Regulation of G1 progression by the PTEN tumor suppressor protein is linked to inhibition of the phosphatidylinositol 3-kinase/Akt pathway

The PTEN-AKT3 signaling cascade as a therapeutic target in melanoma

Melanocytes undergo extensive genetic changes during transformation into aggressive melanomas. These changes deregulate genes whose aberrant activity promotes the development of this disease. The phosphoinositide-3-kinase (PI3K) and mitogen-activated protein (MAP) kinase pathways are two key signaling cascades that have been found to play prominent roles in melanoma development. These pathways relay extra-cellular signals via an ordered series of consecutive phosphorylation events from cell surface throughout the cytoplasm and nucleus regulating diverse cellular processes including proliferation, survival, invasion and angiogenesis. It is generally accepted that therapeutic agents would need to target these two pathways to be an effective therapy for the long-term treatment of advanced-stage melanoma patients. This review provides an overview of the PI3 kinase pathway focusing specifically on two members of the pathway, called PTEN and Akt3, which play important roles in melanoma development. Mechanisms leading to deregulation of these two proteins and therapeutic implications of targeting this signaling cascade to treat melanoma are detailed in this review.

TSC2 deficiency increases PTEN via HIF1alpha

Substantial evidence suggests roles of TSC2 and PTEN in the development of cancer predisposition syndromes. Loss of TSC2 results in benign tumors, neurological disorders, and angiomyolipomas. We found that PTEN mRNA and protein levels are elevated in Tsc2(-/-) mouse embryo fibroblasts with concomitant reduction in Akt phosphorylation. Reconstitution of TSC2 in Tsc2(-/-) mouse embryo fibroblasts decreases PTEN levels. Interestingly, increased HIF1alpha activity present in Tsc2 null cells is required for PTEN transcription and protein expression. We identified a canonical hypoxia-responsive element in the PTEN promoter, which regulates the transcription of this tumor suppressor protein in a TSC2-dependent manner. Finally, we demonstrate a positive correlation between expression of HIF1alpha and PTEN in renal angiomyolipomas from TSC patients. Our results reveal a unique function of HIF1alpha in up-regulation of PTEN and provide a new mechanism of reduced Akt phosphorylation in Tsc2 null cells. These data suggest that PTEN may safeguard against developing malignant tumors in patients with TSC deficiency.

PTEN suppression of YY1 induces HIF-2 activity in von-Hippel-Lindau-null renal-cell carcinoma

Despite recent advances in cancer therapies, metastatic renal cell carcinoma (RCC) remains difficult to treat. Most RCCs result from inactivation of the von Hippel Lindau (VHL) tumor suppressor, leading to stable expression of Hypoxia-Inducible Factor-alpha (HIF-1alpha, -2alpha, -3alpha) and the induction of downstream target genes, including those responsible for angiogenesis and metastasis. While VHL is inactivated in the majority of RCC cases, expression of the PTEN tumor suppressor is reduced in about 30% of cases. PTEN functions to antagonize PI3K/Akt/mTOR signaling, thereby controlling cell growth and survival. Activation of PI3K/Akt/mTOR leads to increased HIF-1alpha expression in certain cancer cells, supporting the rationale of using mTOR inhibitors as anti-cancer agents. Notably, HIF-2alpha, rather than HIF-1alpha, has been shown to play a critical role in renal tumorigenesis. To investigate whether HIF-2alpha is similarly regulated by the PI3K pathway in VHL(-/-)RCC cells, we manipulated PI3K signaling using PTEN overexpression and siRNA knockdown studies and pharmacologic inhibition of PI3K or Akt. Our data support a novel role for wild-type PTEN in promoting HIF-2alpha activity in VHL null RCC cells. This mechanism is unique to the cellular environment in which HIF-2alpha expression is deregulated, resulting from the loss of VHL function. Our data show that PTEN induces HIF-2alpha transcriptional activity by inhibiting expression of Yin Yang 1 (YY1), which acts as a novel corepressor of HIF-2alpha. Further, PTEN suppression of YY1 is mediated through antagonism of PI3K signaling. We conclude that wild-type PTEN relieves the repressive nature of YY1 at certain HIF-2alpha target promoters and that this mechanism may promote early renal tumorigenesis resulting from VHL inactivation by increasing HIF-2alpha activity.

Enhancement of BK(Ca) channel activity induced by hydrogen peroxide: involvement of lipid phosphatase activity of PTEN

Large-conductance calcium and voltage-dependent potassium (BK(Ca)) channel is an important determinant of vascular tone. It is activated by hydrogen peroxide (H(2)O(2)) which occurs in various physiological and pathological processes. However, the regulation mechanism is not fully understood. In the present study, the mSlo in the presence or absence of hbeta1 were cotransfected with the PTEN(wt), PTEN(C124S), PTEN(G129E) in HEK 293 cells. Typical BK(Ca) channel currents could be recorded in cell-attached configurations. We found that PTEN(wt) reduced the H(2)O(2)-induced BK(Ca) channel activation during the initial 10 min treatment. In contrast, coexpression with catalytically inactive PTEN(C124S)/PTEN(G129E) mutants that lack lipid phosphatase activity produced no regulation on the H(2)O(2)-induced BK(Ca) channel activation. These results demonstrated that PTEN regulated the H(2)O(2)-induced BK(Ca) channel activation through phosphatidylinositol 3-phosphatse. However, the inhibitory effect of PTEN on the H(2)O(2)-induced BK(Ca) channel activation was attenuated when cells were treated with H(2)O(2) at concentrations higher than 100 microM or at 100 microM for long-term treatment. In addition, the p-AKT expression level in PTEN(wt) overexpressing cells was lower than that in control cells, and the increase of cytoplasmic free calcium concentration ([Ca(2+)](i)) induced by H(2)O(2) was also inhibited. These findings may elucidate a new mechanism for H(2)O(2)-induced BK(Ca) channel activation and provide some evidences for the role of PTEN on vasodilation induced by H(2)O(2).

p85 Associates with unphosphorylated PTEN and the PTEN-associated complex

The lipid phosphatase PTEN functions as a tumor suppressor by dephosphorylating the D3 position of phosphoinositide-3,4,5-trisphosphate, thereby negatively regulating the phosphoinositide 3-kinase (PI3K)/AKT signaling pathway. In mammalian cells, PTEN exists either as a monomer or as a part of a >600-kDa complex (the PTEN-associated complex [PAC]). Previous studies suggest that the antagonism of PI3K/AKT signaling by PTEN may be mediated by a nonphosphorylated form of the protein resident within the multiprotein complex. Here we show that PTEN associates with p85, the regulatory subunit of PI3K. Using newly generated antibodies, we demonstrate that this PTEN-p85 association involves the unphosphorylated form of PTEN engaged within the PAC and also includes the p110beta isoform of PI3K. The PTEN-p85 association is enhanced by trastuzumab treatment and linked to a decline in AKT phosphorylation in some ERBB2-amplified breast cancer cell lines. Together, these results suggest that integration of p85 into the PAC may provide a novel means of downregulating the PI3K/AKT pathway.

Blockade of PAR1 signaling with cell-penetrating pepducins inhibits Akt survival pathways in breast cancer cells and suppresses tumor survival and metastasis

Protease-activated receptor 1 (PAR1) is a G protein-coupled receptor that is not expressed in normal breast epithelia but is up-regulated in invasive breast carcinomas. In the present study, we found that matrix metalloprotease-1 (MMP-1) robustly activates the PAR1-Akt survival pathway in breast carcinoma cells. This process is blocked by a cell-penetrating lipopeptide "pepducin," P1pal-7, which is a potent inhibitor of cell viability in breast carcinoma cells expressing PAR1. Both a MMP-1 inhibitor and P1pal-7 significantly promote apoptosis in breast tumor xenografts and inhibit metastasis to the lungs by up to 88%. Dual therapy with P1pal-7 and Taxotere inhibits the growth of MDA-MB-231 xenografts by 95%. Consistently, biochemical analysis of xenograft tumors treated with P1pal-7 or MMP-1 inhibitor showed attenuated Akt activity. Ectopic expression of constitutively active Akt rescues breast cancer cells from the synergistic cytotoxicity of P1pal-7 and Taxotere, suggesting that Akt is a critical component of PAR1-dependent cancer cell viability. Together, these findings indicate that blockade of MMP1-PAR1 signaling may provide a benefit beyond treatment with Taxotere alone in advanced, metastatic breast cancer.

Differences in regulation of tight junctions and cell morphology between VHL mutations from disease subtypes

Background

In von Hippel-Lindau (VHL) disease, germline mutations in the VHL tumor suppressor gene cause clear cell renal carcinomas, hemangioblastomas, and pheochromocytomas. The VHL gene product is part of an ubiquitin E3 ligase complex and hypoxia-inducible factor alpha (HIF-α) is a key substrate, although additional VHL functions have been described. A genotype-phenotype relationship exists in VHL disease such that specific VHL mutations elicit certain subsets of these tumors. Here, we examine VHL genotype-phenotype correlations at the cellular level, focusing on the regulation of tight junctions and cell morphology.

Methods

Wild-type and various mutant VHL proteins representing VHL disease subtypes were stably expressed in 3 VHL-negative renal carcinoma cell lines. Using these cell lines, the roles of various VHL-associated cellular functions in regulation of cell morphology were investigated.

Results

As a whole, type 1 mutants varied greatly from type 2 mutants, demonstrating high levels of HIF-2α, cyclin D1 and α5 integrin, lower p27 levels, and a spindly, fibroblastic cellular appearance. Type 2 mutations demonstrated an epithelial morphology similar to wild-type VHL in the majority of the renal cell lines used. Knockdown of p27 in cells with wild-type VHL led to perturbations of both epithelial morphology and ZO-1 localization to tight junctions. ZO-1 localization correlated well with VHL disease subtypes, with greater mislocalization observed for genotypes associated with a higher risk of renal carcinoma. HIF-2α knockdown in 786-O partially restored ZO-1 localization, but did not restore an epithelial morphology.

Conclusion

VHL has both HIF-α dependent and HIF-α independent functions in regulating tight junctions and cell morphology that likely impact the clinical phenotypes seen in VHL disease.

PTEN and p27 expression in mature T-cell and NK-cell neoplasms

PTEN is a tumor suppressor gene located on chromosome 10q23 and is amongst the most commonly mutated genes in human cancers. The lipid phosphatase activity of Pten enables it to dephosphorylate PIP3, thereby antagonizing growth factor stimulated PI3-kinase signaling mediated by AKT/PKB. The growth inhibition effect of PTEN has been shown to be mediated by p27 which is one of the important effector molecules downstream of the AKT pathway. Recently the importance of the Pten and AKT pathway in the regulation of the immune system and development of hematological malignancies has been shown. Loss of Pten and p27 expressions were examined immunohistochemically in 45 patients with peripheral T- and NK-cell lymphoma. Partial or complete loss of Pten was detected in 66.7% of the cases of anaplastic large cell lymphoma (ALCL) compared to only 12.5% of all other mature T-/NK-cell lymphomas combined. Loss of p27 was identified in 64.9% of cases, which showed a positive correlation with Pten loss. In this study, we showed that loss of Pten is more frequent in ALCL as compared to other mature T-/NK-cell lymphomas, which strongly correlates with the loss of p27 expression. Our findings provide further evidence for the importance of the deregulation of the PI3K-AKT pathway in ALCL.

mTOR and HIF-1alpha-mediated tumor metabolism in an LKB1 mouse model of Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS) is a familial cancer disorder due to inherited loss of function mutations in the LKB1/ STK11 serine/threonine kinase. PJS patients develop gastrointestinal hamartomas with 100% penetrance often in the second decade of life, and demonstrate an increased predisposition toward the development of a number of additional malignancies. Among mitogenic signaling pathways, the mammalian-target of rapamycin complex 1 (mTORC1) pathway is hyperactivated in tissues and tumors derived from LKB1-deficient mice. Consistent with a central role for mTORC1 in these tumors, rapamycin as a single agent results in a dramatic suppression of preexisting GI polyps in LKB1+/- mice. However, the key targets of mTORC1 in LKB1-deficient tumors remain unknown. We demonstrate here that these polyps, and LKB1- and AMPK-deficient mouse embryonic fibroblasts, show dramatic up-regulation of the HIF-1alpha transcription factor and its downstream transcriptional targets in an rapamycin-suppressible manner. The HIF-1alpha targets hexokinase II and Glut1 are up-regulated in these polyps, and using FDG-PET, we demonstrate that LKB1+/- mice show increased glucose utilization in focal regions of their GI tract corresponding to these gastrointestinal hamartomas. Importantly, we demonstrate that polyps from human Peutz-Jeghers patients similarly exhibit up-regulated mTORC1 signaling, HIF-1alpha, and GLUT1 levels. Furthermore, like HIF-1alpha and its target genes, the FDG-PET signal in the GI tract of these mice is abolished by rapamycin treatment. These findings suggest a number of therapeutic modalities for the treatment and detection of hamartomas in PJS patients, and potential for the screening and treatment of the 30% of sporadic human lung cancers bearing LKB1 mutations.

Piceatannol, a natural stilbene from grapes, induces G1 cell cycle arrest in androgen-insensitive DU145 human prostate cancer cells via the inhibition of CDK activity

We have examined whether and by what mechanism piceatannol inhibits cell cycle progression in DU145 cells. The treatment of cells with piceatannol for 24h resulted in an increase in the percentage of cells in G1 phase and dose-dependent decreases in [(3)H]thymidine incorporation, as well as in protein levels of cyclin A, cyclin D1, and cyclin-dependent kinase (CDK)2 and CDK4. Piceatannol exerted no effect on the levels of p21(WAF1/CIP1) or p27(KIP1). Piceatannol reduced CDK4 and CDK2 activity. These results indicate that delaying G1 cell cycle progression contributes to the piceatannol-mediated inhibition of DU145 cell growth, which may be mediated via the inhibition of CDK activity.

Prdx1 inhibits tumorigenesis via regulating PTEN/AKT activity

It is widely accepted that reactive oxygen species (ROS) promote tumorigenesis. However, the exact mechanisms are still unclear. As mice lacking the peroxidase peroxiredoxin1 (Prdx1) produce more cellular ROS and die prematurely of cancer, they offer an ideal model system to study ROS-induced tumorigenesis. Prdx1 ablation increased the susceptibility to Ras-induced breast cancer. We, therefore, investigated the role of Prdx1 in regulating oncogenic Ras effector pathways. We found Akt hyperactive in fibroblasts and mammary epithelial cells lacking Prdx1. Investigating the nature of such elevated Akt activation established a novel role for Prdx1 as a safeguard for the lipid phosphatase activity of PTEN, which is essential for its tumour suppressive function. We found binding of the peroxidase Prdx1 to PTEN essential for protecting PTEN from oxidation-induced inactivation. Along those lines, Prdx1 tumour suppression of Ras- or ErbB-2-induced transformation was mediated mainly via PTEN.

The endogenous inhibitor of Akt, CTMP, is critical to ischemia-induced neuronal death

Dysregulation of Akt signaling is important in a broad range of diseases that includes cancer, diabetes and heart disease. The role of Akt signaling in brain disorders is less clear. We found that global ischemia in intact rats triggered expression and activation of the Akt inhibitor CTMP (carboxyl-terminal modulator protein) in vulnerable hippocampal neurons and that CTMP bound and extinguished Akt activity and was essential to ischemia-induced neuronal death. Although ischemia induced a marked phosphorylation and nuclear translocation of Akt, phosphorylated Akt was not active in post-ischemic neurons, as assessed by kinase assays and phosphorylation of the downstream targets GSK-3beta and FOXO3A. RNA interference-mediated depletion of CTMP in a clinically relevant model of stroke restored Akt activity and rescued hippocampal neurons. Our results indicate that CTMP is important in the neurodegeneration that is associated with stroke and identify CTMP as a therapeutic target for the amelioration of hippocampal injury and cognitive deficits.

Epidermal growth factor receptor signaling synergizes with Hedgehog/GLI in oncogenic transformation via activation of the MEK/ERK/JUN pathway

Persistent activation of the Hedgehog (HH)/GLI signaling pathway has been implicated in the development of a number of human cancers. The GLI zinc finger transcription factors act at the end of the HH signaling cascade to control gene expression, and recent studies have shown that the activity of GLI proteins can be additionally modified by integration of distinct signals, such as the MEK/extracellular signal-regulated kinase (ERK) and phosphinositide-3 kinase (PI3K)/AKT pathway. However, little is known about the identity of the upstream activators of these HH/GLI interacting signaling pathways in cancer. Here, we provide evidence that integration of the HH/GLI and epidermal growth factor receptor (EGFR) pathway synergistically induces oncogenic transformation, which depends on EGFR-mediated activation of the RAS/RAF/MEK/ERK but not of the PI3K/AKT pathway. EGFR/MEK/ERK signaling induces JUN/activator protein 1 activation, which is essential for oncogenic transformation, in combination with the GLI activator forms GLI1 and GLI2. Furthermore, pharmacologic inhibition of EGFR and HH/GLI efficiently reduces growth of basal cell carcinoma (BCC) cell lines derived from mice with activated HH/GLI signaling. The results identify the synergistic integration of GLI activator function and EGFR signaling as a critical step in oncogenic transformation and provide a molecular basis for therapeutic opportunities relying on combined inhibition of the HH/GLI and EGFR/MEK/ERK/JUN pathway in BCC.

Measurement of PTEN activity in vivo by imaging phosphorylated Akt

This chapter describes an indirect approach to measure PTEN's lipid phosphatase activity in vivo. PTEN counteracts phosphatidylinositol 3-kinase action in dephosphorylating 3-phosphorylated phosphoinositides. Therefore, PtdIns(3,4,5)P3-dependent activation and phosphorylation of the survival kinase Akt can be used as readout for cellular PTEN activity. Here we have outlined a detailed procedure employing a phosphoserine-specific anti-Akt antibody to examine the content of phosphorylated Akt by immunofluorescence and its dependence on PTEN activity.

Role of the tumor suppressor PTEN in antioxidant responsive element-mediated transcription and associated histone modifications

Coordinated regulation of PI3-kinase (PI3K) and the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN) plays a pivotal role in various cell functions. PTEN is deficient in many cancer cells, including Jurkat human leukemia. Here, we demonstrate that the status of PTEN determines cellular susceptibility to oxidative stress through antioxidant-responsive element (ARE)-mediated transcription of detoxification genes. We found that ferritin H transcription was robustly induced in tert-butylhydroquinone (t-BHQ)-treated Jurkat cells via an ARE, and it was due to PTEN deficiency. Chromatin immunoprecipitation assays revealed that p300/CREB-binding protein (CBP) histone acetyltransferases and Nrf2 recruitment to the ARE and Bach1 release were blocked by the PI3K inhibitor LY294002, along with the partial inhibition of Nrf2 nuclear accumulation. Furthermore, acetylations of histone H3 Lys9 and Lys18, and deacetylation of Lys14 were associated with the PI3K-dependent ARE activation. Consistently, PTEN restoration in Jurkat cells inhibited t-BHQ-mediated expression of ferritin H and another ARE-regulated gene NAD(P)H:quinone oxidoreductase 1. Conversely, PTEN knockdown in K562 cells enhanced the response to t-BHQ. The PTEN status under t-BHQ treatment affected hydrogen peroxide-mediated caspase-3 cleavage. The PI3K-dependent ferritin H induction was observed by treatment with other ARE-activating agents ethoxyquin and hemin. Collectively, the status of PTEN determines chromatin modifications leading to ARE activation.

Identification of nuclear export inhibitors with potent anticancer activity in vivo

The export protein CRM1 is required for the nuclear export of a wide variety of cancer-related "cargo" proteins including p53, c-Abl, and FOXO-3A. Leptomycin B (LMB) is a highly specific inhibitor of CRM1 with significant in vitro potency but limited in vivo efficacy due to toxicity. We now report a series of semisynthetic LMB derivatives showing substantially improved therapeutic windows. Exposure of cancer cells to these compounds leads to a rapid and prolonged block of nuclear export and apoptosis. In contrast to what is observed in cancer cells, these agents induce cell cycle arrest, but not apoptosis, in normal lung fibroblasts. These new nuclear export inhibitors (NEI) maintain the high potency of LMB, are up to 16-fold better tolerated than LMB in vivo, and show significant efficacy in multiple mouse xenograft models. These NEIs show the potential of CRM1 inhibitors as novel and potent anticancer agents.

Breast cancer-associated fibroblasts confer AKT1-mediated epigenetic silencing of Cystatin M in epithelial cells

The interplay between histone modifications and promoter hypermethylation provides a causative explanation for epigenetic gene silencing in cancer. Less is known about the upstream initiators that direct this process. Here, we report that the Cystatin M (CST6) tumor suppressor gene is concurrently down-regulated with other loci in breast epithelial cells cocultured with cancer-associated fibroblasts (CAF). Promoter hypermethylation of CST6 is associated with aberrant AKT1 activation in epithelial cells, as well as the disabled INNP4B regulator resulting from the suppression by CAFs. Repressive chromatin, marked by trimethyl-H3K27 and dimethyl-H3K9, and de novo DNA methylation is established at the promoter. The findings suggest that microenvironmental stimuli are triggers in this epigenetic cascade, leading to the long-term silencing of CST6 in breast tumors. Our present findings implicate a causal mechanism defining how tumor stromal fibroblasts support neoplastic progression by manipulating the epigenome of mammary epithelial cells. The result also highlights the importance of direct cell-cell contact between epithelial cells and the surrounding fibroblasts that confer this epigenetic perturbation. Because this two-way interaction is anticipated, the described coculture system can be used to determine the effect of epithelial factors on fibroblasts in future studies.

A phosphorylation-dependent intramolecular interaction regulates the membrane association and activity of the tumor suppressor PTEN

The PI 3-phosphatase PTEN (phosphatase and tensin homologue deleted on chromosome 10), one of the most important tumor suppressors, must associate with the plasma membrane to maintain appropriate steady-state levels of phosphatidylinositol 3,4,5-triphosphate. Yet the mechanism of membrane binding has received little attention and the key determinants that regulate localization, a phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding motif and a cluster of phosphorylated C-terminal residues, were not included in the crystal structure. We report that membrane binding requires PIP(2) and show that phosphorylation regulates an intramolecular interaction. A truncated version of the enzyme, PTEN(1-351), bound strongly to the membrane, an effect that was reversed by co-expression of the remainder of the molecule, PTEN(352-403). The separate fragments associated in vitro, an interaction dependent on phosphorylation of the C-terminal cluster, a portion of the PIP(2) binding motif, integrity of the phosphatase domain, and the CBR3 loop. Our investigation provides direct evidence for a model in which PTEN switches between open and closed states and phosphorylation favors the closed conformation, thereby regulating localization and function. Small molecules targeting these interactions could potentially serve as therapeutic agents in antagonizing Ras or PI3K-driven tumors. The study also stresses the importance of determining the structure of the native enzyme.

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.

FoxO1 mediates PTEN suppression of androgen receptor N- and C-terminal interactions and coactivator recruitment

FoxO (mammalian forkhead subclass O) proteins are transcription factors acting downstream of the PTEN (phosphatase and tensin homolog deleted on chromosome 10) tumor suppressor. Their activity is negatively regulated by AKT-mediated phosphorylation. Our previous studies showed that the transcriptional activity of the androgen receptor (AR) was inhibited by PTEN in an AKT-sensitive manner. Here, we report the repression of the activity of the full-length AR and its N-terminal domain by FoxO1 and the participation of FoxO1 in AR inhibition by PTEN. Ectopic expression of active FoxO1 decreased the transcriptional activity of AR as well as androgen-induced cell proliferation and production of prostate-specific antigen. FoxO1 knock down by RNA interference increased the transcriptional activity of the AR in PTEN-intact cells and relieved its inhibition by ectopic PTEN in PTEN-null cells. Mutational analysis revealed that FoxO1 fragment 150-655, which contains the forkhead box and C-terminal activation domain, was required for AR inhibition. Mammalian two-hybrid and glutathione-S-transferase pull-down assays demonstrated that the inhibition of AR activity by PTEN through FoxO1 involved the interference of androgen-induced interaction of the N- and C-termini of the AR and the recruitment of the p160 coactivators to its N terminus and to the androgen response elements of natural AR target genes. These studies reveal new mechanisms for the inhibition of AR activity by PTEN-FoxO axis and establish FoxO proteins as important nuclear factors that mediate the mutual antagonism between AR and PTEN tumor suppressor in prostate cancer cells.

Phosphatase activity of nuclear PTEN is required for CDX2-mediated intestinal differentiation of gastric carcinoma

The PTEN tumor suppressor localizes predominantly to the cytoplasm, where it negatively regulates the phosphatidylinositol 3-kinase-AKT signaling pathway; however, the biological significance of nuclear PTEN in gastric carcinoma (GC) remains unknown. In this study, transduction of recombinant PTEN into GC-derived TMK-1 cells promoted PTEN nuclear localization with increased mRNA levels of CDX2 and intestinal claudins (CLDN3 and CLDN4), whereas the G129E phosphatase 'dead' mutant had no effect. In GC tissue samples, tumors with nuclear PTEN expression frequently demonstrated the intestinal-type claudin phenotype. Our results suggested that nuclear localization of PTEN is important for determining intestinal differentiation of GCs.

Fish oil targets PTEN to regulate NFkappaB for downregulation of anti-apoptotic genes in breast tumor growth

The molecular mechanism for the beneficial effect of fish oil on breast tumor growth is largely undefined. Using the xenograft model in nude mice, we for the first time report that the fish oil diet significantly increased the level of PTEN protein in the breast tumors. In addition, the fish oil diet attenuated the PI 3 kinase and Akt kinase activity in the tumors leading to significant inhibition of NFkappaB activation. Fish oil diet also prevented the expression of anti-apoptotic proteins Bcl-2 and Bcl-XL in the breast tumors with concomitant increase in caspase 3 activity. To extend these findings we tested the functional effects of DHA and EPA, the two active omega-3 fatty acids of fish oil, on cultured MDA MB-231 cells. In agreement with our in vivo data, DHA and EPA treatment increased PTEN mRNA and protein expression and inhibited the phosphorylation of p65 subunit of NFkappaB in MDA MB-231 cells. Furthermore, DHA and EPA reduced expression of Bcl-2 and Bcl-XL. NFkappaB DNA binding activity and NFkappaB-dependent transcription of Bcl-2 and Bcl-XL genes were also prevented by DHA and EPA treatment. Finally, we showed that PTEN expression significantly inhibited NFkappaB-dependent transcription of Bcl-2 and Bcl-XL genes. Taken together, our data reveals a novel signaling pathway linking the fish oil diet to increased PTEN expression that attenuates the growth promoting signals and augments the apoptotic signals, resulting in breast tumor regression.

PTEN: a new guardian of the genome

The phosphatase and tensin homolog deleted on chromosome 10 (PTEN) tumor suppressor is a phosphatase that antagonizes the phosphoinositol-3-kinase/AKT signaling pathway and suppresses cell survival as well as cell proliferation. PTEN is the second most frequently mutated gene in human cancer after p53. Germline mutations of PTEN have been found in cancer susceptibility syndromes, such as Cowden syndrome, in which over 80% of patients have mutations of PTEN. Homozygous deletion of Pten causes embryonic lethality, suggesting that PTEN is essential for embryonic development. Mice heterozygous for Pten develop spontaneous tumors in a variety of organs comparable with the spectrum of its mutations in human cancer. The mechanisms of PTEN functions in tumor suppression are currently under intense investigation. Recent studies demonstrate that PTEN plays an essential role in the maintenance of chromosomal stability and that loss of PTEN leads to massive alterations of chromosomes. The tumor suppressor p53 is known as a guardian of the genome that mediates the cellular response to environmental stress, leading to cell cycle arrest or cell death. Through completely different mechanisms, PTEN also protects the genome from instability. Thus, we propose that PTEN is a new guardian of the genome. In this review, we will discuss new discoveries on the role of PTEN in tumor suppression and explore mechanisms by which PTEN maintains genomic stability.

The role of PTEN signaling perturbations in cancer and in targeted therapy

The PTEN tumor suppressor was discovered by its homozygous deletion and other mutations in cancer. Since then, PTEN has been shown to be a non-redundant, evolutionarily conserved phosphatase whose function affects diverse cellular progresses such as cell cycle progression, cell proliferation, chemotaxis, apoptosis, aging, muscle contractility, DNA damage response, angiogenesis and cell polarity. In accordance with its ability to influence multiple crucial cellular processes, PTEN has a major role in the pathogenesis of numerous diseases such as diabetes, autism and almost every cancer examined. This review will discuss the diverse ways in which PTEN signaling is modified in cancer, and how these changes correlate with and might possibly affect the action of targeted chemotherapy.

Heterogeneous nuclear ribonucleoprotein A1 regulates cyclin D1 and c-myc internal ribosome entry site function through Akt signaling

The translation of the cyclin D1 and c-myc mRNAs occurs via internal ribosome entry site (IRES)-mediated initiation under conditions of reduced eIF-4F complex formation and Akt activity. Here we identify hnRNP A1 as an IRES trans-acting factor that regulates cyclin D1 and c-myc IRES activity, depending on the Akt status of the cell. hnRNP A1 binds both IRESs in vitro and in intact cells and enhances in vitro IRES-dependent reporter expression. Akt regulates this IRES activity by inducing phosphorylation of hnRNP A1 on serine 199. Serine 199-phosphorylated hnRNP A1 binds to the IRESs normally but is unable to support IRES activity in vitro. Reducing expression levels of hnRNP A1 or overexpressing a dominant negative version of the protein markedly inhibits rapamycin-stimulated IRES activity in cells and correlated with redistribution of cyclin D1 and c-myc transcripts from heavy polysomes to monosomes. Importantly, knockdown of hnRNP A1 also renders quiescent Akt-containing cells sensitive to rapamycin-induced G(1) arrest. These results support a role for hnRNP A1 in mediating rapamycin-induced alterations of cyclin D1 and c-myc IRES activity in an Akt-dependent manner and provide the first direct link between Akt and the regulation of IRES activity.

Combination of PTEN and gamma-ionizing radiation enhances cell death and G(2)/M arrest through regulation of AKT activity and p21 induction in non-small-cell lung cancer cells

PURPOSE

To identify the role of phosphatase and tensin homolog deleted on chromosome 10 (PTEN) during gamma-ionizing radiation (gamma-IR) treatment for non-small-cell lung cancer cells.

METHODS AND MATERIALS

Wild-type PTEN or mutant forms of PTEN plasmids were transfected to construct stable transfectants of the NCI-H1299 non-small-cell lung cancer cell line. Combined effects of PTEN expression and IR treatment were tested using immunoblot, clonogenic, and cell-counting assays. Related signaling pathways were studied with immunoblot and kinase assays.

RESULTS

At steady state, stable transfectants showed almost the same proliferation rate but had different AKT phosphorylation patterns. When treated with gamma-IR, wild-type PTEN transfectants showed higher levels of cell death compared with mock vector or mutant transfectants, and showed increased G(2)/M cell-cycle arrest accompanied by p21 induction and CDK1 inactivation. NCI-H1299 cells were treated with phosphosinositide-3 kinase (PI3K)/AKT pathway inhibitor (LY29002), resulting in reduced AKT phosphorylation levels. Treatment of NCI-H1299 cells with LY29002 and gamma-IR resulted in increased cell-cycle arrest and p21 induction. Endogenous wild-type PTEN-containing NCI-H460 cells were treated with PTEN-specific siRNA and then irradiated with gamma-IR: however reduced PTEN levels did not induce cell-cycle arrest or p21 expression.

CONCLUSIONS

Taken together, these findings indicate that PTEN may modulate cell death or the cell cycle via AKT inactivation by PTEN and gamma-IR treatment. We also propose that a PTEN-PI3K/AKT-p21-CDK1 pathway could regulate cell death and the cell cycle by gamma-IR treatment.

Endothelial cell-derived bone morphogenetic proteins control proliferation of neural stem/progenitor cells

Neurogenesis persists in the adult brain subventricular zone where neural stem/progenitor cells (NSPCs) lie close to brain endothelial cells (BECs). We show in mouse that BECs produce bone morphogenetic proteins (BMPs). Coculture of embryonic and adult NSPCs with BECs activated the canonical BMP/Smad pathway and reduced their proliferation. We demonstrate that coculture with BECs in the presence of EGF and FGF2 induced a reversible cell cycle exit of NSPCs (LeX+) and an increase in the amount of GFAP/LeX-expressing progenitors thought to be stem cells. Levels of the phosphatidylinositol phosphatase PTEN were upregulated in NSPCs after coculture with BECs, or treatment with recombinant BMP4, with a concomitant reduction in Akt phosphorylation. Silencing Smad5 with siRNA or treatment with Noggin, a BMP antagonist, demonstrated that upregulation of PTEN in NSPCs required BMP/Smad signaling and that this pathway regulated cell cycle exit of NSPCs. Therefore, BECs may provide a feedback mechanism to control the proliferation of NSPCs.

Differential effects of energy stress on AMPK phosphorylation and apoptosis in experimental brain tumor and normal brain

Background

AMP-activated protein kinase (AMPK) is a known physiological cellular energy sensor and becomes phosphorylated at Thr-172 in response to changes in cellular ATP levels. Activated AMPK acts as either an inducer or suppressor of apoptosis depending on the severity of energy stress and the presence or absence of certain functional tumor suppressor genes.

Results

Here we show that energy stress differentially affects AMPK phosphorylation and cell-death in brain tumor tissue and in tissue from contra-lateral normal brain. We compared TSC2 deficient CT-2A mouse astrocytoma cells with syngeneic normal astrocytes that were grown under identical condition in vitro. Energy stress induced by glucose withdrawal or addition of 2-deoxyglucose caused more ATP depletion, AMPK phosphorylation and apoptosis in CT-2A cells than in the normal astrocytes. Under normal energy conditions pharmacological stimulation of AMPK caused apoptosis in CT-2A cells but not in astrocytes. TSC2 siRNA treated astrocytes are hypersensitive to apoptosis induced by energy stress compared to control cells. AMPK phosphorylation and apoptosis were also greater in the CT-2A tumor tissue than in the normal brain tissue following implementation of dietary energy restriction. Inefficient mTOR and TSC2 signaling, downstream of AMPK, is responsible for CT-2A cell-death, while functional LKB1 may protect normal brain cells under energy stress.

Conclusion

Together these data demonstrates that AMPK phosphorylation induces apoptosis in mouse astrocytoma but may protect normal brain cells from apoptosis under similar energy stress condition. Therefore, using activator of AMPK along with glycolysis inhibitor could be a potential therapeutic approach for TSC2 deficient human malignant astrocytoma.

Characterization of an Akt kinase inhibitor with potent pharmacodynamic and antitumor activity

Akt kinases 1, 2, and 3 are important regulators of cell survival and have been shown to be constitutively active in a variety of human tumors. GSK690693 is a novel ATP-competitive, low-nanomolar pan-Akt kinase inhibitor. It is selective for the Akt isoforms versus the majority of kinases in other families; however, it does inhibit additional members of the AGC kinase family. It causes dose-dependent reductions in the phosphorylation state of multiple proteins downstream of Akt, including GSK3 beta, PRAS40, and Forkhead. GSK690693 inhibited proliferation and induced apoptosis in a subset of tumor cells with potency consistent with intracellular inhibition of Akt kinase activity. In immune-compromised mice implanted with human BT474 breast carcinoma xenografts, a single i.p. administration of GSK690693 inhibited GSK3 beta phosphorylation in a dose- and time-dependent manner. After a single dose of GSK690693, >3 micromol/L drug concentration in BT474 tumor xenografts correlated with a sustained decrease in GSK3 beta phosphorylation. Consistent with the role of Akt in insulin signaling, treatment with GSK690693 resulted in acute and transient increases in blood glucose level. Daily administration of GSK690693 produced significant antitumor activity in mice bearing established human SKOV-3 ovarian, LNCaP prostate, and BT474 and HCC-1954 breast carcinoma xenografts. Immunohistochemical analysis of tumor xenografts after repeat dosing with GSK690693 showed reductions in phosphorylated Akt substrates in vivo. These results support further evaluation of GSK690693 as an anticancer agent.

Fatty acid synthase gene is up-regulated by hypoxia via activation of Akt and sterol regulatory element binding protein-1

The fatty acid synthase (FAS) gene is significantly up-regulated in various types of cancers, and blocking the FAS expression results in apoptosis of tumor cells. Therefore, FAS is considered to be an attractive target for anticancer therapy. However, the molecular mechanism by which the FAS gene is up-regulated in tumor cells is poorly understood. We found that FAS was significantly up-regulated by hypoxia, which was also accompanied by reactive oxygen species (ROS) generation in human breast cancer cell lines. The FAS expression was also activated by H(2)O(2), whereas N-acetyl-L-cystein, a ROS inhibitor, suppressed the expression. We also found that the hypoxia significantly up-regulated sterol regulatory-element binding protein (SREBP)-1, the major transcriptional regulator of the FAS gene, via phosphorylation of Akt followed by activation of hypoxia-inducible factor 1 (HIF1). Moreover, our results of reporter assay and chromatin immunoprecipitation analysis indicate that SREBP-1 strongly bound to the SREBP binding site/E-box sequence on the FAS promoter under hypoxia. In our xenograft mouse model, FAS was strongly expressed in the hypoxic regions of the tumor. In addition, our results of immunohistochemical analysis for human breast tumor specimens indicate that the expressions of both FAS and SREBP-1 were colocalized with hypoxic regions in the tumors. Furthermore, we found that hypoxia-induced chemoresistance to cyclophosphamide was partially blocked by a combination of FAS inhibitor and cyclophosphamide. Taken together, our results indicate that FAS gene is up-regulated by hypoxia via activation of the Akt and HIF1 followed by the induction of the SREBP-1 gene, and that hypoxia-induced chemoresistance is partly due to the up-regulation of FAS.

PTEN regulates p300-dependent hypoxia-inducible factor 1 transcriptional activity through Forkhead transcription factor 3a (FOXO3a)

The tumor suppressor PTEN is mutated or deleted in many tumors, causing the activation of the PI3K pathway. Here, we show that the loss of PTEN increases the transcriptional activity of hypoxia-inducible factor 1 (HIF-1) through the inactivation of Forkhead transcription factors (FOXO) in PTEN-null cells. Reintroduction of PTEN into the nucleus, overexpression of a nonphosphorylatable FOXO3a, which accumulates in the nucleus, or inhibition of nuclear export of FOXO3a by leptomycin B represses HIF-1 transcriptional activity in PTEN-null cells. HIF-1 transcriptional activity increases in PTEN-positive cells depleted of FOXO3a with siRNA. PTEN and FOXO3a regulate the transactivation domain of HIF-1alpha. Chromatin immunoprecipitation indicates that FOXO3a complexes with HIF-1alpha and p300 on the Glut-1 promoter, a HIF-1 target gene. Overexpression of p300 reverses FOXO3a-mediated repression of HIF-1 transcriptional activity. Coimmunoprecipitation and GAL4-HIF-1alpha transactivation assays reveal that FOXO3a interferes with p300-dependent HIF-1 transcriptional activity. Thus, FOXO3a negatively regulates HIF-1 transcriptional activity.

STAT3 regulates ABCA3 expression and influences lamellar body formation in alveolar type II cells

ATP-Binding Cassette A3 (ABCA3) is a lamellar body associated lipid transport protein required for normal synthesis and storage of pulmonary surfactant in type II cells in the alveoli. In this study, we demonstrate that STAT3, activated by IL-6, regulates ABCA3 expression in vivo and in vitro. ABCA3 mRNA and immunostaining were decreased in adult mouse lungs in which STAT3 was deleted from the respiratory epithelium (Stat3(Delta/Delta) mice). Consistent with the role of STAT3, intratracheal IL-6 induced ABCA3 expression in vivo. Decreased ABCA3 and abnormalities in the formation of lamellar bodies, the intracellular site of surfactant lipid storage, were observed in Stat3(Delta/Delta) mice. Expression of SREBP1a and 1c, SCAP, ABCA3, and AKT mRNAs was inhibited by deletion of Stat3 in type II cells isolated from Stat3(Delta/Delta) mice. The activities of PI3K and AKT were required for normal Abca3 gene expression in vitro. AKT activation induced SREBP expression and increased the activity of the Abca3 promoter in vitro, consistent with the role of STAT3 signaling, at least in part via SREBP, in the regulation of ABCA3. ABCA3 expression is regulated by IL-6 in a pathway that includes STAT3, PI3K, AKT, SCAP, and SREBP. Activation of STAT3 after exposure to IL-6 enhances ABCA3 expression, which, in turn, influences pulmonary surfactant homeostasis.

The RNA-binding protein KSRP promotes decay of beta-catenin mRNA and is inactivated by PI3K-AKT signaling

β-catenin plays an essential role in several biological events including cell fate determination, cell proliferation, and transformation. Here we report that β-catenin is encoded by a labile transcript whose half-life is prolonged by Wnt and phosphatidylinositol 3-kinase–AKT signaling. AKT phosphorylates the mRNA decay-promoting factor KSRP at a unique serine residue, induces its association with the multifunctional protein 14-3-3, and prevents KSRP interaction with the exoribonucleolytic complex exosome. This impairs KSRP's ability to promote rapid mRNA decay. Our results uncover an unanticipated level of control of β-catenin expression pointing to KSRP as a required factor to ensure rapid degradation of β-catenin in unstimulated cells. We propose KSRP phosphorylation as a link between phosphatidylinositol 3-kinase–AKT signaling and β-catenin accumulation.

During mammalian development and adulthood, β-catenin regulates the transcription of a family of genes with multiple essential roles in cell proliferation and differentiation. β-catenin also plays a role in cancer when it carries mutations that result in uncontrolled β-catenin function. Here, we report that the lifetime of the β-catenin–encoding transcript is under regulatory control. We show that specific cellular signals relevant to proper mammalian development and implicated in tumor formation can prolong β-catenin transcript half-life, leading to the accumulation of β-catenin protein. We identify a molecular mechanism for this prolongation by showing that a protein factor responsible for β-catenin transcript instability (and thus degradation) is impaired by phosphorylation, a chemical modification. When this factor is impaired, β-catenin mRNA and protein accumulate. Our results point to an unanticipated control of β-catenin levels through regulation of its transcript half-life in response to signals related to proliferation and differentiation.

The authors show that the half-life of β-catenin mRNA is prolonged by PI3K-AKT signaling, revealing a new level of control on β-catenin.

Electronegative LDL circulating in smokers impairs endothelial progenitor cell differentiation by inhibiting Akt phosphorylation via LOX-1

Endothelial progenitor cells (EPCs), important for endothelial regeneration and vasculogenesis, are reduced by cigarette smoking. To elucidate the mechanisms, we examined the effects of electronegative LDL, circulating in chronic smokers, on EPC differentiation. Using ion-exchange chromatography, we purified smoker LDL into five subfractions, L1-L5. In matched, nonsmoking healthy subjects, L5, the most electronegative subfraction, was either absent or scanty. Sustained L5 treatment inhibited CD31 and KDR expression and EPC differentiation, whereas L1-L4 had no effect. L5 also inhibited telomerase activity to accelerate EPC senescence in correlation with reduced Akt phosphorylation. Transfection of day 3 EPCs with dominant negative Akt constructs inhibited CD31 and KDR expression, stalled EPC differentiation, and promoted early senescence. In contrast, transfection with constitutively active Akt rendered the EPCs resistant to L5, allowing normal maturation. L5 upregulated the lectin-like oxidized low density lipoprotein receptor 1 (LOX-1), and pretreatment of EPCs with TS20, a LOX-1-neutralizing antibody, blocked internalization of L5 by EPCs and prevented L5-mediated inhibition of EPC differentiation. Mixing L5 with L1 to physiological L5/L1 ratios did not attenuate L5's effects. These findings suggest that cigarette smoking is associated with the formation of L5, which inhibits EPC differentiation by impairing Akt phosphorylation via the LOX-1 receptor.

Determining risk of biochemical recurrence in prostate cancer by immunohistochemical detection of PTEN expression and Akt activation

PURPOSE

A considerable fraction of patients who undergo radical prostatectomy as treatment for primary prostate cancer experience biochemical recurrence detected by elevated serum levels of prostate-specific antigen. In this study, we investigate whether loss of expression of the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and the phosphorylated form of the cell survival protein Akt (pAkt) predicts biochemical recurrence.

EXPERIMENTAL DESIGN

Expression of PTEN and pAkt was detected by immunohistochemistry in paraffin-embedded prostate cancer tissue obtained from men undergoing radical prostatectomy. Outcome was determined by 60-month follow-up determining serum prostate-specific antigen levels.

RESULTS

By itself, PTEN was not a good predictor of biochemical recurrence; however, in combination with pAkt, it was a better predictor of the risk of biochemical recurrence compared with pAkt alone. Ninety percent of all cases with high pAkt and negative PTEN were recurrent whereas 88.2% of those with low pAkt and positive PTEN were nonrecurrent. In addition, high Gleason scores resulted in reduced protection from decreased pAkt and increased PTEN. By univariate logistic regression, pAkt alone gives an area under the receiver-operator characteristic curve of 0.82 whereas the area under the receiver-operator characteristic curve for the combination of PTEN, pAkt, and Gleason based on a stepwise selection model is 0.89, indicating excellent discrimination.

CONCLUSIONS

Our results indicate that loss of PTEN expression, together with increased Akt phosphorylation and Gleason score, is of significant predictive value for determining, at the time of prostatectomy, the risk of biochemical recurrence.

Role of the integrin-linked kinase (ILK) in determining neuronal polarity

The establishment of axon-dendrite polarity in mammalian neurons has recently been shown to involve the kinases Akt and GSK-3beta. Here we report the function of the integrin-linked kinase (ILK) in neuronal polarization. ILK distribution is differential: with more of it present in the axonal tips than that in the dendritic tips of a polarized neuron. Inactivation of ILK by chemical inhibitors, a kinase-inactive mutant or siRNAs inhibited axon formation, whereas a kinase hyperactive ILK mutant induced the formation of multiple axons. Biochemical studies indicate that ILK is upstream of Akt and GSK-3beta. Manipulations of multiple intracellular components indicate that ILK is functionally upstream of Akt and GSK-3beta but downstream of PI3K in neuronal polarity. These results reveal a key role of ILK in the formation of neuronal polarity and suggest a signaling pathway important for neuronal polarity.

Specific amino acid dependency regulates the cellular behavior of melanoma

Relative specific amino acid dependency is one of the metabolic abnormalities of melanoma cells and metabolic studies of this dependency are in their infancy. Herein, we review the current studies in this area and present new information that adds to the understanding of how tyrosine (Tyr) and phenylalanine (Phe) dependency as well as other amino acids regulate the cell behaviors of melanoma cells. Amino acid dependency of human melanoma cells is multifactorial and restricting Tyr and Phe to melanoma triggers a series of alterations in metabolic and signaling pathways in a time-ordered fashion to alter different cellular behaviors. For example, at early time points, the reduction of Tyr and Phe alters metabolic reactions quantitatively or qualitatively. The alterations include modulation of integrin/focal adhesion kinase (FAK)/G protein pathways and the plasminogen activator (PA)/PA inhibitor pathways to inhibit tumor cell invasion. At later time periods, a further drop in intracellular amino acids induces more metabolic alterations to impact the FAK/Ras/Raf and Bcl-2 pathways leading to apoptosis. The threshold effects and the targeting of multiple pathways by restriction of specific amino acids provide a connection between the metabolic alterations and signaling pathways that modulate the cellular behaviors of melanoma cells. Decoding the metabolic alterations that connect amino acid concentration to the crucial step(s) in signaling is important and an exciting area of cancer research.

Application and interpretation of FISH in biomarker studies

Emerging genomic and proteomic data is creating new opportunities to identify novel biomarkers that will have pathway-specific therapeutic impact on cancer progression. Molecular cytogenetic and fluorescence in situ hybridization (FISH) methods have been primarily used in discovery genetic research laboratories until recently. New automated analytical platforms based on FISH technologies and tissue microarray methods are providing a rapid means to determine the impact of consistent genomic aberrations in clinical trials, and in studies designed to investigate differential chemotherapeutic response.

A short N-terminal sequence of PTEN controls cytoplasmic localization and is required for suppression of cell growth

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is an important negative regulator of cell growth and a tumor suppressor. Its growth-attenuating activity is based on the dephosphorylation of phosphatidylinositol 3,4,5-trisphosphate (PIP3), an essential second messenger for the phosphoinositide 3-kinase/Akt signaling pathway. This activity may require localization of PTEN to cytoplasmic membranes. Yet PTEN can also localize to the cell nucleus where its functions remain unclear. Here we present data that define a short sequence in the N-terminal region of PTEN required for cytoplasmic localization. We will refer to this sequence as cytoplasmic localization signal (CLS). It could function as a non-canonical signal for nuclear export or as a cytoplasmic retention signal of PTEN. Mutations within the CLS induce nuclear localization and impair growth suppressive activities of PTEN while preserving lipid phosphatase activity. We propose that nuclear localization of PTEN is not compatible with plasma membrane-targeted growth suppressive functions of PTEN.

Phosphatase PTEN is inactivated in bovine aortic endothelial cells exposed to cyclic strain

Hemodynamic forces, including cyclic strain (CS) and shear stress (SS), have been recognized as important modulators of vascular cell morphology and function. PTEN (also known as MMAC1/TEP1) is a lipid phosphatase that leads to a decrease in intracellular phosphatidylinositol 3,4,5 trisphosphate (PIP3) and therefore can modulate the stimulating effect of phosphatidylinositol 3-kinase (PI3K). In this study, we focused on the upstream regulators of the PI3K-Akt pathway by assessing Akt, PTEN, casein kinase 2 (CK2) (a kinase that catalyzes phosphorylation of PTEN), and PI3K activity in endothelial cells (EC) exposed to CS. The activity of phospho-PTEN (n = 4) and phospho-CK2 (n = 4) increased in a time-dependent fashion, reaching maximal activity by 10 min of CS stimulation. The peak of phospho-Akt activity (n = 4) occurred later, at 60 min. Akt activity was altered by transfection of EC with dominant negative PTEN plasmids. Furthermore, CS increased PIP3 immunoreactivity in a time-dependent manner, reaching maximal activity after 60 min of CS stimulation, and these effects were affected by transfection of EC with dominant negative PTEN plasmids. Inhibition of PTEN activity had no effect on CS-mediated cell proliferation but inhibited CS-mediated suppression of apoptosis.

A small molecule inhibitor for phosphatase and tensin homologue deleted on chromosome 10 (PTEN)

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN), a phosphoinositide 3-phosphatase, is an important regulator of insulin-dependent signaling. The loss or impairment of PTEN results in an antidiabetic impact, which led to the suggestion that PTEN could be an important target for drugs against type II diabetes. Here we report the design and validation of a small- molecule inhibitor of PTEN. Compared with other cysteine-based phosphatases, PTEN has a much wider active site cleft enabling it to bind the PtdIns(3,4,5)P3 substrate. We have exploited this feature in the design of vanadate scaffolds complexed to a range of different organic ligands, some of which show potent inhibitory activity. A vanadyl complexed to hydroxypicolinic acid was found to be a highly potent and specific inhibitor of PTEN that increases cellular PtdIns(3,4,5)P3 levels, phosphorylation of Akt, and glucose uptake in adipocytes at nanomolar concentrations. The findings presented here demonstrate the applicability of a novel and specific chemical inhibitor against PTEN in research and drug development.

Interphase FISH analysis of PTEN in histologic sections shows genomic deletions in 68% of primary prostate cancer and 23% of high-grade prostatic intra-epithelial neoplasias

Prostate cancer (CaP) is characterized by the accumulation of both genetic and epigenetic alterations that transform premalignant lesions to invasive carcinoma. However, the molecular events underlying this critical transition are poorly understood. One of the important genes that might play a role in CaP development is the PTEN gene. At the present time, there has been no systematic analysis of the incidence of genomic PTEN deletion by fluorescence in situ hybridization (FISH) in CaP and associated preneoplastic histologic lesions. This study assesses the frequency of PTEN deletion by interphase FISH analysis in CaP and prostatic intra-epithelial neoplasia (PIN). Dual-color FISH was performed using DNA probes for bands 10q23.3 (PTEN locus) and chromosome 10 centromere using 35 radical prostatectomy specimens. PTEN deletions were not found in 3/3 of stroma, 6/6 samples of benign glandular epithelium, and 12/12 samples of low-grade PIN. However, PTEN deletions were found in 3/13 (23%) of high-grade PIN and 24/35 (68%) of CaP. Concordance was observed between PTEN deletion status and the overall cellular PTEN protein expression levels, as assessed by immunohistochemistry. The high frequency of PTEN deletion observed in CaP versus precursor lesions implicates a pivotal role for PTEN haploinsufficiency in the transition from preneoplastic PIN to CaP. Moreover, this observation is an important consideration for novel therapeutic trials in CaP in which biologic efficacy is influenced by the activity level of PTEN. These findings draw attention to the usefulness of this relatively simple FISH assay for future applications in clinical laboratories.

Selective amino acid restriction targets mitochondria to induce apoptosis of androgen-independent prostate cancer cells

Relative specific amino acid dependency is one of the metabolic abnormalities of cancer cells, and restriction of specific amino acids induces apoptosis of prostate cancer cells. This study shows that restriction of tyrosine and phenylalanine (Tyr/Phe), glutamine (Gln), or methionine (Met), modulates Raf and Akt survival pathways and affects the function of mitochondria in DU145 and PC3, in vitro. These three restrictions inhibit energy production (ATP synthesis) and induce generation of reactive oxygen species (ROS). Restriction of Tyr/Phe or Met in DU145 and Met in PC3 reduces mitochondrial membrane potential (DeltaPsim) and induces caspase-dependent and -independent apoptosis. In DU145, Tyr/Phe or Met restriction reduces activity of Akt, mitochondrial distribution of phosphorylated Raf and apoptosis inducing factor (AIF), and increases mitochondrial distribution of Bak. Mitochondrial Bcl-XL is increased in Tyr/Phe-restricted but decreased in Met-restricted cells. Under Tyr/Phe or Met restriction, reduced mitochondrial Raf does not inactivate the pro-apoptotic function of Bak. Tyr/Phe restriction also inhibits Bcl-2 and Met restriction inhibits Bcl-XL in mitochondria. These comprehensive actions damage the integrity of the mitochondria and induce apoptosis of DU145. In PC3, apoptosis induced by Met restriction was not associated with alterations in intracellular distribution of Raf, Bcl-2 family proteins, or AIF. All of the amino acid restrictions inhibited Akt activity in this cell line. We conclude that specific amino acid restriction differentially interferes with homeostasis/balance between the Raf and Akt survival pathways and with the interaction of Raf and Bcl-2 family proteins in mitochondria to induce apoptosis of DU145 and PC3 cells.

Expression of the PTEN tumor suppressor gene in malignant mammary gland tumors of dogs

OBJECTIVE

To determine whether changes in expression level of the phosphatase and tensin homolog deleted on the chromosome 10 (PTEN) gene are associated with malignant transformation in mammary gland tumors in dogs.

SAMPLE POPULATION

Specimens of 5 benign and 8 malignant mammary gland tumors and 2 unaffected mammary glands from dogs.

PROCEDURE

The open reading frame (ORF) sequence of PTEN gene in each specimen was analyzed via a direct-sequencing method; expression levels of PTEN gene were quantified via a competitive reverse transcription (RT)-PCR method.

RESULTS

Compared with findings in clinically normal samples, amounts of PTEN mRNA were increased 2- to 4-fold in 4 of the 5 benign mammary gland tumor samples. In contrast, PTEN expression was remarkably low in 4 of the 8 malignant tumor samples (approx 12% to 37% of the level in unaffected mammary gland specimens). Gene amplification via the RT-PCR method with total RNA prepared from malignant tumor samples as a template yielded 3 bands that were smaller than the full-length ORF product of PTEN gene; in 2 of those 3 RT-PCR products, exons 6 and 7 or exons 3 to 8 were absent. No mutation was detected in the full-length ORF product of PTEN gene.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that a decreased level of PTEN gene expression (compared with unaffected mammary gland tissue) is associated with malignancy in canine mammary tumors. Analysis of PTENgene expression level in dogs with mammary gland tumors may provide useful prognostic information.

Epigenetic silencing of the PTEN gene in melanoma

Phosphatase and tensin homologue deleted from chromosome 10 (PTEN) seems to be an important tumor suppressor gene in melanoma. Because the PTEN gene is only infrequently deleted or mutated, and because the PTEN protein is low to absent in a significant number of melanomas, we investigated alternative methods of epigenetic silencing. We did quantitative positional methylation analysis (pyrosequencing) on 37 sera from melanoma patients and on 21 pairs of corresponding sera and melanoma specimens in addition to Taqman reverse transcription-PCR. We report significant positional PTEN promoter methylation in 62% of circulating DNA isolated from sera of patients with metastatic melanoma. The percentage of methylation of a selected CpG island in blood showed a correlation with methylation levels in the corresponding melanoma tissue. Moreover, high percentages of PTEN methylation were associated with low PTEN transcription levels. Using the demethylation agent 5-aza-2'-deoxycytidine, reduced methylation and a corresponding increase in PTEN protein were observed in BLM melanoma cells, leading to reduced AKT activity in an in vitro kinase assay. In summary, epigenetic PTEN silencing seems to be a relevant mechanism of inactivating this tumor suppressor gene in melanoma that may promote melanoma development by derepression of the AKT pathway.

Novel antioxidant technology for prostate cancer chemoprevention and treatment

The mechanisms underlying prostate carcinogenesis are not firmly elucidated. An exciting area of research in this regard asks whether prostate cancer results from the consequences of lifelong exposure of prostate tissue to oxidative stress. This article reviews the laboratory-based literature on oxidative stress and its possible role in prostate carcinogenesis. The progression of clinical studies focusing on the relationship between antioxidant supplementation and risk of developing prostate cancer are discussed, along with the patent literature since 2003 involving novel antioxidant technology applicable to prostate cancer prevention and treatment. In particular, recently published in vitro experiments with a novel alpha-tocopherol analogue, 2,2,5,7,8-pentamethyl-6-chromonal, which characterised its unique spectrum of antioxidant and antiandrogen properties in prostate cancer cell lines, is discussed. In addition, recent patent applications and supporting findings from the literature surrounding: i) cisplatin tocopherol compounds; ii) coix seed soft capsules with vitamin E; iii) vitamin E succinate (alpha-tocopheryl succinate); iv) lycopene preparations with other carotenoids; v) compounds of the ginger family; vi) novel aryl-carbaldehyde oxime derivatives; vii) novel phenyl quinoline derivatives; and viii) resveratrol, its derivatives and preparations thereof are discussed as they relate to prostate cancer chemoprevention and treatment.