The transmembrane glycoprotein SHPS‐1 binds the protein tyrosine phosphatase SHP‐2 and serves as its substrate. Although SHPS‐1 has been implicated in growth factor‐ and cell adhesion‐induced signaling, its biological role has remained unknown. Fibroblasts homozygous for expression of an SHPS‐1 mutant lacking most of the cytoplasmic region of this protein exhibited increased formation of actin stress fibers and focal adhesions. They spread more quickly on fibronectin than did wild‐type cells, but they were defective in subsequent polarized extension and migration. The extent of adhesion‐induced activation of Rho, but not that of Rac, was also markedly reduced in the mutant cells. Activation of the Ras–extracellular signal‐regulated kinase signaling pathway and of c‐Jun N‐terminal kinases by growth factors was either unaffected or enhanced in the mutant fibroblasts. These results demonstrate that SHPS‐1 plays crucial roles in integrin‐mediated cytoskeletal reorganization, cell motility and the regulation of Rho, and that it also negatively modulates growth factor‐induced activation of mitogen‐activated protein kinases.

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),...

Receptors coupled to the inhibitory G protein Gi, such as that for lysophosphatidic acid (LPA), have been shown to activate MAP kinase through a RAS-dependent pathway. However, LPA (but not insulin) has now been shown to activate MAP kinase in a RAS-independent manner in CHO cells that overexpress a dominant-negative mutant of the guanine nucleotide exchange protein SOS (CHO-DeltaSOS cells). LPA also induced the activation of MAP kinase kinase (MEK), but not that of RAF1, in CHO-DeltaSOS cells. The RAS-independent activation of MAP kinase by LPA was blocked by inhibitors of phosphatidylinositol 3-kinase (PI3K) or by overexpression of a dominant-negative mutant of the gamma isoform of PI3K. Furthermore, LPA induced the activation of the atypical zeta isoform of protein kinase C (PKC-zeta) in CHO-DeltaSOS cells in a manner that was sensitive to wortmannin or to the dominant-negative mutant of PI3Kgamma, and overexpression of a dominant-negative mutant of PKC-zeta inhibited LPA-induced activation of MAP kinase. These observations indicate that Gi protein-coupled receptors induce activation of MEK and MAP kinase through a RAS-independent pathway that involves PI3Kgamma-dependent activation of atypical PKC-zeta.

Our results demonstrate that NIH-3T3 cells overexpressing a mutant form of the Igf1r gene, in which arginine at 481 is substituted by glutamine, lead to reduced levels of the fold increase of IGF-IR beta-subunit phosphorylation as well as ERK1/2 and Akt phosphorylation and was accompanied by decreased cell proliferation. These results are postulated to be the cause of intrauterine and postnatal growth retardation in the described patients.

SHP-2, a SRC homology 2 domain-containing protein tyrosine phosphatase, mediates activation of Ras and mitogen-activated protein kinase by various mitogens and cell adhesion. Inhibition of endogenous SHP-2 by overexpression of a catalytically inactive (dominant negative) mutant in Chinese hamster ovary cells or Rat-1 ®broblasts has now been shown to induce a marked change in cell morphology (from elongated to less polarized) that is accompanied by substantial increases in the numbers of actin stress ®bers and focal adhesion contacts. Overexpression of the SHP-2 mutant also increased the strength of cell-substratum adhesion and resulted in hyperphosphorylation of SHPS-1, a substrate of SHP-2 that contributes to cell adhesioninduced signaling. Inhibition of SHP-2 also markedly increased the rate of cell attachment to and cell spreading on extracellular matrix proteins such as ®bronectin and vitronectin, eects that were accompanied by enhancement of adhesion-induced tyrosine phosphorylation of paxillin and p130Cas. In addition, cell migration mediated by ®bronectin or vitronectin, but not that induced by insulin, was impaired by overexpression of the SHP-2 mutant. These results suggest that SHP-2 plays an important role in the control of cell shape by contributing to cytoskeletal organization, and that it is an important regulator of integrin-mediated cell adhesion, spreading, and migration as well as of tyrosine phosphorylation of focal adhesion contact-associated proteins. Oncogene (2000) 19, 75 ± 84.