The tumor suppressor WARTS activates the Omi/HtrA2-dependent pathway of cell death

Drosophila tumor suppressor WARTS (Wts) is anevolutionallyconservedserine/threoninekinaseandparti-cipates in a signaling complex that regulates bothproliferationandapoptosistoensurethepropersizeandshapeofthefly.Humancounterpartsofthiscomplexhavebeenfoundtobefrequentlydownregulatedormutatedincancers. WARTS, a human homolog of Wts, is alsoknownastumorsuppressorandmitoticregulator,butitsmolecularimplicationsintumorigenesisarestillobscure.Here,weshowthatWARTSbindsviaitsC-terminustothePDZdomainofaproapoptoticserineproteaseOmi/HtrA2. Depletion of WARTS inhibited Omi/HtrA2-mediatedcelldeath,whereasoverexpressionofWARTSpromoted this process. Furthermore, WARTS can en-hancetheproteaseactivityofOmi/HtrA2bothin vivo andin vitro.ActivationofOmi/HtrA2-mediatedcelldeathisthus a potential mechanism for the tumor suppressiveactivityofWARTS.Oncogene (2005)24,5287–5298.doi:10.1038/sj.onc.1208682;publishedonline27June2005Keywords: mitotic kinases; mitochondria; apoptosis;PDZdomain;IAPs;caspases;cancerIntroductionCell proliferation and apoptosis are precisely coordi-nated to form proper shape and size of organs duringmetazoan development (Conlon and Raff, 1999). Lossof this coordination leads not only to developmentalabnormalitiesbutalsototumorigenesis(Abrams,2002;Green and Evan, 2002). A recent screen of Drosophilamutants that result in tissue overgrowth and largerorgans identified several genes such as warts (wts),salvador (sav) and hippo (hpo) (Tapon et al., 2002;Harveyet al.,2003;Pantalacciet al.,2003;Udanet al.,2003; Wu et al., 2003). Importantly, recent findingsprovideevidencethatmammaliancounterpartsofthesegenesinvolvingthesizecheckpointnetworkfunctionaretumor suppressors. For instance, mice deficient inmammalianorthologofwts (alsoknownaslats)developmalignant tumors (St John et al., 1999), and a humanhomologofWts,WARTS,isdownregulatedormutatedin the subset of human soft-tissue sarcomas (Hisaokaet al., 2002). Furthermore, the human ortholog of sav,WW45, is mutated in several cancer cell lines (Taponet al.,2002).Drosophila wts,sav andhpo aregeneticallylinked,andtheencodedproteinsarelikelytofunctionasacomplex(Taponet al.,2002;Harveyet al.,2003;Pantalacciet al.,2003;Udanet al.,2003;Wuet al.,2003).Wtsphysicallyinteracts with the WW domain of Sav, and theinteractionofWtswithSavfacilitatesitsphosphoryla-tion by Hpo. Dysfunction of this complex results inacceleratedproliferationanddefectiveapoptosis,whicharelinked toanirregularcontrol ofbothcyclinEandDrosophila inhibitor of apoptosis protein (DIAP).Although all three mutants show similar overgrowthphenotypes, null wts mutant has the most severeoutcome, suggesting that Wts is the key player of thispathway (Wu et al., 2003). Therefore, it can bespeculatedthatthedownstreamtargetsofWtsplayanimportantroleintheregulationofcellproliferationandcelldeath.WARTS is a serine–threonine kinase that shares ahighdegreeofsequencehomologywithmembersofthemyotonic dystrophy protein kinase (DMPK) family(Justice et al., 1995; Xu et al., 1995; Nishiyama et al.,1999; Tao et al., 1999), many of which participate inmitoticevents.WehavepreviouslyshownthatWARTSforms a regulatory complex that includes zyxin, apromoter of actin filament assembly, on the mitoticapparatus,andthatitplaysapivotalroleinthecontrolofmitoticprogression(Hirotaet al.,2000).RecentstudyhasalsoshownthatWARTSisinvolvedincytokinesisby regulating actin polymerization (Yang et al., 2004).Furthermore,theexpressionofakinase-inactiveformofWARTS induced mitotic aberration followed by tetra-ploidization (Iida et al., 2004). We thus proposed thatWARTS contributes to normal cell division processes