Negative regulation of PKB/Akt-dependent cell survival by the tumor suppressor PTEN

PTEN 2, a Golgi-associated testis-specific homologue of the PTEN tumor suppressor lipid phosphatase

The tumor suppressor PTEN is a phosphatidylinositol phospholipid phosphatase, which indirectly down-regulates the activity of the protein kinase B/Akt survival kinases. Examination of sequence data bases revealed the existence of a highly conserved homologue of PTEN. This homologue, termed PTEN 2, contained an extended amino-terminal domain having four potential transmembrane motifs, a lipid phosphatase domain, and a potential lipid-binding C2 domain. Transcript analysis demonstrated that PTEN 2 is expressed only in testis and specifically in secondary spermatocytes. In contrast to PTEN, PTEN 2 was localized to the Golgi apparatus via the amino-terminal membrane-spanning regions. Molecular modeling suggested that PTEN 2 is a phospholipid phosphatase with potential specificity for the phosphate at the 3 position of inositol phosphates. Enzymatic analysis of PTEN 2 revealed substrate specificity that is similar to PTEN, with a preference for the dephosphorylation of the phosphatidylinositol 3,5-phosphate phospholipid, a known mediator of vesicular trafficking. Together, these data suggest that PTEN 2 is a Golgi-localized, testis-specific phospholipid phosphatase, which may contribute to the terminal stages of spermatocyte differentiation.

Tumor suppressor PTEN inhibits nuclear accumulation of beta-catenin and T cell/lymphoid enhancer factor 1-mediated transcriptional activation

beta-Catenin is a protein that plays a role in intercellular adhesion as well as in the regulation of gene expression. The latter role of beta-catenin is associated with its oncogenic properties due to the loss of expression or inactivation of the tumor suppressor adenomatous polyposis coli (APC) or mutations in beta-catenin itself. We now demonstrate that another tumor suppressor, PTEN, is also involved in the regulation of nuclear beta-catenin accumulation and T cell factor (TCF) transcriptional activation in an APC-independent manner. We show that nuclear beta-catenin expression is constitutively elevated in PTEN null cells and this elevated expression is reduced upon reexpression of PTEN. TCF promoter/luciferase reporter assays and gel mobility shift analysis demonstrate that PTEN also suppresses TCF transcriptional activity. Furthermore, the constitutively elevated expression of cyclin D1, a beta-catenin/TCF-regulated gene, is also suppressed upon reexpression of PTEN. Mechanistically, PTEN increases the phosphorylation of beta-catenin and enhances its rate of degradation. We define a pathway that involves mainly integrin-linked kinase and glycogen synthase kinase 3 in the PTEN-dependent regulation of beta-catenin stability, nuclear beta-catenin expression, and transcriptional activity. Our data indicate that beta-catenin/TCF-mediated gene transcription is regulated by PTEN, and this may represent a key mechanism by which PTEN suppresses tumor progression.

Antagonism between PTEN/MMAC1/TEP-1 and androgen receptor in growth and apoptosis of prostatic cancer cells

PTEN/MMAC1/TEP-1 (PTEN) tumor suppressor and androgen receptor play important roles in prostatic tumorigenesis by exerting opposite effects on homeostasis of prostatic epithelium. Here, we describe a mutual repression and selective dominance between PTEN and the androgen receptor (AR) in the growth and the apoptosis of prostatic cancer cells. On the one hand, PTEN and an inhibitor of phosphoinositide 3-kinase repressed the transcriptional activity of the AR as well as androgen-induced cell proliferation and production of prostate-specific antigen. On the other hand, androgens protected prostate cancer cells from PTEN-induced apoptosis in an AR-dependent manner. Whereas the repression of the transcriptional activity of the AR by PTEN is likely to involve the down-regulation of AKT, androgens protected prostate cancer cells from PTEN-induced apoptosis without an effect on AKT activity, demonstrating a differential involvement of AKT in the interaction between PTEN and the AR. Our data suggest that the loss of PTEN function may induce tumorigenesis through unopposed activity of the AR as well as contribute to the resistance of prostate cancers to androgen ablation therapy.

Transient activation of Jun N-terminal kinases and protection from apoptosis by the insulin-like growth factor I receptor can be suppressed by dicumarol

The insulin-like growth factor I receptor (IGF-IR) activated by its ligands insulin-like growth factor (IGF)-I or IGF-II mediates suppression of apoptosis and contributes to tumorigenesis and cell growth. Here we investigated the activation of the stress-activated protein kinases including Jun N-terminal Kinases and p38 MAPK by IGF-I in interleukin-3-dependent FL5.12 lymphocytic cells that overexpress the IGF-IR (FL5.12/WT). We have shown previously that IGF-I protects these cells from apoptosis induced by interleukin-3 withdrawal but does not promote proliferation. IGF-I induced a rapid and transient activation of JNK that peaked at 40 min that was paralleled by a transient and robust phosphorylation of c-Jun. p38 was constitutively phosphorylated in FL5.12/WT cells. Activation of the JNK pathway by IGF-I occurred in the presence of phosphatidylinositol 3-kinase inhibitors and could be enhanced by anisomycin. Analysis of a series of FL5.12 cells expressing mutated IGF-IRs and analysis of 32D/IGF-IR cells showed that neither the C terminus of the receptor nor IRS-1 and IRS-2 were required for JNK activation, although tyrosine 950 was essential for full activation. The JNK inhibitor dicumarol suppressed IGF-I-mediated activation of JNK and phosphorylation of c-Jun but did not affect p38 and IkappaB phosphorylation or activation of AKT. IGF-I-mediated protection from apoptosis in FL5.12/WT cells was completely suppressed by dicumarol and partially suppressed by a p38 inhibitor. In the breast carcinoma cell line MCF-7, treatment with dicumarol also induced apoptosis. These data indicate that transient activation of JNK by IGF-I is mediated by signals that are distinct from those leading to phosphatidylinositol 3-kinase and AKT activation. The data further suggest that the SAPK pathways contribute to suppression of apoptosis by the IGF-IR.

Phosphoinositide 3-kinases in T lymphocyte activation

Biochemical experiments have established that the metabolism of inositol phospholipids by phosphoinositide 3-kinases (PI3Ks) and lipid-phosphatases is triggered by many receptors that control T lymphocyte function, including antigen-receptors, costimulatory molecules, cytokines and chemokines. Novel effectors of PI3K have been identified in the immune system and shown to be important in the control of lymphocyte activation. Moreover, key lipid-phosphatases have been identified that act to terminate or modulate PI3K signalling in cells of the immune system.

An Arabidopsis inositol phospholipid kinase strongly expressed in procambial cells: synthesis of PtdIns(4,5)P2 and PtdIns(3,4,5)P3 in insect cells by 5-phosphorylation of precursors

We have cloned a phosphatidylinositol-4-phosphate 5-kinase (PIP5K) cDNA (AtP5K1) from Arabidopsis thaliana. By the application of cell permeabilization and short-term nonequilibrium labelling we show that expression of AtP5K1 in Baculovirus-infected insect (Spodoptera frugiperda) cells directs synthesis of PtdIns(4,5)P2 and PtdIns(3,4,5)P3. The same phosphoinositides were produced by isolated whole-cell membrane fractions of AtP5K1-expressing insect cells. Their synthesis was not affected by adding defined precursor lipids, that is PtdIns(3)P, PtdIns(4)P, PtdIns(3,4)P2, or PtdIns(4,5)P2, in excess, indicating that substrates for the plant enzyme were not limiting in vivo. Enzymatic dissection of lipid headgroups revealed that AtP5K1-directed synthesis of PtdIns(4,5)P2 and PtdIns(3,4,5)P3 proceeds via 5-phosphorylation of precursors. Analysis of promoter-reporter gene (beta-glucuronidase) fusions in transgenic plants revealed that expression of the AtP5K1 gene is strongest in vascular tissues of leaves, flowers, and roots, namely in cells of the lateral meristem, that is the procambium. Single-cell sampling of sap from flower stem meristem tissue and neighbouring phloem cells, when coupled to reverse transcriptase--polymerase chain reaction, confirmed preferential expression of AtP5K1 in procambial tissue. We hypothesize that AtP5K1, like animal and yeast PIP5K, may be involved in the control of cell proliferation.

Frequent loss of PTEN expression is linked to elevated phosphorylated Akt levels, but not associated with p27 and cyclin D1 expression, in primary epithelial ovarian carcinomas

PTEN (MMAC1/TEP1), a tumor suppressor gene on chromosome subband 10q23.3, is variably mutated and/or deleted in a variety of human cancers. Germline mutations in PTEN, which encode a dual-specificity phosphatase, have been implicated in at least two hamartoma tumor syndromes that exhibit some clinical overlap, Cowden syndrome and Bannayan-Riley-Ruvalcaba syndrome. Among several series of ovarian cancers, the frequency of loss of heterozygosity (LOH) of markers flanking and within PTEN, is approximately 30 to 50%, and the somatic intragenic PTEN mutation frequency is <10%. In this study, we screened primary adenocarcinomas of the ovary for LOH of polymorphic markers within and flanking the PTEN gene and for intragenic mutations of the PTEN gene and compared them to PTEN expression using immunohistochemistry. Furthermore, we sought to detect the expression of the presumed downstream targets of PTEN, such as P-Akt, p27, and cyclin D1 by immunohistochemistry. LOH at 10q23 was observed in 29 of 64 (45%) cases. Of the 117 samples, 6 somatic intragenic PTEN mutations, 1 germline mutation, and 1 novel polymorphism were found in 7 (6%) patients. Immunostaining of 49 ovarian cancer samples revealed that 13 (27%) were PTEN immunostain-negative, 25 (51%) had reduced staining, and the rest (22%) were PTEN expression-positive. Among the 44 informative tumors assessed for 10q23 LOH and PTEN immunostaining, there was an association between 10q23 LOH and decreased or absent staining (P = 0.0317). Of note, there were five (11%) tumors with neither mutation nor deletion that exhibited no PTEN expression and 10 (25%) others without mutation or deletion but had decreased PTEN expression. Among the 49 tumors available for immunohistochemistry, 28 (57%) showed P-Akt-positive staining, 24 (49%) had decreased p27 staining, and cyclin D1 was overexpressed in 35 (79%) cases. In general, P-Akt expression was inversely correlated with PTEN expression (P = 0.0083). These data suggest that disruption of PTEN by several mechanisms, allelic loss, intragenic mutation, or epigenetic silencing, all contribute to epithelial ovarian carcinogenesis, and that epigenetic silencing is a significant mechanism. The Akt pathway is prominently involved, but clearly not in all cases. Surprisingly, despite in vitro demonstration that p27 and cyclin D1 lies downstream of PTEN and Akt, there was no correlation between p27 and cyclin D1 expression and PTEN or P-Akt status. Thus, in vivo, although PTEN and Akt play a prominent role in ovarian carcinogenesis, p27 and cyclin D1 might not be the primary downstream targets. Alternatively, these observations could also suggest that pathways involving other than Akt, p27 and cyclin D1 that lie downstream of PTEN play roles in ovarian carcinogenesis.

The PTEN tumor suppressor protein inhibits tumor necrosis factor-induced nuclear factor kappa B activity

Nuclear factor kappaB (NF-kappaB) transcriptionally activates genes that promote immunity and cell survival. Activation of NF-kappaB is induced by an IkappaB kinase (IKK) complex that phosphorylates and promotes dissociation of IkappaB from NF-kappaB, which then translocates into the nucleus. Activation of phosphatidylinositol (PI) 3-kinase/Akt signaling by tumor necrosis factor (TNF) activates IKK and NF-kappaB. The present study shows that PTEN, a tumor suppressor that inhibits PI 3-kinase function, impairs TNF activation of Akt and the IKK complex in 293 cells. Transient expression of PTEN suppressed IKK activation and TNF-induced NF-kappaB DNA binding and transactivation. Studies were conducted with PC-3 prostate cancer cells that do not express PTEN and DU145 prostate cancer cells that express PTEN. TNF activated Akt in PC-3 cells, but not in DU145 cells, and the ability of TNF to activate NF-kappaB was blocked by pharmacological inhibition of PI 3-kinase activity in PC-3 cells, but not in DU145 cells. Expression of PTEN in PC-3 cells to a level comparable with that endogenously present in DU145 cells inhibited TNF activation of NF-kappaB. The cell type-specific ability of PTEN to negatively regulate the PI 3-kinase/AKT/NF-kappaB pathway may be important to its tumor suppressor activity.

Frequent allelic loss at 10q23 but low incidence of PTEN mutations in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare, highly metastatic skin tumor of neuroectodermal origin. The disease shares clinical and histopathological features with small cell lung carcinoma (SCLC). The genetic mechanisms underlying the development and tumor progression of MCC are poorly understood. We recently showed by comparative genomic hybridization (CGH) that the pattern of chromosomal abnormalities in MCC resembles that of SCLC. One of the most frequently observed losses involved the entire chromosome 10 or partial loss of the chromosome 10 long arm (33% of examined MCC cases). The PTEN tumor-suppressor gene has been mapped to 10q23.3 and was shown to be mutated in a variety of human cancers including SCLC. Germline PTEN mutations have been observed in familial predisposing cancer syndromes including Cowden disease. Interestingly, an association between Cowden syndrome and Merkel cell carcinoma has been reported. To study the possible role of PTEN in MCC oncogenesis, loss of heterozygosity (LOH) analysis for the 10q23 region was performed on 26 MCC tumor samples from 23 MCC patients. The PTEN locus was deleted in 9 of 21 (43%) informative MCC tumor samples [7 of 18 (39%) MCC patients]. Despite this high frequency of LOH at 10q23, mutation and homozygous deletion screening of the PTEN gene revealed only one tumor with a nonsense mutation and a second with a homozygous deletion of exon 9. These data suggest that either alternative mechanisms lead to inactivation of the PTEN gene or that other tumor-suppressor genes at chromosome 10 are implicated in the development of MCC.

Differential effects of phosphatidylinositol-3/Akt-kinase inhibition on apoptotic sensitization to cytokines in LNCaP and PCc-3 prostate cancer cells

Alterations in phosphatidylinositol 3'-kinase (PI3'-kinase) and Akt activation frequently occur in prostate cancer and may disrupt apoptotic induction by such cytokines as tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL). To examine the role of PI3' phosphorylation in the cellular response to cytokines, two prostate cancer cell lines with constitutively activated PI3'-kinase cascades (LNCaP and PC-3) were examined for direct sensitivity to cytokines. TNF or TRAIL alone failed to activate apoptosis in either LNCaP or PC-3 cells, and drug-mediated inhibition of the PI3k/Akt cascade caused only minimal activation of apoptosis in either cell line. Suppression of PI3'-kinase/Akt signaling markedly enhanced the apoptotic activity of both TNF and TRAIL in LNCaP cells but not in PC-3 cells. Adenovirus-mediated PTEN/MMAC1 expression in LNCaP cells reduced Akt activation, activated apoptosis, and sensitized cells to TNF but not to TRAIL. Together, these results suggest that PI3'-kinase signaling inhibits both TNF-mediated and TRAIL-mediated apoptosis but may represent one of several apoptotic resistance mechanisms that inhibit cytokine-mediated killing of prostate cancer cells.

Transcriptional repression by RB-E2F and regulation of anchorage-independent survival

Mutations that lead to anchorage-independent survival are a hallmark of tumor cells. Adhesion of integrin receptors to extracellular matrix activates a survival signaling pathway in epithelial cells where Akt phosphorylates and blocks the activity of proapoptotic proteins such as the BCL2 family member Bad, the forkhead transcription factor FKHRL-1, and caspase 9. Insulin-like growth factor 1 (IGF-1) is a well-established epithelial cell survival factor that also triggers activation of Akt and can maintain Akt activity after cells lose matrix contact. It is not until IGF-1 expression diminishes (~16 h after loss of matrix contact) that epithelial cells deprived of matrix contact undergo apoptosis. This suggests that IGF-1 expression is linked to cell adhesion and that it is the loss of IGF-1 which dictates the onset of apoptosis after cells lose matrix contact. Here, we examine the linkage between cell adhesion and IGF-1 expression. While IGF-1 is able to maintain Akt activity and phosphorylation of proapoptotic proteins in cells that have lost matrix contact, Akt is not able to phosphorylate and inactivate another of its substrates, glycogen synthase kinase 3beta (GSK-3beta), under these conditions. The reason for this appears to be a rapid translocation of active Akt away from GSK-3beta when cells lose matrix contact. One target of GSK-3beta is cyclin D, which is turned over in response to this phosphorylation. Therefore, cyclin D is rapidly lost when cells are deprived of matrix contact, leading to a loss of cyclin-dependent kinase 4 activity and accumulation of hypophosphorylated, active Rb. This facilitates assembly of a repressor complex containing histone deacetylase (HDAC), Rb, and E2F that blocks transcription of the gene for IGF-1, leading to loss of Akt activity, accumulation of active proapoptotic proteins, and apoptosis. This feedback loop containing GSK-3beta, cyclin D, HDAC-Rb-E2F, and IGF-1 then determines how long Akt will remain active after cells lose matrix contact, and thus it serves to regulate the onset of apoptosis in such cells.

PTEN mutations in malignant gliomas and their relation with meningeal gliomatosis

A putative tumor suppressor, the PTEN gene at chromosome 10q23. was identified and found to be mutated in many different human tumors. PTEN was recently found to be also involved in focal cell adhesion and cell migration. To identify the role of PTEN gene in malignant gliomas. we used PCR-SSCP and direct sequencing methods to examine 44 malignant gliomas comprising 29 cases without and 15 cases with meningeal gliomatosis. In malignant gliomas without meningeal gliomatosis, 2/29 (7%) of the cases showed alteration of the PTEN gene. In contrast, 5/15 (33%) of malignant gliomas with meningeal gliomatosis cases showed this alteration. These findings indicate that PTEN gene mutation contributes not only to the neoplastic evolution in gliomas but also to the meningeal dissemination of glioma cells.

Phosphoinositide 3-kinase signalling pathways

Phosphoinositide 3-kinases (PI3Ks) phosphorylate the 3′-OH position of the inositol ring of inositol phospholipids, producing three lipid products: PtdIns(3)P, PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3). These lipids bind to the pleckstrin homology (PH) domains of proteins and control the activity and subcellular localisation of a diverse array of signal transduction molecules. Three major classes of signalling molecule are regulated by binding of D-3 phosphoinositides to PH domains: guanine-nucleotide-exchange proteins for Ρ family GTPases, the TEC family tyrosine kinases such as BTK and ITK in B and T lymphocytes, respectively, and the AGC superfamily of serine/threonine protein kinases. These molecules are activated by a variety of extracellular stimuli and have been implicated in a wide range of cellular processes, including cell cycle progression, cell growth, cell motility, cell adhesion and cell survival.

PTEN controls tumor-induced angiogenesis

Mutations of the tumor suppressor PTEN, a phosphatase with specificity for 3-phosphorylated inositol phospholipids, accompany progression of brain tumors from benign to the most malignant forms. Tumor progression, particularly in aggressive and malignant tumors, is associated with the induction of angiogenesis, a process termed the angiogenic switch. Therefore, we tested whether PTEN regulates tumor progression by modulating angiogenesis. U87MG glioma cells stably reconstituted with PTEN cDNA were tested for growth in a nude mouse orthotopic brain tumor model. We observed that the reconstitution of wild-type PTEN had no effect on in vitro proliferation but dramatically decreased tumor growth in vivo and prolonged survival in mice implanted intracranially with these tumor cells. PTEN reconstitution diminished phosphorylation of AKT within the PTEN-reconstituted tumor, induced thrombospondin 1 expression, and suppressed angiogenic activity. These effects were not observed in tumors reconstituted with a lipid phosphatase inactive G129E mutant of PTEN, a result that provides evidence that the lipid phosphatase activity of PTEN regulates the angiogenic response in vivo. These data provide evidence that PTEN regulates tumor-induced angiogenesis and the progression of gliomas to a malignant phenotype via the regulation of phosphoinositide-dependent signals.

PTEN and myotubularin phosphoinositide phosphatases: bringing bioinformatics to the lab bench

Phosphoinositides play an integral role in a diverse array of cellular signaling processes. Although considerable effort has been directed toward characterizing the kinases that produce inositol lipid second messengers, the study of phosphatases that oppose these kinases remains limited. Current research is focused on the identification of novel lipid phosphatases such as PTEN and myotubularin, their physiologic substrates, signaling pathways and links to human diseases. The use of bioinformatics in conjunction with genetic analyses in model organisms will be essential in elucidating the roles of these enzymes in regulating phosphoinositide-mediated cellular signaling.

Molecular modeling studies of the Akt PH domain and its interaction with phosphoinositides

The serine-threonine protein kinase Akt is a direct downstream target of phosphatidylinositol 3-kinase (PI3-K). The PI3-K-generated phospholipids regulate Akt activity via directly binding to the Akt PH domain. The binding of PI3-K-generated phospholipids is critical to the relocalization of Akt to the plasma membrane, which plays an important role in the process of Akt activation. Activation of the PI3-K/Akt signaling pathway promotes cell survival. To elucidate the structural basis of the interaction of PI3-K-generated phospholipids with the Akt PH domain with the objective of carrying out structure-based drug design, we modeled the three-dimensional structure of the Akt PH domain. Comparative modeling-based methods were employed, and the modeled Akt structure was used in turn to construct structural models of Akt in complex with selected PI3-K-generated phospholipids using the computational docking approach. The model of the Akt PH domain consists of seven beta-strands forming two antiparallel beta-sheets capped by a C-terminal alpha-helix. The beta1-beta2, beta3-beta4, and beta6-beta7 loops form a positively charged pocket that can accommodate the PI3-K-generated phospholipids in a complementary fashion through specific hydrogen-bonding interactions. The residues Lys14, Arg25, Tyr38, Arg48, and Arg86 form the bottom of the binding pocket and specifically interact with the 3- and 4-phophate groups of the phospholipids, while residues Thr21 and Arg23 are situated at the wall of the binding pocket and bind to the 1-phosphate group. The predicted binding mode is consistent with known site-directed mutagenesis data, which reveal that mutation of these crucial residues leads to the loss of Akt activity. Moreover, our model can be used to predict the binding affinity of PI3-K-generated phospholipids and rationalize the specificity of the Akt PH domain for PI(3,4)P2, as opposed to other phospholipids such as PI(3)P and PI(3,4,5)P3. Taken together, our modeling studies provide an improved understanding of the molecular interactions present between the Akt PH domain and the PI3-K-generated phospholipids, thereby providing a solid structural basis for the design of novel, high-affinity ligands useful in modulating the activity of Akt.

PTEN: life as a tumor suppressor

PTEN, a tumor suppressor located at chromosome 10q23, is mutated in a variety of sporadic cancers and in two autosomal dominant hamartoma syndromes. PTEN is a phosphatase which dephosphorylates phosphatidylinositol (3,4,5)-triphosphate (PtdIns-3,4,5-P3), an important intracellular second messenger, lowering its level within the cell. By dephosphorylating PtdIns-3,4,5-P3, PTEN acts in opposition to phosphatidylinositol 3-kinase (PI3K), which has a pivotal role in the creation of PtdIns-3,4,5-P3. PtdIns-3,4,5-P3 is necessary for the activation of Akt, a serine/threonine kinase involved in cell growth and survival. By blocking the activation of Akt, PTEN regulates cellular processes such as cell cycling, translation, and apoptosis. In this review, we will discuss the identification of PTEN, its mutational status in cancer, its role as a regulator of PI3K, and its domain structure.

Loss of expression of human spectrin src homology domain binding protein 1 is associated with 10p loss in human prostatic adenocarcinoma

The gene encoding human spectrin Src homology domain binding protein 1, or Hssh3bp1, which is a marker of macropinocytic vesicles and a potential regulator of macropinocytosis, co-localizes to a YAC containing chromosome 10p sequences at loci D10S89 and D10S111 that are frequently deleted in prostate tumors. Expression of Hssh3bp1 was evaluated at the protein level in 17 paired normal and malignant prostate tumor samples using the monoclonal antibody 2G8 to Hssh3bp1. These experiments demonstrated that 4/6 tumors (67%) with 10p deletion failed to express Hssh3bp1 protein compared to 5/11 (46%) tumors with intact 10p. Thus, loss of Hssh3bp1 expression is concordant with allelic loss of adjacent 10p sequences in human prostate tumors. In addition, two prostate tumor cell lines contain an exon skipping mutation in the Hssh3bp1 gene that leads to the abnormal splicing of the mRNA and loss of a portion of Abl tyrosine kinase SH3 domain binding site in the protein. These data are consistent with a role for Hssh3bp1 as a candidate tumor suppressor gene inactivated during prostate tumorigenesis.

Activation of Akt (protein kinase B) in mammary epithelium provides a critical cell survival signal required for tumor progression

Activation of Akt by the phosphatidylinositol 3'-OH kinase (PI3K) results in the inhibition of proapoptotic signals and the promotion of survival signals (L. P. Kane et al., Curr. Biol. 9:601-604, 1999; G. J. Kops et al., Nature 398:630-634, 1999). Evidence supporting the importance of the PI3K/Akt signaling pathway in tumorigenesis stems from experiments with transgenic mice bearing polyomavirus middle T antigen under the control of the mouse mammary tumor virus long terminal repeat promoter. Mammary epithelium-specific expression of polyomavirus middle T antigen results in the rapid development of multifocal metastatic mammary tumors, whereas transgenic mice expressing a mutant middle T antigen decoupled from the phosphatidylinositol 3'-OH kinase (MTY315/322F) develop extensive mammary gland hyperplasias that are highly apoptotic. To directly assess the role of Akt in mammary epithelial development and tumorigenesis, we generated transgenic mice expressing constitutively active Akt (HAPKB308D473D or Akt-DD). Although expression of Akt-DD interferes with normal mammary gland involution, tumors were not observed in these strains. However, coexpression of Akt-DD with MTY315/322F resulted in a dramatic acceleration of mammary tumorigenesis correlated with reduced apoptotic cell death. Furthermore, coexpression of Akt-DD with MTY315/322F resulted in phosphorylation of the FKHR forkhead transcription factor and translational upregulation of cyclin D1 levels. Importantly, we did not observe an associated restoration of wild-type metastasis levels in the bitransgenic strain. Taken together these observations indicate that activation of Akt can contribute to tumor progression by providing an important cell survival signal but does not promote metastatic progression.

Overexpression of SH2-containing inositol phosphatase 2 results in negative regulation of insulin-induced metabolic actions in 3T3-L1 adipocytes via its 5'-phosphatase catalytic activity

Phosphatidylinositol (PI) 3-kinase plays an important role in various metabolic actions of insulin including glucose uptake and glycogen synthesis. Although PI 3-kinase primarily functions as a lipid kinase which preferentially phosphorylates the D-3 position of phospholipids, the effect of hydrolysis of the key PI 3-kinase product PI 3,4,5-triphosphate [PI(3,4,5)P3] on these biological responses is unknown. We recently cloned rat SH2-containing inositol phosphatase 2 (SHIP2) cDNA which possesses the 5'-phosphatase activity to hydrolyze PI(3,4,5)P3 to PI 3,4-bisphosphate [PI(3,4)P2] and which is mainly expressed in the target tissues of insulin. To study the role of SHIP2 in insulin signaling, wild-type SHIP2 (WT-SHIP2) and 5'-phosphatase-defective SHIP2 (Delta IP-SHIP2) were overexpressed in 3T3-L1 adipocytes by means of adenovirus-mediated gene transfer. Early events of insulin signaling including insulin-induced tyrosine phosphorylation of the insulin receptor beta subunit and IRS-1, IRS-1 association with the p85 subunit, and PI 3-kinase activity were not affected by expression of either WT-SHIP2 or Delta IP-SHIP2. Because WT-SHIP2 possesses the 5'-phosphatase catalytic region, its overexpression marked by decreased insulin-induced PI(3,4,5)P3 production, as expected. In contrast, the amount of PI(3,4,5)P3 was increased by the expression of Delta IP-SHIP2, indicating that Delta IP-SHIP2 functions in a dominant-negative manner in 3T3-L1 adipocytes. Both PI(3,4,5)P3 and PI(3,4)P2 were known to possibly activate downstream targets Akt and protein kinase C lambda in vitro. Importantly, expression of WT-SHIP2 inhibited insulin-induced activation of Akt and protein kinase C lambda, whereas these activations were increased by expression of Delta IP-SHIP2 in vivo. Consistent with the regulation of downstream molecules of PI 3-kinase, insulin-induced 2-deoxyglucose uptake and Glut4 translocation were decreased by expression of WT-SHIP2 and increased by expression of Delta IP-SHIP2. In addition, insulin-induced phosphorylation of GSK-3beta and activation of PP1 followed by activation of glycogen synthase and glycogen synthesis were decreased by expression of WT-SHIP2 and increased by the expression of Delta IP-SHIP2. These results indicate that SHIP2 negatively regulates metabolic signaling of insulin via the 5'-phosphatase activity and that PI(3,4,5)P3 rather than PI(3,4)P2 is important for in vivo regulation of insulin-induced activation of downstream molecules of PI 3-kinase leading to glucose uptake and glycogen synthesis.

Structural organization of the mouse phosphatidylinositol 3-kinase p110d gene

Phosphatidylinositol 3-kinases are a family of dual specificity lipid/protein kinases. The products of PI3K's, phosphatidylinositol(3,4,5) triphosphate and phosphatidylinositol(3,4) bisphosphate, act as second messengers connecting activated transmembrane receptors to signaling pathways that control gene transcription, proliferation, transformation, programmed cell death, adhesion, migration and vesicular transport. There is evidence that different isoforms of PI3K's activate specific signaling pathways and are thus responsible for integrating cellular responses. The elucidation of the genomic structure of the catalytic subunits is a necessary step for the investigation of the function of PI3K isoforms by inactivation of the gene in vivo. The structural organization of p110alpha, beta, and gamma genes has been previously reported. Here we report the cloning, sequencing, and structural organization of the mouse p110delta gene from a murine 129/Sv genomic library. The p110delta gene consists of 22 exons and spans over 13 kb. Comparison of the genomic structure with that of p110alpha, beta, and gamma demonstrates that the p110delta gene shares its exon structure with p110beta, the most closely related PI3K at the amino acid level.

Molecular mechanisms of ATP and insulin synergistic stimulation of coronary artery smooth muscle growth

Coronary artery disease (CAD) is the major cause of death in diabetics. Abnormal proliferation of coronary artery smooth muscle cells (CASMC) leads to intimal thickening in CAD. We examined signaling mechanisms involved in the mitogenic effect of ATP and insulin on CASMC. ATP and insulin individually stimulated DNA synthesis by 4- and 2-fold, respectively; however, they acted synergistically to stimulate an increase of 17-fold over basal. A similar synergistic stimulation of extracellular signal-regulated kinase (ERK) and mitogen-activated protein or ERK kinase activities was observed (ATP, 7-fold; insulin, 2-fold; and ATP + insulin, 16-fold over basal). However, the combination of ATP and insulin stimulated only an additive activation of Raf (ATP, 5-fold; insulin, <2-fold; and ATP + insulin, 8-fold over basal) and Ras (ATP, 5-fold; insulin, 2-fold; and ATP + insulin, 8-fold over basal). Thus convergence of ATP and insulin signals appears to be at the level of Ras and Raf. In addition, insulin stimulated activation of Akt (also known as protein kinase B) (10-fold over basal), whereas ATP had little effect. However, when ATP and insulin were added in combination, ATP dramatically reduced the insulin-stimulated Akt activation (2-fold above basal). Thus these results are consistent with ATP relieving an insulin-induced Akt-dependent inhibitory effect on the ERK signaling pathway, leading to synergistic stimulation of CASMC proliferation.

Deficiency of Pten accelerates mammary oncogenesis in MMTV-Wnt-1 transgenic mice

BACKGROUND

Germline mutations in the tumor suppressor PTEN predispose human beings to breast cancer, and genetic and epigenetic alterations of PTEN are also detected in sporadic human breast cancer. Germline Pten mutations in mice lead to the development of a variety of tumors, but mammary carcinomas are infrequently found, especially in mice under the age of six months.

RESULTS

To better understand the role of PTEN in breast tumor development, we have crossed Pten heterozygous mice to MMTV-Wnt-1 transgenic mice that routinely develop ductal carcinomas in the mammary gland. Female Wnt-1 transgenics heterozygous for Pten developed mammary tumors earlier than Wnt-1 transgenics that were wild type for Pten. In most tumors arising in Pten heterozygotes, the Pten wild-type allele was lost, suggesting that cells lacking Pten function have a growth advantage over cells retaining a wild type allele. Tumors with LOH contained high levels of activated AKT/PKB, a downstream target of the PTEN/PI3K pathway.

CONCLUSIONS

An animal model has been developed in which the absence of Pten collaborates with Wnt-1 to induce ductal carcinoma in the mammary gland. This animal model may be useful for testing therapies specific for tumors deregulated in the PTEN/PI3K/AKT pathway.

Mutational analysis of the PTEN gene in endometrial carcinoma and hyperplasia

To determine the potential role of PTEN in the process of endometrial carcinogenesis, we examined a series of endometrial carcinoma and hyperplasia of the uterine corpus for the presence of a PTEN mutation. The entire coding region of the gene was screened for the presence of mutations by single-strand conformation polymorphism analysis, and mutations were confirmed by sequencing. We detected mutations in 14 of 57 endometrial carcinomas (13 of 50 endometrioid adenocarcinomas and 1 of 7 nonendometrioid adenocarcinomas) and 7 of 73 endometrial hyperplasias (1 of 24 simple hyperplasias without atypia, none of 16 complex hyperplasias without atypia, and 6 of 33 complex hyperplasias with atypia). Most (88%) mutations were clustered in exons 5, 7, and 8. Of the 24 mutations detected in 21 cases, 12 were frameshifts, 9 were nonsense, 2 were missense, and 1 was a silent mutation. Patients with a PTEN mutation had a better prognosis than those with no PTEN mutation. The presence of PTEN mutations in hyperplasia suggests that PTEN inactivation may occur as an initiating event in endometrial carcinogenesis and is involved in the development of cytologic atypia in hyperplasia.

Inhibition of mammary epithelial apoptosis and sustained phosphorylation of Akt/PKB in MMTV-IGF-II transgenic mice

IGF-II is a growth factor implicated in human cancers and animal tumor models. While the mitogenic properties of IGF-II are well documented, its ability to suppress apoptosis in vivo has never been proven. We generated independent MMTV-IGF-II transgenic mice to examine the control of epithelial apoptosis at the morphological, cellular and molecular levels during the physiological event of postlactation mammary involution. Transgenic IGF-II expression was achieved in mammary epithelium and increased IGF-II bioactivity was confirmed by phosphorylation of the insulin receptor substrate-1, a signaling molecule downstream of the type I IGF receptor. IGF-II overexpression induced a delay in mammary involution, as evident by increased mammary gland to body weight ratios and persistence of both functionally intact lobulo-alveoli and mammary epithelial cellularity. The delayed mammary involution resulted from a significant reduction in mammary epithelial apoptosis, and not from increased epithelial proliferation. Recombinant IGF-II pellets implanted into involuting mammary glands of wild-type mice provided further evidence that IGF-II protein inhibited local epithelial apoptosis. At the molecular level, phosphorylated Akt/PKB, but not Erk1 or Erk2, persisted in IGF-II overexpressors and temporally correlated with reduced epithelial apoptosis. Levels of the phosphatase PTEN were unaltered in the transgenic tissue suggesting that the maintenance of Akt/PKB phosphorylation resulted from sustained phosphorylation rather than altered dephosphorylation of PIP-3. Together, this data reveal that IGF-II inhibits apoptosis in vivo and this effect correlates with prolonged phosphorylation of Akt/PKB

Heterogeneous lack of expression of the tumour suppressor PTEN protein in human neoplastic tissues

PTEN, a tumour suppressor gene located at chromosome 10q23 and commonly mutated or deleted in a variety of tumours, encodes a dual-specific/phosphatidylinositol-3,4,5-triphosphate (PIP3) phosphatase. We report the generation of an anti-PTEN monoclonal antibody (MAb) that recognises an epitope at the C-terminus of PTEN, and describe the heterogeneous lack of expression of the PTEN protein in human tumour tissues, as demonstrated by immunohistochemical methods. Our anti-PTEN MAb provides a useful tool for the study of PTEN protein expression in tumour samples, in the search for tumour prognostic molecular markers.

Differential roles for alpha(M)beta(2) integrin clustering or activation in the control of apoptosis via regulation of akt and ERK survival mechanisms

The role of integrins in leukocyte apoptosis is unclear, some studies suggest enhancement, others inhibition. We have found that beta(2)-integrin engagement on neutrophils can either inhibit or enhance apoptosis depending on the activation state of the integrin and the presence of proapoptotic stimuli. Both clustering and activation of alpha(M)beta(2) delays spontaneous, or unstimulated, apoptosis, maintains mitochondrial membrane potential, and prevents cytochrome c release. In contrast, in the presence of proapoptotic stimuli, such as Fas ligation, TNFalpha, or UV irradiation, ligation of active alpha(M)beta(2) resulted in enhanced mitochondrial changes and apoptosis. Clustering of inactive integrins did not show this proapoptotic effect and continued to inhibit apoptosis. This discrepancy was attributed to differential signaling in response to integrin clustering versus activation. Clustered, inactive alpha(M)beta(2) was capable of stimulating the kinases ERK and Akt. Activated alpha(M)beta(2) stimulated Akt, but not ERK. When proapoptotic stimuli were combined with either alpha(M)beta(2) clustering or activation, Akt activity was blocked, allowing integrin activation to enhance apoptosis. Clustered, inactive alpha(M)beta(2) continued to inhibit stimulated apoptosis due to maintained ERK activity. Therefore, beta(2)-integrin engagement can both delay and enhance apoptosis in the same cell, suggesting that integrins can play a dual role in the apoptotic progression of leukocytes.

Forkhead transcription factor FKHR-L1 modulates cytokine-dependent transcriptional regulation of p27(KIP1)

Interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating factor regulate the survival, proliferation, and differentiation of hematopoietic lineages. Phosphatidylinositol 3-kinase (PI3K) has been implicated in the regulation of these processes. Here we investigate the molecular mechanism by which PI3K regulates cytokine-mediated proliferation and survival in the murine pre-B-cell line Ba/F3. IL-3 was found to repress the expression of the cyclin-dependent kinase inhibitor p27(KIP1) through activation of PI3K, and this occurs at the level of transcription. This transcriptional regulation occurs through modulation of the forkhead transcription factor FKHR-L1, and IL-3 inhibited FKHR-L1 activity in a PI3K-dependent manner. We have generated Ba/F3 cell lines expressing a tamoxifen-inducible active FKHR-L1 mutant [FKHR-L1(A3):ER*]. Tamoxifen-mediated activation of FKHR-L1(A3):ER* resulted in a striking increase in p27(KIP1) promoter activity and mRNA and protein levels as well as induction of the apoptotic program. The level of p27(KIP1) appears to be critical in the regulation of cell survival since mere ectopic expression of p27(KIP1) was sufficient to induce Ba/F3 apoptosis. Moreover, cell survival was increased in cytokine-starved bone marrow-derived stem cells from p27(KIP1) null-mutant mice compared to that in cells from wild-type mice. Taken together, these observations indicate that inhibition of p27(KIP1) transcription through PI3K-induced FKHR-L1 phosphorylation provides a novel mechanism of regulating cytokine-mediated survival and proliferation.

Forkhead transcription factors are critical effectors of cell death and cell cycle arrest downstream of PTEN

PTEN acts as a tumor suppressor, at least in part, by antagonizing phosphoinositide 3-kinase (PI3K)/Akt signaling. Here we show that Forkhead transcription factors FKHRL1 and FKHR, substrates of the Akt kinase, are aberrantly localized to the cytoplasm and cannot activate transcription in PTEN-deficient cells. Restoration of PTEN function restores FKHR to the nucleus and restores transcriptional activation. Expression of a constitutively active form of FKHR that cannot be phosphorylated by Akt produces the same effect as reconstitution of PTEN on PTEN-deficient tumor cells. Specifically, activated FKHR induces apoptosis in cells that undergo PTEN-mediated cell death and induces G(1) arrest in cells that undergo PTEN-mediated cell cycle arrest. Furthermore, both PTEN and constitutively active FKHR induce p27(KIP1) protein but not p21. These data suggest that Forkhead transcription factors are critical effectors of PTEN-mediated tumor suppression.

Alternative splicing of the human PTEN/MMAC1/TEP1 gene

The human tumour suppressor gene PTEN/MMAC1/TEP1 encodes a lipid and protein phosphatase. Using RT-PCR, alternatively spliced forms of PTEN mRNA, encoding full-length PTEN and two forms of the protein truncated at the C-terminal end, were detected in normal human tissue. Cultured tumour and non-tumour cell lines show similar splicing patterns.

Regulation of angiogenesis and tumorigenesis by signal transduction cascades: lessons from benign and malignant endothelial tumors

Oncogenes and tumor suppressor genes are implicated in the regulation of the angiogenic switch. Much of the data accumulated to date uses NIH 3T3 cells, which are deficient in the tumor suppressor gene p16, as models for these studies. We have used a novel system, derived by sequential introduction of a temperature-sensitive SV40 large T antigen and oncogenic H-ras, to study the angiogenic switch. The results from our studies differ from those using NIH3T3 cells, but have been confirmed by multiple other groups. The data from all of these studies suggest that there is synergy between inactivation of the p53 tumor suppressor gene and activation of the phosphoinositol-3-kinase pathway (PI-3-K), as well as synergy between inactivation of the p16 tumor suppressor gene and activation of the MAP kinase pathway. These findings suggest that there are predictable behaviors of tumors that may be assessed by the status of p53 or p16 in a biopsy, and that these predictable changes in signal transduction may be useful both prognostically and in the design of rationally based drug therapy of benign and malignant tumors.

Loss of PTEN expression leading to high Akt activation in human multiple myelomas

Mouse plasma cell tumor (PCT) and human multiple myeloma (MM) are terminal B-cell malignancies sharing many similarities. Our recent work demonstrated that activation of the insulin-like growth factor receptor (IGF-IR)/insulin receptor substrate (IRS)/phosphatidylinositol 3′ kinase (PI 3′K) pathway was evident in the tumor lines derived from both species. Although PI 3′K activity was higher in mouse tumor lines than that in human tumors, activation of Akt serine/threonine kinase was markedly lower in mouse lines. This discrepancy prompted us to test the status of PTEN tumor suppressor gene, as it has been shown to be a negative regulator of PI 3′K activity. Although all the mouse lines expressed intact PTEN, 2 of the 4 human lines (Δ47 and OPM2) possessing the highest Akt activity lost PTEN expression. Sequencing analysis demonstrated that the PTEN gene contains a deletion spacing from exon 3 to exon 5 or 6 in the Δ47 line and from exon 3 to 7 in the OPM2 line. Restoration of PTEN expression suppressed IGF-I–induced Akt activity, suggesting that loss of PTEN is responsible for uncontrolled Akt activity in these 2 lines. Despite the expression of PTEN with the concomitant low Akt activity in all mouse PCT lines, their p70S6K activities were generally higher than those in 3 human MM lines, arguing for specific negative regulation of Akt, but not p70S6K by PTEN. These results suggest that p70S6K and Akt may be differentially used by the plasma cell tumors derived from mice and humans, respectively.

Loss of PTEN expression leading to high Akt activation in human multiple myelomas

Mouse plasma cell tumor (PCT) and human multiple myeloma (MM) are terminal B-cell malignancies sharing many similarities. Our recent work demonstrated that activation of the insulin-like growth factor receptor (IGF-IR)/insulin receptor substrate (IRS)/phosphatidylinositol 3′ kinase (PI 3′K) pathway was evident in the tumor lines derived from both species. Although PI 3′K activity was higher in mouse tumor lines than that in human tumors, activation of Akt serine/threonine kinase was markedly lower in mouse lines. This discrepancy prompted us to test the status of PTEN tumor suppressor gene, as it has been shown to be a negative regulator of PI 3′K activity. Although all the mouse lines expressed intact PTEN, 2 of the 4 human lines (Δ47 and OPM2) possessing the highest Akt activity lost PTEN expression. Sequencing analysis demonstrated that the PTEN gene contains a deletion spacing from exon 3 to exon 5 or 6 in the Δ47 line and from exon 3 to 7 in the OPM2 line. Restoration of PTEN expression suppressed IGF-I–induced Akt activity, suggesting that loss of PTEN is responsible for uncontrolled Akt activity in these 2 lines. Despite the expression of PTEN with the concomitant low Akt activity in all mouse PCT lines, their p70S6K activities were generally higher than those in 3 human MM lines, arguing for specific negative regulation of Akt, but not p70S6K by PTEN. These results suggest that p70S6K and Akt may be differentially used by the plasma cell tumors derived from mice and humans, respectively.

Identification of a novel PTEN intronic deletion in Li-Fraumeni syndrome and its effect on RNA processing

Germline mutations of the TP53 tumor suppressor gene account for the predisposition to cancer observed in many Li-Fraumeni syndrome (LFS) families. A causative genetic factor in the remaining families that harbor no TP53 mutations remains to be elucidated. The PTEN phosphatase tumor suppressor gene is mutated in human cancers observed in LFS. There also exists some phenotypic overlap in the occurrence of cancers in LFS and Cowden's disease (CD), for which germline PTEN mutations are believed to be responsible. We hypothesized that PTEN may be altered in some TP53-wild-type LFS families. We examined LFS primary patient lymphocytes for PTEN alterations using SSCP and sequence analysis. A novel intronic deletion was found in two unrelated individuals, adjacent to the splice acceptor site of PTEN exon 4. Based on an in vitro mRNA processing assay this alteration is predicted to be a polymorphism. The in vivo effects of this proximal splice site deletion are unknown and a genetic cause for the cancers in these families remains to be elucidated. Germline mutations of PTEN were not detected in other families, suggesting that alterations of this tumor suppressor gene do not account for the cancers observed in the subset of LFS individuals with wild-type germline TP53.

Distinct roles for PI3K in proliferation and survival of oligodendrocyte progenitor cells

Phosphoinositol 3-kinase (PI3K) is a downstream effector for multiple ligand-activated receptors and modulates cell responses through activation of its target protein kinase B (Akt). We examined the roles of PI3K-Akt signaling in a primary glial (oligodendrocyte) progenitor cell culture system that is ligand-dependent for cell proliferation, survival, and prevention of differentiation. We demonstrate that PI3K and Akt (Ser-473 phosphorylation) are activated in response to platelet-derived growth factor but not basic fibroblast growth factor-2 (FGF2) and that distinct forms of PI3K are activated in early progenitors and later-maturation pro-oligodendroblasts as identified by their sensitivity to wortmannin. By establishing conditions to examine effects on cell proliferation and survival independently, we demonstrate that PI3K is necessary for a full mitogenic response and that PI3K is also necessary for early progenitor survival. Our results therefore demonstrate that PI3K-Akt signaling independently regulates proliferation and survival, that the form of PI3K is distinct in early progenitors and pro-oligodendroblasts, and that FGF2 does not activate this pathway in either primary glial cell population.

Regulation of death receptor-mediated apoptosis pathways

Apoptosis or programmed cell death can be induced by a variety of stimuli including activation of death receptors. This subgroup of the TNF/NGF-receptor-superfamily activates caspases, a family of aspartyl-specific cysteine-proteases, which are the main executioners of apoptosis. Depending on the cell type, signalling pathways downstream of the death receptors can be modulated by different proteins such as Bcl-2, FLIPs, chaperones and kinases. Deregulation of apoptosis has been associated with diseases as cancer, autoimmunity and AIDS. Therefore, the identification of modulators of apoptosis has several therapeutic implications.

Akt-mediated survival of oligodendrocytes induced by neuregulins

Neuregulins have been implicated in a number of events in cells in the oligodendrocyte lineage, including enhanced survival, mitosis, migration, and differentiation. At least two signaling pathways have been shown to be involved in neuregulin signaling: the phosphatidylinositol (PI)-3 kinase and the mitogen-activated protein kinase pathways. In the present studies, we examined the signaling pathway involved in the survival function of heregulin, focusing on heregulin-induced changes in Akt activity in cultured glial cells, and the consequences of Akt activation in cells in the oligodendrocyte lineage. Heregulin binds erbB receptors, and in our studies, primary cultures of both oligodendrocyte progenitor cells and differentiating oligodendrocytes expressed erbB2, erbB3, and erbB4 receptors. In C6 glioma cells and primary cultures of oligodendrocytes, heregulin induced time- and dose-dependent Akt phosphorylation at Ser(473) in a wortmannin-sensitive manner. To investigate further the signaling pathway for heregulin in glial cells, BAD was overexpressed in C6 glioma cells. In these cells, heregulin induced phosphorylation of BAD at Ser(136). Apoptosis of oligodendrocyte progenitor cells induced by growth factor deprivation was effectively blocked by heregulin in a wortmannin-sensitive manner. Overexpression of dominant negative Akt but not of wild-type Akt by adenoviral gene transfer in primary cultures of both oligodendrocytes and their progenitors induced significant apoptosis through activation of the caspase cascade. The present data suggest that the survival function of heregulin is mediated through the PI-3 kinase/Akt pathway in cells in the oligodendrocyte lineage and that the Akt pathway may be quite important for survival of cells in this lineage.

Analysis of the PTEN gene mutation in polyposis syndromes and sporadic gastrointestinal tumors in Japanese patients

PURPOSE

PTEN is a candidate tumor suppressor gene for mutations which are responsible for Cowden disease. Recently, it has been shown that PTEN is mutated in several human neoplasms. To investigate the role of PTEN in tumorigenesis, we screened its mutation in Japanese patients with gastrointestinal polyposis and various sporadic tumors.

METHODS

The entire coding region of PTEN was screened by single strand conformational polymorphism or direct sequencing for somatic mutations in 16 gingival papillomas, 4 juvenile polyps, 10 esophageal papillomas, and 20 colorectal cancers and for germline mutations in three patients with Cowden disease (including one with Lhermitte-Duclos disease) and one patient each with juvenile polyposis syndrome, Turcot's syndrome, and Cronkhite-Canada syndrome.

RESULTS

Germline mutations were found in all cases of Cowden disease. One mutation was a nonsense mutation at codon 130 (CGA-->TGA), and the other two were splice site mutations at the 5' site of intron 4 and the 3' site of intron 8. We could not detect germline mutations in other patients with gastrointestinal polyposis or somatic mutations in sporadic tumors.

CONCLUSIONS

We confirmed previous reports that germline mutations in PTEN are responsible for Cowden disease. However, somatic mutations of PTEN may not play a major role in tumorigenesis, at least in colorectal cancers, esophageal papillomas and gingival papillomas.

Hepatocyte growth factor promotes cell survival from fas-mediated cell death in hepatocellular carcinoma cells via Akt activation and Fas-death-inducing signaling complex suppression

The Akt/PI-3 kinase pathway is a system essential for cell survival. In the current study, we showed that hepatocyte growth factor (HGF) activates the Akt/PI-3 kinase pathway to suppress Fas-mediated cell death in human hepatocellular carcinoma (HCC; 3 lines; SK-Hep1, HLE, and Chang Liver cell lines), hepatoblastoma (1 line; HepG2), and embryonic hepatocyte (1 line; WRL). Five tested cell lines showed the resistance to Fas-mediated cell death by the pretreatment of HGF. This HGF-induced cell survival was suppressed by wortmannin (Akt/PI-3 kinase pathway inhibitor), suggesting an involvement of Akt. When cells were pretreated with HGF, Fas-mediated cell death was suppressed, followed by Akt phosphorylation at Ser473. Fas-death-inducing signaling complex (DISC) formation, especially FADD and caspase 8 interaction, was suppressed by HGF and the suppression of the Akt/PI-3 kinase pathway by transient expression of PTEN, resulting in acquisition of Fas-DISC formation and Fas-mediated cell death in HGF-treated cells. We suggest that HGF promotes cell survival in hepatocyte-derived cell lines (HCC, hepatoblastoma, and embryonic hepatocyte) from Fas-mediated cell death via Fas-DISC suppression as a result of Akt activation.

The Akt/protein kinase B-dependent anti-apoptotic pathway and the mitogen-activated protein kinase cascade are alternatively activated in human glioblastoma multiforme

We investigated the activation of two important signal transduction pathways in human glioblastoma cells and found a constitutive phosphorylation of either Akt or mitogen-activated protein kinase (MAPK) under serum free conditions. In all but one cell line Wortmannin-sensitive activation of Akt could be attributed to the loss of functional PTEN protein. All cell lines with Akt activation exhibited only weak phosphorylation of the MAPK signal pathway, whereas those without constitutive Akt activation demonstrated high levels of phosphorylated MAPK under serum free conditions. Our data might indicate the presence of two functional subtypes of glioblastoma multiforme, since Akt and MAPK are involved in cellular survival and proliferation signalling, respectively.

Novel strategies and therapeutics for the treatment of prostate carcinoma

BACKGROUND

An increased understanding of the biology of prostate carcinoma has led to the clinical evaluation of mechanism-based and targeted therapies. Modulating the immune system has been pursued through the use of both active and passive immunity as well as the ex vivo genetic manipulation of effector cells. A variety of gene therapies has been proposed not only to replace defective genes but to localize activation of prodrugs. Angiogenesis and tumor invasion also have been targeted, as have cell cycling and signal transduction. Strategies promoting apoptosis and augmenting differentiation are also under study.

METHODS

This study is a review of current clinical strategies using biologic, immunologic, and genetic approaches for the treatment of prostate carcinoma.

RESULTS

The clinical development of therapy targeting differentiation, apoptosis, cell signaling, angiogenesis, metastasis, immune surveillance, and others are in various stages of clinical development. A disease states model is used to discuss treatment groups, outcome measures, and other trial design elements in relation to specific therapeutic strategies.

CONCLUSIONS

Development of novel agents requires consideration of where in the natural history of the disease they should be applied. In addition, understanding the genetic and molecular alterations that occur as the disease progresses from a localized to a metastatic state, and from androgen dependence to independence, is necessary. Clinical trial design will require consideration of cytostatic and cytotoxic effects, the status of pathways not directly targeted, and potentially unexpected influences on prostate specific antigen expression by these agents.

Detection of PTEN nonsense mutation and psiPTEN expression in central nervous system high-grade astrocytic tumors by a yeast-based stop codon assay

We have developed a new yeast-based assay for the detection of PTEN nonsense mutation, and applied it to a total of 42 astrocytic tumors. The assay utilizes homologous recombination of PCR-amplified PTEN cDNA samples to a yeast vector which expresses an in-frame PTEN::ADE2 chimera protein. An allele of nonsense mutation in the sample PTEN mRNA gives a truncated chimera protein in a yeast cell, resulting in the formation of a red colony. The assay and subsequent sequence analysis demonstrated nonsense mutations as red colonies of more than 10% in one of 10 anaplastic astrocytomas and six of 18 glioblastomas, but none in six pilocytic astrocytomas or in eight astrocytomas. Sequence analysis of white colonies showed one missense mutation in a glioblastoma. Interestingly, four of seven nonsense mutations were frame-shifts due to exon skipping. In addition, pink colonies were found in one of six pilocytic astrocytomas, three of eight astrocytomas, two of 10 anaplastic astrocytomas, and 10 of 18 glioblastomas. Sequence analysis of the pink colonies revealed a sequence similar to those reported as psiPTEN/PTH2. By testing mRNA and genomic DNA, it was found to be a processed pseudogene which was transcribed. The psiPTEN expression was complementary to PTEN mutation, for 14 of 18 glioblastomas showed either PTEN mutation or psiPTEN expression and only one case showed both PTEN mutation and psiPTEN expression (P<0.046), suggesting a pathological role of psiPTEN expression as an alternative to PTEN mutation in glioblastomas.

Deficiency of PTEN in Jurkat T cells causes constitutive localization of Itk to the plasma membrane and hyperresponsiveness to CD3 stimulation

Pleckstrin homology (PH) domain binding to D3-phosphorylated phosphatidylinositides (PI) provides a reversible means of recruiting proteins to the plasma membrane, with the resultant change in subcellular localization playing a key role in the activation of multiple intracellular signaling pathways. Previously we found that the T-cell-specific PH domain-containing kinase Itk is constitutively membrane associated in Jurkat T cells. This distribution was unexpected given that the closely related B-cell kinase, Btk, is almost exclusively cytosolic. In addition to constitutive membrane association of Itk, unstimulated JTAg T cells also exhibited constitutive phosphorylation of Akt on Ser-473, an indication of elevated basal levels of the phosphatidylinositol 3-kinase (PI3K) products PI-3,4-P(2) and PI-3,4,5-P(3) in the plasma membrane. Here we describe a defect in expression of the D3 phosphoinositide phosphatase, PTEN, in Jurkat and JTAg T cells that leads to unregulated PH domain interactions with the plasma membrane. Inhibition of D3 phosphorylation by PI3K inhibitors, or by expression of PTEN, blocked constitutive phosphorylation of Akt on Ser-473 and caused Itk to redistribute to the cytosol. The PTEN-deficient cells were also hyperresponsive to T-cell receptor (TCR) stimulation, as measured by Itk kinase activity, tyrosine phosphorylation of phospholipase C-gamma1, and activation of Erk compared to those in PTEN-replete cells. These data support the idea that PH domain-mediated association with the plasma membrane is required for Itk activation, provide evidence for a negative regulatory role of PTEN in TCR stimulation, and suggest that signaling models based on results from Jurkat T-cell lines may underestimate the role of PI3K in TCR signaling.

Profiling the downstream genes of tumor suppressor PTEN in lung cancer cells by complementary DNA microarray

The phosphatase and tensin homology deleted on chromosome 10 (PTEN) is a tumor suppressor gene with sequence homology to tyrosine phosphatases and the cytoskeletal proteins tensin and auxilin. PTEN has recently been shown to inhibit cell migration and the spreading and formation of focal adhesions. This study investigated the role of PTEN in carcinoma invasion in a lung-cancer cell line and examined the downstream genes regulated by PTEN. We have previously established a cell-line model in human lung adenocarcinoma with different invasive abilities and metastatic potentials. Examining PTEN gene expression in these cell lines, we found that a homozygous deletion in exon 5 is associated with high invasive ability. We then constructed stable constitutive and inducible wild-type PTEN-overexpressed transfectants in the highly invasive cell line CL(1-5). We found that an overexpression of PTEN can inhibit invasion in lung cancer cells. To further explore the downstream genes regulated by PTEN, a high-density complementary DNA (cDNA) microarray technique was used to profile gene changes after PTEN overexpression. Our results indicate a panel of genes that can be modulated by PTEN. PTEN overexpression downregulated genes, including integrin alpha(6), laminin beta(3), heparin-binding epidermal growth factor-like growth factor, urokinase-type plasminogen activator, myb protein B, Akt2, and some expressed sequence tag (EST) clones. In contrast, PTEN overexpression upregulated protein phosphatase 2A1B, ubiquitin protease (unph), secreted phosphoprotein 1, leukocyte elastase inhibitor, nuclear factor-kappaB, cyclic adenosine monophosphate response element binding protein, DNA ligase 1, heat shock protein 90, and some EST genes. Northern hybridization and flow cytometry analysis also confirmed that PTEN overexpression results in the reduced expression of the integrin alpha(6) subunit. The results of this study indicate that PTEN overexpression may inhibit lung cancer invasion by downregulation of a panel of genes including integrin alpha(6). The cDNA microarray technique may be an effective tool to study the downstream function of a tumor suppressor gene.