Nuclear Ptdlns(3,4,5)P3 signaling: an ongoing story

The Role of PTEN in Epithelial–Mesenchymal Transition

Simple Summary

The PTEN phosphatase is a ubiquitously expressed tumor suppressor, which inhibits the PI3K/AKT pathway in the cell. The PI3K/AKT pathway is considered to be one of the main signaling pathways that drives the proliferation of cancer cells. Furthermore, the same pathway controls the epithelial–mesenchymal transition (EMT). EMT is an evolutionarily conserved developmental program, which, upon aberrant reactivation, is also involved in the formation of cancer metastases. Importantly, metastasis is the leading cause of cancer-associated deaths. In this review, we discuss the literature data that highlight the role of PTEN in EMT. Based on this knowledge, we speculate about new possible strategies for cancer treatment.

Abstract

Phosphatase and Tensin Homolog deleted on Chromosome 10 (PTEN) is one of the critical tumor suppressor genes and the main negative regulator of the PI3K pathway. PTEN is frequently found to be inactivated, either partially or fully, in various malignancies. The PI3K/AKT pathway is considered to be one of the main signaling cues that drives the proliferation of cells. Perhaps it is not surprising, then, that this pathway is hyperactivated in highly proliferative tumors. Importantly, the PI3K/AKT pathway also coordinates the epithelial–mesenchymal transition (EMT), which is pivotal for the initiation of metastases and hence is regarded as an attractive target for the treatment of metastatic cancer. It was shown that PTEN suppresses EMT, although the exact mechanism of this effect is still not fully understood. This review is an attempt to systematize the published information on the role of PTEN in the development of malignant tumors, with a main focus on the regulation of the PI3K/AKT pathway in EMT.

PTEN Nuclear Functions

For years, clinical and basic researchers have been aware of the presence of PTEN in the nucleus in cell culture, animal models, and both healthy and diseased human tissues. Despite the early recognition of nuclear PTEN, the understanding of the mechanisms of its nuclear localization, function in the nucleus, and importance in biology and human disease has been lacking. Over the last decade, emerging concepts for the complex involvement of nuclear PTEN in a variety of processes, including genome maintenance and DNA repair, cell-cycle control, gene expression, and DNA replication, are illuminating what could prove to be the key path toward a full understanding of PTEN function in health and disease. Dysregulation of nuclear PTEN is now considered an important aspect of the etiology of many pathologic conditions, prompting reconsideration of the therapeutic approaches aimed at countering the consequences of PTEN deficiency. This new knowledge is fueling the development of innovative therapeutic modalities for a broad spectrum of human conditions, from cancer and metabolic diseases, to neurological disorders and autism.

Nuclear phosphoinositides and their impact on nuclear functions

Polyphosphoinositides (PPIn) are important lipid molecules whose levels are de-regulated in human diseases such as cancer, neurodegenerative disorders and metabolic syndromes. PPIn are synthesized and degraded by an array of kinases, phosphatases and lipases which are localized to various subcellular compartments and are subject to regulation in response to both extra- and intracellular cues. Changes in the activities of enzymes that metabolize PPIn lead to changes in the profiles of PPIn in various subcellular compartments. Understanding how subcellular PPIn are regulated and how they affect downstream signaling is critical to understanding their roles in human diseases. PPIn are present in the nucleus, and their levels are changed in response to various stimuli, suggesting that they may serve to regulate specific nuclear functions. However, the lack of nuclear downstream targets has hindered the definition of which pathways nuclear PPIn affect. Over recent years, targeted and global proteomic studies have identified a plethora of potential PPIn-interacting proteins involved in many aspects of transcription, chromatin remodelling and mRNA maturation, suggesting that PPIn signalling within the nucleus represents a largely unexplored novel layer of complexity in the regulation of nuclear functions.

Nuclear phosphoinositides: location, regulation and function

Lipid signalling in human disease is an important field of investigation and stems from the fact that phosphoinositide signalling has been implicated in the control of nearly all the important cellular pathways including metabolism, cell cycle control, membrane trafficking, apoptosis and neuronal conduction. A distinct nuclear inositide signalling metabolism has been identified, thus defining a new role for inositides in the nucleus, which are now considered essential co-factors for several nuclear processes, including DNA repair, transcription regulation, and RNA dynamics. Deregulation of phoshoinositide metabolism within the nuclear compartment may contribute to disease progression in several disorders, such as chronic inflammation, cancer, metabolic, and degenerative syndromes. In order to utilize these very druggable pathways for human benefit there is a need to identify how nuclear inositides are regulated specifically within this compartment and what downstream nuclear effectors process and integrate inositide signalling cascades in order to specifically control nuclear function. Here we describe some of the facets of nuclear inositide metabolism with a focus on their relationship to cell cycle control and differentiation.

Different conformations of phosphatase and tensin homolog, deleted on chromosome 10 (PTEN) protein within the nucleus and cytoplasm of neurons

PTEN is a critical gene involved in the regulation of many cellular processes. The product of this gene has dual phosphatase activity and is able to dephosphorylate the 5′ end of the phosphatidylinositol (3,4,5)-trisphosphate. Within the cellular nucleus, this protein has been associated with regulation of the expression of many genes, although the mechanism of this regulation remains unclear. In this paper, two specific oligonucleotide aptamers were developed and selected, using the SELEX procedure, according to their ability to detect the PTEN protein in different subcellular compartments of neurons. While one aptamer was able to detect PTEN in the nucleus, the other recognized PTEN in the cytoplasm. The recognition pattern of PTEN by both aptamers was confirmed using antibodies in western blots of the proteins purified from mouse cerebellar homogenates and subcellular fractions. Additionally, we demonstrated that the two aptamers recognized different epitopes of the target peptide. The results presented here could not be fully explained by the canonical phosphatase structure of PTEN, suggesting the existence of different conformations of phosphatase in the nucleus and the cytoplasm.

Purinergic receptor-mediated rapid depletion of nuclear phosphorylated Akt depends on pleckstrin homology domain leucine-rich repeat phosphatase, calcineurin, protein phosphatase 2A, and PTEN phosphatases

Akt is an important oncoprotein, and data suggest a critical role for nuclear Akt in cancer development. We have previously described a rapid (3-5 min) and P2X7-dependent depletion of nuclear phosphorylated Akt (pAkt) and effects on downstream targets, and here we studied mechanisms behind the pAkt depletion. We show that cholesterol-lowering drugs, statins, or extracellular ATP, induced a complex and coordinated response in insulin-stimulated A549 cells leading to depletion of nuclear pAkt. It involved protein/lipid phosphatases PTEN, pleckstrin homology domain leucine-rich repeat phosphatase (PHLPP1 and -2), protein phosphatase 2A (PP2A), and calcineurin. We employed immunocytology, immunoprecipitation, and proximity ligation assay techniques and show that PHLPP and calcineurin translocated to the nucleus and formed complexes with Akt within 3 min. Also PTEN translocated to the nucleus and then co-localized with pAkt close to the nuclear membrane. An inhibitor of the scaffolding immunophilin FK506-binding protein 51 (FKBP51) and calcineurin, FK506, prevented depletion of nuclear pAkt. Furthermore, okadaic acid, an inhibitor of PP2A, prevented the nuclear pAkt depletion. Chemical inhibition and siRNA indicated that PHLPP, PP2A, and PTEN were required for a robust depletion of nuclear pAkt, and in prostate cancer cells lacking PTEN, transfection of PTEN restored the statin-induced pAkt depletion. The activation of protein and lipid phosphatases was paralleled by a rapid proliferating cell nuclear antigen (PCNA) translocation to the nucleus, a PCNA-p21(cip1) complex formation, and cyclin D1 degradation. We conclude that these effects reflect a signaling pathway for rapid depletion of pAkt that may stop the cell cycle.

The regulatory subunits of PI3K, p85alpha and p85beta, interact with XBP-1 and increase its nuclear translocation

Despite the fact that X-box binding protein-1 (XBP-1) is one of the main regulators of the unfolded protein response (UPR), the modulators of XBP-1 are poorly understood. Here, we show that the regulatory subunits of phosphotidyl inositol 3-kinase (PI3K), p85alpha (encoded by Pik3r1) and p85beta (encoded by Pik3r2) form heterodimers that are disrupted by insulin treatment. This disruption of heterodimerization allows the resulting monomers of p85 to interact with, and increase the nuclear translocation of, the spliced form of XBP-1 (XBP-1s). The interaction between p85 and XBP-1s is lost in ob/ob mice, resulting in a severe defect in XBP-1s translocation to the nucleus and thus in the resolution of endoplasmic reticulum (ER) stress. These defects are ameliorated when p85alpha and p85beta are overexpressed in the liver of ob/ob mice. Our results define a previously unknown insulin receptor signaling pathway and provide new mechanistic insight into the development of ER stress during obesity.

Angiotensin-(1-7)-angiotensin-converting enzyme 2 attenuates reactive oxygen species formation to angiotensin II within the cell nucleus

The angiotensin (Ang) type 1 receptor (AT(1)R) is highly expressed on renal nuclei and stimulates reactive oxygen species (ROS). It is not known whether other functional components of the Ang system regulate the nuclear Ang II-AT(1)R ROS pathway. Therefore, we examined the expression of Ang receptors in nuclei isolated from the kidneys of young adult (1.5 years) and older adult (3.0 to 5.0 years) sheep. Binding studies in renal nuclei revealed the AT(2)R as the predominant receptor subtype ( approximately 80%) in young sheep, with the Ang-(1-7) (AT(7)R; Mas protein) and AT(1)R antagonists competing for the remaining sites. Conversely, in older sheep, the AT(1)R accounted for approximately 85% of nuclear sites, whereas the Ang type 2 receptor and AT(7)R subtypes comprise approximately 20% of remaining sites. Ang II increased nuclear ROS to a greater extent in older (97+/-22%; n=6) versus young animals (7+/-2%; P=0.01; n=4), and this was abolished by an AT(1)R antagonist. The AT(7)R antagonist D-Ala(7)-Ang-(1-7) increased ROS formation to Ang II by approximately 2-fold (174+/-5% versus 97+/-22%; P<0.05) in older adults. Immunoblots of renal nuclei revealed protein bands for the AT(7)R and Ang-converting enzyme 2 (ACE2), which metabolizes Ang II to Ang-(1-7). The ACE2 inhibitor MLN4760 also exacerbated the Ang II-dependent formation of ROS (156+/-15%) and abolished the generation of Ang-(1-7) from Ang II. We conclude that an ACE2-Ang-(1-7)-AT(7)R pathway modulates Ang II-dependent ROS formation within the nucleus, providing a unique protective mechanism against oxidative stress and cell damage.

CoA synthase is in complex with p85alphaPI3K and affects PI3K signaling pathway

The complex interplay between cellular signaling and metabolism in eukaryotic cells just start to emerge. Coenzyme A (CoA) and its derivatives play a key role in cell metabolism and also participate in regulatory processes. CoA synthase (CoASy) is a mitochondria-associated enzyme which mediates two final stages of de novo CoA biosynthesis. Here, we report that CoASy is involved in signaling events in the cell and forms a functional complex with p85alphaPI3K in vivo. Importantly, observed interaction of endogenous CoASy and p85alphaPI3K is regulated in a growth factor dependent manner. Surprisingly, both catalytic p110alpha and regulatory p85alpha subunits of PI3K were detected in mitochondrial fraction where mitochondria-localized p85alphaPI3K was found in complex with CoASy. Unexpectedly, significant changes of PI3K signaling pathway activity were observed in experiments with siRNA-mediated CoASy knockdown pointing on the role of CoA biosynthetic pathway in signal transduction.

The angiotensin II-AT1 receptor stimulates reactive oxygen species within the cell nucleus

We and others have reported significant expression of the Ang II Type 1 receptor (AT1R) on renal nuclei; thus, the present study assessed the functional pathways and distribution of the intracellular AT1R on isolated nuclei. Ang II (1nM) stimulated DCF fluorescence, an intranuclear indicator of reactive oxygen species (ROS), while the AT1R antagonist losartan or the NADPH oxidase (NOX) inhibitor DPI abolished the increase in ROS. Dual labeling of nuclei with antibodies against nucleoporin 62 (Nup62) and AT1R or the NADPH oxidase isoform NOX4 revealed complete overlap of the Nup62 and AT1R (99%) by flow cytometry, while NOX4 was present on 65% of nuclei. Treatment of nuclei with a PKC agonist increased ROS while the PKC inhibitor GF109203X or PI3 kinase inhibitor LY294002 abolished Ang II stimulation of ROS. We conclude that the Ang II-AT1R-PKC axis may directly influence nuclear function within the kidney through a redox sensitive pathway.

Spatio-temporal dynamics of phosphatidylinositol-3,4,5-trisphosphate signalling

Many effects of insulin, insulin-like growth factors and other receptor stimuli are mediated via the phospholipid second messenger phosphatidylinositol-3,4,5-trisphosphate (PIP(3)). PIP(3) is formed by the activity of phosphoinositide 3-kinases in the plasma membrane, where it serves to recruit signalling proteins. These proteins coordinate complex events leading to changes in cell metabolism, growth, movement and survival. Over the past decade, new techniques for measurements of PIP(3) in the plasma membrane of individual living cells have markedly improved our understanding of the role of this messenger in a variety of cellular processes. This review summarises the mechanisms involved in formation and degradation of PIP(3) in insulin-responsive cells, how PIP(3) can be measured in individual cells as well as accumulating evidence that the plasma membrane PIP(3) concentration undergoes complex spatio-temporal patterns in many types of cells, with particular emphasis on autocrine insulin-induced PIP(3) oscillations in pancreatic beta-cells.

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

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

PIKK-ing a new partner: a new role for PKB in the DNA damage response

The protein kinase PKB/Akt has long been associated with regulating signaling pathways that promote cell survival and cell growth, for example, in response to growth factors. In contrast, the DNA-dependent protein kinase (DNA-PK) is required for the repair of DNA damage and for cell survival after exposure to DNA-damaging agents, such as ionizing radiation. In a recent paper published in Molecular Cell, Hemmings and colleagues provide evidence that DNA-PK is required for the activation of PKB in response to exposure to ionizing radiation, suggesting that these two protein kinases may act together to promote survival after DNA damage.

Expression of yeast lipid phosphatase Sac1p is regulated by phosphatidylinositol-4-phosphate

Background

Phosphoinositides play a central role in regulating processes at intracellular membranes. In yeast, a large number of phospholipid biosynthetic enzymes use a common mechanism for transcriptional regulation. Yet, how the expression of genes encoding lipid kinases and phosphatases is regulated remains unknown.

Results

Here we show that the expression of lipid phosphatase Sac1p in the yeast Saccharomyces cerevisiae is regulated in response to changes in phosphatidylinositol-4-phosphate (PI(4)P) concentrations. Unlike genes encoding enzymes involved in phospholipid biosynthesis, expression of the SAC1 gene is independent of inositol levels. We identified a novel 9-bp motif within the 5' untranslated region (5'-UTR) of SAC1 that is responsible for PI(4)P-mediated regulation. Upregulation of SAC1 promoter activity correlates with elevated levels of Sac1 protein levels.

Conclusion

Regulation of Sac1p expression via the concentration of its major substrate PI(4)P ensures proper maintenance of compartment-specific pools of PI(4)P.

Insulin-increased prolactin gene expression requires actin treadmilling: potential role for p21 activated kinase

Insulin-increased prolactin gene transcription in GH4 cells was enhanced by binding on fibronectin. This was mediated by receptor-like protein tyrosine phosphatase alpha, which activated Src, Rho, and phosphatidylinositol 3-kinase. It suggested that insulin signaling to gene transcription was partly dependent on actin rearrangement. This was confirmed through studies using inhibitors of actin treadmilling. Cytochalasin D, jasplakinolide, latrunculin B, and swinholide A altered the actin cytoskeleton of GH4 cells, as assessed by Alexa Fluor phalloidin staining, and inhibited insulin-increased prolactin gene transcription. These reagents did not affect the controls. Nor was it due to a gross defect of insulin signaling because activation/translocation of glycogen synthase kinase 3beta and mammalian target of rapamycin were not affected. Expression of wild-type and mutant actin treadmilling agents, Cdc42, TC10, neuronal Wiskott-Aldrich syndrome protein, and Nck, indicated that they were essential to insulin-increased prolactin gene expression, and suggested that activation of p21 associated kinase (PAK) might also be essential to this process. PAK expression also increased and PAK mutants decreased prolactin promoter activity in insulin-treated cells. The activation of PAK in the presence of inhibitors was also consistent with a role in activation of insulin-increased prolactin gene expression. Finally, small interfering RNA-mediated reduction of PAK decreased the effect of insulin on prolactin gene expression. Thus, it is likely that insulin activation of actin treadmilling through Cdc42/TC10 and neuronal Wiskott-Aldrich syndrome protein activates PAK and prolactin gene transcription.

Statins and ATP regulate nuclear pAkt via the P2X7 purinergic receptor in epithelial cells

Many studies have documented P2X7 receptor functions in cells of mesenchymal origin. P2X7 is also expressed in epithelial cells and its role in these cells remains largely unknown. Our data indicate that P2X7 regulate nuclear pAkt in epithelial cells. We show that low concentration of atorvastatin, a drug inhibiting HMG-CoA reductase and cholesterol metabolism, or the natural agonist extracellular ATP rapidly decreased the level of insulin-induced phosphorylated Akt in the nucleus. This effect was seen within minutes and was inhibited by P2X7 inhibitors. Experiments employing P2X7 siRNA and HEK293 cells heterologously expressing P2X7 and in vivo experiments further supported an involvement of P2X7. These data indicate that extracellular ATP and statins via the P2X7 receptor modulate insulin-induced Akt signaling in epithelial cells.

PTEN, more than the AKT pathway

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN)/phosphatidylinositol 3-kinase (PI3K)/AKT constitute an important pathway regulating the signaling of multiple biological processes such as apoptosis, metabolism, cell proliferation and cell growth. PTEN is a dual protein/lipid phosphatase and its main substrate phosphatidyl-inositol 3,4,5 triphosphate (PIP3) is the product of PI3K. Increase in PIP3 recruits AKT to the membrane where is activated by other kinases also dependent on PIP3. Many components of this pathway have been described as causal forces in cancer. PTEN activity is lost by mutations, deletions or promoter methylation silencing at high frequency in many primary and metastatic human cancers. Germ line mutations of PTEN are found in several familial cancer predisposition syndromes. Recently, many activating mutations in the PI3KCA gene (coding for the p110alpha catalytic subunit of PI3K) have been described in human tumors. Activation of PI3K and AKT are reported to occur in breast, ovarian, pancreatic, esophageal and other cancers. Genetically modified mice confirm these PTEN activities. Tissue-specific deletions of PTEN usually provoke cancer. Moreover, an absence of PTEN cooperates with an absence of p53 to promote cancer. However, we have observed very different results with the expression of activated versions of AKT in several tissues. Activated AKT transgenic lines do not develop tumors in breast or prostate tissues and do not cooperate with an absence of p53. This data suggest that an AKT-independent mechanism contributes to PTEN tumorigenesis. Crosses with transgenic mice expressing possible PTEN targets indicate that neither cyclin D1 nor p53 are these AKT-independent targets. However, AKT is more than a passive bridge toward PTEN tumorigenesis, since its expression not only allows but also enforces and accelerates the tumorigenic process in combination with other oncogenes.

Localization of agonist-sensitive PtdIns(3,4,5)P3 reveals a nuclear pool that is insensitive to PTEN expression

Phosphatidylinositol (3,4,5) trisphosphate [PtdIns(3,4,5)P3] is a lipid second messenger, produced by Type I phosphoinositide 3-kinases (PI 3-kinases), which mediates intracellular responses to many growth factors. Although PI 3-kinases are implicated in events at both the plasma membrane and intracellular sites, including the nucleus, direct evidence for the occurrence of PtdIns(3,4,5)P3 at non-plasma membrane locations is limited. We made use of the pleckstrin homology (PH) domain of general receptor for phosphoinositides (Grp1) to detect PtdIns(3,4,5)P3 in an on-section labeling approach by quantitative immunogold electron microscopy. Swiss 3T3 cells contained low levels of PtdIns(3,4,5)P3 that increased up to 15-fold upon stimulation with platelet-derived growth factor (PDGF). The signal was sensitive to PI 3-kinase inhibitors and present mainly at plasma membranes, including lamellipodia, and in a surprisingly large pool within the nuclear matrix. Comparatively little labeling was observed in endomembranes. A similar distribution of PtdIns(3,4,5)P3 was observed in U87MG cells, which lack the PtdIns(3,4,5)P3 phosphatase, PTEN. Re-expression of PTEN into U87MG cells ablated plasma membrane PtdIns(3,4,5)P3, but not the nuclear pool of this lipid even when PTEN was targeted to nuclei. These data have important implications for the versatility of PI 3-kinase signaling and for the proposed functions of PTEN in the nucleus.

Autism, asthma, inflammation, and the hygiene hypothesis

Inflammation and the genes, molecules, and biological pathways that lead to inflammatory processes influence many important and disparate biological processes and disease states that are quite often not generally considered classical inflammatory or autoimmune disorders. These include development, reproduction, aging, tumor development and tumor rejection, cardiovascular pathologies, metabolic disorders, as well as neurological and psychiatric disorders. This paper compares parallel aspects of autism and inflammatory disorders with an emphasis on asthma. These comparisons include epidemiological, morphometric, molecular, and genetic aspects of both disease types, contributing to a hypothesis of autism in the context of the immune based hygiene hypothesis. This hypothesis is meant to address the apparent rise in the prevalence of autism in the population.

PTEN enters the nuclear age

Regulation of the PTEN tumor suppressor protein is poorly understood. In this issue, Wang et al. (2007) and Trotman et al. (2007) describe how ubiquitination regulates PTEN stability and its nuclear localization. Additionally, Shen et al. (2007) report that a nuclear pool of PTEN helps to maintain chromosomal stability.

PTEN Regulation, a Novel Function for the p85 Subunit of Phosphoinositide 3-Kinase

Timely regulation of phosphatidylinositol-3,4-bisphosphate [PI(3,4)P2] and phosphatidylinositol-3,4,5-trisphosphate [PI(3,4,5)P3] abundance in cells is essential for the control of cellular homeostasis. The concentrations of these lipids are low in quiescent cells but rapidly and transiently increase following growth factor receptor (GFR) stimulation, which triggers cellular metabolic changes, proliferation, survival, and motility. Class I(A) phosphatidylinositol 3-kinase (PI3K), which is composed of a p85 (regulatory) and p110 (catalytic) subunits, is the enzyme generating PI(3,4)P2 and PI(3,4,5)P3 following GFR stimulation. Although the steps in GFR-induced activation of PI3K , are relatively well known, the mechanisms for subsequent 3-polyphospho-PI down-regulation are less understood. Examination of frequent genetic alterations in human cancer showed that PTEN (phosphatase with tensin homology on chromosome 10) is the major enzyme that decreases PI(3,4)P2 and PI(3,4,5)P3 cell content. Nonetheless, interpretation of the complexity of PTEN regulation remains a matter of debate. The recent description of diminished PTEN activity in liver-conditional knockout mice lacking the p85alpha PI3K regulatory subunit reveals a previously unknown p85alpha-dependent negative-feedback pathway that controls PI(3,4)P2 and PI(3,4,5)P3 half-life by regulating PTEN.

Cytoplasmic activated protein kinase Akt regulates lipid-droplet accumulation in Drosophila nurse cells

The insulin/insulin-like growth factor signalling (IIS) cascade performs a broad range of evolutionarily conserved functions, including the regulation of growth, developmental timing and lifespan, and the control of sugar, protein and lipid metabolism. Recently, these functions have been genetically dissected in the fruit fly Drosophila melanogaster, revealing a crucial role for cell-surface activation of the downstream effector kinase Akt in many of these processes. However, the mechanisms regulating lipid metabolism and the storage of lipid during development are less well characterized. Here, we use the nutrient-storing nurse cells of the fly ovary to study the cellular effects of intracellular IIS components on lipid accumulation. These cells normally store lipid in a perinuclear pool of small neutral triglyceride-containing droplets. We find that loss of the IIS signalling antagonist PTEN, which stimulates cell growth in most developing tissues, produces a very different phenotype in nurse cells, inducing formation of highly enlarged lipid droplets. Furthermore, we show that the accumulation of activated Akt in the cytoplasm is responsible for this phenotype and leads to a much higher expression of LSD2, the fly homologue of the vertebrate lipid-storage protein perilipin. Our work therefore reveals a signalling mechanism by which the effect of insulin on lipid metabolism could be regulated independently of some of its other functions during development and adulthood. We speculate that this mechanism could be important in explaining the well-established link between obesity and insulin resistance that is observed in Type 2 diabetes.

Nuclear localization of PTEN by a Ran-dependent mechanism enhances apoptosis: Involvement of an N-terminal nuclear localization domain and multiple nuclear exclusion motifs

The targeting of the tumor suppressor PTEN protein to distinct subcellular compartments is a major regulatory mechanism of PTEN function, by controlling its access to substrates and effector proteins. Here, we investigated the molecular basis and functional consequences of PTEN nuclear/cytoplasmic distribution. PTEN accumulated in the nucleus of cells treated with apoptotic stimuli. Nuclear accumulation of PTEN was enhanced by mutations targeting motifs in distinct PTEN domains, and it was dependent on an N-terminal nuclear localization domain. Coexpression of a dominant negative Ran GTPase protein blocked PTEN accumulation in the nucleus, which was also affected by coexpression of importin alpha proteins. The lipid- and protein-phosphatase activity of PTEN differentially modulated PTEN nuclear accumulation. Furthermore, catalytically active nuclear PTEN enhanced cell apoptotic responses. Our findings indicate that multiple nuclear exclusion motifs and a nuclear localization domain control PTEN nuclear localization by a Ran-dependent mechanism and suggest a proapoptotic role for PTEN in the cell nucleus.