PI 3-Kinases (phosphoinositide 3-kinases, PI3Ks) are family of lipid kinases capable of phosphorylating the 3'OH of the inositol ring of phosphoinositides. They are responsible for coordinating a diverse range of cell functions including proliferation, cell survival, degranulation, vesicular trafficking and cell migration, which in turn are involved in cancer. The PI 3-kinases are grouped into three distinct classes: I, II, and III. Class I PI3-kinase is further divided into two groups: PI3K-IA and PI3K-IB.
PI3Ks are heterodimers composed of various combinations of catalytic and regulator subunit isoforms. Each subunit is encoded by a specific gene, also belonging to a different chromosome.
A detailed list of the precise collocation of these genes is seen below.
CHEMICAL STRUCTURE AND IMAGES
Class IA PI3K heterodimers contain specific isoforms of the 85 kDa adaptor subunit that facilitates interaction with receptor tyrosine kinases (RTK) and either an alpha, beta or delta p110 catalytic subunit (p110α, p110β, or p110γ). Class IB PI3K heterodimers contain a p101 regulatory subunit that responds to specific GPCR-associated G-protein, βγ-subunits and a gamma p110 (p110γ) catalytic subunit. Class IA and IB PI3Ks are activated by Ras-GTPase and a variety of other signaling molecules. For instance, the FGFreceptor and TrkA receptors activate PI3-kinase through the FRS-2:Grb2:Gab1 complex.
Class IA PI3Ks are tightly associated with a 85 kDa regulatory subunit called p85. Class I PI3K regulatory subunits are derived from three genes. P85α and two alternative transcripts, p55α and p50α are derived from the gene PIK3R1. P85β is derived from gene PIK3R2. A third gene produces p55γ (p55PIK). P85α and P85β are ubiquitously expressed. The smaller isoforms are tissue specific. P85 contains a Src homology 3 (SH3) domain, a breakpoint-cluster region homology (BH) domain between two proline-rich regions, and two C-terminal SH2 domains separated by an inter-SH2 (iSH2) region, which tightly binds p85 to the catalytic subunit. Since PI3K has multiple protein-interaction domains, p85 is able to interact with several signaling molecules simultaneously, allowing for significant fine tuning of PI3K activity.
The SH3 domain of PI3K has homologues found in many intracellular signaling proteins. It mediates protein-protein interactions by binding to proline-rich motifs in target proteins forming multimeric signaling complexes. Of note, the SH3 domain interacts with Src (Src homology 2/α-collagen-related), CDC42GAP (Cdc42 GTPase-activating protein) and the proto-oncogene product Cbl. SH3 binds to proline rich ligands via a network of hydrophobic and hydrogen bond interactions, particularly with the conserved residues Trp 55, Pro 70, and Tyr 73.
It is in common agreement that is actually loss of the inhibitory interaction of the p85 regulatory subunit that appears to be the basis for one common type of oncogenic mutation in the p110 γ catalytic subunit.
A recent study, based on X-ray crystal structure of the complex of the ABD with the iSH2 domain and an experimental model of the catalytic core of PI3K α, proposed a model of how the regulatory subunit would interact with the full length p110 α catalytic subunit.
- Structure of the p110/p85 heterodimeric phosphoinositide 3-kinase (PI3K), A. Berndt, W-C Hon, D. Svergun and R. Williams, 2008
Protein Aminoacids Percentage
- PI3K – From the Bench to the Clinic and Back, Bart Vanhaesebroeck, Peter K. Vogt, Christian Rommelc, 2011
Early work showed that a phosphatidylinositol kinase activity co-purified with various viral oncoproteins expressed in mammalian cells (Macara et al. 1984; Sugimoto et al. 1984) and that cellular transformation mediated by such oncoproteins was to some extent dependent on the association with this lipid kinase activity (Whitman et al. 1985). This oncoprotein-associated lipid kinase could phosphorylate phosphatidylinositol on the 3-OH position of the inositol ring, hereby generating PI3P, a novel type of phosphoinositide (Whitman et al. 1988). This finding was followed by the discovery of PIP3 (phosphatidylinositol(3,4,5)trisphosphate; PIP3) in GPCR-stimulated neutrophils (Traynor-Kaplan et al. 1988; Traynor-Kaplan et al. 1989) and upon acute stimulation with tyrosine kinase agonists (Auger et al. 1989; Hawkins et al. 1992; Jackson et al. 1992). It was not known at the time that agonist-stimulated PI3K is a heterodimer made up of a p110 catalytic subunit and a regulatory subunit, namely p85 in the case of class IA PI3Ks and p101 in the case of the class IB p110γ. Early studies very much focused on a tyrosine-phosphorylated 85 kD protein found in PDGF-stimulated or polyoma middle T-transformed cells which associated with PI3K activity (Courtneidge and Heber 1987; Kaplan et al. 1987). This protein turned out to be the p85 regulatory subunit of PI3K, and its cDNA was cloned by several groups (Escobedo et al. 1991; Otsu et al. 1991; Skolnik et al. 1991). Protein microsequencing allowed the design of oligonucleotide probes to isolate the first cDNA of a PI3K catalytic subunit, namely p110α (Hiles et al. 1992). This work revealed that the sequence of p110 was closely homologous to that of the product of vps34, a S. cerevisiae gene involved in endosomal sorting of proteins towards the vacuole, the yeast equivalent of the mammalian lysosome (Herman and Emr 1990).
ACTIVATION AND CELLULAR FUNCTION
- Activities of Phosphoinositide Kinase-3 (PI3K), Thomas Angel, 1999
Inactive PI3Ks are rapidly activated in the presence of extracellular stimuli. Such stimuli, as discussed previously, include growth factor receptors with intrinsic protein tyrosine kinase activity, which display pYXXM motifs for p85 docking, as well as receptor substrates which are phosphorylated and interact with PI3K regulatory subunits like nSH2. PI3K can be additionally activated in cooperative processes like translocation to the plasma membrane where lipid substrates are available and by binding GTP loaded Ras to the catalytic subunit.
The preferred substrate of class I, PI3-kinases is phosphoinositide(4,5)bisphosphate (PIP2). This is also a substrate for members of the PI-phospholipase C family and the product of PTEN dephosphorylation of PtdIns(3,4,5)P3. Phosphorylation of PIP2 by PI3-kinase generates PtdIns(3,4,5)P3. PtdIns(3,4,5)P3 and its 5’-dephosphorylation product, PtdIns(3,4,)P2, are important second messengers that coordinate to promote cell survival, growth, protein synthesis, mitosis, and motility. PtdIns(3,4,)P2 is also produced by Class II, PI3-kinases from PtdIns(4)P. Cell survival, mitosis, and protein synthesis are promoted by PI3-kinase-dependent activation of the PDK/AKT pathway. PtdIns(3,4,5)P3 produced by PI3-kinase is also involved with cell motility via regulation of the Rho-GTPases, RhoA, Rac-1 and Cdc42.
Class II, PI3-kinases preferentially phosphorylates phosphatidylinositol (PI) and PtdIns(4)P to form PtdIns(3)P and PtdIns(3,4)P2, respectively. Class II, PI3-kinases also phosphorylate PtdIns(4,5)P2 in the presence of phosphatidylserine (PS). Class III, PI-kinases preferentially phosphorylate phoshatidylinositol (PtdIns) to form phosphoinositol-3-P (PtdIns(3)P). PtdIns(3)P has important roles in vesicular and protein trafficking. Class III, PI3-kinase is involved in targeting lysosomal enzymes to the endocytic pathway.
PI3K AND HUMAN DISEASES
- PI3K-Akt pathway: its functions and alterations in human cancer, Osaki M, Oshimura M, Ito H, 2004
- Targeting the PI3K–AKT–mTOR pathway: progress, pitfalls, and promises, Timothy A Yap, Michelle D Garrett, Mike I Walton, 2008
In recent years, it has been reported that alterations to the PI3K-Akt signaling pathway are frequent in human cancer . The class IA PI 3-kinase p110α is mutated in many forms of displasia. Many of these mutations cause the kinase to be more active. The PtdIns(3,4,5)P3 phosphatase PTEN that antagonises PI 3-kinase signaling is absent from many tumours. Constitutive activation of the PI3K-Akt pathway occurs due to amplification of the PIK3C gene encoding PI3K or the Akt gene, or as a result of mutations in components of the pathway, for example PTEN (phosphatase and tensin homologue deleted on chromosome 10), which inhibit the activation of Akt. Several small molecules designed to specifically target PI3K-Akt have been developed, and induced cell cycle arrest or apoptosis in human cancer cells in vitro and in vivo .
Activated PI3K generates a lipid second messenger, phosphatidylinositol-3,4,5-trisphosphate (PIP3), causing translocation of Akt to the plasma membrane where it becomes phosphorylated and activated. The balance of cellular PIP3 is regulated primarily by a phosphatase called PTEN that reduces PIP3 levels thereby lowering Akt activity. In melanomas, decreased PTEN activity elevates PIP3 levels resulting in Akt activation. Active Akt then phosphorylates downstream cellular proteins that promote melanoma cell proliferation and survival. Recently, Akt3 was discovered to be the predominant isoform activated in sporadic melanomas. Levels of activity increased during melanoma progression with metastatic melanomas having the highest activity.
Role of PTEN
The study presented below has identified phosphatidylinositol 3,4,5-trisphosphate RAC exchanger 2a (P-REX2a) as a PTEN-interacting protein. P-REX2a mRNA was more abundant in human cancer cells and significantly increased in tumors with wild-type PTEN that expressed an activated mutant of PIK3CA encoding the p110 subunit of phosphoinositide 3-kinase subunit α (PI3Kα). P-REX2a inhibited PTEN lipid phosphatase activity and stimulated the PI3K pathway only in the presence of PTEN. P-REX2a stimulated cell growth and cooperated with a PIK3CA mutant to promote growth factor–independent proliferation and transformation. Depletion of P-REX2a reduced amounts of phosphorylated AKT and growth in human cell lines with intact PTEN. Thus, P-REX2a is a component of the PI3K pathway that can antagonize PTEN in cancer cells.
- Activation of the PI3K Pathway in Cancer Through Inhibition of PTEN by Exchange Factor P-REX2a, Barry Fine, Cindy Hodakoski, Susan Koujak, 2009
- PI3K protects against myocardial infarction-induced heart failure: identification of PI3K-regulated miRNA and mRNA, Lin RC, Weeks KL, Gao XM, 2010
- Targeted inhibition of phosphoinositide 3-kinase activity as a novel strategy to normalize beta-adrenergic receptor function in heart failure, Perrino C, Rockman HA, Chiariello M, 2006
Activation ok PI3K leads to phosphorylation of membrane lipids and generation of second messengers such as phosphatidylinositol-3,4,5-triphosphate. The p110 α isoform of PI3K is activated by receptor tyrosine kinases such as IGF-1 receptor. This isoform of PI3K helps mediate physiologic hypertrophy in response to exercise training, promotes cell survival, inhibits cardiac fibrosis and attenuates pathologic hypertrophy. By contrast, activation of the p110γ isoform of PI3K exerts detrimental effects by promoting internalization of β-adrenergic receptors and inhibiting sarco/endoplasmic reticulum Ca2+-ATPase activity. Desensitization of β-adrenergic receptors contributes to sluggish responses of a failing heart to exercise. Chronic overstimulation leads to decreases in the number and responsiveness of β-adrenergic receptors and a defect in coupling to adenylyl cyclase.
- Diabetes and Insulin Signaling: A New Strategy to Promote Pancreatic b Cell Survival, Olivia May, 2010
Strategies for maintaining pancreatic β cell survival are becoming evident to scientists through examining the intricate phosphatidylinositol 3-kinase (PI3K) pathway, which is known to control glucose homeostasis via insulin signaling. Coincidently, promoting pancreatic β cell survival, through regulating protein translation, is also mediated by PI3K signaling in response to levels of available nutrients.
Luca Boggione; matricola 735531
Giacomo Baima; matricola 735507