PTEN modulates cell cycle progression and cell survival by regulating phosphatidylinositol 3,4,5,-trisphosphate and Akt/protein kinase B signaling pathway

Inactivation of PTEN is associated with a low p27Kip1 protein expression in breast carcinoma

BACKGROUND

It was previously demonstrated that PTEN protein expression is reduced in 67 of 236 (28%) breast carcinomas. Recent experimental studies suggested that the cell cycle inhibitor p27Kip1 (p27) is a downstream mediator through which PTEN negatively regulates cell cycle progression.

METHODS

The immunohistochemic expression of p27 and PTEN protein expression was evaluated in a series of 228 invasive ductal carcinomas of the breast.

RESULTS

PTEN protein expression was found to have decreased in 65 of 228 (29%) cases, while the nuclear accumulation of p27 protein was low in 99 of 228 (43%) cases. A reduced PTEN protein expression correlated significantly (P = 0.0214) with a low p27 protein expression. Univariate analysis indicated that the patients demonstrating a combined decrease in PTEN and p27 protein expression have a significantly (P = 0.0044) worse disease-free survival (DFS) than those with other combinations of these two protein expression patterns, while multivariate analysis indicated that the lymph node status, MIB-1 counts, and the combination of PTEN/p27 protein expression (P = 0.0452) are independently significant prognostic factors for DFS.

CONCLUSIONS

A reduced PTEN protein expression correlated significantly with a low p27 protein expression in breast carcinoma. The finding that the patients with a combined decrease in both protein expressions had a poor prognosis thus suggests that a combined loss of PTEN and p27 function is associated with an aggressive phenotype in breast carcinoma.

PTEN negatively regulates neural stem cell self-renewal by modulating G0-G1 cell cycle entry

Previous studies have demonstrated that a small subpopulation of brain tumor cells share key characteristics with neural stem/progenitor cells in terms of phenotype and behavior. These findings suggest that brain tumors might contain "cancer stem cells" that are critical for tumor growth. However, the molecular pathways governing such stem cell-like behavior remain largely elusive. Our previous study suggests that the phosphatase and tensin homologue deleted on chromosome 10 (PTEN) tumor suppressor gene, one of the most frequently mutated genes in glioblastomas, restricts neural stem/progenitor cell proliferation in vivo. In the present study, we sought to determine the role of PTEN in long-term maintenance of stem cell-like properties, cell cycle entry and progression, and growth factor dependence and gene expression. Our results demonstrate an enhanced self-renewal capacity and G(0)-G(1) cell cycle entry and decreased growth factor dependency of Pten null neural/stem progenitor cells. Therefore, loss of PTEN leads to cell physiological changes, which collectively are sufficient to increase the pool of self-renewing neural stem cells and promote their escape from the homeostatic mechanisms of proliferation control.

The tumor suppressor PTEN is necessary for human Sprouty 2-mediated inhibition of cell proliferation

Sprouty family proteins are novel regulators of growth factor actions. Human Sprouty 2 (hSPRY2) inhibits the proliferation of a number of different cell types. However, the mechanisms involved in the anti-proliferative actions of hSPRY2 remain to be elucidated. Here we have demonstrated that hSPRY2 increases the amount of the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and decreases its phosphorylation. The resultant increase in PTEN activity is reflected in decreased activation of Akt by epidermal growth factor and serum. Consistent with increased PTEN activity, in hSPRY2-expressing cells, the progression of cells from the G1 to S phase is decreased. By using PTEN null primary mouse embryonic fibroblasts and their isogenic controls as well as small interfering RNA against PTEN, we demonstrated that PTEN is necessary for hSPRY2 to inhibit Akt activation by epidermal growth factor as well as cell proliferation. Overall, we concluded that hSPRY2 mediates its anti-proliferative actions by altering PTEN content and activity.

Regulatory networks in embryo-derived pluripotent stem cells

Mammalian development requires the specification of over 200 cell types from a single totipotent cell. Investigation of the regulatory networks that are responsible for pluripotency in embryo-derived stem cells is fundamental to understanding mammalian development and realizing therapeutic potential. Extracellular signals and second messengers modulate cell-autonomous regulators such as OCT4, SOX2 and Nanog in a combinatorial complexity. Knowledge of this circuitry might reveal how to achieve phenotypic changes without the genetic manipulation of Oct4, Nanog and other toti/pluripotency-associated genes.

Role of the phosphoinositide 3-kinase pathway in mouse embryonic stem (ES) cells

Mouse ES (embryonic stem) cells maintain pluripotency with robust proliferation in vitro. ES cells share some similarities with cancer cells, such as anchorage-independent growth, loss of contact inhibition and tumour formation. After differentiation, ES cells lose pluripotency and tumorigenicity. Recent studies showed that the PI3K (phosphoinositide 3-kinase) pathway is important for proliferation, survival and maintenance of pluripotency in ES cells. The PI3K pathway is activated by growth factors and cytokines including insulin and leukaemia inhibitory factor. In addition to these exogenous factors, the PI3K pathway is endogenously activated by the constitutively active Ras family protein ERas (ES cell-expressed Ras). The PI3K pathway utilizes multiple downstream effectors including mTOR (mammalian target of rapamycin), which we have shown to be essential for proliferation in mouse ES cells and early embryos.

Akt phosphorylation associates with LOH of PTEN and leads to chemoresistance for gastric cancer

Growth factor receptor-mediated signal transduction has been implicated in conferring resistance to conventional chemotherapy on cancer cells. We describe a pathway that involves AKT/PI3K to mediate chemoresistance in gastric cancer patients. Primary gastric carcinoma tissues and corresponding normal mucosa were obtained from 76 gastric cancer patients who underwent surgery in the Department of Surgery II in Kyushu University Hospital from the years 1996-2000. AKT activation was investigated by immunostaining with a phosphorylation-specific antibody, and LOH (loss of heterozygosity) of PTEN was studied in the same samples. AKT was phosphorylated in 22 cases (28.9%) of gastric cancer cases. AKT and phosphorylated AKT were not correlated with any clinicopathological factor. We found that the gastric cancer patients who had higher AKT phosphorylation (activated AKT) seemed to have LOH of PTEN (p = 0.0008). When the chemotherapeutic sensibilities of these patients were studied in an MTT assay, it was found that the activated AKT was associated with increased resistance to multiple chemotherapeutic agents (5-fluorouracil, adriamycin, mitomycin C and cis-platinum). The results of our study indicate that AKT activation and LOH of PTEN plays an important role in conferring a broad-spectrum chemoresistance in gastric cancer patients. It also indicates that AKT may therefore be a novel molecular target for therapies or chemosensitivity tests that improve the outcomes of gastric cancer patients.

Cytokine signalling in embryonic stem cells

Cytokines play a central role in maintaining self-renewal in mouse embryonic stem (ES) cells through a member of the interleukin-6 type cytokine family termed leukemia inhibitory factor (LIF). LIF activates the JAK-STAT3 pathway through the class I cytokine receptor gp130, which forms a trimeric complex with LIF and the class I cytokine receptor LIF receptor beta. STAT3 has been shown to play a crucial role in self-renewal in mouse ES cells probably by induction of c-myc expression. Thus, ablation of STAT3 activation leads to differentiation. However, important connections between STAT3 and other signalling pathways have been documented. In addition, gp130 activation leads to both PI3K and Src activation. The canonical Wnt pathway is sufficient to maintain self-renewal of both human ES cells and mouse ES cells. It seems quite possible that the main pathway maintaining self-renewal in ES cells is the Wnt pathway, while the LIF-JAK-STAT3 pathway is present in mouse cells as an adaptation for sustaining self-renewal during embryonic diapause, a condition of delayed implantation in mammals. In keeping with this scenario, the Wnt pathway has been shown to elevate the level of c-myc. Thus, the two pathways seem to converge on c-myc as a common target to promote self-renewal. Whereas LIF does not seem to stimulate self-renewal in human embryonic stem cells it cannot be excluded that other cytokines are involved. The pleiotropic actions of the increasing number of cytokines and receptors signalling via JAKs, STATs and SOCS exhibit considerable redundancy, compensation and plasticity in stem cells in accordance with the view that stem cells are governed by quantitative variations in strength and duration of signalling events known from other cell types rather than qualitatively different stem cell-specific factors.

Upregulated function of phosphatidylinositol-3-kinase in genetically hypertensive rats: a moderator of arterial hypercontractility

1. The growth enzyme phosphatidylinositol 3-kinase (PI3K) was recently implicated in the mediation of arterial spontaneous tone, an event observed in arteries from hypertensive, but not normotensive, subjects that contributes to changes in total peripheral resistance in the hypertensive state. We have shown this occurrence in experimentally induced models of hypertension. However, because the majority of hypertension is genetically based, it is important to demonstrate a similar change in genetically hypertensive animals. 2. Aorta from spontaneously hypertensive rats (SHR; systolic blood pressure = 183 +/- 4 mmHg) and Wistar Kyoto (WKY) rats (115 +/- 2 mmHg) were isolated for the measurement of isometric contractile force. Aorta from SHR displayed small increases (approximately 5% maximum phenylephrine (PE)-induced contraction) in spontaneous tone, whereas aorta from WKY rats displayed none. The non-selective PI3K inhibitor LY294002 (20 micromol/L) and the selective inhibitor of the p110delta catalytic subunit of PI3K IC87114 (20 micromol/L) caused a fall of basal tone in SHR aorta (20 +/- 7 and 24 +/- 6% of the initial PE contraction, respectively), but did not alter tone in arteries from WKY rats. LY294002, but not IC87114, normalized the increased potency of noradrenaline (NA) observed in aorta from SHR (-log EC50 values for NA in the presence of vehicle in WKY rats and SHR 7.5 +/- 0.1 and 7.8 +/- 0.1, respectively (P < 0.05); -log EC(50) values for NA in the presence of LY294002 in WKY rats and SHR 7.0 +/- 0.1 and 7.0 +/- 0.1, respectively). 3. Biochemical expression of the p110 catalytic and p85 regulator subunits of PI3K in western analyses revealed no difference in expression of the regulatory p85alpha or p110alpha protein subunits between WKY rats and SHR; p110gamma was not detected. In contrast, p110delta expression was increased greater than 30% in aorta from SHR compared with WKY rats (827.6 +/- 88.5 vs 576.8 +/- 53.4 arbitrary densitometry units, respectively). Immunohistochemical analyses revealed expression of the p110delta isoform in the smooth muscle of arteries. 4. These data underscore the relevance of an enzyme historically classified as one committed to growth/anti-apoptosis in modifying contractility and supports involvement of PI3K in genetically based hypertension.

Molecular targets of opiate drug abuse in neuroAIDS

Opiate drug abuse, through selective actions at mu-opioid receptors (MOR), exacerbates the pathogenesis of human immunodeficiency virus-1 (HIV-1) in the CNS by disrupting glial homeostasis, increasing inflammation, and decreasing the threshold for pro-apoptotic events in neurons. Neurons are affected directly and indirectly by opiate-HIV interactions. Although most opiates drugs have some affinity for kappa (KOR) and/or delta (DOR) opioid receptors, their neurotoxic effects are largely mediated through MOR. Besides direct actions on the neurons themselves, opiates directly affect MOR-expressing astrocytes and microglia. Because of their broad-reaching actions in glia, opiate abuse causes widespread metabolic derangement, inflammation, and the disruption of neuron-glial relationships, which likely contribute to neuronal dysfunction, death, and HIV encephalitis. In addition to direct actions on neural cells, opioids modulate inflammation and disrupt normal intercellular interactions among immunocytes (macrophages and lymphocytes), which on balance further promote neuronal dysfunction and death. The neural pathways involved in opiate enhancement of HIV-induced inflammation and cell death, appear to involve MOR activation with downstream effects through PI3-kinase/Akt and/or MAPK signaling, which suggests possible targets for therapeutic intervention in neuroAIDS.

PTEN represses RNA Polymerase I transcription by disrupting the SL1 complex

PTEN is a tumor suppressor whose function is frequently lost in human cancer. It possesses a lipid phosphatase activity that represses the activation of PI3 kinase/Akt signaling, leading to decreased cell growth, proliferation, and survival. The potential for PTEN to regulate transcription of the large rRNAs by RNA polymerase I (RNA Pol I) was investigated. As increased synthesis of rRNAs is a hallmark of neoplastic transformation, the ability of PTEN to control the transcription of rRNAs might be crucial for its tumor suppressor function. The expression of PTEN in PTEN-deficient cells represses RNA Pol I transcription, while decreasing PTEN expression enhances transcription. PTEN-mediated repression requires its lipid phosphatase activity and is independent of the p53 status of the cell. This event can be uncoupled from PTEN's ability to regulate the cell cycle. RNA Pol I is regulated through PI3 kinase/Akt/mammalian target of rapamycin/S6 kinase, and the expression of constitutively activated S6 kinase is able to abrogate transcription repression by PTEN. No change in the expression of the RNA Pol I transcription components, upstream binding factor or SL1, was observed upon PTEN expression. However, chromatin immunoprecipitation assays demonstrate that PTEN differentially reduces the occupancy of the SL1 subunits on the rRNA gene promoter. Furthermore, PTEN induces dissociation of the SL1 subunits. Together, these results demonstrate that PTEN represses RNA Pol I transcription through a novel mechanism that involves disruption of the SL1 complex.

Molecular pathology of prostate cancer

The molecular pathology of prostate cancer is complex; not only are multiple genes involved in its pathogenesis, but additional environmental factors such as diet and inflammation are also involved. The exhaustive research into prostate cancer to date has demonstrated a complex interaction of multiple genes and environmental factors, some of which may be more important in individual prostate cancer cases. This is an exciting era, with the emergence of new investigative tools such as DNA microarray technology and the application of the field of proteomics to the study of human cancers. Knowledge of genetic changes underlying the initiation, development, and progression of prostate cancer is accumulating rapidly. With increasing knowledge, it may be possible to distinguish indolent from aggressive prostate tumours by molecular fingerprinting. This review discusses the most consistently reported molecular pathological findings in hereditary and sporadic prostate cancer, together with new concepts and technologies.

Molecular pathology of prostate cancer

The incidence of prostate cancer has increased in Japan recently and is developing into a life-threatening disease for many Japanese men. This is a result of several convergent factors including the adoption of a Western lifestyle, the widespread use of prostate-specific antigen (PSA) testing, and an increased population of advanced years in Japanese men. Although there is much information to date relating to molecular events underlying the etiology of prostate cancer, it is still unclear as to how and when these genetic alterations occur in each step of tumorigenesis. One fruitful area of investigation has been in the analysis of chromosomal abnormalities commonly observed in prostate cancer. However, no single candidate gene has been definitely identified in cancer initiation and/or progression; in addition, less research has been devoted to understanding the molecular events that underlie tumor histogenesis in terms of likely precursor lesions, such as prostatic intraepithelial neoplasia (PIN). This article reviews the current knowledge of the molecular pathology of prostate cancer, including its histogenesis, genetic and epigenetic alterations, and hereditary factors.

Redox regulation of PTEN by S-nitrosothiols

PTEN (phosphatase with sequence homology to tensin) is a phosphatidylinositol 3,4,5-trisphosphate phosphatase that regulates many cellular processes. Activity of the enzyme is dependent on the redox state of the active site cysteine such that oxidation by H2O2 leads to inhibition. Because S-nitrosothiols are known to modify enzymes containing reactive cysteines, we hypothesized that S-nitrosothiols would oxidize PTEN and inhibit its phosphatase activity. In the present study, we show that S-nitrosocysteine (CSNO), S-nitrosoglutathione (GSNO), and S-nitroso-N-acetylpenicillamine (SNAP) reversibly oxidized recombinant PTEN. In addition, CSNO led to concentration- and time-dependent oxidation of endogenous cellular PTEN. However, in contrast, GSNO and SNAP were effective only when coincubated with cysteine, suggesting that these nitrosothiols must react with cysteine to form CSNO, which can be transferred across cell membranes. Oxidation of cellular PTEN resulted from thiol modification and led to reversible inhibition of phosphatase activity. Although oxidation of PTEN by H2O2 led to formation of an intramolecular disulfide, oxidation of PTEN by CSNO seemed to lead to formation of a mixed disulfide. Glutathionylation of cellular proteins by incubating cells with diamide or incubating cellular extracts with GSSG oxidized PTEN in a manner similar to that of CSNO. Overall, these data demonstrate for the first time that S-nitrosothiols oxidatively modify PTEN, leading to reversible inhibition of its phosphatase activity, and suggest that the oxidized species is a mixed disulfide.

Phospho-akt expression is associated with a favorable outcome in non-small cell lung cancer

Akt, a Serine/Threonine protein kinase, mediates growth factor-associated cell survival. Constitutive activation of Akt (phosphorylated Akt, P-Akt) has been observed in several human cancers, including lung cancer and may be associated with poor prognosis and chemotherapy and radiotherapy resistance. The clinical relevance of P-Akt in non-small cell lung cancer (NSCLC) is not well described. In the present study, we examined 82 surgically resected snap-frozen and paraffin-embedded stage I to IIIA NSCLC samples for P-Akt and Akt by Western blotting and for P-Akt by immunohistochemistry. P-Akt protein levels above the median, measured using reproducible semiquantitative band densitometry, correlated with a favorable outcome (P = 0.007). Multivariate analysis identified P-Akt as a significant independent favorable prognostic factor (P = 0.004). Although associated with a favorable prognosis, high P-Akt levels correlated with high tumor grade (P = 0.02). Adenocarcinomas were associated with low P-Akt levels (P = 0.039). Akt was not associated with either outcome or clinicopathologic variables. Cytoplasmic (CP-Akt) and nuclear (NP-Akt) P-Akt tumor cell staining was detected in 96% and 42% of cases, respectively. Both CP-Akt and NP-Akt correlated with well-differentiated tumors (P = 0.008 and 0.017, respectively). NP-Akt also correlated with nodal metastases (P = 0.022) and squamous histology (P = 0.037).These results suggest P-Akt expression is a favorable prognostic factor in NSCLC. Immunolocalization of P-Akt, however, may be relevant as NP-Akt was associated with nodal metastases, a known poor prognostic feature in this disease. P-Akt may be a potential novel therapeutic target for the management of NSCLC.

Somatic induction of Pten loss in a preclinical astrocytoma model reveals major roles in disease progression and avenues for target discovery and validation

High-grade astrocytomas are invariably deadly and minimally responsive to therapy. Pten is frequently mutated in aggressive astrocytoma but not in low-grade astrocytoma. However, the Pten astrocytoma suppression mechanisms are unknown. Here we introduced conditional null alleles of Pten (Pten(loxp/loxp)) into a genetically engineered mouse astrocytoma model [TgG(deltaZ)T121] in which the pRb family proteins are inactivated specifically in astrocytes. Pten inactivation was induced by localized somatic retroviral (MSCV)-Cre delivery. Depletion of Pten function in adult astrocytoma cells alleviated the apoptosis evoked by pRb family protein inactivation and also induced tumor cell invasion. In primary astrocytes derived from TgG(deltaZ)T121; Pten(loxp/loxp) mice, Pten deficiency resulted in a marked increase in cell invasiveness that was suppressed by inhibitors of protein kinase C (PKC) or of PKC-zeta, specifically. Finally, focal induction of Pten deficiency in vivo promoted angiogenesis in affected brains. Thus, we show that Pten deficiency in pRb-deficient astrocytoma cells contributes to tumor progression via multiple mechanisms, including suppression of apoptosis, increased cell invasion, and angiogenesis, all of which are hallmarks of high-grade astrocytoma. These studies not only provide mechanistic insight into the role of Pten in astrocytoma suppression but also describe a valuable animal model for preclinical testing that is coupled with a primary cell-based system for target discovery and drug screening.

Fc- and complement-receptor activation stimulates cell cycle progression of macrophage cells from G1 to S

Phagocytosis of microorganisms by macrophages is an important host defense mechanism. While studying the phagocytosis of the human pathogenic fungus Cryptococcus neoformans, we noted that macrophage-like J774 cells with ingested fungal cells had frequent mitotic figures. By analyzing the relative proportion of phagocytic cells as a function of cell cycle phase, we observed an increase in S phase cells after Fc-mediated phagocytosis of polystyrene beads, live or heat-killed C. neoformans. This result was confirmed by increased nuclear BrdU incorporation after Fc-mediated phagocytosis. The induced progression to S phase was observed after both Fc- and complement-mediated phagocytosis of live yeasts. Fc-mediated stimulation of cell division did not require ingestion, because it could be triggered by incubating cells in IgG1-coated plates. Phagocytosis-mediated stimulation of replication was confirmed in vitro using primary bone marrow macrophages and in vivo for peritoneal macrophages. We conclude that phagocytosis of microbes or inert particles can stimulate macrophages to enter S phase and commence cell division. This observation suggests a potential mechanism for increasing the number of effector cells after microbial ingestion, but can also promote the spread of infection.

Increased expression of Mcl-1 is required for protection against serum starvation in phosphatase and tensin homologue on chromosome 10 null mouse embryonic fibroblasts, but repression of Bim is favored in human glioblastomas

Inactivating mutations in the tumor suppressor gene phosphatase and tensin homologue on chromosome 10 (PTEN) result in elevated levels of phosphatidylinositol (3,4,5)-trisphosphate, activation of protein kinase B (PKB), and protection against apoptotic insults such as withdrawal of survival factors. Protection may arise through the inhibition of the pro-apoptotic protein Bim, which is normally repressed by a PKB-dependent mechanism. Here we show that PTEN-/- immortalized mouse embryonic fibroblasts (MEFs) exhibit elevated PKB phosphorylation and are resistant to serum withdrawal-induced death, but exhibit normal Bim expression following withdrawal of serum. In contrast, expression of Mcl-1, a prosurvival member of the Bcl-2 family, was elevated in PTEN-/- MEFs. Transient or stable overexpression of Mcl-1 in PTEN+/- MEFs conferred resistance to serum withdrawal, whereas ablating expression of Mcl-1 in PTEN-/- MEFs, using RNA interference, abolished their resistance to serum withdrawal-induced apoptosis. To determine if Mcl-1 is selected for overexpression in human tumors we examined human glioblastoma cell lines but found that loss of PTEN had no effect on Mcl-1 expression. In contrast, two of three PTEN-/- glioblastoma cell lines exhibited low expression of Bim, which was refractory to serum withdrawal. These results indicate that the resistance of PTEN-/- MEFs to serum withdrawal is largely due to the up-regulation of Mcl-1 but that loss of PTEN in tumor cell lines is more complex and may favor de-regulation of different apoptotic regulators such as Bim.

Feedback inhibition of Akt signaling limits the growth of tumors lacking Tsc2

The PTEN and TSC2 tumor suppressors inhibit mammalian target of rapamycin (mTOR) signaling and are defective in distinct hamartoma syndromes. Using mouse genetics, we find that Pten and Tsc2 act synergistically to suppress the severity of a subset of tumors specific to loss of each of these genes. Interestingly, we find that the slow-growing tumors specific to Tsc2+/- mice exhibit defects in signaling downstream of Akt. However, Pten haploinsufficiency restores Akt signaling in these tumors and dramatically enhances their severity. This study demonstrates that attenuation of the PI3K-Akt pathway in tumors lacking TSC2 contributes to their benign nature.

Gas6 induces proliferation in prostate carcinoma cell lines expressing the Axl receptor

Axl is a tyrosine kinase receptor and although it is expressed in malignancy such as leukemia, colon cancer, melanoma, endometrial, prostate and thyroid cancers, its role has not been completely elucidated yet and appears to be complex. The ligand of Axl, Gas6, is a 75 KDa multimodular protein with an N-terminal gamma-carboxy-glutamic acid that is essential for binding. Gas6 has a mitogenic effect on several normal cell lines. The receptor Axl is expressed in primary prostate carcinoma and in prostate cancer cell lines as such as PC-3 and DU 145. We demonstrated a mitogenic activity determined by Gas6/Axl interaction in these undifferentiated metastatic human prostatic cancer cell lines. This effect is proportional to Axl expression, not due to inhibition of apoptosis, and induces AKT and MAPK phosphorylation. However, only MEK phosphorylation seems to be essential for growth signaling. Our results suggest that Axl overexpression and activation by Gas6 could be involved in progression of prostate neoplastic disease.

PTEN deletion in Bergmann glia leads to premature differentiation and affects laminar organization

Development of the central nervous system is controlled by both intrinsic and extrinsic signals that guide neuronal migration to form laminae. Although defects in neuronal mobility have been well documented as a mechanism for abnormal laminar formation, the role of radial glia, which provide the environmental cues, in modulating neuronal migration is less clear. We provide evidence that loss of PTEN in Bergmann glia leads to premature differentiation of this crucial cell population and subsequently to extensive layering defects. Accordingly, severe granule neuron migration defects and abnormal laminar formation are observed. These results uncover an unexpected role for PTEN in regulating Bergmann glia differentiation, as well as the importance of time-dependent Bergmann glia differentiation during cerebellar development.

Akt-1 expression level regulates CNS precursors

Although most cells in the embryonic mouse cortex express the serine-threonine kinase Akt-1, a small population of progenitors expresses Akt-1 protein at a higher level. To determine the functional significance of this difference, we used a retrovirus to increase Akt-1 expression in cortical progenitors. Increased Akt expression enhanced Akt activation after growth factor stimulation of progenitors. In vivo, it promoted retention in progenitor layers, the ventricular zone and subventricular zone. In vitro, it enhanced proliferation and survival, but did not impair migration. Moreover, it increased the proportion of stem cells, defined by a self-renewal assay. These effects did not depend on the Akt substrate p21(Cip1). In contrast, rapamycin, an inhibitor of mTOR (mammalian target of rapamycin), altered effects of elevated Akt-1 selectively: it eliminated the increase in stem cells and reduced the proliferative response, but had no effect on survival. The ability of elevated Akt-1 to increase the self-renewing population therefore depends on a rapamycin-sensitive mechanism (presumably inhibition of mTOR activity) but not on p21(Cip1), and can be distinguished from its effects on the proliferation and survival of other types of progenitors. Our findings suggest that expression of a high level of Akt-1 by a subpopulation of cortical progenitors biases their responses to extrinsic signals to increase their survival, proliferation, and/or self-renewal. Heterogeneity in Akt-1 level among progenitors could therefore allow cells that share a microenvironment to respond differently to the same extrinsic signals.

PTEN as an effector in the signaling of antimigratory G protein-coupled receptor

PTEN, a tumor suppressor phosphatase, is important in the regulation of cell migration and invasion. Physiological regulation of PTEN (phosphatase and tensin homolog deleted on chromosome 10) by cell surface receptors has not been described. Here, we show that the bioactive lipid sphingosine 1-phosphate (S1P), which acts through the S1P2 receptor (S1P2R) G protein-coupled receptor (GPCR) to inhibit cell migration, utilizes PTEN as a signaling intermediate. S1P2R inhibition of cell migration is abrogated by dominant-negative PTEN expression. S1P was unable to efficiently inhibit the migration of Pten(DeltaloxP/DeltaloxP) mouse embryonic fibroblasts; however, the antimigratory effect was restored upon the expression of PTEN. S1P2R activation of Rho GTPase is not affected in Pten(DeltaloxP/DeltaloxP) cells, and dominant-negative Rho GTPase reversed S1P inhibition of cell migration in WT cells but not in Pten(DeltaloxP/DeltaloxP) cells, suggesting that PTEN acts downstream of the Rho GTPase. Ligand activation of the S1P2R receptor stimulated the coimmunoprecipitation of S1P2R and PTEN. Interestingly, S1P2R signaling increased PTEN phosphatase activity in membrane fractions. Furthermore, tyrosine phosphorylation of PTEN was stimulated by S1P2R signaling. These data suggest that the S1P2R receptor actively regulates the PTEN phosphatase by a Rho GTPase-dependent pathway to inhibit cell migration. GPCR regulation of PTEN maybe a general mechanism in signaling events of cell migration and invasion.

PTEN function: how normal cells control it and tumour cells lose it

The PTEN (phosphatase and tensin homologue deleted on chromosome 10) tumour suppressor is a PI (phosphoinositide) 3-phosphatase that can inhibit cellular proliferation, survival and growth by inactivating PI 3-kinase-dependent signalling. It also suppresses cellular motility through mechanisms that may be partially independent of phosphatase activity. PTEN is one of the most commonly lost tumour suppressors in human cancer, and its deregulation is also implicated in several other diseases. Here we discuss recent developments in our understanding of how the cellular activity of PTEN is regulated, and the closely related question of how this activity is lost in tumours. Cellular PTEN function appears to be regulated by controlling both the expression of the enzyme and also its activity through mechanisms including oxidation and phosphorylation-based control of non-substrate membrane binding. Therefore mutation of PTEN in tumours disrupts not only the catalytic function of PTEN, but also its regulatory aspects. However, although mutation of PTEN is uncommon in many human tumour types, loss of PTEN expression seems to be more frequent. It is currently unclear how these tumours lose PTEN expression in the absence of mutation, and while some data implicate other potential tumour suppressors and oncogenes in this process, this area seems likely to be a key focus of future research.

Transient strong reduction of PTEN expression by specific RNAi induces loss of adhesion of the cells

The tumor suppressor gene pten encodes a lipid phosphatase that dephosphorylates D3 of phosphatidylinositol(3,4,5)trisphosphate, producing phosphatidylinositol(4,5)bisphosphate. Although PTEN has been implicated in cell adhesion and migration, the underlying molecular mechanism is unknown. To investigate the role of PTEN in cell adhesion, we designed three different siRNAs (siRNA PTEN-a, siRNA PTEN-b, and siRNA PTEN-c) and transfected into 293T cells. Two days later, only the cells transfected with siRNA PTEN-b became round and detached from the culture dishes, whereas cells transfected with a control siRNA against GFP or the two other siRNAs against PTEN did not. Evaluation of the RNAi effect revealed that siRNA PTEN-b inhibited >95% of PTEN expression, the most effective among the three siRNAs. To check for non-specific effects such as interferon response and inhibition of off-target genes, we then used quantitative PCR analysis and DNA microarray analysis. None was detected, indicating that the RNAi system was highly specific. Immunofluorescence studies using PTEN-knockdown HeLa cells revealed that the loss of adhesion was accompanied by a reduction in the number of focal adhesion plaques and disorganization of the actin cytoskeleton. Transient and near-complete loss of PTEN expression induces loss of adhesion of the cells.

DJ-1, a novel regulator of the tumor suppressor PTEN

The phosphatidylinositol 3' kinase (PI3'K) pathway, which regulates cell survival, is antagonized by the PTEN tumor suppressor. The regulation of PTEN is unclear. A genetic screen of Drosophila gain-of-function mutants identified DJ-1 as a suppressor of PTEN function. In mammalian cells, DJ-1 underexpression results in decreased phosphorylation of PKB/Akt, while DJ-1 overexpression leads to hyperphosphorylation of PKB/Akt and increased cell survival. In primary breast cancer samples, DJ-1 expression correlates negatively with PTEN immunoreactivity and positively with PKB/Akt hyperphosphorylation. In 19/23 primary non-small cell lung carcinoma samples, DJ-1 expression was increased compared to paired nonneoplastic lung tissue, and correlated positively with relapse incidence. DJ-1 is thus a key negative regulator of PTEN that may be a useful prognostic marker for cancer.

Enforced Bcl-2 expression overrides serum and feeder cell requirements for mouse embryonic stem cell self-renewal

Leukemia inhibitory factor (LIF) is required, but not sufficient, for pluripotent mouse embryonic stem (ES) cell expansion in vitro in the absence of serum or a feeder cell layer, suggesting that additional signals are provided by serum or feeders that are necessary to support self-renewal. Here we show that transgenic ES cell lines expressing Bcl-2, an antiapoptotic protein, continue to self-renew in serum- and feeder-free conditions when supplemented with LIF; even in the absence of bone morphogenic proteins. Bcl-2-expressing clones sustain the characteristics of undifferentiated, pluripotent ES cells during long-term culture, and maintain their potential to differentiate into mature cell types. These results suggest that LIF and Bcl-2 overexpression are sufficient to expand these mouse pluripotent stem cells in vitro.

Adenovirus-Mediated Intraarterial Delivery of PTEN Inhibits Neointimal Hyperplasia

Objective— Phosphoinositide (PI) 3-kinase promotes vascular smooth muscle cell (VSMC) responses necessary for neointimal hyperplasia. We recently demonstrated that the inositol 3-phosphatase PTEN is expressed in VSMCs and that its overexpression inhibits these cellular responses. The purpose of this study was to determine the effects of adenovirus-mediated overexpression of PTEN on neointimal hyperplasia in vivo in the rat carotid injury model.

Methods and Results— Rat carotid arteries were balloon-injured and treated with a recombinant control adenovirus (AdEV) (n=6), an adenovirus encoding wild-type PTEN (AdPTEN) (n=8), or phosphate-buffered saline (sham) (n=5). Injured vessels demonstrated PTEN overexpression by Western blotting and immunohistochemistry after AdPTEN treatment. Neointimal hyperplasia was assessed 2 weeks after balloon injury and adenovirus administration. Compared with controls, AdPTEN treatment significantly decreased neointimal area and percent stenosis. To investigate the mechanisms of action of AdPTEN, vessels were harvested 3 days after balloon injury and virus infection. AdPTEN significantly increased medial cell apoptosis while decreasing proliferation of the remaining viable cells.

Conclusions— PTEN overexpression potently inhibits neointimal hyperplasia through induction of apoptosis and inhibition of medial cell proliferation. These findings suggest that modulation of PTEN expression or activity may be a viable approach to treat neointimal hyperplasia.

Establishment of oocyte population in the fetal ovary: primordial germ cell proliferation and oocyte programmed cell death

Strict control of cell proliferation and cell loss is essential for the coordinated functions of different cell populations in complex multicellular organisms. Oogenesis is characterized by a first phase occurring during embryo-fetal life and in common with spermatogenesis, during which mitotic proliferation of the germline stem cells, the primordial germ cells (PGC), prevails over germ cell death. The result is the formation of a relatively high number of germ cells depending on the species, ready to enter sex specific differentiation. In the female, PGC enter into meiosis and become oocytes, thereby ending their stem cell potential. After entering into meiosis in the fetal ovary, oocytes pass through leptotene, zygotene and pachytene stages before arresting in the last stage of meiotic prophase I, the diplotene or dictyate stage at about the time of birth. The most part of oocytes die during the fetal period or shortly after birth. It is widely accepted that in mammals a female is born with a fixed number of oocytes within the ovaries, which over the years progressively decreases without possibility for renewal. Once the oocyte reserve has been exhausted, ovarian senescence, driving what is referred to as the menopause in women, rapidly ensues. The fertile lifespan of a female depends by the size of the oocyte pool at birth and the rapidity of the oocyte pool depletion. Which mechanisms control PGC proliferation? Why do most of the oocytes die during fetal life and what are the mechanisms of such massive degeneration? Is it possible to prolong the lifespan of a female by reducing oocyte lost during the fetal life? This review reports some of the most recent results obtained in an attempt to answer these questions.

The use of P53, PTEN, and C-erbB-2 to differentiate uterine serous papillary carcinoma from endometrioid endometrial carcinoma

Endometrial adenocarcinoma is the most common pelvic genital malignancy in the western world. The most common subtype of endometrial cancer is endometrioid endometrial carcinoma (EEC), which has a relatively good prognosis. Uterine serous papillary carcinoma (USPC) is also a subtype of endometrial carcinoma. This is an aggressive carcinoma with the majority of patients presenting at stage 3-4 and has a worse prognosis stage for stage when compared with EEC. In addition, the treatment of USPC is more extensive than that of EEC, and therefore definitive diagnosis before surgery ensures the optimum management for the patient. This study aims to determine whether P53, C-erbB-2, and PTEN antibodies have a use in the diagnosis and distinction of these cancers. We created tissue microarrays for 35 cases of EEC and 25 cases of USPC, and then we assessed the immunohistochemical expression of P53, C-erbB-2, and PTEN. There was significantly greater expression of P53 in USPC than that in EEC. However, neither C-erbB-2 nor PTEN showed any significant difference in expression between the two carcinomas. P53 may have a role in the diagnosis of USPC, but neither C-erbB-2 nor PTEN would be useful as part of a diagnostic panel.

Cellular lifespan and senescence signaling in embryonic stem cells

Most mammalian cells when placed in culture will undergo a limited number of cell divisions before entering an unresponsive non-proliferating state termed senescence. However, several pathways that are activated singly or in concert can allow cells to bypass senescence at least for limited periods. These include the telomerase pathway required to maintain telomere ends, the p53 and Rb pathways required to direct senescence in response to DNA damage, telomere shortening and mitogenic signals, and the insulin-like growth factor--Akt pathway that may regulate lifespan and cell proliferation. In this review, we summarize recent findings related to these pathways in embryonic stem (ES) cells and suggest that ES cells are immortal because these pathways are tightly regulated.

Phosphorylation-independent stabilization of p27kip1 by the phosphoinositide 3-kinase pathway in glioblastoma cells

The PTEN tumor suppressor gene is a frequent target of somatic mutation, particularly in glioblastoma multiform and prostate cancer. The expression of PTEN in PTEN-mutant glioblastoma cells leads to a cell cycle arrest in G(0)/G(1) that is mediated at least partially by increased p27(kip1) levels. Here we show that p27(kip1) is not regulated by transcriptional control but that p27(kip1) protein shows increased stability after inhibition of the phosphoinositide (PI) 3-kinase pathway. Because p27(kip1) protein stability is known to be regulated by phosphorylation, we have examined modifications in the phosphorylation pattern after PI 3-kinase inhibition. Biochemical evidence suggests that p27(kip1) is phosphorylated on several serine residues, including Ser-10 and Ser-178, but that phosphorylation is unaltered by PI 3-kinase activity. This is further confirmed by the inducible expression of p27(kip1) phosphorylation site mutants, suggesting that p27(kip1) is destabilized in a phosphorylation-independent manner by the PI 3-kinase pathway at the G(1)/S transition.

Conserved and divergent paths that regulate self-renewal in mouse and human embryonic stem cells

The past few years have seen remarkable progress in our understanding of embryonic stem cell (ES cell) biology. The necessity of examining human ES cells in culture, coupled with the wealth of genomic data and the multiplicity of cell lines available, has enabled researchers to identify critical conserved pathways regulating self-renewal and identify markers that tightly correlate with the ES cell state. Comparison across species has suggested additional pathways likely to be important in long-term self-renewal of ES cells including heterochronic genes, microRNAs, genes involved in telomeric regulation, and polycomb repressors. In this review, we have discussed information on molecules known to be important in ES cell self-renewal or blastocyst development and highlighted known differences between mouse and human ES cells. We suggest that several additional pathways required for self-renewal remain to be discovered and these likely include genes involved in antisense regulation, microRNAs, as well as additional global repressive pathways and novel genes. We suggest that cross species comparisons using large-scale genomic analysis tools are likely to reveal conserved and divergent paths required for ES cell self-renewal and will allow us to derive ES lines from species and strains where this has been difficult.

Regulation of Embryonic Stem Cell Self-renewal by Phosphoinositide 3-Kinase-dependent Signaling

The maintenance of murine embryonic stem (ES) cell self-renewal is regulated by leukemia inhibitory factor (LIF)-dependent activation of signal transducer and activator of transcription 3 (STAT3) and LIF-independent mechanisms including Nanog, BMP2/4, and Wnt signaling. Here we demonstrate a previously undescribed role for phosphoinositide 3-kinases (PI3Ks) in regulation of murine ES cell self-renewal. Treatment with the reversible PI3K inhibitor, LY294002, or more specific inhibition of class I(A) PI3K via regulated expression of dominant negative Deltap85, led to a reduction in the ability of LIF to maintain self-renewal, with cells concomitantly adopting a differentiated morphology. Inhibition of PI3Ks reduced basal and LIF-stimulated phosphorylation of PKB/Akt, GSK3alpha/beta, and S6 proteins. Importantly, LY294002 and Deltap85 expression had no effect on LIF-induced phosphorylation of STAT3 at Tyr(705), but did augment LIF-induced phosphorylation of ERKs in both short and long term incubations. Subsequently, we demonstrate that inhibition of MAP-Erk kinases (MEKs) reverses the effects of PI3K inhibition on self-renewal in a time- and dose-dependent manner, suggesting that the elevated ERK activity observed upon PI3K inhibition contributes to the functional response we observe. Surprisingly, upon long term inhibition of PI3Ks we observed a reduction in phosphorylation of beta-catenin, the target of GSK-3 action in the canonical Wnt pathway, although no consistent alterations in cytosolic levels of beta-catenin were observed, indicating this pathway is not playing a major role downstream of PI3Ks. Our studies support a role for PI3Ks in regulation of self-renewal and increase our understanding of the molecular signaling components involved in regulation of stem cell fate.

PTENless means more

Recent studies indicate that certain key molecules that are vital for various developmental processes, such as Wnt, Shh, and Notch, cause cancer when dysregulated. PTEN, a tumor suppressor that antagonizes the PI3 kinase pathway, is the newest one on the list. The biological function of PTEN is evolutionarily conserved from C. elegans to humans, and the PTEN-controlled signaling pathway regulates cellular processes crucial for normal development, including cell proliferation, soma growth, cell death, and cell migration. In this review, we will focus on the function of PTEN in murine development and its role in regulating stem cell self-renewal and proliferation. We will summarize the organomegaly phenotypes associated with Pten tissue-specific deletion and discuss how PTEN controls organ size, a fundamental aspect of development. Last, we will review the role of PTEN in hormone-dependent, adult-onset mammary and prostate gland development.

Imatinib mesylate (Gleevec) inhibits ovarian cancer cell growth through a mechanism dependent on platelet-derived growth factor receptor alpha and Akt inactivation

PURPOSE

We identified the platelet-derived growth factor receptor alpha (PDGFRalpha) as an ovarian cancer-specific gene by microarray hybridization using primary cultures. The purpose of this study is to evaluate whether disruption of the platelet-derived growth factor-regulated growth pathway by Imatinib mesylate (Gleevec), a partially selective PDGFR inhibitor, inhibits growth of ovarian cancer cells expressing PDGFR.

EXPERIMENTAL DESIGN

To investigate the effects of Imatinib mesylate in ovarian cancer, we established an in vitro model by immortalizing primary ovarian cells, which express endogenous PDGFR, and we evaluated the effects of Imatinib on cell proliferation. In addition, we investigated the involvement of Akt in mediating Imatinib-inhibited cell growth inhibition.

RESULTS

We found that 39% of ovarian tumors express PDGFR by immunohistochemistry. We showed that Imatinib inhibits the growth of ovarian cancer cells in a PDGFR-specific manner, at clinically relevant concentrations (IC(50) < 1 micro M). Imatinib inhibits the growth of three primary ovarian cultures and two immortalized cultures (PDGFR positive), but has no effects on SkOv3 and CaOv3 cell lines (PDGFR negative). Imatinib exerts antiproliferative effects by arresting cells at G(0)-G(1) and preventing progression through S phase. Imatinib inhibits both PDGFRalpha and Akt phosphorylation at a concentration of 1 micro M. Stable expression of constitutively active Akt induces partial resistance to PDGFR inhibition in ovarian cancer cells, as demonstrated by cell proliferation assay and cell cycle analysis.

CONCLUSIONS

Our data indicate that Imatinib mesylate inhibits the growth of ovarian cancer cells through PDGFR inactivation. In addition, our results suggest that constitutive Akt activation modulates sensitivity to Imatinib in ovarian cancer cells.

Self-renewal of teratocarcinoma and embryonic stem cells

Pluripotent stem cells derived from preimplantation embryos, primordial germ cells or teratocarcinomas are currently unique in undergoing prolonged symmetrical self-renewal in culture. For mouse embryonic stem (ES) cells, self-renewal is dependent on signals from the cytokine leukaemia inhibitory factor (LIF) and from either serum or bone morphogenetic proteins (BMPs). In addition to the extrinsic regulation of gene expression, intrinsic transcriptional determinants are also required for maintenance of the undifferentiated state. These include Oct4, a member of the POU family of homeodomain proteins and a second recently identified homeodomain protein, Nanog. When overexpressed, Nanog allows ES cells to self-renew in the absence of the otherwise obligatory LIF and BMP signals. Although Nanog can act independent of the LIF signal, a contribution of both pathways provides maximal self-renewal efficiency. Nanog function also requires Oct4. Here, we review recent progress in ES cell self-renewal, relate this to the biology of teratocarcinomas and offer testable hypotheses to expose the mechanics of ES cell self-renewal.

Cytoplasmic mislocalization of p27Kip1 protein is associated with constitutive phosphorylation of Akt or protein kinase B and poor prognosis in acute myelogenous leukemia

Cyclin-dependent kinase inhibitor p27Kip1 functions at the nuclear level by binding to cyclin E/cyclin-dependent kinase-2. It was shown that Akt or protein kinase B (Akt/PKB)-dependent phosphorylation of p27Kip1 led to the cytoplasmic mislocalization of p27Kip1, suggesting the potential abrogation of its activity. Here, we evaluated the localization of p27Kip1 protein in leukemic blasts in relation to Akt/PKB phosphorylation and clinical outcomes in acute myelogenous leukemia (AML). Western blot analysis of the nuclear and cytoplasmic fractions revealed a heterogenous localization pattern of p27Kip1 in AML. Cytoplasmic mislocalization of p27Kip1 was significantly associated with the constitutive serine(473) Akt/PKB phosphorylation in AML cells (P < 0.05). Transfection of U937 cells with an expression construct encoding the constitutively active form of Akt/PKB resulted in a remarkable increase in the levels of cytoplasmic p27Kip1. Whereas the transfection of U937 cells with a construct encoding dominant-negative Akt/PKB resulted in a recovery of nuclear localization of p27Kip1. Both the disease-free survival and overall survival are significantly shorter in AML cases with high cytoplasmic to nuclear ratio of p27Kip1 localization compared with the cases with low cytoplasmic to nuclear ratio (P = 0.0353, P = 0.0023, respectively). Multivariate analysis indicated that the cytoplasmic to nuclear ratio of p27Kip1 localization was an independent prognostic variable for both disease-free survival and overall survival (P = 0.043, P = 0.008, respectively). These findings additionally extend our understanding of the role of p27Kip1 in AML, and buttress the case of p27Kip1 mislocalization as a prognostic indicator and Akt/PKB/p27Kip1 pathway as a ready target for antileukemia therapy.

Role of PTEN and Akt in the regulation of growth and apoptosis in human osteoblastic cells

Cancer cells are characterized by either an increased ability to proliferate or a diminished capacity to undergo programmed cell death. PTEN is instrumental in regulating the balance between growth and death in several cell types and has been described as a tumor suppressor. The chromosome arm on which PTEN is located is deleted in a subset of human osteosarcoma tumors. Therefore, we predicted that the loss of PTEN expression was contributing to increased Akt activation and the subsequent growth and survival of osteosarcoma tumor cells. Immunoblot analyses of several human osteosarcoma cell lines and normal osteoblasts revealed relatively abundant levels of PTEN. Furthermore, stimulation of cell growth or induction of apoptosis in osteosarcoma cells failed to affect PTEN expression or activity. Therefore, routine regulation of osteosarcoma cell growth and survival appears to be independent of changes in PTEN. Subsequently, the activation of a downstream target of PTEN activity, the survival factor Akt, was analyzed. Inappropriate activation of Akt could bypass the negative regulation by PTEN. Analyses of Akt expression in several osteosarcoma cell lines and normal osteoblasts revealed uniformly low basal levels of phosphorylated Akt. The levels of phosphorylated Akt did not increase following growth stimulation. In addition, osteosarcoma cell growth was unaffected by inhibitors of phosphatidylinositol-3 kinase, an upstream activator of the Akt signaling pathway. These data further suggest that the Akt pathway is not the predominant signaling cascade required for osteoblastic growth. However, inhibition of PTEN activity resulted in increased levels of Akt phosphorylation and enhanced cell proliferation. These data suggest that while abundant levels of PTEN normally maintain Akt in an inactive form in osteoblastic cells, the Akt signaling pathway is intact and functional.

Gefitinib (ZD1839) Monotherapy as a Salvage Regimen for Previously Treated Advanced Non-Small Cell Lung Cancer

PURPOSE

A worldwide compassionate-use program has enabled >42,000 patients with advanced non-small cell lung cancer (NSCLC) to receive gefitinib treatment. Here we report the outcome of gefitinib therapy in patients who enrolled in the "Iressa" Expanded Access Program at the Samsung Medical Center.

EXPERIMENTAL DESIGN

Patients with advanced or metastatic NSCLC who had progressed after prior systemic chemotherapy and for whom no other treatment option was available were eligible to receive gefitinib treatment as part of the Expanded Access Program. A post hoc assessment of potential prognostic factors for response and survival was performed by multivariate analysis.

RESULTS

All 111 evaluable patients had stage IV disease; most patients had a baseline performance status of 2 [n = 52 (47%)] or 3 [n = 18 (16%)] and had received >/=2 prior chemotherapy regimens (56%). The objective response rate was 26%, the disease control rate (measured over >/=8 weeks) was 40%, and the 1-year survival rate was 44%. Adenocarcinoma histology was associated with better response and disease control rates, and a performance status of 0-2 was also associated with a better disease control rate. Both of these factors, as well as female gender, were significantly associated with longer survival. Gefitinib was well tolerated; the most common adverse event was grade 1 skin rash.

CONCLUSIONS

Gefitinib demonstrated significant antitumor activity and a favorable tolerability profile in this series of NSCLC patients with poor prognosis.

The LKB1 tumor suppressor negatively regulates mTOR signaling

Germline mutations in LKB1, TSC2, or PTEN tumor suppressor genes result in hamartomatous syndromes with shared tumor biological features. The recent observations of LKB1-mediated activation of AMP-activated protein kinase (AMPK) and AMPK inhibition of mTOR through TSC2 prompted us to examine the biochemical and biological relationship between LKB1 and mTOR regulation. Here, we report that LKB1 is required for repression of mTOR under low ATP conditions in cultured cells in an AMPK- and TSC2-dependent manner, and that Lkb1 null MEFs and the hamartomatous gastrointestinal polyps from Lkb1 mutant mice show elevated signaling downstream of mTOR. These findings position aberrant mTOR activation at the nexus of these germline neoplastic conditions and suggest the use of mTOR inhibitors in the treatment of Peutz-Jeghers syndrome.

Protein kinase B/AKT 1 plays a pivotal role in insulin-like growth factor-1 receptor signaling induced 3T3-L1 adipocyte differentiation

During 3T3-L1 preadipocyte differentiation induction, the insulin-stimulated insulin-like growth factor-1 (IGF-1) receptor signal is responsible for the induction of adipocyte differentiation. Treatment with inhibitors of phosphatidylinositol 3-kinase, LY294002 or wortmannin, leads to the complete blockade of adipocyte differentiation in 3T3-L1 preadipocytes. Of the three factors (1-methyl-3-isobutylxanthine, dexamethasone, and insulin) inducing 3T3-L1 preadipocyte differentiation, only insulin was able to activate the phosphatidylinositol 3-kinase-protein kinase B/Akt signal cascade. In 3T3-L1 preadipocytes, protein kinase B/Akt 1 RNA interference not only suppressed the expression of protein kinase B/Akt 1 but also blocked hormone-induced adipocyte differentiation. In these protein kinase B/Akt 1 RNA interference cells, the signal molecules upstream of protein kinase B/Akt 1, such as IGF-1 receptor and insulin receptor substrate-1, were normally activated by insulin stimulation, whereas insulin-stimulated phosphorylation of forkhead transcription factor (FKHR), which is a downstream molecule of PKB/Akt 1, was blocked. Thus, protein kinase B/Akt 1 is an important signal mediator in IGF-1 receptor signal cascade for inducing adipocyte differentiation.

ARK5 is a tumor invasion-associated factor downstream of Akt signaling

AMP-activated protein kinases (AMPKs) are a class of serine/threonine protein kinases that are activated by an increase in intracellular AMP concentration. They are a sensitive indicator of cellular energy status and have been found to promote tumor cell survival during nutrient starvation. We recently identified a novel AMPK catalytic subunit family member, ARK5, whose activation is directly regulated by Akt, which, in turn, has been reported to be a key player in tumor malignancy. In this study, we attempted to determine whether ARK5 is involved in tumor malignancy under regulation by Akt. Matrigel invasion assays demonstrated that both overexpressed and endogenous ARK5 showed strong activity dependent on Akt. In addition, ARK5 expression induced activation of matrix metalloproteinase 2 (MMP-2) and MMP-9 following new expression of membrane type 1 MMP (MT1-MMP), and the MT1-MMP expression induced by ARK5 was initiated by rapamycin-sensitive signaling. In nude mice, ARK5 expression was associated with a significant increase in tumor growth and significant suppression of necrosis in tumor tissue. Interestingly, only the ARK5-overexpressing PANC-1 cell line (P/ARK) tumor showed invasion and metastasis in nude mice, although Akt was activated in tumors derived from both P/ARK and its parental cell line. We report that a novel AMPK catalytic subunit family member, ARK5, plays a key role in tumor malignancy downstream of Akt.

Pathological and molecular mechanisms of prostate carcinogenesis: implications for diagnosis, detection, prevention, and treatment

Prostate cancer is an increasing threat throughout the world. As a result of a demographic shift in population, the number of men at risk for developing prostate cancer is growing rapidly. For 2002, an estimated 189,000 prostate cancer cases were diagnosed in the U.S., accompanied by an estimated 30,200 prostate cancer deaths [Jemal et al., 2002]. Most prostate cancer is now diagnosed in men who were biopsied as a result of an elevated serum PSA (>4 ng/ml) level detected following routine screening. Autopsy studies [Breslow et al., 1977; Yatani et al., 1982; Sakr et al., 1993], and the recent results of the Prostate Cancer Prevention Trial (PCPT) [Thompson et al., 2003], a large scale clinical trial where all men entered the trial without an elevated PSA (<3 ng/ml) were subsequently biopsied, indicate the prevalence of histologic prostate cancer is much higher than anticipated by PSA screening. Environmental factors, such as diet and lifestyle, have long been recognized contributors to the development of prostate cancer. Recent studies of the molecular alterations in prostate cancer cells have begun to provide clues as to how prostate cancer may arise and progress. For example, while inflammation in the prostate has been suggested previously as a contributor to prostate cancer development [Gardner and Bennett, 1992; Platz, 1998; De Marzo et al., 1999; Nelson et al., 2003], research regarding the genetic and pathological aspects of prostate inflammation has only recently begun to receive attention. Here, we review the subject of inflammation and prostate cancer as part of a "chronic epithelial injury" hypothesis of prostate carcinogenesis, and the somatic genome and phenotypic changes characteristic of prostate cancer cells. We also present the implications of these changes for prostate cancer diagnosis, detection, prevention, and treatment.

Integrin-linked kinase (ILK) regulation of the cell viability in PTEN mutant glioblastoma and in vitro inhibition by the specific COX-2 inhibitor NS-398

We report the increased activity and expression of the ILK protein in human glioblastomas and demonstrate that ILK activity is regulated by PTEN. The transfection of wild type-PTEN into the glioblastoma cell line U-251 MG altered the localization of ILK in the cell membrane; transfection with PTEN down-regulated PKB/Akt-Ser-473 phosphorylation via the inhibition of ILK-signaling. Our results suggest that ILK is critical for the PTEN-sensitive regulation of PKB/Akt-dependent cell survival. The selective COX-2 inhibitor NS-398 was found capable of down-regulating ILK and PKB/Akt phosphorylation. Our data indicate that inhibition of ILK signaling may be beneficial in the treatment of PTEN-deficient glioblastoma.

Nuclear targeting of Akt enhances kinase activity and survival of cardiomyocytes

Heart failure is associated with death of cardiomyocytes leading to loss of contractility. Previous studies using membrane-targeted Akt (myristolated-Akt), an enzyme involved in antiapoptotic signaling, showed inhibition of cell death and prevention of pathogenesis induced by cardiomyopathic stimuli. However, recent studies by our group have found accumulation of activated Akt in the nucleus, suggesting that biologically relevant target(s) of Akt activity may be located there. To test this hypothesis, a targeted Akt construct was created to determine the antiapoptotic action of nuclear Akt accumulation. Nuclear localization of the adenovirally encoded Akt construct was confirmed by confocal microscopy. Cardiomyocytes expressing nuclear-targeted Akt showed no evidence of morphological remodeling such as altered myofibril density or hypertrophy. Nuclear-targeted Akt significantly elevated levels of phospho-Akt and kinase activity and inhibited apoptosis as effectively as myristolated-Akt in hypoxia-induced cell death. Transgenic overexpression of nuclear-targeted Akt did not result in hypertrophic remodeling, altered cardiomyocyte DNA content or nucleation, or enhanced phosphorylation of typical cytoplasmic Akt substrates, yet transgenic hearts were protected from ischemia-reperfusion injury. Gene array analyses demonstrated changes in the transcriptional profile of Akt/nuc hearts compared with nontransgenic controls distinct from prior characterizations of Akt expression in transgenic hearts. Collectively, these experiments show that targeting of Akt to the nucleus mediates inhibition of apoptosis without hypertrophic remodeling, opening new possibilities for therapeutic applications of nuclear-targeted Akt to inhibit cell death associated with heart disease.

Liver-specific deletion of negative regulator Pten results in fatty liver and insulin hypersensitivity [corrected]

In the liver, insulin controls both lipid and glucose metabolism through its cell surface receptor and intracellular mediators such as phosphatidylinositol 3-kinase and serine-threonine kinase AKT. The insulin signaling pathway is further modulated by protein tyrosine phosphatase or lipid phosphatase. Here, we investigated the function of phosphatase and tension homologue deleted on chromosome 10 (PTEN), a negative regulator of the phosphatidylinositol 3-kinase/AKT pathway, by targeted deletion of Pten in murine liver. Deletion of Pten in the liver resulted in increased fatty acid synthesis, accompanied by hepatomegaly and fatty liver phenotype. Interestingly, Pten liver-specific deletion causes enhanced liver insulin action with improved systemic glucose tolerance. Thus, deletion of Pten in the liver may provide a valuable model that permits the study of the metabolic actions of insulin signaling in the liver, and PTEN may be a promising target for therapeutic intervention for type 2 diabetes.

Liver-specific deletion of negative regulator Pten results in fatty liver and insulin hypersensitivity [corrected]

In the liver, insulin controls both lipid and glucose metabolism through its cell surface receptor and intracellular mediators such as phosphatidylinositol 3-kinase and serine-threonine kinase AKT. The insulin signaling pathway is further modulated by protein tyrosine phosphatase or lipid phosphatase. Here, we investigated the function of phosphatase and tension homologue deleted on chromosome 10 (PTEN), a negative regulator of the phosphatidylinositol 3-kinase/AKT pathway, by targeted deletion of Pten in murine liver. Deletion of Pten in the liver resulted in increased fatty acid synthesis, accompanied by hepatomegaly and fatty liver phenotype. Interestingly, Pten liver-specific deletion causes enhanced liver insulin action with improved systemic glucose tolerance. Thus, deletion of Pten in the liver may provide a valuable model that permits the study of the metabolic actions of insulin signaling in the liver, and PTEN may be a promising target for therapeutic intervention for type 2 diabetes.

Down-regulation of the tumor suppressor PTEN by the tumor necrosis factor-alpha/nuclear factor-kappaB (NF-kappaB)-inducing kinase/NF-kappaB pathway is linked to a default IkappaB-alpha autoregulatory loop

The PTEN (phosphatase and tensin homolog deleted on chromosome ten) tumor suppressor gene affects multiple cellular processes including cell growth, proliferation, and cell migration by antagonizing phosphatidylinositol 3-kinase (PI3K). However, mechanisms by which PTEN expression is regulated have not been studied extensively. Similar to PTEN, tumor necrosis factor-alpha (TNF-alpha) affects a wide spectrum of diseases including inflammatory processes and cancer by acting as a mediator of apoptosis, inflammation, and immunity. In this study, we show that treatment of cancer cell lines with TNF-alpha decreases PTEN expression. In addition, overexpression of TNF-alpha downstream signaling targets, nuclear factor-kappaB (NF-kappaB)-inducing kinase (NIK) and p65 nuclear factor NF-kappaB, lowers PTEN expression, suggesting that TNF-alpha-induced down-regulation of PTEN is mediated through a TNF-alpha/NIK/NF-kappaB pathway. Down-regulation of PTEN by NIK/NF-kappaB results in activation of the PI3K/Akt pathway and augmentation of TNF-alpha-induced PI3K/Akt stimulation. Importantly, we demonstrate that this effect is associated with a lack of an inhibitor of kappaB (IkappaB)-alpha autoregulatory loop. Moreover, these findings suggest the interaction between PI3K/Akt and NF-kappaB via transcriptional regulation of PTEN and offer one possible explanation for increased tumorigenesis in systems in which NF-kappaB is chronically activated. In such a tumor system, these findings suggest a positive feedback loop whereby Akt activation of NF-kappaB further stimulates Akt via down-regulation of the PI3K inhibitor PTEN.

Early onset of neoplasia in the prostate and skin of mice with tissue-specific deletion of Pten

PTEN is a tumor suppressor gene mutated in various advanced human neoplasias, including glioblastomas and prostate, breast, endometrial, and kidney cancers. This tumor suppressor is a lipid phosphatase that negatively regulates cell survival and proliferation mediated by phosphatidylinositol 3-kinase/protein kinase B signaling. Using the Cre-loxP system, we selectively inactivated Pten in murine tissues in which the MMTV-LTR promoter is active, resulting in hyperproliferation and neoplastic changes in Pten-null skin and prostate. These phenotypes had early onset and were completely penetrant. Abnormalities in Pten mutant skin consisted of mild epidermal hyperplasia, whereas prostates from these mice exhibited high-grade prostatic intraepithelial neoplasia (HGPIN) that frequently progressed to focally invasive cancer. These data demonstrate that Pten is an important physiological regulator of growth in the skin and prostate. Further, the early onset of HGPIN in Pten mutant males is unique to this animal model and implicates PTEN mutations in the initiation of prostate cancer. Consistent with high PTEN mutation rates in human prostate tumors, these data indicate that PTEN is a critical tumor suppressor in this organ.