National Human Genome Research Institute, Cancer Genetics Branch, National Institutes of Health, Bethesda, MD, USA
Regulation of PTEN by p85α: The PI3K-AKT signal transduction pathway is antagonized by the activity of the PTEN phosphatase, which dephosphorylates PIP3 to generate PIP2. Chagpar et al., (2010) demonstrated that p85α binds directly to PTEN via the p85α-SH3-BH domains. Cells expressing a synthetic mutant of p85α that abolished the p85α-PTEN interaction exhibited increased AKT activation following stimulation by growth factors. Chagpar et al., thus proposed that p85α can bind to PTEN and enhance PTEN activity. Subsequently, Cheung et al., (2011) demonstrated that compared to wildtype p85α, a tumor-associated mutant (p85α-E160X) that introduces a premature stop codon within the BH domain, was associated with reduced stability of the PTEN protein. Treatment of cells expressing the p85α-E160X mutant with a proteosome inhibitor lead to a modest increase in PTEN levels, further suggesting that the p85α-PTEN interaction prevents proteosomal degradation of PTEN and thus increases PTEN stability. The regulation of PTEN activity by p85α accounts for the increased insulin sensitivity observed in PIK3R1-/- or p85α-/- mice (Mauvais-Jarvis et al., 2002; Brachmann et al., 2005; Taniguchi et al., 2006; Taniguchi et al., 2010; Chagpar et al., 2010).
Receptor trafficking: p85α has GAP (GTP-ase Activating Protein) activity towards the Rab4, Rab5, Rac1, and Cdc42 small GTPases and, to a lesser extent, towards the Rab6 GTPase. The GAP activity of p85α resides within the BH domain. Within the BH domain, Arg151 and Arg274 are important for maximal GAP activity of p85α. The regulation of Rab4 and Rab5 activity by p85α has been implicated in the endosomal trafficking of activated PDGFR; cells expressing a synthetic mutant (p85α-Arg274A) exhibited delayed degradation of activated PDGFR, prolonged activation of the MAPK and AKT signalling pathways, and the capacity to transform NIH 3T3 cells (Chamberlain et al., 2004; Chamberlain et al., 2008; Chamberlain et al., 2010).
Regulation of the unfolded protein response: p85α interacts with XBP-1s, a transcription factor that regulates the unfolded protein response following endoplasmic reticulum stress, and facilitates the relocation of XBP-1s to the nucleus (Park et al., 2010a; Winnay et al., 2010).
p55α and p50α isoforms: Involved in insulin signaling (Inuki et al., 1997; Chen et al., 2004).
Mutation spectrum: Among 107 nonsynonymous, somatic mutations of PIK3R1 catalogued in the COSMIC database (v59 release, May 23rd, 2012) (Forbes et al., 2010), 43% (46 of 107) of mutations are in-frame insertions/deletions, 14% (15 of 107) are nonsense mutations, 11.2% (12 of 107) are frameshift mutations, 31.8% (34 of 107) are missense mutations. The majority (68.2%, 73 of 107) of all somatic mutations in PIK3R1 localize to animo acid residues within the iSH2 domain, which is shared by all four protein isoforms encoded by PIK3R1.
Altered functional properties of mutant proteins: Biochemical and cellular studies of tumor-associated p85α mutants have revealed functional differences between mutant p85α and wild type p85α, as well as functional differences among various p85α mutants (Philp et al., 2001; Jaiswal et al., 2009; Sun et al., 2010; Cheung et al., 2011; Urick et al., 2011).
- Transforming properties: Sun et al., (2010) evaluated the ability of nine tumor-associated mutant p85α proteins to transform chicken embryo fibroblasts. The p85α-KS459delN and p85α-DKRMNS560del mutants had the highest efficiency of transformation, the p85α-R574fs and p85α-T576del mutants had intermediate efficiency of transformation, and the p85α-D560Y, p85α-N564K p85α-W583del, p85α-E439del, and p85α-G376R mutants were only weakly tansforming (Sun et al., 2010). Transformation was mediated by p110α but not by p110α, p110α, or p110α (Sun et al., 2010). Each of the nine p85α mutants analyzed by Sun et al., retained the ability to bind p110α and resulted in hyperphosphorylation of AKT (T308) and 4E-BP1 when exogenously expressed in CEF cells (Sun et al., 2010). Eight of the tumor-associated mutants analyzed by Sun et al., (2010) localized to the iSH2 domain; one mutant (p85α-G376R) localized to the nSH2 domain. Jaiswal et al., showed that the p85α-D560Y, p85α-N564D, and p85α-QYL579delL mutants were capable of promoting both the IL-3 independent growth and anchorage-independent growth of BaF3 cells (Jaiswal et al., 2009). Similarily, Cheung et al., showed that the p85α-R574fs, p85α-T576del, p85α-E160X, p85α-R348X, and p85α-R503W mutants induced IL3-independent growth of BaF3 cells (Cheung et al., 2011).
- Altered p110α-binding: Truncating mutants of p85α that lack all or part of the the iSH2 domain (p85α-E160X, p85α-R162X, p85α-L380fs, p85α-R348X, p85α-K511VfsX2) fail to bind to p110α (Jaiswal et al., 2009; Cheung et al., 2011; Urick et al., 2011). In contrast, small in-frame deletions or missense mutations within the iSH2 domain retained the ability to bind p110α (Jaiswal et al., 2009; Cheung et al., 2011; Urick et al., 2011).
- Increased PI3K activity: Jaiswal et al., (2009) showed that p85α mutants that were capable of binding p110α were able to stabilize p110α. p85α/p110α holoenzymes composed of the p85α-N564 or p85α-QYL579delL mutants had increased lipid kinase activity compared with the wildtype-p85α/p110α holoenzyme (Jaiswal et al., 2009). Holoenzymes consisting of mutant p85α and p110α or p110α also exhibited increased kinase activity (Jaiswal et al., 2009). Philp et al. (2001) described a recurrent intronic PIK3R1 mutation in ovarian cancer cells; the mutation caused skipping of exon 13, resulting in deletion of residues 551-670 within the iSH2/cSH2 domains of p85α, and was associated with increased PI3K activity.
- Hyperphosphorylation of AKT: Mutants of p85α that retained the ability to bind p110α also lead to increased phosphorylation of AKT (Jaiswal et al., 2009; Cheung et al., 2011; Urick et al., 2011).
- Dysregulation of PTEN stability: Cheung et al., (2011) showed that the p85α-E160X mutant, which was present in an endometrial tumor and truncates p85α within the BH domain, is associated with reduced stability of the PTEN protein.
Daphne W Bell
PIK3R1 (phosphoinositide-3-kinase, regulatory subunit 1 (alpha))
Atlas Genet Cytogenet Oncol Haematol. 2012-05-01
Online version: http://atlasgeneticsoncology.org/gene/41717/pik3r1-(phosphoinositide-3-kinase-regulatory-subunit-1-(alpha))