Bilirubin-induced cell death during continuous and intermittent phototherapy and in the dark.
OBJECTIVE
To compare continuous and intermittent light exposure in the presence of bilirubin with respect to cellular damage. Furthermore, it was of interest to characterize the nature of cellular toxicity of bilirubin in the dark.


METHODS
A murine lymphoma cell line, L5178Y-R (LY-R), was exposed to solutions of bilirubin (160 microM) supplemented with human serum albumin (200 microM) and irradiated with phototherapy light (Philips 20W/52) at a constant total dose of approximately 500 kJ/m2. The irradiation was given either as intermittent or continuous treatment with light of variable irradiance. The three lower irradiance levels were clinically relevant. Cells treated with bilirubin were also kept in the dark for various periods of time. Cell toxicity was determined by measuring apoptosis and necrosis. Apoptosis was measured by terminal deoxynucleotide transferase and propidium iodide staining assay, while trypan blue assay was used for detection of necrosis.


RESULTS
There was no difference (n = 6, p > 0.05) between continuous and intermittent irradiation in the induction of early and late apoptotic cell death. Necrosis was more pronounced after intermittent treatment. Bilirubin dark toxicity was observed and classified as both apoptotic and necrotic.


CONCLUSIONS
Continuous and intermittent light exposure caused the same degree of apoptotic cell death, while the cells underwent more necrotic death after intermittent exposure. Bilirubin was cytotoxic in the dark by both cell death mechanisms.
Roll
Ellen Bruzell
EB
Norwegian Radiation Protection Authority, Østerås, Norway. ebr@niom.no
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects*; D001663:Bilirubin / Q000494:pharmacology*; D016923:Cell Death / Q000187:drug effects / Q000528:radiation effects; D066298:In Vitro Techniques; D008214:Lymphocytes / Q000187:drug effects / Q000528:radiation effects; D051379:Mice; D018462:Photobiology; D010789:Phototherapy / Q000379:methods*; D011336:Probability; D012307:Risk Factors; D014407:Tumor Cells, Cultured
D001663:Bilirubin