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Cyclooxygenase-2 Inhibition Induces Apoptosis Signaling via Death Receptors and Mitochondria in Hepatocellular Carcinoma

¹ Institute of Pathology and ² Department of General Surgery, University of Heidelberg; ³ Department of Internal Medicine IV, Hepatology and Gastroenterology and ⁴ Department of Internal Medicine I, Endocrinology and Metabolism, University Hospital Heidelberg; ⁵ Division of Apoptosis Regulation-D040 and ⁶ Tumor Immunology Program, German Cancer Research Center (DKFZ), Heidelberg, Germany; ⁷ Center for Molecular Medicine (ZMMK), University of Cologne, Cologne, Germany: ⁸ Department of Medicine I, Johannes Gutenberg University, Mainz, Germany; and ⁹ Department of Internal Medicine III, University Hospital Aachen, Aachen, Germany

Requests for reprints: Martina Müller, Department of Internal Medicine IV, Hepatology and Gastroenterology, University Hospital Heidelberg, Im Neuenheimer Feld 410, 69120 Heidelberg, Germany. Phone: 49-6221-5638795; Fax: 49-6221-564395; E-mail: martina_mueller-schilling{at}med.uni-heidelberg.de.

Inhibition of cyclooxygenase (COX)-2 elicits chemopreventive and therapeutic effects in solid tumors that are coupled with the induction of apoptosis in tumor cells. We investigated the mechanisms by which COX-2 inhibition induces apoptosis in hepatocellular carcinoma (HCC) cells. COX-2 inhibition triggered expression of the CD95, tumor necrosis factor (TNF)-R, and TNF-related apoptosis-inducing ligand (TRAIL)-R1 and TRAIL-R2 death receptors. Addition of the respective specific ligands further increased apoptosis, indicating that COX-2 inhibition induced the expression of functional death receptors. Overexpression of a dominant-negative Fas-associated death domain mutant reduced COX-2 inhibitor-mediated apoptosis. Furthermore, our findings showed a link between COX-2 inhibition and the mitochondrial apoptosis pathway. COX-2 inhibition led to a rapid down-regulation of myeloid cell leukemia-1 (Mcl-1), an antiapoptotic member of the Bcl-2 family, followed by translocation of Bax to mitochondria and cytochrome c release from mitochondria. Consequently, overexpression of Mcl-1 led to inhibition of COX-2 inhibitor-mediated apoptosis. Furthermore, blocking endogenous Mcl-1 function using a small-interfering RNA approach enhanced COX-2 inhibitor-mediated apoptosis. It is of clinical importance that celecoxib acted synergistically with chemotherapeutic drugs in the induction of apoptosis in HCC cells. The clinical relevance of these results is further substantiated by the finding that COX-2 inhibitors did not sensitize primary human hepatocytes toward chemotherapy-induced apoptosis. In conclusion, COX-2 inhibition engages different apoptosis pathways in HCC cells stimulating death receptor signaling, activation of caspases, and apoptosis originating from mitochondria. (Cancer Res 2006; 66(14): 7059-66)

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