1. ¹Section of Thoracic Oncology, Surgery Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD, USA
2. ²Molecular Imaging Laboratory, Clinical Center, National Institutes of Health, Bethesda, MD, USA
3. ³Department of Thoracic and Cardiovascular Surgery, MD Anderson Cancer Center, Houston, TX, USA
4. ⁴Section of Thoracic Surgery, Division of Cardiothoracic Surgery, DeWitt Daughtry Family Department of Surgery, University of Miami, Jackson Memorial Hospital, Miami, FL, USA

Correspondence: Dr DM Nguyen, Section of Thoracic Surgery, Division of Cardiothoracic Surgery, DeWitt Daughtry Family Department of Surgery, University of Miami, Jackson Memorial Hospital (ET 3072), 1611 NW 12th Street, Miami, FL 33134, USA. E-mail: dnguyen4@med.miami.edu

⁵Research Fellow, Howard Hughes Medical Research Institute.

⁶Research Fellow, Clinical Research Training Program, NIH.

Received 23 March 2007; Revised 11 October 2007; Accepted 10 November 2007; Published online 29 February 2008.

Abstract

Despite adequately expressing tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors DR4/DR5, malignant cells are frequently refractory to the cytotoxic effect of this apoptosis-inducing ligand. The susceptibility of cancer cells to TRAIL can be potentiated by cisplatin (CDDP). This study was designed to evaluate the ability of cisplatin to enhance the cytotoxic effect of TRAIL gene therapy using the recombinant adenovirus-mediated tumor-selective expression of membrane-bound green fluorescence protein (GFP)-TRAIL fusion protein (AdVgTRAIL) on thoracic cancer cells and to elucidate the putative mechanisms responsible for this synergistic combination effect. While causing little death of cultured thoracic cancer cells by itself, AdVgTRAIL in combination with CDDP, on the other hand, mediated profound supra-additive cytotoxicity and apoptosis via a strong bystander effect. CDDP/AdVgTRAIL-induced cytotoxicity was completely abrogated either by the pancaspase inhibitor zVAD-fmk or by the selective caspase 9 inhibitor or by transient knockdown of caspase 9 by siRNA, indicating that this process was caspase-mediated and mitochondria-dependent. This was confirmed by the observation that Bcl2 overexpression protected the cells from combination-induced cytotoxicity. Robust activation of caspase 8 activity in combination-treated cells was blocked by overexpression of Bcl2, indicating that caspase 8 activation was secondary to the mitochondria-mediated amplification feedback loop. Combining CDDP with AdVgTRAIL greatly enhances its tumoricidal efficacy in cultured thoracic cancer cells in vitro. The two agents interact to mediate profound activation of caspase cascade via recruitment of the mitochondria and positive feedback loop. The CDDP/AdVgTRAIL combination also exhibits a strong antitumor effect in in vivo animal model of human cancer xenografts.