Negative Regulation of Platelet Clearance and of the Macrophage Phagocytic Response by the Transmembrane Glycoprotein SHPS-1

Abstract

SHPS-1 is a receptor-type glycoprotein that binds and activates the protein-tyrosine phosphatases SHP-1 and SHP-2, and thereby negatively modulates intracellular signaling initiated by various cell surface receptors coupled to tyrosine kinases. SHPS-1 also regulates intercellular communication in the neural and immune systems through its association with CD47 (integrin-associated protein) on adjacent cells. Furthermore, recent studies with fibroblasts derived from mice expressing an SHPS-1 mutant that lacks most of the cytoplasmic region suggested that the intact protein contributes to cytoskeletal function. Mice homozygous for this SHPS-1 mutation have now been shown to manifest thrombocytopenia. These animals did not exhibit a defect in megakaryocytopoiesis or in platelet production. However, platelets were cleared from the bloodstream more rapidly in the mutant mice than in wild-type animals. Furthermore, peritoneal macrophages from the mutant mice phagocytosed red blood cells more effectively than did those from wild-type mice; in addition, they exhibited an increase both in the rate of cell spreading and in the formation of filopodia-like structures at the cell periphery. These results indicate that SHPS-1 both contributes to the survival of circulating platelets and down-regulates the macrophage phagocytic response.SHPS-1 is a transmembrane glycoprotein that is abundant in neural and myeloid tissues (1-6). This molecule is also known as SIRP␣1 (7), BIT (8), MFR (9), and p84 neural adhesion molecule (10). The cytoplasmic region of SHPS-1 contains two immunoreceptor tyrosine-based inhibitory motifs, which recruit and activate the Src homology 2 domain-containing protein-tyrosine phosphatases SHP-1 and SHP-2 in a phosphorylation-dependent manner (1, 7, 11). The putative extracellular region of this protein comprises three immunoglobulin (Ig)-like domains, of which the most amino-terminal, IgV-like domain associates with the ligand CD47, also known as integrin-associated protein (6,12,13).Tyrosine phosphorylation of SHPS-1 is induced by soluble growth factors (1,7,14,15), integrin-mediated cell adhesion (16 -18), or cross-linking of Fc␥ receptors (19). Overexpression of SHPS-1 inhibits the activation of extracellular signal-regulated kinases induced by growth factors such as insulin, epidermal growth factor, and platelet-derived growth factor (7); it also inhibits promotion of the motility and survival of glioblastoma cells by epidermal growth factor (20). Furthermore, SHPS-1 inhibits IgE-induced mediator secretion and cytokine synthesis by mast cells (21). These observations suggest that SHPS-1, presumably by recruiting SHP-1 or SHP-2, negatively modulates a wide range of cellular activation signals initiated by tyrosine kinase-coupled receptors. However, the physiological significance of these observations remains unclear.Recent studies have suggested that SHPS-1, through its association with CD47, contributes to cellular functions that depend on intercellular communication, including T cell activation (13),...

Cd47-Sirpα interaction and IL-10 constrain inflammation-induced macrophage phagocytosis of healthy self-cells

“…Moreover, we found that thioglycollate, a reagent commonly used to elicit macrophages in the peritoneum, activates PEM phagocytosis toward RBCs (Fig. 3H), explaining the seemingly discrepant results reported by others (28).…”

“…Resolution of at least two puzzles remains paramount: first, why macrophages do not attack self in mice deficient of Cd47 or Sirpα? Despite that the Cd47-Sirpα-mediated inhibition is missing, mice deficient of Cd47 (26), Sirpα (established in this study), or the Sirpα cytoplasmic domain (27) are relatively healthy, manifesting no or only minor phenotypes (9,18,28) that suggest macrophage phagocytosis toward self-cells. Adoptive transfer of Cd47-null RBCs into these mutant mice also failed to induce rapid elimination (1) (also shown in this study).…”

Anti-CD47 antibodies promote phagocytosis and inhibit the growth of human myeloma cells

“…6,9 CD47 on target cells interacts with SIRPa on macrophages and transmits a 'don't eat me' signal through SH2 domain-containing phosphatases, SHP-1 and -2, and results in the inhibition of target cell phagocytosis. 7,28,29 CD47 is expressed at lower levels on bone marrow resident hematopoietic stem cells (HSCs) than on circulating HSCs. However, when bone marrow HSCs were mobilized out of bone marrow, they upregulated CD47.…”

Resistance of B16 Melanoma Cells to CD47-induced Negative Regulation of Motility as a Result of Aberrant N-Glycosylation of SHPS-1

“…Given the role of SHPS-1 in cytoskeletal reorganization (20,21) and the fact that B16F10 cells are highly motile whereas Melan-a cells are not (30), we focused our attention on the effect of CD47 binding to SHPS-1 on cell migration. Quantitative analysis with a Boyden chamber assay revealed that the number of Melan-a cells that migrated through a membrane coated with fibronectin was reduced by ϳ20% in the presence of CD47-Fc (Fig.…”

“…SHPS-1 negatively or positively regulates intracellular signaling initiated either by tyrosine kinase-coupled receptors for growth factors or by cell adhesion to extracellular matrix proteins (18,19). With the use of mice lacking most of the cytoplasmic region of SHPS-1 (20,21), we recently showed that this protein plays an important role both in integrin-mediated cytoskeletal reorganization and in cell migration. In addition, studies with dominant negative or loss-of-function mutant proteins have established SHP-2 as a signaling component essential for cell adhesion and migration as well as cytoskeletal organization (22)(23)(24)(25)(26).…”