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Protein–protein interactions (PPIs) represent an enormous source of opportunity for therapeutic intervention. We and others have recently pinpointed key rules that will help in identifying the next generation of innovative drugs to tackle this challenging class of targets within the next decade. We used these rules to design an oriented chemical library corresponding to a set of diverse “PPI-like” modulators with cores identified as privileged structures in therapeutics. In this work, we purchased the resulting 1664 structurally diverse compounds and evaluated them on a series of representative protein–protein interfaces with distinct “druggability” potential using homogeneous time-resolved fluorescence (HTRF) technology. For certain PPI classes, analysis of the hit rates revealed up to 100 enrichment factors compared with nonoriented chemical libraries. This observation correlates with the predicted “druggability” of the targets. A specific focus on selectivity profiles, the three-dimensional (3D) molecular modes of action resolved by X-ray crystallography, and the biological activities of identified hits targeting the well-defined “druggable” bromodomains of the bromo and extraterminal (BET) family are presented as a proof-of-concept. Overall, our present study illustrates the potency of machine learning-based oriented chemical libraries to accelerate the identification of hits targeting PPIs. A generalization of this method to a larger set of compounds will accelerate the discovery of original and potent probes for this challenging class of targets.

Protein–protein interactions organize the localization, clustering, signal transduction, and degradation of cellular proteins and are therefore implicated in numerous biological functions. These interactions are mediated by specialized domains able to bind to modified or unmodified peptides present in binding partners. Among the most broadly distributed protein interaction domains, PSD95-disc large-zonula occludens (PDZ) domains are usually able to bind carboxy-terminal sequences of their partners. In an effort to accelerate the discovery of PDZ domain interactions, we have constructed an array displaying 96% of the human PDZ domains that is amenable to rapid two-hybrid screens in yeast. We have demonstrated that this array can efficiently identify interactions using carboxy-terminal sequences of PDZ domain binders such as the E6 oncoviral protein and protein kinases (PDGFRβ, BRSK2, PCTK1, ACVR2B, and HER4); this has been validated via mass spectrometry analysis. Taking advantage of this array, we show that PDZ domains of Scrib and SNX27 bind to the carboxy-terminal region of the planar cell polarity receptor Vangl2. We also have demonstrated the requirement of Scrib for the promigratory function of Vangl2 and described the morphogenetic function of SNX27 in the early Xenopusembryo. The resource presented here is thus adapted for the screen of PDZ interactors and, furthermore, should facilitate the understanding of PDZ-mediated functions.

The integrin α6β1 and its main ligand laminin-111 are overexpressed in glioblastoma, as compared with normal brain tissue, suggesting they may be involved in glioblastoma malignancy. To address this question, we stably expressed the α6 integrin subunit in the U87 cell line via retroviral-mediated gene transfer. We show that cell surface expression of the α6β1 integrin led to dramatic changes in tumor U87 cell behavior, both in vitroand in vivo. Nude mice receiving either subcutaneous or intracerebral inoculation of α6β1-expressing cells developed substantially more voluminous tumors than mice injected with control cells. The difference in tumor growth was associated with a marked increase in vascularization in response to α6β1 integrin expression and may also be related to changes in the balance between cell proliferation and survival. Indeed, expression of α6β1 enhanced proliferation and decreased apoptosis of U87 cells both in the tumor and in vitro. Additionally, we demonstrate that α6β1 is implicated in glioblastoma cell migration and invasion and that laminin-111 might mediate dissemination of α6β1-positive cells in vivo. Our results highlight for the first time the considerable role of the integrin α6β1 in glioma progression.

We present a general strategy for the dominant negative reduction in the levels of type-1 membrane-bound heterodimeric proteins within the secretory pathway through fusion of the soluble ectodomain of one of the partners to the transmembrane-cytosolic tail of the lysosomal protein Lamp1. Thus, in human embryonic kidney (HEK)-293 cells, overexpression of an integrin β3Lamp1 chimera resulted in a drastic reduction of its endogenous partner, the integrin αv subunit. The mechanism involves the formation in the endoplasmic reticulum of a αv/β3Lamp1 complex that is subsequently sorted towards a lysosomal/endosomal degradation pathway. The specificity of this approach is afforded by the invariance in the levels of the endogenous integrins α5 and β1 as compared with control cells. Conversely overexpression of integrin β3 in HEK-293 cells led to an increased level of αvβ3 at the cell surface. Functionally β3Lamp1 and β3 overexpressors exhibit decreased and increased adhesion to vitronectin, respectively, as well as diminished cellular aggregation. The application of this technology should enable the analysis of the functional importance of homodimers or heterodimers in the cell types of choice and the identification of novel partner proteins by proteomic approaches.

Integrin-mediated adhesion of cells to extracellular matrix proteins has been shown to activate various intracellular signaling events. In the present study, we demonstrate that the addition of a monoclonal antibody raised against the β4 integrin subunit in the culture medium of a clone derived from the colon adenocarcinoma cell line LoVo specifically results in stimulation of cell migration and invasion through reconstituted basement membrane matrices. Moreover, an increase in MMP-2 activity is observed. Conversely, monoclonal anti-α6 and anti-β1 have no effect on MMP-2 expression. The s.c. co-injection of adenocarcinoma cells with antibodies raised against the β4 integrin subunit to immunosuppressed newborn rats gives rise to tumors displaying altered and disorganized peri-tumoral basement membranes compared with tumors obtained when cells are injected with adenocarcinoma cells alone. Higher metastatic capacity of cells results when they are co-injected with antibodies to the β4 integrin subunit. Our results suggest that the β4 subunit of α6β4 integrin, a laminin receptor in colon adenocarcinoma, may be responsible for the specific signals which stimulate cell motility, expression of MMP-2 and tumor invasion. Int. J. Cancer 85:850–856, 2000. © 2000 Wiley-Liss, Inc.

Several integrin α subunits undergo post-translational endoproteolytic processing at pairs of basic amino acids that is mediated by the proprotein convertase furin. Here we ask whether other convertase family members can participate in these processing events. We therefore examined the endoproteolysis rate of the integrin subunits pro-α5, α6 and αv by recombinant furin, proprotein convertase (PC)5A, paired basic amino acid converting enzyme (PACE)4, PC1, PC2 and PC7 in vitro and/or ex vivo after overexpression in LoVo cells that were deficient in furin activity. We found that 60-fold more PC1 than furin was needed to produce 50% cleavage of pro-α subunit substrates in vitro; the defective pro-α chain endoproteolysis in LoVo cells was not rescued by overexpression of PC1 or PC2. No endoproteolysis occurred with PC7 either in vitro or ex vivo, although similar primary sequences of the cleavage site are found in integrins and in proteins efficiently processed by PC7, which suggests that a particular conformation of the cleavage site is required for optimal convertase-substrate interactions. In vitro, 50% cleavage of pro-α subunits was obtained with one-third of the amount of PC5A and PACE4 than of furin. In LoVo cells, PC5A remained more active than furin, PACE4 activity was quite low, and PC5B, which differs from PC5A by a C-terminal extension containing a transmembrane domain, was very inefficient in processing integrin α-subunit precursors. In conclusion, these results indicate that integrin α-subunit endoproteolytic processing involves the redundant function of furin and PC5A and to a smaller extent PACE4, but not of PC1, PC2, PC5B or PC7.

The recruitment of osteoblast progenitors involves their migration and attachment to the sites of bone formation through interactions with matrix proteins. In a time-limited cell attachment assay, coated laminin-1 inhibits the adhesion of most rat calvaria cells but attaches specifically to osteoprogenitors, as quantified by the number of bone colonies (nodules) formed in the cultures. In order to determine the molecular mechanisms involved in osteoprogenitor attachment to laminin-1, we investigated the effects of laminin-5, a N-truncated laminin variant. In contrast to laminin-1, laminin-5 increased (1.5-fold) rat calvaria cell attachment and did not display any specific affinity for osteoprogenitors. In competition experiments on laminin-5, blocking antibodies directed against either the integrin chain β1 or the C-terminal portion of laminin-5, as well as thermic denaturation of the protein at 80°C, inhibited rat calvaria cell attachment, suggesting the implication of integrin α3β1 binding to the conformation-dependent C-terminal end of laminin-5. Stepwise thermic denaturation did not suppress the anti-adhesive activity of laminin-1, while osteoprogenitor recruitment was abolished after denaturation above 60°C, suggesting that different domains are involved in these two effects. The anti-β1 antibody further decreased RC cell attachment to laminin-1, providing evidence for concomitant anti-adhesive and β1-dependent cell attachment activities. Blocking of β1 integrin subunit did not, however, reduce osteoprogenitor recruitment. Finally, purified elastase digestion fragment E1+, encompassing the N-terminal short arms of laminin-1, reproduced the effects of the complete molecule in the assay, while C-terminal fragment E8 did not display any cell attachment or osteoprogenitor recruitment properties. In conclusion, the anti-adhesive and osteoprogenitor-selective effects of laminin-1 on rat calvaria cell populations are distinct, β1-integrin-independent properties mapping to the short arms of the molecule and thus not displayed by the truncated laminin-5.

The modulation by PKC activators and inhibitors of adhesion, spreading, migration, actin cytoskeleton organization, and focal complex formation in keratinocytes attaching to type I collagen was studied. Two actin microfilament networks, stress fibers and cortical actin, could be distinguished on the basis of cellular distribution and opposite regulation by growth factors, tyrosine kinase inhibitors, and PKC activators. Stress fiber formation was stimulated by growth factors and by PMA (100 ng/ml) and these stimulations were blocked by tyrosine kinase inhibitors (0.3 mMgenistein and 1 μMherbimycin A). By contrast, the cortical network occurred in quiescent cells, was unaffected by tyrosine kinase inhibitors, and was broken down after PKC activation by PMA. Spreading, migration, and actin polymerization were completely blocked while adhesion efficacy was significantly decreased by three specific PKC inhibitors. Half-inhibition of migration was obtained with 0.025, 1, and 3 μMconcentrations of calphostin C, chelerytrine chloride, andd-erythro-sphingosine, respectively, which are concentrations close to those known to inhibit the PKC kinase functionin vitro.Paxillin clustering, which was observed even in the presence of tyrosine kinase inhibitors, disappeared only when actin polymerization was completely impaired, i.e., in cells treated with PKC inhibitors or with both tyrosine kinase inhibitors and PMA, which indicated that focal complex formation was highly dependent on microfilament reorganization. The analysis of these data underscores a major regulation function of PKC in the molecular events involved in growth factor and adhesion-dependent regulation of microfilament dynamics.

In the present report the biosynthesis of the integrin α-chains endowed with constitutive endoproteolytic cleavage was evaluated in LoVo cells where furin, a subtilisin-like convertase involved in post-translational endoproteolytic processing, is not functional. It was found that cell-surface α3, α6 and αv subunits were not processed endoproteolytically into heavy and light chains as they were in HT29-D4 cells, a furin-competent cell line. Complete removal of N-linked oligosaccharides and pulse–chase experiments confirmed that the cleavage of the α6 integrin subunit occurring 45 min after translation in HT29 cells did not take place in LoVo cells. Apart from cleavage deficiency, α6 subunit glycosylation, association with β4 subunits and targeting to the plasma membrane seemed comparable in LoVo and HT29 cells. The pro-α6 and the pro-α3 subunits immunopurified from LoVo cells were highly sensitive to endoproteolysis by recombinant furin. Furin cleavage was calcium dependent and resulted in the conversion of the 140 kDa pro-α6 into a 120 kDa heavy chain. These results suggest strongly that furin is involved in the endoproteolytic processing of cleavable integrin α subunits.

The production of laminin by 14-day fetal rat intestinal endoderm and mesenchyme was investigated. The amount of neosynthesized laminin was measured after purification using affinity chromatography. Chain composition of laminin was analyzed by immunoblotting and immunofluorescence staining. The data show that both embryonic intestinal tissue components synthesize laminin and that A and B1/B2 chains were detected in both endodermal and mesenchymal cells. The cellular source of laminin found at the epithelial basement membrane has been studied by immunocytochemistry in rat/chick or mouse/chick interspecies hybrid intestines taken at various stages of development. Immunodetection of the whole laminin molecule and of the individual A and B1/B2 chains by rodent-specific polyclonal and monoclonal antibodies at the basement membrane level in these hybrid intestines revealed (a) laminin molecules, which originate from both mesenchymal and endodermal cells; (b) deposition of A and B1/B2 chains by endodermal cells, regardless of the stage of growth of the hybrid intestines; and (c) asynchronous deposition of the various chains of laminin into the basement membrane by the mesenchyme. B1/B2 chains are deposited concomitant with contact with the epithelium, whereas A chains appear only later (13 days after grafting). These data reinforce the suggestion from previous studies that cooperation between epithelium and mesenchyme is necessary for the formation of a complete basement membrane in the developing intestine.

We have previously described an inverse relationship between Cdx1and Cdx2mRNA levels and the extent of dysplasia and severity of clinical outcome in colorectal carcinoma, suggesting that altered expression of these genes was associated with colorectal carcinogenesis or tumor progression. To investigate further their involvement in the physiopathology of colorectal cancer, HT29 colon carcinoma cells that show very lowCdxexpression were transfected with Cdx1and/or Cdx2cDNA to elicit their overexpression. Growth rate, tumorigenicity, resistance to apoptosis, and migration potential of the corresponding cells were analyzed. Growth rate of cells overexpressing Cdx2decreased by half, whereas overexpression of Cdx1had no effect. However, cells overexpressing both Cdxs had a growth rate reduced to 20% of control. In cells overexpressing Cdx1orCdx2, tumorigenicity and resistance to apoptosis induced by serum starvation, ceramide, or staurosporine were not changed compared with control cells; yet phorbol ester-stimulated cell migration was decreased by 50%. In cells overexpressing both Cdx1andCdx2, tumorigenicity was decreased by 50%, resistance to apoptosis was significantly lowered, and stimulated cell migration was further decreased to 15% of control compared with cells expressingCdx1or Cdx2. Finally, cells overexpressing both Cdxs showed strongly decreased Bcl-2 expression, which could account for their increased sensitivity to apoptosis. These findings show that, in HT29 cells, both Cdx1andCdx2genes must be expressed to reduce tumorigenic potential, to increase sensitivity to apoptosis, and to reduce cell migration, suggesting that the two genes control the normal phenotype by independent pathways. This may explain why loss of Cdx1or Cdx2expression is associated with tumor development and invasiveness in colorectal tumors.

Fibronectin is a major component of the extracellular matrix of adherent layers of human long-term marrow cultures where it may stabilize the extracellular matrix network and provide adhesion sites for primitive hemopoietic cells. This study was devised to analyze the role of adherent cell populations in fibronectin synthesis, matrix assembly, and degradation. In cultures performed under the conditions described by Gartner and Kaplan, immunopreci-pitation after metabolic labeling showed that adherent cells synthesized a fibronectin variant comprising the EDa domain and lacking the EDb one. Vascular smooth musclelike stromal cells were the cell subset responsible for this synthesis. Once synthesized by stromal cells, EDa + fibronectin was secreted into the supernatant and incorporated into the extracellular matrix. The cumulation in the extracellular matrix was predominant by weeks 5 and 6 of culture, when a decrease in the stromal cell intracytoplasmic content of fibronectin was observed. Stromal cells from a transformed cell line, L20ri —, were also able to synthesize the EDa + fibronectin variant, although for these cells the assembly into the extracellular matrix was partly impaired. Besides stromal cells, other cell types participated in fibronectin synthesis: early-adhering granulomonocytic cells and macrophages appearing later in culture were able to synthesize an EDa —, EDb — fibronectin variant, clearly distinct from the EDa + variant produced by stromal cells. Studies on cultures in which macrophage growth was stimulated at the expense of stromal cells by adding granulocyte-macrophage colony-stimulating factor (50 ng/mL) to the culture medium showed a striking decrease in amounts of fibronectin measured in the adherent layer. This decrease was caused by a lack of incorporation of fibronectin in the extracellular matrix, disclosing a major difference between stromal cells and macrophages in terms of matrix assembly. This study confirms the similarity between stromal cells and vascular smooth muscle cells, because in vivo subendothelial intimal aortic smooth muscle cells and cultured smooth muscle cells from the aortic media express the EDa +, EDb — fibronectin variant. Furthermore, our results suggest that the level of fibronectin in adherent layers is regulated by stromal cells and macrophages. The balance between these two cell populations may therefore be crucial for the local control of hemopoiesis by regulating the extracellular fibronectin available for the adhesion of hematopoietic cells. Our data indicate that it may be essential to study the adhesion of stem cells to EDa +, EDb — fibronectin instead of EDa —, EDb — soluble fibronectin, as found in human plasma.