Familial cylindromatosis is an autosomal dominant predisposition to tumours of skin appendages called cylindromas. Familial cylindromatosis is caused by mutations in a gene encoding the CYLD protein of previously unknown function. Here we show that CYLD is a deubiquitinating enzyme that negatively regulates activation of the transcription factor NF-kappaB by specific tumour-necrosis factor receptors (TNFRs). Loss of the deubiquitinating activity of CYLD correlates with tumorigenesis. CYLD inhibits activation of NF-kappaB by the TNFR family members CD40, XEDAR and EDAR in a manner that depends on the deubiquitinating activity of CYLD. Downregulation of CYLD by RNA-mediated interference augments both basal and CD40-mediated activation of NF-kappaB. The inhibition of NF-kappaB activation by CYLD is mediated, at least in part, by the deubiquitination and inactivation of TNFR-associated factor 2 (TRAF2) and, to a lesser extent, TRAF6. These results indicate that CYLD is a negative regulator of the cytokine-mediated activation of NF-kappaB that is required for appropriate cellular homeostasis of skin appendages.

Epstein–Barr virus latent membrane protein 1 (LMP1) activation of NF-κB is critical for Epstein–Barr virus-infected B lymphocyte survival. LMP1 activates the IκB kinase complex and NF-κB through two cytoplasmic signaling domains that engage tumor necrosis factor receptor-associated factor (TRAF)1/2/3/5 or TRADD and RIP. We now use cells lacking expression of TRAF2, TRAF5, TRAF6, IKKα, IKKβ, IKKγ, TAB2, IL-1 receptor-associated kinase (IRAK)1, or IRAK4 to assess their roles in LMP1-mediated NF-κB activation. LMP1-induced RelA nuclear translocation was similar in IKKα knockout (KO) and WT murine embryo fibroblasts (MEFs) but substantially deficient in IKKβ KO MEFs. NF-κB-dependent promoter responses were also substantially deficient in IKKβ KO MEFs but were hyperactive in IKKα KO MEFs. More surprisingly, NF-κB responses were near normal in TRAF2 and TRAF5 double-KO MEFs, IKKγ KO MEFs, TAB2 KO MEFs, and IRAK4 KO MEFs but were highly deficient in TRAF6 KO MEFs and IRAK1 KO HEK293 cells. Consistent with the importance of TRAF6, LMP1-induced NF-κB activation in HEK293 cells was inhibited by expression of dominant-negative TAB2 and Ubc13 alleles. These data extend a role for IKKα in IKKβ regulation, identify an unusual IKKβ-dependent and IKKγ-independent NF-κB activation, and indicate that IRAK1 and TRAF6 are essential for LMP1-induced NF-κB activation.

The cylindromatosis (CYLD) gene is mutated in human tumors of skin appendages. It encodes a deubiquitylating enzyme (CYLD) that is a negative regulator of the NF-B and JNK signaling pathways, in vitro. However, the tissue-specific function and regulation of CYLD in vivo are poorly understood. We established a genetically tractable animal model to initiate a systematic investigation of these issues by characterizing an ortholog of CYLD in Drosophila. Drosophila CYLD is broadly expressed during development and, in adult animals, is localized in the fat body, ovaries, testes, digestive tract and specific areas of the nervous system. We demonstrate that the protein product of Drosophila CYLD (CYLD), like its mammalian counterpart, is a deubiquitylating enzyme. Impairment of CYLD expression is associated with altered fat body morphology in adult flies, increased triglyceride levels and increased survival under starvation conditions. Furthermore, flies with compromised CYLD expression exhibited reduced resistance to bacterial infections. All mutant phenotypes described were reversible upon conditional expression of CYLD transgenes. Our results implicate CYLD in a broad range of functions associated with fat homeostasis and host defence in Drosophila.