Secondary lymphoid tissue and costimulation-blockade resistant rejection: A nonhuman primate renal transplant study.

Naïve T cell activation requires antigen presentation combined with costimulation through CD28, both of which optimally occur in secondary lymphoid tissues such as lymph nodes and the spleen. Belatacept impairs CD28 costimulation by binding its ligands, CD80 and CD86, and in doing so, impairs de novo alloimmune responses. However, in most patients belatacept is ineffective in preventing allograft rejection when used as a monotherapy, and adjuvant therapy is required for control of costimulation-blockade resistant rejection (CoBRR). In rodent models, impaired access to secondary lymphoid tissues has been demonstrated to reduce alloimmune responses to vascularized allografts. Here we show that surgical maneuvers, lymphatic ligation, and splenectomy, designed to anatomically limit access to secondary lymphoid tissues, control CoBRR and facilitate belatacept monotherapy in a nonhuman primate model of kidney transplantation without adjuvant immunotherapy. We further demonstrate that animals sustained on belatacept monotherapy progressively develop an increasingly naïve T and B cell repertoire, an effect that is accelerated by splenectomy and lost at the time of belatacept withdrawal and rejection. These pilot data inform the role of secondary lymphoid tissues on the development of CoBRR and the use of costimulation molecule-focused therapies.

The role of human CD46 in early xenoislet engraftment in a dual transplant model.

BACKGROUND: Membrane cofactor protein CD46 attenuates the complement cascade by facilitating cleavage of C3b and C4b. In solid organ xenotransplantation, organs expressing CD46 have been shown to resist hyperacute rejection. However, the incremental value of human CD46 expression for islet xenotransplantation remains poorly defined. METHODS: This study attempted to delineate the role of CD46 in early neonatal porcine islet engraftment by comparing Gal-knocked out (GKO) and hCD46-transgenic (GKO/CD46) islets in a dual transplant model. Seven rhesus macaques underwent dual transplant and were sacrificed at 1 hour (n = 4) or 24 hours (n = 3). Both hemilivers were recovered and fixed for immunohistochemistry (CD46, insulin, neutrophil elastase, platelet, IgM, IgG, C3d, C4d, CD68, Caspase 3). Quantitative immunohistochemical analysis was performed using the Aperio Imagescope. RESULTS: Within 1 hour of intraportal infusion of xenografts, no differences were observed between the two types of islets in terms of platelet, antibody, or complement deposition. Cellular infiltration and islet apoptotic activity were also similar at 1 hour. At 24 hours, GKO/CD46 islets demonstrated significantly less platelet deposition (P = 0.01) and neutrophil infiltration (P = 0.01) compared to GKO islets. In contrast, C3d (P = 0.38) and C4d (P = 0.45) deposition was equal between the two genotypes. CONCLUSIONS: Our findings suggest that expression of hCD46 on NPIs potentially provides a measurable incremental survival advantage in vivo by reducing early thrombo-inflammatory events associated with instant blood-mediated inflammatory reaction (IBMIR) following intraportal islet infusion.

Novel bone morphogenetic protein signaling through Smad2 and Smad3 to regulate cancer progression and development.

The bone morphogenetic protein (BMP) signaling pathways have important roles in embryonic development and cellular homeostasis, with aberrant BMP signaling resulting in a broad spectrum of human disease. We report that BMPs unexpectedly signal through the canonical transforming growth factor ß (TGF-ß)-responsive Smad2 and Smad3. BMP-induced Smad2/3 signaling occurs preferentially in embryonic cells and transformed cells. BMPs signal to Smad2/3 by stimulating complex formation between the BMP-binding TGF-ß superfamily receptors, activin receptor-like kinase (ALK)3/6, and the Smad2/3 phosphorylating receptors ALK5/7. BMP signaling through Smad2 mediates, in part, dorsoventral axis patterning in zebrafish embryos, whereas BMP signaling through Smad3 facilitates cancer cell invasion. Consistent with increased BMP-mediated Smad2/3 signaling during cancer progression, Smad1/5 and Smad 2/3 signaling converge in human cancer specimens. Thus, the signaling mechanisms used by BMPs and TGF-ß superfamily receptors are broader than previously appreciated.

Deep sequencing of the small RNA transcriptome of normal and malignant human B cells identifies hundreds of novel microRNAs.

A role for microRNA (miRNA) has been recognized in nearly every biologic system examined thus far. A complete delineation of their role must be preceded by the identification of all miRNAs present in any system. We elucidated the complete small RNA transcriptome of normal and malignant B cells through deep sequencing of 31 normal and malignant human B-cell samples that comprise the spectrum of B-cell differentiation and common malignant phenotypes. We identified the expression of 333 known miRNAs, which is more than twice the number previously recognized in any tissue type. We further identified the expression of 286 candidate novel miRNAs in normal and malignant B cells. These miRNAs were validated at a high rate (92%) using quantitative polymerase chain reaction, and we demonstrated their application in the distinction of clinically relevant subgroups of lymphoma. We further demonstrated that a novel miRNA cluster, previously annotated as a hypothetical gene LOC100130622, contains 6 novel miRNAs that regulate the transforming growth factor-ß pathway. Thus, our work suggests that more than a third of the miRNAs present in most cellular types are currently unknown and that these miRNAs may regulate important cellular functions.

ALK5 phosphorylation of the endoglin cytoplasmic domain regulates Smad1/5/8 signaling and endothelial cell migration.

Endoglin, an endothelial cell-specific transforming growth factor-beta (TGF-beta) superfamily coreceptor, has an essential role in angiogenesis. Endoglin-null mice have an embryonic lethal phenotype due to defects in angiogenesis and mutations in endoglin result in the vascular disease hereditary hemorrhagic telangiectasia type I. Increased endoglin expression in the proliferating endothelium of tumors has been correlated with metastasis, tumor grade and decreased survival. Although endoglin is thought to regulate TGF-beta superfamily signaling in endothelial cells through regulating the balance between two TGF-beta-responsive pathways, the activin receptor-like kinase 5 (ALK5)/Smad2/3 pathway and the activin receptor-like kinase 1 (ALK1)/Smad1/5/8 pathway, the mechanism by which endoglin regulates angiogenesis has not been defined. Here, we investigate the role of the cytoplasmic domain of endoglin and its phosphorylation by ALK5 in regulating endoglin function in endothelial cells. We demonstrate that the cytoplasmic domain of endoglin is basally phosphorylated by ALK5, primarily on serines 646 and 649, in endothelial cells. Functionally, the loss of phosphorylation at serine 646 resulted in a loss of endoglin-mediated inhibition of Smad1/5/8 signaling in response to TGF-beta and endothelial cell migration, whereas loss of phosphorylation at both serines 646 and 649 resulted in a loss of endoglin-mediated inhibition of Smad1/5/8 signaling in response to bone morphogenetic protein-9. Taken together, these results support endoglin phosphorylation by ALK5 as an important mechanism for regulating TGF-beta superfamily signaling and migration in endothelial cells.

The type III TGF-beta receptor suppresses breast cancer progression.

The TGF-beta signaling pathway has a complex role in regulating mammary carcinogenesis. Here we demonstrate that the type III TGF-beta receptor (TbetaRIII, or betaglycan), a ubiquitously expressed TGF-beta coreceptor, regulated breast cancer progression and metastasis. Most human breast cancers lost TbetaRIII expression, with loss of heterozygosity of the TGFBR3 gene locus correlating with decreased TbetaRIII expression. TbetaRIII expression decreased during breast cancer progression, and low TbetaRIII levels predicted decreased recurrence-free survival in breast cancer patients. Restoring TbetaRIII expression in breast cancer cells dramatically inhibited tumor invasiveness in vitro and tumor invasion, angiogenesis, and metastasis in vivo. TbetaRIII appeared to inhibit tumor invasion by undergoing ectodomain shedding and producing soluble TbetaRIII, which binds and sequesters TGF-beta to decrease TGF-beta signaling and reduce breast cancer cell invasion and tumor-induced angiogenesis. Our results indicate that loss of TbetaRIII through allelic imbalance is a frequent genetic event during human breast cancer development that increases metastatic potential.

The type III transforming growth factor-beta receptor negatively regulates nuclear factor kappa B signaling through its interaction with beta-arrestin2.

Transforming growth factor-beta (TGF-beta) increases or decreases nuclear factor kappa B (NFkappaB) signaling in a context-dependent manner through mechanisms that remain to be defined. The type III transforming growth factor-beta receptor (TbetaRIII) is a TGF-beta superfamily co-receptor with emerging roles in both mediating and regulating TGF-beta superfamily signaling. We have previously reported a novel interaction of TbetaRIII with the scaffolding protein, beta-arrestin2, which results in TbetaRIII internalization and downregulation of TGF-beta signaling. beta-arrestin2 also scaffolds interacting receptors with the mitogen-activated protein kinase and NFkappaB-signaling pathways. Here, we demonstrate that TbetaRIII, through its interaction with beta-arrestin2, negatively regulates NFkappaB signaling in MCF10A breast epithelial and MDA-MB-231 breast cancer cells. Increasing TbetaRIII expression reduced NFkappaB-mediated transcriptional activation and IkappaBalpha degradation, whereas a TbetaRIII mutant unable to interact with beta-arrestin2, TbetaRIII-T841A, had no effect. In a reciprocal manner, short hairpin RNA-mediated silencing of either TbetaRIII expression or beta-arrestin2 expression increased NFkappaB-mediated transcriptional activation and IkappaBalpha degradation. Functionally, TbetaRIII-mediated repression of NFkappaB signaling is important for TbetaRIII-mediated inhibition of breast cancer cell migration. These studies define a mechanism through which TbetaRIII regulates NFkappaB signaling and expand the roles of this TGF-beta superfamily co-receptor in regulating epithelial cell homeostasis.

Bone morphogenetic proteins induce pancreatic cancer cell invasiveness through a Smad1-dependent mechanism that involves matrix metalloproteinase-2.

Bone morphogenetic proteins (BMPs) have an emerging role in human cancers. Here we demonstrate that the BMP-signaling pathway is intact and functional in human pancreatic cancer cells, with several BMP signaling components and transcriptional targets upregulated in human pancreatic cancer specimens compared with normal pancreatic tissue. Functionally, multiple BMP family members, including BMP-2, BMP-4 and BMP-7, induce an epithelial to mesenchymal transition (EMT) in the human pancreatic cancer cell line Panc-1, as demonstrated by morphological alterations and loss of E-cadherin expression. BMP-mediated EMT results in an increase in invasiveness of Panc-1 cells, in part through increased expression and activity of matrix metalloproteinase (MMP)-2, a known mediator of pancreatic cancer cell invasiveness. Accompanying EMT, BMP reduces expression of the transforming growth factor (TGF)-beta superfamily receptor, transforming growth factor-beta type III receptor (TbetaRIII), for which we have previously demonstrated loss of expression during pancreatic cancer progression. Maintaining TbetaRIII expression inhibits BMP-mediated invasion and suppresses Smad1 activation. Further, Smad1 is required for BMP-induced invasiveness and partially responsible for BMP-mediated increases in MMP-2 activity. These data suggest that BMP signaling, through Smad1 induction and upregulation of MMP-2, is an important mediator of pancreatic cancer invasiveness and a potential therapeutic target for treating this deadly disease.

Loss of betaglycan expression in ovarian cancer: role in motility and invasion.

The transforming growth factor-beta (TGF-beta) superfamily members, TGF-beta, activin, and inhibin, all have prominent roles in regulating normal ovarian function. Betaglycan, or the type III TGF-beta receptor, is a coreceptor that regulates TGF-beta, activin, and inhibin signaling. Here, we show that betaglycan expression is frequently decreased or lost in epithelial derived ovarian cancer at both the mRNA and protein level, with the degree of loss correlating with tumor grade. Treatment of ovarian cancer cell lines with the methyltransferase inhibitor 5-aza-2-deoxycytidine and the histone deacetylase inhibitor trichostatin A resulted in significant synergistic induction of betaglycan message levels and increased betaglycan protein expression, indicating that epigenetic silencing may play a role in the loss of betaglycan expression observed in ovarian cancer. Although restoring betaglycan expression in Ovca429 ovarian cancer cells is not sufficient to restore TGF-beta-mediated inhibition of proliferation, betaglycan significantly inhibits ovarian cancer cell motility and invasiveness. Furthermore, betaglycan specifically enhances the antimigratory effects of inhibin and the ability of inhibin to repress matrix metalloproteinase levels in these cells. These results show, for the first time, epigenetic regulation of betaglycan expression in ovarian cancer, and a novel role for betaglycan in regulating ovarian cancer motility and invasiveness.

The type III transforming growth factor-beta receptor as a novel tumor suppressor gene in prostate cancer.

The transforming growth factor-beta (TGF-beta) signaling pathway has an important role in regulating normal prostate epithelium, inhibiting proliferation, differentiation, and both androgen deprivation-induced and androgen-independent apoptosis. During prostate cancer formation, most prostate cancer cells become resistant to these homeostatic effects of TGF-beta. Although the loss of expression of either the type I (TbetaRI) or type II (TbetaRII) TGF-beta receptor has been documented in approximately 30% of prostate cancers, most prostate cancers become TGF-beta resistant without mutation or deletion of TbetaRI, TbetaRII, or Smads2, 3, and 4, and thus, the mechanism of resistance remains to be defined. Here, we show that type III TGF-beta receptor (TbetaRIII or betaglycan) expression is decreased or lost in the majority of human prostate cancers as compared with benign prostate tissue at both the mRNA and protein level. Loss of TbetaRIII expression correlates with advancing tumor stage and a higher probability of prostate-specific antigen (PSA) recurrence, suggesting a role in prostate cancer progression. The loss of TbetaRIII expression is mediated by the loss of heterozygosity at the TGFBR3 genomic locus and epigenetic regulation of the TbetaRIII promoter. Functionally, restoring TbetaRIII expression in prostate cancer cells potently decreases cell motility and cell invasion through Matrigel in vitro and prostate tumorigenicity in vivo. Taken together, these studies define the loss of TbetaRIII expression as a common event in human prostate cancer and suggest that this loss is important for prostate cancer progression through effects on cell motility, invasiveness, and tumorigenicity.

TbetaRIII suppresses non-small cell lung cancer invasiveness and tumorigenicity.

The transforming growth factor-beta (TGF-beta) superfamily has essential roles in lung development, regulating cell proliferation, branching morphogenesis, differentiation and apoptosis. Although most lung cancers become resistant to the tumor suppressor effects of TGF-beta, and loss or mutation of one of the components of the TGF-beta signaling pathway, including TbetaRII, Smad2 and Smad4 have been reported, mutations are not common in non-small cell lung cancer (NSCLC). Here we demonstrate that the TGF-beta superfamily co-receptor, the type III TGF-beta receptor (TbetaRIII or betaglycan) is lost in the majority of NSCLC specimens at the mRNA and protein levels, with loss correlating with increased tumor grade and disease progression. Loss of heterozygosity at the TGFBR3 genomic locus occurs in 38.5% of NSCLC specimens and correlates with decreased TbetaRIII expression, suggesting loss of heterozygosity as one mechanism for TbetaRIII loss. In the H460 cell model of NSCLC, restoring TbetaRIII expression decreased colony formation in soft agar. In the A549 cell model of NSCLC, restoring TbetaRIII expression significantly decreased cellular migration and invasion through Matrigel, in the presence and absence of TGF-beta1, and decreased tumorigenicity in vivo. In a reciprocal manner, shRNA-mediated silencing of endogenous TbetaRIII expression enhanced invasion through Matrigel. Mechanistically, TbetaRIII functions, at least in part, through undergoing ectodomain shedding, generating soluble TbetaRIII, which is able to inhibit cellular invasiveness. Taken together, these results support TbetaRIII as a novel tumor suppressor gene that is commonly lost in NSCLC resulting in a functional increase in cellular migration, invasion and anchorage-independent growth of lung cancer cells.

Expression of the type III TGF-beta receptor is negatively regulated by TGF-beta.

The type III transforming growth factor-beta receptor (TbetaRIII or betaglycan) is a ubiquitously expressed transforming growth factor-beta (TGF-beta) superfamily coreceptor with essential roles in embryonic development. Recent studies have defined a role for TbetaRIII in the pathogenesis of human cancers, with frequent loss of TbetaRIII expression at the message and protein level. Mechanisms for the loss of TbetaRIII expression remain to be fully defined. Advanced human cancers often have elevated circulating levels of TGF-beta1. Here, we define a specific role for TGF-beta1 in negatively regulating TbetaRIII at the message level in breast and ovarian cancer models. TGF-beta1 decreased TbetaRIII message and protein levels in ovarian (Ovca420) and breast cancer (MDA-MB-231) cell lines in both a dose- and time-dependent manner. TGF-beta1-mediated TbetaRIII repression is mediated by the type I TGF-beta receptor/Smad2/3 pathway as the activin receptor-like kinase 5 (ALK5) inhibitor, SB431542, abrogated this effect, while the expression of constitutively active ALK5 was sufficient to repress TbetaRIII expression. Mechanistically, TGF-beta1 does not affect TbetaRIII messenger RNA (mRNA) stability, but instead directly regulates the TbetaRIII promoter. We define alternative promoters for the TGFBR3 gene, a distal and proximal promoter. Although both promoters are active, only the proximal promoter was responsive and negatively regulated by TGF-beta1 and constitutively active ALK5. Taken together, these studies define TGF-beta1-mediated downregulation of TbetaRIII mRNA expression through effects on the ALK5/Smad2/3 pathway on the TGFBR3 gene proximal promoter as a potential mechanism for decreased TbetaRIII expression in human cancers.

The type III TGF-beta receptor signals through both Smad3 and the p38 MAP kinase pathways to contribute to inhibition of cell proliferation.

Transforming growth factor beta (TGFbeta) has an important role as a negative regulator of cellular proliferation. The type III transforming growth factor beta receptor (TbetaRIII) has an emerging role as both a TGFbeta superfamily co-receptor and in mediating signaling through its cytoplasmic domain. In L6 myoblasts, TbetaRIII expression enhanced TGFbeta1-mediated growth inhibition, with this effect mediated, in part, by the TbetaRIII cytoplasmic domain. The effects of TbetaRIII were not due to altered ligand presentation or to differences in Smad2 phosphorylation. Instead, TbetaRIII specifically increased Smad3 phosphorylation, both basal and TGFbeta-stimulated Smad3 nuclear localization and Smad3-dependent activation of reporter genes independent of its cytoplasmic domain. Conversely, SB431542, a type I transforming growth factor beta receptor (TbetaRI) inhibitor, as well as dominant-negative Smad3 specifically and significantly abrogated the effects of TbetaRIII on TGFbeta1-mediated inhibition of proliferation. TbetaRIII also specifically increased p38 phosphorylation, and SB203580, a p38 kinase inhibitor, specifically and significantly abrogated the effects of TbetaRIII/TGFbeta1-mediated inhibition of proliferation in L6 myoblasts and in primary human epithelial cells. Importantly, treatment with the TbetaRI and p38 inhibitors together had additive effects on abrogating TbetaRIII/TGFbeta1-mediated inhibition of proliferation. In a reciprocal manner, short hairpin RNA-mediated knockdown of endogenous TbetaRIII in various human epithelial cells attenuated TGFbeta1-mediated inhibition of proliferation. Taken together, these data demonstrate that TbetaRIII contributes to and enhances TGFbeta-mediated growth inhibition through both TbetaRI/Smad3-dependent and p38 mitogen-activated protein kinase pathways.

Role of TGF-ß receptor III localization in polarity and breast cancer progression.

The majority of breast cancers originate from the highly polarized luminal epithelial cells lining the breast ducts. However, cell polarity is often lost during breast cancer progression. The type III transforming growth factor-ß cell surface receptor (TßRIII) functions as a suppressor of breast cancer progression and also regulates the process of epithelial-to-mesenchymal transition (EMT), a consequence of which is the loss of cell polarity. Many cell surface proteins exhibit polarized expression, being targeted specifically to the apical or basolateral domains. Here we demonstrate that TßRIII is basolaterally localized in polarized breast epithelial cells and that disruption of the basolateral targeting of TßRIII through a single amino acid mutation of proline 826 in the cytosolic domain results in global loss of cell polarity through enhanced EMT. In addition, the mistargeting of TßRIII results in enhanced proliferation, migration, and invasion in vitro and enhanced tumor formation and invasion in an in vivo mouse model of breast carcinoma. These results suggest that proper localization of TßRIII is critical for maintenance of epithelial cell polarity and phenotype and expand the mechanisms by which TßRIII prevents breast cancer initiation and progression.

Ectodomain shedding of TßRIII is required for TßRIII-mediated suppression of TGF-ß signaling and breast cancer migration and invasion.

The type III transforming growth factor ß (TGF-ß) receptor (TßRIII), also known as betaglycan, is the most abundantly expressed TGF-ß receptor. TßRIII suppresses breast cancer progression by inhibiting migration, invasion, metastasis, and angiogenesis. TßRIII binds TGF-ß ligands, with membrane-bound TßRIII presenting ligand to enhance TGF-ß signaling. However, TßRIII can also undergo ectodomain shedding, releasing soluble TßRIII, which binds and sequesters ligand to inhibit downstream signaling. To investigate the relative contributions of soluble and membrane-bound TßRIII on TGF-ß signaling and breast cancer biology, we defined TßRIII mutants with impaired (ΔShed-TßRIII) or enhanced ectodomain shedding (SS-TßRIII). Inhibiting ectodomain shedding of TßRIII increased TGF-ß responsiveness and abrogated TßRIII's ability to inhibit breast cancer cell migration and invasion. Conversely, expressing SS-TßRIII, which increased soluble TßRIII production, decreased TGF-ß signaling and increased TßRIII-mediated inhibition of breast cancer cell migration and invasion. Of importance, SS-TßRIII-mediated increases in soluble TßRIII production also reduced breast cancer metastasis in vivo. Taken together, these studies suggest that the ratio of soluble TßRIII to membrane-bound TßRIII is an important determinant for regulation of TßRIII- and TGF-ß-mediated signaling and biology.

TGF-ß regulates LARG and GEF-H1 during EMT to affect stiffening response to force and cell invasion.

Recent studies implicate a role for cell mechanics in cancer progression. The epithelial-to-mesenchymal transition (EMT) regulates the detachment of cancer cells from the epithelium and facilitates their invasion into stromal tissue. Although classic EMT hallmarks include loss of cell-cell adhesions, morphology changes, and increased invasion capacity, little is known about the associated mechanical changes. Previously, force application on integrins has been shown to initiate cytoskeletal rearrangements that result in increased cell stiffness and a stiffening response. Here we demonstrate that transforming growth factor ß (TGF-ß)-induced EMT results in decreased stiffness and loss of the normal stiffening response to force applied on integrins. We find that suppression of the RhoA guanine nucleotide exchange factors (GEFs) LARG and GEF-H1 through TGF-ß/ALK5-enhanced proteasomal degradation mediates these changes in cell mechanics and affects EMT-associated invasion. Taken together, our results reveal a functional connection between attenuated stiffness and stiffening response and the increased invasion capacity acquired after TGF-ß-induced EMT.

Endocardial cell epithelial-mesenchymal transformation requires Type III TGFß receptor interaction with GIPC.

An early event in heart valve formation is the epithelial-mesenchymal transformation (EMT) of a subpopulation of endothelial cells in specific regions of the heart tube, the endocardial cushions. The Type III TGFß receptor (TGFßR3) is required for TGFß2- or BMP-2-stimulated EMT in atrioventricular endocardial cushion (AVC) explants in vitro but the mediators downstream of TGFßR3 are not well described. Using AVC and ventricular explants as an in vitro assay, we found an absolute requirement for specific TGFßR3 cytoplasmic residues, GAIP-interacting protein, C terminus (GIPC), and specific Activin Receptor-Like Kinases (ALK)s for TGFßR3-mediated EMT when stimulated by TGFß2 or BMP-2. The introduction of TGFßR3 into nontransforming ventricular endocardial cells, followed by the addition of either TGFß2 or BMP-2, results in EMT. TGFßR3 lacking the entire cytoplasmic domain, or only the 3C-terminal amino acids that are required to bind GIPC, fails to support EMT in response to TGFß2 or BMP-2. Overexpression of GIPC in AVC endocardial cells enhanced EMT while siRNA-mediated silencing of GIPC in ventricular cells overexpressing TGFßR3 significantly inhibited EMT. Targeting of specific ALKs by siRNA revealed that TGFßR3-mediated EMT requires ALK2 and ALK3, in addition to ALK5, but not ALK4 or ALK6. Taken together, these data identify GIPC, ALK2, ALK3, and ALK5 as signaling components required for TGFßR3-mediated endothelial cell EMT.

Endoglin promotes transforming growth factor beta-mediated Smad 1/5/8 signaling and inhibits endothelial cell migration through its association with GIPC.

Transforming growth factor beta (TGF-beta) signals through two distinct pathways to regulate endothelial cell proliferation, migration, and angiogenesis, the ALK-1/Smad 1/5/8 and ALK-5/Smad2/3 pathways. Endoglin is a co-receptor predominantly expressed in endothelial cells that participates in TGFbeta-mediated signaling with ALK-1 and ALK-5 and regulates critical aspects of cellular and biological responses. The embryonic lethal phenotype of knock-out mice because of defects in angiogenesis and disease-causing mutations resulting in human vascular diseases both support essential roles for endoglin, ALK-1, and ALK-5 in the vasculature. However, the mechanism by which endoglin mediates TGF-beta signaling through ALK-1 and ALK-5 has remained elusive. Here we describe a novel interaction between endoglin and GIPC, a scaffolding protein known to regulate cell surface receptor expression and trafficking. Co-immunoprecipitation and immunofluorescence confocal studies both demonstrate a specific interaction between endoglin and GIPC in endothelial cells, mediated by a class I PDZ binding motif in the cytoplasmic domain of endoglin. Subcellular distribution studies demonstrate that endoglin recruits GIPC to the plasma membrane and co-localizes with GIPC in a TGFbeta-independent manner, with GIPC-promoting cell surface retention of endoglin. Endoglin specifically enhanced TGF-beta1-induced phosphorylation of Smad 1/5/8, increased a Smad 1/5/8 responsive promoter, and inhibited endothelial cell migration in a manner dependent on the ability of endoglin to interact with GIPC. These studies define a novel mechanism for the regulation of endoglin signaling and function in endothelial cells and demonstrate a new role for GIPC in TGF-beta signaling.

Beta-arrestin 2 mediates endocytosis of type III TGF-beta receptor and down-regulation of its signaling.

beta-Arrestins bind to activated seven transmembrane-spanning (7TMS) receptors (G protein-coupled receptors) after the receptors are phosphorylated by G protein-coupled receptor kinases (GRKs), thereby regulating their signaling and internalization. Here, we demonstrate an unexpected and analogous role of beta-arrestin 2 (betaarr2) for the single transmembrane-spanning type III transforming growth factor-beta (TGF-beta) receptor (TbetaRIII, also referred to as betaglycan). Binding of betaarr2 to TbetaRIII was also triggered by phosphorylation of the receptor on its cytoplasmic domain (likely at threonine 841). However, such phosphorylation was mediated by the type II TGF-beta receptor (TbetaRII), which is itself a kinase, rather than by a GRK. Association with betaarr2 led to internalization of both receptors and down-regulation of TGF-beta signaling. Thus, the regulatory actions of beta-arrestins are broader than previously appreciated, extending to the TGF-beta receptor family as well.