mediates intracellular signals originating from the Ang II type 1 (AT1) receptor in a Src kinase-dependent manner, we examined whether IGF-1 receptor transactivation was also required for cortactin phosphorylation. Treatment of quiescent smooth muscle cells with Ang II resulted in both cortactin phosphorylation and its translocation to the plasma membrane. Both events were prevented by 1-(1,1-dimethylethyl)-1-(4-methylphenyl)-1H-pyrazolo(3,4-d)pyrimidin-4-amine (PP1), a Src kinase inhibitor, and by AG1024, an inhibitor of the IGF-1 receptor tyrosine kinase. Additionally, PP1 and AG1024 blocked the association of cortactin with actin-related protein (Arp) 3, an actin nucleation factor. These results indicate that Src kinase and the IGF-1 receptor kinase are necessary for activating cortactin. Phosphorylation of Src kinase in Ang II-treated cells was subsequently examined and was shown to be prevented by AG1024. Furthermore, Src kinase phosphorylation was blocked by inhibitors of protein kinase C (PKC), but not by inhibitors of phosphatidylinositol (PI) 3-kinase. These data establish that IGF-1 receptor transactivation is required for Src kinase-mediated cortactin phosphorylation and cytoskeletal reorganization in response to Ang II.

GIT1 mediates VEGF-induced podosome formation in endothelial cells: critical role for PLCgamma

OBJECTIVE

We and others showed that tyrosine kinase receptors (TKRs) such as the epidermal growth factor receptor stimulate G protein-coupled receptor (GPCR) kinase-interacting protein 1 (GIT1) phosphorylation via c-Src, which is required for phospholipase C-gamma (PLCgamma) activation, indicating that GIT1 participates in TKR signaling. VEGF is the most important TKR in endothelial cells (ECs); essential for cell survival, migration, and angiogenesis. Podosomes, actin-rich structures, were found to contribute to EC migration, tissue invasion, and matrix remodeling, suggesting a role for podosomes in angiogenesis. Because GIT1 is a substrate of c-Src, and podosome formation is c-Src dependent, we hypothesized that GIT1 plays an important role in VEGF-induced EC podosome formation and cell migration.

METHODS AND RESULTS

Exposure of ECs to VEGF for 30 minutes stimulated GIT1 colocalization with podosomes. Depletion of GIT1 by siRNA significantly decreased VEGF-induced podosome formation. A key role for PLCgamma was suggested by several experiments. Double staining PLCgamma and actin showed colocalization of PLCgamma with podosomes. Podosome formation was dramatically reduced by PLCgamma inhibitor U73122, Src inhibitor PP2, or expression of dominant negative small GTPases. Therefore, VEGF-induced EC podosome formation is dependent on Src, GIT1, PLCgamma, and small GTPases. In addition, matrix metalloprotease 2 (MMP2) and MT-MMP1 were detected at sites of VEGF-induced podosomes. Depletion of GIT1 by siRNA also significantly inhibited VEGF-induced MMP2 activation and extracellular matrix (ECM) degradation. Therefore, GIT1 mediates VEGF-induced matrix metalloproteinase (MMP) activation and ECM degradation by regulating podosome formation. Finally, depletion of GIT1 by siRNA significantly decreased VEGF-induced cell migration.

CONCLUSIONS

These data indicate that GIT1 is an essential mediator for VEGF-induced EC podosome formation and cell migration via PLCgamma.

Role of phospholipase Cgamma1 in cell spreading requires association with a beta-Pix/GIT1-containing complex, leading to activation of Cdc42 and Rac1

The significance of multiprotein signaling complexes in cell motility is becoming increasingly important. We have previously shown that phospholipase Cgamma1 (PLCgamma1) is critical for integrin-mediated cell spreading and motility (N. Jones et al., J. Cell Sci. 118:2695-2706, 2005). In the current study we show that, on a basement membrane-type matrix, PLCgamma1 associates with the adaptor protein GIT1 and the Rac1/Cdc42 guanine exchange factor beta-Pix; GIT1 and beta-Pix form tight complexes independently of PLCgamma1. The association of PLCgamma1 with the complex requires both GIT1 and beta-Pix and the specific array region (gammaSA) of PLCgamma1. Mutations of PLCgamma1 within the gammaSA region reveal that association with this complex is essential for the phosphorylation of PLCgamma1 and the progression to an elongated morphology after integrin engagement. Short interfering RNA (siRNA) depletion of either beta-Pix or GIT1 inhibited cell spreading in a fashion similar to that seen with siRNA against PLCgamma1. Furthermore, siRNA depletion of PLCgamma1, beta-Pix, or GIT1 inhibited Cdc42 and Rac1 activation, while constitutively active forms of Cdc42 or Rac1, but not RhoA, were able to rescue the elongation of these cells. Signaling of the PLCgamma1/GIT1/beta-Pix complex to Cdc42/Rac1 was found to involve the activation of calpains, calcium-dependent proteases. Therefore, we propose that the association of PLCgamma1 with complexes containing GIT1 and beta-Pix is essential for its role in integrin-mediated cell spreading and motility. As a component of this complex, PLCgamma1 is also involved in the activation of Cdc42 and Rac1.

Dynamic recruitment of PAK1 to the immunological synapse is mediated by PIX independently of SLP-76 and Vav1

T cell receptor engagement activates p21-activated kinase 1 (PAK1) through a LAT-SLP-76-Nck-Vav-Rac-dependent pathway. A second independent pathway involving a GIT-PIX-PAK1 trimolecular complex is also activated by T cell receptor ligation. Here we show a Vav-independent pathway exists that leads to PAK1 activation. In addition, PAK1, PIX and GIT1 were recruited to the T cell-antigen-presenting cell contact site independently of SLP-76 and Vav1. PAK1 recruitment to the T cell-antigen-presenting cell interface required interaction with PIX, which also led to optimal PLC-gamma1 activation and T cell receptor-dependent transcriptional responses. These data indicate that a pathway involving the GIT-PIX-PAK1 complex has a crucial function in PAK1 activation by recruiting PAK1 to the immunological synapse.

Hypoxia and the regulation of mitogen-activated protein kinases: gene transcription and the assessment of potential pharmacologic therapeutic interventions

Oxygen is an environmental/developmental signal that regulates cellular energetics, growth, and differentiation processes. Despite its central role in nearly all higher life processes, the molecular mechanisms for sensing oxygen levels and the pathways involved in transducing this information are still being elucidated. Altering gene expression is the most fundamental and effective way for a cell to respond to extracellular signals and/or changes in its microenvironment. During development, the expression of specific sets of genes is regulated spatially (by position/morphogenetic gradients) and temporally, presumably via the sensing of molecular oxygen available within the microenvironment. Regulation of signaling responses is governed by transcription factors that bind to control regions (consensus sequences) of target genes and alter their expression in response to specific signals. Complex signal transduction during hypoxia (deficiency of oxygen in inspired gases or in arterial blood and/or in tissues) involves the coupling of ligand-receptor interactions to many intracellular events. These events basically include phosphorylations by tyrosine kinases and/or serine/threonine kinases, such as those of mitogen-activated protein kinases (MAPKs), a superfamily of kinases responsive to stress nonhomeostatic conditions. Protein phosphorylations imposed during hypoxia change enzyme activities and protein conformations, and the eventual outcome is rather complex, comprising of an alteration in cellular activity and changes in the programming of genes expressed within the responding cells. These molecular changes serve as signals that are crucial for cell survival under contingent conditions imposed during hypoxia. This review correlates current concepts of hypoxic sensing pathways with hypoxia-related phosphorylation mechanisms mediated by MAPKs via the genetic and pharmacologic regulation/manipulation of specific transcription factors and related cofactors.