TM4SF1 is a molecular facilitator that distributes cargo proteins intracellularly in endothelial cells in support of blood vessel formation.

Transmembrane-4 L-six family member-1 (TM4SF1) is an atypical tetraspanin that is highly and selectively expressed in proliferating endothelial cells and plays an essential role in blood vessel development. TM4SF1 forms clusters on the cell surface called TMED (TM4SF1-enriched microdomains) and recruits other proteins that internalize along with TM4SF1 via microtubules to intracellular locations including the nucleus. We report here that tumor growth and wound healing are inhibited in Tm4sf1-heterozygous mice. Investigating the mechanisms of TM4SF1 activity, we show that 12 out of 18 signaling molecules examined are recruited to TMED on the surface of cultured human umbilical vein endothelial cells (HUVEC) and internalize along with TMED; notable among them are PLCγ and HDAC6. When TM4SF1 is knocked down in HUVEC, microtubules are heavily acetylated despite normal levels of HDAC6 protein, and, despite normal levels of VEGFR2, are unable to proliferate. Together, our studies indicate that pathological angiogenesis is inhibited when levels of TM4SF1 are reduced as in Tm4sf1-heterozygous mice; a likely mechanism is that TM4SF1 regulates the intracellular distribution of signaling molecules necessary for endothelial cell proliferation and migration.

Intracellular distribution of TM4SF1 and internalization of TM4SF1-antibody complex in vascular endothelial cells.

Transmembrane-4 L-six family member-1 (TM4SF1) is a small plasma membrane-associated glycoprotein that is highly and selectively expressed on the plasma membranes of tumor cells, cultured endothelial cells, and, in vivo, on tumor-associated endothelium. Immunofluorescence microscopy also demonstrated TM4SF1 in cytoplasm and, tentatively, within nuclei. With monoclonal antibody 8G4, and the finer resolution afforded by immuno-nanogold transmission electron microscopy, we now demonstrate TM4SF1 in uncoated cytoplasmic vesicles, nuclear pores and nucleoplasm. Because of its prominent surface location on tumor cells and tumor-associated endothelium, TM4SF1 has potential as a dual therapeutic target using an antibody drug conjugate (ADC) approach. For ADC to be successful, antibodies reacting with cell surface antigens must be internalized for delivery of associated toxins to intracellular targets. We now report that 8G4 is efficiently taken up into cultured endothelial cells by uncoated vesicles in a dynamin-dependent, clathrin-independent manner. It is then transported along microtubules through the cytoplasm and passes through nuclear pores into the nucleus. These findings validate TM4SF1 as an attractive candidate for cancer therapy with antibody-bound toxins that have the capacity to react with either cytoplasmic or nuclear targets in tumor cells or tumor-associated vascular endothelium.

TM4SF1: a new vascular therapeutic target in cancer.

Transmembrane-4 L-six family member-1 (TM4SF1) is a small plasma membrane glycoprotein that regulates cell motility and proliferation. TM4SF1 is an attractive cancer target because of its high expression in both tumor cells and on the vascular endothelial cells lining tumor blood vessels. We generated mouse monoclonal antibodies against human TM4SF1 in order to evaluate their therapeutic potential; 13 of the antibodies we generated reacted with extracellular loop-2 (EL2), TM4SF1's larger extracellular, lumen-facing domain. However, none of these antibodies reacted with mouse TM4SF1, likely because the EL2 of mouse TM4SF1 differs significantly from that of its human counterpart. Therefore, to test our antibodies in vivo, we employed an established model of engineered human vessels in which human endothelial colony-forming cells (ECFC) and human mesenchymal stem cells (MSC) are incorporated into Matrigel plugs that are implanted subcutaneously in immunodeficient nude mice. We modified the original protocol by (1) preculturing human ECFC on laminin, fibronectin, and collagen-coated plates, and (2) increasing the ECFC/MSC ratio. These modifications significantly increased the human vascular network in Matrigel implants. Two injections of one of our anti-TM4SF1 EL2 monoclonal antibodies, 8G4, effectively eliminated the human vascular component present in these plugs; they also abrogated human PC3 prostate cancer cells that were incorporated into the ECFC/MSC Matrigel mix. Together, these studies provide a mouse model for assessing tumor xenografts that are supplied by a human vascular network and demonstrate that anti-TM4SF1 antibodies such as 8G4 hold promise for cancer therapy.

FOXO1-mediated activation of Akt plays a critical role in vascular homeostasis.

RATIONALE: Forkhead box-O transcription factors (FoxOs) transduce a wide range of extracellular signals, resulting in changes in cell survival, cell cycle progression, and several cell type-specific responses. FoxO1 is expressed in many cell types, including endothelial cells (ECs). Previous studies have shown that Foxo1 knockout in mice results in embryonic lethality at E11 because of impaired vascular development. In contrast, somatic deletion of Foxo1 is associated with hyperproliferation of ECs. Thus, the precise role of FoxO1 in the endothelium remains enigmatic. OBJECTIVE: To determine the effect of endothelial-specific knockout and overexpression of FoxO1 on vascular homeostasis. METHODS AND RESULTS: We show that EC-specific disruption of Foxo1 in mice phenocopies the full knockout. Although endothelial expression of FoxO1 rescued otherwise Foxo1-null animals, overexpression of constitutively active FoxO1 resulted in increased EC size, occlusion of capillaries, elevated peripheral resistance, heart failure, and death. Knockdown of FoxO1 in ECs resulted in marked inhibition of basal and vascular endothelial growth factor-induced Akt-mammalian target of rapamycin complex 1 (mTORC1) signaling. CONCLUSIONS: Our findings suggest that in mice, endothelial expression of FoxO1 is both necessary and sufficient for embryonic development. Moreover, FoxO1-mediated feedback activation of Akt maintains growth factor responsive Akt/mTORC1 activity within a homeostatic range.

Human white adipose tissue vasculature contains endothelial colony-forming cells with robust in vivo vasculogenic potential.

Blood-derived endothelial colony-forming cells (ECFCs) have robust vasculogenic potential that can be exploited to bioengineer long-lasting human vascular networks in vivo. However, circulating ECFCs are exceedingly rare in adult peripheral blood. Because the mechanism by which ECFCs are mobilized into circulation is currently unknown, the reliability of peripheral blood as a clinical source of ECFCs remains a concern. Thus, there is a need to find alternative sources of autologous ECFCs. Here we aimed to determine whether ECFCs reside in the vasculature of human white adipose tissue (WAT) and to evaluate if WAT-derived ECFCs have equal clinical potential to blood-derived ECFCs. We isolated the complete endothelial cell (EC) population from intact biopsies of normal human subcutaneous WAT by enzymatic digestion and selection of CD31(+) cells. Subsequently, we extensively compared WAT-derived EC phenotype and functionality to bonafide ECFCs derived from both umbilical cord blood and adult peripheral blood. We demonstrated that human WAT is indeed a dependable source of ECFCs with indistinguishable properties to adult peripheral blood ECFCs, including hierarchical clonogenic ability, large expansion potential, stable endothelial phenotype, and robust in vivo blood vessel-forming capacity. Considering the unreliability and low rate of occurrence of ECFCs in adult blood and that biopsies of WAT can be obtained with minimal intervention in an ambulatory setting, our results indicate WAT as a more practical alternative to obtain large amounts of readily available autologous ECFCs for future vascular cell therapies.

Induction of erythropoiesis using human vascular networks genetically engineered for controlled erythropoietin release.

For decades, autologous ex vivo gene therapy has been postulated as a potential alternative to parenteral administration of recombinant proteins. However, achieving effective cellular engraftment of previously retrieved patient cells is challenging. Recently, our ability to engineer vasculature in vivo has allowed for the introduction of instructions into tissues by genetically modifying the vascular cells that build these blood vessels. In the present study, we genetically engineered human blood-derived endothelial colony-forming cells (ECFCs) to express erythropoietin (EPO) under the control of a tetracycline-regulated system, and generated subcutaneous vascular networks capable of systemic EPO release in immunodeficient mice. These ECFC-lined vascular networks formed functional anastomoses with the mouse vasculature, allowing direct delivery of recombinant human EPO into the bloodstream. After activation of EPO expression, erythropoiesis was induced in both normal and anemic mice, a process that was completely reversible. This approach could relieve patients from frequent EPO injections, reducing the medical costs associated with the management of anemia. We propose this ECFC-based gene-delivery strategy as a viable alternative technology when routine administration of recombinant proteins is needed.