title: Formation of Tankyrase Inhibitor-Induced Degradasomes Requires Proteasome Activity.
abstract: In canonical Wnt signaling, the protein levels of the key signaling mediator β-catenin are under tight regulation by the multimeric destruction complex that mediates proteasomal degradation of β-catenin. In colorectal cancer, destruction complex activity is often compromised due to mutations in the multifunctional scaffolding protein Adenomatous Polyposis Coli (APC), leading to a stabilization of β-catenin. Recently, tankyrase inhibitors (TNKSi), a novel class of small molecule inhibitors, were shown to re-establish a functional destruction complex in APC-mutant cancer cell lines by stabilizing AXIN1/2, whose protein levels are usually kept low via poly(ADP-ribosyl)ation by the tankyrase enzymes (TNKS1/2). Surprisingly, we found that for the formation of the morphological correlates of destruction complexes, called degradasomes, functional proteasomes are required. In addition we found that AXIN2 is strongly upregulated after 6 h of TNKS inhibition. The proteasome inhibitor MG132 counteracted TNKSi-induced degradasome formation and AXIN2 stabilization, and this was accompanied by reduced transcription of AXIN2. Mechanistically we could implicate the transcription factor FoxM1 in this process, which was recently shown to be a transcriptional activator of AXIN2. We observed a substantial reduction in TNKSi-induced stabilization of AXIN2 after siRNA-mediated depletion of FoxM1 and found that proteasome inhibition reduced the active (phosphorylated) fraction of FoxM1. This can explain the decreased protein levels of AXIN2 after MG132 treatment. Our findings have implications for the design of in vitro studies on the destruction complex and for clinical applications of TNKSi.
authors.affiliation: Centre for Cancer Biomedicine, Faculty of Medicine, Oslo University Hospital, Oslo, Norway.|Department of Molecular Cell Biology, Institute for Cancer Research, Oslo University Hospital, Oslo, Norway.
authors.forename: Nina Marie
authors.initials: NM
authors.lastname: Pedersen
authors.affiliation: Centre for Cancer Biomedicine, Faculty of Medicine, Oslo University Hospital, Oslo, Norway.|Department of Molecular Cell Biology, Institute for Cancer Research, Oslo University Hospital, Oslo, Norway.
authors.forename: Tor Espen
authors.initials: TE
authors.lastname: Thorvaldsen
authors.affiliation: Centre for Cancer Biomedicine, Faculty of Medicine, Oslo University Hospital, Oslo, Norway.|Department of Molecular Cell Biology, Institute for Cancer Research, Oslo University Hospital, Oslo, Norway.
authors.forename: Sebastian Wolfgang
authors.initials: SW
authors.lastname: Schultz
authors.affiliation: Centre for Cancer Biomedicine, Faculty of Medicine, Oslo University Hospital, Oslo, Norway.|Department of Molecular Cell Biology, Institute for Cancer Research, Oslo University Hospital, Oslo, Norway.
authors.forename: Eva Maria
authors.initials: EM
authors.lastname: Wenzel
authors.affiliation: Centre for Cancer Biomedicine, Faculty of Medicine, Oslo University Hospital, Oslo, Norway.|Department of Molecular Cell Biology, Institute for Cancer Research, Oslo University Hospital, Oslo, Norway.
authors.forename: Harald
authors.initials: H
authors.lastname: Stenmark
mesh_terms: D025601:Adenomatous Polyposis Coli Protein / Q000235:genetics / Q000378:metabolism; D060466:Axin Protein / Q000235:genetics / Q000378:metabolism; D018938:Caco-2 Cells; D045744:Cell Line, Tumor; D004791:Enzyme Inhibitors / Q000494:pharmacology; D000072278:Forkhead Box Protein M1 / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D046988:Proteasome Endopeptidase Complex / Q000187:drug effects / Q000378:metabolism*; D055550:Protein Stability; D059748:Proteolysis / Q000187:drug effects; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D038501:Tankyrases / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D060449:Wnt Signaling Pathway; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism
chemical_list: D025601:Adenomatous Polyposis Coli Protein; D060466:Axin Protein; D004791:Enzyme Inhibitors; D000072278:Forkhead Box Protein M1; D007976:Leupeptins; D046988:Proteasome Endopeptidase Complex; D034741:RNA, Small Interfering; D038501:Tankyrases; D051176:beta Catenin; C555337:APC protein, human; C555451:AXIN1 protein, human; C491761:AXIN2 protein, human; C495270:CTNNB1 protein, human; C107585:FOXM1 protein, human; C437334:TNKS2 protein, human; C418109:TNKS protein, human; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde