SUMMARY The p53 tumor suppressor is a key mediator of cellular responses to various stresses. Here we show that under conditions of basal physiologic and cell-culture stress, p53 inhibits expression of the CD44 cell-surface molecule via binding to a non-canonical p53-binding sequence in the CD44 promoter. This interaction enables an untransformed cell to respond to stress-induced, p53-dependent cytostatic and apoptotic signals that would otherwise be blocked by the actions of CD44. In the absence of p53 function, the resulting de-repressed CD44 expression is essential for the growth and tumor-initiating ability of highly tumorigenic mammary epithelial cells. In both tumorigenic and non-tumorigenic cells, CD44’s expression is positively regulated by p63, a paralogue of p53. Our data indicate that CD44 is a key tumor-promoting agent in transformed tumor cells lacking p53 function. They also suggest that the de-repression of CD44 resulting from inactivation of p53 can potentially aid the survival of immortalized, premalignant, cells.

The avidity of integrin adhesion receptors for extracellular ligands is subject to dynamic regulation by intracellular programs that have yet to be elucidated. We describe here a protein, cytohesin-1, which specifically interacts with the intracellular portion of the integrin beta 2 chain (CD18). The molecule shows homology to the yeast SEC7 gene product and bears a pleckstrin homology (PH) domain. Overexpression of either the full-length cytohesin-1 or the SEC7 domain induces beta 2 integrin-dependent binding of Jurkat cells to ICAM-1, whereas expression of the isolated cytohesin-1 PH domain inhibits T cell receptor-stimulated adhesion. Similar inhibition is not exhibited by PH domains taken from other proteins, showing that the interaction is specific and that individual PH domains are capable of discriminating between alternative targets.

We investigated the influence of normal cell phenotype on the neoplastic phenotype by comparing tumors derived from two different normal human mammary epithelial cell populations, one of which was isolated using a new culture medium. Transformation of these two cell populations with the same set of genetic elements yielded cells that formed tumor xenografts exhibiting major differences in histopathology, tumorigenicity, and metastatic behavior. While one cell type (HMECs) yielded squamous cell carcinomas, the other cell type (BPECs) yielded tumors closely resembling human breast adenocarcinomas. Transformed BPECs gave rise to lung metastases and were up to 10(4)-fold more tumorigenic than transformed HMECs, which are nonmetastatic. Hence, the pre-existing differences between BPECs and HMECs strongly influence the phenotypes of their transformed derivatives.

Transforming growth factor-beta1 (TGF-beta1) is a critical cytokine for cell proliferation and differentiation. It is secreted by many cells in a latent pro-form (LTGF-beta1) from which biologically active TGF-beta1 is released by an in vivo mechanism that is not known. Here we show that the mannose-6-phosphate/insulin-like growth factor II-receptor (M6P/IGFII-R), which binds LTGF-beta1, complexes with urokinase (plasminogen activator)-receptor (uPA-R) on the surface of human monocytes and directly binds plasminogen (Plg). Plasmin generated from Plg in the complex mediates release of TGF-beta1 when M6P/IGFII-R is associated with uPA-R. Thus, this interaction of M6P/IGFII-R and uPA-R suggests a potential mechanism for the generation of TGF-beta1 by cells.

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When confined to a primary site (early stage disease), the five-year survival rate from colon cancer is ϳ90%. However, when disseminated from primary sites (metastatic), survival drops precipitously to 8%. These dramatic survival statistics underscore the need to define the pathobiology of the metastatic process to devise novel interventions to prevent or inhibit colon cancer spread. Hematogenous metastasis occurs as a highly regulated cascade of events, initiated by the escape of tumor cells from the primary site into the blood stream and culminating in the formation of secondary colonies in distant organs. This process critically involves the binding of circulating colon cancer cells to the endothelium of target tissue(s) under fluid shear conditions, as well as the dynamic formation of leukocyte/cancer cell emboli, each of which are directed by selectin-selectin ligand interactions.The selectin family of adhesion molecules, E-, P-, and L-selectin, are Ca 2ϩ -dependent lectins that bind sialofucosylated carbohydrate structures, the prototypes of which are sialyl Lewis X (sLe x ) and sialyl Lewis A (sLe a ) (1, 2). E-and P-selectin are typically inducible endothelial molecules (P-selectin is also expressed on activated platelets), and L-selectin expression is restricted to leukocytes (3). There are multiple reports that tumor cell expression of E-selectin ligand(s) promotes the metastatic spread of numerous cancer types in vivo, including colon cancer (4 -10). Colon carcinoma cells also tether and roll under dynamic flow conditions on E-selectin purified and immobilized on plastic (11, 12), as well as E-selectin presented by cytokine-stimulated human umbilical vein endothelial cells (HUVEC) 2 (11-13). Independently, L-selectin has been shown to promote colon cancer metastasis in vivo (14), presumably mediated by the physical association of leukocytes with tumor cells resulting in leukocyte-colon cancer cell emboli (15, 16). Although expression of E-selectin ligand(s) on tumor cells itself promotes metastasis, one model holds that selectins work cooperatively to promote the spread of cancer; leukocyte L-selectin engagement of relevant tumor ligand(s) mediates the formation of leukocyte-tumor aggregates, which possess heightened binding capacity to endothelium in an E-selectindependent manner (16).Colon carcinoma cells express selectin ligands, and there is abundant evidence that selectin-dependent adhesive events are central to the metastatic process (4 -10,14). However, these selectin ligands have yet to be fully characterized or identified other than by general classifications (e.g. mucins). We recently identified the sialofucosylated hematopoietic cell E-and L-selectin ligand (HCELL) glycoform of CD44 on the LS174T colon carcinoma cell line and demonstrated its function as a high affinity E-and L-selectin ligand using the blot rolling assay (17, 18). However, these studies did not specifically address the relative contribution(s) of HCELL to the observed potent E-and L-selectin ligand activity of intact LS174...

Leukocyte migration to sites of inflammation is a multistep process involving transient adhesion to the endothelium followed by cell surface-controlled proteolysis for transmigration through the vessel wall and chemotactic movement within tissues. One of the key players in this machinery appears to be the urokinase-type plasminogen activator (uPA)/uPA receptor system. The role of uPA and its receptor (CD87) in plasminogen (Plg) activation, cell adhesion, and chemotaxis is well established; however, less is known of how these activities are regulated. Here we provide evidence that the mannose 6-phosphate/insulin-like growth factor 2 receptor (CD222) controls CD87-mediated functions. Expression of human CD222 in CD222؊/؊ mouse fibroblasts downregulated Plg activation, cell adhesion, and chemotaxis induced by the uPA/CD87 system. In addition, we demonstrate that the N-terminal region of CD222, which is similar to the Plg-binding site of streptokinase, plays a crucial role in binding of CD87 and Plg. A peptide derived from this region in CD222 is able to disrupt the physical interaction of CD222 with CD87 and, furthermore, mimics the inhibitory effects of CD222 on CD87 functions. Taken together, our results indicate a novel role for CD222 in regulation of fibrinolysis, cell adhesion, and migration.

Transforming growth factor-β (TGF-β), a key modulator of endothelial cell apoptosis, must be activated from the latent form (LTGF-β) to induce biological responses. In the present study, we report activation of TGF-β by functional and physical co-operation of the mannose-6-phosphate/insulin-like-growth-factor-II receptor (CD222) and the urokinase-type plasminogen activator receptor (CD87). We show that endothelial cells express CD222 and CD87 in a membrane complex and demonstrate that the association of these two receptors is essential for the release of active TGF-β in the transduced mouse fibroblast used as model cells. By contrast, smooth-muscle cells, which express CD222 and CD87 at similar density to endothelial cells but not in complexed form, do not activate TGF-β. We also have found that mini-plasminogen is a high-affinity ligand for CD222 and is essential for the activation of TGF-β by the CD87-CD222 complex to induce apoptosis in endothelial cells. This specific mechanism of TGF-β-mediated apoptosis in endothelial cells is thus a potential novel target to be considered for treatment of pathological vascular disorders (e.g. tumor angiogenesis).