Unique biochemical properties of the protein tyrosine phosphatase activity of PTEN—Demonstration of different active site structural requirements for phosphopeptide and phospholipid phosphatase activities of PTEN

Missense PTEN mutations of the active site residues Asp-92, Cys-124 and Gly-129 contribute to Cowden syndrome. How their mutations affect phospholipid phosphatase activity and tumor suppressor function of PTEN has been defined. In this study, we investigated how their mutations affect the kinetics and catalytic mechanism of PTEN phosphoprotein phosphatase activity. Our data suggest that PTEN catalysis of phosphoprotein dephosphorylation follows a two-step mechanism with Cys-124 transiently phosphorylated to form the phosphoenzyme intermediate. In spite of this, we were unable to trap the genuine phosphoenzyme intermediate; instead, we unexpectedly discovered a novel phosphotransfer reaction in which the phosphate group is transferred from a tyrosyl phosphopeptide to PTEN to form a unique phosphorylated protein. Even though the finding is novel, the phosphotransfer reaction is likely an in vitro non-enzymatic reaction. Kinetic analysis revealed that mutation of Asp-92 has negligible impacts on phosphopeptide phosphatase activity of PTEN, suggesting that Asp-92 does not participate in the phosphopeptide dephosphorylation reaction. The results also imply that allosteric regulators facilitating the recruitment of Asp-92 to participate in catalysis will increase the activity of PTEN in dephosphorylating phosphoprotein and phosphopeptide substrates. Furthermore, kinetic analysis revealed that the G129E mutation has different effects on phospholipid and phosphoprotein phosphatase activities. Taken together, the data show that while the two phosphatase activities of PTEN follow a similar catalytic mechanism, they have notable differences in the requirements of the active site structure.     

A Functional Dissection of PTEN N-Terminus: Implications in PTEN Subcellular Targeting and Tumor Suppressor Activity

Spatial regulation of the tumor suppressor PTEN is exerted through alternative plasma membrane, cytoplasmic, and nuclear subcellular locations. The N-terminal region of PTEN is important for the control of PTEN subcellular localization and function. It contains both an active nuclear localization signal (NLS) and an overlapping PIP2-binding motif (PBM) involved in plasma membrane targeting. We report a comprehensive mutational and functional analysis of the PTEN N-terminus, including a panel of tumor-related mutations at this region. Nuclear/cytoplasmic partitioning in mammalian cells and PIP3 phosphatase assays in reconstituted S. cerevisiae defined categories of PTEN N-terminal mutations with distinct PIP3 phosphatase and nuclear accumulation properties. Noticeably, most tumor-related mutations that lost PIP3 phosphatase activity also displayed impaired nuclear localization. Cell proliferation and soft-agar colony formation analysis in mammalian cells of mutations with distinctive nuclear accumulation and catalytic activity patterns suggested a contribution of both properties to PTEN tumor suppressor activity. Our functional dissection of the PTEN N-terminus provides the basis for a systematic analysis of tumor-related and experimentally engineered PTEN mutations.     

Crystal structure of the PTEN tumor suppressor: implications for its phosphoinositide phosphatase activity and membrane association.

The PTEN tumor suppressor is mutated in diverse human cancers and in hereditary cancer predisposition syndromes. PTEN is a phosphatase that can act on both polypeptide and phosphoinositide substrates in vitro. The PTEN structure reveals a phosphatase domain that is similar to protein phosphatases but has an enlarged active site important for the accommodation of the phosphoinositide substrate. The structure also reveals that PTEN has a C2 domain. The PTEN C2 domain binds phospholipid membranes in vitro, and mutation of basic residues that could mediate this reduces PTEN's membrane affinity and its ability to suppress the growth of glioblastoma tumor cells. The phosphatase and C2 domains associate across an extensive interface, suggesting that the C2 domain may serve to productively position the catalytic domain on the membrane.     

Inactivation of platelet-derived growth factor receptor by the tumor suppressor PTEN provides a novel mechanism of action of the phosphatase

PTEN, mutated in a variety of human cancers, is a dual specificity protein phosphatase and also possesses D3-phosphoinositide phosphatase activity on phosphatidylinositol 3,4,5-tris-phosphate (PIP(3)), a product of phosphatidylinositol 3-kinase. This PIP(3) phosphatase activity of PTEN contributes to its tumor suppressor function by inhibition of Akt kinase, a direct target of PIP(3). We have recently shown that Akt regulates PDGF-induced DNA synthesis in mesangial cells. In this study, we demonstrate that expression of PTEN in mesangial cells inhibits PDGF-induced Akt activation leading to reduction in PDGF-induced DNA synthesis. As a potential mechanism, we show that PTEN inhibits PDGF-induced protein tyrosine phosphorylation with concomitant dephosphorylation and inactivation of tyrosine phosphorylated and activated PDGF receptor. Recombinant as well as immunopurified PTEN dephosphorylates autophosphorylated PDGF receptor in vitro. Expression of phosphatase deficient mutant of PTEN does not dephosphorylate PDGF-induced tyrosine phosphorylated PDGF receptor. Rather its expression increases tyrosine phosphorylation of PDGF receptor. Furthermore, expression of PTEN attenuated PDGF-induced signal transduction including phosphatidylinositol 3-kinase and Erk1/2 MAPK activities. Our data provide the first evidence that PTEN is physically associated with platelet-derived growth factor (PDGF) receptor and that PDGF causes its dissociation from the receptor. Finally, we show that both the C2 and tail domains of PTEN contribute to binding to the PDGF receptor. These data demonstrate a novel aspect of PTEN function where it acts as an effector for the PDGF receptor function and negatively regulates PDGF receptor activation.     

Regulation of dauer larva development in Caenorhabditis elegans by daf-18, a homologue of the tumour suppressor PTEN

The tumour suppressor gene PTEN (also called MMAC1 or TEP1) is somatically mutated in a variety of cancer types [1] [2] [3] [4]. In addition, germline mutation of PTEN is responsible for two dominantly inherited, related cancer syndromes called Cowden disease and Bannayan-Ruvalcaba-Riley syndrome [4]. PTEN encodes a dual-specificity phosphatase that inhibits cell spreading and migration partly by inhibiting integrin-mediated signalling [5] [6] [7]. Furthermore, PTEN regulates the levels of phosphatidylinositol 3,4,5-trisphosphate (PIP3) by specifically dephosphorylating position 3 on the inositol ring [8]. We report here that the dauer formation gene daf-18 is the Caenorhabditis elegans homologue of PTEN. DAF-18 is a component of the insulin-like signalling pathway controlling entry into diapause and adult longevity that is regulated by the DAF-2 receptor tyrosine kinase and the AGE-1 PI 3-kinase [9]. Others have shown that mutation of daf-18 suppresses the life extension and constitutive dauer formation associated with daf-2 or age-1 mutants. Similarly, we show that inactivation of daf-18 by RNA-mediated interference mimics this suppression, and that a wild-type daf-18 transgene rescues the dauer defect. These results indicate that PTEN/daf-18 antagonizes the DAF-2-AGE-1 pathway, perhaps by catalyzing dephosphorylation of the PIP3 generated by AGE-1. These data further support the notion that mutations of PTEN contribute to the development of human neoplasia through an aberrant activation of the PI 3-kinase signalling cascade.     

PIP3 Waves and PTEN Dynamics in the Emergence of Cell Polarity

In a motile eukaryotic cell, front protrusion and tail retraction are superimposed on each other. To single out mechanisms that result in front to tail or in tail to front transition, we separated the two processes in time using cells that oscillate between a full front and a full tail state. State transitions were visualized by total internal reflection fluorescence microscopy using as a front marker PIP3 (phosphatidylinositol [3,4,5] tris-phosphate), and as a tail marker the tumor-suppressor PTEN (phosphatase tensin homolog) that degrades PIP3. Negative fluctuations in the PTEN layer of the membrane gated a local increase in PIP3. In a subset of areas lacking PTEN (PTEN holes), PIP3 was amplified until a propagated wave was initiated. Wave propagation implies that a PIP3 signal is transmitted by a self-sustained process, such that the temporal and spatial profiles of the signal are maintained during passage of the wave across the entire expanse of the cell membrane. Actin clusters were remodeled into a ring along the perimeter of the expanding PIP3 wave. The reverse transition of PIP3 to PTEN was linked to the previous site of wave initiation: where PIP3 decayed first, the entry of PTEN was primed.     

PTEN interactions with focal adhesion kinase and suppression of the extracellular matrix-dependent phosphatidylinositol 3-kinase/Akt cell survival pathway.

The tumor suppressor PTEN is a phosphatase with sequence homology to tensin. PTEN dephosphorylates phosphatidylinositol 3,4, 5-trisphosphate (PIP3) and focal adhesion kinase (FAK), and it can inhibit cell growth, invasion, migration, and focal adhesions. We investigated molecular interactions of PTEN and FAK in glioblastoma and breast cancer cells lacking PTEN. The PTEN trapping mutant D92A bound wild-type FAK, requiring FAK autophosphorylation site Tyr397. In PTEN-mutated cancer cells, FAK phosphorylation was retained even in suspension after detachment from extracellular matrix, accompanied by enhanced PI 3-K association with FAK and sustained PI 3-K activity, PIP3 levels, and Akt phosphorylation; expression of exogenous PTEN suppressed all five properties. PTEN-mutated cells were resistant to apoptosis in suspension, but most of the cells entered apoptosis after expression of exogenous PTEN or wortmannin treatment. Moreover, overexpression of FAK in PTEN-transfected cells reversed the decreased FAK phosphorylation and PI 3-K activity, and it partially rescued PIP3 levels, Akt phosphorylation, and PTEN-induced apoptosis. Our results show that FAK Tyr397 is important in PTEN interactions with FAK, that PTEN regulates FAK phosphorylation and molecular associations after detachment from matrix, and that PTEN negatively regulates the extracellular matrix-dependent PI 3-K/Akt cell survival pathway in a process that can include FAK.     

Co-existence of high levels of the PTEN protein with enhanced Akt activation in renal cell carcinoma

Recruiting Akt to the membrane-bound phosphatidylinositol (3,4,5) trisphosphate (PIP3) is required for Akt activation. While PI3 kinase (PI3K) produces PIP3, PTEN dephosphorylates the 3-position phosphate from PIP3, thereby directly inhibiting Akt activation. PTEN is the dominant PIP3 phosphatase, as knockdown of PTEN results in increases in Akt activation in mice. The PTEN tumor suppressor gene is frequently mutated in a variety of human cancers, consistent with an inverse correlation between levels of the PTEN protein and Akt activation. We have examined PTEN expression and Akt activation in 35 primary clear cell renal cell carcinomas RCCs (ccRCCs) and 9 papillary RCCs (pRCCs) and their respective non-tumor kidney tissues. The PTEN protein was reduced in 16 ccRCCs (16/35=45.7%) and 8 pRCCs (8/9=88.9%). In these RCCs, 25.0% (4/16) of ccRCCs and 25.0% (2/8) of pRCCs expressed elevated Akt activation. 19 ccRCCc (19/35=54.3%) expressed comparable or higher levels of PTEN. Of these ccRCCs, 31.6% (6/19) showed increases in Akt activation. As PTEN dominantly inhibits Akt activation, the coexistence of high levels of the PTEN protein with enhanced Akt activation suggests the existence of novel mechanisms which attenuate PTEN function in ccRCC. These mechanisms may reduce PTEN function or increase PIP3 production.     

PTEN: The down side of PI 3-kinase signalling

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

NHERFs, NEP, MAGUKs, and more: interactions that regulate PTEN

This year marks the 10th anniversary of the discovery of the PTEN/MMAC1/TEP1 tumor suppressor gene (hereafter referred to as PTEN), one of the most commonly mutated genes in cancer. PTEN encodes a lipid phosphatase that dephosphorylates phosphoinositide-3,4,5-triphosphate (PIP(3)), thereby counteracting mitogenic signaling pathways driven by phosphoinositol-3-kinases (PI3K). By opposing PI3K signaling, PTEN inhibits the activation of the critical PI3K effector proteins Akt1-3 (also known as protein kinase B or PKB). Given its central role in antagonizing PI3K signaling, one might expect that like PI3K, the activity of the PTEN protein would be highly regulated by numerous protein/protein interactions. However, surprisingly little is known about such interactions. This fact, combined with the generally accepted notion that phosphatases are less exquisitely regulated than kinases, has led to the idea that PTEN may function in a relatively unregulated fashion. Here we review the identities and proposed functions of known PTEN-interacting proteins, and point out avenues of investigation that we hope may be fruitful in identifying important new mechanisms of PTEN regulation in mammalian cells.     

Direct identification of PTEN phosphorylation sites

The PTEN tumor suppressor gene encodes a phosphatidylinositol 3'-phosphatase that is inactivated in a high percentage of human tumors, particularly glioblastoma, melanoma, and prostate and endometrial carcinoma. Previous studies showed that PTEN is a seryl phosphoprotein and a substrate of protein kinase CK2 (CK2). However, the sites in PTEN that are phosphorylated in vivo have not been identified directly, nor has the effect of phosphorylation on PTEN catalytic activity been reported. We used mass spectrometric methods to identify Ser(370) and Ser(385) as in vivo phosphorylation sites of PTEN. These sites also are phosphorylated by CK2 in vitro, and phosphorylation inhibits PTEN activity towards its substrate, PIP3. We also identify a novel in vivo phosphorylation site, Thr(366). Following transient over-expression, a fraction of CK2 and PTEN co-immunoprecipitate. Moreover, pharmacological inhibition of CK2 activity leads to decreased Akt activation in PTEN+/+ but not PTEN-/- fibroblasts. Our results contrast with previous assignments of PTEN phosphorylation sites based solely on mutagenesis approaches, suggest that CK2 is a physiologically relevant PTEN kinase, and raise the possibility that CK2-mediated inhibition of PTEN plays a role in oncogenesis.     

PtdIns(3,4,5)P(3)-dependent and -independent roles for PTEN in the control of cell migration

BACKGROUND: Phosphatase and tensin homolog (PTEN) mediates many of its effects on proliferation, growth, survival, and migration through its PtdIns(3,4,5)P(3) lipid phosphatase activity, suppressing phosphoinositide 3-kinase (PI3K)-dependent signaling pathways. PTEN also possesses a protein phosphatase activity, the role of which is less well characterized. RESULTS: We have investigated the role of PTEN in the control of cell migration of mesoderm cells ingressing through the primitive streak in the chick embryo. Overexpression of PTEN strongly inhibits the epithelial-to-mesenchymal transition (EMT) of mesoderm cells ingressing through the anterior and middle primitive streak, but it does not affect EMT of cells located in the posterior streak. The inhibitory activity on EMT is completely dependent on targeting PTEN through its C-terminal PDZ binding site, but can be achieved by a PTEN mutant (PTEN G129E) with only protein phosphatase activity. Expression either of PTEN lacking the PDZ binding site or of the PTEN C2 domain, or inhibition of PI3K through specific inhibitors, does not inhibit EMT, but results in a loss of both cell polarity and directional migration of mesoderm cells. The PTEN-related protein TPTE, which normally lacks any detectable lipid and protein phosphatase activity, can be reactivated through mutation, and only this reactivated mutant leads to nondirectional migration of these cells in vivo. CONCLUSIONS: PTEN modulates cell migration of mesoderm cells in the chick embryo through at least two distinct mechanisms: controlling EMT, which involves its protein phosphatase activity; and controlling the directional motility of mesoderm cells, through its lipid phosphatase activity.     

Mutation of a unique aspartate residue abolishes the catalytic activity but not substrate binding of the mouse N-methylpurine-DNA glycosylase (MPG)

N-Methylpurine-DNA glycosylase (MPG) initiates base excision repair in DNA by removing a variety of alkylated purine adducts. Although Asp was identified as the active site residue in various DNA glycosylases based on the crystal structure, Glu-125 in human MPG (Glu-145 in mouse MPG) was recently proposed to be the catalytic residue. Mutational analysis for all Asp residues in a truncated, fully active MPG protein showed that only Asp-152 (Asp-132 in the human protein), which is located near the active site, is essential for catalytic activity. However, the substrate binding was not affected in the inactive Glu-152, Asn-152, and Ala-152 mutants. Furthermore, mutation of Asp-152 did not significantly affect the intrinsic tryptophan fluorescence of the enzyme and the far UV CD spectra, although a small change in the near UV CD spectra of the mutants suggests localized conformational change in the aromatic residues. We propose that in addition to Glu-145 in mouse MPG, which functions as the activator of a water molecule for nucleophilic attack, Asp-152 plays an essential role either by donating a proton to the substrate base and, thus, facilitating its release or by stabilizing the steric configuration of the active site pocket.     

Structure and mutational analysis of the PhoN protein of Salmonella typhimurium provide insight into mechanistic details

The Salmonella typhimurium PhoN protein is a nonspecific acid phosphatase and belongs to the phosphatidic acid phosphatase type 2 (PAP2) superfamily. We report here the crystal structures of phosphate-bound PhoN, the PhoN-tungstate complex, and the T159D mutant of PhoN along with functional characterization of three mutants: L39T, T159D, and D201N. Invariant active site residues, Lys-123, Arg-130, Ser-156, Gly-157, His-158, and Arg-191, interact with phosphate and tungstate oxyanions. Ser-156 also accepts a hydrogen bond from Thr-159. The T159D mutation, surprisingly, severely diminishes phosphatase activity, apparently by disturbing the active site scaffold: Arg-191 is swung out of the active site resulting in conformational changes in His-158 and His-197 residues. Our results reveal a hitherto unknown functional role of Arg-191, namely, restricting the active conformation of catalytic His-158 and His-197 residues. Consistent with the conserved nature of Asp-201 in the PAP2 superfamily, the D201N mutation completely abolished phosphatase activity. On the basis of this observation and in silico analysis we suggest that the crucial mechanistic role of Asp-201 is to stabilize the positive charge on the phosphohistidine intermediate generated by the transfer of phosphoryl to the nucleophile, His-197, located within hydrogen bond distance to the invariant Asp-201. This is in contrast to earlier suggestions that Asp-201 stabilizes His-197 and the His197-Asp201 dyad facilitates formation of the phosphoenzyme intermediate through a charge-relay system. Finally, the L39T mutation in the conserved polyproline motif (39LPPPP43) of dimeric PhoN leads to a marginal reduction in activity, in contrast to the nearly 50-fold reduction observed for monomeric Prevotella intermedia acid phosphatase, suggesting that the varying quaternary structure of PhoN orthologues may have functional significance.     

A mutant form of PTEN linked to autism

The tumor suppressor, phosphatase, and tensin homologue deleted on chromosome 10 (PTEN), is a phosphoinositide (PI) phosphatase specific for the 3‐position of the inositol ring. PTEN has been implicated in autism for a subset of patients with macrocephaly. Various studies identified patients in this subclass with one normal and one mutated PTEN gene. We characterize the binding, structural properties, activity, and subcellular localization of one of these autism‐related mutants, H93R PTEN. Even though this mutation is located at the phosphatase active site, we find that it affects the functions of neighboring domains. H93R PTEN binding to phosphatidylserine‐bearing model membranes is 5.6‐fold enhanced in comparison to wild‐type PTEN. In contrast, we find that binding to phosphatidylinositol‐4,5‐bisphosphate (PI(4,5)P₂) model membranes is 2.5‐fold decreased for the mutant PTEN in comparison to wild‐type PTEN. The structural change previously found for wild‐type PTEN upon interaction with PI(4,5)P₂, is absent for H93R PTEN. Consistent with the increased binding to phosphatidylserine, we find enhanced plasma membrane association of PTEN‐GFP in U87MG cells. However, this enhanced plasma membrane association does not translate into increased PI(3,4,5)P₃ turnover, since in vivo studies show a reduced activity of the H93R PTEN‐GFP mutant. Because the interaction of PI(4,5)P₂ with PTEN's N‐terminal domain is diminished by this mutation, we hypothesize that the interaction of PTEN's N‐terminal domain with the phosphatase domain is impacted by the H93R mutation, preventing PI(4,5)P₂ from inducing the conformational change that activates phosphatase activity.     

PTEN enters the nucleus by diffusion

Despite much evidence for phosphatidylinositol phosphate (PIP)-triggered signaling pathways in the nucleus, there is little understanding of how the levels and activities of these proteins are regulated. As a first step to elucidating this problem, we determined whether phosphatase and tensin homolog deleted on chromosome 10 (PTEN) enters the nucleus by passive diffusion or active transport. We expressed various PTEN fusion proteins in tsBN2, HeLa, LNCaP, and U87MG cells and determined that the largest PTEN fusion proteins showed little or no nuclear localization. Because diffusion through nuclear pores is limited to proteins of 60,000 Da or less, this suggests that nuclear translocation of PTEN occurs via diffusion. We examined PTEN mutants, seeking to identify a nuclear localization signal (NLS) for PTEN. Mutation of K13 and R14 decreased nuclear localization, but these amino acids do not appear to be part of an NLS. We used fluorescence recovery after photobleaching (FRAP) to demonstrate that GFP-PTEN can passively pass through nuclear pores. Diffusion in the cytoplasm is retarded for the PTEN mutants that show reduced nuclear localization. We conclude that PTEN enters the nucleus by diffusion. In addition, sequestration of PTEN in the cytoplasm likely limits PTEN nuclear translocation.     

Tumour suppressor PTEN activity is differentially inducible by myo-inositol phosphates

Tumour evolution and efficacy of treatments are controlled by the microenvironment, the composition of which is primarily dependent on the angiogenic reaction to hypoxic stress. Tumour angiogenesis normalization is a challenge for adjuvant therapy strategies to chemo-, radio- and immunotherapeutics. Myo-inositol trispyrophosphate (ITPP) appears to provide the means to alleviate hypoxia in the tumour site by a double molecular mechanism. First, it modifies the properties of red blood cells (RBC) to release oxygen (O₂) in the hypoxic sites more easily, leading to a rapid and stable increase in the partial pressure of oxygen (pO₂). And second, it activates the endothelial phosphatase and tensin homologue deleted on Chromosome 10 (PTEN). The hypothesis that stable normalization of the vascular system is due to the PTEN, a tumour suppressor and phosphatase which controls the proper angiogenic reaction was ascertained. Here, by direct biochemical measurements of PTEN competitive activity in relation to PIP2 production, we show that the kinetics are complex in terms of the activation/inhibition effects of ITPP with an inverted consequence towards the kinase PI3K. The use of the surface plasmon resonance (SPR) technique allowed us to demonstrate that PTEN binds inositol derivatives differently but weakly. This method permitted us to reveal that PTEN is highly sensitive to the local concentration conditions, especially that ITPP increases the PTEN activity towards PIP3, and importantly, that PTEN affinity for ITPP is considerably increased by the presence of PIP3, as occurs in vivo. Our approach demonstrates the validity of using ITPP to activate PTEN for stable vessel normalization strategies.     

Thioredoxin-1 binds to the C2 domain of PTEN inhibiting PTEN's lipid phosphatase activity and membrane binding: a mechanism for the functional loss of PTEN's tumor suppressor activity

Thioredoxin-1 (Trx-1) is a 12 kDa redox protein that is overexpressed in a large number of human tumors. Elevated Trx-1 is associated with increased tumor cell proliferation, inhibited apoptosis, aggressive tumor growth, and decreased patient survival. The molecular mechanisms for the promotion of tumorigenesis by Trx-1 are not known. PTEN is a major tumor suppressor of human cancer that acts by hydrolyzing membrane phosphatidylinositol (PtdIns)-3-phosphates, thus, preventing the activation of the survival signaling kinase Akt by PtdIns-3-kinase. We show that Trx-1 binds in a redox dependent manner to PTEN to inhibit its PtdIns-3-phosphatase activity which results in increased Akt activation in cells. Molecular docking and site-specific mutation studies show that the binding of Trx-1 to PTEN occurs through a disulfide bond between the active site Cys(32) of Trx-1 and Cys(212) of the C2 domain of PTEN leading to steric interference by bound Trx-1 of the catalytic site of PTEN and of the C2 lipid membrane-binding domain. The results of the study suggest that the increased levels of Trx-1 in human tumors could lead to functional inhibition of PTEN tumor suppressor activity providing an additional mechanism for tumorigenesis with loss of PTEN activity.     

BCR-ABL inactivates cytosolic PTEN through Casein Kinase II mediated tail phosphorylation

The tumor suppressive function of PTEN is exerted within 2 different cellular compartments. In the cytosol-membrane, it negatively regulates PI3K-AKT pathway through the de-phosphorylation of phosphatidylinositol (3,4,5)-triphosphate (PIP3), therefore blocking one of the major signaling transduction pathways in tumorigenesis. In the nucleus, PTEN controls genomic stability and cellular proliferation through phosphatase independent mechanisms. Importantly, impairments in PTEN cellular compartmentalization, changes in protein levels and post-transductional modifications affect PTEN tumor suppressive functions. Targeting mechanisms that inactivate PTEN promotes apoptosis induction of cancer cells, without affecting normal cells, with appealing therapeutic implications. Recently, we have shown that BCR-ABL promotes PTEN nuclear exclusion by favoring HAUSP mediated PTEN de-ubiquitination in Chronic Myeloid Leukemia. Here, we show that nuclear exclusion of PTEN is associated with PTEN inactivation in the cytoplasm of CML cells. In particular, BCR-ABL promotes Casein Kinase II-mediated PTEN tail phosphorylation with consequent inhibition of the phosphatase activity toward PIP3. Targeting Casein Kinase II promotes PTEN reactivation with apoptosis induction. We therefore propose a novel BCR-ABL/CKII/PTEN pathway as a potential target to achieve synthetic lethality with tyrosine kinase inhibitors.     

PTEN and myotubularin phosphatases: from 3-phosphoinositide dephosphorylation to disease

The phosphatase and tensin homolog deleted on chromosome ten (PTEN) and myotubularin (MTM1) represent subfamilies of protein tyrosine phosphatases whose principal physiological substrates are D3-phosphorylated inositol phospholipids. As lipid phosphatases, PTEN- and MTM1-related (MTMR) proteins dephosphorylate the products of phosphoinositide 3-kinases and antagonize downstream effectors that utilize 3-phosphoinositides as ligands for protein targeting domains or allosteric activation. Here, we describe the cellular mechanisms of PTEN and MTMR function and their role in the etiology of cancer and other human diseases.