Increased radiation-induced apoptosis in mouse thymus in the absence of metallothionein

Metallothionein (MT) has been shown to protect cells from free radical indu ced DNA damage after exposure to copper, hydrogen peroxide and also radiati on. In order to study the role of MT in radiation induced apoptosis, age-ma tched male control mice (C57BL/6J), MT-I overexpressing (MT-I*) and MT-null transgenic mice were exposed to whole body cobalt 60 gamma-irradiation at 0, 5, or 10 Gy, and their thymus were removed 24 h later. The basal levels of MT and zinc concentrations in the thymus were measured by (109)Cadmium-h eme assay and atomic absorption spectrophotometry, respectively. The MT exp ression after radiation was determined by immunohistochemical staining usin g a polyclonal antibody to MT. The extent of apoptosis in thymocytes was de termined by histology (H&E stain). DNA was isolated from the thymus, and DN A fragmentation was determined by agarose gel electrophoresis. The results showed that the basal level of MT protein in MT-I* thymus was 2.4-fold high er than control mice, and that MT was inducible in both MT-I* and control C 57BL6 thymus after radiation exposure. Minimal MT protein was detected in M T-null mice thymus before or after radiation, while, a significantly higher number of apoptotic cells and DNA fragmentation were found in MT-null thym us after whole body irradiation. These data demonstrated a protective role for MT in radiation-induced apoptosis in mouse thymus. (C) 1999 Elsevier Sc ience Ireland Ltd. All rights reserved.