Shc and FAK differentially regulate cell motility and directionality modulated by PTEN

Phosphoinositide phosphatases in cell biology and disease

Phosphoinositides are essential signaling molecules linked to a diverse array of cellular processes in eukaryotic cells. The metabolic interconversions of these phospholipids are subject to exquisite spatial and temporal regulation executed by arrays of phosphatidylinositol (PtdIns) and phosphoinositide-metabolizing enzymes. These include PtdIns- and phosphoinositide-kinases that drive phosphoinositide synthesis, and phospholipases and phosphatases that regulate phosphoinositide degradation. In the past decade, phosphoinositide phosphatases have emerged as topics of particular interest. This interest is driven by the recent appreciation that these enzymes represent primary mechanisms for phosphoinositide degradation, and because of their ever-increasing connections with human diseases. Herein, we review the biochemical properties of six major phosphoinositide phosphatases, the functional involvements of these enzymes in regulating phosphoinositide metabolism, the pathologies that arise from functional derangements of individual phosphatases, and recent ideas concerning the involvements of phosphoinositide phosphatases in membrane traffic control.

The mechanisms underlying fibroblast apoptosis regulated by growth factors during wound healing

While investigating the mechanisms underlying cell death during wound healing processes, we uncovered the pro-apoptotic effects of basic fibroblast growth factor (bFGF) on granulation tissue fibroblasts following pretreatment with transforming growth factor (TGF)-beta1 in vitro. bFGF induced caspase-3 activation and apoptosis in TGF-beta1-pretreated granulation tissue-derived fibroblasts (GF-1) following bFGF treatment for 48 and 96 h. In contrast, fibroblasts that had been treated in the same manner and that originated from the uninjured dermis did not display apoptosis, indicating that the mechanisms underlying apoptosis events in fibroblasts that originate from normal dermal and wound tissues differ. In this process, we also found that bFGF inhibited Akt phosphorylation at serine 473 and induced a rapid loss of phosphorylation of focal adhesion kinase (FAK) at tyrosine 397 in pretreated GF-1 cells, an event that coincided with the dissociation of phosphorylated FAK from the focal adhesions. Therefore, inhibition of survival signals relayed via the disrupted focal adhesion structures and inactivated Akt following bFGF treatment may lead to apoptosis in GF-1 cells pretreated with TGF-beta1. Pretreatment of GF-1 with TGF-beta1 followed by the addition of bFGF resulted in significantly greater inhibition of phosphorylation of Akt and FAK compared to treatment with TGF-beta1 or bFGF alone. The combinatorial treatment also led to proteolysis of FAK and inhibition of FAK and Akt protein expression in GF-1 cells. These findings demonstrated a significant role for the two cytokines in apoptosis of granulation tissue fibroblasts during wound healing. In vivo studies also confirmed a marked decline in phosphorylation and protein expression of Akt and FAK in bFGF-injected skin wounds. These results led to the hypothesis that temporal activation of TGF-beta1 and bFGF at the injury site promotes apoptosis in granulation tissue fibroblasts, an event that is critical for the termination of proliferative granulation tissue formation.

Identification of novel PTEN-binding partners: PTEN interaction with fatty acid binding protein FABP4

PTEN is a tumor suppressor with dual protein and lipid-phosphatase activity, which is frequently deleted or mutated in many human advanced cancers. Recent studies have also demonstrated that PTEN is a promising target in type II diabetes and obesity treatment. Using C-terminal PTEN sequence in pEG202-NLS as bait, yeast two-hybrid screening on Mouse Embryo, Colon Cancer, and HeLa cDNA libraries was carried out. Isolated positive clones were validated by mating assay and identified through automated DNA sequencing and BLAST database searches. Sequence analysis revealed a number of PTEN-binding proteins linking this phosphatase to a number of different signaling cascades, suggesting that PTEN may perform other functions besides tumor-suppressing activity in different cell types. In particular, the interplay between PTEN function and adipocyte-specific fatty-acid-binding protein FABP4 is of notable interest. The demonstrable tautology of PTEN to FABP4 suggested a role for this phosphatase in the regulation of lipid metabolism and adipocyte differentiation. This interaction was further studied using coimmunoprecipitation and gel-filtration assays. Finally, based on Biacore assay, we have calculated the K(D) of PTEN-FABP4 complex, which is around 2.8 microM.

PTEN augments SPARC suppression of proliferation and inhibits SPARC-induced migration by suppressing SHC-RAF-ERK and AKT signaling

SPARC (secreted protein acidic and rich in cysteine) is expressed in all grades of astrocytoma, including glioblastoma (GBM). SPARC suppresses glioma growth but promotes migration and invasion by mediating integrin and growth factor receptor-regulated kinases and their downstream effectors. PTEN (phosphatase and tensin homolog deleted on chromosome 10), which is commonly lost in primary GBMs, negatively regulates proliferation and migration by inhibiting some of the same SPARC-mediated signaling pathways. This study determined whether PTEN reconstitution in PTEN-mutant, SPARC-expressing U87MG cells could further suppress proliferation and tumor growth but inhibit migration and invasion in SPARC-expressing cells in vitro and in vivo, and thereby prolong survival in animals with xenograft tumors. In vitro, PTEN reduced proliferation and migration in both SPARC-expressing and control cells, with a greater suppression in SPARC-expressing cells. PTEN reconstitution suppressed AKT activation in SPARC-expressing and control cells but suppressed the SHC-RAF-ERK signaling pathway only in SPARC-expressing cells. Importantly, coexpression of SPARC and PTEN resulted in the smallest, least proliferative tumors with reduced invasive capacity and longer animal survival. Furthermore, direct inhibition of the AKT and SHC-RAF-ERK signaling pathways suppressed the proliferation and migration of SPARC-expressing cells in vitro. These findings demonstrate that PTEN reconstitution or inhibition of signaling pathways that are activated by the loss of PTEN provide potential therapeutic strategies to inhibit SPARC-induced invasion while enhancing the negative effect of SPARC on tumor growth.

Fyn: a novel molecular target in cancer

Fyn is 59-kDa member of the Src family of kinases that is historically associated with T-cell and neuronal signaling in development and normal cellular physiology. Whereas Src has been heavily studied in cancer, less attention has been traditionally awarded to the other Src kinases such as Fyn. Our group has shown that Fyn is particularly upregulated in prostate cancer in contrast to the alternative members of the Src family. This suggests that it may mediate several important processes attributed to Src kinases in prostate cancer and other malignancies. These functions include not only cellular growth and proliferation but also morphogenesis and cellular motility. Together, these suggest a role for Fyn in both progression and metastasis. As several agents in clinical development affect Fyn activation, understanding the role that Fyn plays in cancer is of great importance in oncology. Cancer 2010. (c) 2010 American Cancer Society.

Phosphatidylinositol 3-kinase (PI3K) pathway activation in bladder cancer

The phosphatidylinositol 3-kinase (PI3K) pathway is a critical signal transduction pathway that regulates multiple cellular functions. Aberrant activation of this pathway has been identified in a wide range of cancers. Several pathway components including AKT, PI3K and mTOR represent potential therapeutic targets and many small molecule inhibitors are in development or early clinical trials. The complex regulation of the pathway, together with the multiple mechanisms by which it can be activated, make this a highly challenging pathway to target. For successful inhibition, detailed molecular information on individual tumours will be required and it is already clear that different tumour types show distinct combinations of alterations. Recent results have identified alterations in pathway components PIK3CA, PTEN, AKT1 and TSC1 in bladder cancer, some of which are significantly related to tumour phenotype and clinical behaviour. Co-existence of alterations to several PI3K pathway genes in some bladder tumours indicates that these proteins may have functions that are not related solely to the known canonical pathway.

The pathobiology of glioma tumors

The ongoing characterization of the genetic and epigenetic alterations in the gliomas has already improved the classification of these heterogeneous tumors and enabled the development of rodent models for analysis of the molecular pathways underlying their proliferative and invasive behavior. Effective application of the targeted therapies that are now in development will depend on pathologists' ability to provide accurate information regarding the genetic alterations and the expression of key receptors and ligands in the tumors. Here we review the mechanisms that have been implicated in the pathogenesis of the gliomas and provide examples of the cooperative nature of the pathways involved, which may influence the initial therapeutic response and the potential for development of resistance.

Association of sporadic breast cancer with PTEN/MMAC1/TEP1 promoter hypermethylation

PTEN/MMAC1/TEP1 encodes a tumor suppressor protein, which regulates cell cycle progression, translation, and apoptosis by blocking the activation of Akt/PKB. The loss of PTEN function increases cell survival and induces tumor invasion. In this study, PTEN promoter status and its correlation with genetic and pathologic parameters were analyzed in genomic DNA from Iranian patients with breast cancer. DNA methylation patterns in the CpG islands were determined by a methylation-specific PCR (MSP) assay. PTEN promoter methylation was found to be present in 37 of 53(70%) tumor tissues and none in 20 normal counterparts. Moreover, promoter methylation was found in patients with heterozygote mutation in the PTEN gene. The pathological history of cancerous tissue sections showed that PTEN gene could be inactivated at the stages III and IV in sporadic breast cancer. These findings suggested that promoter hypermethylation of PTEN might contribute to the progression of sporadic breast cancer in human.

c-Jun induces mammary epithelial cellular invasion and breast cancer stem cell expansion

The molecular mechanisms governing breast tumor cellular self-renewal contribute to breast cancer progression and therapeutic resistance. The ErbB2 oncogene is overexpressed in approximately 30% of human breast cancers. c-Jun, the first cellular proto-oncogene, is overexpressed in human breast cancer. However, the role of endogenous c-Jun in mammary tumor progression is unknown. Herein, transgenic mice expressing the mammary gland-targeted ErbB2 oncogene were crossed with c-jun(f/f) transgenic mice to determine the role of endogenous c-Jun in mammary tumor invasion and stem cell function. The excision of c-jun by Cre recombinase reduced cellular migration, invasion, and mammosphere formation of ErbB2-induced mammary tumors. Proteomic analysis identified a subset of secreted proteins (stem cell factor (SCF) and CCL5) induced by ErbB2 expression that were dependent upon endogenous c-Jun expression. SCF and CCL5 were identified as transcriptionally induced by c-Jun. CCL5 rescued the c-Jun-deficient breast tumor cellular invasion phenotype. SCF rescued the c-Jun-deficient mammosphere production. Endogenous c-Jun thus contributes to ErbB2-induced mammary tumor cell invasion and self-renewal.

PTEN as a unique promising therapeutic target for occupational asthma

The tumor suppressor phosphatase and tensin homologue deleted on chromosome ten (PTEN) dephophorylates phosphatidylinositol 3,4,5-triphosphate (PIP3) and is a key negative regulator of phosphoinositide kinase-3 (PI3K) signaling pathway. PTEN also suppresses cellular motility through mechanisms that may be partially independent of phosphatase activity. PTEN is one of the most commonly lost tumor suppressors in human cancers, and its down-regulation is also implicated in several other diseases including airway inflammatory diseases. There is increasing evidence regarding the protective effects of PTEN on the bronchial asthma which is induced by complex signaling networks. Very recently, as for the occupational asthma (OA) with considerable controversy for its pathobiologic mechanisms, PTEN has been considered as a key molecule which is capable of protecting toluene diisocyanate (TDI)-induced asthma, suggesting that PTEN is located at switching point of various molecular signals in OA. Knowledge of the mechanisms of PTEN regulation/function could direct to the pharmacological manipulation of PTEN. This article reviews the latest knowledge and studies on the roles and mechanisms of PTEN in OA.

An integrin-activating peptide, PHSRN, ameliorates inhibitory effects of conventional peritoneal dialysis fluids on peritoneal wound healing

BACKGROUND

Bioincompatible peritoneal dialysis fluids (PDFs) cause pathological changes in the peritoneal membrane, related to membrane dysfunction and progressive peritoneal fibrosis. We investigated the effects of Pro-His-Ser-Arg-Asn (PHSRN) peptide, one of the fibronectin cell-binding domains that activates integrins and reinforces wound healing, on peritoneal remodelling in a rat peritoneal injury model undergoing peritoneal dialysis.

METHODS

The peritoneal mesothelial monolayer was removed by a stripping procedure in rats receiving conventional high glucose-containing PDF supplemented with or without PHSRN or control His-Ser-Pro-Asn-Hrg (HSPNR) peptides. Effects of PHSRN on cell motility and signalling molecules were examined in cultured rat peritoneal mesothelial cells (RPMCs) and normal rat kidney fibroblasts (NRKs).

RESULTS

The cytokeratin- and HBME-1-positive mesothelial cell monolayer was selectively removed by the procedure. By day 6, HBME-1-positive cells had regenerated to 53.3 +/- 6.5% of the peritoneal surface in the control group. Regeneration of the mesothelial layer was delayed in the PDF group (35.2 +/- 10.2%, P < 0.05), but PHSRN reversed the effects of PDF (51.7 +/- 9.6%, P < 0.05). PDF treatment increased thickening of granulomatous submesothelial tissue and numbers of ED1-, CD31- and alpha-smooth muscle actin-positive cells, but PHSRN ameliorated these effects. HSPNR had no effects on mesothelial regeneration or peritoneal wound healing. PHSRN, but not HSPNR, recovered glucose-induced inhibition of cell motility and phosphorylation of focal adhesion kinase and its downstream p130(Cas) in RPMCs and NRKs.

CONCLUSIONS

These results suggest that PHSRN has beneficial effects on peritoneal regeneration by reducing the inhibitory effects of conventional PDF on integrin-mediated wound healing.

Suppression of cellular proliferation and invasion by the concerted lipid and protein phosphatase activities of PTEN

PTEN is a tumour suppressor with phosphatase activity in vitro against both lipids and proteins and other potential non-enzymatic mechanisms of action. Although the importance of PTEN's lipid phosphatase activity in regulating the PI3K signalling pathway is recognized, the significance of PTEN's other mechanisms of action is currently unclear. In this study, we describe the systematic identification of a PTEN mutant, PTEN Y138L, with activity against lipid, but not soluble substrates. Using this mutant, we provide evidence for the interfacial activation of PTEN against lipid substrates. We also show that when re-expressed at physiological levels in PTEN null U87MG glioblastoma cells, the protein phosphatase activity of PTEN is not required to regulate cellular PtdInsP(3) levels or the downstream protein kinase Akt/PKB. Finally, in three-dimensional Matrigel cultures of U87MG cells similarly re-expressing PTEN mutants, both the protein and lipid phosphatase activities were required to inhibit invasion, but either activity alone significantly inhibited proliferation, albeit only weakly for the protein phosphatase activity. Our data provide a novel tool to address the significance of PTEN's separable lipid and protein phosphatase activities and suggest that both activities suppress proliferation and together suppress invasion.

Phosphatidyl inositol 3-kinase signaling in hypothalamic proopiomelanocortin neurons contributes to the regulation of glucose homeostasis

Recent studies demonstrated a role for hypothalamic insulin and leptin action in the regulation of glucose homeostasis. This regulation involves proopiomelanocortin (POMC) neurons because suppression of phosphatidyl inositol 3-kinase (PI3K) signaling in these neurons blunts the acute effects of insulin and leptin on POMC neuronal activity. In the current study, we investigated whether disruption of PI3K signaling in POMC neurons alters normal glucose homeostasis using mouse models designed to both increase and decrease PI3K-mediated signaling in these neurons. We found that deleting p85alpha alone induced resistance to diet-induced obesity. In contrast, deletion of the p110alpha catalytic subunit of PI3K led to increased weight gain and adipose tissue along with reduced energy expenditure. Independent of these effects, increased PI3K activity in POMC neurons improved insulin sensitivity, whereas decreased PI3K signaling resulted in impaired glucose regulation. These studies show that activity of the PI3K pathway in POMC neurons is involved in not only normal energy regulation but also glucose homeostasis.

Shc is required for ErbB2-induced inhibition of apoptosis but is dispensable for cell proliferation and disruption of cell polarity

Amplification and overexpression of ErbB2 strongly correlates with aggressive breast cancers. A deeper understanding of pathways downstream of ErbB2 signaling that are required for the transformation of human mammary epithelial cells will identify novel strategies for therapeutic intervention in breast cancer. Using an inducible activation of ErbB2 autophosphorylation qsite mutants and the MCF-10A three-dimensional (3D) culture system, we investigated pathways used by ErbB2 to transform the epithelia. We report that ErbB2 induces cell proliferation and loss of 3D organization by redundant mechanisms, whereas it disrupts apical basal polarity and inhibits apoptosis using Tyr 1201 and Tyr 1226/7, respectively. Signals downstream of Tyr 1226/7 were also sufficient to confer paclitaxel resistance. The Tyr 1226/7 binds Shc, and the knockdown of Shc blocks the ability of ErbB2 to inhibit apoptosis and mediate paclitaxel resistance. Tyr 1226/7 is known to activate the Ras/Erk pathway; however, paclitaxel resistance did not correlate with the activation of Erk or Akt, suggesting the presence of a novel mechanism. Thus, our results show that targeting pathways used by ErbB2 to inhibit cell death is a better option than targeting cell proliferation pathways. Furthermore, we identify a novel function for Shc as a regulator of apoptosis and drug resistance in human mammary epithelial cells transformed by ErbB2.

Cholesteatoma growth and proliferation: posttranscriptional regulation by microRNA-21

OBJECTIVES

The goal of this study was to identify novel regulatory mechanisms controlling the growth and proliferation of cholesteatoma. Specifically, the potential role of microRNAs, regulators of protein translation, was studied in cholesteatoma.

STUDY DESIGN

This study represents a molecular biologic investigation characterizing and comparing microRNA and protein expression in cholesteatoma and normal postauricular skin.

METHODS

Cholesteatoma and normal skin were taken from patients at the time of surgery. Tissue was processed for RNA and protein extraction. Real-time reverse-transcriptase-polymerase chain reaction was used to assess levels of human microRNAs, reverse-transcriptase-polymerase chain reaction was used to confirm the presence of upstream regulators, and Western blot analyses were used to assess levels of downstream target proteins.

RESULTS

Among the microRNAs investigated, human microRNA-21 (hsa-miR-21) showed a 4.4-fold higher expression in cholesteatoma as compared with normal skin (p = 0.0011). The downstream targets of hsa-miR-21, PTEN and programmed cell death 4, were found to be greatly reduced in 3 of 4 cholesteatoma samples. Proposed upstream regulators of hsa-miR-21 expression (CD14, interleukin 6R, gp130, and signal transducer and activator of transcription 3) were present in all cholesteatoma tissues.

CONCLUSION

MicroRNAs represent powerful regulators of protein translation, and their dysregulation has been implicated in many neoplastic diseases. This study specifically identified up-regulation of hsa-miR-21 concurrent with down-regulation of potent tumor suppressor proteins PTEN and programmed cell death 4. These proteins control aspects of apoptosis, proliferation, invasion, and migration. The results of this study were used to develop a model for cholesteatoma proliferation through microRNA dysregulation. This model can serve as a template for further study into potential RNA-based therapies for the treatment of cholesteatoma.

PTEN is a mechanosensing signal transducer for myosin II localization in Dictyostelium cells

To investigate the role of PTEN in regulation of cortical motile activity, especially in myosin II localization, eGFP-PTEN and mRFP-myosin II were simultaneously expressed in Dictyostelium cells. PTEN and myosin II co-localized at the posterior of migrating cells and furrow region of dividing cells. In suspension culture, PTEN knockout (pten(-)) cells became multinucleated, and myosin II significantly decreased in amount at the furrow. During pseudopod retraction and cell aspiration by microcapillary, PTEN accumulated at the tips of pseudopods and aspirated lobes prior to the accumulation of myosin II. In pten(-) cells, only a small amount of myosin II accumulated at the retracting pseudopods and aspirated cell lobes. PTEN accumulated at the retracting pseudopods and aspirated lobes even in myosin II null cells and latrunculin B-treated cells though in reduced amounts, indicating that PTEN accumulates partially depending on myosin II and cortical actin. Accumulation of PTEN prior to myosin II suggests that PTEN is an upstream component in signaling pathway to localize myosin II, possibly with mechanosensing signaling loop where actomyosin-driven contraction further augments accumulation of PTEN and myosin II by a positive feedback mechanism.

Indole-3-carbinol prevents PTEN loss in cervical cancer in vivo

Indole-3-carbinol (I3C) is a phytochemical (derived from broccoli, cabbage, and other cruciferous vegetables) with proven anticancer efficacy including the reduction of cervical intraepithelial neoplasia (CIN) and its progression to cervical cancer. In a breast cancer cell line, I3C inhibited cell adhesion, spreading, and invasion associated with an upregulation of the tumor suppressor gene PTEN, suggesting that PTEN is important in inhibition of late stages in the development of cancer. The goal of this study was to determine the expression of PTEN during the development of cervical cancer and whether I3C affected expression of PTEN in vivo. We show diminished PTEN expression during the progression from low-grade to high-grade cervical dysplasia in humans and in a mouse model for cervical cancer, the K14HPV16 transgenic mice promoted with estrogen. The implication is that loss of PTEN function is required for this transition. Additionally, dietary I3C increased PTEN expression in the cervical epithelium of the transgenic mouse, an observation that suggests PTEN upregulation by I3C is one mechanism by which I3C inhibits development of cervical cancer.

A FAK-p120RasGAP-p190RhoGAP complex regulates polarity in migrating cells

Directional motility is a complex process requiring the spatiotemporal integration of signals that regulate cytoskeletal changes, and the establishment of an anteroposterior or polarized cell axis. Focal adhesion kinase (FAK) promotes cell migration, but a molecular role for FAK in promoting cell polarity remains undefined. Here, using wound healing and Golgi-reorientation analyses, we show that fibroblast, endothelial and carcinoma polarity during cell migration requires FAK and is associated with a complex between FAK, p120RasGAP and p190RhoGAP (p190A), leading to p190A tyrosine phosphorylation. Fibronectin-integrin-mediated FAK activation and phosphorylation promote SH2-mediated binding of p120RasGAP to FAK and FAK-mediated p190A tyrosine phosphorylation. The association of p120RasGAP with FAK facilitates the formation of a FAK-p120RasGAP-p190A complex targeted to leading-edge focal adhesions by FAK. Knockdown of p120RasGAP, mutation of FAK Y397 or inhibition of FAK activity prevent the association of FAK with p190A and subsequent tyrosine phosphorylation of p190A, and result in the loss of cell polarity. Because reconstitution of FAK-null fibroblasts with FAK or a Pyk2-FAK chimera restore the normal decrease in RhoA GTP binding upon cell spreading on fibronectin, our studies support a model whereby FAK activity facilitates the recruitment and stabilization of a p120RasGAP-p190A complex at leading-edge focal adhesions connected to the transient inhibition of RhoA activity and the regulation of cell polarity.

Clinicopathological significance of PTEN and Caspase-3 expressions in breast cancer

OBJECTIVE

To investigate the expressions of PTEN and Caspase-3 proteins in human breast carcinoma, and to evaluate their clinicopathological implications during the tumorigenesis and progression of breast cancer.

METHODS

The expressions of PTEN and Caspase-3 proteins in 95 cases of breast cancer and 15 cases of benign breast diseases were investigated immunohistochemically. Correlations between the expression of PTEN protein, Caspase-3 protein, and clinicopathological features of breast cancers were analyzed.

RESULTS

The loss expression rate of PTEN protein in tumor tissues was significantly higher than that in benign breast diseases (33.7% vs. 0, P < 0.01). Analysis of the clinicopathological features showed that PTEN expression level was negatively correlated with TNM stage, histological grade, axillary lymph node status, recurrence, and metastasis (P < 0.05). The positive expression level of Caspase-3 was negatively correlated with TNM stage (P < 0.01), but not related with histological grade, axillary lymph node status, recurrence, or metastasis (P > 0.05). In addition, the expression of PTEN protein had significantly positive correlation with the expression of Caspase-3 protein in breast cancer (P < 0.01).

CONCLUSION

The combination detection of PTEN and Caspase-3 may serve as an important index to estimate the pathobiological behavior and prognosis of breast cancer.

Interactions between PTEN and receptor tyrosine kinase pathways and their implications for glioma therapy

Gliomas are the most common and deadly form of malignant primary brain tumors. Loss of the tumor-suppressor PTEN and activation of the receptor tyrosine kinases (RTKs) EGF receptor, c-Met, PDGF receptor and VEGF receptor are among the most common molecular dysfunctions associated with glioma malignancy. PTEN interacts with RTK-dependent signaling at multiple levels. These include the ability of PTEN to counteract PI3K activation by RTKs, as well as possible effects of PTEN on RTK activation of the MAPK pathway and RTK-dependent gene-expression regulation. Consequently, PTEN expression affects RTK-induced malignancy. Importantly, the PTEN status was recently found to be critical for the outcome of RTK-targeted clinical therapies that have been developed recently. Combining RTK-targeted therapies with therapies aimed at counteracting the effects of PTEN loss, such as mTOR inhibition, might also have therapeutic advantage. This article reviews the known molecular and functional interactions between PTEN and RTK pathways and their implications for glioma therapy.

3',4'-Dihydroxyflavonol down-regulates monocyte chemoattractant protein-1 in smooth muscle: role of focal adhesion kinase and PDGF receptor signalling

BACKGROUND AND PURPOSE

We investigated the effects of a synthetic flavonol, 3',4'-dihydroxyflavonol (DiOHF) on the expression of monocyte chemoattractant protein-1 (MCP-1) in rat vascular smooth muscle cells.

EXPERIMENTAL APPROACH

MCP-1 expression was assessed by quantitative real-time PCR and protein phosphorylation by immunoprecipitation and Western blots.

KEY RESULTS

DiOHF (1-30 micromol x L(-1)) concentration-dependently reduced MCP-1 expression in both quiescent cells and cells stimulated with platelet-derived growth factor (PDGF) or interleukin 1-beta. The effect of DiOHF was associated with a suppression of focal adhesion kinase (FAK)-mediated signalling. In vitro kinase assays demonstrated that DiOHF is a potent inhibitor of FAK kinase activity (EC(50)= 2.4 micromol x L(-1)). Expression of FAK-related non-kinase reduced basal MCP-1 expression, but not that induced by PDGF or interleukin 1-beta. DiOHF also inhibited autophosphorylation of PDGF receptors. The PDGF receptor inhibitor AG-1296 potently suppressed basal and PDGF-induced MCP-1 expression. Inhibition of extracellular signal-regulated kinase activation by DiOHF, either directly or indirectly, may also be involved in its effects on MCP-1 expression. DiOHF had no inhibitory effect on either p38 or nuclear factor-kappaB activation. Moreover, DiOHF inhibited smooth muscle cell spreading (a FAK-mediated response) and proliferation.

CONCLUSIONS AND IMPLICATIONS

This is the first report on a flavonoid compound (DiOHF) that is a potent FAK inhibitor. DiOHF also inhibits PDGF receptor autophosphorylation. These effects underlie the inhibitory action of DiOHF on MCP-1 expression in smooth muscle cells. Our results suggest that DiOHF might be a useful tool for dissection of the (patho)physiological roles of FAK signalling.

Endothelial cell-specific molecule 2 (ECSM2) modulates actin remodeling and epidermal growth factor receptor signaling

Endothelial cell-specific molecules (ECSMs) play a pivotal role in the pathogenesis of many angiogenesis-related diseases. Since its initial discovery, the exact function of human ECSM2 has not been defined. In this study, by database mining, we identified a number of hypothetical proteins across species exhibiting substantial sequence homology to the human ECSM2. We showed that ECSM2 is preferentially expressed in endothelial cells and blood vessels. Their characteristic structures and unique expression patterns suggest that ECSM2 is an evolutionarily conserved gene and may have important functions. We further explored the potential roles of human ECSM2 at the molecular and cellular level. Using a reconstitution mammalian cell system, we demonstrated that ECSM2 mainly resides at the cell membrane, is critically involved in cell-shape changes and actin cytoskeletal rearrangement, and suppresses tyrosine phosphorylation signaling. More importantly, we uncovered that ECSM2 can cross-talk with epidermal growth factor receptor (EGFR) to attenuate the EGF-induced cell migration, possibly via inhibiting the Shc-Ras-ERK (MAP kinase) pathway. Given the importance of growth factor and receptor tyrosine kinase-mediated signaling and cell migration in angiogenesis-related diseases, our findings regarding the inhibitory effects of ECSM2 on EGF-mediated signaling and cell motility may have important therapeutic implications.

Roles of integrins in tumor angiogenesis and lymphangiogenesis

The lifelong dedication of Dr. Judah Folkman to understand how tumors co-opt vasculature to promote tumor growth and spread resulted in the development of an astounding body of knowledge and development of new clinical therapeutics for cancer. Angiogenesis is a critical point in the development and dissemination of most human tumors. Tumor-associated lymphangiogenesis also plays an important role in mediating tumor spread to lymph nodes. The molecular regulations of these processes are complex, and many key molecular families have been implicated in the regulation of angiogenesis and lymphangiogenesis. By regulating cell-cell and cell-matrix contacts, integrins participate in blood and lymphatic vessel growth by promoting endothelial cell migration and survival. Understanding the underlying mechanisms by which integrins promote tumor-associated blood and lymphatic vessel development might provide important modalities for the therapeutic intervention of metastatic spread. This review focuses on the role of integrins in angiogenesis and lymphangiogenesis. Integrins represent potential targets for pharmacological agents and open new avenues for the control of metastatic spread in the treatment of malignancies. This article is dedicated to the memory of Dr. Judah Folkman, an amazing and caring teacher, scientist, physician, and friend.

Interactions between PTEN and the c-Met pathway in glioblastoma and implications for therapy

The tyrosine kinase receptor c-Met and its ligand hepatocyte growth factor (HGF) are frequently overexpressed and the tumor suppressor PTEN is often mutated in glioblastoma. Because PTEN can interact with c-Met-dependent signaling, we studied the effects of PTEN on c-Met-induced malignancy and associated molecular events and assessed the potential therapeutic value of combining PTEN restoration approaches with HGF/c-Met inhibition. We studied the effects of c-Met activation on cell proliferation, cell cycle progression, cell migration, cell invasion, and associated molecular events in the settings of restored or inhibited PTEN expression in glioblastoma cells. We also assessed the experimental therapeutic effects of combining anti-HGF/c-Met approaches with PTEN restoration or mTOR inhibition. PTEN significantly inhibited HGF-induced proliferation, cell cycle progression, migration, and invasion of glioblastoma cells. PTEN attenuated HGF-induced changes of signal transduction proteins Akt, GSK-3, JNK, and mTOR as well as cell cycle regulatory proteins p27, cyclin E, and E2F-1. Combining PTEN restoration to PTEN-null glioblastoma cells with c-Met and HGF inhibition additively inhibited tumor cell proliferation and cell cycle progression. Similarly, combining a monoclonal anti-HGF antibody (L2G7) with the mTOR inhibitor rapamycin had additive inhibitory effects on glioblastoma cell proliferation. Systemic in vivo delivery of L2G7 and PTEN restoration as well as systemic in vivo deliveries of L2G7 and rapamycin additively inhibited intracranial glioma xenograft growth. These preclinical studies show for the first time that PTEN loss amplifies c-Met-induced glioblastoma malignancy and suggest that combining anti-HGF/c-Met approaches with PTEN restoration or mTOR inhibition is worth testing in a clinical setting.

Genetic modelling of the PTEN/AKT pathway in cancer research

The focus on targeted therapies has been fuelled by extensive research on molecular pathways and their role in tumorigenesis. Novel models of human cancer have been created to evaluate the role of specific genes in the different stages of cancer. Currently, mouse modelling of human cancer is possible through the expression of oncogenes, specific genetic mutations or the inactivation of tumour suppressor genes, and these models have begun to provide us with an understanding of the molecular pathways involved in tumour initiation and progression at the physiological level. Additionally, these mouse models serve as an excellent system to evaluate the efficacy of currently developed molecular targeted therapies and identify new potential targets for future therapies. The PTEN/AKT pathway is implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. Deregulation of the PTEN/AKT pathway is a common event in human cancer. Despite the abundant literature, the physiological role of each element of the pathway has begun to be uncovered thanks to genetically engineered mice. This review will summarise some of the key animal models which have helped us to understand this signalling network and its contribution to tumorigenesis.

The mechanism of action of the tumour suppressor gene PTEN

Intracellular levels of phosphorylation are regulated by the coordinated action of protein kinases and phosphatases. Disregulation of this balance can lead to cellular transformation. Here we review knowledge of the mechanisms of one protein phosphatase, the tumour suppressor PTEN/MMAC/TEP 1 apropos its role in tumorigenesis and signal transduction. PTEN plays an important role in the phosphatidyl-inositol-3-kinase (PI3-K) pathway by catalyzing degradation of phosphatidylinositol-(3,4,5)-triphosphate generated by PI3-K. This inhibits downstream targets mainly protein kinase B (PKB/Akt), cell survival and proliferation. PTEN contributes to cell cycle regulation by blockade of cells entering the S phase of the cell cycle, and by upregulation of p27(Kip1) which is recruited into the cyclin E/cdk2 complex. PTEN also modulates cell migration and motility by regulation of the extracellular signal-related kinase - mitogen activated protein kinase (ERK-MAPK) pathway and by dephosphorylation of focal adhesion kinase (FAK). We also emphasize the increasingly important role that PTEN has from an evolutionary point of view. A number of PTEN functions have been elucidated but more information is needed for utilization in clinical application and potential cancer therapy.

PTEN deficiency is a common defect in juvenile myelomonocytic leukemia

The biological hallmark of juvenile myelomonocytic leukemia (JMML) is selective GM-CSF hypersensitivity. We hypothesized that PTEN protein deficiency might lead to insufficient negative growth signals to counter the hyperactive Ras signaling and therefore aid in the acceleration of the malignant transformation of JMML. In screening 34 JMML patients we found: (1) decreased PTEN protein in 67% of patients; (2) significantly lower PTEN mRNA levels in patients compared to controls (p<0.01); (3) a hypermethylated PTEN promoter in 77% of patients; and (4) constitutive-hyperactive Akt and MAPK in 55% and 73% of patients, respectively. These findings suggest that PTEN deficiency is very common in JMML and is in part due to hypermethylation of the PTEN gene promoter.

Regulation of T-cell responses by PTEN

The phosphoinositide 3-kinase (PI3K) signaling pathway plays a critical role in the development, activation, and homeostasis of T cells by modulating the expression of survival and mitogenic factors in response to a variety of stimuli. Ligation of the antigen receptor, costimulatory molecules, and cytokine receptors activate PI3K, resulting in the production of the lipid second messenger phosphatidylinositol-3,4,5-triphosphate (PIP(3)). A number of molecules help to regulate the activity of this pathway, including the lipid phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10). By limiting the amount of PIP(3) available within the cell, PTEN directly opposes PI3K activity and influences the selection of developing thymocytes as well as the activation requirements of mature T cells. T cells with unchecked PI3K activity, as a result of PTEN deficiency, contribute to the development of both autoimmune disease and lymphoma. This review dissects our current understanding of PI3K and PTEN and discusses why appropriate balance of these molecules is necessary to maintain normal T-cell responses.

PTEN: a default gate-keeping tumor suppressor with a versatile tail

The tumor suppressor PTEN controls a variety of biological processes including cell proliferation, growth, migration, and death. As a master cellular regulator, PTEN itself is also subjected to deliberated regulation to ensure its proper function. Defects in PTEN regulation have a profound impact on carcinogenesis. In this review, we briefly discuss recent advances concerning PTEN regulation and how such knowledge facilitates our understanding and further exploration of PTEN biology. The carboxyl-tail of PTEN, which appears to be associated with multiple types of posttranslational regulation, will be under detailed scrutiny. Further, a comparative analysis of PTEN and p53 suggests while p53 needs to be activated to suppress tumorigenesis (a dormant gatekeeper), PTEN is probably a constitutive surveillant against cancer development, thus a default gatekeeper.

Pten deficiency in melanocytes results in resistance to hair graying and susceptibility to carcinogen-induced melanomagenesis

Phosphate and tensin homologue deleted on chromosome 10 (PTEN) is a tumor suppressor gene inactivated in numerous sporadic cancers, including melanomas. To analyze Pten functions in melanocytes, we used the Cre-loxP system to delete Pten specifically in murine pigment-producing cells and generated DctCrePten(flox/flox) mice. Half of DctCrePten(flox/flox) mice died shortly after birth with enlargements of the cerebral cortex and hippocampus. Melanocytes were increased in the dermis of perinatal DctCrePten(flox/flox) mice. When the mutants were subjected to repeated depilations, melanocyte stem cells in the bulge of the hair follicle resisted exhaustion and the mice were protected against hair graying. Although spontaneous melanomas did not form in DctCrePten(flox/flox) mice, large nevi and melanomas developed after carcinogen exposure. DctCrePten(flox/flox) melanocytes were increased in size and exhibited heightened activation of Akt and extracellular signal-regulated kinases, increased expression of Bcl-2, and decreased expression of p27(Kip1). Our results show that Pten is important for the maintenance of melanocyte stem cells and the suppression of melanomagenesis.

p66shc negatively regulates insulin-like growth factor I signal transduction via inhibition of p52shc binding to Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 leading to impaired growth factor receptor-bound protein-2 membrane recruitment

Our previous studies have indicated an essential role of p52shc in mediating IGF-I activation of MAPK in smooth muscle cells (SMC). However, the role of the p66 isoform of shc in IGF-I signal transduction is unclear. In the current study, two approaches were employed to investigate the role of p66shc in mediating IGF-I signaling. Knockdown p66shc by small interfering RNA enhanced IGF-I-stimulated p52shc tyrosine phosphorylation and growth factor receptor-bound protein-2 (Grb2) association, resulting in increased IGF-I-dependent MAPK activation. This was associated with enhanced IGF-I-stimulated cell proliferation. In contrast, knockdown of p66shc did not affect IGF-I-stimulated IGF-I receptor tyrosine phosphorylation. Overexpression of p66shc impaired IGF-I-stimulated p52shc tyrosine phosphorylation and p52shc-Grb2 association. In addition, IGF-I-dependent MAPK activation was also impaired, and SMC proliferation in response to IGF-I was inhibited. IGF-I-dependent cell migration was enhanced by p66shc knockdown and attenuated by p66shc overexpression. Mechanistic studies indicated that p66shc inhibited IGF-I signal transduction via competitively inhibiting the binding of Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2) to SHP substrate-1 (SHPS-1), leading to the disruption of SHPS-1/SHP-2/Src/p52shc complex formation, an event that has been shown previously to be essential for p52shc phosphorylation and Grb2 recruitment. These findings indicate that p66shc functions to negatively regulate the formation of a signaling complex that is required for p52shc activation in response to IGF-I, thus leading to attenuation of IGF-I-stimulated cell proliferation and migration.

Protein tyrosine phosphorylation and protein tyrosine nitration in redox signaling

Reversible phosphorylation of protein tyrosine residues by polypeptide growth factor-receptor protein tyrosine kinases is implicated in the control of fundamental cellular processes including the cell cycle, cell adhesion, and cell survival, as well as cell proliferation and differentiation. During the last decade, it has become apparent that receptor protein tyrosine kinases and the signaling pathways they activate belong to a large signaling network. Such a network can be regulated by various extracellular cues, which include cell adhesion, agonists of G protein-coupled receptors, and oxidants. It is well documented that signaling initiated by receptor protein tyrosine kinases is directly dependent on the intracellular production of oxidants, including reactive oxygen and nitrogen species. Accumulated evidence indicates that the intracellular redox environment plays a major role in the mechanisms underlying the actions of growth factors. Oxidation of cysteine thiols and nitration of tyrosine residues on signaling proteins are described as posttranslational modifications that regulate, positively or negatively, protein tyrosine phosphorylation (PTP). Early observations described the inhibition of PTP activities by oxidants, resulting in increased levels of proteins phosphorylated on tyrosine. Therefore, a redox circuitry involving the increasing production of intracellular oxidants associated with growth-factor stimulation/cell adhesion, oxidative reversible inhibition of protein tyrosine phosphatases, and the activation of protein tyrosine kinases can be delineated.

The protein phosphatase activity of PTEN regulates SRC family kinases and controls glioma migration

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) is mutated or lost in 60% to 70% of advanced gliomas and is associated with malignant phenotypic changes such as migration, which contribute to the morbidity and mortality of this disease. Most of the tumor suppressor function of PTEN has been attributed to its ability to dephosphorylate the second messenger, phosphatidylinositol 3,4,5-triphosphate, resulting in the biological control of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway. Despite recent work suggesting that the protein phosphatase activity of PTEN controls glioma cell migration, the mechanisms by which this occurs are unclear. Herein, we show using glioma cell lines (U87MG and U373MG) stably transfected with wild-type PTEN or catalytically altered mutants of PTEN that PTEN controls integrin-directed migration in a lipid phosphatase, PI3K/AKT-independent manner. Confirming this observation, we show that the stable overexpression of COOH-terminal Src kinase, the physiologic negative regulator of SRC family kinases (SFK), or treatment with the SFK inhibitor PP1 abrogates glioma migration. The results provide direct evidence that the downstream effect of the protein phosphatase activity of PTEN is to suppress SFK and FYN, and to regulate RAC-GTPase activity after alpha(v) integrin stimulation. Furthermore, studying vitronectin-directed migration using (a) Fyn small interfering RNA and (b) astrocytes from Fyn heterozygous (+/-) mice, Pten heterozygous (+/-) mice, Pten and Fyn double heterozygous (+/-) mice, or Fyn knockout (-/-) mice confirmed a role of FYN in alpha(v) integrin-mediated haptotaxis in glial cells. Our combined results provide direct biochemical and genetic evidence that PTEN's protein phosphatase activity controls FYN kinase function in glioma cells and regulates migration in a PI3K/AKT-independent manner.

MAP-kinase activity necessary for TGFbeta1-stimulated mesangial cell type I collagen expression requires adhesion-dependent phosphorylation of FAK tyrosine 397

The signals mediating transforming growth factor beta (TGFbeta)-stimulated kidney fibrogenesis are poorly understood. We previously reported TGFbeta-stimulated, Smad-mediated collagen production by human kidney mesangial cells, and that ERK MAP kinase activity optimizes collagen expression and enhances phosphorylation of the Smad3 linker region. Furthermore, we showed that disrupting cytoskeletal integrity decreases type I collagen production. Focal adhesion kinase (FAK, PTK2) activity could integrate these findings. Adhesion-dependent FAK Y397 phosphorylation was detected basally, whereas FAK Y925 phosphorylation was TGFbeta1-dependent. By immunocytochemistry, TGFbeta1 stimulated the merging of phosphorylated FAK with the ends of thickening stress fibers. Cells cultured on poly-L-lysine (pLL) to promote integrin-independent attachment spread less than those on control substrate and failed to demonstrate focal adhesion (FA) engagement with F-actin. FAK Y397 phosphorylation and ERK activity were also decreased under these conditions. In cells with decreased FAK Y397 phosphorylation from either plating on pLL or overexpressing a FAK Y397F point mutant, serine phosphorylation of the Smad linker region, but not of the C-terminus, was reduced. Y397F and Y925F FAK point mutants inhibited TGFbeta-induced Elk-Gal activity, but only the Y397F mutant inhibited TGFbeta-stimulated collagen-promoter activity. The inhibition by the Y397F mutant or by culture on pLL was prevented by co-transfection of constitutively active ERK MAP kinase kinase (MEK), suggesting that FAK Y397 phosphorylation promotes collagen expression via ERK MAP kinase activity. Finally, Y397 FAK phosphorylation, and both C-terminal and linker-region Smad3 phosphorylation were detected in murine TGFbeta-dependent kidney fibrosis. Together, these data demonstrate adhesion-dependent FAK phosphorylation promoting TGFbeta-induced responses to regulate collagen production.

Phosphoinositide binding to the substrate regulates susceptibility to proteolysis by calpain

Calpain-mediated proteolysis regulates cytoskeletal dynamics and is altered during aging and the progression of numerous diseases or pathological conditions. Although several cytoskeletal proteins have been identified as substrates, how localized calpain activity is regulated and the mechanisms controlling substrate recognition are not clear. In this study, we report that phosphoinositide binding regulates the susceptibility of the cytoskeletal adhesion protein alpha-actinin to proteolysis by calpains 1 and 2. At first, alpha-actinin did not appear to be a substrate for calpain 2; however, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) binding to alpha-actinin resulted in nearly complete proteolysis of the full-length protein, producing stable breakdown products. Calpain 1 was able to cleave alpha-actinin in the absence of phosphoinositide binding; however, PtdIns(3,4,5)P(3) binding increased the rate of proteolysis, and phosphatidylinositol 4,5-diphosphate (PtdIns(4,5)P(2)) binding significantly inhibited cleavage. Phosphoinositide binding appeared to regulate calpain proteolysis of alpha-actinin by modulating the exposure of a highly sensitive cleavage site within the calponin homology 2 domain. In U87MG glioblastoma cells, which contain elevated levels of PtdIns(3,4,5)P(3), alpha-actinin colocalized with calpain within dynamic actin cytoskeletal structures. Furthermore, proteolysis of alpha-actinin producing stable breakdown products was observed in U87MG cells treated with calcium ionophore to activate the calcium-dependent calpains. Additional evidence of PtdIns(3,4,5)P(3)-mediated calpain proteolysis of alpha-actinin was observed in rat embryonic fibroblasts. These results suggest that PtdIns(3,4,5)P(3) binding is a critical determinant for alpha-actinin proteolysis by calpain. In conclusion, phosphoinositide binding to the substrate is a potential mechanism for regulating susceptibility to proteolysis by calpain.

Portrait of PTEN: messages from mutant mice

PTEN is a tumor suppressor gene mutated in many human sporadic cancers and in hereditary cancer syndromes such as Cowden disease. The major substrate of PTEN is phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P3), a second messenger molecule produced following PI3K activation induced by a variety of stimuli. PI(3,4,5)P3 activates the serine-threonine kinase Akt, which is involved in antiapoptosis, proliferation and oncogenesis. In mice, heterozygosity for a null mutation of Pten (Pten(+/-)mice) frequently leads to the development of a variety of cancers and autoimmune disease. Homozygosity for the null mutation (Pten(-/-) mice) results in early embryonic lethality, precluding the functional analysis of Pten in adult tissues and organs. To investigate the physiological functions of Pten in viable mice, we and other groups have used the Cre-loxP system to generate various tissue-specific Pten mutations. The present review will summarize results obtained from the study of conditional mutant mice lacking Pten in specific tissues, and discuss the possible biological and molecular explanations for why Pten deficiency leads to tumorigenesis.

Carbon monoxide donors or heme oxygenase-1 (HO-1) overexpression blocks interleukin-18-mediated NF-kappaB-PTEN-dependent human cardiac endothelial cell death

The objective of this study was to determine whether heme oxygenase-1 (HO-1) or heme metabolites exert cytoprotective effects on interleukin-18-mediated endothelial cell (EC) death. Treatment with interleukin (IL)-18 increased NF-kappaB activation and PTEN induction, suppressed Akt activation, and stimulated EC death. While ectopic expression of p65 enhanced PTEN transcription, adenoviral transduction of dnIkappaB-alpha, dnp65, or dnIKKbeta was inhibitory. Furthermore, IL-18 suppressed HO-1 mRNA expression via enhanced mRNA degradation. Overexpression of HO-1, treatment with HO-1 inducer hemin, or the CO donor cobalt (III) protoporphyrin IX all reversed IL-18-mediated NF-kappaB activation, PTEN induction, Akt suppression, and EC death. Furthermore, hemin induced HO-1 expression, and HO-1 knockdown, HO-1 inhibition, or CO scavengers all reversed the prosurvival effects of hemin. In addition, the CO donors CORM-1 and CORM-3 and the heme metabolites biliverdin and bilirubin attenuated IL-18-induced EC death via a similar signaling pathway. IL-18 induced p38alpha MAPK activation, and suppressed p38beta isoform expression. While p38alpha knockdown attenuated, p38beta knockdown potentiated IL-18-mediated EC death. Hemin and HO-1 reversed IL-18-mediated p38alpha induction and restored p38beta levels. These results demonstrate that IL-18 suppresses HO-1 expression and induces EC death. HO-1 overexpression, HO-1 induction, or treatment with heme metabolites all reverse IL-18-mediated p38alpha MAPK and NF-kappaB activation, PTEN induction, Akt suppression, and EC death. Thus, HO-1 inducers and CO donors may have the therapeutic potential to effectively block IL-18 signaling and reduce IL-18-dependent vascular injury and inflammation.

Cell colonization in degradable 3D porous matrices

Cell colonization is an important in a wide variety of biological processes and applications including vascularization, wound healing, tissue engineering, stem cell differentiation and biosensors. During colonization porous 3D structures are used to support and guide the ingrowth of cells into the matrix. In this review, we summarize our understanding of various factors affecting cell colonization in three-dimensional environment. The structural, biological and degradation properties of the matrix all play key roles during colonization. Further, specific scaffold properties such as porosity, pore size, fiber thickness, topography and scaffold stiffness as well as important cell material interactions such as cell adhesion and mechanotransduction also influence colonization.

Phosphorylation of focal adhesion kinase promotes extravasation of breast cancer cells

Inhibition of focal adhesion kinase (FAK) delays transendothelial migration of breast cancer cells. Here we investigate whether phosphorylation of specific tyrosine residues of FAK (397, 861, and 925) known to control aspects of cell migration on extracellular matrix (ECM), are also involved in transendothelial migration. AU-565 and MDA-MB-231 cells expressing Phe397 FAK show delayed or decreased transendothelial migration, demonstrating the involvement of the FAK autophosphorylation site. Only MDA-MB-231 cells expressing Phe861 FAK exhibit delayed transendothelial migration. Neither MDA-MB-231 nor AU-565 cells expressing Phe925 FAK show a change in transendothelial migration compared to untreated cancer cells. These findings suggest that modified signaling mechanisms regulate cancer cell migration through an endothelial monolayer versus those involved in cell migration on or through ECM.

Src homology domain 2 adaptors affect adherence of Salmonella enterica serovar Typhimurium to non-phagocytic cells

The ability of Salmonella enterica serovar Typhimurium (S. Typhimurium) to penetrate the intestinal epithelium is key to its pathogenesis. Bacterial invasion can be seen as a two-step process initially requiring adherence to the host cell surface followed by internalization into the host cell. Evidence suggests that adherence of S. Typhimurium to host cells is receptor-mediated; however, the host cell receptor(s) has/have not been identified. Internalization of S. Typhimurium absolutely requires the actin cytoskeleton yet only a few of the cytoskeletal components involved in this process have been identified. In order to identify host proteins that may play a role in S. Typhimurium invasion, the recruitment of actin-associated proteins was investigated. The contribution of recruited Src homology 2 adaptor proteins to invasion was further investigated and it was found that, while not involved in bacterial internalization itself, the adaptors Nck and ShcA influenced adherence of S. Typhimurium to non-phagocytic cells.

Hyperplasia and carcinomas in Pten-deficient mice and reduced PTEN protein in human bladder cancer patients

PTEN is a tumor suppressor gene mutated in many human cancers. We used the Cre-loxP system to generate an urothelium-specific null mutation of Pten in mice [FabpCrePten(flox/flox) (FPten(flox/flox)) mice]. Histologic examination revealed that all FPten(flox/flox) mice exhibited urothelial hyperplasia in which component cells showed enlarged nuclei and increased cell size. With time, 10% of FPten(flox/flox) mice spontaneously developed pedicellate papillary transitional cell carcinomas (TCC). This type of tumor also arose in FPten(flox/flox) mice treated with the chemical carcinogen N-butyl-N-(4-hydroxybutyl) nitrosamine. FPten(flox/flox) urothelial cells were hyperproliferative and showed increased activation of the survival signaling molecules Akt and extracellular signal-regulated kinase. In humans, 53% of primary bladder cancer patients exhibited decreased or absent expression of PTEN protein in either the cytoplasm or nucleus of tumor cells. In early bladder cancers, PTEN expression was repressed in 42% of superficial papillary TCC but in only 8% of cases of carcinoma in situ (CIS). In advanced bladder cancers, PTEN protein was significantly reduced (particularly in the nucleus) in 94% of cases, and this decrease in PTEN correlated with disease stage and grade. Thus, PTEN deficiency may contribute to bladder cancer both by initiating superficial papillary TCC and by promoting the progression of CIS to advanced invasive and metastatic forms.

Inhibition of apoptosis by MAD1 is mediated by repression of the PTEN tumor suppressor gene

The MYC/MAX/MAD network of transcriptional regulators controls distinct aspects of cell physiology, including cell proliferation and apoptosis. Within the network MAD proteins antagonize the functions of MYC oncoproteins, and the latter are deregulated in the majority of human cancers. While MYC sensitizes cells to proapoptotic signals, the transcriptional repressor MAD1 inhibits apoptosis in response to a broad range of stimuli, including oncoproteins. The molecular targets of MAD1 that mediate inhibition of apoptosis are not known. Here we describe the phosphatase and tensin homologue deleted on chromosome ten (PTEN) tumor suppressor gene as a target of MAD1. By binding to the proximal promoter region, MAD1 downregulated PTEN expression. PTEN functions as a lipid phosphatase that regulates the phosphatidylinositol 3-kinase/AKT pathway. Indeed MAD1-dependent repression of PTEN led to activation of AKT and subsequent stimulation of the antiapoptotic NF-kappaB pathway. Interfering with AKT function affected the control of Fas-induced apoptosis by MAD1. In addition, knockdown of PTEN using small interfering RNA (siRNA) or the lack of PTEN rendered cells insensitive to inhibition of apoptosis by MAD1. These findings identify the PTEN gene as a target of the MYC-antagonist MAD1 and provide a molecular framework critical for the ability of MAD1 to inhibit apoptosis.

MAGI-2 Inhibits cell migration and proliferation via PTEN in human hepatocarcinoma cells

MAGI-2, a multidomain scaffolding protein, contains nine potential protein-protein interaction modules, including a GuK domain, two WW domains and six PDZ domains. In this study, we examined eight human hepatocarcinoma cell lines (HHCCs) and found that MAGI-2 was expressed only in 7721 cells. After 7721, 7404 and 97H cells were transfected with myc-MAGI-2 plasmid, their migration and proliferation was significantly inhibited, which was associated with downregulation of p-FAK and p-Akt. It is known that p-FAK is a substrate of PTEN and p-Akt can be regulated by PTEN via PIP(3). We demonstrated that PTEN was upregulated after myc-MAGI-2 transfection, which was due to the enhancement of PTEN protein stability rather than mRNA levels. Furthermore, MAGI-2-induced inhibition of cell migration and proliferation was attenuated in 7721 cells with PTEN silence or in PTEN-null cell line U87MG, and PTEN transfection could restore the effect of MAGI-2 in U87MG cells. Finally, the molecular association between PTEN and MAGI-2 was confirmed. Our results suggested that PTEN played a critical role in MAGI-2-induced inhibition of cell migration and proliferation in HHCCs.

PTEN, more than the AKT pathway

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN)/phosphatidylinositol 3-kinase (PI3K)/AKT constitute an important pathway regulating the signaling of multiple biological processes such as apoptosis, metabolism, cell proliferation and cell growth. PTEN is a dual protein/lipid phosphatase and its main substrate phosphatidyl-inositol 3,4,5 triphosphate (PIP3) is the product of PI3K. Increase in PIP3 recruits AKT to the membrane where is activated by other kinases also dependent on PIP3. Many components of this pathway have been described as causal forces in cancer. PTEN activity is lost by mutations, deletions or promoter methylation silencing at high frequency in many primary and metastatic human cancers. Germ line mutations of PTEN are found in several familial cancer predisposition syndromes. Recently, many activating mutations in the PI3KCA gene (coding for the p110alpha catalytic subunit of PI3K) have been described in human tumors. Activation of PI3K and AKT are reported to occur in breast, ovarian, pancreatic, esophageal and other cancers. Genetically modified mice confirm these PTEN activities. Tissue-specific deletions of PTEN usually provoke cancer. Moreover, an absence of PTEN cooperates with an absence of p53 to promote cancer. However, we have observed very different results with the expression of activated versions of AKT in several tissues. Activated AKT transgenic lines do not develop tumors in breast or prostate tissues and do not cooperate with an absence of p53. This data suggest that an AKT-independent mechanism contributes to PTEN tumorigenesis. Crosses with transgenic mice expressing possible PTEN targets indicate that neither cyclin D1 nor p53 are these AKT-independent targets. However, AKT is more than a passive bridge toward PTEN tumorigenesis, since its expression not only allows but also enforces and accelerates the tumorigenic process in combination with other oncogenes.

Analysis of directional cell migration on defined FN gradients: role of intracellular signaling molecules

Directional cell migration on extracellular matrix (ECM) plays important roles in embryonic development and adult organisms. To study the mechanisms and signaling pathways involved in the regulation of directional cell migration, we created defined fibronectin (FN) gradients by using microfluidic systems. We found that fibroblasts exhibited haptotaxis towards higher FN concentration on the gradient. Furthermore, the net movements in the direction of FN gradients correlated with the increase in the slope of the gradient although the overall rate of migration was not correlated. Consistent with previous observations on the uniformly coated surface, local higher FN concentration led to reduced migration rate due to increased spreading. Upon transfection of N-WASP or activated Cdc42, but not FAK or Grb7, the cells showed increased directional migration. However, transfection of FAK, but not the other signaling molecules, led to an increase in the persistence of directional cell migration, which is dependent on the slope of the gradient as well as FAK interaction with PI3K. Together, these studies reveal some novel properties of directional cell migration on defined FN gradient and suggested a role for FAK signaling and N-WASP and Cdc42 in the differential regulation of the persistence and rate of directional cell migration.

Activation signal transduction by beta1 integrin in T cells from patients with systemic lupus erythematosus

OBJECTIVE

Beta 1 integrin is a representative adhesion molecule for cell-cell and cell-extracellular matrix interactions, and it provides costimulatory signals to T cells. However, the relevance of beta1 integrin to T cell activation in systemic lupus erythematosus (SLE) remains unclear. We undertook this study to perform a quantitative and functional analysis of beta1 integrin-mediated signaling to T cells in patients with SLE.

METHODS

Expression of cell surface molecules was assessed by flow cytometric analysis. Engagement of beta1 integrins was performed by crosslinking using a specific monoclonal antibody. To assess tyrosine kinases in beta1 integrin-mediated signaling, the cells were transfected with a wild-type (WT) focal adhesion kinase (FAK), a dominant-negative truncation of the FAK, or a WT PTEN expression plasmid via nucleofection.

RESULTS

Beta 1 integrin expression was significantly up-regulated on peripheral blood T cells from patients with active SLE, particularly those with the complication of World Health Organization class IV nephritis, whereas CD28 was significantly decreased in patients with active SLE compared with normal individuals. Beta 1 integrin expression closely correlated with serum hypocomplementemia. Engagement of beta1 integrin on T cells from patients with active SLE, but not on those from normal individuals, induced cell proliferation as well as CD40L expression on T cells. Up-regulation of CD40L expression and T cell proliferation, induced by beta1 integrin stimulation, were completely inhibited by transfection of the dominant-negative truncations of FAK or WT PTEN.

CONCLUSION

These results suggest that engagement of beta1 integrins on SLE T cells could induce FAK-mediated signaling and subsequent CD40L expression and proliferation. Thus, the beta1 integrin signaling cascade might serve to enhance autoreactive T cell activation.