ORIGINAL ARTICLE E6/E7oncogenesincreaseandtumorsuppressorsdecreasetheproportionof self-renewing neural progenitor cells KPiltti 1 ,LKerosuo 1 ,JHakanen 2 ,MEriksson 3 ,AAngers-Loustau 1 ,SLeppa¨ 3 ,MSalminen 2 , HSariola 1,4 andKWartiovaara 1 1 Developmental Biology, Institute of Biomedicine, University of Helsinki, Helsinki, Finland; 2 Institute of Biotechnology, University of Helsinki, Helsinki, Finland; 3 Molecular and Cancer Biology, Institute of Biomedicine, University of Helsinki, Helsinki, Finland and 4 HUCH Laboratory Diagnostics, Helsinki University Central Hospital, Helsinki, Finland Manyifnotmosttissuesneedacontrollednumberofstem cellstomaintainnormalfunction.Cancercanbeseenasa processofdisturbedtissuehomeostasis,inwhichtoomany cells have or acquire too primitive identity. Here we measured how oncogenes and tumour suppressors affect the differentiation capacity, proportion and characteris- tics of progenitor cells in a model tissue. Neural progenitorcells(NPCs)wereexposedtohumanpapilloma virus E6, E7 or E6/E7 oncogenes, which degrade tumour suppressors p53 and pRb family members, respectively. E6/E7-expressing or p53/ NPCs were able to differentiate, but simultaneously retained high capacity for self-renewal, proliferation, ability to remain multi- potent in conditions promoting differentiation and showed delayed cell cycle exit. These functions were mediated through p53 and pRb family, and involved MEK–ERK signalling. Decreased amount of p53 increased self- renewal and proliferation, whereas pRb affected only proliferation. Our results suggest that the oncogenes increase whereas p53 and pRb family tumour suppressors decrease the number and proportion of progenitor cells. These findings provide one explanation how oncogenes and tumour suppressors control tissue homeostasis and highlighttheirimportanceinstemcellself-renewal,linked both to cancer and life-long tissue turnover. Oncogene (2006) 25, 4880–4889.doi:10.1038/sj.onc.1209492; publishedonline13March2006 Keywords: oncogene; tumor suppressor; progenitor; self-renewal Introduction Theexistenceofstemcellsthroughoutthelifespanofthe individual is due to the ability to self-renew. Upon differentiation, stem cells divide asymmetrically, and produceonecommittedprecursorcellandonestemcell to maintain the stem cell pool. Interestingly, the same mechanismsthatkeepthestemcellsprimitiveareoften activatedinmalignanttransformation,inwhichthecells acquire uncontrolled capacity for self-renewal (Pardal et al.,2003).Thesecancerandnormalstemcellsshare several characteristics and often express the same markersandfactors(Singh et al.,2003). Oneofthemostimportantquestionsincancerandstem cell biology is the mechanism of self-renewal. As several virus oncogenes (Dyson et al., 1993; Harris et al., 1998; Whyte et al., 1988; DeCaprio 1999) contribute to tumorigenesis, we addressed the question: what happens if the cells with self-renewal capacity are exposed to viral oncogenes?Forthis,wetransducedneuralprogenitorcells (NPCs)withHPV16E6andE7oncogenes.HPV16E6/E7 is a classical oncogene complex (Fehrmann and Laimins, 2003)andanestablishedriskfactorforHPV-relatedcervix cancers and head and neck squamous cell carcinomas (Walboomers et al., 1999; van Houten et al., 2001). In HPV-infected cells, E6 and E7 oncogenes degrade the tumour suppressor p53 and pRb/p107/p130, respectively (Scheffner et al., 1990; Davies et al., 1993; Boyer et al., 1996) and interfere with several other cellular activities (reviewedbyMunger et al.,2004).E6/E7immortalizeand transformatleastkeratinocytes,epithelialandendothelial cells(Hawley-Nelson et al.,1989;Coursen et al.,1997). Duringtheirdifferentiationtoneurons,NPCsunder- go changes that are considered normally irreversible. These include cell cycle exit, which involves the activation of tumour suppressors p53 (Miller et al., 2003) and pRb (Slack et al., 1993; Slack et al., 1998). Both of them are important at various times of brain development(reviewedbyChoiandDonehower,1999), andtheirdeficiencyfrequentlyresultsinbrainanomalies (Armstrong et al., 1995) or embryonic death (Macleod et al.,1996;Slack et al.,1998).TheroleofpRbfamily membersinterminaldifferentiationofneuronsseemsto be complex, as pRb has both cell cycle (cell-autono- mous) and differentiation controlling functions (non- cell-autonomous) (Liu et al., 2004). Other Rb family members,p107andp130,havealsoaroleindifferentia- tion (Callaghan et al., 1999; Liu et al., 2004). The arguablymostimportantfunctionofp53inneuronsis to prevent cell cycle re-entry (Miller et al., 2003). Interestingly, increasing the amount of p53 seems to accelerate the ageing of several tissues (Tyner et al., Received 30 September 2005; revised 20 December 2005; accepted 24 January2006;publishedonline13March2006 Correspondence:DrKWartiovaara,DevelopmentalBiology,Institute of Biomedicine, PO Box 63, University of Helsinki, FIN-00014 Helsinki,Finland. E-mail:kirmo.wartiovaara@helsinki.fi Oncogene (2006) 25, 4880–4889 & 2006 Nature Publishing Group All rights reserved 0950-9232/06 $30.00 www.nature.com/onc