Cytoplasmic p27Kip1 counteracts the pro-apoptotic function of the open conformation of PTEN by retention and destabilization of PTEN outside of the nucleus

miR-101-3p-mediated role of PDZK1 in hepatocellular carcinoma progression and the underlying PI3K/Akt signaling mechanism

BACKGROUND

The molecular targets and associated mechanisms of hepatocellular carcinoma (HCC) have been widely studied, but the roles of PDZK1 in HCC are unclear. Therefore, the aim of this study is to explore the role and associated mechanisms of PDZK1 in HCC.

RESULTS

It was found that the expression of PDZK1 in HCC tissues was higher than that in paired paracancerous tissues. High expression of PDZK1 was associated with lymph node metastasis, degree of differentiation, and clinical stage. Upregulation of PDZK1 in HCC cells affected their proliferation, migration, invasion, apoptosis, and cell cycle, and also induced PI3K/AKT activation. PDZK1 is a downstream target gene of miR-101-3p. Accordingly, increase in the expression of miR-101-3p reversed the promotive effect of PDZK1 in HCC. Moreover, PDZK1 was found to accelerate cell proliferation and promote the malignant progression of HCC via the PI3K/AKT pathway.

CONCLUSION

Our study indicated that the miR-101-3p/PDZK1 axis plays a role in HCC progression and could be beneficial as a novel biomarker and new therapeutic target for HCC treatment.

Functional analysis of PTEN variants of unknown significance from PHTS patients unveils complex patterns of PTEN biological activity in disease

Heterozygous germline mutations in PTEN gene predispose to hamartomas and tumors in different tissues, as well as to neurodevelopmental disorders, and define at genetic level the PTEN Hamartoma Tumor Syndrome (PHTS). The major physiologic role of PTEN protein is the dephosphorylation of phosphatidylinositol (3,4,5)-trisphosphate (PIP3), counteracting the pro-oncogenic function of phosphatidylinositol 3-kinase (PI3K), and PTEN mutations in PHTS patients frequently abrogate PTEN PIP3 catalytic activity. PTEN also displays non-canonical PIP3-independent functions, but their involvement in PHTS pathogeny is less understood. We have previously identified and described, at clinical and genetic level, novel PTEN variants of unknown functional significance in PHTS patients. Here, we have performed an extensive functional characterization of these PTEN variants (c.77 C > T, p.(Thr26Ile), T26I; c.284 C > G, p.(Pro95Arg), P95R; c.529 T > A, p.(Tyr177Asn), Y177N; c.781 C > G, p.(Gln261Glu), Q261E; c.829 A > G, p.(Thr277Ala), T277A; and c.929 A > G, p.(Asp310Gly), D310G), including cell expression levels and protein stability, PIP3-phosphatase activity, and subcellular localization. In addition, caspase-3 cleavage analysis in cells has been assessed using a C2-domain caspase-3 cleavage-specific anti-PTEN antibody. We have found complex patterns of functional activity on PTEN variants, ranging from loss of PIP3-phosphatase activity, diminished protein expression and stability, and altered nuclear/cytoplasmic localization, to intact functional properties, when compared with PTEN wild type. Furthermore, we have found that PTEN cleavage at the C2-domain by the pro-apoptotic protease caspase-3 is diminished in specific PTEN PHTS variants. Our findings illustrate the multifaceted molecular features of pathogenic PTEN protein variants, which could account for the complexity in the genotype/phenotype manifestations of PHTS patients.

Expression of Human PTEN-L in a Yeast Heterologous Model Unveils Specific N-Terminal Motifs Controlling PTEN-L Subcellular Localization and Function

The tumour suppressor PTEN is frequently downregulated, mutated or lost in several types of tumours and congenital disorders including PHTS (PTEN Hamartoma Tumour Syndrome) and ASD (Autism Spectrum Disorder). PTEN is a lipid phosphatase whose activity over the lipid messenger PIP₃ counteracts the stimulation of the oncogenic phosphatidylinositol 3-kinase (PI3K) pathway. Recently, several extended versions of PTEN produced in the cell by alternative translation initiation have been described, among which, PTEN-L and PTEN-M represent the longest isoforms. We previously developed a humanized yeast model in which the expression of PI3K in Saccharomyces cerevisiae led to growth inhibition that could be suppressed by co-expression of PTEN. Here, we show that the expression of PTEN-L and PTEN-M in yeast results in robust counteracting of PI3K-dependent growth inhibition. N-terminally tagged GFP-PTEN-L was sharply localized at the yeast plasma membrane. Point mutations of a putative membrane-binding helix located at the PTEN-L extension or its deletion shifted localization to nuclear. Also, a shift from plasma membrane to nucleus was observed in mutants at basic amino acid clusters at the PIP₂-binding motif, and at the Cα2 and CBR3 loops at the C2 domain. In contrast, C-terminally tagged PTEN-L-GFP displayed mitochondrial localization in yeast, which was shifted to plasma membrane by removing the first 22 PTEN-L residues. Our results suggest an important role of the N-terminal extension of alternative PTEN isoforms on their spatial and functional regulation.

Overexpression of PTEN may increase the effect of pemetrexed on A549 cells via inhibition of the PI3K/AKT/mTOR pathway and carbohydrate metabolism

Lung cancer is one of the leading causes of tumor-associated mortality, and >75% of patients with lung cancer have non-small cell lung cancer (NSCLC). Pemetrexed, a folate antagonist, is a first-line chemotherapy drug for NSCLC that is administered alone or in combination with cisplatin. The present study established in vitro cell models of PTEN inhibition and overexpression, and the effects of the treatment with pemetrexed were investigated in these cell models. Result from the present study demonstrated that treatment with pemetrexed suppressed lung cancer cell proliferation, inhibited mRNA and protein expression levels of anti-apoptotic Bcl2, and increased the mRNA and the protein expression levels of pro-apoptotic p53 and apoptosis regulator BAX. The present study suggested that pemetrexed regulated apoptosis via the inhibition of the mTOR/PI3K/AKT signaling pathway. Additionally, cellular processes associated with the aerobic oxidation of carbohydrates were identified to be significantly inhibited. The present findings suggested that treatment with pemetrexed may exhibit synergistic effects with PTEN on lung cancer cells via the inhibition of the PI3K/AKT/mTOR signaling pathway and through carbohydrate metabolism, and treatment with pemetrexed combined with PTEN overexpression may represent a novel therapeutic strategy for the treatment of NSCLC.

Loss of PTEN Accelerates NKX3.1 Degradation to Promote Prostate Cancer Progression

NKX3.1 is the most commonly deleted gene in prostate cancer and a gatekeeper suppressor. NKX3.1 is a growth suppressor, mediator of apoptosis, inducer of antioxidants, and enhancer of DNA repair. PTEN is a ubiquitous tumor suppressor that is often decreased in prostate cancer during tumor progression. Steady-state turnover of NKX3.1 is mediated by DYRK1B phosphorylation at NKX3.1 serine 185 that leads to polyubiquitination and proteasomal degradation. In this study, we show PTEN is an NKX3.1 phosphatase that protects NKX3.1 from degradation. PTEN specifically opposed phosphorylation at NKX3.1(S185) and prolonged NKX3.1 half-life. PTEN and NKX3.1 interacted primarily in the nucleus as loss of PTEN nuclear localization abrogated its ability to bind to and protect NKX3.1 from degradation. The effect of PTEN on NKX3.1 was mediated via rapid enzyme-substrate interaction. An effect of PTEN on Nkx3.1 gene transcription was seen in vitro, but not in vivo. In gene-targeted mice, Nkx3.1 expression significantly diminished shortly after loss of Pten expression in the prostate. Nkx3.1 loss primarily increased prostate epithelial cell proliferation in vivo. In these mice, Nkx3.1 mRNA was not affected by Pten expression. Thus, the prostate cancer suppressors PTEN and NKX3.1 interact and loss of PTEN is responsible, at least in part, for progressive loss of NKX3.1 that occurs during tumor progression. SIGNIFICANCE: PTEN functions as a phosphatase of NKX3.1, a gatekeeper suppressor of prostate cancer.

Precise definition of PTEN C-terminal epitopes and its implications in clinical oncology

Anti-PTEN monoclonal antibodies (mAb) are arising as important tools for immunohistochemistry (IHC) and protein quantification routine analysis in clinical oncology. Although an effort has been made to document the reliability of tumor tissue section immunostaining by anti-PTEN mAb, and to standardize their IHC use in research and in the clinical practice, the precise topological and biochemical definition of the epitope recognized by each mAb has been conventionally overlooked. In this study, six commercial anti-PTEN mAb have been validated and characterized for sensitivity and specificity by IHC and FISH, using a set of prostate and urothelial bladder tumor specimens, and by immunoblot, using PTEN positive and PTEN negative human cell lines. Immunoblot precise epitope mapping, performed using recombinant PTEN variants and mutations, revealed that all mAb recognized linear epitopes of 6–11 amino acid length at the PTEN C-terminus. Tumor-associated or disease-associated mutations at the PTEN C-terminus did not affect subcellular localization or PIP3 phosphatase activity of PTEN in cells, although resulted in specific loss of reactivity for some mAb. Furthermore, specific mimicking-phosphorylation mutations at the PTEN C-terminal region also abolished binding of specific mAb. Our study adds new evidence on the relevance of a precise epitope mapping in the validation of anti-PTEN mAb for their use in the clinics. This will be substantial to provide a more accurate diagnosis in clinical oncology based on PTEN protein expression in tumors and biological fluids.

A pathogenic role for germline PTEN variants which accumulate into the nucleus

The PTEN gene encodes a master regulator protein that exerts essential functions both in the cytoplasm and in the nucleus. PTEN is mutated in the germline of both patients with heterogeneous tumor syndromic diseases, categorized as PTEN hamartoma tumor syndrome (PHTS), and a group affected with autism spectrum disorders (ASD). Previous studies have unveiled the functional heterogeneity of PTEN variants found in both patient cohorts, making functional studies necessary to provide mechanistic insights related to their pathogenicity. Here, we have functionally characterized a PTEN missense variant [c.49C>G; p.(Gln17Glu); Q17E] associated to both PHTS and ASD patients. The PTEN Q17E variant displayed partially reduced PIP3-catalytic activity and normal stability in cells, as shown using S. cerevisiae and mammalian cell experimental models. Remarkably, PTEN Q17E accumulated in the nucleus, in a process involving the PTEN N-terminal nuclear localization sequence. The analysis of additional germline-associated PTEN N-terminal variants illustrated the existence of a PTEN N-terminal region whose targeting in disease causes PTEN nuclear accumulation, in parallel with defects in PIP3-catalytic activity in cells. Our findings highlight the frequent occurrence of PTEN gene mutations targeting PTEN N-terminus whose pathogenicity may be related, at least in part, with the retention of PTEN in the nucleus. This could be important for the implementation of precision therapies for patients with alterations in the PTEN pathway.

PTEN Inhibition in Human Disease Therapy

The tumor suppressor PTEN is a major homeostatic regulator, by virtue of its lipid phosphatase activity against phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3], which downregulates the PI3K/AKT/mTOR prosurvival signaling, as well as by its protein phosphatase activity towards specific protein targets. PTEN catalytic activity is crucial to control cell growth under physiologic and pathologic situations, and it impacts not only in preventing tumor cell survival and proliferation, but also in restraining several cellular regeneration processes, such as those associated with nerve injury recovery, cardiac ischemia, or wound healing. In these conditions, inhibition of PTEN catalysis is being explored as a potentially beneficial therapeutic intervention. Here, an overview of human diseases and conditions in which PTEN inhibition could be beneficial is presented, together with an update on the current status of specific small molecule inhibitors of PTEN enzymatic activity, their use in experimental models, and their limitations as research or therapeutic drugs.

Lapatinib resistance in HER2+ cancers: latest findings and new concepts on molecular mechanisms

In the era of new and mostly effective molecular targeted therapies, human epidermal growth factor receptor 2 positive (HER2+) cancers are still intractable diseases. Lapatinib, a dual epidermal growth factor receptor (EGFR) and HER2 tyrosine kinase inhibitor, has greatly improved breast cancer prognosis in recent years after the initial introduction of trastuzumab (Herceptin). However, clinical evidence indicates the existence of both primary unresponsiveness and secondary lapatinib resistance, which leads to the failure of this agent in HER2+ cancer patients. It remains a major clinical challenge to target the oncogenic pathways with drugs having low resistance. Multiple pathways are involved in the occurrence of lapatinib resistance, including the pathways of receptor tyrosine kinase, non-receptor tyrosine kinase, autophagy, apoptosis, microRNA, cancer stem cell, tumor metabolism, cell cycle, and heat shock protein. Moreover, understanding the relationship among these mechanisms may contribute to future tumor combination therapies. Therefore, it is of urgent necessity to elucidate the precise mechanisms of lapatinib resistance and improve the therapeutic use of this agent in clinic. The present review, in the hope of providing further scientific support for molecular targeted therapies in HER2+ cancers, discusses about the latest findings and new concepts on molecular mechanisms underlying lapatinib resistance.

Fine-Tuning of Pten Localization and Phosphatase Activity Is Essential for Zebrafish Angiogenesis

The lipid- and protein phosphatase PTEN is an essential tumor suppressor that is highly conserved among all higher eukaryotes. As an antagonist of the PI3K/Akt cell survival and proliferation pathway, it exerts its most prominent function at the cell membrane, but (PIP3-independent) functions of nuclear PTEN have been discovered as well. PTEN subcellular localization is tightly controlled by its protein conformation. In the closed conformation, PTEN localizes predominantly to the cytoplasm. Opening up of the conformation of PTEN exposes N-terminal and C-terminal regions of the protein that are required for both interaction with the cell membrane and translocation to the nucleus. Lack of Pten leads to hyperbranching of the intersegmental vessels during zebrafish embryogenesis, which is rescued by expression of exogenous Pten. Here, we observed that expression of mutant PTEN with an open conformation rescued the hyperbranching phenotype in pten double homozygous embryos and suppressed the increased p-Akt levels that are characteristic for embryos lacking Pten. In addition, in pten mutant and wild type embryos alike, open conformation PTEN induced stalled intersegmental vessels, which fail to connect with the dorsal longitudinal anastomotic vessel. Functional hyperactivity of open conformation PTEN in comparison to wild type PTEN seems to result predominantly from its enhanced recruitment to the cell membrane. Enhanced recruitment of phosphatase inactive mutants to the membrane did not induce the stalled vessel phenotype nor did it rescue the hyperbranching phenotype in pten double homozygous embryos, indicating that PTEN phosphatase activity is indispensable for its regulatory function during angiogenesis. Taken together, our data suggest that PTEN phosphatase activity needs to be carefully fine-tuned for normal embryogenesis and that the control of its subcellular localization is a key mechanism in this process.

Assessing PTEN Subcellular Localization

PTEN subcellular localization is fundamental in the execution of the distinct PTEN biological activities, including not only its PI(3,4,5)P3 phosphatase activity when associated to membranes but also its subcellular compartment-specific interactions with regulatory and effector proteins, including those exerted in the nucleus. As a consequence, PTEN subcellular localization is tightly regulated in vivo by both intrinsic and extrinsic mechanisms. The plasma membrane/nucleus/cytoplasm partitioning of PTEN has been the focus of several studies, both from a mechanistic and from a disease-association point of view. Here, we summarize the current knowledge on PTEN plasma membrane/nucleus/cytoplasm distribution, and present subcellular fractionation, immunofluorescence, and immunohistochemical methods to study the distribution and shuttling of PTEN between these subcellular compartments.

Mislocalization of p27 to the cytoplasm of breast cancer cells confers resistance to anti-HER2 targeted therapy

As a cell cycle inhibitor and tumor suppressor, p27 is frequently misregulated in human cancers. Increased degradation is the most common mechanism of misregulation, however in some cancers, p27 is mislocalized from its cell cycle inhibitory location in the nucleus, to the cytoplasm. In normal cells cytoplasmic p27 has functions that are distinct from its cell cycle-regulatory nuclear functions. Therefore, an important question is whether localization of p27 to the cytoplasm in tumor cells is primarily a mechanism for cancelling its inhibitory effect on cell proliferation, or whether cytoplasmic p27 has more direct oncogenic actions. To study p27 mislocalization in human cancers we screened a panel of common breast cancer cell lines. We observed that p27 accumulated in the cytoplasm exclusively in cell lines that are Her2+. To address the significance of p27 mislocalization in Her2+ breast cancer cells we interrogated the cellular response to the dual-Her2/EGFR kinase inhibitor, lapatinib. Knockdown of p27 using shRNA sensitized Her2+ cells to lapatinib-induced apoptosis. Moreover, expression of a constitutively cytoplasmic form of p27 (p27ΔNLS) reversed the lapatinib-induced apoptosis, suggesting that cytoplasmic p27 contributed to lapatinib resistance in Her2+ breast cancer cells by suppressing apoptosis. Our results suggest that p27 localization may be useful as a predictive biomarker of therapeutic response in patients with Her2+ breast cancers.

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.

Lung neuroendocrine tumors: correlation of ubiquitinylation and sumoylation with nucleo-cytosolic partitioning of PTEN

Background

The tumor suppressor phosphatase and tensin homolog (PTEN) is a pleiotropic enzyme, inhibiting phosphatidyl-inositol-3 kinase (PI3K) signaling in the cytosol and stabilizing the genome in the nucleus. Nucleo-cytosolic partitioning is dependent on the post-translational modifications ubiquitinylation and sumoylation. This cellular compartmentalization of PTEN was investigated in lung neuroendocrine tumors (lung NET).

Methods

Tumor tissues from 192 lung NET patients (surgical specimens = 183, autopsies = 9) were investigated on tissue microarrays. PTEN was H-scored by two investigators in nucleus and cytosol using the monoclonal antibody 6H2.1. Results were correlated with immunoreactivity for USP7 (herpes virus-associated ubiquitin-specific protease 7) and SUMO2/3 (small ubiquitin-related modifier protein 2/3) as well as PTEN and p53 FISH gene status. Clinico-pathologic data including overall survival, proliferation rate and diagnostic markers (synaptophysin, chromogranin A, Mib-1, TTF-1) were recorded.

Results

The multicentre cohort included 58 typical carcinoids (TC), 42 atypical carcinoids (AC), 32 large cell neuroendocrine carcinomas (LCNEC) and 60 small cell lung carcinomas (SCLC). Carcinoids were smaller in size and had higher synaptophysin and chromogranin A, but lower TTF-1 expressions. Patients with carcinoids were predominantly female and 10 years younger than patients with LCNEC/SCLC. In comparison to the carcinoids, LCNEC/SCLC tumors presented a stronger loss of nuclear and cytosolic PTEN associated with a loss of PTEN and p53. Concomitantly, a loss of nuclear USP7 but increase of nuclear and cytosolic SUMO2/3 was found. Loss of nuclear and cytosolic PTEN, loss of nuclear USP7 and increase of cytosolic SUMO2/3 thus correlated with poor survival. Among carcinoids, loss of cytosolic PTEN was predominantly found in TTF1-negative larger tumors of male patients. Among SCLC, loss of both cytosolic and nuclear PTEN but not proliferation rate or tumor size delineated a subgroup with poorer survival (all p-values <0.05).

Conclusions

Cellular ubiquitinylation and sumoylation likely influence the functional PTEN loss in high grade lung NET. Both nuclear and cytosolic PTEN immunoreactivity should be considered for correlation with clinico-pathologic parameters.

Effect of p27 gene combined with Pientzehuang ([characters: see text]) on tumor growth in osteosarcoma-bearing nude mice

OBJECTIVE

To observe the effect of p27 gene recombinant adenovirus combined with Chinese medicine Pientzehuang ([characters: see text]) on the growth of xenografted human osteosarcoma in nude mice.

METHODS

Tissue transplantation was used to construct the orthotopic model of human osteosarcoma Saos-2 cell in nude mice. Thirty tumor-bearing nude mice were randomly divided into 5 groups with 6 mice in each group: blank control group (model of osteosarcoma), empty vector group (recombinant adeno-associated virus-multiple cloning site), Pientzehuang group, p27 gene group and combined treatment group (p27 gene combined with Pientzehuang). The effect of combined treatment on human osteosarcoma was analyzed through the tumor formation, tumor volume and inhibition rate of tumor growth. The expression of p27 was measured by immunohistochemical staining and Western blot.

RESULTS

The orthotopic model of osteosarcoma in nude mice was successfully constructed. The general appearance of tumor-bearing nude mice in Pientzehuang and p27 gene groups was markedly improved compared with the blank control group; and in the combined treatment group it was significantly improved compared with the Pientzehuang and p27 gene groups. The tumor growth in the Pientzehuang and p27 gene groups was significantly inhibited compared with the blank control group P<0.05); while in the combined treatment group it was markedly inhibited compared with the Pientzehuang and p27 gene groups (P<0.05). The rates of tumor growth inhibition were 34.1%, 56.5% and 63.8% in the Pientzehuang, p27 gene and combined treatment groups, respectively. Meanwhile, the protein expression of p27 gene in the p27 gene group was significantly increased compared with the blank control group (P<0.05); and it was significantly increased in the combined treatment group compared with the p27 gene and Pientzehuang groups (P<0.05).

CONCLUSION

p27 gene introduced by adenovirus combined with Pientzehuang can inhibit the growth of human osteosarcoma cell Saos-2 in nude mice.

Toxicity of functional nano-micro zinc oxide tetrapods: impact of cell culture conditions, cellular age and material properties

With increasing production and applications of nanostructured zinc oxide, e.g., for biomedical and consumer products, the question of safety is getting more and more important. Different morphologies of zinc oxide structures have been synthesized and accordingly investigated. In this study, we have particularly focused on nano-micro ZnO tetrapods (ZnO-T), because their large scale fabrication has been made possible by a newly introduced flame transport synthesis approach which will probably lead to several new applications. Moreover, ZnO-T provide a completely different morphology then classical spherical ZnO nanoparticles. To get a better understanding of parameters that affect the interactions between ZnO-T and mammalian cells, and thus their biocompatibility, we have examined the impact of cell culture conditions as well as of material properties on cytotoxicity. Our results demonstrate that the cell density of fibroblasts in culture along with their age, i.e., the number of preceding cell divisions, strongly affect the cytotoxic potency of ZnO-T. Concerning the material properties, the toxic potency of ZnO-T is found to be significantly lower than that of spherical ZnO nanoparticles. Furthermore, the morphology of the ZnO-T influenced cellular toxicity in contrast to surface charges modified by UV illumination or O₂ treatment and to the material age. Finally, we have observed that direct contact between tetrapods and cells increases their toxicity compared to transwell culture models which allow only an indirect effect via released zinc ions. The results reveal several parameters that can be of importance for the assessment of ZnO-T toxicity in cell cultures and for particle development.

PINK1 regulates histone H3 trimethylation and gene expression by interaction with the polycomb protein EED/WAIT1

Mutations in PTEN-induced putative kinase 1 (PINK1) gene are associated to early-onset recessive forms of Parkinson disease. PINK1 function is related to mitochondria homeostasis, but the molecular pathways in which PINK1 is involved are largely unknown. Here, we report the identification of the embryonic ectoderm development polycomb histone-methylation modulator (EED/WAIT1) as a PINK1-interacting and -regulated protein. The PINK1:EED/WAIT1 physical interaction was mediated by the PINK1 kinase domain and the EED/WAIT1 40 amino acid ending with tryptophan and aspartate (WD40)-repeat region, and PINK1 phosphorylated EED/WAIT1 in vitro. PINK1 associated with EED/WAIT1 in cells and relocated EED/WAIT1 to the mitochondria. This interaction reduced the trimethylation of lysine 27 from histone H3, which affected polycomb-regulated gene transcription during RA differentiation of SH-SY5Y human neuroblastoma cells. Our findings unveil a pathway by which PINK1 regulates histone methylation and gene expression through the polycomb repressor complex.

PTPs emerge as PIPs: protein tyrosine phosphatases with lipid-phosphatase activities in human disease

Protein tyrosine phosphatases (PTPs) constitute a family of key homeostatic regulators, with wide implications on physiology and disease. Recent findings have unveiled that the biological activity of PTPs goes beyond the dephosphorylation of phospho-proteins to shut down protein tyrosine kinase-driven signaling cascades. Substrates dephosphorylated by clinically relevant PTPs extend to phospholipids and phosphorylated carbohydrates as well. In addition, non-catalytic functions are also used by PTPs to regulate essential cellular functions. Consequently, PTPs have emerged as novel potential therapeutic targets for human diseases, including cancer predispositions, myopathies and neuropathies. In this review, we highlight recent advances on the multifaceted role of lipid-phosphatase PTPs in human pathology, with an emphasis on hereditary diseases. The involved PTP regulatory networks and PTP modulatory strategies with potential therapeutic application are discussed.

Differential regulation of cyclin-dependent kinase inhibitors in neuroblastoma cells

Gastrin-releasing peptide (GRP) and its receptor (GRP-R) are highly expressed in undifferentiated neuroblastoma, and they play critical roles in oncogenesis. We previously reported that GRP activates the PI3K/AKT signaling pathway to promote DNA synthesis and cell cycle progression in neuroblastoma cells. Conversely, GRP-R silencing induces cell cycle arrest. Here, we speculated that GRP/GRP-R signaling induces neuroblastoma cell proliferation via regulation of cyclin-dependent kinase (CDK) inhibitors. Surprisingly, we found that GRP/GRP-R differentially induced expressions of p21 and p27. Silencing GRP/GRP-R decreased p21, but it increased p27 expressions in neuroblastoma cells. Furthermore, we found that the intracellular localization of p21 and p27 in the nuclear and cytoplasmic compartments, respectively. In addition, we found that GRP/GRP-R silencing increased the expression and accumulation of PTEN in the cytoplasm of neuroblastoma cells where it co-localized with p27, thus suggesting that p27 promotes the function of PTEN as a tumor suppressor by stabilizing PTEN in the cytoplasm. GRP/GRP-R regulation of CDK inhibitors and tumor suppressor PTEN may be critical for tumoriogenesis of neuroblastoma.

Interference with the PTEN-MAST2 interaction by a viral protein leads to cellular relocalization of PTEN

PTEN (phosphatase and tensin homolog deleted on chromosome 10) and MAST2 (microtubule-associated serine and threonine kinase 2) interact with each other through the PDZ domain of MAST2 (MAST2-PDZ) and the carboxyl-terminal (C-terminal) PDZ domain-binding site (PDZ-BS) of PTEN. These two proteins function as negative regulators of cell survival pathways, and silencing of either one promotes neuronal survival. In human neuroblastoma cells infected with rabies virus (RABV), the C-terminal PDZ domain of the viral glycoprotein (G protein) can target MAST2-PDZ, and RABV infection triggers neuronal survival in a PDZ-BS-dependent fashion. These findings suggest that the PTEN-MAST2 complex inhibits neuronal survival and that viral G protein disrupts this complex through competition with PTEN for binding to MAST2-PDZ. We showed that the C-terminal sequences of PTEN and the viral G protein bound to MAST2-PDZ with similar affinities. Nuclear magnetic resonance structures of these complexes exhibited similar large interaction surfaces, providing a structural basis for their binding specificities. Additionally, the viral G protein promoted the nuclear exclusion of PTEN in infected neuroblastoma cells in a PDZ-BS-dependent manner without altering total PTEN abundance. These findings suggest that formation of the PTEN-MAST2 complex is specifically affected by the viral G protein and emphasize how disruption of a critical protein-protein interaction regulates intracellular PTEN trafficking. In turn, the data show how the viral protein might be used to decipher the underlying molecular mechanisms and to clarify how the subcellular localization of PTEN regulates neuronal survival.