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Transcriptional Profiling Identifies Altered Intracellular Labile Iron Homeostasis as a Contributing Factor to the Toxicity of Adaphostin: Decreased Vascular Endothelial Growth Factor Secretion Is Independent of Hypoxia-Inducible Factor-1 Regulation

Authors' Affiliations: ¹ SAIC Frederick, Inc., Screening Technologies Branch, Laboratory of Functional Genomics, National Cancer Institute Frederick, Frederick, Maryland and ² Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Rockville, Maryland

Requests for reprints: Anne Monks, SAIC Frederick, Inc., National Cancer Institute Frederick, P.O. Box B, Frederick MD, 21702. Phone: 301-846-5528; Fax: 301-846-6081; E-mail: monks{at}dtpax2.ncifcrf.gov.

Purpose: Adaphostin was developed as an inhibitor of the p210^bcr-abl tyrosine kinase, but as its activity is not limited to tumor cell lines containing this translocation, transcriptional profiling was used as a tool to elucidate additional mechanisms responsible for adaphostin cytotoxicity.

Experimental design: Profiles of drug-induced transcriptional changes were measured in three hematopoietic cell lines following 1 and 10 µmol/L adaphostin for 2 to 6 hours and then confirmed with real-time reverse transcription-PCR (2-24 hours). These data indicated altered iron homeostasis, and this was confirmed experimentally. Alteration of vascular endothelial growth factor (VEGF) secretion through hypoxia-inducible factor-1 (HIF-1) regulation was also investigated.

Results: Drug-induced genes included heat shock proteins and ubiquitins, but an intriguing response was the induction of ferritins. Measurement of the labile iron pool showed release of chelatable iron immediately after treatment with adaphostin and was quenched with the addition of an iron chelator. Pretreatment of cells with desferrioxamine and N-acetyl-cysteine reduced but did not ablate the sensitivity of the cells to adaphostin, and desferrioxamine was able to modulate adaphostin-induced activation of p38 and inactivation of AKT. VEGF secretion was shown to be reduced in cell lines after the addition of adaphostin but was not dependent on HIF-1.

Conclusions: Adaphostin-induced cytotoxicity is caused in part by a rapid release of free iron, leading to redox perturbations and cell death. Despite this, reduced VEGF secretion was found to be independent of regulation by the redox responsive transcription factor HIF-1. Thus, adaphostin remains an interesting agent with the ability to kill tumor cells directly and modulate angiogenesis.

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