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| PHLPP1alpha and PHLPP1beta protein structure. |
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Description | The PHLPP1alpha and PHLPP1beta proteins both contain a pleckstrin homology (PH) domain, a series of leucine-rich repeats (LRR), a PP2C phosphatase domain, and a C-terminal PDZ (post synaptic density protein [PSD95], Drosophila disc large tumor suppressor [DlgA], and zonula occludens-1 protein [zo-1]) binding motif. In addition, PHLPP1beta has a putative Ras association (RA) domain near its N-terminus. PHLPP1alpha is composed of 1205 amino acids and has a molecular weight of approximately 133 kDa, while PHLPP1beta has 1717 amino acids and a molecular weight of approximately 185 kDa. (The related isoform PHLPP2 has a domain structure similar to that of PHLPP1beta). |
Expression | PHLPP1 is expressed in most human cancer cell lines and all mouse tissues examined so far. PHLPP1beta appears to be more abundant than PHLPP1alpha. Rat PHLPP1beta (termed SCOP for Suprachiasmatic nucleus circadian oscillatory protein) is also expressed in the suprachiasmatic nucleus, where its mRNA expression oscillates in a circadian fashion. |
Localisation | PHLPP1 appears to be localized throughout the cell. |
Function | PHLPP1 is a phosphatase that specifically dephosphorylates the hydrophobic motif (HM) of Akt and conventional/novel PKC isoforms. HM phosphorylation is important for the function of both kinases. For Akt, phosphorylation at serine 473, the HM site, allows full activation of the kinase and subsequent phosphorylation of its downstream substrates. For PKC, phosphorylation of the HM (serine 660 in PKCbetaII) increases protein stability; once the HM is dephosphorylated, two other important regulatory sites on the kinase (the activation loop and the turn motif) are rendered more sensitive to dephosphorylation by other phosphatases. The dephosphorylated PKC is then shunted to the detergent-insoluble fraction of the cell, where it is degraded. PHLPP1 therefore functions to decrease the activity of both Akt and PKC, albeit by different mechanisms. While PHLPP1 and its family member PHLPP2 have similar functions, their specificity for Akt isoforms differs. PHLPP1 preferentially binds and dephosphorylates Akt2 and Akt3, resulting in decreased phosphorylation of a set of Akt targets that includes GSK-3beta, TSC2, and FoxO, as well as and GSK3a. PHLPP2, on the other hand, binds and dephosphorylates Akt1 and Akt3, resulting in downregulation of an overlapping yet distinct set of downstream targets: GSK-3beta, TSC2, and FoxO, as well as TSC2 and p27. Interestingly, PHLPP1's regulation of its protein substrates appears to be regulated by its protein-protein interaction domains. PHLPP1 lacking a C-terminal PDZ ligand is unable to dephosphorylate Akt, whereas deletion of PHLPP1's PH domain decreases its ability to dephosphorylate PKC. Since PHLPP1 downregulates the pro-survival kinase Akt, it is not surprising that this phosphatase plays roles in apoptosis and suppression of cellular proliferation. siRNA-mediated reduction of PHLPP1 causes increased apoptosis in a number of cell lines, whereas overexpression of PHLPP1 decreases proliferation in LN229, a glioblastoma cell line, and suppresses its ability to form tumors in nude mice. PHLPP1 also regulates the phosphorylation and activity of ERK; it has been suggested to interact directly with the nucleotide-free form of K-Ras and thus suppress the Ras/Raf/MEK/ERK pathway. This pathway is important for the regulation of learning and memory, and overexpression of rat PHLPP1beta in the hippocampus of transgenic mice abolishes memory for novel objects. In addition, training for hippocampus-based learning prompts calpain protease-mediated degradation of PHLPP1. Together, these results suggest that proper regulation of PHLPP1 in certain neurons is crucial for memory formation. |
Homology | PHLPP is a highly conserved phosphatase; its earliest orthologue is the yeast protein CYR1. In addition to a PP2C phosphatase domain, a leucine-rich repeat, and a Ras association domain, CYR1 contains an adenylate cyclase domain near its C terminus. Though invertebrates have only one PHLPP gene, most vertebrates have genes for both PHLPP1 and PHLPP2. |
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Entity | Glioblastoma multiforme |
Oncogenesis | PHLPP1 overexpression in human LN229 cells limits their ability to form tumors in a xenograft model. Various human glioblastoma cell lines respond to PHLPP1 knockdown with increased Akt phosphorylation. In addition, mRNA expression of both PHLPP1 and PHLPP2 are decreased by around 30% in patient glioblastoma samples (relative to normal brain). |
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Entity | Colorectal cancer |
Cytogenetics | 18q21.33, the chromosomal locus containing the gene for PHLPP1 as well as the putative tumor suppressors BCL2 and Maspin, commonly undergoes loss of heterozygosity in colon cancers. |
Oncogenesis | Overexpression of PHLPP1 or PHLPP2 in the human colon cancer cell lines HCT-116 and HT29 causes decreased expression of PKC and decreased phosphorylation of Akt. Cells overexpressing PHLPP exhibit decreased proliferation and were less able to induce tumors in nude mice. Conversely, DLD1 cells, which express high levels of PHLPP, respond to PHLPP1 or PHLPP2 knockdown with increased Akt phosphorylation, PKC stability, and proliferation. |
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Entity | Leukemia |
Disease | Chronic lymphocytic leukemia, chronic myelogenous leukemia |
Oncogenesis | PHLPP1 mRNA expression is frequently reduced to undetectable levels in patients with chronic lymphocytic leukemia (CLL). About 50% of CLL patients have loss of chromosomal region 13q14, and about 50% of these show drastically reduced PHLPP1 expression. In chronic myelogenous leukemia (CML), PHLPP mRNA levels may also be decreased, albeit by a different mechanism. Bcr-Abl, the fusion protein responsible for CML, downregulates PHLPP1 and PHLPP2 mRNA levels; decreasing PHLPP levels interferes with the efficacy of Bcr-Abl inihibitors, including Gleevec, in CML cell lines. |
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PHLPP and a second isoform, PHLPP2, differentially attenuate the amplitude of Akt signaling by regulating distinct Akt isoforms. |
Brognard J, Sierecki E, Gao T, Newton AC. |
Mol Cell. 2007 Mar 23;25(6):917-31. |
PMID 17386267 |
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PHLiPPing the switch on Akt and protein kinase C signaling. |
Brognard, J, Newton AC. |
Trends Endocrinol Metab 2008 Aug;19(6):223-30. (REVIEW) |
PMID 18511290 |
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Comprehensive genomic characterization defines human glioblastoma genes and core pathways. |
Cancer Genome Atlas Research Network. |
Nature. 2008 Oct 23;455(7216):1061-8. |
PMID 18772890 |
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The phosphatase PHLPP controls the cellular levels of protein kinase C. |
Gao T, Brognard J, Newton AC. |
J Biol Chem. 2008 Mar 7;283(10):6300-11. |
PMID 18162466 |
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PHLPP: a phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth. |
Gao T, Furnari F, Newton AC. |
Mol Cell. 2005 Apr 1;18(1):13-24. |
PMID 15808505 |
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Depletion of pleckstrin homology domain leucine-rich repeat protein phosphatase 1 and 2 by Bcr-Abl promotes chronic myelogenous leukemia cell proliferation through continuous phosphorylation of Akt isoforms. |
Hirano I, Nakamura S, Yokota D, Ono T, Shigeno K, Fujisawa S, Shinjo K, Ohnishi K. |
J Biol Chem. 2009 Mar 4. [Epub ahead of print] |
PMID 19261608 |
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Loss of PHLPP expression in colon cancer: role in proliferation and tumorigenesis. |
Liu J, Weiss HL, Rychahou P, Jackson LN, Evers BM, Gao T. |
Oncogene. 2009 Feb 19;28(7):994-1004. |
PMID 19079341 |
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PHLPPing it off: phosphatases get in the Akt. |
Mendoza MC, Blenis J. |
Mol Cell. 2007 Mar 23;25(6):798-800. (REVIEW) |
PMID 17386258 |
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Integrated genomic profiling of chronic lymphocytic leukemia identifies subtypes of deletion 13q14. |
Ouillette P, Erba H, Kujawski L, Kaminski M, Shedden K, Malek SN. |
Cancer Res. 2008 Feb 15;68(4):1012-21. |
PMID 18281475 |
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Suprachiasmatic nucleus circadian oscillatory protein, a novel binding partner of K-Ras in the membrane rafts, negatively regulates MAPK pathway. |
Shimizu K, Okada M, Nagai K, Fukada Y. |
J Biol Chem. 2003 Apr 25;278(17):14920-5. Epub 2003 Feb 19. |
PMID 12594205 |
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Proteolytic degradation of SCOP in the hippocampus contributes to activation of MAP kinase and memory. |
Shimizu K, Phan T, Mansuy IM, Storm DR. |
Cell. 2007 Mar 23;128(6):1219-29. |
PMID 17382888 |
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Neuronal and glioma-derived stem cell factor induces angiogenesis within the brain. |
Sun L, Hui AM, Su Q, Vortmeyer A, Kotliarov Y, Pastorino S, Passaniti A, Menon J, Walling J, Bailey R, Rosenblum M, Mikkelsen T, Fine HA. |
Cancer Cell. 2006 Apr;9(4):287-300. |
PMID 16616334 |
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