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