Identification of protein-tyrosine phosphatase 1B as the major tyrosine phosphatase activity capable of dephosphorylating and activating c-Src in several human breast cancer cell lines

Calpain 2 and PTP1B function in a novel pathway with Src to regulate invadopodia dynamics and breast cancer cell invasion

Invasive cancer cells form dynamic adhesive structures associated with matrix degradation called invadopodia. Calpain 2 is a calcium-dependent intracellular protease that regulates adhesion turnover and disassembly through the targeting of specific substrates such as talin. Here, we describe a novel function for calpain 2 in the formation of invadopodia and in the invasive abilities of breast cancer cells through the modulation of endogenous c-Src activity. Calpain-deficient breast cancer cells show impaired invadopodia formation that is rescued by expression of a truncated fragment of protein tyrosine phosphatase 1B (PTP1B) corresponding to the calpain proteolytic fragment, which indicates that calpain modulates invadopodia through PTP1B. Moreover, PTP1B activity is required for efficient invadopodia formation and breast cancer invasion, which suggests that PTP1B may modulate breast cancer progression through its effects on invadopodia. Collectively, our experiments implicate a novel signaling pathway involving calpain 2, PTP1B, and Src in the regulation of invadopodia and breast cancer invasion.

PTP1B regulates cortactin tyrosine phosphorylation by targeting Tyr446

The emergence of protein-tyrosine phosphatase 1B (PTP1B) as a potential drug target for treatment of diabetes, obesity, and cancer underlies the importance of understanding its full range of cellular functions. Here, we have identified cortactin, a central regulator of actin cytoskeletal dynamics, as a substrate of PTP1B. A trapping mutant of PTP1B binds cortactin at the phosphorylation site Tyr(446), the regulation and function of which have not previously been characterized. We show that phosphorylation of cortactin Tyr(446) is induced by hyperosmolarity and potentiates apoptotic signaling during prolonged hyperosmotic stress. This study advances the importance of Tyr(446) in the regulation of cortactin and provides a potential mechanism to explain the effects of PTP1B on processes including cell adhesion, migration, and tumorigenesis.

Apoptosis of estrogen-receptor negative breast cancer and colon cancer cell lines by PTP alpha and src RNAi

We show that siRNA-mediated suppression of protein tyrosine phosphatase alpha (PTP alpha) reduces Src activity 2 to 4-fold in breast, colon and other human cancer cell lines. Src and PTP alpha RNAi induced apoptosis in estrogen receptor (ER)-negative breast cancer and colon cancer cells, but not in immortalized noncancerous breast cells, ER-positive breast cancer cells or other cancer cell types tested. RNAi of other Src family members (Fyn and Yes) or of PTP1B, a phosphatase previously suggested to be an activator of Src in breast cancer, had no effect. Although further tests with primary tumor tissues are required, the unexpected correlation between ER status and Src/PTP alpha dependence in breast cancer cell lines may be important for planning therapeutic strategies, and the insensitivity of normal breast cells to the RNAi highlights the potential of PTP alpha, which may be easier to target than Src, as a therapeutic target in ER-negative breast cancer.

ER stress is associated with dedifferentiation and an epithelial-to-mesenchymal transition-like phenotype in PC Cl3 thyroid cells

Conditions perturbing the homeostasis of the endoplasmic reticulum (ER) cause accumulation of unfolded proteins and trigger ER stress. In PC Cl3 thyroid cells, thapsigargin and tunicamycin interfered with the folding of thyroglobulin, causing accumulation of this very large secretory glycoprotein in the ER. Consequently, mRNAs encoding BiP and XBP-1 were induced and spliced, respectively. In the absence of apoptosis, differentiation of PC Cl3 cells was inhibited. mRNA and protein levels of the thyroid-specific genes encoding thyroglobulin, thyroperoxidase and the sodium/iodide symporter and of the genes encoding the thyroid transcription factors TTF-1, TTF-2 and Pax-8 were dramatically downregulated. These effects were, at least in part, transcriptional. Moreover, they were selective and temporally distinct from the general and transient PERK-dependent translational inhibition. Thyroid dedifferentiation was accompanied by changes in the organization of the polarized epithelial monolayer. Downregulation of the mRNA encoding E-cadherin, and upregulation of the mRNAs encoding vimentin, α-smooth muscle actin, α(1)(I) collagen and SNAI1/SIP1, together with formation of actin stress fibers and loss of trans-epithelial resistance were found, confirming an epithelial-mesenchymal transition (EMT). The thyroid-specific and epithelial dedifferentiation by thapsigargin or tunicamycin were completely prevented by the PP2 inhibitor of Src-family kinases and by stable expression of a dominant-negative Src. Together, these data indicate that ER stress induces dedifferentiation and an EMT-like phenotype in thyroid cells through a Src-mediated signaling pathway.

PTP1B contributes to the oncogenic properties of colon cancer cells through Src activation

Src-specific activity has been reported to be elevated in a high percentage of colon cancer cell lines and tumors, but the underlying mechanisms are largely unknown. In this study, we report that, in the seven cancer cell lines tested, Src-specific activity was elevated (5.2- to 18.7-fold) relative to normal colon cells (FHC). This activation of Src correlated with reduced phosphorylation at Y530 of Src, whereas there was no significant change in the level of phosphorylation at Y419. The membrane tyrosine phosphatase activity for a Src family-specific phosphopeptide substrate FCP (Fyn COOH-terminal peptide phosphorylated by Csk) was greatly increased in the cancer cells and was attributed to PTP1B in most of the cell lines. Membrane PTP1B protein levels were also greatly increased. Overexpression of PTP1B increased Src specific activity in colon cancer cells by reducing phosphorylation at Y530 of Src. It also increased anchorage-independent cell growth and this increase was blocked by the Src inhibitor PP2 and Src small interfering RNA (siRNA). Down-regulating PTP1B activity by PTP1B inhibitor CinnGEL 2Me or knocking down PTP1B using siRNA also reduced Src kinase activity and colony formation ability of colon cancer cells. PTP1B siRNA reduced tumor growth in nonobese diabetic/severe combined immunodeficient mice. This study suggests that (a) PTP1B can act as an important activator of Src in colon cancer cells via dephosphorylation at Y530 of Src and (b) elevated levels of PTP1B can increase tumorigenicity of colon cancer cells by activating Src.

The role of Src in prostate cancer

The Src family kinases (SFKs) are the largest family of nonreceptor protein tyrosine kinases and are responsible for signal transduction during many cellular activities, including differentiation, adhesion, and migration. Aberrant Src/SFK activity has been widely implicated in cancer development. Several lines of evidence indicate a role for SFKs in the development of prostate cancer, e.g. SFK overexpression in prostate cancer cell lines and tissues and reduced cancer cell proliferation, invasion, and migration following Src inhibition. In particular, Src may be involved in androgen-independent growth during advanced stages of disease. Src signaling is also a key pathway during normal and dysregulated bone functioning, and bone metastases are responsible for substantial morbidity in advanced prostate cancer. Src/SFK inhibition therefore represents a potentially useful therapeutic strategy for patients with various stages of prostate cancer. To date, four Src inhibitors have reached clinical trials. Of these, the broadest range of in vitro prostate cancer data are available for dasatinib, which inhibits several SFKs as well as other tyrosine kinases. Src inhibitors may be specifically evaluated in prostate cancer clinical trials in the near future.

Recent development of small molecular specific inhibitor of protein tyrosine phosphatase 1B

Protein tyrosine phosphatases (PTPs), a large family of signaling enzymes, play essential roles in intracellular signal transduction by regulating the cellular level of tyrosine phosphorylation to control cell growth and differentiation, metabolism, cell migration, gene transcription, ion-channel activity, immune response, cell apoptosis, and bone development. Among all PTPs, protein tyrosine phosphatase 1B (PTP1B) plays a seminal role in cellular signaling and in many human diseases, including cancer, diabetes, and obesity. Therefore, small molecular inhibitors of PTP1B can be promising drug candidates. Because of the structural homologies in many families of PTPs, it is a challenging task to find inhibitors specific to each PTP. Recent studies suggested that secondary binding pockets or peripheral binding sites around the conserved active site should be exploited to design novel potent and selective PTP1B inhibitors. In this review, we discuss the structural and biological features of small molecular PTP1B-specific inhibitors, with particular emphasis on small molecular inhibitors targeting PTP1B over the other PTPs that have been synthesized in the past 4 years.

Tonic B-cell and viral ITAM signaling: context is everything

The presence of an immunoreceptor tyrosine-based activation motif (ITAM) makes immunoreceptors different from other signaling receptors, like integrins, G-coupled protein receptors, chemokine receptors, and growth factor receptors. This unique motif has the canonical sequence D/Ex(0-2)YxxL/Ix(6-8)YxxL/I, where x represents any amino acid and is present at least once in all immunoreceptor complexes. Immunoreceptors can promote survival, activation, and differentiation by transducing signals through these highly conserved motifs. Traditionally, ITAM signaling is thought to occur in response to ligand-induced aggregation, although evidence indicates that ligand-independent tonic signaling also provides functionally relevant signals. The majority of proteins containing ITAMs are transmembrane proteins that exist as part of immunoreceptor complexes. However, oncogenic viruses also have ITAM-containing proteins. In this review, we discuss what is known about tonic signaling by both cellular and viral ITAM-containing proteins and speculate what we might learn from each context.

Growth inhibition and induction of early apoptosis by arenicolsterol A, a novel cytotoxic enolic sulphated sterol from the marine annelid, Arenicola cristata

Arenicolsterol A (ASA), a novel cytotoxic enolic sulphated sterol, was isolated from the marine annelid, Arenicola cristata (AC). Growth inhibition of this compound on cancer cell lines was determined by MTT assay and suppression of tumour stem cells colony formation. The results showed that ASA was selectively cytotoxic on HeLa cell line (IC(50) = 6.00 +/- 1.16 micromol L(- 1) on HeLa cell line, IC(50) = 10.85 +/- 0.97 micromol L(- 1) on 929 cell line and 14.72 +/- 1.55 micromol L(- 1) on NCI-h6 cell line). In addition, the apoptosis induced by ASA was verified from monitoring the stainability with Annexin V and propidium iodine by a fluorescence-activated cell sorter. The experimental data confirmed that ASA could induce apoptosis in HeLa cells by arresting early stage in apoptosis. Meanwhile, the apoptosis was found to be correlative with the inhibition of the protein tyrosine phosphatases (cdc25A, cdc25B, JSP1, etc). Therefore, ASA might be a novel promising precursor of anticancer medicines.

Vascular signaling through cholesterol-rich domains: implications in hypertension

PURPOSE OF REVIEW

Lipid rafts are emerging as key players in the integration of cellular responses. Alterations in these highly regulated signaling cascades are important in structural, mechanical and functional abnormalities that underlie vascular pathological processes. The present review focuses on recent advances in signal transduction through caveolae/lipid rafts, implicated in hypertensive processes.

RECENT FINDINGS

Caveolae/lipid rafts function as sites of dynamic regulatory events in receptor-induced signal transduction. Mediators of vascular function, including G-protein coupled receptors, Src family tyrosine kinases, receptor tyrosine kinases, protein phosphatases and nitric oxide synthase, are concentrated within these microdomains. The assembly of functionally active nicotinamide adenine dinucleotide phosphate oxidase and subsequent reactive oxygen species production are also dependent on interactions within the caveolae/lipid rafts. Recent findings have also demonstrated the importance of actin-cytoskeleton and focal adhesion sites for protein interactions with caveolae/lipid raft.

SUMMARY

Many vascular signaling processes are altered in hypertension. Whether these events involve lipid rafts/caveolae remains unclear. A better understanding of how signaling molecules compartmentalize in lipid rafts/caveolae will provide further insights into molecular mechanisms underlying vascular damage in cardiovascular disease.

Proteomic approaches to studying protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) constitute a large family of enzymes that play key roles in cell signaling. Deregulation of PTP activity results in aberrant tyrosine phosphorylation, which has been linked to the etiology of several human diseases, including cancer. Since phosphate removal by the PTPs can both enhance and antagonize cellular signaling, it is essential to elucidate the physiological context in which PTPs operate. Two powerful proteomic approaches have been developed to rapidly establish the exact functional roles for every PTP, both in normal cellular physiology and in pathogenic conditions. In the first, an affinity-based substrate-trapping approach has been employed for PTP substrate identification. Identification and characterization of specific PTP-substrate interactions will associate functions with PTP as well as implicate PTP to specific signaling pathways. In the second, a number of activity-based PTP probes have been developed that can provide a direct readout of the functional state of the PTPs in complex proteomes. The ability to profile the entire PTP family on the basis of changes in their activity is expected to yield new functional insights into pathways regulated by the PTPs and contribute to the discovery of PTPs as novel therapeutic targets. Effective application of these proteomic techniques will accelerate the functional characterization of PTPs, thereby facilitating our understanding of PTPs in cell signaling and in diseases.

PTP1B and TC-PTP: novel roles in immune-cell signaling

It has recently been demonstrated that the protein tyrosine phosphatase (PTP) PTP1B and the T-cell PTP (TC-PTP) target several substrates involved in immune cell signaling. Recent data have furthered the view of these 2 PTP members as key regulators of the immune response. This review will focus on the substrate specificities of PTP1B and TC-PTP and their roles in immune cell signaling, and will discuss some new data implicating PTP1B and TC-PTP in myeloid development.

Protein tyrosine phosphatase function: the substrate perspective

It is now well established that the members of the PTP (protein tyrosine phosphatase) superfamily play critical roles in fundamental biological processes. Although there has been much progress in defining the function of PTPs, the task of identifying substrates for these enzymes still presents a challenge. Many PTPs have yet to have their physiological substrates identified. The focus of this review will be on the current state of knowledge of PTP substrates and the approaches used to identify them. We propose experimental criteria that should be satisfied in order to rigorously assign PTP substrates as bona fide. Finally, the progress that has been made in defining the biological roles of PTPs through the identification of their substrates will be discussed.

A brake becomes an accelerator: PTP1B--a new therapeutic target for breast cancer

The protein tyrosine phosphatase PTP1B, previously recognized for its role in downregulating insulin and leptin signaling, has now been shown to function as a positive regulator of signaling events associated with breast tumorigenesis. Inhibitors of PTP1B that have been developed as drug candidates for treatment of diabetes and obesity may offer new avenues for the treatment of breast cancer.

Protein tyrosine phosphatase 1B deficiency or inhibition delays ErbB2-induced mammary tumorigenesis and protects from lung metastasis

We investigated the role of protein tyrosine phosphatase 1B (PTP1B) in mammary tumorigenesis using both genetic and pharmacological approaches. It has been previously shown that transgenic mice with a deletion mutation in the region of Erbb2 encoding its extracellular domain (referred to as NDL2 mice, for 'Neu deletion in extracellular domain 2') develop mammary tumors that progress to lung metastasis. However, deletion of PTP1B activity in the NDL2 transgenic mice either by breeding with Ptpn1-deficient mice or by treatment with a specific PTP1B inhibitor results in significant mammary tumor latency and resistance to lung metastasis. In contrast, specific overexpression of PTP1B in the mammary gland leads to spontaneous breast cancer development. The regulation of ErbB2-induced mammary tumorigenesis by PTB1B occurs through the attenuation of both the MAP kinase (MAPK) and Akt pathways. This report provides a rationale for the development of PTP1B as a new therapeutic target in breast cancer.

The role of intracellular signaling in insulin-mediated regulation of drug metabolizing enzyme gene and protein expression

Endogenous factors, including hormones, growth factors and cytokines, play an important role in the regulation of hepatic drug metabolizing enzyme expression in both physiological and pathophysiological conditions. Diabetes, fasting, obesity, protein-calorie malnutrition and long-term alcohol consumption produce changes in hepatic drug metabolizing enzyme gene and protein expression. This difference in expression alters the metabolism of xenobiotics, including procarcinogens, carcinogens, toxicants and therapeutic agents, potentially impacting the efficacy and safety of therapeutic agents, and/or resulting in drug-drug interactions. Although the mechanisms by which xenobiotics regulate drug metabolizing enzymes have been studied intensively, less is known regarding the cellular signaling pathways and components which regulate drug metabolizing enzyme gene and protein expression in response to hormones and cytokines. Recent findings, however, have revealed that several cellular signaling pathways are involved in hormone- and growth factor-mediated regulation of drug metabolizing enzymes. Our laboratory has reported that insulin and growth factors regulate drug metabolizing enzyme gene and protein expression, including cytochromes P450 (CYP), glutathione S-transferases (GST) and microsomal epoxide hydrolase (mEH), through receptors which are members of the large receptor tyrosine kinase (RTK) family, and by downstream effectors such as phosphatidylinositol 3-kinase, mitogen activated protein kinase (MAPK), Akt/protein kinase B (PKB), mammalian target of rapamycin (mTOR), and the p70 ribosomal protein S6 kinase (p70S6 kinase). Here, we review current knowledge of the signaling pathways implicated in regulation of drug metabolizing enzyme gene and protein expression in response to insulin and growth factors, with the goal of increasing our understanding of how disease affects these signaling pathways, components, and ultimately gene expression and translational control.

Inhibitors of protein tyrosine phosphatases: next-generation drugs?

The protein tyrosine phosphatases (PTPs) constitute a family of closely related key regulatory enzymes that dephosphorylate phosphotyrosine residues in their protein substrates. Malfunctions in PTP activity are linked to various diseases, ranging from cancer to neurological disorders and diabetes. Consequently, PTPs have emerged as promising targets for therapeutic intervention in recent years. In this review, general aspects of PTPs and the development of small-molecule inhibitors of PTPs by both academic research groups and pharmaceutical companies are discussed. Different strategies have been successfully applied to identify potent and selective inhibitors. These studies constitute the basis for the future development of PTP inhibitors as drugs.

Association of Hck genetic polymorphisms with gene expression and COPD

Polymorphonuclear leukocytes (PMNs) are major effector cells in the chronic airway inflammation in chronic obstructive pulmonary disease (COPD). PMN degranulation is associated with degradation of extracellular matrix and tissue damage. Hck is an essential molecule in the signaling pathway regulating PMN degranulation. We hypothesized that polymorphisms affect the expression level of Hck, which, in turn, modulates PMN mediator release and tissue damage and influences the development of COPD. Here we systematically investigated genetic tag polymorphisms of the Hck gene, Hck mRNA and protein expression pattern in PMNs, and PMN mediator release (myeloperoxidase) in 60 healthy white subjects, and assessed their association with the use of several genetic models. The association of genetic polymorphisms with COPD-related phenotypes was determined in the lung healthy study cohort (LHS). We identified a novel 15 bp insertion/deletion polymorphism (8,656 L/S) in intron 1 of the Hck gene, which was associated with differential expression of Hck protein and PMN myeloperoxidase release. In the LHS cohort, there was significant interaction between the 8,656 L/S polymorphism and smoking on baseline lung function and 8,656 L/S was associated with bronchodilator response. These data suggest that the insertion/deletion polymorphism could be a functional polymorphism of the Hck gene, may contribute to COPD pathogenesis and modify COPD-related phenotypes.

Tyrosine phosphatases epsilon and alpha perform specific and overlapping functions in regulation of voltage-gated potassium channels in Schwann cells

Tyrosine phosphatases (PTPs) epsilon and alpha are closely related and share several molecular functions, such as regulation of Src family kinases and voltage-gated potassium (Kv) channels. Functional interrelationships between PTPepsilon and PTPalpha and the mechanisms by which they regulate K+ channels and Src were analyzed in vivo in mice lacking either or both PTPs. Lack of either PTP increases Kv channel activity and phosphorylation in Schwann cells, indicating these PTPs inhibit Kv current amplitude in vivo. Open probability and unitary conductance of Kv channels are unchanged, suggesting an effect on channel number or organization. PTPalpha inhibits Kv channels more strongly than PTPepsilon; this correlates with constitutive association of PTPalpha with Kv2.1, driven by membranal localization of PTPalpha. PTPalpha, but not PTPepsilon, activates Src in sciatic nerve extracts, suggesting Src deregulation is not responsible exclusively for the observed phenotypes and highlighting an unexpected difference between both PTPs. Developmentally, sciatic nerve myelination is reduced transiently in mice lacking either PTP and more so in mice lacking both PTPs, suggesting both PTPs support myelination but are not fully redundant. We conclude that PTPepsilon and PTPalpha differ significantly in their regulation of Kv channels and Src in the system examined and that similarity between PTPs does not necessarily result in full functional redundancy in vivo.

Inhibition of phosphotyrosine phosphatase 1B causes resistance in BCR-ABL-positive leukemia cells to the ABL kinase inhibitor STI571

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of BCR-ABL-mediated transformation in vitro and in vivo. To investigate whether PTP1B modulates the biological effects of the abl kinase inhibitor STI571 in BCR-ABL-positive cells, we transfected Philadelphia chromosome-positive (Ph+) chronic myeloid leukemia cell-derived K562 cells with either wild-type PTP1B (K562/PTP1B), a substrate-trapping dominant-negative mutant PTP1B (K562/D181A), or empty vector (K562/mock). Cells were cultured with or without STI571 and analyzed for its effects on proliferation, differentiation, and apoptosis. In both K562/mock and K562/PTP1B cells, 0.25 to 1 mumol/L STI571 induced dose-dependent growth arrest and apoptosis, as measured by a decrease of cell proliferation and an increase of Annexin V-positive cells and/or of cells in the sub-G(1) apoptotic phase. Western blot analysis showed increased protein levels of activated caspase-3 and caspase-8 and induction of poly(ADP-ribose) polymerase cleavage. Low concentrations of STI571 promoted erythroid differentiation of these cells. Conversely, K562/D181A cells displayed significantly lower PTP1B-specific tyrosine phosphatase activity and were significantly less sensitive to STI571-induced growth arrest, apoptosis, and erythroid differentiation. Pharmacologic inhibition of PTP1B activity in wild-type K562 cells, using bis(N,N-dimethylhydroxamido)hydroxooxovanadate, attenuated STI571-induced apoptosis. Lastly, comparison of the STI571-sensitive Ph+ acute lymphoblastic leukemia cell line SupB15 with a STI571-resistant subline revealed significantly decreased PTP1B activity and enhanced BCR-ABL phosphorylation in the STI571-resistant SupB15 cells. In conclusion, functional PTP1B is involved in STI571-induced growth and cell cycle arrest, apoptosis, and differentiation, and attenuation of PTP1B function may contribute to resistance towards STI571.

Src tyrosine kinase as a chemotherapeutic target: is there a clinical case?

Src tyrosine kinase was the first protooncogene described. It has been found to be overexpressed and activated in a large number of different cancers. Cellular Src has been shown to activate a number of different effectors that are involved in different aspects of cancer biology such as metastasis, cell cycle regulation and cell survival. Despite this, Src inhibitors have not entered the regular arsenal of chemotherapeutics. This article reviews some of the biology, rationale, in vitro and in vivo preclinical evidence, and some very early clinical trials demonstrating efficacy of Src inhibitors.

ER-bound PTP1B is targeted to newly forming cell-matrix adhesions

Here, we define the mechanism through which protein tyrosine phosphatase 1B (PTP1B) is targeted to cell-matrix adhesion sites. Green fluorescent protein (GFP)-labeled PTP1B bearing the substrate-trapping mutation D181A was found in punctate structures in lamellae. The puncta co-localized with focal adhesion kinase (FAK) and Src, and defined the distal tips of cell-matrix adhesion sites identified with paxillin and vinculin. PTP1B is largely associated with the external face of the endoplasmic reticulum (ER) and the puncta develop from ER projections over cell-matrix adhesion sites, a process dependent on microtubules. Deletion of the ER-targeting sequence resulted in cytosolic localization and altered the distribution of PTP1B at cell-matrix foci, whereas mutations disrupting interactions with Src homology 3 (SH3) domains, and the insulin and cadherin receptors had no effect. PTP1B recognizes substrates within forming adhesion foci as revealed by its preferential association with paxillin as opposed to zyxin-containing foci. Our results suggest that PTP1B targets to immature cell-matrix foci in newly forming lamellae by dynamic extensions of the ER and contributes to the maturation of these sites.

Protein-tyrosine phosphatases and cancer

Tyrosine phosphorylation is an important signalling mechanism in eukaryotic cells. In cancer, oncogenic activation of tyrosine kinases is a common feature, and novel anticancer drugs have been introduced that target these enzymes. Tyrosine phosphorylation is also controlled by protein-tyrosine phosphatases (PTPs). Recent evidence has shown that PTPs can function as tumour suppressors. In addition, some PTPs, including SHP2, positively regulate the signalling of growth-factor receptors, and can be oncogenic. An improved understanding of how these enzymes function and how they are regulated might aid the development of new anticancer agents.

Phosphorylated α-Actinin and Protein-tyrosine Phosphatase 1B Coregulate the Disassembly of the Focal Adhesion Kinase·Src Complex and Promote Cell Migration

The focal adhesion kinase (FAK) is a key regulator of cell migration. Phosphorylation at Tyr-397 activates FAK and creates a binding site for Src family kinases. FAK phosphorylates the cytoskeletal protein alpha-actinin at Tyr-12. Here we report that protein-tyrosine phosphatase 1B (PTP 1B) is an alpha-actinin phosphatase. PTP 1B-dependent dephosphorylation of alpha-actinin was seen in COS-7 cells and PTP 1B-null fibroblasts reconstituted with PTP 1B. Furthermore, we show that coexpression of wild-type alpha-actinin and PTP 1B causes dephosphorylation at Tyr-397 in FAK. No dephosphorylation was observed in cells coexpressing the alpha-actinin phosphorylation mutant Y12F and PTP 1B. Furthermore, the phosphorylation at four other sites in FAK was not altered by PTP 1B. In addition, we found that phosphorylated alpha-actinin bound to Src and reduced the binding of FAK to Src. The dephosphorylation at Tyr-397 in FAK triggered by wild-type alpha-actinin and PTP 1B caused a significant increase in cell migration. We propose that phosphorylated alpha-actinin disrupts the FAK x Src complex exposing Tyr-397 in FAK to PTP 1B. These findings uncover a novel feedback loop involving phosphorylated alpha-actinin and PTP 1B that regulates FAK x Src interaction and cell migration.

Regulation of the catalytic activity of PTP1B: Roles for cell adhesion, tyrosine residue 66, and proline residues 309 and 310

The reversible phosphorylation of proteins on tyrosine residues is fundamental to a variety of intracellular signaling pathways and is controlled by the actions of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). While much progress has been made in understanding the regulation of PTKs, there is still relatively little known concerning the regulation of PTPs. Using immune complex phosphatase assays, we demonstrated that the enzymatic activity of the nonreceptor type PTP, PTP1B, is regulated by cell adhesion. Placing primary human foreskin fibroblasts (HFFs) in suspension leads to a distinct increase in PTP1B activity, whereas the readhesion of suspended HFFs onto fibronectin or collagen I inhibited activity. To gain insight into the mechanisms involved, we analyzed recombinant forms of PTP1B mutated at potential regulatory sites. Our results indicated that tyrosine residue 66 is essential for maintaining activity at 37 degrees C. We also found that the C-terminal region of PTP1B and localization to the endoplasmic reticulum are not required for the inhibition of activity by cell adhesion. However, analysis of PA-PTP1B, in which alanines are substituted for prolines 309 and 310, revealed an important role for these residues as the catalytic activity of this mutant did not decrease following readhesion onto collagen I. Since the binding of p130cas and Src to PTP1B is dependent upon these proline residues, we assayed the regulation of PTP1B in mouse embryo fibroblasts deficient in these proteins. We found that neither p130cas nor Src is required for the inhibition of PTP1B activity by adhesion to extracellular matrix proteins. Additionally, pretreatment with cytochalasin D did not prevent the reduction of PTP1B activity when cells adhered to collagen I, indicating that cell spreading is not required for this regulation. The control of the catalytic activity of PTP1B by cell adhesion demonstrated in this study is likely to have important implications for growth factor and insulin signaling.

Intracellular reactive oxygen species activate Src tyrosine kinase during cell adhesion and anchorage-dependent cell growth

Src tyrosine kinases are central components of adhesive responses and are required for cell spreading onto the extracellular matrix. Among other intracellular messengers elicited by integrin ligation are reactive oxygen species, which act as synergistic mediators of cytoskeleton rearrangement and cell spreading. We report that after integrin ligation, the tyrosine kinase Src is oxidized and activated. Src displays an early activation phase, concurrent with focal adhesion formation and driven mainly by Tyr527 dephosphorylation, and a late phase, concomitant with reactive oxygen species production, cell spreading, and integrin-elicited kinase oxidation. In addition, our results suggest that reactive oxygen species are key mediators of in vitro and in vivo v-Src tumorigenic properties, as both antioxidant treatments and the oxidant-insensitive C245A and C487A Src mutants greatly decrease invasivity, serum-independent and anchorage-independent growth, and tumor onset. Therefore we propose that, in addition to the known phosphorylation/dephosphorylation circuitry, redox regulation of Src activity is required during both cell attachment to the extracellular matrix and tumorigenesis.

Involvement of the small protein tyrosine phosphatases TC-PTP and PTP1B in signal transduction and diseases: from diabetes, obesity to cell cycle, and cancer

As in other fields of biomedical research, the use of gene-targeted mice by homologous recombination in embryonic stem cells has provided important findings on the function of several members of the protein tyrosine phosphatase (PTP) family. For instance, the phenotypic characterization of knockout mice has been critical in understanding the sites of action of the related PTPs protein tyrosine phosphatase 1B (PTP1B) and T-cell-PTP (TC-PTP). By their increased insulin sensitivity and insulin receptor hyperphosphorylation, PTP1B null mice demonstrated a clear function for this enzyme as a negative regulator of insulin signaling. As well, TC-PTP has also been recently involved in insulin signaling in vitro. Importantly, the high identity in their amino acid sequences suggests that they must be examined simultaneously as targets of drug development. Indeed, they possess different as well as overlapping substrates, which suggest complementary and overlapping roles of both TC-PTP and PTP1B. Here, we review the function of PTP1B and TC-PTP in diabetes, obesity, and processes related to cancer.

Cytoplasmic protein tyrosine phosphatases, regulation and function: the roles of PTP1B and TC-PTP

PTP1B and TC-PTP are closely related protein tyrosine phosphatases, sharing 74% homology in their catalytic domain. However, their cellular localization, function, and regulation are found to be different. Their substrate specificity has implicated these enzymes in various signaling pathways, regulating metabolism, proliferation and cytokine signaling. For instance, PTP1B has been shown to regulate the activation of cytokine receptors through the dephosphorylation of specific members of the JAK family, namely JAK2 and TYK2, whereas TC-PTP is involved in the modulation of cytokine signaling via JAK1 and JAK3 molecules. Gene-targeting approaches will help us to unravel the physiological functions of these enzymes.

Sensitivity of protein tyrosine phosphatase activity to the redox environment, cytochrome C, and microperoxidase

Protein tyrosine phosphatase activity depends on a catalytic thiolate group on an acidic cysteine residue that is sensitive to reactive oxygen species. Representative of this family of enzymes is protein tyrosine phosphatase 1B (PTP1B), a major target for type 2 diabetes therapy. PTP1B is sensitive to hydrogen peroxide (H2O2) in vitro and in cells. It is also sensitive to glutathionylation by glutathione disulfide (GSSG). The sensitivity of PTP1B to the redox state of its environment was partially characterized in vitro by examination of phosphatase activity in the presence of various concentrations of glutathione (GSH) and GSSG. Enzyme sensitivity to glutathionylation was dependent on the amount of available thiol groups and increased as GSH concentration was increased. The half-inhibitory concentration for H2O2 was much less than that of GSSG in the presence of low concentrations of GSH, indicating that reaction with H2O2 is much more likely than is glutathionylation by GSSG. PTP1B and a related oxidant-sensitive phosphatase, PTEN, were also sensitive to the lipid peroxidation by-product 4-hydroxynonenal. Furthermore, PTP1B was inhibited by cytochrome c and microperoxidase. Taken together, these data suggest that not only H2O2, but also a variety of redox-active metabolites and hemes can oxidatively inactivate PTPs with potentially profound implications for signal transduction.

The role of protein-tyrosine phosphatase 1B in integrin signaling

Protein-tyrosine phosphatase 1B (PTP1B) is a key negative regulator of insulin and leptin signaling and a novel therapeutic target for the treatment of type 2 diabetes, obesity, and other associated metabolic syndromes. Because PTP1B regulates multiple signal pathways and it can both enhance and antagonize a cellular event, it is important to establish the physiological relevance of PTP1B in these processes. In this study, we utilize potent and selective PTP1B inhibitors to delineate the role of PTP1B in integrin signaling. We show that down-regulation of PTP1B activity with small molecule inhibitors suppresses cell spreading and migration to fibronectin, increases Tyr(527) phosphorylation in Src, and decreases phosphorylation of FAK, p130(Cas), and ERK1/2. In addition, PTP1B "substrate-trapping" mutants bind Tyr(527)-phosphorylated Src and protect it from dephosphorylation by endogenous PTP1B. These results establish that PTP1B promotes integrin-mediated responses in fibroblasts by dephosphorylating the inhibitory pTyr(527) and thereby activating the Src kinase. We also show that PTP1B forms a complex with Src and p130(Cas), and that the proline-rich motif PPRPPK (residues 309-314) in PTP1B is essential for the complex formation. We suggest that the specificity of PTP1B for Src pTyr(527) is mediated by protein-protein interactions involving the docking protein p130(Cas) with both Src and PTP1B in addition to the interactions between the PTP1B active site and the pTyr(527) motif.

Protein tyrosine phosphatases and breast cancer

Protein tyrosine phosphatases (PTPs) consist of a large family of related enzymes, including the group of classical PTPs with its two main subgroups, the transmembrane receptor-type (RPTPs) and the intracellular or non-transmembrane PTPs. Published data on the expression and function of a panel of these enzymes in normal and cancerous breast tissues are discussed in this review. While most studies, albeit on different enzymes, have tended to agree on the evidence for an increased PTP expression in breast cancer, any connection between PTP expression and the enzymes' role in cancer development and progression remains largely open to interpretation. Concomitant increases of protein tyrosine kinase (PTK) and PTP activities in many cancers further indicate that a complex dysregulation in the balance of tyrosine phosphorylation could be responsible for major alterations in various cellular processes controlling tissue homeostasis. In particular, any relationship between the expression of PTPs and their specific diverse roles in the regulation of cell growth and apoptosis in breast cancer needs to be addressed in major fundamental, preclinical and clinical studies.

Src tyrosine kinase regulates insulin-induced activation of protein kinase C (PKC) delta in skeletal muscle

Insulin stimulation of skeletal muscle results in rapid activation of protein kinase Cdelta (PKCdelta), which is associated with its tyrosine phosphorylation and physical association with insulin receptor (IR). The mechanisms underlying tyrosine phosphorylation of PKCdelta have not been determined. In this study, we investigated the possibility that the Src family of nonreceptor tyrosine kinases may be involved upstream insulin signaling. Studies were done on differentiated rat skeletal myotubes in primary culture. Insulin caused an immediate stimulation of Src and induced its physical association with both IR and PKCdelta. Inhibition of Src by treatment with the Src family inhibitor PP2 reduced insulin-stimulated Src-PKCdelta association, PKCdelta tyrosine phosphorylation and PKCdelta activation. PP2 inhibition of Src also decreased insulin-induced IR tyrosine phosphorylation, IR-PKCdelta association and association of Src with both PKCdelta and IR. Finally, inhibition of Src decreased insulin-induced glucose uptake. We conclude that insulin activates Src tyrosine kinase, which regulates PKCdelta activity. Thus, Src tyrosine kinase may play an important role in insulin-induced tyrosine phosphorylation of both IR and PKCdelta. Moreover, both Src and PKCdelta appear to be involved in IR activation and subsequent downstream signaling.

Selective regulation of tumor necrosis factor-induced Erk signaling by Src family kinases and the T cell protein tyrosine phosphatase

The proinflammatory cytokine tumor necrosis factor (TNF) modulates cellular responses through the mitogen-activated protein kinase (MAPK) and nuclear factor-kappaB (NF-kappaB) signaling pathways, but the molecular mechanisms underlying MAPK activation are unknown. T cell protein tyrosine phosphatase (TCPTP) is essential for hematopoietic development and negatively regulates inflammatory responses. Using TCPTP-deficient fibroblasts, we show here that TCPTP regulates TNF-induced MAPK but not NF-kappaB signaling. TCPTP interacted with the adaptor protein TRAF2, and dephosphorylated and inactivated Src tyrosine kinases to suppress downstream signaling through extracellular signal-regulated kinases and production of interleukin 6. These results link TCPTP and Src tyrosine kinases to the selective regulation of TNF-induced MAPK signaling and identify a previously unknown mechanism for modulating inflammatory responses mediated by TNF.

Peroxynitrite-Dependent Upregulation of Src Kinases in Red Blood Cells: Strategies to Study the Activation Mechanisms

Several studies have demonstrated that treatment of cells with oxidants, and in particular with peroxynitrite, may cause the upregulation of tyrosine phosphorylation signaling. In erythrocytes, peroxynitrite induces tyrosine phosphorylation of the major intrinsic membrane protein, band 3. A closer look at the enzymes involved revealed that the effect of peroxynitrite was due to the inhibition of phosphotyrosine phosphatases and/or to the activation of src kinases. The activity of src kinases is modulated not only by phosphatases and other kinases but also through redox modification of cysteine residues: Peroxynitrite can, thus, affect src kinase activity by means of direct and indirect mechanisms. In this chapter, we describe the different pathways leading to src kinase activation and the experimental procedures that can be performed to reveal the activation mechanism. The aim is to provide a more general strategy adaptable to different cell types and different oxidants.

Bombesin and angiotensin II rapidly stimulate Src phosphorylation at Tyr-418 in fibroblasts and intestinal epithelial cells through a PP2-insensitive pathway

Src is activated in response to a variety of growth factors and hormones that bind G protein-coupled receptors (GPCRs), and its activity is regulated by phosphorylation at key sites, including the autophosphorylation site Tyr-418 and the inhibitory site Tyr-529. To better understand the mechanisms controlling Src activation, we examined Src phosphorylation in Swiss 3T3 fibroblasts stimulated with bombesin and in IEC-18 intestinal epithelial cells stimulated with angiotensin II (Ang II). Phosphorylation at Src Tyr-418, the activation loop site, was rapidly and markedly increased after GPCR agonist addition in both cell types. However, treatment of intact cells with the selective Src family kinase inhibitor PP2, at concentrations which abolished Src-mediated phosphorylation of focal adhesion kinase (FAK) at Tyr-577, unexpectedly led to increased phosphorylation at Src Tyr-418 and diminished phosphorylation at Tyr-529. In Swiss 3T3 cells, PP2 enhanced Tyr-418 phosphorylation after 1 min of bombesin stimulation, while in IEC-18 cells, PP2 increased Ang II-stimulated Tyr-418 phosphorylation at all times tested. These results imply that a distinct, non-Src family kinase may be responsible for phosphorylating Src at Tyr-418 in intact fibroblasts and epithelial cells stimulated by GPCR agonists.

Oncogenic mutations reduce the stability of SRC kinase

The oncogenic potential of the viral tyrosine kinase v-Src is due to its constitutive activity. Unlike the highly homologous cellular c-Src kinase, a C-terminal deletion of the regulatory tail and numerous point mutations make the viral kinase uncontrollable. To determine the basis of these differences, we analysed the structure and stability of v-Src and c-Src in vitro. We show that the stability of v-Src against unfolding and irreversible aggregation is significantly lower than that of c-Src. Furthermore, in v-Src hydrophobic residues are more exposed already in the native state. In consequence, v-Src was found to be inactive close to physiological temperatures. We thus suggest that the ensemble of mutations that transform c-Src into the oncogenic variant cause a concomitant destabilisation of the kinase.

SRC gene expression in human cancer: the role of transcriptional activation

Human pp60c-Src (or c-Src) is a 60 kDa nonreceptor tyrosine kinase encoded by the SRC gene and is the cellular homologue to the potent transforming v-Src viral oncogene. c-Src functions at the hub of a vast array of signal transduction cascades that influence cellular proliferation, differentiation, motility, and survival. c-Src activation has been documented in upwards of 50% of tumors derived from the colon, liver, lung, breast, and pancreas. Therefore, a major focus has been to understand the mechanisms of c-Src activation in human cancer. Early studies concentrated on post-translational mechanisms that lead to increased c-Src kinase activity, which often correlated with overexpression of c-Src protein. More recently, the discovery of an activating SRC mutation in a small subset of advanced colon tumors has been reported. In addition, elevated SRC transcription has been identified as yet another mechanism contributing significantly to c-Src activation in a subset of human colon cancer cell lines. Interestingly, histone deacetylase (HDAC) inhibitors, agents with well-documented anti-cancer activity, repress SRC transcription in a wide variety of human cancer cell lines. Analysis of the mechanisms behind HDAC inhibitor mediated repression could be utilized in the future to specifically inhibit SRC gene expression in human cancer.

The interplay between Src and integrins in normal and tumor biology

Src family nonreceptor protein tyrosine kinases transduce signals that control normal cellular processes such as cell proliferation, adhesion and motility. Normally, cellular Src is held in an inactive state, but in several cancer types, abnormal events lead to elevated kinase activity of the protein and cause pleiotropic cellular responses inducing transformation and metastasis. A prerequisite of the ability of a cancer cell to undergo metastasis into distant tissues is to penetrate surrounding extracellular matrices. These processes are facilitated by the integrin family of cell adhesion molecules. As is the case with Src, altered integrin activity or substrate affinity can contribute to the neoplastic phenotype. Therefore, understanding the interplay between Src and integrin function has been of intense interest over the past few years. This review focuses on the role of Src and integrin signaling in normal cells and how this is deregulated in human cancer. We will identify the key players in the integrin-mediated signaling pathways involved in cell motility and apoptosis, such as FAK, paxillin and p130(CAS), and discuss how Src signaling affects the formation of focal adhesions and the extracellular matrix.

On the cross-regulation of protein tyrosine phosphatases and receptor tyrosine kinases in intracellular signaling

Intracellular signaling proteins are very often regulated by site-specific phosphorylation. For example, growth factor receptors in eukaryotic cells contain intrinsic tyrosine kinase activity and use inter- and intra-molecular interactions to recruit and orient potential protein substrates for phosphorylation. Equally important in determining the magnitude and kinetics of such a response is protein dephosphorylation, catalysed by phosphatase enzymes. A growing body of evidence indicates that certain protein tyrosine phosphatases (PTPs), like tyrosine kinases, are affected by intermolecular interactions that alter the specific activity or localization of their catalytic domains. Using a detailed kinetic modeling framework, we theoretically explore the regulation of PTPs through their association with receptor tyrosine kinases, as noted for the Src homology 2-domain-containing PTPs, SHP-1 and -2. Receptor-PTP binding, in turn, is expected to influence the phosphorylation pattern of those receptors and proteins they associate with, and we show how PTPs might serve to co- or counter-regulate parallel pathways in a signaling network.

Src kinase mediates angiotensin II-dependent increase in pulmonary endothelial nitric oxide synthase

We have previously demonstrated that angiotensin II (Ang II) stimulates nitric oxide (NO) production in bovine pulmonary artery endothelial cells (BPAECs) by increasing NO synthase (NOS) expression via the type 2 receptor. The purpose of this study was to identify the Ang II-dependent signaling pathway that mediates this increase in endothelial NOS (eNOS). The Ang II-dependent increase in eNOS expression is prevented when BPAECs are pretreated with the tyrosine kinase inhibitors, herbimycin A and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine, which also blocked Ang II-dependent mitogen-activated protein kinase (MAPK) kinase/extracellular-regulated protein kinase (MEK)-1 and MAPK phosphorylation, suggesting that Src is upstream of MAPK in this pathway. Transfection of BPAECs with an Src dominant negative mutant cDNA prevented the Ang II-dependent Src activation and increase in eNOS protein expression. PD98059, a MEK-1 inhibitor, prevented the Ang II-dependent phosphorylation of extracellular-regulated protein kinases 1 and 2 and increase in eNOS expression. Neither AG1478, an epidermal growth factor receptor kinase inhibitor, nor AG1295, a platelet derived growth factor receptor kinase inhibitor, had any effect on Ang II-stimulated Src activity, MAPK activation, or eNOS expression. Pertussis toxin prevented the Ang II-dependent increase in Src activity, MAPK activation, and eNOS expression. These data suggest that Ang II stimulates Src tyrosine kinase via a pertussis toxin-sensitive pathway, which in turn activates the MAPK pathway, resulting in increased eNOS protein expression in BPAECs.

Molecular dissection of egg fertilization signaling with the aid of tyrosine kinase-specific inhibitor and activator strategies

Fertilization is triggered by sperm-egg interaction and fusion that initiate a transient rise(s) in the free intracellular calcium ([Ca(2+)](i)) that is responsible for a series of biochemical and cell biological events, so-called "egg activation". Calcium-dependent egg activation leads to the initiation of developmental program that culminates in the birth of individuals. A growing body of knowledge has uncovered the molecular mechanisms underlying sperm-induced transient [Ca(2+)](i) increase(s) to some extent; namely, in most animals so far studied, a second messenger inositol 1,4,5-trisphosphate (IP(3)) seems to play a pivotal role in inducing [Ca(2+)](i) transient(s) at fertilization. However, signaling mechanisms used by sperm to initiate IP(3)-[Ca(2+)](i) transient pathway have not been elucidated. To approach this problem, we have employed African clawed frog, Xenopus laevis, as a model animal and conducted experiments designed specifically to determine the role of the Src family protein-tyrosine kinases (SFKs or Src family PTKs) in the sperm-induced egg activation. This review compiles information about the use of PTK-specific inhibitors and activators for analyzing signal transduction events in egg fertilization. Specifically, we focus on molecular identification of Xenopus Src and the signaling mechanism of the Src-dependent egg activation that has been established recently. We also summarize recent advances in understanding the role of the Src family kinases in egg fertilization of other model organisms, and discuss future directions of the field.

Protein tyrosine phosphatase epsilon activates Yes and Fyn in Neu-induced mammary tumor cells

The receptor-type form of protein tyrosine phosphatase epsilon (RPTP) is among the few tyrosine phosphatases that can support the transformed phenotype of tumor cells. Accordingly, cells from mammary epithelial tumors induced by activated Neu in mice genetically lacking RPTP appear morphologically less transformed and exhibit reduced proliferation. The effect of RPTP in these cells is mediated at least in part by its ability to activate Src, the prototypic member of a family of related kinases. We show here that RPTP is a physiological activator of two additional Src family kinases, Yes and Fyn. Activities of both kinases are inhibited in mammary tumor cells lacking RPTP, and phosphorylation at their C-terminal inhibitory tyrosines is increased. In agreement, opposite effects on activities and phosphorylation of Yes and Fyn are observed following increased expression of PTP. RPTP also forms stable complexes with either kinase, providing physical opportunity for their activation by RPTP. Surprisingly, expression of Yes or of Fyn does not rescue the morphological phenotype of RPTP-deficient tumor cells in contrast with the strong ability of Src to do so. We conclude that RPTP activates Src, Yes, and Fyn, but that these related kinases play distinct roles in Neu-induced mammary tumor cells.

The role of protein tyrosine phosphatase 1B in Ras signaling

Protein tyrosine phosphatase (PTP) 1B has been implicated as a negative regulator of multiple signaling pathways downstream of receptor tyrosine kinases. Inhibition of this enzyme was initially thought to potentially lead to increased oncogenic signaling and tumorigenesis. Surprisingly, we show that platelet-derived growth factor-stimulated extracellular-regulated kinase signaling in PTP1B-deficient cells is not significantly hyperactivated. Moreover, these cells exhibit decreased Ras activity and reduced proliferation by way of previously uncharacterized pathways. On immortalization, PTP1B-deficient fibroblasts display increased expression of Ras GTPase-activating protein (p120RasGAP). Furthermore, we demonstrate that p62Dok (downstream of tyrosine kinase) is a putative substrate of PTP1B and that tyrosine phosphorylation of p62Dok is indeed increased in PTP1B-deficient cells. Consistent with the decreased Ras activity in cells lacking PTP1B, introduction of constitutively activated Ras restored extracellular-regulated kinase signaling and their proliferative potential to those of WT cells. These results indicate that loss of PTP1B can lead to decreased Ras signaling, despite enhanced signaling of other pathways. This finding may in part explain the absence of increased tumor incidence in PTP1B-deficient mice. Thus, PTP1B can positively regulate Ras activity by acting on pathways distal to those of receptor tyrosine kinases.