PHLPP
Encyclopedia
The PHLPP isoforms are a pair of protein phosphatase
s, PHLPP1
and PHLPP2
, which 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.
PHLPP dephosphorylates Ser
-473 (the hydrophobic motif) in Akt, thus partially inactivating the kinase.
In addition, PHLPP dephosphorylates conventional and novel members of the protein kinase C family at their hydrophobic motifs, corresponding to Ser-660 in PKCβII.
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 repeat
s, 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. 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.
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.
, AKT2
, and AKT3
. These enzyme
s are members of the serine/threonine-specific protein kinase family .
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.
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 which 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 which 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.
The role of Akt3 is less clear, though it appears to be predominantly expressed in brain. It has been reported that mice lacking Akt3 have small brains.
) 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 PDK1 phosphorylates.
(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.
) 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. 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. 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 inihibitors, including Gleevec, in CML cell lines.
Finally, both Akt and PKC are known to be tumor promoters, suggesting that their negative regulator PHLPP may act as a tumor suppressor.
Phosphatase
A phosphatase is an enzyme that removes a phosphate group from its substrate by hydrolysing phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl group . This action is directly opposite to that of phosphorylases and kinases, which attach phosphate groups to their...
s, PHLPP1
PHLPP (gene)
PH domain and leucine rich repeat protein phosphatase, also known as PHLPP, is an enzyme which in humans is encoded by the PHLPP gene.- Further reading :...
and PHLPP2
PHLPPL
PH domain and leucine rich repeat protein phosphatase-like, also known as PHLPPL, is an enzyme which in humans is encoded by the PHLPPL gene.- Further reading :...
, which are important regulators of Akt
AKT
Akt, also known as Protein Kinase B , is a serine/threonine protein kinase that plays a key role in multiple cellular processes such as glucose metabolism, cell proliferation, apoptosis, transcription and cell migration.-Family members:...
serine-threonine kinases (Akt1
AKT1
RAC-alpha serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT1 gene. Multiple alternatively spliced transcript variants have been found for this gene.- Function :...
, Akt2
AKT2
RAC-beta serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT2 gene.-Interactions:AKT2 has been shown to interact with TCL1A, APPL1, SH3RF1 and CHUK.-Further reading:...
, Akt3
AKT3
RAC-gamma serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT3 gene.-Interactions:AKT3 has been shown to interact with Protein kinase Mζ.-Further reading:...
) and conventional/novel protein kinase C
Protein kinase C
Protein kinase C also known as PKC is a family of 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. PKC enzymes in turn are activated by signals such as increases in...
(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.
PHLPP dephosphorylates Ser
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
-473 (the hydrophobic motif) in Akt, thus partially inactivating the kinase.
In addition, PHLPP dephosphorylates conventional and novel members of the protein kinase C family at their hydrophobic motifs, corresponding to Ser-660 in PKCβII.
Domain structure
PHLPP is a member of the PPM family of phosphatases, which requires magnesiumMagnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
or manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
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
Pleckstrin homology domain
Pleckstrin homology domain is a protein domain of approximately 120 amino acids that occurs in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton....
, a series of leucine-rich repeat
Leucine-rich repeat
A leucine-rich repeat is a protein structural motif that forms an α/β horseshoe fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine...
s, a PP2C phosphatase domain, and a C-terminal PDZ
PDZ domain
The PDZ domain is a common structural domain of 80-90 amino-acids found in the signaling proteins of bacteria, yeast, plants, viruses and animals...
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. 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.
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 AKT1AKT1
RAC-alpha serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT1 gene. Multiple alternatively spliced transcript variants have been found for this gene.- Function :...
and AKT3
AKT3
RAC-gamma serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT3 gene.-Interactions:AKT3 has been shown to interact with Protein kinase Mζ.-Further reading:...
, whereas PHLPP1 is specific for AKT2
AKT2
RAC-beta serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT2 gene.-Interactions:AKT2 has been shown to interact with TCL1A, APPL1, SH3RF1 and CHUK.-Further reading:...
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.
The Akt family of kinases
In humans, there are three genes in the Akt family: AKT1AKT1
RAC-alpha serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT1 gene. Multiple alternatively spliced transcript variants have been found for this gene.- Function :...
, AKT2
AKT2
RAC-beta serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT2 gene.-Interactions:AKT2 has been shown to interact with TCL1A, APPL1, SH3RF1 and CHUK.-Further reading:...
, and AKT3
AKT3
RAC-gamma serine/threonine-protein kinase is an enzyme that in humans is encoded by the AKT3 gene.-Interactions:AKT3 has been shown to interact with Protein kinase Mζ.-Further reading:...
. These enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s are members of the serine/threonine-specific protein kinase family .
Akt1 is involved in cellular survival pathways and inhibition of apoptotic
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
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
Oncogene
An oncogene is a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels.An oncogene is a gene found in the chromosomes of tumor cells whose activation is associated with the initial and continuing conversion of normal cells into cancer...
in the transforming retrovirus
Retrovirus
A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...
, AKT8.
Akt2 is important in the insulin signaling pathway. It is required to induce glucose transport.
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 which 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 which do not have Akt2, but have normal Akt1, have mild growth deficiency and display a diabetic phenotype (insulin resistance
Insulin resistance
Insulin resistance is a physiological condition where the natural hormone insulin becomes less effective at lowering blood sugars. The resulting increase in blood glucose may raise levels outside the normal range and cause adverse health effects, depending on dietary conditions. Certain cell types...
), again consistent with the idea that Akt2 is more specific for the insulin receptor
Insulin receptor
In molecular biology, the insulin receptor is a transmembrane receptor that is activated by insulin. It belongs to the large class of tyrosine kinase receptors....
signaling pathway.
The role of Akt3 is less clear, though it appears to be predominantly expressed in brain. It has been reported that mice lacking Akt3 have small brains.
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 (PDK1PDK1
[Pyruvate dehydrogenase [lipoamide]] kinase isozyme 1, mitochondrial is an enzyme that in humans is encoded by the PDK1 gene. It codes for an isozyme of pyruvate dehydrogenase kinase ....
) and PDK2
PDK2
Pyruvate dehydrogenase kinase isoform 2 also known as [pyruvate dehydrogenase [lipoamide]] kinase isozyme 2, mitochondrial is an enzyme that in humans is encoded by the PDK2 gene. PDK2 is an isozyme of pyruvate dehydrogenase kinase....
. 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 PDK1 phosphorylates.
Dephosphorylation of protein kinase C
PHLPP1 and 2 also dephosphorylate the hydrophobic motifs of two classes of the protein kinase CProtein kinase C
Protein kinase C also known as PKC is a family of 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. PKC enzymes in turn are activated by signals such as increases in...
(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
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
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.
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 (LOHLoss of heterozygosity
Loss of heterozygosity in a cell is the loss of normal function of one allele of a gene in which the other allele was already inactivated. This term is mostly used in the context of oncogenesis; after an inactivating mutation in one allele of a tumor suppressor gene occurs in the parent's germline...
) 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. 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. Recent studies have also shown that Bcr-Abl, the fusion protein responsible for chronic myelogenous leukemia (CML
Chronic myelogenous leukemia
Chronic myelogenous leukemia , also known as chronic granulocytic leukemia , is a cancer of the white blood cells. It is a form of leukemia characterized by the increased and unregulated growth of predominantly myeloid cells in the bone marrow and the accumulation of these cells in the blood...
), downregulates PHLPP1 and PHLPP2 levels, and that decreasing PHLPP levels interferes with the efficacy of Bcr-Abl inihibitors, including Gleevec, in CML cell lines.
Finally, both Akt and PKC are known to be tumor promoters, suggesting that their negative regulator PHLPP may act as a tumor suppressor.