Constitutive Overexpression of Bcl-2, Survivin and ER Stress Chaperone GRP-78 Confers Intrinsic Radioresistance in Human Hepatocellular Carcinoma Cells: Insight into the Mechanistic Pathways Involved

Chih-Hsiung Wu, Yih-Huei Uen, Chun-Te Ho, Ya-Ting Tseng, Tsan-Zon Liu, Jeng-Fong Chiou, Stephen Wan Leung

Cancer Center and Department of Radiation Oncology, Taipei Medical University and Hospital, Taipei, Chinese Taipei.
Department of General Surgery, Chi-Mei Foundation Medical Center, Tainan, Chinese Taipei.
Department of Radiology and Nuclear Medicine, Yuan’s General Hospital, Kaohsiung, Chinese Taipei.
Department of Surgery, Taipei Medical University-Shuang Ho Hospital, Taipei, Chinese Taipei.
Graduate Institute of Medical Biotechnology, Chang-Gung University, Gueishan, Chinese Taipei.
Graduate Institute of Medical Sciences, Taipei Medical University, Taipei, Chinese Taipei.

DOI: 10.4236/jct.2013.42A048                

Abstract

We present evidence here that abundantly expressed b-catenin-triggered NF-kB-dependent upregulation of inducible nitric oxide synthase(iNOS) found in hepatoma Mahlavu cells (RT-resistant variant designated as RR-Mal), but not in Hep 3B cells (RT-sensitive variant designated as RS-3B) is a key element contribrting to the radioresisitance through the activation of two prominent radioprotective pathways. First, high iNOS expression found in RR-Mal, but not in RS-3B cells was found to perturb calcium homeostasis that triggered ER stress response leading to the overproduction of ER chaperone GRP-78 via robust generation of cleaved ATF-6a (50 kDa) subunits and their nuclear translocation. Meanwhile, both abundantly expressed NF-κB and COX-2 found in RR-Mal cells could also provoke an increased production of PGE2 resulting in robust production of Bcl-2. Interestingly, when RR-Mal cells were treated with PDTC (a NF-κB inhibitor) or celecoxib (a COX-2 inhibitor), a concentration-dependent downregulation of Bcl-2 could be demonstrated implying that Bcl-2 overexpression was indeed mediated through NF-κB/Cox-2/PGE2 pathway. Importantly, we also unveiled that siRNA-mediated silencing of survivin in RR-Mal cells could result in a concomitant downregulation of GRP-78 due to a severe inhibition of ATF-6a (50 kDa) expression. Taken together, our data demonstrate that constitutively overexpressed b-catenin/NF-κB/iNOS and NF-κB/COX-2/PGE2 pathways that overproducing GRP-78, survivin and Bcl-2 expressions are responsible for radioresistance acquisition in RR-Mal cells. Thus, both pathways could be served as potential targets for overcoming radioresistance.

Keywords

Radioresistance; Hepatoma Cells; Inducible Nitric Oxide Synthase; Bcl-2; Survivin; GRP-78

Conflicts of Interest

The authors declare no conflicts of interest.

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