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c-Myc–Induced Chemosensitization Is Mediated by Suppression of Cyclin D1 Expression and Nuclear Factor-B Activity in Pancreatic Cancer Cells

Authors' Affiliation: Department of Pathology, Wayne State University School of Medicine, Detroit, Michigan

Requests for reprints: Joshua D. Liao, Hormel Institute, University of Minnesota, 801 16^th Avenue NE, Austin, MN 55912. Phone: 507-437-9665; Fax: 507-437-9606; E-mail: djliao{at}hi.umn.edu.

Purpose: Pancreatic cancer is a highly aggressive disease that remains refractory to various chemotherapeutic agents. Because the proto-oncogene c-myc can modulate apoptosis in response to cytotoxic insults and is commonly overexpressed in pancreatic cancer, we investigated the value of c-myc as a potential modulator of cellular response to various chemotherapeutic agents.

Experimental Design: Stable overexpression or small interfering RNA (siRNA)–mediated knockdown of c-myc and restoration of cyclin D1 were done in the Ela-myc pancreatic tumor cell line. Cell viability after cisplatin treatment of c-myc–overexpressing, control, and siRNA-transfected cells was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and drug-induced apoptosis was measured by DNA fragmentation, sub-G₁, and poly(ADP-ribose) polymerase cleavage analyses. Protein expression profile after cisplatin treatment was determined by Western blotting and DNA binding activity of nuclear factor-B was examined by electrophoretic mobility shift assay.

Results: Ectopic overexpression of c-myc in murine and human pancreatic cancer cell lines, Ela-myc and L3.6pl, respectively, resulted in increased sensitivity to cisplatin and other chemotherapeutic drugs. Increased sensitivity to cisplatin in c-myc–overexpressing cells was due, in part, to the marked increase in cisplatin-induced apoptosis. Conversely, down-regulation of c-myc expression in stable c-myc–overexpressing cells by c-myc siRNA resulted in decreased sensitivity to cisplatin-induced cell death. These results indicate an important role of c-myc in chemosensitivity of pancreatic cancer cells. The c-myc–induced cisplatin sensitivity correlated with inhibition of nuclear factor B activity, which was partially restored by ectopic cyclin D1 overexpression.

Conclusions: Our results suggest that the c-myc–dependent sensitization to chemotherapy-induced apoptosis involves suppression of cyclin D1 expression and nuclear factor B activity.