CAAT/enhancer binding protein homologous protein-dependent death receptor 5 induction is a major component of SHetA2-induced apoptosis in lung cancer cells

The flexible heteroarotinoids (Flex-Het) represent a novel type of atypical retinoids lacking activity in binding to and transactivating retinoid receptors. Preclinical studies have shown that Flex-Hets induce apoptosis of cancer cells while sparing normal cells and exhibit anticancer activity in vivo with improved therapeutic ratios over conventional retinoid receptor agonists. Flex-Hets have been shown to induce apoptosis through activation of the intrinsic apoptotic pathway. The present study has revealed a novel mechanism underlying Flex-Het-induced apoptosis involving induction of death receptor 5 (DR5). The representative Flex-Het SHetA2 effectively inhibited the growth of human lung cancer cells in cell culture and in mice. SHetA2 induced apoptosis, which could be abrogated by silencing caspase-8 expression, indicating that ShetA2 triggers a caspase-8-dependent apoptosis. Accordingly, SHetA2 up-regulated DR5 expression, including cell surface levels of DR5, and augmented tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. Importantly, small interfering RNA (siRNA)-mediated blockade of DR5 induction conferred cell resistance to SHetA2-induced apoptosis, as well as SHetA2/TRAIL-induced apoptosis. These results show that DR5 induction is a key component of apoptosis induced by SHetA2 or by SHetA2 combined with TRAIL. SHetA2 exerted CAAT/enhancer-binding protein homologous protein (CHOP)-dependent transactivation of the DR5 promoter. Consistently, SHetA2 induced CHOP expression, which paralleled DR5 up-regulation, whereas siRNA-mediated blockage of CHOP induction prevented DR5 up-regulation, indicating CHOP-dependent DR5 up-regulation by SHetA2. Collectively, we conclude that CHOP-dependent DR5 up-regulation is a key event mediating SHetA2-induced apoptosis.