The tyrosine phosphatase SHP-1 negatively regulates cytotrophoblast proliferation in first-trimester human placenta by modulating EGFR activation

Insulin-like growth factors (IGFs) influence placental cell (cytotrophoblast) kinetics. We recently reported that the protein tyrosine phosphatase (PTP) SHP-2 positively regulates IGF actions in the placenta. In other systems, the closely related PTP, SHP-1, functions as a negative regulator of signaling events but its role in the placenta is still unknown. We examined the hypothesis that SHP-1 negatively regulates IGF actions in the human placenta. Immunohistochemical (IHC) analysis demonstrated that SHP-1 is abundant in cytotrophoblast. SHP-1 expression was decreased in first-trimester placental explants using siRNA; knockdown did not alter IGF-induced proliferation but it significantly enhanced proliferation in serum-free conditions, revealing that placental growth is endogenously regulated. Candidate regulators were determined by using antibody arrays, Western blotting, and IHC to examine the activation status of multiple receptor tyrosine kinases (RTKs) in SHP-1-depleted explants; amongst the alterations observed was enhanced activation of EGFR, suggesting that SHP-1 may interact with EGFR to inhibit proliferation. The EGFR tyrosine kinase inhibitor PD153035 reversed the elevated proliferation seen in the absence of SHP-1. This study demonstrates a role for SHP-1 in human trophoblast turnover and establishes SHP-1 as a negative regulator of EGFR activation. Targeting placental SHP-1 expression may provide therapeutic benefits in common pregnancy conditions with abnormal trophoblast proliferation.     

Integrin-mediated signal transduction

Integrins, expressed on virtually every cell type, are proteins that mediate cellular interactions with components of the extracellular matrix (ECM) and cell surface integral plasma membrane proteins. In addition, integrins interact with the cytoskeleton and through this process participate in cell migration, tissue organization, cell growth, haemostasis, inflammation, target recognition of lymphocytes and the differentiation of many cell types. Signals generated from ligand-integrin interactions are propagated via the integrin cytoplasmic tails to signal transduction pathways within the cell (outside-in signalling). Information from within the cell can also be transmitted to the outside via integrin affinity modulation (inside-out signalling). Protein tyrosine phosphorylation has a central role in integrin-initiated cell signalling, leading to cytoskeletal organization and focal adhesion formation. This review will examine the current understanding of integrin function, focusing on the intracellular consequences of integrin-ligand interaction.     

The role of ras and other low molecular weight guanine nucleotide (GTP)-binding proteins during hematopoietic cell differentiation

Recent progress in the understanding of signal transduction and gene regulation in hematopoietic cells has shown that many intracellular signalling pathways are modulated by low molecular weight guanine nucleotide (GTP)-binding proteins (LMWGs). LMWGs act as molecular switches for regulating a wide range of signal-transduction pathways in virtually all cells. In hematopoietic cells, LMWGs have been shown to participate in essential functions such as growth control, differentiation, cytoskeletal organization, cytokine and chemoattractant-induced signalling events, reduced nicotinamide adenine dinucleotide phosphate oxidase activity, intracellular vesicle transport and secretion. In human leukemias, myelodysplastic syndromes and myeloproliferative disorders, Ras activation occurs by point mutations, overexpression or by alteration of NF-1 Ras-GTPase activating protein (GAP). These are postinitiation events in leukemia but may modulate growth-factor-dependent and independent leukemic growth. Two animal models of mutated N-ras expression resulting in myelodysplastic and myeloproliferative features are discussed. The role of Ras in organ development is discussed in the context of transgenic knockout mice. More LMWG functions will certainly be identified as we gain a better understanding of regulatory pathways modulating myeloid signal transduction. This review will summarize our current understanding of this rapidly advancing area of research.     

Signal regulation by family conspiracy

The signal regulating proteins (SIRPs) are a family of ubiquitously expressed transmembrane glycoproteins composed of two subgroups: SIRPα and SIRPβ, containing more than ten members. SIRPα has been shown to inhibit signalling through a variety of receptors including receptor tyrosine kinases and cytokine receptors. This function involves protein tyrosine kinases and is dependent on immunoreceptor tyrosine-based inhibition motifs which recruit key protein tyrosine phosphatases to the membrane. Negative regulation by SIRPα may also involve its ligand, CD47, in a bi-directional signalling mechanism. The SIRPβ subtype has no cytoplasmic domain but instead associates with at least one other transmembrane protein (DAP-12, or KARAP). DAP-12 possesses immunoreceptor tyrosine-based activation motifs within its cytoplasmic domain that are thought to link SIRPβ to activating machinery. SIRPα and SIRPβ thus have complementary roles in signal regulation and may conspire to tune the response to a stimulus. Received 6 July 2000; revised 2 August 2000; accepted 5 August 2000     

Crossing paths: interactions between the cell death machinery and growth factor survival signals

Cytokines and growth factors play a crucial role in the maintenance of haematopoietic homeostasis. They transduce signals that regulate the competing commitments of haematopoietic stem cells, quiescence or proliferation, retention of stem cell pluripotency or differentiation, and survival or demise. When the balance between these commitments and the requirements of the organisms is disturbed, particularly when it favours survival and proliferation, cancer may result. Cell death provoked by loss of growth factor signalling is regulated by the Bcl-2 family of apoptosis regulators, and thus survival messages transduced by growth factors must regulate the activity of these proteins. Many aspects of direct interactions between cytokine signalling and regulation of apoptosis remain elusive. In this review, we explore the mechanisms by which cytokines, in particular Interleukin-3 and granulocyte–macrophage colony-stimulating factor, promote cell survival and suppress apoptosis as models of how cytokine signalling and apoptotic pathways intersect.     

Sphingolipids in mammalian cell signalling

Sphingolipids and their metabolites, ceramide, sphingosine and sphingosine-1-phosphate, are involved in a variety of cellular processes including differentiation, cellular senescence, apoptosis and proliferation. Ceramide is the main second messenger, and is produced by sphingomyelinase-induced hydrolysis of sphingomyelin and by de novo synthesis. Many stimuli, e.g. growth factors, cytokines, G protein-coupled receptor agonists and stress (UV irradiation) increase cellular ceramide levels. Sphingomyelin in the plasma membrane is located primarily in the outer (extracellular) leaflet of the bilayer, whilst sphingomyelinases are found at the inner (cytosolic) face and within lysosomes/endosomes. Such cellular compartmentalisation restricts the site of ceramide production and subsequent interaction with target proteins. Glycosphingolipids and sphingomyelin together with cholesterol are major components of specialised membrane microdomains known as lipid rafts, which are involved in receptor aggregation and immune responses. Many signalling molecules, for example Src family tyrosine kinases and glycosylinositolphosphate-anchored proteins, are associated with rafts, and disruption of these domains affects cellular responses such as apoptosis. Sphingosine and sphingosine-1-phosphate derived from ceramide are also signalling molecules. In particular, sphingosine-1-phosphate is involved in proliferation, differentiation and apoptosis. Sphingosine-1-phosphate can act both extracellularly through endothelial-differentiating gene (EDG) family G protein-coupled receptors and intracellularly through direct interactions with target proteins. The importance of sphingolipid signalling in cardiovascular development has been reinforced by recent reports implicating EDG receptors in the regulation of embryonic cardiac and vascular morphogenesis. Received 16 May 2001; received after revision 29 June 2001; accepted 3 July 2001     

How do Shp2 mutations that oppositely influence its biochemical activity result in syndromes with overlapping symptoms?

Activating and inactivating mutations of SHP-2 are responsible, respectively, for the Noonan (NS) and the LEOPARD (LS) syndromes. Clinically, these developmental disorders overlap greatly, resulting in the apparent paradox of similar diseases caused by mutations that oppositely influence SHP-2 phosphatase activity. While the mechanisms remain unclear, recent functional analysis of SHP-2, along with the identification of other genes involved in NS and in other related syndromes (neurofibromatosis-1, Costello and cardio-facio-cutaneous syndromes), strongly suggest that Ras/MAPK represents the major signaling pathway deregulated by SHP-2 mutants. We discuss the idea that, with the exception of LS mutations that have been shown to exert a dominant negative effect, all disease-causing mutations involved in Ras/MAPK-mediated signaling, including SHP-2, might lead to enhanced MAPK activation. This suggests that a narrow range of MAPK signaling is required for appropriate development. We also discuss the possibility that LS mutations may not simply exhibit dominant negative activity. Received 30 November 2006; received after revision 8 February 2007; accepted 13 March 2007     

Suppressor of cytokine signaling 1 blocks mitosis in human melanoma cells

Hypermethylation of SOCS genes is associated with many human cancers, suggesting a role as tumor suppressors. As adaptor molecules for ubiquitin ligases, SOCS proteins modulate turnover of numerous target proteins. Few SOCS targets identified so far have a direct role in cell cycle progression; the mechanism by which SOCS regulate the cell cycle thus remains largely unknown. Here we show that SOCS1 overexpression inhibits in vitro and in vivo expansion of human melanoma cells, and that SOCS1 associates specifically with Cdh1, triggering its degradation by the proteasome. Cells therefore show a G1/S transition defect, as well as a secondary blockade in mitosis and accumulation of cells in metaphase. SOCS1 expression correlated with a reduction in cyclin D/E levels and an increase in the tumor suppressor p19, as well as the CDK inhibitor p53, explaining the G1/S transition defect. As a result of Cdh1 degradation, SOCS1-expressing cells accumulated cyclin B1 and securin, as well as apparently inactive Cdc20, in mitosis. Levels of the late mitotic Cdh1 substrate Aurora A did not change. These observations comprise a hitherto unreported mechanism of SOCS1 tumor suppression, suggesting this molecule as a candidate for the design of new therapeutic strategies for human melanoma.     

Signalling via caveolin: involvement in the cross-talk between phosphoinositolglycans and insulin

In recent years, a number of cross-talk systems have been identified which feed into the insulin signalling cascade at the level of insulin receptor substrate (IRS) tyrosine phosphorylation, e.g., receptor and non-receptor tyrosine kinases and G-protein-coupled receptors. At the molecular level, a number of negative modulator and feedback systems somehow interacting with the beta-subunit (catecholamine-, phorbolester-, or tumor necrosis factor-alpha-induced serine/threonine phosphorylation, carboxy-terminal trimming by a thiol-dependent protease, association of inhibitory/regulatory proteins such as RAD, PC1, PED, alpha2-HS-glycoprotein) have been identified as candidate mechanisms for the impairment of insulin receptor function by elevations in the activity and/or amount of the corresponding modification enzymes/inhibitors. Both decreased responsiveness and sensitivity of the insulin receptor beta-subunit for insulin-induced tyrosine autophosphorylation have been demonstrated in several cellular and animal models of metabolic insulin resistance as well as in the adipose tissue and skeletal muscle of diabetic patients and obese Pima Indians compared to non-obese subjects. Therefore, induction of the insulin signalling cascade by bypassing the defective insulin receptor kinase may be useful for the therapy of non-insulin dependent diabetes mellitus. During the past two decades, phosphoinositolglycans (PIGs) of various origin have been demonstrated to exert potent insulin-mimetic metabolic effects upon incubation with cultured or isolated muscle and adipose cells. However, it remained to be elucidated whether these compounds actually manage to trigger insulin signalling and if so at which level of hierarchy within the signalling cascade the site of interference is located. Recent studies using isolated rat adipocytes and chemically synthesized PIG compounds point to IRS1/3 tyrosine phosphorylation by p59Lyn kinase as the site of cross-talk, the negative regulation of which by interaction with caveolin is apparently abrogated by PIG. This putative mechanism is thus compatible with the recently formulated caveolin signalling hypothesis, the supporting data for which are reviewed here. Though we have not obtained experimental evidence for the involvement of PIG in physiological insulin action, the potential cross-talk between insulin and PIG signalling, including the caveolae/detergent-insoluble glycolipid-enriched rafts as the compartments where the corresponding signalling components are concentrated, thus represent novel targets for signal transduction therapy.