title: Protein phosphatase 2A, a negative regulator of the ERK signaling pathway, is activated by tyrosine phosphorylation of putative HLA class II-associated protein I (PHAPI)/pp32 in response to the antiproliferative lectin, jacalin.
abstract: Protein phosphatase 2A (PP2A) is a family of mammalian serine/threonine phosphatases that is involved in the control of many cellular functions including those mediated by extracellular signal-regulated kinase (ERK) signaling. While investigating the reversible antiproliferative effect of the dietary lectin, jacalin, which binds the Thomsen-Friedenreich antigen (galactose beta1-3 N-acetylgalactosamine alpha-), we have found that this lectin (30 microg/ml) induces rapid, transient, tyrosine phosphorylation of putative human HLA-DR-associated protein I (PHAPI, also known as the tumor suppressor pp32) in HT29 human colon cancer cells. This is accompanied by the release of PP2A from association with PHAPI, allowing increased phosphatase activity of PP2A (by 42 +/- 10% at 10 min) and consequent complete dephosphorylation of the ERK kinase, MEK1/2, by 10 min and of ERK1/2 by 60 min. PHAPI knockdown by RNA interference abolished the effects of jacalin on PP2A activation and MEK inhibition. Thus phosphorylation of PHAPI/pp32 is a critical regulatory step in PP2A activation and ERK signaling.
authors.affiliation: Departments of Medicine and Human Anatomy and Cell Biology, University of Liverpool, Liverpool L69 3GA, UK. lgyu@liv.ac.uk
authors.forename: Lu-Gang
authors.initials: LG
authors.lastname: Yu
authors.forename: Len C
authors.initials: LC
authors.lastname: Packman
authors.forename: Mike
authors.initials: M
authors.lastname: Weldon
authors.forename: Jane
authors.initials: J
authors.lastname: Hamlett
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Rhodes
mesh_terms: D019073:HT29 Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D048369:MAP Kinase Kinase 1; D020935:MAP Kinase Signaling System; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009687:Nuclear Proteins; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D037121:Plant Lectins / Q000494:pharmacology*; D011485:Protein Binding; D054648:Protein Phosphatase 2; D011506:Proteins / Q000378:metabolism*; D016601:RNA-Binding Proteins; D014443:Tyrosine / Q000378:metabolism
chemical_list: C087425:ANP32A protein, human; D047908:Intracellular Signaling Peptides and Proteins; D009687:Nuclear Proteins; D037121:Plant Lectins; D011506:Proteins; D016601:RNA-Binding Proteins; C043696:jacalin; D014443:Tyrosine; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D048369:MAP Kinase Kinase 1; C482125:MAP2K1 protein, human; D020929:Mitogen-Activated Protein Kinase Kinases; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2