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

Motif analysis of the tumor suppressor gene MMAC/PTEN identifies tyrosines critical for tumor suppression and lipid phosphatase activity

The tumor suppressor gene, MMAC/PTEN, has phosphatase, C2, and PDZ-binding domains as well as potential sites of regulation by phosphorylation, including tyrosine phosphorylation, which may contribute to its ability to modulate cell growth and viability. Several obvious and significant motifs were found in MMAC/PTEN, including most notably, a catalytic domain of tyrosine phosphatase (IHCxxGxxRS/T) and several potential tyrosine phosphorylation sites. To examine the functional significance of tyrosine phosphorylation of MMAC/PTEN, retroviral constructs were generated with mutations at two putative tyrosine phosphorylation sites (Y240A/Y240F and Y315A/Y315F). Stable expression of wild-type MMAC/PTEN in U251 human glioma cells (which do not normally produce a functional MMAC/PTEN gene product) resulted in a significant reduction of tumor growth in nude mice, decreased growth rate, saturation density, and colony formation in vitro, as well as dephosphorylation of D3-phosphorylated phosphatidylinositols (PtdIns) in vitro. Mutation of Y240 or Y315 to either alanine or phenylalanine abrogated the ability of MMAC/PTEN to alter growth rate, saturation density, and colony formation in vitro. The ability of MMAC/PTEN to limit tumor growth in nude mice was markedly decreased but not abrogated by mutation of Y240 or Y315 to alanine. Thus, Y240 and Y315 are required for MMAC/PTEN to decrease tumor growth in vitro and in vivo. In contrast to wild-type MMAC/PTEN, mutant MMAC/PTEN containing Y240A or Y315A was unable to dephosphorylate D3-phosphorylated PtdIns in vitro. Thus, Y240A and Y315A are involved in the ability of MMAC/PTEN to dephosphorylate PtdIns and regulate tumor cell growth in vitro and in vivo.

Platelet-derived growth factor-BB-mediated glycosaminoglycan synthesis is transduced through Akt

Previously we have demonstrated that the phosphoinositide 3-kinase (PI-3K) signal-transduction pathway mediates platelet-derived growth factor (PDGF)-BB-induced glycosaminoglycan (GAG) synthesis in fetal lung fibroblasts. In the present study we further investigated the signal-transduction pathway(s) that results in PDGF-BB-induced GAG synthesis. Over-expression of a soluble PDGF beta-receptor as well as a mutated form of the beta-receptor, unable to bind PI-3K, diminished GAG synthesis in fetal lung fibroblasts subsequent to PDGF-BB stimulation. The PI-3K inhibitor wortmannin blocked PDGF-BB-induced Akt activity as well as significantly diminishing PDGF-BB-mediated GAG synthesis. Expression of dominant-negative PI-3K also abrogated Akt activity and GAG synthesis. Furthermore, expression of dominant-negative Akt abrogated endogenous Akt activity, Rab3D phosphorylation and GAG synthesis, whereas expression of constitutively activated Akt stimulated Rab3D phosphorylation and GAG synthesis in the absence of PDGF-BB. Over-expression of wild-type PTEN (phosphatase and tensin homologue deleted in chromosome 10) inhibited Akt activity and concomitantly attenuated GAG synthesis in fibroblasts stimulated with PDGF-BB. These data suggest that Akt is an integral protein involved in PDGF-BB-mediated GAG regulation in fetal lung fibroblasts.

PTEN augments staurosporine-induced apoptosis in PTEN-null Ishikawa cells by downregulating PI3K/Akt signaling pathway

Staurosporine is a potent apoptosis inducer, but its mechanism remains to be clarified. We investigated the involvement of PTEN in staurosporine-induced apoptosis. Ishikawa cells, from an endometrial carcinoma cell line, expressed a high amount of PTEN mRNA but did not express the PTEN protein because of protein truncations. We isolated clones expressing the steady-state level of the PTEN protein from PTEN-null Ishikawa cells by transfection. The obtained clones showed reduced proliferative activity and reduced anchorage-independent cell growth with the augmented p27(Kip1). These cell lines were sensitized to apoptosis by staurosporine. A low concentration of UCN-01 did not affect apoptosis, but a high concentration augmented apoptosis in the PTEN-expressing clone. Alpha-sphingosine and H-7 did not affect apoptosis in these cell lines. PI3K inhibition augmented staurosporine-induced apoptosis in the parental cell line, but not in the PTEN-expressing clone. In the clone, phosho-Akt/PKB and phospho-Bad (Ser-136) were downregulated. Staurosporine reduced the levels of phospho-Akt/PKB and phospho-Bad (Ser-136) in all the cell lines, but the reduction was most significant in the PTEN-expressing clone. These results suggest that inhibition of the PI3K/Akt/PKB signaling pathway might be associated with staurosporine-induced apoptosis in Ishikawa cells.

The Raf/MEK/ERK signal transduction cascade as a target for chemotherapeutic intervention in leukemia

The Raf/MEK/ERK (MAPK) signal transduction cascade is a vital mediator of a number of cellular fates including growth, proliferation and survival, among others. The focus of this review centers on the MAPK signal transduction pathway, its mechanisms of activation, downstream mediators of signaling, and the transcription factors that ultimately alter gene expression. Furthermore, negative regulators of this cascade, including phosphatases, are discussed with an emphasis placed upon chemotherapeutic intervention at various points along the pathway. In addition, mounting evidence suggests that the PI3K/Akt pathway may play a role in the effects elicited via MAPK signaling; as such, potential interactions and their possible cellular ramifications are discussed.

Protean PTEN: form and function

Germline mutations distributed across the PTEN tumor-suppressor gene have been found to result in a wide spectrum of phenotypic features. Originally shown to be a major susceptibility gene for both Cowden syndrome (CS), which is characterized by multiple hamartomas and an increased risk of breast, thyroid, and endometrial cancers, and Bannayan-Riley-Ruvalcaba syndrome, which is characterized by lipomatosis, macrocephaly, and speckled penis, the PTEN hamartoma tumor syndrome spectrum has broadened to include Proteus syndrome and Proteus-like syndromes. Exon 5, which encodes the core motif, is a hotspot for mutations likely due to the biology of the protein. PTEN is a major lipid 3-phosphatase, which signals down the PI3 kinase/AKT pro-apoptotic pathway. Furthermore, PTEN is a protein phosphatase, with the ability to dephosphorylate both serine and threonine residues. The protein-phosphatase activity has also been shown to regulate various cell-survival pathways, such as the mitogen-activated kinase (MAPK) pathway. Although it is well established that PTEN's lipid-phosphatase activity, via the PI3K/AKT pathway, mediates growth suppression, there is accumulating evidence that the protein-phosphatase/MAPK pathway is equally important in the mediation of growth arrest and other crucial cellular functions.

Farnesyl thiosalicylic acid inhibits the growth of melanoma cells through a combination of cytostatic and pro-apoptotic effects

Novel classes of drug that interfere with the signalling of the small G-protein Ras, the so-called Ras antagonists, are showing much promise as novel anti-cancer agents. In this study, we demonstrate that the novel Ras antagonist farnesylthiosalicylic acid (FTS) inhibits the growth of Colo 853 melanoma cells through a combination of cytostatic and pro-apoptotic effects. Furthermore, these phenomena are seen under conditions of cell attachment and in the presence of serum. Treatment of Colo 853 cells with FTS led to time-dependent inhibition of constitutive Akt, retinoblastoma protein (pRB) and ERK activity, with a concurrent loss of Akt expression. Inhibition of Akt and ERK activity induces apoptosis in other human cancer cell lines. Here it is demonstrated that inhibition of Akt, or ERK and Akt in combination, leads to cell cycle arrest but not apoptosis in melanoma cells. FTS treatment was also found to upregulate activity of the stress-activated p38 MAP kinase. Inhibition of p38 MAP kinase, using the selective inhibitor SB 203580, followed by FTS treatment, significantly increased the proportion of apoptotic cells after 72 hr, possibly suggesting a modulatory role for p38 MAP kinase in FTS-induced melanoma cell apoptosis.

PTEN: The down side of PI 3-kinase signalling

The PTEN tumour suppressor protein is a phosphoinositide 3-phosphatase that, by metabolising phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)), acts in direct antagonism to growth factor stimulated PI 3-kinases. A wealth of data has now illuminated pathways that can be controlled by PTEN through PtdIns(3,4,5)P(3), some of which, when deregulated, give a selective advantage to tumour cells. Early studies of PTEN showed that its activity was able to promote cell cycle arrest and apoptosis and inhibit cell motility, but more recent data have identified other functional consequences of PTEN action, such as effects on the regulation of angiogenesis. The structure of PTEN includes several features not seen in related protein phosphatases, which adapt the enzyme to act efficiently as a lipid phosphatase, including a C2 domain tightly associated with the phosphatase domain, and a broader and deeper active site pocket. Several pieces of data indicate that PTEN is a principal regulator of the cellular levels of PtdIns(3,4,5)P(3), but work is only just beginning to uncover mechanisms by which the cellular activity of PTEN can be controlled. There also remains the vexing question of whether any of PTEN's cellular functions reflect its evolutionary roots as a member of the protein tyrosine phosphatase superfamily.

Specific inhibition of PTEN expression reverses hyperglycemia in diabetic mice

Signaling through the phosphatidylinositol 3'-kinase (PI3K) pathway is crucial for metabolic responses to insulin, and defects in PI3K signaling have been demonstrated in type 2 diabetes. PTEN (MMAC1) is a lipid/protein phosphatase that can negatively regulate the PI3K pathway by dephosphorylating phosphatidylinositol (3,4,5)-triphosphate, but it is unclear whether PTEN is physiologically relevant to insulin signaling in vivo. We employed an antisense oligonucleotide (ASO) strategy in an effort to specifically inhibit the expression of PTEN. Transfection of cells in culture with ASO targeting PTEN reduced PTEN mRNA and protein levels and increased insulin-stimulated Akt phosphorylation in alpha-mouse liver-12 (AML12) cells. Systemic administration of PTEN ASO once a week in mice suppressed PTEN mRNA and protein expression in liver and fat by up to 90 and 75%, respectively, and normalized blood glucose concentrations in db/db and ob/ob mice. Inhibition of PTEN expression also dramatically reduced insulin concentrations in ob/ob mice, improved the performance of db/db mice during insulin tolerance tests, and increased Akt phosphorylation in liver in response to insulin. These results suggest that PTEN plays a significant role in regulating glucose metabolism in vivo by negatively regulating insulin signaling.

Regulation of cell polarity during eukaryotic chemotaxis: the chemotactic compass

Phosphatidylinositol 3-kinase lipid products and the Rho GTPases play a central role in transmitting information from chemotactic receptors to the effectors of cell polarity, and recent advances in the field have allowed us to understand these roles more clearly. Emergent properties of positive and negative regulation of these molecules may account for the establishment of cell polarity during chemotaxis for a wide range of cells from Dictyostelium to fibroblasts to neutrophils.

Control of cell cycle exit and entry by protein kinase B-regulated forkhead transcription factors

AFX-like Forkhead transcription factors, which are controlled by phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signaling, are involved in regulating cell cycle progression and cell death. Both cell cycle arrest and induction of apoptosis are mediated in part by transcriptional regulation of p27(kip1). Here we show that the Forkheads AFX (FOXO4) and FKHR-L1 (FOXO3a) also directly control transcription of the retinoblastoma-like p130 protein and cause upregulation of p130 protein expression. Detailed analysis of p130 regulation demonstrates that following Forkhead-induced cell cycle arrest, cells enter G(0) and become quiescent. This is shown by a change in phosphorylation of p130 to G(0)-specific forms and increased p130/E2F-4 complex formation. Most importantly, long-term Forkhead activation causes a sustained but reversible inhibition of proliferation without a marked increase in apoptosis. As for the activity of the Forkheads, we also show that protein levels of p130 are controlled by endogenous PI3K/PKB signaling upon cell cycle reentry. Surprisingly, not only nontransformed cells, but also cancer cells such as human colon carcinoma cells, are forced into quiescence by Forkhead activation. We therefore propose that Forkhead inactivation by PKB signaling in quiescent cells is a crucial step in cell cycle reentry and contributes to the processes of transformation and regeneration.

DNA mismatch repair deficiency accelerates endometrial tumorigenesis in Pten heterozygous mice

PTEN mutation and microsatellite instability are two of the most common genetic alterations in uterine endometrioid carcinoma. Furthermore, previous studies have suggested an association between the two alterations, however the basis and consequence of the association is not understood. Recently it has been shown that 100% of female Pten(+/-) mice develop complex atypical hyperplasia by 32 weeks of age that progresses to endometrial carcinoma in approximately 20 to 25% of mice at 40 weeks. In an attempt to expand this mouse model of endometrial tumorigenesis and to further our understanding of the association betweenPten mutations and DNA mismatch repair deficiency, we generated Ptenheterozygous, Mlh1-null (mismatch repair deficient) mice. Significantly, the majority ofPten(+/-)/Mlh1(-/-)mice developed polypoid lesions in the endometrium at 6 to 9 weeks of age. By 14 to 18 weeks, all of the double-mutant mice had lesions histologically similar to those seen inPten(+/-) mice, and two of them exhibited invasive disease. Moreover, the frequency of loss of the wild-type Pten allele in the double-mutant mice at 14 to 18 weeks was similar to that seen in lesions from 40-week-old Pten(+/-) mice. Taken together, our results indicate that DNA mismatch repair deficiency can accelerate endometrial tumorigenesis inPten heterozygous mice and suggests that loss of the wild-type Pten allele is involved in the development/progression of tumors in this setting.

PTEN modulates vascular endothelial growth factor-mediated signaling and angiogenic effects

Phosphatidylinositol 3-kinase is activated by vascular endothelial growth factor (VEGF), and many of the angiogenic cellular responses of VEGF are regulated by the lipid products of phosphatidylinositol 3-kinase. The tumor suppressor PTEN has been shown to down-regulate phosphatidylinositol 3-kinase signaling, yet the effects of PTEN on VEGF-mediated signaling and angiogenesis are unknown. Inhibition of endogenous PTEN in cultured endothelial cells by adenovirus-mediated overexpression of a dominant negative PTEN mutant (PTEN-C/S) enhanced VEGF-mediated Akt phosphorylation, and this effect correlated with decreases in caspase-3 cleavage, caspase-3 activity, and DNA degradation after induction of apoptosis with tumor necrosis factor-alpha. Overexpression of PTEN-C/S also enhanced VEGF-mediated endothelial cell proliferation and migration. In contrast, overexpression of wild-type PTEN inhibited the anti-apoptotic, proliferative, and chemotactic effects of VEGF. Moreover, PTEN-C/S increased the length of vascular sprouts in the rat aortic ring assay and modulated VEGF-mediated tube formation in an in vitro angiogenesis assay, whereas PTEN-wild type inhibited these effects. Taken together, these findings demonstrate that PTEN potently modulates VEGF-mediated signaling and function and that PTEN is a viable target in therapeutic approaches to promote or inhibit angiogenesis.

PTEN blocks tumor necrosis factor-induced NF-kappa B-dependent transcription by inhibiting the transactivation potential of the p65 subunit

PTEN is a lipid phosphatase responsible for down-regulating the phosphoinositide 3-kinase product phosphatidylinositol 3,4,5-triphosphate. Phosphatidylinositol 3,4,5-triphosphate is involved in the activation of the anti-apoptotic effector target, Akt. Although the Akt pathway has been implicated in regulating NF-kappaB activity, it is controversial as to whether Akt activates NF-kappaB predominantly through mechanisms that regulate nuclear translocation or transactivation potential. In this report, we utilized PTEN as a natural biological inhibitor of Akt activity to study the effects on tumor necrosis factor (TNF)-induced activation of NF-kappaB. We found that the reintroduction of PTEN into prostate cells inhibited TNF-stimulated NF-kappaB transcriptional activity. PTEN failed to block TNF-induced IKK activation, IkappaBalpha degradation, p105 processing, p65 (RelA) nuclear translocation, and DNA binding of NF-kappaB. However, PTEN inhibited NF-kappaB-dependent transcription by blocking the ability of TNF to stimulate the transactivation domain of the p65 subunit. PTEN also inhibited the transactivation potential of the cyclic AMP-response element-binding protein, but this was not observed for c-Jun. The transactivation potential of p65 following TNF stimulation could be rescued from PTEN-dependent repression by re-introducing expression constructs encoding activated forms of phosphoinositide 3-kinase, Akt, or Akt and IKK. The ability of PTEN to inhibit the TNF-induced transactivation function of p65 is important, because expression of PTEN blocked TNF-stimulated NF-kappaB-dependent gene expression, thus sensitizing cells to TNF-induced apoptosis. Maintenance of the PTEN tumor suppressor protein is therefore required to modulate Akt activity and to concomitantly control the transcriptional activity of the anti-apoptotic transcription factor NF-kappaB.

PTEN and myotubularin: novel phosphoinositide phosphatases

Protein tyrosine phosphatases (PTPs) are a diverse group of enzymes that contain a highly conserved active site motif, Cys-x5-Arg (Cx5R). The PTP superfamily enzymes, which include tyrosine-specific, dual specificity, low-molecular-weight, and Cdc25 phosphatases, are key mediators of a wide variety of cellular processes, including growth, metabolism, differentiation, motility, and programmed cell death. The PTEN/MMAC1/TEP1 gene was originally identified as a candidate tumor suppressor gene located on human chromosome 10q23; it encodes a protein with sequence similarity to PTPs and tensin. Recent studies have demonstrated that PTEN plays an essential role in regulating signaling pathways involved in cell growth and apoptosis, and mutations in the PTEN gene are now known to cause tumorigenesis in a number of human tissues. In addition, germ line mutations in the PTEN gene also play a major role in the development of Cowden and Bannayan-Zonana syndromes, in which patients often suffer from increased risk of breast and thyroid cancers. Biochemical studies of the PTEN phosphatase have revealed a molecular mechanism by which tumorigenesis may be caused in individuals with PTEN mutations. Unlike most members of the PTP superfamily, PTEN utilizes the phosphoinositide second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP3), as its physiologic substrate. This inositol lipid is an important regulator of cell growth and survival signaling through the Ser/Thr protein kinases PDK1 and Akt. By specifically dephosphorylating the D3 position of PIP3, the PTEN tumor suppressor functions as a negative regulator of signaling processes downstream of this lipid second messenger. Mutations that impair PTEN function result in a marked increase in cellular levels of PIP3 and constitutive activation of Akt survival signaling pathways, leading to inhibition of apoptosis, hyperplasia, and tumor formation. Certain structural features of PTEN contribute to its specificity for PIP3, as well as its role(s) in regulating cellular proliferation and apoptosis. Recently, myotubularin, a second PTP superfamily enzyme associated with human disease, has also been shown to utilize a phosphoinositide as its physiologic substrate.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Living with lethal PIP3 levels: viability of flies lacking PTEN restored by a PH domain mutation in Akt/PKB

The phosphoinositide phosphatase PTEN is mutated in many human cancers. Although the role of PTEN has been studied extensively, the relative contributions of its numerous potential downstream effectors to deregulated growth and tumorigenesis remain uncertain. We provide genetic evidence in Drosophila melanogaster for the paramount importance of the protein kinase Akt [also called protein kinase B (PKB)] in mediating the effects of increased phosphatidylinositol 3,4,5-trisphosphate (PIP3) concentrations that are caused by the loss of PTEN function. A mutation in the pleckstrin homology (PH) domain of Akt that reduces its affinity for PIP3 sufficed to rescue the lethality of flies devoid of PTEN activity. Thus, Akt appears to be the only critical target activated by increased PIP3 concentrations in Drosophila.

Phosphatidylinositol (3,4,5)P3 is essential but not sufficient for protein kinase B (PKB) activation; phosphatidylinositol (3,4)P2 is required for PKB phosphorylation at Ser-473: studies using cells from SH2-containing inositol-5-phosphatase knockout mice

Using bone marrow derived mast cells from SH2-containing inositol-5-phosphatase (SHIP) +/+ and minus sign/minus sign mice, we found that the loss of SHIP leads to a dramatic increase in Steel Factor (SF)-stimulated phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)), a substantial reduction in PI(3,4)P(2), and no change in PI(4,5)P(2) levels. We also found that SF-induced activation of protein kinase B (PKB) is increased and prolonged in SHIP -/- cells, due in large part to more PKB associating with the plasma membrane in these cells. Pretreatment of SHIP -/- cells with 25 microm LY294002 resulted in complete inhibition of SF-induced PI(3,4)P(2), while still yielding PI(3,4,5)P(3) levels similar to those achieved in SHIP+/+ cells. This offered a unique opportunity to study the regulation of PKB by PI(3,4,5)P(3), in the absence of PI(3,4)P(2). Under these conditions, PKB activity was markedly reduced compared with that in SF-stimulated SHIP+/+ cells, even though more PKB localized to the plasma membrane. Although phosphoinositide-dependent kinase 1 mediated phosphorylation of PKB at Thr-308 was unaffected by LY294002, phosphorylation at Ser-473 was dramatically reduced. Moreover, intracellular delivery of PI(3,4)P(2) to LY294002-pretreated, SF-stimulated SHIP -/- cells increased phosphorylation of PKB at Ser-473 and increased PKB activity. These results are consistent with a model in which SHIP serves as a regulator of both activity and subcellular localization of PKB.

Cooperativity of Nkx3.1 and Pten loss of function in a mouse model of prostate carcinogenesis

Mouse models have provided significant insights into the molecular mechanisms of tumor suppressor gene function. Here we use mouse models of prostate carcinogenesis to demonstrate that the Nkx3.1 homeobox gene undergoes epigenetic inactivation through loss of protein expression. Loss of function of Nkx3.1 in mice cooperates with loss of function of the Pten tumor suppressor gene in cancer progression. This cooperativity results in the synergistic activation of Akt (protein kinase B), a key modulator of cell growth and survival. Our findings underscore the significance of interactions between tissue-specific regulators such as Nkx3.1 and broad-spectrum tumor suppressors such as Pten in contributing to the distinct phenotypes of different cancers.

Astrocyte inactivation of the pRb pathway predisposes mice to malignant astrocytoma development that is accelerated by PTEN mutation

We have inactivated pRb, p107, and p130 in astrocytes by transgenic expression of T(121) (a truncated SV40 T antigen) under the GFAP promoter. Founder mice died perinatally with extensive expansion of neural precursor and anaplastic astrocyte populations. In astrocytes, aberrant proliferation and extensive apoptosis were induced. Using a conditional allele of T(121), early lethality was circumvented, and adult mice developed high-grade astrocytoma, in which regions of decreased apoptosis expressed activated Akt. Indeed, astrocytoma development was accelerated in a PTEN(+/-), but not p53(+/-), background. These studies establish a highly penetrant preclinical model for astrocytoma based on events observed in the human disease and further provide insight into the role of PTEN mutation in astrocytoma progression.

Cell metabolism in the regulation of programmed cell death

Increased cellular metabolism and resistance to apoptosis are two hallmarks of cell transformation. Recent progress in the understanding of the role of mitochondria in controlling apoptosis has brought attention to the links between elements of the apoptotic machinery and cellular metabolism. Here, we review the coordinated effects of growth factor withdrawal on bioenergetics and programmed cell death, and discuss the metabolic consequences of genes that prevent apoptosis, including the BCL2 family of genes and AKT.

Phosphoinositide-specific inositol polyphosphate 5-phosphatase IV inhibits Akt/protein kinase B phosphorylation and leads to apoptotic cell death

Phosphoinositide-specific inositol polyphosphate 5- phosphatase IV has the affinity for PI(3,4,5)P(3) (K(m) = 0.65 microM) that is approximately 10-fold greater than the other inositol polyphosphate 5-phosphatases, which use this substrate including SHIP, OCRL, and 5ptase II, suggesting that it may be important in controlling intracellular levels of this metabolite. We created cell lines stably expressing the enzyme to study its effect on cell function. We found that overexpression of 5ptase IV in 293 cells caused the rapid depletion of both PI(4,5)P(2) and PI(3,4,5)P(3) in cells with corresponding increases in the products, PI(4)P and PI(3,4)P(2), changing the balance of two phosphoinositol products of phosphoinositide 3-kinase, PI(3,4)P(2) and PI(3,4,5)P(3), in the cell. One of the targets of these phosphoinositides is the serine/threonine kinase Akt, which plays an important role in the control of apoptosis. We were able to address the relative roles of PI(3,4)P(2) and PI(3,4,5)P(3) in the activation of Akt by selective depletion of these phosphoinositides in cells stably transfected with 5ptase IV and inositol polyphosphate 4-phosphatase (4ptase I). In cells transfected with 4ptase I, the level of PI(3,4)P(2) was reduced, and PI(3,4,5)P(3) was increased. Expression of the two enzymes had the opposite effect on the phosphorylation of Akt in response to stimulation with growth factors or heat shock. Akt phosphorylation was inhibited in cells expressing 5ptase IV but increased in 4ptase I cells and correlated with the intracellular level of PI(3,4,5)P(3) and not that of PI(3,4)P(2). The inhibition of Akt phosphorylation in cells expressing 5ptase IV makes them highly susceptible to FAS-induced apoptosis, whereas overexpressing of the 4ptase I protects cells from apoptosis. Our results place 5ptase IV as a relevant biological regulator of PI3K/Akt pathway in cells.

PTEN regulates tumor cell adhesion of colon carcinoma cells under dynamic conditions of fluid flow

The regulation of integrin-mediated cell adhesion and its stabilization involves different phosphorylation and dephosphorylation events. Focal adhesion kinase (FAK) has been recently found to be a substrate of the dual-specific phosphatase PTEN in glioma cells, where it appears to be involved in regulation of cell spreading and migration as part of focal adhesions. We have investigated the role of PTEN in cell adhesion of HT-29 human colon carcinoma cells under static and hydrodynamic conditions of fluid flow. PTEN coprecipitated with FAK and paxillin dependent on the formation of adhesions to collagens. This corresponded with an adhesion-dependent increase in Tyr-phosphatase activity of PTEN. Using preparations of native FAK and PTEN from HT-29 cells in a specific Tyr-phosphatase assay FAK was identified as substrate for this dephosphorylation. If expression of PTEN was reduced using antisense oligonucleotides cell adhesion under dynamic conditions of laminar flow, but not under static conditions was significantly increased. In addition, cell spreading was increased in cells with reduced PTEN expression. We conclude that PTEN appears to be involved in the regulation of integrin-mediated adhesion through dephosphorylation of FAK. This phosphatase might play a role as a negative regulator for the formation of stable HT-29 cell adhesion to extracellular matrix.

PTEN protects p53 from Mdm2 and sensitizes cancer cells to chemotherapy

The PTEN tumor suppressor protein inhibits phosphatidylinositol 3-kinase (PI3K)/Akt signaling that promotes translocation of Mdm2 into the nucleus. When restricted to the cytoplasm, Mdm2 is degraded. The ability of PTEN to inhibit the nuclear entry of Mdm2 increases the cellular content and transactivation of the p53 tumor suppressor protein. Retroviral transduction of PTEN into U87MG (PTEN null) glioblastoma cells increases p53 activity and expression of p53 target genes and induces cell cycle arrest. U87MG/PTEN glioblastoma cells are more sensitive than U87MG/PTEN null cells to death induced by etoposide, a chemotherapeutic agent that induces DNA damage. Previously, tumor suppressor proteins have been supposed to act individually to suppress cancers. Our results establish a direct connection between the activities of two major tumor suppressors and show that they act together to respond to stresses and malignancies. PTEN protects p53 from survival signals, permitting p53 to function as a guardian of the genome. By virtue of its capacity to protect p53, PTEN can sensitize tumor cells to chemotherapy that relies on p53 activity. p53 induces PTEN gene expression, and here it is shown that PTEN protects p53, indicating that a positive feedback loop may amplify the cellular response to stress, damage, and cancer.

LATS1 tumor suppressor regulates G2/M transition and apoptosis

The LATS1 gene is a mammalian member of the novel lats tumor suppressor family. Both lats mosaic flies and LATS1 deficient mice spontaneously develop tumors. Our previous studies have shown that inactivation of Drosophila lats leads to up-regulation of cyclin A in the fly, and the human LATS1 protein associates with CDC2 in early mitosis in HeLa cells, suggesting that the lats gene family may negatively regulate cell proliferation by modulating CDC2/Cyclin A activity. We demonstrate here that transduction of the human breast cancer cell MCF-7 with recombinant LATS1 adenovirus (Ad-LATS1), but not with EGFP adenovirus (Ad-EGFP), inhibits in vitro cell proliferation. Ectopic expression of LATS1 in MCF-7 cells specifically down-regulates Cyclin A and Cyclin B protein levels and dramatically reduces CDC2 kinase activity, leading to a G2/M blockade. Furthermore, Ad-LATS1 suppresses anchorage-independent growth of MCF-7 cells in soft agar and tumor formation in athymic nude mice. We also demonstrate that ectopic expression of LATS1 in MCF-7 cells and human lung cancer cell H460 up-regulates the level of BAX proteins and induces apoptosis. Finally, we show that LATS1 kinase activity is required for its ability to inhibit cell growth and induce apoptosis. The results indicate that the LATS1 tumor suppressor may play an important role in the control of human tumor development and that LATS1 suppresses tumorigenesis by negatively regulating cell proliferation and modulating cell survival.

Ansamycin antibiotics inhibit Akt activation and cyclin D expression in breast cancer cells that overexpress HER2

Ansamycin antibiotics, such as 17-allylaminogeldanamycin (17-AAG), bind to Hsp90 and regulate its function, resulting in the proteasomal degradation of a subset of signaling proteins that require Hsp90 for conformational maturation. HER2 is a very sensitive target of these drugs. Ansamycins cause RB-dependent G1 arrest that is associated with loss of D-cyclins via a PI3 kinase, Akt dependent pathway. Downregulation of D-cyclin was due, in part, to loss of Akt expression in response to drug. Moreover, in HER2 overexpressing breast cancer cells, 17-AAG caused rapid inhibition of Akt activity prior to any change in Akt protein. Ansamycins caused rapid degradation of HER2 and a concomitant loss in HER3 associated PI3 kinase activity. This led to a loss of Akt activity, dephosphorylation of Akt substrates, and loss of D-cyclin expression. Introduction into cells of a constitutively membrane bound form of PI3 kinase prevented the effects of the drug on Akt activity and D-cyclins. Thus, in breast cancer cells with high HER2, Akt activation by HER2/HER3 heterodimers is required for D-cyclin expression. In murine xenograft models, non-toxic doses of 17-AAG markedly reduced the expression of HER2 and phosphorylation of Akt and inhibited tumor growth. Thus, pharmacological inhibition of Akt activation is achievable with ansamycins and may be useful for the treatment of HER2 driven tumors.

TC21 mediates transformation and cell survival via activation of phosphatidylinositol 3-kinase/Akt and NF-kappaB signaling pathway

The signaling pathways of TC21-mediated transformation and cell survival are not well-established. In this study, we have investigated the role of PI3-K/Akt signaling pathway in oncogenic-TC21-mediated transformation and cell survival. We found that oncogenic-TC21 stimulated the PI3-K activity. This was associated with the activation of Akt, a key component of PI3-K signaling pathway. We also found that TC21 interacted and formed complex with PI3-K. Mutations in the GTP-binding region of TC21, which enhanced GTP-binding potential of this protein, also stimulated its association with PI3-K, suggesting that PI3-K may preferentially interact with the GTP-bound form. Suppression of PI3-K and Akt by specific inhibitors LY294002 and Wortmannin reversed TC21-induced transformation. Likewise, inhibition of PI3-K activity by the PI3-K phosphotase PTEN reduced TC21-mediated focus formation in NIH3T3 cells. Investigation of TC21's effect on cell survival revealed that mutant-TC21 expressing cells were more resistant to etoposide- and cisplatin-induced cell death, and this was associated with the activation of anti-apoptotic protein NF-kappaB, a downstream target of Akt. Treatment of PI3-K inhibitor LY294002 significantly suppressed TC21-mediated NF-kappaB activation. In conclusion, we have identified PI3-K as an effector of TC21 and demonstrated that the PI3-K/Akt signaling pathway plays important roles in TC21-mediated transformation and cell survival.

Integrins play a critical role in mechanical stress-induced p38 MAPK activation

Mechanical stress activates various hypertrophic responses, including activation of mitogen-activated protein kinases (MAPKs) in cardiac myocytes. Stretch activated extracellular signal-regulated kinases partly through secreted humoral growth factors, including angiotensin II, whereas stretch-induced activation of c-Jun NH(2)-terminal kinases and p38 MAPK was independent of angiotensin II. In this study, we examined the role of integrin signaling in stretch-induced activation of p38 MAPK in cardiomyocytes of neonatal rats. Overexpression of the tumor suppressor PTEN, which inhibits outside-in integrin signaling, strongly suppressed stretch-induced activation of p38 MAPK. Overexpression of focal adhesion kinase (FAK) antagonized the effects of PTEN, and both tyrosine residues at 397 and 925 of FAK were necessary for its effects. Stretch induced tyrosine phosphorylation and activation of FAK and Src. Stretch-induced activation of p38 MAPK was abolished by overexpression of FAT and CSK, which are inhibitors of the FAK and Src families, respectively, and was suppressed by overexpression of a dominant-negative mutant of Ras. Mechanical stretch-induced increase in protein synthesis was suppressed by SB202190, a p38 MAPK inhibitor. These results suggest that mechanical stress activates p38 MAPK and induces cardiac hypertrophy through the integrin-FAK-Src-Ras pathway in cardiac myocytes.

Interleukin-2-mediated survival and proliferative signals are uncoupled in T lymphocytes that fail to divide after activation

T lymphocytes are heterogeneous with respect to their ability to proliferate following activation in vitro and in vivo. Approximately 30% of T lymphocytes fail to progress through the cell cycle, despite showing evidence of an activated state. The population of T lymphocytes that remains undivided during a primary stimulation has been shown to be refractory to restimulation via the TCR and fails to proliferate in response to IL-2. In an in vitro model of T-cell deletion following clonal expansion, we demonstrate that T lymphocytes that do not progress through the cell cycle during primary stimulation have a sevenfold greater survival advantage compared with T lymphocytes that have divided. Progression through multiple division cycles is associated with down-regulation of Bcl-2 during a postactivation period of growth factor withdrawal. However this alone does not account for diminished survival, as constitutive expression of a Bcl-2 transgene did not restore survival to the levels seen in undivided cells. Engagement of the IL-2 receptor on these undivided activated T lymphocytes leads to enhanced survival and up-regulation of Bcl-2 and Bcl-xL. Surprisingly, while IL-2 also induces phosphorylation of Akt, it does not initiate cell cycle progression in this population of primary undivided cells. Our data provide evidence that a T cell's survival capacity is linked to its proliferative behavior. Furthermore, our results provide the first report of a population of T cells, in which the IL-2 receptor-mediated signaling pathways leading to survival and proliferation are naturally uncoupled.

Monocyte survival factors induce Akt activation and suppress caspase-3

A number of inflammatory cytokines and growth factors promote monocyte survival; however, the biochemical events stimulated by these factors are poorly defined. We previously showed that the monocyte survival factor macrophage colony-stimulating factor (M-CSF) activated monocyte survival through a PI 3-kinase-dependent pathway resulting in the phosphorylation of Akt and the suppression of the activation of caspase-3. Because other cytokines and bacterial cell wall products also induce monocyte survival, we hypothesized that these factors may also suppress caspase-3 and caspase-9 activation and activate Akt in human monocytes. To test this hypothesis, we found that interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, lipopolysaccharide (LPS), granulocyte macrophage-colony-stimulating factor (GM-CSF), and IL-18 appeared to suppress DNA fragmentation, caspase-9, and caspase-3 activation in human monocytes. Moreover, these stimuli appeared to induce the serine and threonine phosphorylation of Akt, which was reduced by the PI 3-kinase inhibitor LY294002. Using in vitro kinase assays, M-CSF appeared to induce more Akt activity than did the other survival factors. Treatment of monocytes with either LY294002 or wortmannin resulted in caspase-3 activation in the presence of these survival factors. These results suggest that monocyte survival factors may suppress DNA fragmentation, caspase-9, and caspase-3 activation in a PI 3-kinase-dependent manner, perhaps through the activation of Akt.

A novel conditional Akt 'survival switch' reversibly protects cells from apoptosis

The anti-apoptotic Akt kinase is commonly activated by survival factors following plasma membrane relocalization attributable to the interaction of its pleckstrin homology (PH) domain with phosphatidylinositol 3-kinase (PI3K)-generated PI3,4-P(2) and PI3,4,5-P(3). Once activated, Akt can prevent or delay apoptosis by phosphorylation-dependent inhibition or activation of multiple signaling molecules involved in apoptosis, such as BAD, caspase-9, GSK3, and NF-kappaB and forkhead family transcription factors. Here, we describe and characterize a novel, conditional Akt controlled by chemically induced dimerization (CID). In this approach, the Akt PH domain has been replaced with the rapamycin (and FK506)-binding domain, FKBP12, to make F3-DeltaPH.Akt. To effect membrane recruitment, a myristoylated rapamycin-binding domain from FRAP/mTOR, called M-FRB, binds to lipid permeable rapamycin (and non-bioactive synthetic 'rapalogs'), leading to reversible heterodimerization of M-FRB with FKBP-DeltaPH.Akt. Like endogenous c-Akt, we show that the kinase activity of membrane-localized F3-DeltaPH.Akt correlates strongly with phosphorylation at T308 and S473; however, unlike c-Akt, phosphorylation and activation of inducible Akt (iAkt) is largely PI3K independent. CID-mediated activation of iAkt results in phosphorylation of GSK3, and contributes to NF-kappaB activation in vivo in a dose-sensitive manner. Finally, in Jurkat T cells stably expressing iAkt, CID-induced Akt activation rescued cells from apoptosis triggered by multiple apoptotic stimuli, including staurosporine, anti-Fas antibodies, PI3K inhibitors and the DNA damaging agent, etoposide. This novel inducible Akt should be useful for identifying new Akt substrates and for reversibly protecting tissue from apoptosis due to ischemic injury or immunological attack.

Integrin-linked kinase, a promising cancer therapeutic target: biochemical and biological properties

Integrin-linked kinase (ILK) is an ankyrin repeat-containing Ser/Thr kinase that interacts with the cytoplasmic domains of beta(1) and beta(3) integrins. ILK is widely expressed in tissues throughout the body, and, as might be expected, appears to mediate a diversity of functions relating to its role in coupling integrins and growth factor receptors to downstream signaling pathways. Through its downstream targets protein kinase B/Akt and glycogen synthase kinase-3beta, ILK appears to be involved in several oncogenesis-related events, including suppression of apoptosis and promotion of cell survival, as well as cell migration and invasion. Over-expression of ILK in epithelial cells results in anchorage-independent cell growth with increased cell cycle progression. Inoculation of nude mice with ILK over-expressing cells leads to tumor formation. Furthermore, increased ILK expression and activity have been correlated with malignancy in several human tumor types, including breast, prostate, brain, and colon carcinomas. Based on these findings, ILK represents an excellent therapeutic target for the prevention of tumor progression. Here, we provide an overview of the physical and biochemical properties of ILK, and present data describing the impact of small-molecule ILK inhibitors on several ILK-mediated cellular functions.

Protein kinase B (Akt) regulation and function in T lymphocytes

Protein kinase B (PKB) [1-5] is a serine/threonine kinase that is activated by cytokines, antigen receptors, the costimulator CD28 and chemokines in lymphocytes. [6-11] PKB is thus poised to contribute to a variety of immune activation responses. A number of functions have been ascribed to PKB in different cell lineages including the regulation of cell survival, cytokine gene induction and cell cycle progression. In the present article the mechanisms that control PKB activity in T lymphocytes will be reviewed and the function of this kinase in the immune system will be discussed.

Unravelling novel intracellular pathways in cell-based assays

The pharmaceutical industry is currently facing several challenges to identify and develop novel drug targets. Traditional drug discovery focussed on a small number of well-characterized gene products. Recently, this picture has changed with the completion of the draft sequence of the human genome, which has led to the identification of thousands of novel genes with unknown or poorly understood function. To cope with this overwhelming number of potential drug target candidates, new strategies for the elucidation of gene function, as well as their involvement in intracellular pathways, are required.

Inhibition of protein kinase B/Akt. implications for cancer therapy

Protein kinase B (PKB, also called Akt) is an important regulator of cell proliferation and survival. Amplification of genes encoding PKB isoforms has been found in several types of human cancers. In addition, mutations in the phosphatase and tensin homolog deleted on chromosome ten (PTEN), one of the most frequently mutated tumor suppressor genes, results in elevated PKB activity. PKB has a wide range of cellular targets, and the oncogenicity of PKB arises from activation of both proliferative and anti-apoptotic signaling. Furthermore, PKB contributes to tumor progression by promoting cell invasiveness and angiogenesis. These observations establish PKB as an attractive target for cancer therapy. A cellular inhibitor of PKB, termed carboxyl-terminal modulator protein, reverts the phenotype of viral akt-transformed cells, suggesting that a specific PKB inhibitor will be useful in the treatment of tumors with elevated PKB activity. Since inhibition of PKB activity induces apoptosis in a range of mammalian cells, a PKB inhibitor may be effective, in combination with other anticancer drugs, for the treatment of tumors with other mutations.

Cyclin-dependent kinase 5 prevents neuronal apoptosis by negative regulation of c-Jun N-terminal kinase 3

Cyclin-dependent kinase 5 (cdk5) is a serine/threonine kinase activated by associating with its neuron-specific activators p35 and p39. Analysis of cdk5(-/-) and p35(-/-) mice has demonstrated that both cdk5 and p35 are essential for neuronal migration, axon pathfinding and the laminar configuration of the cerebral cortex, suggesting that the cdk5-p35 complex may play a role in neuron survival. However, the targets of cdk5 that regulate neuron survival are unknown. Here, we show that cdk5 directly phosphorylates c-Jun N-terminal kinase 3 (JNK3) on Thr131 and inhibits its kinase activity, leading to reduced c-Jun phosphorylation. Expression of cdk5 and p35 in HEK293T cells inhibits c-Jun phosphorylation induced by UV irradiation. These effects can be restored by expression of a catalytically inactive mutant form of cdk5. Moreover, cdk5-deficient cultured cortical neurons exhibit increased sensitivity to apoptotic stimuli, as well as elevated JNK3 activity and c-Jun phosphorylation. Taken together, these findings show that cdk5 may exert its role as a key element by negatively regulating the c-Jun N-terminal kinase/stress-activated protein kinase signaling pathway during neuronal apoptosis.

Reduced PTEN expression in the pancreas overexpressing transforming growth factor-beta 1

PTEN is a candidate tumour suppressor gene and frequently mutated in multiple cancers, however, not in pancreatic cancer. Recently, it has been demonstrated that PTEN expression is regulated by TGF-beta1. Using TGF-beta1 transgenic mice (n=7) and wildtype littermates (n=6), as well as pancreatic tissues obtained from organ donors (n=10) and patients with pancreatic cancer (n=10), we assessed the expression of PTEN by means of immunohistochemistry and semiquantitative PCR analysis. In addition, PANC-1 cells were treated with TGF-beta1 in vitro and the levels of PTEN mRNA were determined in these cells. In human pancreatic cancers PTEN mRNA levels were significantly decreased (P<0.05). In addition, in the pancreas of TGF-beta1 transgenic mice the expression of PTEN was significantly reduced (P<0.01), as compared to wildtype littermates and incubation of PANC-1 cells with TGF-beta1 decreased PTEN mRNA levels after 24 h. Inasmuch as TGF-beta1 decreases PTEN expression in human pancreatic cancer cells and human pancreatic cancers overexpress TGF-beta1, the reduced expression of PTEN in pancreatic cancer may be mediated by TGF-beta1 overexpression. Thus, although PTEN is not mutated in pancreatic cancers, the reduction of its expression may give pancreatic cancer cells an additional growth advantage.

Animal models of tumor-suppressor genes

The development of cancer requires multiple genetic alterations perturbing distinct cellular pathways. In human cancers, these alterations often arise owing to mutations in tumor-suppressor genes whose normal function is to either inhibit the proliferation, apoptosis, or differentiation of cells, or maintain their genomic integrity. Mouse models for tumor suppressors frequently provide definitive evidence for the antitumorigenic functions of these genes. In addition, animal models permit the identification of previously unsuspected roles of these genes in development and differentiation. The availability of null and tissue-specific mouse mutants for tumor-suppressor genes has greatly facilitated our understanding of the mechanisms leading to cancer. In this review, we describe mouse models for tumor-suppressor genes.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

PTEN sensitizes prostate cancer cells to death receptor-mediated and drug-induced apoptosis through a FADD-dependent pathway

The PTEN tumor suppressor is frequently mutated in human tumors. Loss of PTEN function is associated with constitutive survival signaling through the phosphatidylinositol-3 kinase/Akt pathway. Therefore, we asked if reconstitution of PTEN function would lead to the reversal of resistance to apoptosis in prostate cancer cells. Adenovirus-mediated expression of PTEN completely suppressed constitutive Akt activation in LNCaP prostate cancer cells and enhanced apoptosis induced by a broad range of apoptotic stimuli. PTEN expression sensitized cells to death receptor-mediated apoptosis induced by tumor necrosis factor, anti-Fas antibody, and TRAIL. PTEN also sensitized cells to non-receptor mediated apoptosis induced by a kinase inhibitor staurosporine and chemotherapeutic agents mitoxantrone and etoposide. PTEN-mediated apoptosis was accompanied by caspase-3 and caspase-8 activation and was inhibited by a broad specificity caspase inhibitor Z-VAD-fmk. Bcl-2 overexpression also blocked PTEN-mediated apoptosis. Lipid phosphatase activity of PTEN is required for apoptosis as the PTEN G129E mutant selectively deficient in lipid phosphatase activity was unable to sensitize cells to apoptosis. PTEN-mediated apoptosis involves a FADD-dependent pathway for both death receptor-mediated and drug-induced apoptosis as coexpression of a dominant negative FADD mutant blocked PTEN-mediated apoptosis. Since in death receptor signaling, FADD mediates activation of caspase-8, which in turn cleaves BID, and since caspase-8 is activated in PTEN-mediated apoptosis, we examined BID cleavage in PTEN-mediated apoptosis. PTEN facilitated BID cleavage after treatment with low doses of staurosporine and mitoxantrone. BID cleavage was inhibited by dominant negative FADD. Taken together, these data are consistent with the hypothesis that PTEN promotes drug-induced apoptosis by facilitating caspase-8 activation and BID cleavage through a FADD-dependent pathway.

CD38 is associated with lipid rafts and upon receptor stimulation leads to Akt/protein kinase B and Erk activation in the absence of the CD3-zeta immune receptor tyrosine-based activation motifs

T lymphocytes can be activated via the T cell receptor (TCR) or by triggering through a number of other cell surface structures, including the CD38 co-receptor molecule. Here, we show that in TCR+ T cells that express a CD3-zeta lacking the cytoplasmic domain, cross-linking with CD38- or CD3-specific monoclonal antibodies induces tyrosine phosphorylation of CD3-epsilon, zeta-associated protein-70, linker for activation of T cells, and Shc. Moreover, in these cells, anti-CD38 or anti-CD3 stimulation leads to protein kinase B/Akt and Erk activation, suggesting that the CD3-zeta-immunoreceptor tyrosine-based activation motifs are not required for CD38 signaling in T cells. Interestingly, in unstimulated T cells, lipid rafts are highly enriched in CD38, including the T cells lacking the cytoplasmic tail of CD3-zeta. Moreover, CD38 clustering by extensive cross-linking with an anti-CD38 monoclonal antibody and a secondary antibody leads to an increased resistance of CD38 to detergent solubilization, suggesting that CD38 is constitutively associated with membrane rafts. Consistent with this, cholesterol depletion with methyl-beta-cyclodextrin substantially reduces CD38-mediated Akt activation while enhancing CD38-mediated Erk activation. CD38/raft association may improve the signaling capabilities of CD38 via formation of protein/lipid domains to which signaling-competent molecules, such as immunoreceptor tyrosine-based activation motif-bearing CD3 molecules and protein-tyrosine kinases, are recruited.

Functional roles of Akt signaling in mouse skin tumorigenesis

The mouse skin carcinogenesis protocol is a unique model for understanding the molecular events leading to oncogenic transformation. Mutations in the Ha-ras gene, and the presence of functional cyclin D1 and the EGF receptor, have proven to be important in this system. However, the signal transduction pathways connecting these elements during mouse skin carcinogenesis are poorly understood. This paper studies the relevance of the Akt and ERK pathways in the different stages of chemically induced mouse skin tumors. Akt activity increases throughout the entire process, and its early activation is detected prior to increased cyclin D1 expression. ERK activity rises only during the later stages of malignant conversion. The observed early increase in Akt activity appears to be due to raised PI-3K activity. Other factors acting on Akt such as ILK activation and decreased PTEN phosphatase activity appear to be involved at the conversion stage. To further confirm the involvement of Akt in this process, PB keratinocytes were transfected with Akt and subsequently injected into nude mice. The expression of Akt accelerates tumorigenesis and contributes to increased malignancy of these keratinocytes as demonstrated by the rate of appearance, the growth and the histological characteristics of the tumors. Collectively, these data provide evidence that Akt activation is one of the key elements during the different steps of mouse skin tumorigenesis.

Hamartomatous polyposis syndromes: genetic pathways

Hamartomatous polyposis syndromes are a group of clinically distinct but perhaps genetically related disorders in which the predominant finding is multiple hamartomatous polyps in the gastrointestinal tract. These syndromes are transmitted in an autosomal dominant fashion to offspring, but sporadic forms do exist in which the biological parents lack an identified germline mutation. Despite the nondysplastic histologic characteristics of their polyps, each hamartomatous syndrome carries an elevated risk for cancer at specific organ sites. Several genes have been identified as mutated in the germline from these syndromes, and they provide clues to the pathogenesis of the polyps and may explain some of the elevated cancer risk. Pathways involved in the hamartomatous syndromes include those of vascular endothelial growth factor, the transforming growth factor beta superfamily, and antagonizing the effects of Akt/protein kinase B.

Induction of apoptosis in cancer: new therapeutic opportunities

Autonomous cell proliferation is one of the hallmarks of cancer cells, driven by activated growth-promoting oncogenes. However, deregulated activation of these oncogenes also triggers apoptosis via multiple pathways. Among them, the ARF-p53 pathway appears to play a major role in mediating oncogene-induced apoptosis. Consequently, suppression of apoptosis by inactivation of p53 and other tumor suppressors is central to tumor development. These findings have broad implications in understanding cancer genetics and therapy. They help define the roles for oncogenes and tumor suppressor genes in tumorigenesis. Furthermore, the notion that cancer cells often carry specific defects in apoptotic pathways but are inherently sensitive to apoptosis as a result of deregulated proliferation, offers numerous opportunities for manipulating apoptosis in directions of clinical application.

Akt mediates Rac/Cdc42-regulated cell motility in growth factor-stimulated cells and in invasive PTEN knockout cells

Growth factors promote cell survival and cell motility, presumably through the activation of Akt and the Rac and Cdc42 GTPases, respectively. Because Akt is dispensable for Rac/Cdc42 regulation of actin reorganization, it has been assumed that Rac and Cdc42 stimulate cell motility independent of Akt in mammalian cells. However, in this study we demonstrate that Akt is essential for Rac/Cdc42-regulated cell motility in mammalian fibroblasts. A dominant-negative Akt inhibits cell motility stimulated by Rac/Cdc42 or by PDGF treatment, without affecting ruffling membrane-type actin reorganization. We have confirmed a previous report that Akt is activated by expression of Rac and Cdc42 and also observed colocalization of endogenous phosphorylated Akt with Rac and Cdc42 at the leading edge of fibroblasts. Importantly, expression of active Akt but not the closely related kinase SGK is sufficient for increasing cell motility. This effect of Akt is cell autonomous and not mediated by inhibition of GSK3. Finally, we found that dominant-negative Akt but not SGK reverses the increased cell motility phenotype of fibroblasts lacking the PTEN tumor suppressor gene. Taken together, these results suggest that Akt promotes cell motility downstream of Rac/Cdc42 in growth factor-stimulated cells and in invasive PTEN-deficient cells.

Negative regulation of neural stem/progenitor cell proliferation by the Pten tumor suppressor gene in vivo

The mechanisms controlling neural stem cell proliferation are poorly understood. Here we demonstrate that the PTEN tumor suppressor plays an important role in regulating neural stem/progenitor cells in vivo and in vitro. Mice lacking PTEN exhibited enlarged, histoarchitecturally abnormal brains, which resulted from increased cell proliferation, decreased cell death, and enlarged cell size. Neurosphere cultures revealed a greater proliferation capacity for tripotent Pten-/- central nervous system stem/progenitor cells, which can be attributed, at least in part, to a shortened cell cycle. However, cell fate commitments of the progenitors were largely undisturbed. Our results suggest that PTEN negatively regulates neural stem cell proliferation.