Samy Ashkar

Receptor-Ligand Interaction Between CD44 and Osteopontin (Eta-1)

The CD44 family of surface receptors regulates adhesion, movement, and activation of normal and neoplastic cells. The cytokine osteopontin (Eta-1), which regulates similar cellular functions, was found to be a protein ligand of CD44. Osteopontin induces cellular chemotaxis but not homotypic aggregation, whereas the inverse is true for the interaction between CD44 and a carbohydrate ligand, hyaluronate. The different responses of cells after CD44 ligation by either osteopontin or hyaluronate may account for the independent effects of CD44 on cell migration and growth. This mechanism may also be exploited by tumor cells to promote metastasis formation.

Phosphorylation-dependent interaction of osteopontin with its receptors regulates macrophage migration and activation

Neutrophil‐independent macrophage responses are a prominent part of delayed‐type immune and healing processes and depend on T cell‐secreted cytokines. An important mediator in this setting is the phosphoprotein osteopontin, whose secretion by activated T cells confers resistance to infection by several intracellular pathogens through recruitment and activation of macrophages. Here, we analyze the structural basis of this activity following cleavage of the phosphoprotein by thrombin into two fragments. An interaction between the C‐terminal domain of osteopontin and the receptor CD44 induces macrophage chemotaxis, and engagement of β3‐integrin receptors by a nonoverlapping N‐terminal osteopontin domain induces cell spreading and subsequent activation. Serine phosphorylation of the osteopontin molecule on specific sites is required for functional interaction with integrin but not CD44 receptors. Thus, in addition to regulation of intracellular enzymes and substrates, phosphorylation also regulates the biological activity of secreted cytokines. These data, taken as a whole, indicate that the activities of distinct osteopontin domains are required to coordinate macrophage migration and activation and may bear on incompletely understood mechanisms of delayed‐type hypersensitivity, wound healing, and granulomatous disease.

Signal Transduction of Mechanical Stimuli Is Dependent on Microfilament Integrity: Identification of Osteopontin as a Mechanically Induced Gene in Osteoblasts

Mechanical perturbation has been shown to modulate a wide variety of changes in second message signals and patterns of gene expression in osteoblasts. Embryonic chick osteoblasts were subjected to a dynamic spatially uniform biaxial strain (1.3% applied strain) at 0.25 Hz for a single 2‐h period, and osteopontin (OPN), an Arg‐Gly‐Asp (RGD)‐containing protein, was shown to be a mechanoresponsive gene. Expression of opn mRNA reached a maximal 4‐fold increase 9 h after the end of the mechanical perturbation that was not inhibited by cycloheximide, thus demonstrating that mechanoinduction of opn expression is a primary response through the activation of pre‐existing transcriptional factors. The signal transduction pathways, which mediated the increased expression of opn in response to mechanical stimuli, were shown to be dependent on the activation of a tyrosine kinase(s) and protein kinase A (PKA) or a PKA‐like kinase. Selective inhibition of protein kinase C (PKC) had no effect on the mechanoinduction of osteopontin even though opn has been demonstrated to be an early response gene to phorbol 12‐myristate 13‐acetate (PMA) stimulation. Mechanotransduction was dependent on microfilament integrity since cytochalasin‐D blocked the up‐regulation of the opn expression; however, microfilament disruption had no effect on the PMA induction of the gene. The microtubule component of the cytoskeleton was not related to the mechanism of signal transduction involved in controlling opn expression in response to mechanical stimulation since colchicine did not block opn expression. Mechanical stimulus was shown to activate focal adhesion kinase (FAK), which specifically became associated with the cytoskeleton after mechanical perturbation, and its association with the cytoskeleton was dependent on tyrosine kinase activity. In conclusion, these results demonstrate that the signal transduction pathway for mechanical activation of opn is uniquely dependent on the structural integrity of the microfilament component of the cytoskeleton. In contrast, the PKC pathway, which also activates this gene in osteoblasts, acts independently of the cytoskeleton in the transduction of its activity.

Molecular mechanisms of tumor dissemination in primary and metastatic brain cancers

Cancer is characterized by dysregulated growth control, overcoming of replicative senescence, and metastasis formation. Tumor dissemination distinguishes malignant from benign neoplasms and is mediated by homing receptors, their ligands, and proteinases. The homing receptor CD44 is frequently expressed on primary brain tumors and brain metastases. Its engagement by osteopontin physiologically induces macrophage chemotaxis, a mechanism that may be utilized by metastatic brain tumors in the process of dissemination. In host defense, osteopontin and its receptors, CD44 and integrin alpha(V)beta(3), play key roles in mediating delayed type hypersensitivity responses by activating macrophages to induce Th1 cytokines while inhibiting Th2 cytokines. Other metastasis associated gene products similarly contribute to host defenses. Hence, cancer spread is regulated by a set of developmentally non-essential genes which physiologically mediate stress responses, inflammation, wound healing, and neovascularization. Function of the relevant gene products is extensively modified post-transcriptionally and their dysregulation in cancer occurs on the levels of expression and splicing. Consistent patterns of organ preference by malignancies of particular tissue origin suggest a necessary connection between loss of growth control and senescence genes and expression of genes mediating the dissemination of tumor cells.

Stress response genes: the genes that make cancer metastasize

Abstract. Cancer is characterized by dysregulated growth control, overcoming of replicative senescence, and metastasis formation. The topology of cancer spread is mediated by a set of developmentally nonessential genes which are physiologically involved in stress responses, inflammation, wound healing, and neovascularization. The function of these gene products is extensively modified posttranscriptionally. In cancer, metastasis genes are dysregulated at the levels of expression or splicing. These genes constitute a unique group of cancer-related biomolecules.