Radiation-induced apoptosis and gene expression in neonatal kidney and testis with and without protein synthesis inhibition

Purpose: To analyse the incidence of radiation-induced apoptosis, expression of two apoptosis-related genes, Bcl-2 and p53, and post-radiation levels of cell proliferation in the neonatal rat (4-5 days old) kidney and testis. Materials and methods: Apoptosis was quantified in control or treated kidney or testis at 2, 4, 6, 8 and 24 h after 5 Gy of whole body X-irradiation (n = 4 per group). Morphology (light and electron microscopy) and DNA gel electrophoresis were used to assess apoptosis. Temporal and spatial expression of Bcl-2 or p53 were analysed using immunohistochemistry. Administration of cycloheximide (1.5 mg/kg) was used to determine whether new protein synthesis had a role in induction of apoptosis. Tritiated thymidine uptake and autoradiography were used to indicate alterations in cell proliferation (radiolabel administered Ih prior to tissue collection) or S-phase cells undergoing radiation-induced apoptosis (radiolabel administered Ih prior to irradiation). Results: Apoptosis peaked at 4 h in the testis and 6 h in the kidney and was significantly higher in the renal nephrogenic zone than in the testis (p < 0.05). Mitosis was almost completely negated after irradiation in both tissues. A higher proportion (almost fivefold) of the apoptotic cells died in S phase in the kidney than in the testis. Cycloheximide negated induction of apoptosis in the kidney, and markedly decreased apoptosis in the testis. Bcl-2 expression was highest in the differentiated zone of control kidneys and increased after irradiation in the nephrogenic zone, particularly near foci of apoptosis in developing nephrons. In the control testis, Sertoli cells had moderate expression of Bcl-2. After irradiation, there was complete absence of Bcl-2 expression in apoptotic Sertoli cells, with surviving cells increasing Bcl-2 expression. Irradiated kidney had more intense nuclear p53 expression compared with controls. In the testis, p53 that was present in controls continued to be expressed in surviving cells but not apoptotic cells in radiation-treated animals. Conclusions: Unique differences can be identified between the incidence and biomolecular control of radiation-induced apoptosis in the normal neonatal kidney and testis. These results may find application for minimizing damage to these normal neonatal tissues in the development of, for example, cancer treatment regimens