Journal of the National Cancer Institute Advance Access originally published online on August 26, 2008
JNCI Journal of the National Cancer Institute 2008 100(17):1233-1246; doi:10.1093/jnci/djn239
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Published by Oxford University Press 2008.
ARTICLES |
Induction of Apoptosis in Human Cancer Cells by Candidaspongiolide, a Novel Sponge Polyketide
Affiliations of authors: Tumor Hypoxia Laboratory, SAIC-Frederick, Inc. (DT, BU, GM), and Screening Technologies Branch, Developmental Therapeutics Program (JHC, TLM, RHS), National Cancer Institute at Frederick, Frederick, MD; Laboratory of Marine Biochemistry, University of Tokyo, Tokyo, Japan (SM, NF), Experimental Chemotherapy Laboratory, Regina Elena Cancer Institute, Rome, Italy (DT, DDB)
Correspondence to: Giovanni Melillo, MD, DTP-Tumor Hypoxia Laboratory, Bldg 432, Rm 218, National Cancer Institute, Frederick, MD 21702 (e-mail: melillog{at}ncifcrf.gov).
Background: Candidaspongiolide (CAN), a novel polyketide from a marine sponge, is the active component of a mixture that was found to be potently cytotoxic in the National Cancer Institute’s 60-cell-line screen.
Methods: Effects of CAN on U251 glioma and HCT116 colorectal cancer cells and on normal fibroblasts were assessed using radiolabeling studies to measure protein synthesis, clonogenic assays to measure cell survival, flow cytometry of annexin V– and propidium iodide–stained cells to measure apoptosis, and western blots in the presence or absence of specific inhibitors to assess accumulation and phosphorylation of potential downstream target proteins.
Results: CAN inhibited protein synthesis and potently induced apoptosis in both U251 and HCT116 cells, the latter in part by a caspase 12–dependent pathway. For example, 25%–30% of U251 or HCT116 cells became apoptotic after 24 hours of treatment with 100 nM CAN. CAN also rapidly induced sustained phosphorylation of eukaryotic translation initiation factor-2 (eIF2)-
at Ser51 and of the translation elongation factor eEF2 at Thr56, which could contribute to its dose-dependent inhibition of protein synthesis. Stable expression of dominant-negative eIF2 was sufficient to prevent CAN-induced eIF2 phosphorylation and induction of apoptosis but insufficient to prevent inhibition of protein synthesis. CAN induction of eIF2 phosphorylation did not occur by a classic endoplasmic reticulum stress pathway. However, an inhibitor of and small-interfering RNAs to the double-stranded RNA–dependent protein kinase PKR prevented CAN-mediated eIF2 phosphorylation and apoptosis, respectively. Although CAN inhibited protein synthesis in both cancer cells and normal human fibroblasts, it induced eIF2 phosphorylation and apoptosis only in cancer cells.
Conclusions: CAN triggers PKR/eIF2/caspase 12–dependent apoptosis and inhibits protein synthesis in cancer cells but only inhibits protein synthesis in normal cells.
| CONTEXT AND CAVEATS Prior knowledge Candidaspongiolide (CAN), a tedanolide macrolide, was discovered to be the active compound in an extract from marine sponges that killed glioma and melanoma cells in the NCI 60-cell-line screen. The mechanism of cytotoxicity was not known, although related compounds have been reported to inhibit protein synthesis. Study design Cell and molecular assays were done to examine the effects of CAN on protein synthesis and apoptosis pathways in U251 glioma cells and HCT116 colorectal cancer cells, as well as in normal cells. Contribution
CAN inhibited protein synthesis and induced apoptosis in the tested cancer cell lines by a mechanism that included activation of caspase 12 and protein kinase PKR and inhibitory phosphorylation of eukaryotic initiation factor-2 (eIF2)- Implications The in vitro activity of CAN raises the possibility that it will have antitumor activity against tumors in vivo. Limitations Further testing of CAN's safety and efficacy in treating tumors in mouse models will be necessary before its usefulness in humans can be evaluated. From the Editors
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Manuscript received November 5, 2007; revised June 5, 2008; accepted June 13, 2008.
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J Natl Cancer Inst 2008 100: 1191.