CDB15:0001404 SPP1 — CD44

ABSTRACT: Previously we have shown that two members of the newly named SIBLING (small integrin-binding ligand, N-linked glycoproteins) family of proteins, bone sialoprotein, and osteopontin, bound first to a cell surface receptor and then to complement Factor H thereby blocking the lytic activity of the alternative pathway of complement. Another member of this family, dentin matrix protein 1, is shown in this paper to be very similar to osteopontin in that it can bind strongly to Factor H (K(a) approximately 1 nm) and block the lytic activity through either the vitronectin receptor (alpha(V)beta(3) integrin) or CD44. Binding of Factor H to SIBLING localized to the cells surface was demonstrated by fluorescence-activated cell sorting. Extensive overlapping fragment analyses suggests that both dentin matrix protein 1 and osteopontin interact with cell surface CD44 through their amino termini. Similar fragments of bone sialoprotein, like the intact protein, did not functionally interact with CD44. All three proteins are shown to act in conjunction with Factor I, a serum protease that, when complexed to appropriate cofactors, stops the lytic pathway by digesting the bound C3b in a series of proteolytic steps. These results show that at least three members of this family confer membrane cofactor protein-like activity (MCP or CD46) upon cells expressing RGD-binding integrins or CD44. The required order of the assembly of the complex suggests that this cofactor activity is limited to short diffusional distances.

ABSTRACT: 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 beta(3)-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.

ABSTRACT: Tumor heterogeneity presents a formidable challenge in understanding the mechanisms driving tumor progression and metastasis. The heterogeneity of hepatocellular carcinoma (HCC) in cellular level is not clear.