Poly(ADP-ribose) synthetase activity during bleomycin-induced lung fibrosis in hamsters

Abstract

Bleomycin damages cellular DNA and is a potent inducer of pulmonary fibrosis. It has been shown to act through a superoxide-mediated mechanism. We are interested in determining the biochemical mechanisms involved in fibrosis and in this preliminary study we have examined the temporal relationship between early biochemical events associated with DNA damage and fibrosis, in lungs of hamsters after administration of 0.75 unit of bleomycin. The activities of poly(ADP-ribose) synthetase, an enzyme associated with DNA repair, inducible superoxide dismutase (SOD) and prolyl hydroxylase as well as the tissue levels of NAD + and hydroxyproline in the lung were determined. All three enzyme activities expressed as per milligram DNA or per lung, increased upon bleomycin treatment over the saline-administered controls. Lung poly(ADP-ribose) synthetase activity which is sensitive to DNA breaks, increased first (24% over control in 1 day, P < 0.0001), attained the maximum value on the 5th day (952% over control, P < 0.0001), and started to decline thereafter and approached near the control value on 14th day. Bleomycin treatment induced a rapid change in the level of lung NAD +. After 1 day the level of NAD + was reduced by 42% compared to the control ( P < 0.001), further declined to 65% ( P < 0.001) on the 3rd day, and stayed at that level until the 7th day. On the 14th day, however, the NAD + level was still lower (29%, P < 0.05) but approaching the value in the control animals. The activity of prolyl hydroxylase showed significant increase on the 3rd day (50% over control, P < 0.0001) after bleomycin administration. The enzyme activity continued to increase until the end of the experiment (490% of control, P < 0.0001, on Day 14). The content of undialyzable hydroxyproline, a marker for collagen, was also increased significantly in the lung tissue on the 3rd day (30% over control, P < 0.05), continued to increase and reached the highest level on the 14th day (71% over control, P < 0.001). A significant increase in the activity of SOD (19% over control, P < 0.001) was seen on the 5th day which continued to increase and attained the highest value on Day 14 (115% over control, P < 0.0001). These data indicate that poly(ADP-ribose) synthesis in the lung is increased upon bleomycin administration and suggest that the toxicity of the drug in vivo is mediated through damage to cellular DNA which initiates a DNA repair response in the lung.