Epidemiological studies report a negative association between circulating
bilirubin concentrations and the risk for
cancer and
cardiovascular disease. Structurally related
tetrapyrroles also possess in vitro anti-genotoxic activity and may prevent mutation prior to
malignancy. Furthermore, few data suggest that
tetrapyrroles exert
anti-carcinogenic effects via induction of cell cycle arrest and apoptosis. To further investigate whether
tetrapyrroles provoke DNA-damage in human
cancer cells, they were tested in the single cell gel electrophoresis assay (SCGE). Eight
tetrapyrroles (unconjugated
bilirubin,
bilirubin ditaurate,
biliverdin,
biliverdin-/
bilirubin dimethyl ester,
urobilin,
stercobilin and
protoporphyrin) were added to cultured Caco2 and HepG2 cells and their effects on comet formation (% tail
DNA) were assessed. Flow cytometric assessment (apoptosis/
necrosis, cell cycle, intracellular radical species generation) assisted in revealing underlying mechanisms of intracellular action. Cells were incubated with
tetrapyrroles at concentrations of 0.5, 5 and 17μM for 24h. Addition of 300μM tertiary-butyl
hydroperoxide to cells served as a positive control.
Tetrapyrrole incubation mostly resulted in increased DNA-damage (comet formation) in Caco2 and HepG2 cells.
Tetrapyrroles that are concentrated within the intestine, including
protoporphyrin,
urobilin and
stercobilin, led to significant comet formation in both cell lines, implicating the compounds in inducing DNA-damage and apoptosis in
cancer cells found within organs of the digestive system.