Abstract
OBJECTIVE: To examine whether pharmacologically relevant zinc-binding agents are capable of depleting X-linked inhibitor of apoptosis protein in tumor cells. Our prior work reveals that treatment with zinc-chelating agents induces selective downregulation of the X-linked inhibitor of apoptosis protein in cancer cells of various origins. A precursor of the heme synthetic pathway, 5-aminolevulinic acid, is metabolized to protoporphyrin IX, which is highly reactive with zinc. We assessed whether modified versions of 5-aminolevulinic acid with lipophilic side chains can enhance efficacy and selectivity with respect to protoporphyrin IX accumulation, X-linked inhibitor of apoptosis protein depletion, and tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis in human castration-resistant prostate cancer cells. METHODS: Seven modified versions of 5-aminolevulinic acid (5 esters and 2 amides) were synthesized. Levels of endogenous protoporphyrin IX were examined by flow cytometry. X-linked inhibitor of apoptosis protein expression was examined by Western blotting. terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling assay was used to assess cell apoptosis. Results were compared qualitatively. RESULTS: Accumulation of endogenous protoporphyrin IX by castration-resistant prostate cancer cells was shown to be directly related to the carbon chain length of the esterified 5-aminolevulinic acid derivatives. In fact, treatment with 5-aminolevulinic acid-HE was superior to that achieved by 5-aminolevulinic acid with respect to X-linked inhibitor of apoptosis protein downregulation. 5-aminolevulinic acid and 5-aminolevulinic acid-HE in combination with tumor necrosis factor-related apoptosis-inducing ligand significantly enhanced apoptotic cell death in castration-resistant prostate cancer cell lines. CONCLUSION: Esterified derivatives of 5-aminolevulinic acid alone or in combination with other agents may provide therapeutic opportunities in the treatment of castration-resistant prostate cancer by harnessing apoptotic pathways that are triggered by cellular zinc imbalance.