The insulin-like growth factor I receptor regulates glucose transport by astrocytes.

Previous findings indicate that reducing brain insulin-like growth factor I receptor (IGF-IR) activity promotes ample neuroprotection. We now examined a possible action of IGF-IR on brain glucose transport to explain its wide protective activity, as energy availability is crucial for healthy tissue function. Using (18) FGlucose PET we found that shRNA interference of IGF-IR in mouse somatosensory cortex significantly increased glucose uptake upon sensory stimulation. In vivo microscopy using astrocyte specific staining showed that after IGF-IR shRNA injection in somatosensory cortex, astrocytes displayed greater increases in glucose uptake as compared to astrocytes in the scramble-injected side. Further, mice with the IGF-IR knock down in astrocytes showed increased glucose uptake in somatosensory cortex upon sensory stimulation. Analysis of underlying mechanisms indicated that IGF-IR interacts with glucose transporter 1 (GLUT1), the main facilitative glucose transporter in astrocytes, through a mechanism involving interactions with the scaffolding protein GIPC and the multicargo transporter LRP1 to retain GLUT1 inside the cell. These findings identify IGF-IR as a key modulator of brain glucose metabolism through its inhibitory action on astrocytic GLUT1 activity. GLIA 2016;64:1962-1971.

A role for astrocytic insulin-like growth factor I receptors in the response to ischemic insult.

Increased neurotrophic support, including insulin-like growth factor I (IGF-I), is an important aspect of the adaptive response to ischemic insult. However, recent findings indicate that the IGF-I receptor (IGF-IR) in neurons plays a detrimental role in the response to stroke. Thus, we investigated the role of astrocytic IGF-IR on ischemic insults using tamoxifen-regulated Cre deletion of IGF-IR in glial fibrillary acidic protein (GFAP) astrocytes, a major cellular component in the response to injury. Ablation of IGF-IR in astrocytes (GFAP-IGF-IR KO mice) resulted in larger ischemic lesions, greater blood-brain-barrier disruption and more deteriorated sensorimotor coordination. RNAseq detected increases in inflammatory, cell adhesion and angiogenic pathways, while the expression of various classical biomarkers of response to ischemic lesion were significantly increased at the lesion site compared to control littermates. While serum IGF-I levels after injury were decreased in both control and GFAP-IR KO mice, brain IGF-I mRNA expression show larger increases in the latter. Further, greater damage was also accompanied by altered glial reactivity as reflected by changes in the morphology of GFAP astrocytes, and relative abundance of ionized calcium binding adaptor molecule 1 (Iba 1) microglia. These results suggest a protective role for astrocytic IGF-IR in the response to ischemic injury.

PGE2 induces angiogenesis via MT1-MMP-mediated activation of the TGFbeta/Alk5 signaling pathway.

The development of a new vascular network is essential for the onset and progression of many pathophysiologic processes. Cyclooxygenase-2 displays a proangiogenic activity in in vitro and in vivo models, mediated principally through its metabolite prostaglandin E(2) (PGE(2)). Here, we provide evidence for a novel signaling route through which PGE(2) activates the Alk5-Smad3 pathway in endothelial cells. PGE(2) induces Alk5-dependent Smad3 nuclear translocation and DNA binding, and the activation of this pathway involves the release of active TGFbeta from its latent form through a process mediated by the metalloproteinase MT1-MMP, whose membrane clustering is promoted by PGE(2). MT1-MMP-dependent transforming growth factor beta (TGFbeta) signaling through Alk5 is also required for PGE(2)-induced endothelial cord formation in vitro, and Alk5 kinase activity is required for PGE(2)-induced neovascularization in vivo. These findings identify a novel signaling pathway linking PGE(2) and TGFbeta, 2 effectors involved in tumor growth and angiogenesis, and reveal potential targets for the treatment of angiogenesis-related disorders.

MT1-MMP collagenolytic activity is regulated through association with tetraspanin CD151 in primary endothelial cells.

MT1-MMP plays a key role in endothelial function, as underscored by the angiogenic defects found in MT1-MMP deficient mice. We have studied the molecular interactions that underlie the functional regulation of MT1-MMP. At lateral endothelial cell junctions, MT1-MMP colocalizes with tetraspanin CD151 (Tspan 24) and its associated partner alpha3beta1 integrin. Biochemical and FRET analyses show that MT1-MMP, through its hemopexin domain, associates tightly with CD151, thus forming alpha3beta1 integrin/CD151/MT1-MMP ternary complexes. siRNA knockdown of HUVEC CD151 expression enhanced MT1-MMP-mediated activation of MMP2, and the same activation was seen in ex vivo lung endothelial cells isolated from CD151-deficient mice. However, analysis of collagen degradation in these experimental models revealed a diminished MT1-MMP enzymatic activity in confined areas around the cell periphery. CD151 knockdown affected both MT1-MMP subcellular localization and its inclusion into detergent-resistant membrane domains, and prevented biochemical association of the metalloproteinase with the integrin alpha3beta1. These data provide evidence for a novel regulatory role of tetraspanin microdomains on the collagenolytic activity of MT1-MMP and indicate that CD151 is a key regulator of MT1-MMP in endothelial homeostasis.

Regulation of MT1-MMP Activity through Its Association with ERMs.

Membrane-bound proteases play a key role in biology by degrading matrix proteins or shedding adhesion receptors. MT1-MMP metalloproteinase is critical during cancer invasion, angiogenesis, and development. MT1-MMP activity is strictly regulated by internalization, recycling, autoprocessing but also through its incorporation into tetraspanin-enriched microdomains (TEMs), into invadopodia, or by its secretion on extracellular vesicles (EVs). We identified a juxtamembrane positively charged cluster responsible for the interaction of MT1-MMP with ERM (ezrin/radixin/moesin) cytoskeletal connectors in breast carcinoma cells. Linkage to ERMs regulates MT1-MMP subcellular distribution and internalization, but not its incorporation into extracellular vesicles. MT1-MMP association to ERMs and insertion into TEMs are independent phenomena, so that mutation of the ERM-binding motif in the cytoplasmic region of MT1-MMP does not preclude its association with the tetraspanin CD151, but impairs the accumulation and coalescence of CD151/MT1-MMP complexes at actin-rich structures. Conversely, gene deletion of CD151 does not impact on MT1-MMP colocalization with ERM molecules. At the plasma membrane MT1-MMP autoprocessing is severely dependent on ERM association and seems to be the dominant regulator of the enzyme collagenolytic activity. This newly characterized MT1-MMP/ERM association can thus be of relevance for tumor cell invasion.

Astrocytes require insulin-like growth factor I to protect neurons against oxidative injury.

Oxidative stress is a proposed mechanism in brain aging, making the study of its regulatory processes an important aspect of current neurobiological research. In this regard, the role of the aging regulator insulin-like growth factor I (IGF-I) in brain responses to oxidative stress remains elusive as both beneficial and detrimental actions have been ascribed to this growth factor. Because astrocytes protect neurons against oxidative injury, we explored whether IGF-I participates in astrocyte neuroprotection and found that blockade of the IGF-I receptor in astrocytes abrogated their rescuing effect on neurons. We found that IGF-I directly protects astrocytes against oxidative stress (H 2O 2). Indeed, in astrocytes but not in neurons, IGF-I decreases the pro-oxidant protein thioredoxin-interacting protein 1 and normalizes the levels of reactive oxygen species. Furthermore, IGF-I cooperates with trophic signals produced by astrocytes in response to H 2O 2 such as stem cell factor (SCF) to protect neurons against oxidative insult. After stroke, a condition associated with brain aging where oxidative injury affects peri-infarcted regions, a simultaneous increase in SCF and IGF-I expression was found in the cortex, suggesting that a similar cooperative response takes place in vivo. Cell-specific modulation by IGF-I of brain responses to oxidative stress may contribute in clarifying the role of IGF-I in brain aging.

Functional interplay between endothelial nitric oxide synthase and membrane type 1 matrix metalloproteinase in migrating endothelial cells.

Nitric oxide (NO) is essential for vascular homeostasis and is also a critical modulator of angiogenesis; however, the molecular mechanisms of NO action during angiogenesis remain elusive. We have investigated the potential relationship between NO and membrane type 1-matrix metalloproteinase (MT1-MMP) during endothelial migration and capillary tube formation. Endothelial NO synthase (eNOS) colocalizes with MT1-MMP at motility-associated structures in migratory human endothelial cells (ECs); moreover, NO is produced at these structures and is released into the medium during EC migration. We have therefore addressed 2 questions: (1) the putative regulation of MT1-MMP by NO in migratory ECs; and (2) the requirement for MT1-MMP in NO-induced EC migration and tube formation. NO upregulates MT1-MMP membrane clustering on migratory human ECs, and this is accompanied by increased degradation of type I collagen substrate. MT1-MMP membrane expression and localization are impaired in lung ECs from eNOS-deficient mice, and these cells also show impaired migration and tube formation in vitro. Inhibition of MT1-MMP with a neutralizing antibody impairs NOinduced tube formation by human ECs, and NO-induced endothelial migration and tube formation are impaired in lung ECs from mice deficient in MT1-MMP. MT1-MMP thus appears to be a key molecular effector of NO during the EC migration and angiogenic processes, and is a potential therapeutic target for NO-associated vascular disorders.

MT1-MMP proinvasive activity is regulated by a novel Rab8-dependent exocytic pathway.

MT1-matrix metalloproteinase (MT1-MMP) is one of the most critical factors in the invasion machinery of tumor cells. Subcellular localization to invasive structures is key for MT1-MMP proinvasive activity. However, the mechanism driving this polarized distribution remains obscure. We now report that polarized exocytosis of MT1-MMP occurs during MDA-MB-231 adenocarcinoma cell migration into collagen type I three-dimensional matrices. Polarized trafficking of MT1-MMP is triggered by beta1 integrin-mediated adhesion to collagen, and is required for protease localization at invasive structures. Localization of MT1-MMP within VSV-G/Rab8-positive vesicles, but not in Rab11/Tf/TfRc-positive compartment in invasive cells, suggests the involvement of the exocytic traffic pathway. Furthermore, constitutively active Rab8 mutants induce MT1-MMP exocytic traffic, collagen degradation and invasion, whereas Rab8- but not Rab11-knockdown inhibited these processes. Altogether, these data reveal a novel pathway of MT1-MMP redistribution to invasive structures, exocytic vesicle trafficking, which is crucial for its role in tumor cell invasiveness. Mechanistically, MT1-MMP delivery to invasive structures, and therefore its proinvasive activity, is regulated by Rab8 GTPase.

MT1-MMP: universal or particular player in angiogenesis?

Tumorigenesis involves not only tumor cells that become transformed but also the peritumoral stroma which reacts inducing inflammatory and angiogenic responses. Angiogenesis, the formation of new capillaries from preexisting vessels, is an absolute requirement for tumor growth and metastasis, and it can be induced and modulated by a wide variety of soluble factors. During angiogenesis, quiescent endothelial cells are activated and they initiate migration by degrading the basement membranes through the action of specific proteases, in particular of matrix metalloproteinases (MMPs). Among these, the membrane type 1-matrix metalloproteinase (MT1-MMP) has been identified as a key player during the angiogenic response. In this review, we will summarize the role of MT1-MMP in angiogenesis and the regulatory mechanisms of this protease in endothelial cells. Since our recent findings have suggested that MT1-MMP is not universally required for angiogenesis, we hypothesize that the regulation and participation of MT1-MMP in angiogenesis may depend on the nature of the angiogenic stimulus. Experiments aimed at testing this hypothesis have shown that similarly to the chemokine stromal cell-derived factor-1 (SDF-1)/CXCL12, lipopolysaccharide (LPS) seems to induce the formation of capillary tubes by human or mouse endothelial cells (ECs) in an MT1-MMP-independent manner. The implications of these findings in the potential use of MT1-MMP inhibitors in cancer therapy are discussed.

Membrane type 1-matrix metalloproteinase is regulated by chemokines monocyte-chemoattractant protein-1/ccl2 and interleukin-8/CXCL8 in endothelial cells during angiogenesis.

We have investigated the putative role and regulation of membrane type 1-matrix metalloproteinase (MT1-MMP) in angiogenesis induced by inflammatory factors of the chemokine family. The absence of MT1-MMP from null mice or derived mouse lung endothelial cells or the blockade of its activity with inhibitory antibodies resulted in the specific decrease of in vivo and in vitro angiogenesis induced by CCL2 but not CXCL12. Similarly, CCL2- and CXCL8-induced tube formation by human endothelial cells (ECs) was highly dependent on MT1-MMP activity. CCL2 and CXCL8 significantly increased MT1-MMP surface expression, clustering, activity, and function in human ECs. Investigation of the signaling pathways involved in chemokine-induced MT1-MMP activity in ECs revealed that CCL2 and CXCL8 induced cortical actin polymerization and sustained activation of phosphatidylinositol 3-kinase (PI3K) and the small GTPase Rac. Inhibition of PI3K or actin polymerization impaired CCL2-induced MT1-MMP activity. Finally, dimerization of MT1-MMP was found to be enhanced by CCL2 in ECs in a PI3K- and actin polymerization-dependent manner. In summary, we identify MT1-MMP as a molecular target preferentially involved in angiogenesis mediated by CCL2 and CXCL8, but not CXCL12, and suggest that MT1-MMP dimerization might be an important mechanism of its regulation during angiogenesis.

Membrane type 1-matrix metalloproteinase is involved in migration of human monocytes and is regulated through their interaction with fibronectin or endothelium.

Membrane type 1-matrix metalloproteinase (MT1-MMP) is involved in endothelial and tumor-cell migration, but its putative role in leukocyte migration has not been characterized yet. Here, we demonstrate that anti-MT1-MMP monoclonal antibody (mAb) impaired monocyte chemotactic protein-1 (MCP-1)-stimulated monocyte migration on fibronectin (FN), vascular cell adhesion molecule-1 (VCAM-1), and intercellular adhesion molecule-1 (ICAM-1). In addition, monocyte transmigration through tumor necrosis factor-alpha (TNF-alpha)-activated endothelium is also inhibited by anti-MT1-MMP mAb. Therefore, regulation of MT1-MMP in human peripheral blood monocytes was investigated. First, MT1-MMP clustering was observed at motility-associated membrane protrusions of MCP-1-stimulated monocytes migrating on FN, VCAM-1, or ICAM-1 and at the leading edge, together with profilin, of monocytes transmigrating through activated endothelial cells. In addition, up-regulation of MT1-MMP expression was induced in human monocytes upon attachment to FN in a manner dependent on alpha4beta1 and alpha5beta1 integrins. Binding of monocytes to TNF-alpha-activated human endothelial cells as well as to VCAM-1 or ICAM-1 also resulted in an increase of MT1-MMP expression. These findings correlated with an enhancement of MT1-MMP fibrinolytic activity in monocytes bound to FN, VCAM-1, or ICAM-1. Our data show that MT1-MMP is required during human monocyte migration and endothelial transmigration and that MT1-MMP localization, expression, and activity are regulated in monocytes upon contact with FN or endothelial ligands, pointing to a key role of MT1-MMP in monocyte recruitment during inflammation.