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Signal transduction at the cellular level refers to the movement of signals from outside the cell to inside. The movement of signals can be simple, like that associated with receptor molecules of the acetylcholine class: receptors that constitute channels which, upon ligand interaction, allow signals to be passed in the form of small ion movement, either into or out of the cell. These ion movements result in changes in the electrical potential of the cells that, in turn, propagates the signal along the cell. More complex signal transduction involves the coupling of ligand-receptor interactions to many intracellular events. These events include phosphorylations by tyrosine kinases and/or serine/threonine kinases. Protein phosphorylations change enzyme activities and protein conformations. The eventual outcome is an alteration in cellular activity and changes in the program of genes expressed within the responding cells.
Please refer to the page on Growth Factors for descriptions of the growth factors described in this page and the explanation of their abbreviations.
back to the topSignal transducing receptors are of three general classes:
1. Receptors that penetrate the plasma membrane and have intrinsic enzymatic activity. Receptors that have intrinsic enzymatic activities include those that are tyrosine kinases (e.g. PDGF, insulin, EGF and FGF receptors), tyrosine phosphatases (e.g. CD45 [cluster determinant-45] protein of T cells and macrophages), guanylate cyclases (e.g. natriuretic peptide receptors) and serine/threonine kinases (e.g. activin and TGF-β receptors). Receptors with intrinsic tyrosine kinase activity are capable of autophosphorylation as well as phosphorylation of other substrates. Additionally, several families of receptors lack intrinsic enzyme activity, yet are coupled to intracellular tyrosine kinases by direct protein-protein interactions (see below).
2. Receptors that are coupled, inside the cell, to GTP-binding and hydrolyzing proteins (termed G-proteins). Receptors of the class that interact with G-proteins all have a structure that is characterized by 7 transmembrane spanning domains. These receptors are termed serpentine receptors. Examples of this class are the adrenergic receptors, odorant receptors, and certain hormone receptors (e.g. glucagon, angiotensin, vasopressin and bradykinin).
3. Receptors that are found intracellularly and upon ligand binding migrate to the nucleus where the ligand-receptor complex directly affects gene transcription. Because this class of receptors is intracellular and functions in the nucleus as transcription factors they are commonly referred to as the nuclear receptors. Receptors of this class include the large family of steroid and thyroid hormone receptors. Receptors in this class have a ligand-binding domain, a DNA-binding domain and a transcriptional activator domain.
back to the topThe proteins encoding RTKs contain four major domains:
An extracellular ligand binding domain.
An intracellular tyrosine kinase domain.
An intracellular regulatory domain.
A transmembrane domain.
The amino acid sequences of the tyrosine kinase domains of RTKs are highly conserved with those of cAMP-dependent protein kinase (PKA) within the ATP binding and substrate binding regions. Some RTKs have an insertion of non-kinase domain amino acids into the kinase domain termed the kinase insert. RTK proteins are classified into families based upon structural features in their extracellular portions (as well as the presence or absence of a kinase insert) which include the cysteine rich domains, immunoglobulin-like domains, leucine-rich domains, Kringle domains, cadherin domains, fibronectin type III repeats, discoidin I-like domains, acidic domains, and EGF-like domains. Based upon the presence of these various extracellular domains the RTKs have been sub-divided into at least 14 different families.
| Class | Examples | Structural Features of Class |
| I | EGF receptor, NEU/HER2, HER3 | cysteine-rich sequences |
| II | insulin receptor, IGF-1 receptor | cysteine-rich sequences; characterized by disulfide-linked heterotetramers |
| III | PDGF receptors, c-Kit | contain 5 immunoglobulin-like domains; contain the kinase insert |
| IV | FGF receptors | contain 3 immunoglobulin-like domains as well as the kinase insert; acidic domain |
| V | vascular endothelial cell growth factor (VEGF) receptor | contain 7 immunoglobulin-like domains as well as the kinase insert domain |
| VI | hepatocyte growth factor (HGF) and scatter factor (SF) receptors | heterodimeric like the class II receptors except that one of the two protein subunits is completely extracellular. The HGF receptor is a proto-oncogene that was originally identified as the MET oncogene |
| VII | neurotrophin receptor family (TRKA, TRKB, TRKC) and NGF receptor | contain no or few cysteine-rich domains; NGFR has leucine rich domain |
Many receptors that have intrinsic tyrosine kinase activity as well as the tyrosine kinases that are associated with cell surface receptors contain tyrosines residues, that upon phosphorylation, interact with other proteins of the signaling cascade. These other proteins contain a domain of amino acid sequences that are homologous to a domain first identified in the SRC proto-oncogene. These domains are termed SH2 domains (SRC homology domain 2). Another conserved protein-protein interaction domain identified in many signal transduction proteins is related to a third domain in SRC identified as the SH3 domain.
The interactions of SH2 domain containing proteins with RTKs or receptor associated tyrosine kinases leads to tyrosine phosphorylation of the SH2 containing proteins. The result of the phosphorylation of SH2 containing proteins that have enzymatic activity is an alteration (either positively or negatively) in that activity. Several SH2 containing proteins that have intrinsic enzymatic activity include phospholipase Cγ (PLCγ), the proto-oncogene RAS associated GTPase activating protein (rasGAP), phosphatidylinositol-3-kinase (PI3K), protein phosphatase-1C (PTP1C), as well as members of the SRC family of protein tyrosine kinases (PTKs).
back to the topThere are numerous intracellular PTKs that are responsible for phosphorylating a variety of intracellular proteins on tyrosine residues following activation of cellular growth and proliferation signals. There is now recognized two distinct families of non-receptor PTKs. The archetypal PTK family is related to the SRC protein. The SRC protein is a tyrosine kinase first identified as the transforming protein in Rous sarcoma virus. Subsequently, a cellular homolog was identifed. Numerous proto-oncogenes were identified as the transforming proteins carried by retroviruses. The second family is related to the Janus kinase (JAK).
Most of the proteins of both families of non-receptor PTKs couple to cellular receptors that lack enzymatic activity themselves. This class of receptors includes all of the cytokine receptors (eg the interleukin-2 receptor, IL2R) as well as the CD4 and CD8 cell surface glycoproteins of T cells and the T cell antigen receptor (TCR). This mode of coupling receptors to intracellular PTKs suggests a split form of RTK.
Another example of receptor-signaling through protein interaction involves the insulin receptor (IR). This receptor has intrinsic tyrosine kinase activity but does not directly interact, following autophosphorylation, with enzymatically active proteins containing SH2 domains (e.g. PI3K or PLCγ). Instead, the principal IR substrate is a protein termed IRS-1. IRS-1 contains several motifs that resemble SH2 binding consensus sites for the catalytically active subunit of PI3K. These domains allow complexes to form between IRS-1 and PI3K. This model suggests that IRS-1 acts as a docking or adapter protein to couple the IR to SH2 containing signaling proteins.
Additional adapter proteins have been identified, the most commonly occurring being a protein termed growth factor receptor-binding protein 2, GRB2.
An example of an alteration in receptor activity in response to association with an intracellular PTK is the nicotinic acetylcholine receptor (AChR). These receptors comprise an ion channel consisting of four distinct subunits (α, β, γ, and δ). The β, γ, and δ subunits are tyrosine phosphorylated in response to acetylcholine binding which leads to an increase in the rate of desensitization to acetylcholine.
back to the topThe receptors for the TGF-β superfamily of ligands have intrinsic serine/threonine kinase activity. A more complete description of the TGF-β signaling cascade can be found in the Signaling by Wnts and the TGFs-β/BMP Families page.
There are more than 30 multifunctional proteins of the TGF-β superfamily which also includes the activins, inhibins and the bone morphogenetic proteins (BMPs). This superfamily of proteins can induce and/or inhibit cellular proliferation or differentiation and regulate migration and adhesion of various cell types. The signaling pathways utilized by the TGF-β, activin and BMP receptors are different than those for receptors with intrinsic tyrosine kinase activity or that associate with intracellular tyrosine kinases.
At least 17 RSTKs have been isolated and can be divided into 2 subfamilies identified as the type I and type II receptors. Ligands first bind to the type II receptors which then leads to interaction with the type I receptors. When the complex between ligand and the 2 receptor subtypes forms, the type II receptor phosphorylates the type I receptor leading to initiation of the signaling cascade. One predominant effect of TGF-β is regulation of progression through the cell cycle. One nuclear protein involved in the responses of cells to TGF-β is the proto-oncogene, MYC which directly affects the expression of genes harboring MYC-binding elements.
back to the topThe are several serine/threonine kinases that function in signal transduction pathways. The two more commonly known are cAMP-dependent protein kinase (PKA) and protein kinase C (PKC). Activities of PKA are described in numerous pages in this web site, e.g. see the Glycogen Metabolism page. Additional serine/threonine kinases important for signal transduction are the mitogen activated protein kinases (MAP kinases).
PKC was originally identified as a serine/threonine kinase that was maximally active in the presence of diacylglycerols (DAG) and calcium ion. It is now known that there are at least ten proteins of the PKC family. Each of these enzymes exhibits specific patterns of tissue expression and activation by lipid and calcium. PKCs are involved in the signal transduction pathways initiated by certain hormones, growth factors and neurotransmitters. The phosphorylation of various proteins, by PKC, can lead to either increased or decreased activity. Of particular importance is the phosphorylation of the EGF receptor by PKC which down-regulates the tyrosine kinase activity of the receptor. This effectively limits the length of the cellular responses initiated through the EGF receptor.
MAP kinases were identified by virtue of their activation in response to growth factor stimulation of cells in culture, hence the name mitogen activated protein kinases. MAP kinases are also called ERKs for extracellular-signal regulated kinases. On the basis of in vitro substrates the MAP kinases have been variously called microtubule associated protein-2 kinase (MAP-2 kinase), myelin basic protein kinase (MBP kinase), ribosomal S6 protein kinase (RSK-kinase: i.e. a kinase that phosphorylates a kinase) and EGF receptor threonine kinase (ERT kinase). All of these proteins have similar biochemical properties, immuno-crossreactivities, amino acid sequence and ability to in vitro phosphorylate similar substrates.
Maximal MAP kinase activity requires that both tyrosine and threonine residues are phosphorylated. This indicates that MAP kinases act as switch kinases that transmits information of increased intracellular tyrosine phosphorylation to that of serine/threonine phosphorylation. Although MAP kinase activation was first observed in response to activation of the EGF, PDGF, NGF and insulin receptors, other cellular stimuli such as T cell activation (which signals through the LCK [Lystra cell kinase] tyrosine kinase), phorbol esters (that function through activation of PKC), thrombin, bombesin and bradykinin (that function through G-proteins) as well as N-methyl-D-aspartate (NMDA) receptor activation and electrical stimulation rapidly induce tyrosine phosphorylation of MAP kinases.
MAP kinases are, however, not the direct substrates for RTKs nor receptor associated tyrosine kinases but are in fact activated by an additional class of kinases termed MAP kinase kinases (MAPK kinases) and MAPK kinase kinases (MAPKK kinases). One of the MAPK kinases has been identified as the proto-oncogenic serine/threonine kinase, RAF. Ultimate targets of the MAP kinases are several transcriptional regulators e.g. serum response factor (SRF), and the proto-oncogenes FOS, MYC and JUN as well as members of the steroid/thyroid hormone receptor super family of proteins.
back to the topPhospholipases and phospholipids are involved in the processes of transmitting ligand-receptor induced signals from the plasma membrane to intracellular proteins. The primary protein affected by the activation of phospholipases is PKC which is maximally active in the presence of calcium ion and DAG. The generation of DAG occurs in response to agonist activation of various phospholipases. The principal mediators of PKC activity are receptors coupled to activation of phospholipase C-γ (PLCγ). PLCγ contains SH2 domains that allow it to interact with tyrosine phosphorylated RTKs. This allows PLC-γ to be intimately associated with the signal transduction complexes of the membrane as well as membrane phospholipids that are its substrates. Activation of PLCγ leads primarily to the hydrolysis of membrane phosphatidylinositol bisphosphate (PIP2) leading to an increase in intracellular DAG and inositol trisphosphate (IP3). The released IP3 interacts with intracellular membrane receptors leading to an increased release of stored calcium ions. Together, the increased DAG and intracellular free calcium ion concentrations lead to increased activity of PKC.
Recent evidence indicates that phospholipases D and A2 (PLD and PLA2) also are involved in the sustained activation of PKC through their hydrolysis of membrane phosphatidylcholine (PC). PLD action on PC leads to the release of phosphatidic acid which in turn is converted to DAG by a specific phosphatidic acid phosphomonoesterase. PLA2 hydrolyzes PC to yield free fatty acids and lysoPC both of which have been shown to potentiate the DAG mediated activation of PKC. Of medical significance is the ability of phorbol ester tumor promoters to activate PKC directly. This leads to elevated and unregulated activation of PKC and the consequent disruption in normal cellular growth and proliferation control leading ultimately to neoplasia.
PI3K is tyrosine phosphorylated, and subsequently activated, by various RTKs and receptor-associated PTKs. PI3K is a heterodimeric protein containing an 85 kDa and 110 kDa subunits. The p85 subunit contains SH2 domains that interact with activated receptors or other receptor-associated PTKs and is itself subsequently tyrosine phosphorylated and activated. The 85 kDa subunit is non-catalytic, however, it does contain a domain homologous to GTPase activating (GAP) proteins. It is the 110 kDa subunit that is enzymatically active. PI3K, associates with and is activated by, the PDGF, EGF, insulin, IGF-1, HGF and NGF receptors. PI3K phosphorylates various phosphatidylinositols at the 3 position of the inositol ring. This activity generates additional substrates for PLCγ allowing a cascade of DAG and IP3 to be generated by a single activated RTK or other protein tyrosine kinases.
Lysophospholipids (LPs) are minor lipid components compared to the major membrane phospholipids such as phosphatidylcholline (PC), phosphatidylethanolamine (PE), and sphingomyelin. The LPs were originally presumed to be simple metabolic intermediates in the de novo biosynthesis of phospholipids. However, subsequent studies demonstrated that the LPs exhibited biological properties resembling those of extracellular growth factors or signaling molecules. The most biologically significant LPs are lysophosphatidic acid (LPA), lysophosphatidylcholine (LPC), sphingosine 1-phosphate (S1P), and sphingosylphosphorylcholine (SPC). Each of these LPs functions via interaction with specific G-protein coupled receptors (GPCRs) leading to autocrine or paracrine effects. The first LP receptor identified was called LPA1 because it bound LPA. The first GPCR shown to bind S1P was called S1P1.
Currently there are fifteen characterized LP receptors. Because several of the LP receptors were independently identified in unrelated assays, there are several different names for some members of this receptor family. In particular, there is a group of genes that were originally identified as GPCRs and called endothelial differentiation genes (EDGs) that were later found to be the same as several of the LP receptors. Thus LPA1 is also known as EDG-2, LPA2 as EDG-4, and LPA3 as EDG-7. S1P1 is also known as EDG-1, S1P2 as EDG-5, S1P3 as EDG-3, S1P4 as EDG-6, and S1P5 as EDG-8. Activation of the LPA receptors triggers several different downstream signaling cascades. These include activation of MAP kinase (MAPK), activation of PLCγ, Akt/PKB activation, calcium moblization, release of arachidonic acid, inhibition or activation of adenylate cyclase, and activation of several small GTPases such as Ras, Rho, and Rac. The LPs exert a wide-range of biochemical and physiological responses including platelet activation, smooth muscle contraction, cell growth, and fibroblast proliferation.
LPA is produced by activated platelets, activated adipocytes, neuronal cells, as well as several other cell types. The mode(s) of LPA synthesis intracellularly remains to be fully elucidated. LPA is produced in the serum through the action of several different enzymes including monoacylglycerol kinase, phospholipase A1 (PLA1), secretory phospholipase A2 (sPLA2), and lysophospholipase D (lysoPLD). LysoPLD is also called autotaxin (ATX) which was the name given to a tumor autocrine motility factor. ATX was also shown to be an ecto-nucleotide phosphodiesterase. Degradation of LPA occurs via lysophospholipase, lipid phosphate phosphatase, or LPA acyl transferase (also called endophilin).
S1P is stored in platelets and released upon platelet activation. Synthesis of S1P occurs exclusively from sphingosine via the action of sphingosine kinases. Degradation of S1P occurs through the action of S1P lyases or S1P phosphatases.
back to the topThere are several different classifications of receptors that couple signal transduction to G-proteins. These classes of receptor are termed G-protein coupled receptors, GPCRs. Well over 1000 different GPCRs have been cloned, most being orphan receptors having no as yet identified ligand. Three different classes of GPCR are reviewed:
1. GPCRs that modulate adenylate cyclase activity. One class of adenylate cyclase modulating receptors activate the enzyme leading to the production of cAMP as the second messenger. Receptors of this class include the β-adrenergic, glucagon and odorant molecule receptors. Increases in the production of cAMP leads to an increase in the activity of PKA in the case of β-adrenergic and glucagon receptors. In the case of odorant molecule receptors the increase in cAMP leads to the activation of ion channels. In contrast to increased adenylate cyclase activity, the α1-type adrenergic receptors are coupled to inhibitory G-proteins that repress adenylate cyclase activity upon receptor activation.
2. GPCRs that activate PLCγ leading to hydrolysis of polyphosphoinositides (e.g. PIP2) generating the second messengers, diacylglycerol (DAG) and inositoltrisphosphate (IP3). This class of receptors includes the α2-adrenergic , angiotensin, bradykinin and vasopressin receptors.
3. A novel class of GPCRs are the photoreceptors. This class is coupled to a G-protein termed transducin that activates a phosphodiesterase which leads to a decrease in the level of cGMP. The drop in cGMP then results in the closing of a Na+/Ca2+ channel leading to hyperpolarization of the cell. See the Role of Vitamin A in Vision for more details.
back to the topThe activity of G-proteins with respect to GTP hydrolysis is regulated by a family of proteins termed GTPase activating proteins, GAPs. The proto-oncogenic protein, RAS, is a G-protein involved in the genesis of numerous forms of cancer (when the protein sustains specific mutations). Of particular clinical significance is the fact that oncogenic activation of Ras occurs with higher frequency than any other gene in the development of colorectal cancers. Regulation of RAS GTPase activity is controlled by RASGAP.
There are several other GAP proteins besides RASGAP that are important in signal transduction. There are two clinically important proteins of the GAP family of proteins. One is the gene product of the neurofibromatosis type-1 (NF1) susceptibility locus. The NF1 gene is a tumor suppressor gene and the protein encoded is called neurofibromin. The second is the protein encoded by the BCR locus (break point cluster region gene). The BCR locus is rearranged in the Philadelphia+ chromosome (Ph+) observed with high frequency in chronic myelogenous leukemias (CMLs) and acute lymphocytic leukemias (ALLs).
back to the topThe steroid and thyroid hormone family of receptors are proteins that effectively bypass all of the signal transduction pathways described thus far by residing within the cytoplasm. Additionally, all of the hormone receptors are bi-functional. They are capable of binding hormone as well as directly activating gene transcription. Because these receptors bind ligand intracellularly and then interact with DNA directly they are more commonly called the nuclear receptors.
The steroid/thyroid hormone receptor superfamily [e.g. glucocorticoid (GR), vitamin D (VDR), retinoic acid (RAR) and thyroid hormone TR) receptors] is a class of proteins that reside in the cytoplasm and bind their lipophilic hormone ligands in this compartment as these hormones are capable of freely penetrating the hydrophobic plasma membrane. Upon binding ligand the hormone-receptor complex translocates to the nucleus and binds to specific DNA sequences termed hormone response elements (HREs). The binding of the complex to an HRE results in altered transcription rates of the associated gene.
Analysis of the human genome has revealed 48 nuclear receptor genes. Many of these genes are capable of yielding more than one receptor isoform. The nuclear receptors all contain a ligand-binding domain (LBD) and a DNA-binding domain (DBD). Based upon the sequences of these two domains the nuclear receptor family is divided into six sub-families. Some members of the family bind to DNA as homodimers such as is the case for subfamily III receptors which comprises the steroid receptors such as the estrogen receptor (ER), mineralocorticoid receptor (MR), progesterone receptor (PR), androgen receptor (AR), and the glucocorticoid receptor (GR). Other family members (such as all subfamily I members) bind to DNA as heterodimers through interactions with the retinoid X receptors (RXRs, see below).
In addition to the steroid hormone and thyroid hormone receptors there are numerous additional family members that bind lipophilic ligands. These include the retinoid X receptors (RXRs), the liver X receptors (LXRs), the farnesoid X receptors (FXRs) and the peroxisome proliferator-activated receptors (PPARs).
RXRs: The RXRs represent a class of receptors that bind the retinoid 9-cis-retinoic acid. There are three isotypes of the RXRs: RXRα, RXRβ, and RXRγ and each isotype is composed of several isoforms. The RXRs serve as obligatory heterodimeric partners for numerous members of the nuclear receptor family including those discussed below (PPARs, LXRs, and FXRs). In the absence of a heterodimeric binding partner the RXRs are bound to hormone response elements (HREs) in DNA and are complexed with co-repressor proteins that include a histone deacetylase (HDAC) and silencing mediator of retinoid and thyroid hormone receptor (SMRT) or nuclear receptor corepressor 1 (NCoR).
RXRα is widely expressed with highest levels liver, kidney, spleen, placenta, and skin. The critical role for RXRα in development is demonstrated by the fact that null mice are embryonic lethals. RXRβ is important for spermatogenesis and RXRγ has a restricted expression in the brain and muscle.
PPARs: The PPAR family is composed of three family members: PPARα, PPARβ/δ, and PPARγ. Each of these receptors forms a heterodimer with the RXRs.
The first family member identified was PPARα and it was found by virtue of it binding to the fibrate class of anti-hyperlipidemic drugs or peroxisome proliferators. Subsequently it was shown that PPARα is the endogenous receptor for polyunsaturated fatty acids. PPARα is highly expressed in the liver, skeletal muscle, heart, and kidney. Its function in the liver is to induce hepatic peroxisomal fatty acid oxidation during periods of fasting. Expression of PPARα is also seen in macrophage foam cells and vascular endothelium. Its role in these cells is thought to be the activation of anti-inflammatory and anti-atherogenic effects.
PPARγ is a master regulator of adipogenesis and is most abundantly expressed in adipose tissue. Low levels of expression are also observed in liver and skeletal muscle. PPARγ was identified as the target of the thiazolidinedione (TZD) class of insulin-sensitizing drugs. The mechanism of action of the TZDs is a function of tha activation of PPARγ activity and the consequent activation of adipocytes leading to increased fat storage and secretion of insulin-sensitizing adipocytokines such as adiponectin.
PPARδ is expressed in most tissues and is involved in the promotion of mitochondrial fatty acid oxidation, energy consumption, and thermogenesis. PPARδ serves as the receptor for polyunsaturated fatty acids and VLDLs. Current pharmacologic targeting of PPARδ is aimed at increasing HDL levels in humans since experiments in animals have shown that increased PPARδ levels result in increased HDL and reduced levels of serum triglycerides.
LXRs: There are two forms of the LXRs: LXRα and LXRβ. The LXRs form heterodimers with the RXRs and as such can regulate gene expression either upon binding oxysterols (e.g. 22R-hydroxycholesterol) or 9-cis-retinoic acid. Because the LXRs bind oxysterols they are important in the regulation of whole body cholesterol levels. The function of LXRs in the liver is to mediate cholesterol metabolism by inducing the expression of SREBP-1c. SREBP-1c is a transcription factor involved in the control of the expression of numerous genes including several involved in cholesterol synthesis. Recent evidence also indicates that the LXRs act as sensors for glucose levels since they have been shown to bind glucose leading to activation of the gene encoding the transcription factor carbohydrate-response element-binding protein (ChREBP).
FXRs: There are two genes encoding FXRs identified as FXRα and FXRβ. In humans at least four FXR isoforms have been identified as being derived from the FXRα gene as a result of activation from different promoters and the use of alternative splicing; FXRα1, FXRα2, FXRα3, and FXRα4. The FXR gene is also known as the NR1H4 gene (for nuclear receptor subfamily 1, group H, member 4). The FXR genes are expressed at highest levels in the intestine and liver. FXR forms a heterodimer with members of the RXR family. Following heterodimer formation the complex binds to specific sequences in target genes resulting in regulated expression. One major target of FXR is the small heterodimer partner (SHP) gene. Activation of SHP expression by FXR results in inhibition of transcription of SHP target genes. Of significance to bile acid synthesis, SHP represses the expression of the cholesterol 7-hydroxylase gene (CYP7A1). CYP7A1 is the rate-limiting enzyme in the synthesis of bile acids from cholesterol. The FXRs were originally identified by their ability to bind farensol metabolites. However, subsequent research has demonstrated that FXRs are receptors for bile acids which is the primary mechanism by which bile acids negatively regulate their own expression. In addition to binding bile acids, FXRs have been shown to bind polyunsaturated fatty acids (PUFAs) such as the omega-3 PUFAs docashexaenoic acid (DHA) and α-linolenic acid (ALA). Most recently, FXR has been shown to bind the androgen hormone, androsterone, derived via testosterone metabolism.
PXR: A particular receptor of this family that has been shown to bind numerous structurally unrelated chemicals was originally identified as the pregnane X receptor (PXR). PXR is highly expressed in the liver and is involved in mediating drug-induced multi-drug clearance. For this reason PXR is important in protecting the body from harmful metabolites. An additional physiologically significant function of PXR is in the regulation of bile acid synthesis. PXR is a recognized receptor for lithocholic acid and other bile acid precursors. PXR activation leads to repression of bile acid synthesis due to its physical association with hepatocyte nuclear factor 4α (HNF-4α) causing this transcription factor to no longer be able to associate with the transcriptional co-activator PGC-1α (PPARγ co-activator 1α) which ultimately leads to loss of transcription factor activation of the rate-limiting enzyme of bile acid synthesis CYP7A1 which, as described above, is also the target of FXR action. In addition to regulation of bile acid metabolism, PXR represses the expression of the gluconeogenic enzyme PEPCK.
In addition to the nuclear receptors discussed here additional members are being identified all the time such at the estrogen related receptors (ERRβ and ERRγ), the retinoid-related orphan receptor (RORα), and the constitutive androstane receptor (CAR).
back to the topSubstantial evidence links both tyrosine and serine/threonine phosphorylation with increased cellular growth, proliferation and differentiation. Removal of the incorporated phosphates must be a necessary event in order to turn off the proliferative signals. This suggests that phosphatases may function as anti-oncogenes or growth suppressor genes. The loss of a functional phosphatase involved in regulating growth promoting signals could lead to neoplasia. However, examples are known where dephosphorylation is required for promotion of cell growth. This is particularly true of specialized kinases that are directly involved in regulating cell cycle progression. Therefore, it is difficult to envision all phosphatases as being tumor suppressor genes.
There are two broad classes of protein tyrosine phosphatases (PTPs). One class are transmembrane enzymes which contain the phosphatase activity domain in the intracellular portion of the protein. This class of PTP is commonly called the receptor (R) class of PTP. The other class is intracellularly localized enzymes and are referred to as NT PTPs (for non-transmembrane). Currently over 40 genes have been characterized as encoding one or the other class of PTP. The first transmembrane PTP characterized was the leukocyte common antigen protein, CD45. This protein was shown to have homology to the intracellular PTP, PTP1B. There are at least eight sub-classes of the transmembrane PTPs and ten sub-classes of the NT PTPs.
The clearest studies of a role for transmembrane PTPs in signal transduction have involved the CD45 protein. These studies have shown that CD45 is involved in the regulation of the tyrosine kinase activity of LCK in T cells. As indicated above LCK is associated with T cell antigens CD4 and CD8 generating a split-RTK involved in T cell activation. It is suspected that CD45 dephosphorylates a regulatory tyrosine phosphorylation site in the C-terminus of LCK, thereby, increasing the activity of LCK towards its substrate(s).
The second class of PTPs are the intracellular proteins. The C-terminal residues of most if not all intracellular PTPs are very hydrophobic and suggest these sites are membrane attachment domains of these proteins. One role of intracellular PTPs is in the maturation of Xenopus oocytes in response to hormone. Over expression of PTP1B in oocytes resulted in a marked retardation in the rate of insulin- and progesterone-induced maturation. These results suggest a role for PTP1B in countering the signals leading to cellular activation.
The above observation as well as several others have demonstrated a link between insulin function and PTP-1B. PTP1B directly interacts with the insulin receptor and removes the tyrosine phosphates incorporated by autophosphorylation in response to insulin binding, thereby, negatively affecting the activity of the insulin receptor. Recently the PTP-B gene was disrupted in mice by targeted deletion. Mice lacking a functional PTP-1B gene exhibit increased insulin sensitivity as well as resistance to obesity induced by a high fat diet.
As with the transmembrane PTPs little is known about the regulation of the activity of the intracellular PTPs. Two intracellular PTPs (PTP1C and PTP1D) have been shown to contain SH2 domains. These SH2 domains allow these PTPs to directly interact with tyrosine phosphorylated RTKs and PTKs, thereby, dephosphorylating tyrosines in these proteins. Following receptor stimulation of signal transduction events, the SH2 containing PTPs are directed to several of the RTKs and/or PTKs with the net effect being a termination of the signaling events by tyrosine dephosphorylation.
Other phosphatases that recognize serine and/or threonine phosphorylated proteins also exist in cells. These are referred to as protein serine phosphatases (PSPs). At least 15 distinct PSPs have been identified. The type 2A PSPs exhibit selective substrate specificity towards PKC phosphorylated proteins; in particular serine and threonine phosphorylated receptors. Type 2A PSPs are more effective than other PSPs in dephosphorylating RSKs, proteins that are involved in signaling cascades by phosphorylating ribosomal S6 protein (see above). However, a type 1 PSP is required to dephosphorylate S6 itself.
The type 2A PSPs have 2 subunits (a regulatory and a catalytic) both of which can associate with one of the tumor antigens of the DNA tumor virus, polyoma. Transformation by DNA tumor viruses such as polyoma appears to be mediated by the formation of a signal transduction unit consisting of a virally encoded T antigen and several host encoded proteins. Several host proteins are tyrosine kinases of the SRC family. Polyoma middle T antigen also can bind to PI3K. The association of type 2A PSPs in these complexes may lead to dephosphorylation of regulatory serine/threonine phosphorylated sites resulting in increased signal transduction and subsequent cellular proliferation.
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