Transgenic overexpression of a dominant negative mutant of FADD that, although counterselected during tumor progression, cooperates in L‐myc‐induced tumorigenesis

Abstract

Key words: FADD-DN; death receptor; cell cycle; oncogene cooperationApoptosis as a special form of programmed cell death has been recognized as a fundamental process both throughout embryogenesis and in the adult organism to maintain tissue homeostasis and remove dangerous or surplus cells without inflammation. 1 For a cancer to occur and progress, apoptosis must be inhibited as the tumor cells are constantly exposed to a variety of apoptotic stimuli, e.g., overexpression of apoptosis-inducing oncogenes, hypoxia or lack of survival signals after detachment of metastatic cells from the tumor mass (anoikis). 2 Furthermore, conventional treatment of cancer by chemotherapy and irradiation is based on the selective induction of apoptosis in tumor cells, and tumor resistance to such treatment is a result of the inhibition of drug-or radiation-induced apoptosis. 3 The extrinsic apoptotic pathway is triggered by so-called death receptors, which together with their ligands belong to the TNF/ TNFR superfamily. The death receptor subfamily consists of CD95/Fas/Apo-1, TNFR1, DR3 and the TRAIL receptors DR4/ DR5, DR6, NGFR and EDAR. 4 Death receptors contain in their intracellular part a specific protein-binding motif, the DD, which mediates homophilic interactions with other DD-bearing proteins. FADD is essential for death receptor-induced apoptosis as it is used as an adaptor protein by several DD-containing death receptors. [5][6][7][8][9][10][11][12][13][14][15] In the activated CD95 receptor, FADD binds with its N-terminal DD to the CD95 DD. FADD then recruits the apical caspase-8 via another protein-binding domain in its C terminus, the DED. Within this assembled intracellular receptor complex (DISC 16 ), caspase-8 becomes activated ("induced proximity" 17 ) and starts to proteolyse its targets.Death receptor-mediated cell killing is normally induced by the death receptor ligands and plays an important role during physiological cell death as well as in different antitumor strategies (e.g., cytotoxic T-cell killing 18 ). Consequently, several mutations have been identified in tumors which lead to impairment of death receptor-induced apoptosis. 1,19 Examples include overexpression of the inhibitory FLIP proteins in melanomas, 20 transcriptional downregulation of caspase-8 mRNA by gene deletion or promoter methylation 21 and CD95 mutations in different cancers. [22][23][24] Direct proof of the tumor-suppressing proapoptotic activity of the CD95 receptor has been obtained by in vivo cooperation experiments. Tumor development in L-myc transgenic mice is accelerated when mice are bred onto a CD95-deficient lpr background. 25 Interference with FADD functionality leads to the expected impairment of death receptor killing. 26 -30 Surprisingly, however, not only cell death but also T-cell proliferation is strongly inhibited in lck FADD-DN transgenic mice 28 -30 and in rag1 -/-k.o. animals reconstituted with a FADD -/-k.o. T-cell compartment. 26 The influence of such a FADD-dependent signaling pathway on proliferation is not re...