Induction of apoptosis in human cancer cells by candidaspongiolide, a novel sponge polyketide.
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)-alpha 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 eIF2alpha was sufficient to prevent CAN-induced eIF2alpha phosphorylation and induction of apoptosis but insufficient to prevent inhibition of protein synthesis. CAN induction of eIF2alpha 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 eIF2alpha phosphorylation and apoptosis, respectively. Although CAN inhibited protein synthesis in both cancer cells and normal human fibroblasts, it induced eIF2alpha phosphorylation and apoptosis only in cancer cells.


CONCLUSIONS
CAN triggers PKR/eIF2alpha/caspase 12-dependent apoptosis and inhibits protein synthesis in cancer cells but only inhibits protein synthesis in normal cells.
Trisciuoglio
Daniela
D
Tumor Hypoxia Laboratory, SAIC-Frederick, Inc., National Cancer Institute, Frederick, MD 21702, USA.
Uranchimeg
Badarch
B
Cardellina
John H
JH
Meragelman
Tamara L
TL
Matsunaga
Shigeki
S
Fusetani
Nobuhiru
N
Del Bufalo
Donatella
D
Shoemaker
Robert H
RH
Melillo
Giovanni
G
D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D015153:Blotting, Western; D053454:Caspase 12 / Q000378:metabolism; D045744:Cell Line, Tumor; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation / Q000187:drug effects; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism*; D005434:Flow Cytometry; D005910:Glioma / Q000209:etiology*; D045325:HCT116 Cells; D006801:Humans; D048670:MAP Kinase Kinase 4 / Q000378:metabolism; D018942:Macrolides / Q000494:pharmacology*; D010766:Phosphorylation / Q000187:drug effects; D011161:Porifera; D034741:RNA, Small Interfering; D014162:Transfection; D019892:eIF-2 Kinase / Q000378:metabolism*; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
D000970:Antineoplastic Agents; D015852:Eukaryotic Initiation Factor-2; D018942:Macrolides; D034741:RNA, Small Interfering; C531429:candidaspongiolide; D019892:eIF-2 Kinase; D048051:p38 Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D053454:Caspase 12