Targeted therapy of the XIAP/proteasome pathway overcomes TRAIL-resistance in carcinoma by switching apoptosis signaling to a Bax/Bak-independent 'type I' mode

TRAIL is a promising anticancer agent, capable of inducing apoptosis in a wide range of treatment-resistant tumor cells. In ‘type II' cells, the death signal triggered by TRAIL requires amplification via the mitochondrial apoptosis pathway. Consequently, deregulation of the intrinsic apoptosis-signaling pathway, for example, by loss of Bax and Bak, confers TRAIL-resistance and limits its application. Here, we show that despite resistance of Bax/Bak double-deficient cells, TRAIL-treatment resulted in caspase-8 activation and complete processing of the caspase-3 proenzymes. However, active caspase-3 was degraded by the proteasome and not detectable unless the XIAP/proteasome pathway was inhibited. Direct or indirect inhibition of XIAP by RNAi, Mithramycin A or by the SMAC mimetic LBW-242 as well as inhibition of the proteasome by Bortezomib overcomes TRAIL-resistance of Bax/Bak double-deficient tumor cells. Moreover, activation and stabilization of caspase-3 becomes independent of mitochondrial death signaling, demonstrating that inhibition of the XIAP/proteasome pathway overcomes resistance by converting ‘type II' to ‘type I' cells. Our results further demonstrate that the E3 ubiquitin ligase XIAP is a gatekeeper critical for the ‘type II' phenotype. Pharmacological manipulation of XIAP therefore is a promising strategy to sensitize cells for TRAIL and to overcome TRAIL-resistance in case of central defects in the intrinsic apoptosis-signaling pathway.

Differential regulation of the proapoptotic multidomain protein Bak by p53 and p73 at the promoter level

During apoptosis Bcl-2 proteins control permeabilization of the mitochondrial outer membrane leading to the release of cytochrome c. Essential gatekeepers for cytochrome c release are the proapoptotic multidomain proteins, Bax, and Bak. The expression of Bax is upregulated upon cellular stress by the tumor suppressor p53. Despite the high functional homology of Bax and Bak, little is known about how the bak gene is regulated. To investigate its transcriptional regulation in further detail, we have analyzed a region spanning 8200 bp upstream of the bak start codon (within exon 2) for transcription factor-binding sites, and identified three p53 consensus sites (BS1-3). Reporter gene assays in combination with site-directed mutagenesis revealed that only one putative p53-binding site (BS3) is necessary and sufficient for induction of reporter gene expression by p53. Consistently, p53 induces expression of endogenous Bak. At the mRNA level, induction of Bak expression is weaker than induction of Puma and p21. Interestingly, Bak expression can also be induced by p73 that binds however to each of the three p53-binding sites within the bak promoter region. Our data suggest that expression of Bak can be induced by both, p53 and p73 utilizing different binding sites within the bak promoter.