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CAAT/Enhancer Binding Protein Homologous Protein–Dependent Death Receptor 5 Induction Is a Major Component of SHetA2-Induced Apoptosis in Lung Cancer Cells

¹ Department of Hematology and Medical Oncology, Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia; ² Cardiothoracic Surgery Department, Daping Hospital, Third Military Medical University, Chongqing, P.R. China; ³ Department of Obstetrics and Gynecology, University of Oklahoma Health Science Center, Oklahoma City, Oklahoma; and ⁴ Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma

Requests for reprints: Shi-Yong Sun, Winship Cancer Institute, Emory University School of Medicine, 1365-C Clifton Road NE, C3088, Atlanta, GA 30322. Phone: 404-778-2170; Fax: 404-778-5520; E-mail: shi-yong.sun{at}emoryhealthcare.org.

Key Words: SHetA2 • apoptosis • death receptor 5 • TRAIL • 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. [Cancer Res 2008;68(13):5335–44]