Biopanning and rapid analysis of selective interactive ligands.

Here we introduce a new approach for the screening, selection and sorting of cell-surface-binding peptides from phage libraries. Biopanning and rapid analysis of selective interactive ligands (termed BRASIL) is based on differential centrifugation in which a cell suspension incubated with phage in an aqueous upper phase is centrifuged through a non-miscible organic lower phase. This single-step organic phase separation is faster, more sensitive and more specific than current methods that rely on washing steps or limiting dilution. As a proof-of-principle, we screened human endothelial cells stimulated with vascular endothelial growth factor (VEGF) and constructed a peptide-based ligand-receptor map of the VEGF family. Next, we validated the motif PQPRPL as a novel chimeric ligand mimic that binds specifically to VEGF receptor-1 and to neuropilin-1. BRASIL may prove itself a superior method for probing target cell surfaces with a broad range of potential applications.

Aminopeptidase N is a receptor for tumor-homing peptides and a target for inhibiting angiogenesis.

Phage that display a surface peptide with the NGR sequence motif home selectively to tumor vasculature in vivo. A drug coupled to an NGR peptide has more potent antitumor effects than the free drug [W. Arap et al., Science (Washington DC), 279: 377-380, 1998]. We show here that the receptor for the NGR peptides in tumor vasculature is aminopeptidase N (APN; also called CD13). NGR phage specifically bound to immunocaptured APN and to cells engineered to express APN on their surface. Antibodies against APN inhibited in vivo tumor homing by the NGR phage. Immunohistochemical staining showed that APN expression is up-regulated in endothelial cells within mouse and human tumors. In another tissue that undergoes angiogenesis, corpus luteum, blood vessels also expressed APN, but APN was not detected in blood vessels of various other normal tissues stained under the same conditions. APN antagonists specifically inhibited angiogenesis in chorioallantoic membranes and in the retina and suppressed tumor growth. Thus, APN is involved in angiogenesis and can serve as a target for delivering drugs into tumors and for inhibiting angiogenesis.

Model-Based Design of a Decision Tree for Treating HER2+ Cancers Based on Genetic and Protein Biomarkers.

Human cancers are incredibly diverse with regard to molecular aberrations, dependence on oncogenic signaling pathways, and responses to pharmacological intervention. We wished to assess how cellular dependence on the canonical PI3K vs. MAPK pathways within HER2+ cancers affects responses to combinations of targeted therapies, and biomarkers predictive of their activity. Through an integrative analysis of mechanistic model simulations and in vitro cell line profiling, we designed a six-arm decision tree to stratify treatment of HER2+ cancers using combinations of targeted agents. Activating mutations in the PI3K and MAPK pathways (PIK3CA and KRAS), and expression of the HER3 ligand heregulin determined sensitivity to combinations of inhibitors against HER2 (lapatinib), HER3 (MM-111), AKT (MK-2206), and MEK (GSK-1120212; trametinib), in addition to the standard of care trastuzumab (Herceptin). The strategy used to identify effective combinations and predictive biomarkers in HER2-expressing tumors may be more broadly extendable to other human cancers.

Identification of receptor ligands with phage display peptide libraries.

With the development and maturation of the technology of displaying peptides on bacteriophage, it has become possible to isolate peptide ligands to various targets. In the phage display strategy, up to 10(9) peptides of different permutations are expressed on the surface of filamentous phage. Thus, peptides capable of binding target molecules in vitro and even target tissues in vivo can be identified. In recent years, a series of libraries that display degenerate peptides of different lengths have been constructed, and specific ligands to cell surface receptors, such as integrins, have been isolated. In the in vivo biopanning, peptides targeting distinct organs or tumors have been rescued after intravenous administration of phage libraries into mice. In one application, the isolated peptide ligands have been used to direct a cytotoxic drug to tumor vasculature in mice. Further applications in radioimaging and radiotherapy are being investigated.

CD13/APN is activated by angiogenic signals and is essential for capillary tube formation.

In the hematopoietic compartment, the CD13/APN metalloprotease is one of the earliest markers of cells committed to the myeloid lineage where it is expressed exclusively on the surface of myeloid progenitors and their differentiated progeny. CD13/APN is also found in nonhematopoietic tissues, and its novel expression on the endothelial cells of angiogenic, but not normal, vasculature was recently described. Treatment of animals with CD13/APN inhibitors significantly impaired retinal neovascularization, chorioallantoic membrane angiogenesis, and xenograft tumor growth, indicating that CD13/APN plays an important functional role in vasculogenesis and identifying it as a critical regulator of angiogenesis. To investigate the mechanisms of CD13/APN induction in tumor vasculature, the regulation of CD13/APN by factors contributing to angiogenic progression was studied. In this report, it is shown that endogenous CD13/APN levels in primary cells and cell lines are up-regulated in response to hypoxia, angiogenic growth factors, and signals regulating capillary tube formation during angiogenesis. Transcription of reporter plasmids containing CD13/APN proximal promoter sequences is significantly increased in response to the same angiogenic signals that regulate the expression of the endogenous gene and in human tumor xenografts, indicating that this fragment contains elements essential for the angiogenic induction of CD13/APN expression. Finally, functional antagonists of CD13/APN interfere with tube formation but not proliferation of primary vascular endothelial cells, suggesting that CD13/APN functions in the control of endothelial cell morphogenesis. These studies clearly establish the CD13/APN metalloprotease as an important regulator of endothelial morphogenesis during angiogenesis.

SEM1, a homologue of the split hand/split foot malformation candidate gene Dss1, regulates exocytosis and pseudohyphal differentiation in yeast.

The exocyst is an essential multiprotein complex mediating polarized secretion in yeast. Here we describe a gene, SEM1, that can multicopy-suppress exocyst mutants sec3-2, sec8-9, sec10-2, and sec15-1. SEM1 is highly conserved among eukaryotic species. Its human homologue, DSS1, has been suggested as a candidate gene for the split hand/split foot malformation disorder. SEM1 is not an essential gene. However, its deletion rescued growth of the temperature-sensitive exocyst mutants sec3-2, sec8-9, sec10-1, and sec15-1 at the restrictive temperature. Cell fractionation showed that Sem1p is mainly cytosolic but also associates with the microsomal fraction. In linear sucrose gradients, Sem1p cosedimented with the exocyst component Sec8p. In diploid cells that normally do not form pseudohyphae (S288C background), deletion of SEM1 triggered pseudohyphal growth. This phenotype was abolished after reintroduction of either SEM1 or the mouse homologue Dss1 into the cells. In diploids that have normal capacity for pseudohyphal growth (Sigma1278b background), deletion of SEM1 enhanced filamentous growth. The functionality of both SEM1 and Dss1 in a differentiation process in yeast suggests that Dss1 indeed could be the gene affected in the split hand/split foot malformation disorder. These results characterize SEM1 as a regulator of both exocyst function and pseudohyphal differentiation and suggest a unique link between these two cellular functions in yeast.

An anti-angiogenic state in mice and humans with retinal photoreceptor cell degeneration.

Abnormal angiogenesis accompanies many pathological conditions including cancer, inflammation, and eye diseases. Proliferative retinopathy because of retinal neovascularization is a leading cause of blindness in developed countries. Another major cause of irreversible vision loss is retinitis pigmentosa, a group of diseases characterized by progressive photoreceptor cell degeneration. Interestingly, anecdotal evidence has long suggested that proliferative diabetic retinopathy is rarely associated clinically with retinitis pigmentosa. Here we show that neonatal mice with classic inherited retinal degeneration (Pdeb(rd1)/Pdeb(rd1)) fail to mount reactive retinal neovascularization in a mouse model of oxygen-induced proliferative retinopathy. We also present a comparable human paradigm: spontaneous regression of retinal neovascularization associated with long-standing diabetes mellitus occurs when retinitis pigmentosa becomes clinically evident. Both mouse and human data indicate that reactive retinal neovascularization either fails to develop or regresses when the number of photoreceptor cells is markedly reduced. Our findings support the hypothesis that a functional mechanism underlying this anti-angiogenic state is failure of the predicted up-regulation of vascular endothelial growth factor, although other growth factors may also be involved. Preventive and therapeutic strategies against both proliferative and degenerative retinopathies may emerge from this work.