PHLPP

Last updated
PH domain and leucine-rich repeat protein phosphatase
Identifiers
Symbol PHLPP
Alt. symbolsPHLPP1, PLEKHE1
NCBI gene 23239
HGNC 20610
OMIM 609396
RefSeq XM_166290
UniProt O60346
Other data
EC number 3.1.3.16
Locus Chr. 18 q21.32
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Structures Swiss-model
Domains InterPro
PH domain and leucine rich repeat protein phosphatase-like
Identifiers
Symbol PHLPPL
Alt. symbolsPHLPP2
NCBI gene 23035
HGNC 29149
OMIM 611066
RefSeq NM_015020
UniProt Q6ZVD8
Other data
EC number 3.1.3.16
Locus Chr. 16 q22.2
Search for
Structures Swiss-model
Domains InterPro

The PHLPP isoforms (PH domain and Leucine rich repeat Protein Phosphatases) are a pair of protein phosphatases, PHLPP1 and PHLPP2, that are important regulators of Akt serine-threonine kinases (Akt1, Akt2, Akt3) and conventional/novel protein kinase C (PKC) isoforms. PHLPP may act as a tumor suppressor in several types of cancer due to its ability to block growth factor-induced signaling in cancer cells. [1]

Contents

PHLPP dephosphorylates Ser-473 (the hydrophobic motif) in Akt, thus partially inactivating the kinase. [2]

In addition, PHLPP dephosphorylates conventional and novel members of the protein kinase C family at their hydrophobic motifs, corresponding to Ser-660 in PKCβII. [3]

Domain structure

PHLPP is a member of the PPM family of phosphatases, which requires magnesium or manganese for their activity and are insensitive to most common phosphatase inhibitors, including [okadaic acid]. PHLPP1 and PHLPP2 have a similar domain structure, which includes a putative Ras association domain, a pleckstrin homology domain, a series of leucine-rich repeats, a PP2C phosphatase domain, and a C-terminal PDZ ligand. PHLPP1 has two splice variants, PHLPP1α and PHLPP1β, of which PHLPP1β is larger by approximately 1.5 kilobase pairs. PHLPP1α, which was the first PHLPP isoform to be characterized, lacks the N-terminal portion of the protein, including the Ras association domain. [1] PHLPP's domain structure influences its ability to dephosphorylate its substrates. A PHLPP construct lacking the PH domain is unable to decrease PKC phosphorylation, while PHLPP lacking the PDZ ligand is unable to decrease Akt phosphorylation. [2]

Dephosphorylation of Akt

The phosphatases in the PHLPP family, PHLPP1 and PHLPP2 have been shown to directly dephosphorylate, and therefore inactivate, distinct Akt isoforms, at one of the two critical phosphorylation sites required for activation: Serine473. PHLPP2 dephosphorylates AKT1 and AKT3, whereas PHLPP1 is specific for AKT2 and AKT3. Lack of PHLPP appears to have effects on growth factor-induced Akt phosphorylation. When both PHLPP1 and PHLPP2 are knocked down using siRNA and cells are stimulated using epidermal growth factor, peak Akt phosphorylation at both Serine473 and Threonine308 (the other site required for full Akt activation) is increased dramatically. [4]

The Akt family of kinases

In humans, there are three genes in the Akt family: AKT1, AKT2, and AKT3. These enzymes are members of the serine/threonine-specific protein kinase family (EC 2.7.11.1).

Akt1 is involved in cellular survival pathways and inhibition of apoptotic processes. Akt1 is also able to induce protein synthesis pathways, and is therefore a key signaling protein in the cellular pathways that lead to skeletal muscle hypertrophy, and general tissue growth. Since it can block apoptosis, and thereby promote cell survival, Akt1 has been implicated as a major factor in many types of cancer. Akt (now also called Akt1) was originally identified as the oncogene in the transforming retrovirus, AKT8.

Akt2 is important in the insulin signaling pathway. It is required to induce glucose transport.[ citation needed ]

These separate roles for Akt1 and Akt2 were demonstrated by studying mice in which either the Akt1 or the Akt2 gene was deleted, or "knocked out". In a mouse that is null for Akt1 but normal for Akt2, glucose homeostasis is unperturbed, but the animals are smaller, consistent with a role for Akt1 in growth. In contrast, mice that do not have Akt2 but have normal Akt1 have mild growth deficiency and display a diabetic phenotype (insulin resistance), again consistent with the idea that Akt2 is more specific for the insulin receptor signaling pathway. [5]

The role of Akt3 is less clear, though it appears to be expressed predominantly in brain. It has been reported that mice lacking Akt3 have small brains. [6]

Phosphorylation of Akt by PDK1 and PDK2

Once correctly positioned in the membrane via binding of PIP3, Akt can then be phosphorylated by its activating kinases, phosphoinositide-dependent kinase 1 (PDK1) and PDK2. Serine473, the hydrophobic motif, is phosphorylated in an mTORC2-dependent manner, leading some investigators to hypothesize that mTORC2 is the long-sought PDK2 molecule. Threonine308, the activation loop, is phosphorylated by PDK1, allowing full Akt activation. Activated Akt can then go on to activate or deactivate its myriad substrates via its kinase activity. The PHLPPs therefore antagonize PDK1 and PDK2, since they dephosphorylate the site that PDK2 phosphorylates. [1]

Dephosphorylation of protein kinase C

PHLPP1 and 2 also dephosphorylate the hydrophobic motifs of two classes of the protein kinase C (PKC) family: the conventional PKCs and the novel PKCs. (The third class of PKCs, known as the atypicals, have a phospho-mimetic at the hydrophobic motif, rendering them insensitive to PHLPP.)

The PKC family of kinases consists of 10 isoforms, whose sensitivity to various second messengers is dictated by their domain structure. The conventional PKCs can be activated by calcium and diacylglycerol, two important mediators of G protein-coupled receptor signaling. The novel PKCs are activated by diacylglycerol but not calcium, while the atypical PKCs are activated by neither.

The PKC family, like Akt, plays roles in cell survival and motility. Most PKC isoforms are anti-apoptotic, although PKCδ (a novel PKC isoform) is pro-apoptotic in some systems.

Although PKC possesses the same phosphorylation sites as Akt, its regulation is quite different. PKC is constitutively phosphorylated, and its acute activity is regulated by binding of the enzyme to membranes. Dephosphorylation of PKC at the hydrophobic motif by PHLPP allows PKC to be dephosphorylated at two other sites (the activation loop and the turn motif). This in turn renders PKC sensitive to degradation. Thus, prolonged increases in PHLPP expression or activity inhibit PKC phosphorylation and stability, decreasing the total levels of PKC over time. [1]

Role in cancer

Investigators have hypothesized that the PHLPP isoforms may play roles in cancer, for several reasons. First, the genetic loci coding for PHLPP1 and 2 are commonly lost in cancer. The region including PHLPP1, 18q21.33, commonly undergoes loss of heterozygosity (LOH) in colon cancers, while 16q22.3, which includes the PHLPP2 gene, undergoes LOH in breast and ovarian cancers, Wilms tumors, prostate cancer and hepatocellular carcinoma. [1] Second, experimental overexpression of PHLPP in cancer cell lines tends to decrease apoptosis and increase proliferation, and stable colon and glioblastoma cell lines overexpressing PHLPP1 show decreased tumor formation in xenograft models. [2] [7] Recent studies have also shown that Bcr-Abl, the fusion protein responsible for chronic myelogenous leukemia (CML), downregulates PHLPP1 and PHLPP2 levels, and that decreasing PHLPP levels interferes with the efficacy of Bcr-Abl inhibitors, including Gleevec, in CML cell lines. [8]

Finally, both Akt and PKC are known to be tumor promoters, suggesting that their negative regulator PHLPP may act as a tumor suppressor.

Related Research Articles

<span class="mw-page-title-main">Protein kinase</span> Enzyme that adds phosphate groups to other proteins

A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a functional change of the target protein (substrate) by changing enzyme activity, cellular location, or association with other proteins. The human genome contains about 500 protein kinase genes and they constitute about 2% of all human genes. There are two main types of protein kinase. The great majority are serine/threonine kinases, which phosphorylate the hydroxyl groups of serines and threonines in their targets and most of the others are tyrosine kinases, although additional types exist. Protein kinases are also found in bacteria and plants. Up to 30% of all human proteins may be modified by kinase activity, and kinases are known to regulate the majority of cellular pathways, especially those involved in signal transduction.

A protein phosphatase is a phosphatase enzyme that removes a phosphate group from the phosphorylated amino acid residue of its substrate protein. Protein phosphorylation is one of the most common forms of reversible protein posttranslational modification (PTM), with up to 30% of all proteins being phosphorylated at any given time. Protein kinases (PKs) are the effectors of phosphorylation and catalyse the transfer of a γ-phosphate from ATP to specific amino acids on proteins. Several hundred PKs exist in mammals and are classified into distinct super-families. Proteins are phosphorylated predominantly on Ser, Thr and Tyr residues, which account for 79.3, 16.9 and 3.8% respectively of the phosphoproteome, at least in mammals. In contrast, protein phosphatases (PPs) are the primary effectors of dephosphorylation and can be grouped into three main classes based on sequence, structure and catalytic function. The largest class of PPs is the phosphoprotein phosphatase (PPP) family comprising PP1, PP2A, PP2B, PP4, PP5, PP6 and PP7, and the protein phosphatase Mg2+- or Mn2+-dependent (PPM) family, composed primarily of PP2C. The protein Tyr phosphatase (PTP) super-family forms the second group, and the aspartate-based protein phosphatases the third. The protein pseudophosphatases form part of the larger phosphatase family, and in most cases are thought to be catalytically inert, instead functioning as phosphate-binding proteins, integrators of signalling or subcellular traps. Examples of membrane-spanning protein phosphatases containing both active (phosphatase) and inactive (pseudophosphatase) domains linked in tandem are known, conceptually similar to the kinase and pseudokinase domain polypeptide structure of the JAK pseudokinases. A complete comparative analysis of human phosphatases and pseudophosphatases has been completed by Manning and colleagues, forming a companion piece to the ground-breaking analysis of the human kinome, which encodes the complete set of ~536 human protein kinases.

<span class="mw-page-title-main">Kinase</span> Enzyme catalyzing transfer of phosphate groups onto specific substrates

In biochemistry, a kinase is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule donates a phosphate group to the substrate molecule. This transesterification produces a phosphorylated substrate and ADP. Conversely, it is referred to as dephosphorylation when the phosphorylated substrate donates a phosphate group and ADP gains a phosphate group. These two processes, phosphorylation and dephosphorylation, occur four times during glycolysis.

A mitogen-activated protein kinase is a type of protein kinase that is specific to the amino acids serine and threonine. MAPKs are involved in directing cellular responses to a diverse array of stimuli, such as mitogens, osmotic stress, heat shock and proinflammatory cytokines. They regulate cell functions including proliferation, gene expression, differentiation, mitosis, cell survival, and apoptosis.

In cell biology, Protein kinase C, commonly abbreviated to PKC (EC 2.7.11.13), is a family of protein kinase enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins, or a member of this family. PKC enzymes in turn are activated by signals such as increases in the concentration of diacylglycerol (DAG) or calcium ions (Ca2+). Hence PKC enzymes play important roles in several signal transduction cascades.

<span class="mw-page-title-main">Protein kinase B</span> Set of three serine/threonine-specific protein kinases

Protein kinase B (PKB), also known as Akt, is the collective name of a set of three serine/threonine-specific protein kinases that play key roles in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription, and cell migration.

<span class="mw-page-title-main">Platelet-derived growth factor receptor</span>

Platelet-derived growth factor receptors (PDGF-R) are cell surface tyrosine kinase receptors for members of the platelet-derived growth factor (PDGF) family. PDGF subunits -A and -B are important factors regulating cell proliferation, cellular differentiation, cell growth, development and many diseases including cancer. There are two forms of the PDGF-R, alpha and beta each encoded by a different gene. Depending on which growth factor is bound, PDGF-R homo- or heterodimerizes.

<span class="mw-page-title-main">AKT1</span> Protein-coding gene in the species Homo sapiens

RAC(Rho family)-alpha serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT1 gene. This enzyme belongs to the AKT subfamily of serine/threonine kinases that contain SH2 protein domains. It is commonly referred to as PKB, or by both names as "Akt/PKB".

<span class="mw-page-title-main">PRKCD</span> Protein-coding gene in the species Homo sapiens

Protein kinase C delta type is an enzyme that in humans is encoded by the PRKCD gene.

<span class="mw-page-title-main">BRAF (gene)</span> Protein-coding gene in the species Homo sapiens

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<span class="mw-page-title-main">AKT3</span> Protein-coding gene in the species Homo sapiens

RAC-gamma serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT3 gene.

<span class="mw-page-title-main">Myosin-light-chain phosphatase</span>

Myosin light-chain phosphatase, also called myosin phosphatase (EC 3.1.3.53; systematic name [myosin-light-chain]-phosphate phosphohydrolase), is an enzyme (specifically a serine/threonine-specific protein phosphatase) that dephosphorylates the regulatory light chain of myosin II:

<span class="mw-page-title-main">MAP2K7</span> Protein-coding gene in the species Homo sapiens

Dual specificity mitogen-activated protein kinase kinase 7, also known as MAP kinase kinase 7 or MKK7, is an enzyme that in humans is encoded by the MAP2K7 gene. This protein is a member of the mitogen-activated protein kinase kinase family. The MKK7 protein exists as six different isoforms with three possible N-termini and two possible C-termini.

<span class="mw-page-title-main">PKN2</span> Protein-coding gene in the species Homo sapiens

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<span class="mw-page-title-main">Protein phosphorylation</span> Process of introducing a phosphate group on to a protein

Protein phosphorylation is a reversible post-translational modification of proteins in which an amino acid residue is phosphorylated by a protein kinase by the addition of a covalently bound phosphate group. Phosphorylation alters the structural conformation of a protein, causing it to become either activated or deactivated, or otherwise modifying its function. Approximately 13000 human proteins have sites that are phosphorylated.

The Akt signaling pathway or PI3K-Akt signaling pathway is a signal transduction pathway that promotes survival and growth in response to extracellular signals. Key proteins involved are PI3K and Akt.

<span class="mw-page-title-main">Phosphoinositide-dependent kinase-1</span> Protein-coding gene in the species Homo sapiens

In the field of biochemistry, PDPK1 refers to the protein 3-phosphoinositide-dependent protein kinase-1, an enzyme which is encoded by the PDPK1 gene in humans. It is implicated in the development and progression of melanomas.

<span class="mw-page-title-main">Rho-associated protein kinase</span>

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<span class="mw-page-title-main">Triciribine</span>

Triciribine is a cancer drug which was first synthesized in the 1970s and studied clinically in the 1980s and 1990s without success. Following the discovery in the early 2000s that the drug would be effective against tumours with hyperactivated Akt, it is now again under consideration in a variety of cancers. As PTX-200, the drug is currently in two early stage clinical trials in breast cancer and ovarian cancer being conducted by the small molecule drug development company Prescient Therapeutics.

References

  1. 1 2 3 4 5 Brognard J, Newton AC (August 2008). "PHLiPPing the Switch on Akt and Protein Kinase C Signaling". Trends Endocrinol. Metab. 19 (6): 223–30. doi:10.1016/j.tem.2008.04.001. PMC   2963565 . PMID   18511290.
  2. 1 2 3 Gao T, Furnari F, Newton AC (April 2005). "PHLPP: a phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth". Mol. Cell. 18 (1): 13–24. doi: 10.1016/j.molcel.2005.03.008 . PMID   15808505.
  3. ↑ Gao T, Brognard J, Newton AC (March 2008). "The phosphatase PHLPP controls the cellular levels of protein kinase C". J. Biol. Chem. 283 (10): 6300–11. doi: 10.1074/jbc.M707319200 . PMID   18162466.
  4. ↑ Brognard J, Sierecki E, Gao T, Newton AC (March 2007). "PHLPP and a second isoform, PHLPP2, differentially attenuate the amplitude of Akt signaling by regulating distinct Akt isoforms". Mol. Cell. 25 (6): 917–31. doi: 10.1016/j.molcel.2007.02.017 . PMID   17386267.
  5. ↑ Garofalo RS, Orena SJ, Rafidi K, Torchia AJ, Stock JL, Hildebrandt AL, Coskran T, Black SC, Brees DJ, Wicks JR, McNeish JD, Coleman KG (July 2003). "Severe diabetes, age-dependent loss of adipose tissue, and mild growth deficiency in mice lacking Akt2/PKBβ". J. Clin. Invest. 112 (2): 197–208. doi:10.1172/JCI16885. PMC   164287 . PMID   12843127.
  6. ↑ Dummler B, Tschopp O, Hynx D, Yang ZZ, Dirnhofer S, Hemmings BA (November 2006). "Life with a Single Isoform of Akt: Mice Lacking Akt2 and Akt3 Are Viable but Display Impaired Glucose Homeostasis and Growth Deficiencies". Mol. Cell. Biol. 26 (21): 8042–51. doi:10.1128/MCB.00722-06. PMC   1636753 . PMID   16923958.
  7. ↑ Liu J, Weiss HL, Rychahou P, Jackson LN, Evers BM, Gao T (February 2009). "Loss of PHLPP expression in colon cancer: Role in proliferation and tumorigenesis". Oncogene. 28 (7): 994–1004. doi:10.1038/onc.2008.450. PMC   2921630 . PMID   19079341.
  8. ↑ Hirano I, Nakamura S, Yokota D, Ono T, Shigeno K, Fujisawa S, Shinjo K, Ohnishi K (March 2009). "Depletion of Pleckstrin Homology Domain Leucine-rich Repeat Protein Phosphatases 1 and 2 by Bcr-Abl Promotes Chronic Myelogenous Leukemia Cell Proliferation through Continuous Phosphorylation of Akt Isoforms". J. Biol. Chem. 284 (33): 22155–65. doi: 10.1074/jbc.M808182200 . PMC   2755940 . PMID   19261608.
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