CYLD: A Key Regulator of Hepatocellular Apoptosis, Proliferation and Carcinogenesis

Background and Aims: The tumor supressor CYLD is involved in NFκB and JNK signalling and cell cycle regulation. Full length CYLD (FLCYLD) counteracts NFκB activity and is a cellular adversary of the oncogene Bcl-3 (1). There is high evidence that short splice variant sCYLD positively regulates NFκB signal transduction and cellular homeostasis (2). The aim of our study was to analyse the role of CYLD for receptor mediated hepatocyte apoptosis, proliferation and chemically induced HCC development. Methods: We generated two mouse models for analysis of CYLD in hepatocytes. For the first model CYLD Exon7/8 flox/flox mice were mated with AlbuminCre mice leading to hepatocyte specific deletion of FLCYLD and overexpression of sCYLD (CYLDex7/8 AlbCre) (2). The second model was a complete CYLD knockout (3). We analyzed both hepatocyte apoptosis induction in vitro and in vivo after activation of the death receptors CD95 or TNF-R1. Furthermore we analysed hepatocellular proliferation after partial liver resection and development of HCC after DEN/Phenobarbital treatment. Results: CYLD-/- and CYLDex7/8 AlbCre primary murine hepatocytes (PMH) were less sensitive towards TNFα and Jo2 induced apoptosis compared to WT. In vivo CYLD-/- mice showed lower AST and ALT levels after LPS/D-Gal and Jo2 injection. NFκB activity was increased and JNK activity was decreased in unstimulated as well as in TNFα stimulated PMH in both knockout models. Analysis of Bcl-2 proteins revealed reduced expression of proapoptotic Bid in CYLD-/- hepatocytes. Proliferation rates after partial liver resection were 3-fold enhanced in CYLD-/-. Treatment of CYLDex7/8 AlbCre mice with DEN/Phenobarbital resulted in higher numbers of tumors and HCC nodules after 6 months. Conclusion: Our results suggest an important role of CYLD for the sensitivity of hepatocytes towards death receptor triggering. Ablation of CYLD contributes to enhanced cell survival and proliferation of hepatocytes.

Literatur: (1) Ikeda F, Dikic I: CYLD in Ubiquitin Signaling and Tumor Pathogenesis. Cell, 2006 May 19, 125, 643-645 (2) Hövelmeyer N, Wunderlich FT, Massoumi R, Jakobsen CG, Song J, Wörns MA, Merkwirth C, Kovalenko A, Aumailley M, Strand D, Brüning JC, Galle PR, Wallach D, Fässler R, Waisman A: Regulation of B cell homeostasis and activation by the tumor suppressor gene CYLD. J Exp Med. 2007 Oct 29; 204(11):2615-2627. (3) Massoumi R,1 Chmielarska K, Hennecke K, Pfeifer A, Fässler R: Cyld Inhibits Tumor Cell Proliferation by Blocking Bcl-3- Dependent NF-kB Signaling. Cell. 2006 May 19; 125, 665–677.

Apoptosis - CYLD - NF-kB - Proliferation - hepatocellular carcinoma