KC Ngai, CY Yeung

Bilirubin toxicity was once thought to be an all-or-none phenomenon. Recent reports on cry analysis and brainstem auditory evoked potentials have suggested that the toxicity might be reversible upon the removal of bilirubin by procedures such as exchange transfusions. This study investigated the reversibility of bilirubin cytotoxicity in a tissue culture model. We used glioblastoma as a cultured human cell line to study the effect of bilirubin. A modified MTT [3-(4,5-dimethylthiazol-2-y1)-2,5-diphenyltetrazolium bromide] colorimetric method was used to measure the cell viability. We adopted three study regimes: (1) measuring the cell viability at different times of exposure to bilirubin, (2) adding albumin to increase its binding capacity to recover the bilirubin deposits from the cells, and (3) removing the free bilirubin from the culture system. Removing bilirubin or increasing albumin binding capacity significantly increased cell survival. Addition of albumin resulted in much better cell survival than simple removal of bilirubin. It also significantly prolonged the time required to cause 50% cell death. This observation indicates a reversible process of bilirubin toxicity in the early stage. During the initial period of exposure to bilirubin, there was a gradual increase of cell death with time; beyond a certain time, a drastic reduction of cell survival occurred. Protective effects on cell death by removal of bilirubin or addition of albumin to the system were significantly more obvious in the first hours. The results indicate that bilirubin cytotoxicity is a biphasic phenomenon. The initial phase of toxicity, which was probably at the membrane level, could be reversed. After bilirubin has entered the intracellular compartment, mitochondrial function becomes damaged and at this stage the cytotoxicity is irreversible.