Receptor tyrosine kinase (RTK) overexpression is frequently observed in many human cancers and drives cell division and cell survival. One major pathway activated by these receptors is the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. PI3K activity consists of a p110 catalytic protein that phosphorylates lipids which in turn activate Akt signaling. The p110 protein is stabilized and regulated by another protein, p85.
1. Regulation of PTEN (CIHR)
PTEN is a tumor suppressor protein that counteracts the PI3K pathway, since it is the lipid phosphatase that dephosphorylates PI3K lipid products. We have recently shown that the p85 protein that regulates p110-PI3K activity, also directly binds and positively regulates PTEN lipid phosphatase activity. Thus, p85 is a dual regulator of both the kinase and the phosphatase controlling lipid levels and the resulting Akt signaling. Our results have been developed into a model that explains the paradoxical phenotypes observed in transgenic mice containing reduced p110, PTEN and p85 levels. We are also pursuing experiments to further characterize the regulatory effects of p85 towards PTEN.
2. Regulation of Receptor Endocytosis and Down-regulation
We are studying mechanisms cells use to decrease receptor tyrosine kinase levels via uptake of cell surface receptors into cells (endocytosis) and degradation in lysosomal compartments.
a) Regulation of p85 GAP activity in Rab-mediated endocytosis (CCSRI)
The p85 protein also binds to many activated RTKs and remains bound while receptors are taken up inside cells. We have found that p85 has GAP activity that regulates Rab5 and Rab4 GTPases. Rabs are important for the trafficking of RTK-containing vesicles during endocytosis, receptor deactivation and recycling back to the cell surface, and for receptor sorting for degradation. Defects in the Rab regulatory function of p85 are oncogenic. We are studying this new role for p85 and the effect of mutations within the GAP of p85 that have recently been discovered in human cancer samples.
b) The role of ankyrin3 in receptor-mediated endocytosis (Sask Cancer Agency)
Overexpression of small isoforms of ankyrin3 result in reduced RTK levels through an unknown mechanism. We are studying how ankyrin3 enhances receptor degradation and the role of ubiquitin binding in this process.
c) Trafficking of EGFR/ErbB2 receptors in breast cancer cells (CBCF)
We are studying receptor trafficking and using complementary strategies in an effort to enhance EGFR &/or ErbB2 degradation in breast cancer cells.
Anderson, Deborah H. (2010) p85 plays a critical role in controlling flux through the PI3K/PTEN signaling axis through dual regulation of both p110 (PI3K) and PTEN. Cell Cycle, 9, 2055-2056.
Chagpar, Ryaz B., Philip H. Links, M. Chris Pastor, Levi A. Furber, Andrea D. Hawrysh, M. Dean Chamberlain and Deborah H. Anderson (2010) Direct positive regulation of PTEN by the p85 subunit of phosphatidylinositol 3-kinase. Proc Natl Acad Sci USA, 107, 5471-5476.
Torres, Vicente A., Ainhoa Mielgo, Daniela Barila, Deborah H. Anderson and Dwayne Stupack (2008) Caspase 8 promotes peripheral localization and activation of Rab5. J. Biol. Chem., 283, 36280-36289.
Chamberlain, M. Dean, Jennifer C. Oberg, Levi A. Furber, Sharon F. Poland, Andrea D. Hawrysh, Stacey M. Knafelc, Heidi M. McBride and Deborah H. Anderson (2010) Deregulation of Rab5 and Rab4 proteins in p85R274A-expressing cells alters PDGFR trafficking. Cell Signalling 22, 1562-1575.
Chamberlain, M. Dean, Tim Chan, Jenny C. Oberg, Andrea D. Hawrysh, Kristy M. James, Anurag Saxena, Jim Xiang and Deborah H. Anderson (2008) Disrupted RabGAP function of the p85 subunit of phosphatidylinositol 3`-kinase results in cell transformation. J Biol Chem, 283, 15861-15868.
Ignatiuk, Ashley, Jeremy P. Quickfall, Andrea D. Hawrysh, M. Dean Chamberlain and Deborah H. Anderson (2006) The smaller isoforms of ankyrin 3 bind to the p85 subunit of phosphatidylinositol 3`-kinase and effect PDGF receptor down-regulation. J. Biol. Chem., 281, 5956-64.
Anderson, Deborah H. (2006) Role of Lipids in the MAPK signalling pathway. Progress in Lipids Research, 45, 102-119.
Anderson, Deborah H., and M. Dean Chamberlain (2005) Assay and stimulation of the Rab5 GTPase by the p85a subunit of phosphatidylinositol 3-kinase. Methods in Enzymology, 403, Chapter 48, 552-561.
Chamberlain, M. Dean, and Deborah H. Anderson (2005) Measurement of the Interaction of the p85a subunit of phosphatidylinositol 3-kinase with Rab5. Methods in Enzymology, 403, Chapter 47, 541-552.
Mahon, Elizabeth S., Andrea D. Hawrysh, Ryaz B. Chagpar, Lindsey M. Johnson, and Deborah H. Anderson (2005) A-Raf associates with and regulates platelet-derived growth factor receptor signalling. Cellular Signalling, 17, 857-868.
Johnson, Lindsey M., Kristy M. James, M. Dean Chamberlain, and Deborah H. Anderson (2005) Identification of key residues in the A-Raf kinase important for phosphoinositide lipid binding specificity. Biochemistry, 44, 3432-3440.
Chamberlain, M. Dean, Tangyne R. Berry, M. Chris Pastor and Deborah H. Anderson (2004) The p85α subunit of phosphatidylinositol 3`-kinase binds to and stimulates the GTPase activity of Rab proteins. J. Biol. Chem., 279, 48607-48614.