Faculty Bibliography

Grb2-associated binder 2 (Gab2) is a recently identified member of the Gab/Daughter of sevenless family of docking proteins, which localize, amplify and integrate signaling pathways activated by various receptors including receptor tyrosine kinases (RTKs). To date, Gab2 signaling has been primarily investigated in hematopoietic cells. Here we report marked overexpression of Gab2 in a subset of breast cancer cell lines relative to normal breast epithelial strains and a trend for increased Gab2 expression in estrogen receptor (ER)-positive lines. Overexpression relative to normal ductal epithelium was also observed in some primary breast cancers. In MCF-7 breast cancer cells Gab2 was markedly tyrosine phosphorylated in response to heregulin and also following EGF, insulin or bFGF administration, indicating that a variety of RTKs implicated in breast cancer development or progression couple to this docking protein. In hormone-responsive breast cancer cells, GAB2 mRNA and protein expression were induced by estradiol in a manner sensitive to the pure anti-estrogen ICI 182780, indicating that this regulation is mediated via the ER. Gab2 therefore represents a novel link between steroid and growth factor signaling in breast cancer, and when overexpressed, may modulate the sensitivity of breast cancer cells to these important growth regulators.

The BCR/ABL oncogene causes chronic myelogenous leukemia (CML) in humans and a CML-like disease, as well as lymphoid leukemia, in mice. p210 BCR/ABL is an activated tyrosine kinase that phosphorylates itself and several cellular signaling proteins. The autophosphorylation site tyrosine 177 binds the adaptor Grb2 and helps determine the lineage and severity of BCR/ABL disease: Tyr177 mutation (BCR/ABL-Y177F) dramatically impairs myeloid leukemogenesis, while diminishing lymphoid leukemogenesis. The critical signal(s) from Tyr177 has remained unclear. We report that Tyr177 recruits the scaffolding adaptor Gab2 via a Grb2/Gab2 complex. Compared to BCR/ABL-expressing Ba/F3 cells, BCR/ABL-Y177F cells exhibit markedly reduced Gab2 tyrosine phosphorylation and association of phosphatidylinositol-3 kinase (PI3K) and Shp2 with Gab2 and BCR/ABL, and decreased PI3K/Akt and Ras/Erk activation, cell proliferation, and spontaneous migration. Remarkably, bone marrow myeloid progenitors from Gab2 (-/-) mice are resistant to transformation by BCR/ABL, whereas lymphoid transformation is diminished as a consequence of markedly increased apoptosis. BCR/ABL-evoked PI3K/Akt and Ras/Erk activation also are impaired in Gab2 (-/-) primary myeloid and lymphoid cells. Our results identify Gab2 and its associated proteins as key determinants of the lineage and severity of BCR/ABL transformation.

Receptor tyrosine kinases (RTKs) play distinct roles in multiple biological systems. Many RTKs transmit similar signals, raising questions about how specificity is achieved. One potential mechanism for RTK specificity is control of the magnitude and kinetics of activation of downstream pathways. We have found that the protein tyrosine phosphatase Shp2 regulates the strength and duration of phosphatidylinositol 3'-kinase (PI3K) activation in the epidermal growth factor (EGF) receptor signaling pathway. Shp2 mutant fibroblasts exhibit increased association of the p85 subunit of PI3K with the scaffolding adapter Gab1 compared to that for wild-type (WT) fibroblasts or Shp2 mutant cells reconstituted with WT Shp2. Far-Western analysis suggests increased phosphorylation of p85 binding sites on Gab1. Gab1-associated PI3K activity is increased and PI3K-dependent downstream signals are enhanced in Shp2 mutant cells following EGF stimulation. Analogous results are obtained in fibroblasts inducibly expressing dominant-negative Shp2. Our results suggest that, in addition to its role as a positive component of the Ras-Erk pathway, Shp2 negatively regulates EGF-dependent PI3K activation by dephosphorylating Gab1 p85 binding sites, thereby terminating a previously proposed Gab1-PI3K positive feedback loop. Activation of PI3K-dependent pathways following stimulation by other growth factors is unaffected or decreased in Shp2 mutant cells. Thus, Shp2 regulates the kinetics and magnitude of RTK signaling in a receptor-specific manner.

CD22, a negative regulator of B cell antigen receptor signaling, binds glycoconjugates terminating in alpha2, 6 sialic acid. The physiological ligand(s) for CD22 remain unknown. We asked whether the sialic acid binding domains are necessary for CD22 to function as a negative regulator. We generated two mutants that lack sialic acid binding activity and expressed them in a novel CD22(-/-) murine B cell line. Anti-IgM activated B cells expressing either CD22 mutant had greater Ca(2+) responses than cells expressing wild-type CD22. Each variant also had reduced CD22 tyrosine phosphorylation and Src homology 2 domain-containing protein tyrosine phosphatase-1 association. These data suggest that the alpha2, 6 sialic acid ligand binding activity of CD22 is critical for its negative regulatory functions.

We have developed a mathematical theory that describes the regulation of signaling pathways as a function of a limited number of key parameters. Our analysis includes linear kinase-phosphatase cascades, as well as systems containing feedback interactions, crosstalk with other signaling pathways, and/or scaffolding and G proteins. We find that phosphatases have a more pronounced effect than kinases on the rate and duration of signaling, whereas signal amplitude is controlled primarily by kinases. The simplest model pathways allow amplified signaling only at the expense of slow signal propagation. More complex and realistic pathways can combine high amplification and signaling rates with maintenance of a stable off-state. Our models also explain how different agonists can evoke transient or sustained signaling of the same pathway and provide a rationale for signaling pathway design.

Mice lacking the protein-tyrosine phosphatase PTP1B are hypersensitive to insulin and resistant to obesity. However, the molecular basis for resistance to obesity has been unclear. Here we show that PTP1B regulates leptin signaling. In transfection studies, PTP1B dephosphorylates the leptin receptor-associated kinase, Jak2. PTP1B is expressed in hypothalamic regions harboring leptin-responsive neurons. Compared to wild-type littermates, PTP1B(-/-) mice have decreased leptin/body fat ratios, leptin hypersensitivity, and enhanced leptin-induced hypothalamic Stat3 tyrosyl phosphorylation. Gold thioglucose treatment, which ablates leptin-responsive hypothalamic neurons, partially overcomes resistance to obesity in PTP1B(-/-) mice. Our data indicate that PTP1B regulates leptin signaling in vivo, likely by targeting Jak2. PTP1B may be a novel target to treat leptin resistance in obesity.

When bound by extracellular ligands, receptor tyrosine kinases (RTKs) on the cell surface transmit critical signals to the cell interior. Although signal termination is less well understood, protein tyrosine phosphatase-1B (PTP1B) is implicated in the dephosphorylation and inactivation of several RTKs. However, PTP1B resides on the cytoplasmic surface of the endoplasmic reticulum (ER), so how and when it accesses RTKs has been unclear. Using fluorescence resonance energy transfer (FRET) methods, we monitored interactions between the epidermal- and platelet-derived growth factor receptors and PTP1B. PTP1B-catalyzed dephosphorylation required endocytosis of the receptors and occurred at specific sites on the surface of the ER. Most of the RTKs activated at the cell surface showed interaction with PTP1B after internalization, establishing that RTK activation and inactivation are spatially and temporally partitioned within cells.

The erythro-megakaryoblastic leukemia cell line K562 undergoes erythroid or myeloid differentiation in response to treatment with various inducing agents. We observed that expression of the SH2-containing protein tyrosine phosphatase SHP-1 was induced upon exposure of K562 cells to differentiating agents. Under the same conditions, expression of SHP-2, a close relative of SHP-1, and the more distantly related PTP-1 B remained unchanged. Induction of SHP-1 expression correlates with dephosphorylation of a specific and limited set of tyrosyl phosphoproteins, suggesting that dephosphorylation of these proteins may be important for the differentiation process. Importantly, expression of exogenous SHP-1 inhibits K562 proliferation and alters the adhesion properties of these cells, indicating a more differentiated phenotype. Moreover, SHP-1 is found in a complex with both p210 Bcr-Abl and p190 Bcr-Abl, suggesting that it may regulate Bcr-Abl or Bcr-Abl-associated phosphotyrosyl proteins. Our results indicate that induction of SHP-1 expression is important for K562 differentiation in response to various inducers and raise the possibility that functional inactivation of SHP-1 may play a role in progression to blast crisis in chronic myelogenous leukemia.

Dos/Gab family scaffolding adapters (Dos, Gab1, Gab2) bind several signal relay molecules, including the protein-tyrosine phosphatase Shp-2 and phosphatidylinositol-3-OH kinase (PI(3)K); they are also implicated in growth factor, cytokine and antigen receptor signal transduction. Mice lacking Gab1 die during embryogenesis and show defective responses to several stimuli. Here we report that Gab2-/- mice are viable and generally healthy; however, the response (for example, degranulation and cytokine gene expression) of Gab2-/- mast cells to stimulation of the high affinity immunoglobulin-epsilon (IgE) receptor Fc(epsilon)RI is defective. Accordingly, allergic reactions such as passive cutaneous and systemic anaphylaxis are markedly impaired in Gab2-/- mice. Biochemical analyses reveal that signalling pathways dependent on PI(3)K, a critical component of Fc(epsilon)RI signalling, are defective in Gab2-/- mast cells. Our data identify Gab2 as the principal activator of PI(3)K in response to Fc(epsilon)RI activation, thereby providing genetic evidence that Dos/Gab family scaffolds regulate the PI(3)K pathway in vivo. Gab2 and/or its associated signalling molecules may be new targets for developing drugs to treat allergy.

Previous reports indicate that the expression and/or activity of the protein-tyrosine phosphatase (PTP) LAR are increased in insulin-responsive tissues of obese, insulin-resistant humans and rodents, but it is not known whether these alterations contribute to the pathogenesis of insulin resistance. To address this question, we generated transgenic mice that overexpress human LAR, specifically in muscle, to levels comparable to those reported in insulin-resistant humans. In LAR-transgenic mice, fasting plasma insulin was increased 2.5-fold compared with wild-type controls, whereas fasting glucose was normal. Whole-body glucose disposal and glucose uptake into muscle in vivo were reduced by 39-50%. Insulin injection resulted in normal tyrosyl phosphorylation of the insulin receptor and insulin receptor substrate 1 (IRS-1) in muscle of transgenic mice. However, phosphorylation of IRS-2 was reduced by 62%, PI3' kinase activity associated with phosphotyrosine, IRS-1, or IRS-2 was reduced by 34-57%, and association of p85alpha with both IRS proteins was reduced by 39-52%. Thus, overexpression of LAR in muscle causes whole-body insulin resistance, most likely due to dephosphorylation of specific regulatory phosphotyrosines on IRS proteins. Our data suggest that increased expression and/or activity of LAR or related PTPs in insulin target tissues of obese humans may contribute to the pathogenesis of insulin resistance.