Regulation of the tumor suppressor PTEN by SUMO

SUMOylation regulates AKT1 activity

Serine threonine kinase AKT has a central role in the cell, controlling survival, proliferation, metabolism and angiogenesis. Deregulation of its activity underlies a wide range of pathological situations, including cancer. Here we show that AKT is post-translationally modified by the small ubiquitin-like modifier (SUMO) protein. Interestingly, neither SUMO conjugation nor activation of SUMOylated AKT is regulated by the classical AKT targeting to the cell membrane or by the phosphoinositide 3-kinase pathway. We demonstrate that SUMO induces the activation of AKT, whereas, conversely, down-modulation of the SUMO machinery diminishes AKT activation and cell proliferation. Furthermore, an AKT SUMOylation mutant shows reduced activation, and decreased anti-apoptotic and pro-tumoral activities in comparison with the wild-type protein. These results identify SUMO as a novel key regulator of AKT phosphorylation and activity.

Subcellular targeting and dynamic regulation of PTEN: implications for neuronal cells and neurological disorders

PTEN is a lipid and protein phosphatase that regulates a diverse range of cellular mechanisms. PTEN is mainly present in the cytosol and transiently associates with the plasma membrane to dephosphorylate PI(3,4,5)P3, thereby antagonizing the PI3-Kinase signaling pathway. Recently, PTEN has been shown to associate also with organelles such as the endoplasmic reticulum (ER), the mitochondria, or the nucleus, and to be secreted outside of the cell. In addition, PTEN dynamically localizes to specialized sub-cellular compartments such as the neuronal growth cone or dendritic spines. The diverse localizations of PTEN imply a tight temporal and spatial regulation, orchestrated by mechanisms such as posttranslational modifications, formation of distinct protein–protein interactions, or the activation/recruitment of PTEN downstream of external cues. The regulation of PTEN function is thus not only important at the enzymatic activity level, but is also associated to its spatial distribution. In this review we will summarize (i) recent findings that highlight mechanisms controlling PTEN movement and sub-cellular localization, and (ii) current understanding of how PTEN localization is achieved by mechanisms controlling posttranslational modification, by association with binding partners and by PTEN structural or activity requirements. Finally, we will discuss the possible roles of compartmentalized PTEN in developing and mature neurons in health and disease.

PTEN function: the long and the short of it

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a phosphatase that is frequently altered in cancer. PTEN has phosphatase-dependent and -independent roles, and genetic alterations in PTEN lead to deregulation of protein synthesis, the cell cycle, migration, growth, DNA repair, and survival signaling. PTEN localization, stability, conformation, and phosphatase activity are controlled by an array of protein-protein interactions and post-translational modifications. Thus, PTEN-interacting and -modifying proteins have profound effects on the tumor suppressive functions of PTEN. Moreover, recent studies identified mechanisms by which PTEN can exit cells, via either exosomal export or secretion, and act on neighboring cells. This review focuses on modes of PTEN protein regulation and ways in which perturbations in this regulation may lead to disease.

Interactions of phosphatase and tensin homologue (PTEN) proteins with phosphatidylinositol phosphates: insights from molecular dynamics simulations of PTEN and voltage sensitive phosphatase

The phosphatase and tensin homologue (PTEN) and the Ciona intestinalis voltage sensitive phosphatase (Ci-VSP) are both phosphatidylinositol phosphate (PIP) phosphatases that contain a C2 domain. PTEN is a tumor suppressor protein that acts as a phosphatase on PIP₃ in mammalian cell membranes. It contains two principal domains: a phosphatase domain (PD) and a C2 domain. Despite detailed structural and functional characterization, less is known about its mechanism of interaction with PIP-containing lipid bilayers. Ci-VSP consists of an N-terminal transmembrane voltage sensor domain and a C-terminal PTEN domain, which in turn contains a PD and a C2 domain. The nature of the interaction of the PTEN domain of Ci-VSP with membranes has not been well established. We have used multiscale molecular dynamics simulations to define the interaction mechanisms of PTEN and of the Ci-VSP PTEN domains with PIP-containing lipid bilayers. Our results suggest a novel mechanism of association of the PTEN with such bilayers, in which an initial electrostatics-driven encounter of the protein and bilayer is followed by reorientation of the protein to optimize its interactions with PIP molecules in the membrane. Although a PIP₃ molecule binds close to the active site of PTEN, our simulations suggest a further conformational change of the protein may be required for catalytically productive binding to occur. Ci-VSP interacted with membranes in an orientation comparable to that of PTEN but bound directly to PIP-containing membranes without a subsequent reorientation step. Again, PIP₃ bound close to the active site of the Ci-VSP PD, but not in a catalytically productive manner. Interactions of Ci-VSP with the bilayer induced clustering of PIP molecules around the protein.

Cadmium selectively induces MIP-2 and COX-2 through PTEN-mediated Akt activation in RAW264.7 cells

Increasing evidence demonstrates that cadmium (Cd) induces inflammation, but its mechanisms remain obscure. The present study showed that treatment with CdCl₂ selectively upregulates macrophage inflammatory protein (MIP)-2 and cyclooxygenase (COX)-2 in RAW264.7 cells. Concomitantly, Cd²⁺ markedly elevated the level of phosphorylated Akt in dose- and time-dependent manners. LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K), blocked Cd²⁺-evoked Akt phosphorylation. Correspondingly, LY294002 significantly repressed Cd²⁺-induced upregulation of MIP-2 and COX-2 in RAW264.7 cells. Further experiments showed that treatment with Cd²⁺ significantly reduced the level of PTEN protein in RAW264.7 cells. MG132, a specific proteasome inhibitor, blocked Cd²⁺-induced reduction in PTEN protein as well as Akt phosphorylation, implicating the involvement of proteasome-mediated PTEN degradation. Of interest, Cd²⁺-induced degradation of PTEN protein appears to be associated with PTEN ubiquitination. N-acetylcysteine, a glutathione (GSH) precursor, blocked Cd²⁺-evoked PTEN degradation as well as Akt phosphorylation. By contrast, L-buthionine-S,R-sulfoximine, an inhibitor of cellular GSH synthesis, exacerbated Cd²⁺-induced PTEN degradation and Akt phosphorylation. Alpha-phenyl-N-tert-butylnitrone and vitamin C, two antioxidants, did not prevent from Cd²⁺-induced PTEN degradation and Akt phosphorylation. In conclusion, Cd²⁺ selectively induces MIP-2 and COX-2 through PTEN-mediated PI3K/Akt activation. Cellular GSH depletion mediates Cd²⁺-induced PTEN degradation and subsequent PI3K/Akt activation in macrophages.

Regulation of the transcriptional activation of the androgen receptor by the UXT-binding protein VHL

Loss and/or inactivation of the VHL (von Hippel-Lindau) tumour suppressor causes various tumours. Using a yeast two-hybrid system, we have identified the AR (androgen receptor) co-activator UXT (ubiquitously expressed transcript), as a VHL-interacting protein. GST pull-down and co-immunoprecipitation assays show that UXT interacts with VHL. In addition, UXT recruits VHL to the nucleus. VHL associates with the DBD (DNA-binding domain) and hinge domains of the AR and induces AR ubiquitination. Moreover, VHL interaction with the AR activates AR transactivation upon DHT (dihydrotestosterone) treatment. VHL knockdown inhibits AR ubiquitination and decreases transcriptional activation of the AR. Our data suggest that the VHL-UXT interaction and VHL-induced ubiquitination of AR regulate transcriptional activation of the AR.

Receptor trafficking and the regulation of synaptic plasticity by SUMO

Timely and efficient information transfer at synapses is fundamental to brain function. Synapses are highly dynamic structures that exhibit long-lasting activity-dependent alterations to their structure and transmission efficiency, a phenomenon termed synaptic plasticity. These changes, which occur through alterations in presynaptic release or in the trafficking of postsynaptic receptor proteins, underpin the formation and stabilisation of neural circuits during brain development, and encode, process and store information essential for learning, memory and cognition. In recent years, it has emerged that the ubiquitin-like posttranslational modification SUMOylation is an important mediator of several aspects of neuronal and synaptic function. Through orchestrating synapse formation, presynaptic release and the trafficking of postsynaptic receptor proteins during forms of synaptic plasticity such as long-term potentiation, long-term depression and homeostatic scaling, SUMOylation is being increasingly appreciated to play a central role in neurotransmission. In this review, we outline key discoveries in this relatively new field, provide an update on recent progress regarding the targets and consequences of protein SUMOylation in synaptic function and plasticity, and highlight key outstanding questions regarding the roles of protein SUMOylation in the brain.

Sumoylation in gene regulation, human disease, and therapeutic action

Similar to ubiquitination, sumoylation covalently attaches a small ubiquitin-like modifier (SUMO) protein (92-97 amino acids) to the ε-amino group of a lysine residue. This is quite different from the classically defined post-translational modifications, such as phosphorylation, acetylation, and methylation, which typically add a small chemical group to the targeted residue. Sumoylation has been well studied at the molecular and cellular levels, focusing mostly on site-specific conjugation of human SUMO1, SUMO2, and SUMO3, as well as their homologues in various species. In this short review, we will discuss some recent examples to highlight (a) emerging trends about the coordinated regulation of sumoylation and other post-translational modifications in modulating the function of some transcription factors and pathway-specific regulators, (b) diverse roles of sumoylation in gene regulation implicated in stem cells and different pathogenic conditions, and (c) potential therapeutic strategies related to some of the diseases stated above.

Analysis of SUMOylated proteins using SUMO-traps

SUMO-modified proteins are recognized by SUMO interacting motifs (SIMs), thus triggering diverse cellular responses. Here SIMs were used to develop SUMO-traps to capture endogenous SUMOylated proteins. Our results show that these small peptides are transferable motifs that maintain their SUMO binding capacity when fused to the heterologous carrier protein GST. The tandem disposition of SIMs increases the binding capacity of SUMO-traps to specifically interact with polySUMO but not poly-Ubiquitin chains. We demonstrate that this SUMO capturing system purifies SUMOylated proteins such as IκBα, PTEN, PML or p53 in vitro and in vivo. These properties can be used to explore the many critical functions regulated by protein SUMOylation.

Modification of Akt by SUMO conjugation regulates alternative splicing and cell cycle

Akt/PKB is a key signaling molecule in higher eukaryotes and a crucial protein kinase in human health and disease. Phosphorylation, acetylation, and ubiquitylation have been reported as important regulatory post-translational modifications of this kinase. We describe here that Akt is modified by SUMO conjugation, and show that lysine residues 276 and 301 are the major SUMO attachment sites within this protein. We found that phosphorylation and SUMOylation of Akt appear as independent events. However, decreasing Akt SUMOylation levels severely affects the role of this kinase as a regulator of fibronectin and Bcl-x alternative splicing. Moreover, we observed that the Akt mutant (Akt E17K) found in several human tumors displays increased levels of SUMOylation and also an enhanced capacity to regulate fibronectin splicing patterns. This splicing regulatory activity is completely abolished by decreasing Akt E17K SUMO conjugation levels. Additionally, we found that SUMOylation controls Akt regulatory function at G₁/S transition during cell cycle progression. These findings reveal SUMO conjugation as a novel level of regulation for Akt activity, opening new areas of exploration related to the molecular mechanisms involved in the diverse cellular functions of this kinase.

SUMOylation of p53 mediates interferon activities

There is growing evidence that many host proteins involved in innate and intrinsic immunity are regulated by SUMOylation, and that SUMO contributes to the regulatory process that governs the initiation of the type I interferon (IFN) response. The tumor suppressor p53 is a modulator of the IFN response that plays a role in virus-induced apoptosis and in IFN-induced senescence. Here we demonstrate that IFN treatment increases the levels of SUMOylated p53 and induces cellular senescence through a process that is partially dependent upon SUMOylation of p53. Similarly, we show that vesicular stomatitis virus (VSV) infection induces p53 SUMOylation, and that this modification favors the control of VSV replication. Thus, our study provides evidence that IFN signaling induces p53 SUMOylation, which results in the activation of a cellular senescence program and contributes to the antiviral functions of interferon.

Akt SUMOylation regulates cell proliferation and tumorigenesis

Proto-oncogene Akt plays essential roles in cell proliferation and tumorigenesis. Full activation of Akt is regulated by phosphorylation, ubiquitination, and acetylation. Here we report that SUMOylation of Akt is a novel mechanism for its activation. Systematically analyzing the role of lysine residues in Akt activation revealed that K276, which is located in a SUMOylation consensus motif, is essential for Akt activation. Ectopic or endogenous Akt1 could be modified by SUMOylation. RNA interference-mediated silencing of UBC9 reduced Akt SUMOylation, which was promoted by SUMO E3 ligase PIAS1 and reversed by the SUMO-specific protease SENP1. Although multiple sites on Akt could be SUMOylated, K276 was identified as a major SUMO acceptor site. K276R or E278A mutation reduced SUMOylation of Akt but had little effect on its ubiquitination. Strikingly, these mutations also completely abolished Akt kinase activity. In support of these results, we found that expression of PIAS1 and SUMO1 increased Akt activity, whereas expression of SENP1 reduced Akt1 activity. Interestingly, the cancer-derived mutant E17K in Akt1 that occurs in various cancers was more efficiently SUMOylated than wild-type Akt. Moreover, SUMOylation loss dramatically reduced Akt1 E17K-mediated cell proliferation, cell migration, and tumorigenesis. Collectively, our findings establish that Akt SUMOylation provides a novel regulatory mechanism for activating Akt function.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Role of SUMO-specific protease 2 in reprogramming cellular glucose metabolism

Most cancer cells exhibit a shift in glucose metabolic strategy, displaying increased glycolysis even with adequate oxygen supply. SUMO-specific proteases (SENPs) de-SUMOylate substrates including HIF1α and p53,two key regulators in cancer glucose metabolism, to regulate their activity, stability and subcellular localization. However, the role of SENPs in tumor glucose metabolism remains unclear. Here we report that SUMO-specific protease 2 (SENP2) negatively regulates aerobic glycolysis in MCF7 and MEF cells. Over-expression of SENP2 reduces the glucose uptake and lactate production, increasing the cellular ATP levels in MCF7 cells, while SENP2 knockout MEF cells show increased glucose uptake and lactate production along with the decreased ATP levels. Consistently, the MCF7 cells over-expressing SENP2 exhibit decreased expression levels of key glycolytic enzymes and an increased rate of glucose oxidation compared with control MCF7 cells, indicating inhibited glycolysis but enhanced oxidative mitochondrial respiration. Moreover, SENP2 over-expressing MCF7 cells demonstrated a reduced amount of phosphorylated AKT, whereas SENP2 knockout MEFs exhibit increased levels of phosphorylated AKT. Furthermore, inhibiting AKT phosphorylation by LY294002 rescued the phenotype induced by SENP2 deficiency in MEFs. In conclusion, SENP2 represses glycolysis and shifts glucose metabolic strategy, in part through inhibition of AKT phosphorylation. Our study reveals a novel function of SENP2 in regulating glucose metabolism.

Dyskeratosis congenita mutations in dyskerin SUMOylation consensus sites lead to impaired telomerase RNA accumulation and telomere defects

Mutations in the dyskerin gene (DKC1) cause X-linked dyskeratosis congenita (DC), a rare and fatal premature aging syndrome characterized by defective telomere maintenance. Dyskerin is a highly conserved nucleolar protein, and a component of the human telomerase complex that is essential for human telomerase RNA (hTR) stability. However, its regulation remains poorly understood. Here, we report that dyskerin can be modified by small ubiquitin-like modifiers (SUMOs). We find that human DC-causing mutations in highly conserved dyskerin SUMOylation consensus sites lead to impaired hTR accumulation, telomerase activity and telomere maintenance. Finally, we show that modification of dyskerin by SUMOylation is required for its stability. Our findings provide the first evidence that dyskerin stability is regulated by SUMOylation and that mutations altering dyskerin SUMOylation can lead to defects in telomere maintenance that are characteristics of DC.