Loss of the cylindromatosis tumour suppressor inhibits apoptosis by activating NF-κB

Protein modification by the conjugation of ubiquitin moieties-ubiquitination- plays a major part in many biological processes, including cell cycle and apoptosis(1). The enzymes that mediate ubiquitin-conjugation have been well-studied, but much less is known about the ubiquitin-specific proteases that mediate deubiquitination of cellular substrates(2,3). To study this gene family, we designed a collection of RNA interference vectors to suppress 50 human de-ubiquitinating enzymes, and used these vectors to identify de-ubiquitinating enzymes in cancer-relevant pathways. We report here that inhibition of one of these enzymes, the familial cylindromatosis tumour suppressor gene (CYLD)(4), having no known function, enhances activation of the transcription factor NF-kappaB. We show that CYLD binds to the NEMO (also known as IKKgamma) component of the IkappaB kinase (IKK) complex, and appears to regulate its activity through de-ubiquitination of TRAF2, as TRAF2 ubiquitination can be modulated by CYLD. Inhibition of CYLD increases resistance to apoptosis, suggesting a mechanism through which loss of CYLD contributes to oncogenesis. We show that this effect can be relieved by aspirin derivatives that inhibit NF-kappaB activity(5), which suggests a therapeutic intervention strategy to restore growth control in patients suffering from familial cylindromatosis.