| Summary: | The non-steroidal anti-inflammatory drugs (NSAIDs) are known to have chemopreventative properties against colorectal cancer. Recent evidence suggests the mechanism of action is not through inhibition of cyclo-oxygenase (COX), as previously thought. We hypothesised that the NSAIDs could be acting through the polyamine biosynthetic pathway. Polyamines play an important role in the development of cancer at all stages from initiation through to maintenance of the transformed phenotype. The aim of this study was to investigate if the polyamines play a role in the cytotoxicity of the NSAIDs in human colorectal cancer cells in vitro. All four drugs (salicylate, sulindac, naproxen and SC'236) displayed cytotoxicity in vitro in three human colorectal cancer cell lines (HT-115, DLD-1 and HCT-15) with comparable IC50 concentrations. All NSAIDs decreased cell growth in the COX-null HCT-5 cells. Sulindac and naproxen caused growth inhibition at low doses but induced apoptosis at higher (>IC50) doses. Salicylate only induced apoptosis in HCT-15 cells and only naproxen and SC'236 induced caspase-3 activity. All NSAIDs depleted intracellular polyamine concentrations to 35-60% of control. Ornithine decarboxylase (ODC) activity was increased in HCT-15 cells but inhibited in DLD-1 cells. Polyamine export was increased in response to NSAID treatment and this was accompanied a 340-530% increase in spermidine/spermine N1-acetyltransferase (SSAT) activity and up to a 500% rise in polyamine oxidase (PAO) activity. Spermidine prevented the growth inhibitory effects of salicylate in HCT-15 cells and inhibited the induction of apoptosis caused by all NSAIDs. Polyamine depletion by pre-treatment with a-difluoromethylornithine (DFMO) decreased the effectiveness of the NSAIDs but did not affect the apoptosis induced. Overall this study highlights several key changes in polyamine metabolism that occurred in response to NSAID treatment. It is our conclusion that polyamines provide an alternative COX-independent pathway for the cytotoxicity of the NSAIDs in vitro, although further work to fully elucidate their role is required.
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