Impaired Fas response and autoimmunity in Pten+/- mice

Genetics of melanoma tumorigenesis

Melanoma therapy is moving away from combinatorial approaches and towards newer targeted strategies. With the identification of mutations in various RAS pathway genes, there are now tremendous opportunities to bring inhibitors of RAS signalling to the clinical arena.

Multistep pathogenesis of autoimmune disease

In the immune system, many tolerance checkpoints exist to prevent self-antigens from stimulating the relentless growth of self-reactive B and T lymphocytes. The genes and molecular pathways underpinning these checkpoints overlap with those involved in tumor suppression. As with an inherited predisposition to cancer, inherited defects in self-tolerance genes typically precipitate autoimmune disease stochastically after a latent phase. Multiple mutations, inherited and somatic, may be needed before a self-reactive clone bypasses sequential tolerance checkpoints resulting in the emergence of autoimmune disease.

Cowden syndrome

Cowden syndrome (CS), due to germline mutations of the PTEN tumor-suppressor gene, is an often overlooked cancer predisposition syndrome associated with an increased risk of breast, thyroid, and endometrial cancers, as well as benign manifestations. Germline PTEN mutations also are associated with syndromes that have not been historically connected to an increase in risk for malignancy. These disorders include Bannayan-Riley-Ruvalcaba syndrome (BRRS), Proteus syndrome (PS), and Proteus-like syndrome (PSL). These syndromes can be described under the umbrella of PTEN hamartoma tumor syndrome (PHTS). As one would expect in allelic disorders, there is broad phenotypic overlap in the PHTS; however, the syndromes are clinically distinct. As additional information is discovered about new syndromes of cancer predisposition and their concordant genes, oncologists and allied healthcare providers must maintain vigilance to appropriately identify, and screen, individuals at an increased risk. Although CS is the only PHTS with a clearly documented predisposition to malignancies, pending further data, for precautionary reasons all individuals with a germline PTEN mutation are recommended to follow the cancer surveillance recommendations for CS.

PTEN and Akt expression during growth of human ovarian follicles

PURPOSE

To assess the expression of PTEN and total and phosphorylated Akt in human ovarian follicles during follicular growth.

METHODS

Immunohistochemistry of ovarian tissues and Western blotting and immunofluorescence of primary cultured luteinized granulosa cells for PTEN and Akt.

RESULTS

Immunoreactivity of Akt was found in the oocytes, granulosa cells and theca cells in primordial follicles, follicles at each growing stage and luteal cells. As the follicles grew, staining for PTEN became intense in the granulosa cells, whereas the intensity of phospho-Akt became weak. Western blotting and immunofluorescence analysis using primary cultured granulosa-lutein cells showed Akt and PTEN expression, and phosphorylation of Akt in vitro.

CONCLUSIONS

PTEN and Akt are present in the granulosa cells during folliculogenesis. An increase in PTEN may lead to changes in proliferation and/or differentiation of granulosa cells during follicular growth via regulation of Akt phosphorylation.

B cells in glomerulonephritis: focus on lupus nephritis

The production of pathogenic antibody has been traditionally viewed as the principle contribution of B cells to the pathogenesis of immune-mediated glomerulonephritis. However, it is increasingly appreciated that B cells play a much broader role in such diseases, functioning as antigen-presenting cells, regulators of T cells, dendritic cells, and macrophages and orchestrators of local lymphatic expansion. In this review, we provide an overview of basic B cell biology and consider the evidence implicating B cells in one of the archetypal immune-mediated glomerulonephritides, lupus nephritis.

Targeting phosphoinositide 3-kinase: moving towards therapy

Phosphoinositide 3-kinases (PI3K) orchestrate cell responses including mitogenic signaling, cell survival and growth, metabolic control, vesicular trafficking, degranulation, cytoskeletal rearrangement and migration. Deregulation of the PI3K pathway occurs by activating mutations in growth factor receptors or the PIK3CA locus coding for PI3Kalpha, by loss of function of the lipid phosphatase and tensin homolog deleted in chromosome ten (PTEN/MMAC/TEP1), by the up-regulation of protein kinase B (PKB/Akt), or the impairment of the tuberous sclerosis complex (TSC1/2). All these events are linked to growth and proliferation, and have thus prompted a significant interest in the pharmaceutical targeting of the PI3K pathway in cancer. Genetic targeting of PI3Kgamma (p110gamma) and PI3Kdelta (p110delta) in mice has underlined a central role of these PI3K isoforms in inflammation and allergy, as they modulate chemotaxis of leukocytes and degranulation in mast cells. Proof-of-concept molecules selective for PI3Kgamma have already successfully alleviated disease progress in murine models of rheumatoid arthritis and lupus erythematosus. As targeting PI3K moves forward to therapy of chronic, non-fatal disease, safety concerns for PI3K inhibitors increase. Many of the present inhibitor series interfere with target of rapamycin (TOR), DNA-dependent protein kinase (DNA-PK(cs)) and activity of the ataxia telangiectasia mutated gene product (ATM). Here we review the current disease-relevant knowledge for isoform-specific PI3K function in the above mentioned diseases, and review the progress of >400 recent patents covering pharmaceutical targeting of PI3K. Currently, several drugs targeting the PI3K pathway have entered clinical trials (phase I) for solid tumors and suppression of tissue damage after myocardial infarction (phases I,II).

Shared signaling networks active in B cells isolated from genetically distinct mouse models of lupus

Though B cells play key roles in lupus pathogenesis, the molecular circuitry and its dysregulation in these cells as disease evolves remain poorly understood. To address this, a comprehensive scan of multiple signaling axes using multiplexed Western blotting was undertaken in several different murine lupus strains. PI3K/AKT/mTOR (mTOR, mammalian target of rapamycin), MEK1/Erk1/2, p38, NF-kappaB, multiple Bcl-2 family members, and cell-cycle molecules were observed to be hyperexpressed in lupus B cells in an age-dependent and lupus susceptibility gene-dose-dependent manner. Therapeutic targeting of the AKT/mTOR axis using a rapamycin (sirolimus) derivative ameliorated the serological, cellular, and pathological phenotypes associated with lupus. Surprisingly, the targeting of this axis was associated with the crippling of several other signaling axes. These studies reveal that lupus pathogenesis is contingent upon the activation of an elaborate network of signaling cascades that is shared among genetically distinct mouse models and raise hope that targeting pivotal nodes in these networks may offer therapeutic benefit.

Deletion of PKBalpha/Akt1 affects thymic development

Background

The thymus constitutes the primary lymphoid organ for the majority of T cells. The phosphatidyl-inositol 3 kinase (PI3K) signaling pathway is involved in lymphoid development. Defects in single components of this pathway prevent thymocytes from progressing beyond early T cell developmental stages. Protein kinase B (PKB) is the main effector of the PI3K pathway.

Methodology/Principal Findings

To determine whether PKB mediates PI3K signaling in the thymus, we characterized PKB knockout thymi. Our results reveal a significant thymic hypocellularity in PKBα^−/− neonates and an accumulation of early thymocyte subsets in PKBα^−/− adult mice. Using thymic grafting and fetal liver cell transfer experiments, the latter finding was specifically attributed to the lack of PKBα within the lymphoid component of the thymus. Microarray analyses show that the absence of PKBα in early thymocyte subsets modifies the expression of genes known to be involved in pre-TCR signaling, in T cell activation, and in the transduction of interferon-mediated signals.

Conclusions/Significance

This report highlights the specific requirements of PKBα for thymic development and opens up new prospects as to the mechanism downstream of PKBα in early thymocytes.

[Therapeutic potential of phosphoinositide 3-kinase inhibitors in rheumatoid arthritis]

Activation of PI3K is a key step of intracellular signaling involved in many cellular functions. PI3K inhibitors have shown anti-rheumatic effects on arthritis model in mice. PI3K is a potential therapeutic target of rheumatoid arthritis.

Role of PTEN/PI3K pathway in endothelial cells

PTEN (phosphatase and tensin homologue deleted on chromosome 10) is an important tumour-suppressor gene that encodes a 3-phosphatase. The major substrate of PTEN is PIP(3) (phosphatidylinositol 3,4,5-trisphosphate) generated by the action of PI3Ks (phosphoinositide 3-kinases). Hereditary mutation of PTEN causes tumour-susceptibility diseases such as Cowden disease. We used the Cre-loxP system to generate an endothelial cell-specific mutation of PTEN in mice. Heterozygous mutation of PTEN in endothelial cells enhances postnatal neovascularization, including tumour angiogenesis necessary for tumour growth. This observation suggests that Cowden disease patients are not only at risk for additional tumorigenic mutations due to complete loss of PTEN function, but may also experience accelerated growth of incipient tumours due to enhanced angiogenesis. Homozygous mutation of Pten in murine endothelial cells impairs cardiovascular morphogenesis and is embryonic lethal due to endothelial cell hyperproliferation and impaired vascular remodelling. Additional homozygous mutation of p85alpha, the regulatory subunit of class IA PI3Ks, or p110gamma, the catalytic subunit of the sole class IB PI3K, led to a partial rescue of all phenotypes in our PTEN-deficient mice. Thus inhibition of the PI3K pathway, including the targeting of PI3Kgamma, may be an attractive therapeutic strategy for the treatment of various malignancies.

A costimulation-initiated signaling pathway regulates NFATc1 transcription in T lymphocytes

T cell activation and differentiation is accompanied and mediated by transcriptional reprogramming. The NFATc1 transcription factor is strongly induced upon T cell activation and controls numerous genes involved in the T cell effector function. However, its regulation by physiological stimuli in primary T cells has not been well understood. We previously found that ICOS synergizes with TCR and CD28 to greatly enhance NFATc1 expression in primary T cells. In this study, we have examined the signaling mechanisms whereby costimulation regulates NFATc1 expression. We found that CD28 and ICOS regulate sustained PI3K activity in primary T cells, which is required for NFATc1 up-regulation. CD28 and ICOS costimulation, possibly through Itk, a Tec kinase downstream of the PI3K, enhanced phosphorylation of phospholipase C gamma1 and increased and sustained Ca(2+) flux in T cells. Costimulation of T cells potentiated transcription of the Nfatc1 gene P1 promoter in a PI3K-dependent manner. This work demonstrates an important role for costimulatory receptors in sustaining T cell activation programs leading to Nfatc1 gene transcription and has implications in our understanding of the immune response and tolerance.

Zebrafish pten genes have overlapping and non-redundant functions in tumorigenesis and embryonic development

In human cancer, PTEN (Phosphatase and TENsin homolog on chromosome 10, also referred to as MMAC1 and TEP1) is a frequently mutated tumor suppressor gene. We have used the zebrafish as a model to investigate the role of Pten in embryonic development and tumorigenesis. The zebrafish genome encodes two pten genes, ptena and ptenb. Here, we report that both Pten gene products from zebrafish are functional. Target-selected inactivation of ptena and ptenb revealed that Ptena and Ptenb have redundant functions in embryonic development, in that ptena-/- and ptenb-/- mutants did not show embryonic phenotypes. Homozygous single mutants survived as adults and they were viable and fertile. Double homozygous ptena-/-ptenb-/- mutants died at 5 days post fertilization with pleiotropic defects. These defects were rescued by treatment with the phosphatidylinositol-3-kinase inhibitor, LY294002. Double homozygous embryos showed enhanced cellular proliferation. In addition, cell survival was dramatically enhanced in embryos that lack functional Pten upon gamma-irradiation. Surprisingly, adult ptenb-/- zebrafish developed ocular tumors later in life, despite the expression of ptena in adult eyes. We conclude that whereas Ptena and Ptenb have redundant functions in embryonic development, they apparently do not have completely overlapping functions later in life. These pten mutant zebrafish represent a unique model to screen for genetic and/or chemical suppressors of Pten loss-of-function.

PTEN is destabilized by phosphorylation on Thr366

Although PTEN (phosphatase and tensin homologue deleted on chromosome 10) is one of the most commonly mutated tumour suppressors in human cancers, loss of PTEN expression in the absence of mutation appears to occur in an even greater number of tumours. PTEN is phosphorylated in vitro on Thr366 and Ser370 by GSK3 (glycogen synthase kinase 3) and CK2 (casein kinase 2) respectively, and specific inhibitors of these kinases block these phosphorylation events in cultured cells. Although mutation of these phosphorylation sites did not alter the phosphatase activity of PTEN in vitro or in cells, blocking phosphorylation of Thr366 by either mutation or GSK3 inhibition in glioblastoma cell lines led to a stabilization of the PTEN protein. Our data support a model in which the phosphorylation of Thr366 plays a role in destabilizing the PTEN protein.

T-cell function is partially maintained in the absence of class IA phosphoinositide 3-kinase signaling

The class IA subgroup of phosphoinositide 3-kinase (PI3K) is activated downstream of antigen receptors, costimulatory molecules, and cytokine receptors on lymphocytes. Targeted deletion of individual genes for class IA regulatory subunits severely impairs the development and function of B cells but not T cells. Here we analyze conditional mutant mice in which thymocytes and T cells lack the major class IA regulatory subunits p85alpha, p55alpha, p50alpha, and p85beta. These cells exhibit nearly complete loss of PI3K signaling downstream of the T-cell receptor (TCR) and CD28. Nevertheless, T-cell development is largely unperturbed, and peripheral T cells show only partial impairments in proliferation and cytokine production in vitro. Both genetic and pharmacologic experiments suggest that class IA PI3K signaling plays a limited role in T-cell proliferation driven by TCR/CD28 clustering. In vivo, class IA-deficient T cells provide reduced help to B cells but show normal ability to mediate antiviral immunity. Together these findings provide definitive evidence that class IA PI3K regulatory subunits are essential for a subset of T-cell functions while challenging the notion that this signaling mechanism is a critical mediator of costimulatory signals downstream of CD28.

The AKT-mTOR pathway plays a critical role in the development of leiomyosarcomas

We analyzed the PI3K-AKT signaling cascade in a cohort of sarcomas and found a marked induction of insulin receptor substrate-2 (IRS2) and phosphorylated AKT and a concomitant upregulation of downstream effectors in most leiomyosarcomas. To determine the role of aberrant PI3K-AKT signaling in leiomyosarcoma pathogenesis, we genetically inactivated Pten in the smooth muscle cell lineage by cross-breeding Pten(loxP/loxP) mice with Tagln-cre mice. Mice carrying homozygous deletion of Pten alleles developed widespread smooth muscle cell hyperplasia and abdominal leiomyosarcomas, with a very rapid onset and elevated incidence (approximately 80%) compared to other animal models. Constitutive mTOR activation was restricted to the leiomyosarcomas, revealing the requirement for additional molecular events besides Pten loss. The rapamycin derivative everolimus substantially decelerated tumor growth on Tagln-cre/Pten(loxP/loxP) mice and prolonged their lifespan. Our data show a new and critical role for the AKT-mTOR pathway in smooth muscle transformation and leiomyosarcoma genesis, and support treatment of selected sarcomas by the targeting of this pathway with new compounds or combinations of these with conventional chemotherapy agents.

Mgat5 and Pten interact to regulate cell growth and polarity

Phosphatase and tensin homolog (Pten) phosphatase opposes intracellular phosphoinositide 3-kinase (PI3K)/Akt signaling and is a potent tumor suppressor, while Golgi beta1,6 N-acetylglucosaminyltransferase V (Mgat5) is positively associated with cancer progression and metastasis. beta1,6GlcNAc-branched N-glycans on receptor glycoproteins promote their surface residency and sensitizes cells to growth factor signaling. Here we demonstrate that the Pten heterozygosity in mouse embryonic fibroblasts enhances cell adhesion-dependent PI3K/Akt signaling, cell spreading, and proliferation, while Pten/Mgat5 double mutant cells are normalized. However, planar asymmetry typical of fibroblasts and invasive carcinomas is not fully rescued, suggesting that Mgat5 and Pten function together to regulate the membrane dynamics of PI3K/Akt signaling typical of motile cells. Pten heterozygosity was associated with increased surface beta1,6GlcNAc-branched N-glycans, suggesting positive feedback from PI3K signaling to N-glycan branching. In vivo, Mgat5(-/-) Pten(+/-) and Mgat5(+/-)Pten(+/-)mutant mice showed a small but significant increase in longevity compared with Pten(+/-) mice. Taken together, our results reveal that Mgat5 and Pten interact in an opposing manner to regulate cellular sensitivities to extracelluar growth cues.

Promise and Progress for Functional and Molecular Imaging of Response to Targeted Therapies

Biomarkers to predict or monitor therapy response are becoming essential components of drug developer's armamentaria. Molecular and functional imaging has particular promise as a biomarker for anticancer therapies because it is non-invasive, can be used longitudinally and provides information on the whole patient or tumor. Despite this promise, molecular or functional imaging endpoints are not routinely incorporated into clinical trial design. As the costs of clinical trials and drug development become prohibitively more expensive, the need for improved biomarkers has become imperative and thus, the relatively high cost of imaging is justified. Imaging endpoints, such as Diffusion-Weighted MRI, DCE-MRI and FDG-PET have the potential to make drug development more efficient at all phases, from discovery screening with in vivo pharmacodynamics in animal models through the phase III enrichment of the patient population for potential responders. This review focuses on the progress of imaging responses to new classes of anti-cancer therapies targeted against PI3 kinase/AKT, HIF-1alpha and VEGF. The ultimate promise of molecular and functional imaging is to theragnostically predict response prior to commencement of targeted therapy.

The PTEN phosphatase is essential for long-term depression of hippocampal synapses

Activity-induced long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission are two types of enduring changes in neuronal connections that are believed to underlie learning and memory functions. Here we show that CA1 synapses in hippocampal slices from PTEN-deficient mice exhibit LTP, but are resistant to LTD. PTEN reduces phosphatidylinositol-3-kinase (PI3-K) activity, and pharmacological inhibition of PI3-K restores LTD in PTEN-deficient mice, suggesting that inhibition of PI3-K by PTEN is necessary for LTD induction. These findings demonstrate a pivotal role for PTEN in LTD, and suggest that alterations in PTEN could have an impact on learning and memory processes.

'Vive la Résistance!'--the PI3K-Akt pathway can determine target sensitivity to regulatory T cell suppression

CD4+CD25+ regulatory T (Treg) cells have emerged as important regulators of immune responses but the mechanisms through which Treg cells mediate suppression are still unclear. Recently, several studies have identified murine models of spontaneous autoimmunity or genetically engineered mice in which the Treg cells function normally but the CD4+CD25- T effector (Teff) cells are resistant to Treg-mediated suppression. Here, we postulate that the activation status of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway in Teff cells is a primary determinant of Teff cell sensitivity to Treg cell-mediated suppression, and that when the PI3K-Akt pathway is hyperactivated in Teff cells, these cells are resistant to Treg cell-mediated suppression. We further postulate that this paradigm can mechanistically link abnormalities in the PI3K-Akt pathway to the development of autoimmunity.

PI3K delta and PI3K gamma: partners in crime in inflammation in rheumatoid arthritis and beyond?

Dysregulated signal transduction in innate and adaptive immune cells is known to be associated with the development of various autoimmune and inflammatory diseases. Consequently, targeting intracellular signalling of the pro-inflammatory cytokine network heralds hope for the next generation of anti-inflammatory drugs. Phosphoinositide 3-kinases (PI3Ks) generate lipid-based second messengers that control an array of intracellular signalling pathways that are known to have important roles in leukocytes. In light of the recent progress in the development of selective PI3K inhibitors, and the beneficial effects of these inhibitors in models of acute and chronic inflammatory disorders, we discuss the therapeutic potential of blocking PI3K isoforms for the treatment of rheumatoid arthritis and other immune-mediated diseases.

Pten (phosphatase and tensin homologue gene) haploinsufficiency promotes insulin hypersensitivity

AIMS/HYPOTHESIS

Insulin controls glucose metabolism via multiple signalling pathways, including the phosphatidylinositol 3-kinase (PI3K) pathway in muscle and adipose tissue. The protein/lipid phosphatase Pten (phosphatase and tensin homologue deleted on chromosome 10) attenuates PI3K signalling by dephosphorylating the phosphatidylinositol 3,4,5-trisphosphate generated by PI3K. The current study was aimed at investigating the effect of haploinsufficiency for Pten on insulin-stimulated glucose uptake.

MATERIALS AND METHODS

Insulin sensitivity in Pten heterozygous (Pten(+/-)) mice was investigated in i.p. insulin challenge and glucose tolerance tests. Glucose uptake was monitored in vitro in primary cultures of myocytes from Pten(+/-) mice, and in vivo by positron emission tomography. The phosphorylation status of protein kinase B (PKB/Akt), a downstream signalling protein in the PI3K pathway, and glycogen synthase kinase 3beta (GSK3beta), a substrate of PKB/Akt, was determined by western immunoblotting.

RESULTS

Following i.p. insulin challenge, blood glucose levels in Pten(+/-) mice remained depressed for up to 120 min, whereas glucose levels in wild-type mice began to recover after approximately 30 min. After glucose challenge, blood glucose returned to normal about twice as rapidly in Pten(+/-) mice. Enhanced glucose uptake was observed both in Pten(+/-) myocytes and in skeletal muscle of Pten(+/-) mice by PET. PKB and GSK3beta phosphorylation was enhanced and prolonged in Pten(+/-) myocytes.

CONCLUSIONS/INTERPRETATION

Pten is a key negative regulator of insulin-stimulated glucose uptake in vitro and in vivo. The partial reduction of Pten due to Pten haploinsufficiency is enough to elicit enhanced insulin sensitivity and glucose tolerance in Pten(+/-) mice.

Phosphoinositide 3-kinase signalling and FoxO transcription factors in rheumatoid arthritis

Although the mechanisms leading to the induction of RA (rheumatoid arthritis) are poorly understood, improper activation, proliferation, survival and retention of neutrophils, macrophages, lymphocytes and other leucocytes contribute to perpetuation of inflammation and eventual joint destruction through activation of stromal fibroblast-like synoviocytes. Fundamental studies in developmental biology, cellular biology and immunology have established critical roles for PI3K (phosphoinositide 3-kinase) signal transduction pathways in cellular chemotactic responses, proliferation, apoptosis and survival. Despite profound alteration of these cellular processes in RA, involvement of PI3K signalling pathways in this chronic inflammatory disease, and their assessment as potential therapeutic targets, has until recently received scant attention. This review highlights recent advances in our understanding of PI3K signalling pathways, in particular regulation of FoxO (forkhead box O) transcription factors, and their relevance to RA.

Cutting edge: T cell requirement for CD28 costimulation is due to negative regulation of TCR signals by PTEN

Recent studies suggest that the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) plays a critical role in the maintenance of self-tolerance. Using T cell-specific PTEN knockout mice (PTENDeltaT), we have identified a novel mechanism by which PTEN regulates T cell tolerance. We found that TCR stimulation alone, without CD28 costimulation, is sufficient to induce hyperactivation of the PI3K pathway, which leads to enhanced IL-2 production by naive PTENDeltaT T cells. Importantly, as a result of this increased response to TCR stimulation, PTENDeltaT CD4(+) T cells no longer require CD28 costimulation for in vitro or in vivo expansion. In fact, unlike wild-type T cells, PTENDeltaT CD4(+) T cells are not anergized by delivery of TCR stimulation alone. These data suggest that by negatively regulating TCR signals, PTEN imposes a requirement for CD28 costimulation, thus defining a novel mechanism for its role in self-tolerance.

Genetics of SLE in mice

Genetic studies in spontaneous, induced, and gene-manipulated mouse models of SLE have provided significant insights into the potential number and diversity of genes that can promote, resist, and modify lupus susceptibility. Novel genes and mechanisms of disease pathogenesis have also been identified. Importantly, mouse models have provided an initial view of the genomic landscape of lupus-affecting genes, and have documented the complexities of verifying and determining the role of specific candidate loci and genes. Mouse models of lupus should continue to serve as a vital approach to defining the genetics of SLE.

Critical role of PICT-1, a tumor suppressor candidate, in phosphatidylinositol 3,4,5-trisphosphate signals and tumorigenic transformation

The tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) regulates diverse cellular functions by dephosphorylating the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP(3)). Recent study revealed that PICT-1/GLTSCR2 bound to and stabilized PTEN protein in cells, implicating its roles in PTEN-governed PIP(3) signals. In this study, we demonstrate that RNA interference-mediated knockdown of PICT-1 in HeLa cells down-regulated endogenous PTEN and resulted in the activation of PIP(3) downstream effectors, such as protein kinase B/Akt. Furthermore, the PICT-1 knockdown promoted HeLa cell proliferation; however the proliferation of PTEN-null cells was not altered by the PICT-1 knockdown, suggesting its dependency on PTEN status. In addition, apoptosis of HeLa cells induced by staurosporine or serum-depletion was alleviated by the PICT-1 knockdown in the similar PTEN-dependent manner. Most strikingly, the PICT-1 knockdown in HeLa and NIH3T3 cells promoted anchorage-independent growth, a hallmark of tumorigenic transformation. Furthermore, PICT-1 was aberrantly expressed in 18 (41%) of 44 human neuroblastoma specimens, and the PICT-1 loss was associated with reduced PTEN protein expression in spite of the existence of PTEN mRNA. Collectively, these results suggest that PICT-1 plays a role in PIP(3) signals through controlling PTEN protein stability and the impairment in the PICT-1-PTEN regulatory unit may become a causative factor in human tumor(s).

Genetic background controls tumor development in PTEN-deficient mice

PTEN is one of the most frequently mutated tumor suppressor genes in human cancers. Germ line mutations of PTEN have been detected in three rare autosomal-dominant disorders. However, identical mutations in the PTEN gene may lead to different symptoms that have traditionally been described as different disorders, such as Cowden disease, Lhermitte-Duclos disease, and Bannayan-Zonana syndromes. This lack of genotype-phenotype correlation prompted us to directly test the possible effects of genetic background or modifier genes on PTEN-controlled tumorigenesis using genetically engineered mouse models. In this study, we generated two animal models in which either exon 5 (Pten(Delta5)) or promoter to exon 3 (Pten(-)) of the murine Pten gene were deleted and compared phenotypes associated with individual mutations on two genetic backgrounds. We found that the onset and spectrum of tumor formation depend significantly on the genetic background but less on the type of mutation generated. Our results suggest that PTEN plays a critical role in cancer development, and genetic background may influence the onset, the spectrum, and the progression of tumorigenesis caused by Pten mutation.

The evolution of phosphatidylinositol 3-kinases as regulators of growth and metabolism

Phosphatidylinositol 3-kinases (PI3Ks) evolved from a single enzyme that regulates vesicle trafficking in unicellular eukaryotes into a family of enzymes that regulate cellular metabolism and growth in multicellular organisms. In this review, we examine how the PI3K pathway has evolved to control these fundamental processes, and how this pathway is in turn regulated by intricate feedback and crosstalk mechanisms. In light of the recent advances in our understanding of the function of PI3Ks in the pathogenesis of diabetes and cancer, we discuss the exciting therapeutic opportunities for targeting this pathway to treat these diseases.

PTEN inhibits IL-2 receptor-mediated expansion of CD4+ CD25+ Tregs

One of the greatest barriers against harnessing the potential of CD4+ CD25+ Tregs as a cellular immunotherapy is their hypoproliferative phenotype. We have previously shown that the hypoproliferative response of Tregs to IL-2 is associated with defective downstream PI3K signaling. Here, we demonstrate that targeted deletion of the lipid phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10) regulates the peripheral homeostasis of Tregs in vivo and allows their expansion ex vivo in response to IL-2 alone. PTEN deficiency does not adversely affect either the thymic development or the function of Tregs, which retain their ability to suppress responder T cells in vitro and prevent colitis in vivo. Conversely, reexpression of PTEN in PTEN-deficient Tregs as well as in activated CD4+ T cells inhibits IL-2-dependent proliferation, confirming PTEN as a negative regulator of IL-2 receptor signaling. These data demonstrate that PTEN regulates the "anergic" response of Tregs to IL-2 in vitro and Treg homeostasis in vivo and indicate that inhibition of PTEN activity may facilitate the expansion of these cells for potential use in cellular immunotherapy.

Gene expression analysis reveals a signature of estrogen receptor activation upon loss of Pten in a mouse model of endometrial cancer

Loss of PTEN is the earliest detectable genetic lesion in the endometrioid subtype of endometrial cancer (EEC), a tumor thought to be associated with an increase in unopposed estrogen activity. Pten(+/-) mice develop endometrial neoplastic lesions with full penetrance, despite having normal estrogen levels. We have utilized oligonucleotide arrays to identify the alterations in gene expression patterns associated with loss of Pten and consequent neoplastic transformation of the endometrium. We show that 487 and 330 genes are substantially up- and downregulated, respectively, in Pten(+/-) mice. Several genes whose expression levels are impacted by loss of Pten are associated with pathways and functions that are relevant to the transformation and progression processes. Strikingly, we found that the expression levels of over 100 genes known to be regulated by estrogen receptor alpha (ERalpha) are also altered in the neoplastic uterus from Pten(+/-) mice, thus mimicking a hyperestrogenic environment. These results provide in vivo evidence supporting the hypothesis that loss of Pten and subsequent Akt activation result in the activation of several ERalpha-dependent pathways that, mimicking increased estrogen signaling, may play a pivotal role in the neoplastic process.

Inhibition of the phosphatidylinositol 3'-kinase pathway promotes autocrine Fas-induced death of phosphatase and tensin homologue-deficient prostate cancer cells

Rationally designed therapeutics that target the phosphatidylinositol 3'-kinase (PI3K) cell survival pathway are currently in preclinical and clinical development for cancer therapy. Drugs targeting the PI3K pathway aim to inhibit proliferation, promote apoptosis, and enhance chemosensitivity and radiosensitivity of cancer cells. The phosphatase and tensin homologue (PTEN) phosphatidylinositol 3'-phosphatase is a key negative regulator of the PI3K pathway. Inactivation of the PTEN tumor suppressor results in constitutive activation of the PI3K pathway and is found in approximately 50% of advanced prostate cancers, which correlates with a high Gleason score and poor prognosis. Inhibition of the PI3K pathway leads to apoptosis of prostate cancer cells; however, the precise mechanism by which this occurs is unknown. Here we report that apoptotic cell death of PTEN-deficient LNCaP and PC3 prostate cancer cells induced by the PI3K inhibitor LY294002 can be abrogated by disrupting Fas/Fas ligand (FasL) interactions with recombinant Fas:Fc fusion protein or FasL neutralizing antibody (Nok-1), or by expressing dominant-negative Fas-associated death domain. Furthermore, we find that apoptosis induced by expression of wild-type PTEN, driven by a tetracycline-inducible expression system in LNCaP cells, can be inhibited by blocking Fas/FasL interaction using Fas:Fc or Nok-1. These data show that apoptosis induced by blockade of the PI3K pathway in prostate tumor cells is mediated by an autocrine Fas/FasL apoptotic mechanism and the Fas apoptotic pathway is both necessary and sufficient to mediate apoptosis by PI3K inhibition.

The Selective Protein Kinase C β Inhibitor Enzastaurin Induces Apoptosis in Cutaneous T-Cell Lymphoma Cell Lines through the AKT Pathway

Enzastaurin displays pro-apoptotic properties against a spectrum of malignancies and is currently being investigated in clinical trials. We have investigated the effects of enzastaurin on the viability of the cutaneous T-cell lymphoma cell lines HuT-78 and HH by using 3-(4,5-dimethylthiazol-2-yl)-5(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay, cell cycle analysis, propidium iodide and annexin-V staining, and caspase-3-mediated proteolytic activation. Enzastaurin-treatment decreased cell viability, increased annexin V-FITC-positive cells, and increased the proportion of sub-G1 populations in both cell lines that was not reversed by the T-cell growth stimulating cytokines IL-2, IL-7, IL-15. Enzastaurin-induced cell death involved caspase-3-activated cleavage of poly(ADP-ribose) polymerase that was inhibited by the pan-caspase inhibitor ZVAD-fmk, whereas the increase in sub-G1 population was only partially inhibited by ZVAD-fmk. Furthermore, enzastaurin downregulated AKT activity and its downstream effectors GSK3beta and ribosomal protein S6. The phosphatidylinositol 3-kinase (PI3K)/AKT pathway has been implicated in the growth and survival of hematologic malignancies and inhibition of this pathway is considered as a therapeutic target. Protein kinase C activation contributes to PI3K/AKT activation, but it is unknown how enzastaurin may interfere with signaling through this pathway. These results demonstrate that enzastaurin, at clinically achievable concentrations, induces apoptosis and affects AKT signaling, and provide a rationale for further in vivo studies addressing the therapeutic efficacy in cutaneous T-cell lymphoma patients.

Hypomorphic mutation of PDK1 suppresses tumorigenesis in PTEN(+/-) mice

Many cancers possess elevated levels of PtdIns(3,4,5)P(3), the second messenger that induces activation of the protein kinases PKB/Akt and S6K and thereby stimulates cell proliferation, growth, and survival. The importance of this pathway in tumorigenesis has been highlighted by the finding that PTEN, the lipid phosphatase that breaks down PtdIns(3,4,5)P(3) to PtdIns(4,5)P(2), is frequently mutated in human cancer. Cells lacking PTEN possess elevated levels of PtdIns(3,4,5)P(3), PKB, and S6K activity and heterozygous PTEN(+/-) mice develop a variety of tumors. Knockout of PKBalpha in PTEN-deficient cells reduces aggressive growth and promotes apoptosis, whereas treatment of PTEN(+/-) mice with rapamycin, an inhibitor of the activation of S6K, reduces neoplasia. We explored the importance of PDK1, the protein kinase that activates PKB and S6K, in mediating tumorigenesis caused by the deletion of PTEN. We demonstrate that reducing the expression of PDK1 in PTEN(+/-) mice, markedly protects these animals from developing a wide range of tumors. Our findings provide genetic evidence that PDK1 is a key effector in mediating neoplasia resulting from loss of PTEN and also validate PDK1 as a promising anticancer target for the prevention of tumors that possess elevated PKB and S6K activity.

Genetic disorders of programmed cell death in the immune system

Human genetics offers new possibilities for understanding physiological regulatory mechanisms and disorders of the immune system. Genetic abnormalities of lymphocyte cell death programs have provided insights into mechanisms of receptor biology and principles of immune homeostasis and tolerance. Thus far, there are two major diseases of programmed cell death associated with inherited human mutations: the autoimmune lymphoproliferative syndrome and the caspase-eight deficiency state. We describe the details of their molecular pathogenesis and discuss how these diseases illustrate important concepts in immune regulation and genetics.

p18 Ink4c and Pten constrain a positive regulatory loop between cell growth and cell cycle control

Inactivation of the Rb-mediated G1 control pathway is a common event found in many types of human tumors. To test how the Rb pathway interacts with other pathways in tumor suppression, we characterized mice with mutations in both the cyclin-dependent kinase (CDK) inhibitor p18 Ink4c and the lipid phosphatase Pten, which regulates cell growth. The double mutant mice develop a wider spectrum of tumors, including prostate cancer in the anterior and dorsolateral lobes, with nearly complete penetrance and at an accelerated rate. The remaining wild-type allele of Pten was lost at a high frequency in Pten+/- cells but not in p18+/- Pten+/- or p18-/- Pten+/- prostate tumor cells, nor in other Pten+/- tumor cells, suggesting a tissue- and genetic background-dependent haploinsufficiency of Pten in tumor suppression. p18 deletion, CDK4 overexpression, or oncoviral inactivation of Rb family proteins caused activation of Akt/PKB that was recessive to the reduction of PTEN activity. We suggest that p18 and Pten cooperate in tumor suppression by constraining a positive regulatory loop between cell growth and cell cycle control pathways.

Identification of a tumour suppressor network opposing nuclear Akt function

The proto-oncogene AKT (also known as PKB) is activated in many human cancers, mostly owing to loss of the PTEN tumour suppressor. In such tumours, AKT becomes enriched at cell membranes where it is activated by phosphorylation. Yet many targets inhibited by phosphorylated AKT (for example, the FOXO transcription factors) are nuclear; it has remained unclear how relevant nuclear phosphorylated AKT (pAKT) function is for tumorigenesis. Here we show that the PMLtumour suppressor prevents cancer by inactivating pAKT inside the nucleus. We find in a mouse model that Pml loss markedly accelerates tumour onset, incidence and progression in Pten-heterozygous mutants, and leads to female sterility with features that recapitulate the phenotype of Foxo3a knockout mice. We show that Pml deficiency on its own leads to tumorigenesis in the prostate, a tissue that is exquisitely sensitive to pAkt levels, and demonstrate that Pml specifically recruits the Akt phosphatase PP2a as well as pAkt into Pml nuclear bodies. Notably, we find that Pml-null cells are impaired in PP2a phosphatase activity towards Akt, and thus accumulate nuclear pAkt. As a consequence, the progressive reduction in Pml dose leads to inactivation of Foxo3a-mediated transcription of proapoptotic Bim and the cell cycle inhibitor p27(kip1). Our results demonstrate that Pml orchestrates a nuclear tumour suppressor network for inactivation of nuclear pAkt, and thus highlight the importance of AKT compartmentalization in human cancer pathogenesis and treatment.

The PTEN and INK4A/ARF tumor suppressors maintain myelolymphoid homeostasis and cooperate to constrain histiocytic sarcoma development in humans

Histiocytic sarcoma (HS) is a rare malignant proliferation of histiocytes of uncertain molecular pathogenesis. Here, genetic analysis of coincident loss of Pten and Ink4a/Arf tumor suppressors in the mouse revealed a neoplastic phenotype dominated by a premalignant expansion of biphenotypic myelolymphoid cells followed by the development of HS. Pten protein loss occurred only in the histiocytic portion of tumors, suggesting a stepwise genetic inactivation in the generation of HS. Similarly, human HS showed genetic or epigenetic inactivation of PTEN, p16(INK4A), and p14(ARF), supporting the relevance of this genetically engineered mouse model of HS. These genetic and translational observations establish a cooperative role of Pten and Ink4a/Arf in the development of HS and provide mechanistic insights into the pathogenesis of human HS.

Human Alveolar Macrophages Are Deficient in PTEN

Human alveolar macrophages play a critical role in host defense and in the development of inflammation and fibrosis in the lung. Unlike their precursor cells, blood monocytes, alveolar macrophages are long-lived and tend to be resistant to apoptotic stimuli. In this study, we examined the role of differentiation in altering baseline phosphatidylinositol (PI) 3-kinase/Akt activity. We found that differentiation increased activity of pro-survival PI 3-kinase/Akt while decreasing amounts of the negative PI 3-kinase regulator, PTEN. PTEN is a lipid phosphatase with activity against phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3), the major bioactive product of PI 3-kinase. Examining in vivo differentiation of alveolar macrophages (by comparing blood monocytes to alveolar macrophages from single donors), we found that differentiation resulted in increased baseline reactive oxygen species (ROS) in the alveolar macrophages. This led to a deficiency in PTEN, increased activity of Akt, and prolonged survival of alveolar macrophages. These data support the hypothesis that alterations in ROS levels contribute to macrophage homeostasis by altering the balance between PI 3-kinase/Akt and the phosphatase, PTEN.

Phosphoinositide 3-kinase controls early and late events in mammalian cell division

Phosphoinositide 3-kinase (PI3K) plays a crucial role in triggering cell division. To initiate this process, PI3K induces two distinct routes, of which one promotes cell growth and the other regulates cyclin-dependent kinases. Fine-tuned PI3K regulation is also required for later cell cycle phases. Here, we review the multiple points at which PI3K controls cell division and discuss its impact on human cancer.

Advances in understanding angiogenesis through molecular studies

Tumors, in most cases, need angiogenesis for their sustained growth. A great deal of evidence has suggested that the process of angiogenesis is regulated by the balance between proangiogenic and antiangiogenic factors. Thus, the inhibition of tumor angiogenesis has been considered to be one of the key targets in anticancer therapy, and more than 60 antiangiogenic compounds are currently under clinical evaluation in cancer patients. However, the molecular mechanisms responsible for the activity of many of these antiangiogenic compounds are still not well understood. The recent development of microarray technology has allowed us to investigate the mechanism of action of these inhibitors more rapidly and extensively. With the use of microarray technology, novel molecules and pathways are shown to play a role in angiogenesis. This article also reviews new experimental approaches combined with microarray analysis to identify the molecular pathways involved in tumor-host interactions. Elucidation of the pathways that mediate both angiogenic and antiangiogenic responses will help us to develop better anticancer therapies.

Sphingosine kinase 1–mediated inhibition of Fas death signaling in rheumatoid arthritis B lymphoblastoid cells

OBJECTIVE

It is becoming increasingly apparent that B cells play an important role in the pathogenesis of rheumatoid arthritis (RA). Due to the scarcity of B cells in RA, it has been technically difficult to functionally characterize B cell apoptosis in this disease. As a necessary first step to identify candidate aberrations, we investigated Fas-mediated signaling events in immortalized peripheral blood B lymphoblastoid cell lines (LCLs) from patients with RA and controls.

METHODS

Cell death was determined by the MTS assay, and apoptosis was detected by the TUNEL assay and DNA laddering. Proteolytic activation of caspase 3 was determined by immunoblotting, and its enzymatic activity was determined by a fluorometric technique. Messenger RNA (mRNA) expression was quantified by real-time polymerase chain reaction (PCR) analysis. The functional role of sphingosine kinase (SPHK) was determined by measuring its enzymatic activity, by quantifying the levels of its product, sphingosine 1-phosphate (S1P), and by investigating the ability of the SPHK inhibitor N,N-dimethylsphingosine and isozyme-specific small interfering RNA (siRNA) oligonucleotides to reverse signaling aberrations.

RESULTS

LCLs from patients with RA displayed disease-specific Fas-mediated signal transduction impairment with consequent resistance to cell death. RA LCLs displayed high constitutive SPHK activity and increased levels of S1P. Real-time PCR analysis showed higher SPHK-1 mRNA expression levels in RA patients compared with paired controls. Increased SPHK-1 (but not SPHK-2) mRNA levels were observed in synovial tissue from RA patients. Competitive inhibitors of SPHK reversed the resistance of RA LCLs to Fas-induced apoptosis. Additionally, resistance to Fas-mediated signaling was reversed by siRNA oligonucleotides specific for SPHK-1 but not by oligonucleotides specific for SPHK-2.

CONCLUSION

These findings demonstrate disease-specific resistance to Fas-mediated death signaling in patients with RA and implicate increased SPHK-1 activity as the cause of this aberration.

Class IB-Phosphatidylinositol 3-Kinase (PI3K) Deficiency Ameliorates IA-PI3K-Induced Systemic Lupus but Not T Cell Invasion

Class I PI3K catalyzes formation of 3-poly-phosphoinositides. The family is divided into IA isoforms, activated by Tyr kinases and the IB isoform (PI3Kgamma), activated by G protein-coupled receptors. Mutations that affect PI3K are implicated in chronic inflammation, although the differential contribution of each isoform to pathology has not been elucidated. Enhanced activation of class IA-PI3K in T cells extends CD4+ memory cell survival, triggering an invasive lymphoproliferative disorder and systemic lupus. As both IA- and IB-PI3K isoforms regulate T cell activation, and activated pathogenic CD4+ memory cells are involved in triggering systemic lupus, we examined whether deletion of IB could reduce the pathological consequences of increased IA-PI3K activity. IB-PI3Kgamma deficiency did not abolish invasion or lymphoproliferation, but reduced CD4+ memory cell survival, autoantibody production, glomerulonephritis, and systemic lupus. Deletion of the IB-PI3Kgamma isoform thus decreased survival of pathogenic CD4+ memory cells, selectively inhibiting systemic lupus development. These results validate the PI3Kgamma isoform as a target for systemic lupus erythematosus treatment.

NF-kappaB couples protein kinase B/Akt signaling to distinct survival pathways and the regulation of lymphocyte homeostasis in vivo

Protein kinase B (PKBalpha/Akt1) a PI3K-dependent serine-threonine kinase, promotes T cell viability in response to many stimuli and regulates homeostasis and autoimmune disease in vivo. To dissect the mechanisms by which PKB inhibits apoptosis, we have examined the pathways downstream of PKB that promote survival after cytokine withdrawal vs Fas-mediated death. Our studies show that PKB-mediated survival after cytokine withdrawal is independent of protein synthesis and the induction of NF-kappaB. In contrast, PKB requires de novo gene transcription by NF-kappaB to block apoptosis triggered by the Fas death receptor. Using gene-deficient and transgenic mouse models, we establish that NF-kappaB1, and not c-Rel, is the critical signaling molecule downstream of the PI3K-PTEN-PKB signaling axis that regulates lymphocyte homeostasis.

Constitutively activated CK2 potentially plays a pivotal role in Theileria-induced lymphocyte transformation

Activation of casein kinase II (CK2) was one of the first observations made on how Theileria parasites manipulate host cell signal transduction pathways and we argue that CK2 induction may in fact contribute to many of the different activation events that have been described since 1993 for Theileria-infected lymphocytes such as sustained activation of transcription factors c-Myc and NF-κB. CK2 also contributes to infected lymphocyte survival by inhibiting caspase activation and is probably behind constitutive PI3-K activation by phosphorylating PTEN. Finally, we also discuss how CK2A may act not only as a kinase, but also as a stimulatory subunit for the protein phosphatase PP2A, so dampening down the MEK/ERK and Akt/PKB pathways and for all these reasons we propose CK2 as a central player in Theileria-induced lymphocyte transformation.

Protein phosphatase 2A is a negative regulator of IL-2 production in patients with systemic lupus erythematosus

Decreased IL-2 production in systemic lupus erythematosus (SLE) represents a central component of the disease immunopathology. We report that the message, protein, and enzymatic activity of the catalytic subunit of protein phosphatase 2A (PP2Ac), but not PP1, are increased in patients with SLE regardless of disease activity and treatment and in a disease-specific manner. Treatment of SLE T cells with PP2Ac-siRNA decreased the protein levels and activity of PP2Ac in a specific manner and increased the levels of phosphorylated cAMP response element-binding protein and its binding to the IL2 and c-fos promoters, as well as increased activator protein 1 activity, causing normalization of IL-2 production. Our data document increased activity of PP2A as a novel SLE disease-specific abnormality and define a distinct mechanism whereby it represses IL-2 production. We propose the use of PP2Ac-siRNA as a novel tool to correct T cell IL-2 production in SLE patients.

Phosphatase and tensin homolog regulation of islet growth and glucose homeostasis

The Irs2 branch of the insulin/insulin-like growth factor signaling cascade activates the phosphatidylinositol 3-kinase --> Akt --> Foxo1 cascade in many tissues, including hepatocytes and pancreatic beta-cells. The 3'-lipid phosphatase Pten ordinarily attenuates this cascade; however, its influence on beta-cell growth or function is unknown. To determine whether decreased Pten expression could restore beta-cell function and prevent diabetes in Irs2(-/-) mice, we generated wild type or Irs2 knock-out mice that were haploinsufficient for Pten (Irs2(-/-)::Pten(+/-)). Irs2(-/-) mice develop diabetes by 3 months of age as beta-cell mass declined progressively until insulin production was lost. Pten insufficiency increased peripheral insulin sensitivity in wild type and Irs2(-/-) mice and increased Akt and Foxo1 phosphorylation in the islets. Glucose tolerance improved in the Pten(+/-) mice, although beta-cell mass and circulating insulin levels decreased. Compared with Irs2(-/-) mice, the Irs2(-/-)::Pten(+/-) mice displayed nearly normal glucose tolerance and survived without diabetes, because normal but small islets produced sufficient insulin until the mice died of lymphoproliferative disease at 12 months age. Thus, steps to enhance phosphatidylinositol 3-kinase signaling can promote beta-cell growth, function, and survival without the Irs2 branch of the insulin/insulin-like growth factor signaling cascade.

Activation of AKT kinases in cancer: implications for therapeutic targeting

The AKT1, AKT2, and AKT3 kinases have emerged as critical mediators of signal transduction pathways downstream of activated tyrosine kinases and phosphatidylinositol 3-kinase. An ever-increasing list of AKT substrates has precisely defined the multiple functions of this kinase family in normal physiology and disease states. Cellular processes regulated by AKT include cell proliferation and survival, cell size and response to nutrient availability, intermediary metabolism, angiogenesis, and tissue invasion. All these processes represent hallmarks of cancer, and a burgeoning literature has defined the importance of AKT alterations in human cancer and experimental models of tumorigenesis, continuing the legacy represented by the original identification of v-Akt as the transforming oncogene of a murine retrovirus. Many oncoproteins and tumor suppressors intersect in the AKT pathway, finely regulating cellular functions at the interface of signal transduction and classical metabolic regulation. This careful balance is altered in human cancer by a variety of activating and inactivating mechanisms that target both AKT and interrelated proteins. Reprogramming of this altered circuitry by pharmacologic modulation of the AKT pathway represents a powerful strategy for rational cancer therapy. In this review, we summarize a large body of data, from many types of cancer, indicating that AKT activation is one of the most common molecular alterations in human malignancy. We also review mechanisms of activation of AKT kinases, examples of therapeutic modulation of the AKT pathway in animal models, and the current status of efforts to target molecular components of the AKT pathway for cancer therapy and, possibly, cancer prevention.

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

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