Low-Dose Radiation Exposure with 56MnO2 Powder Changes Gene Expressions in the Testes and the Prostate in Rats

To investigate the biological effects of internal exposure of radioactive ⁵⁶MnO₂ powder, the major radioisotope dust in the soil after atomic bomb explosions, on male reproductive function, the gene expression of the testes and the prostate was examined. Ten-week-old male Wistar rats were exposed to three doses of radioactive ⁵⁶MnO₂ powder (41-100 mGy in whole body doses), stable MnO₂ powder, or external ⁶⁰Co γ-rays (2 Gy). Animals were necropsied on Days 3 and 61 postexposure. The mRNA expressions of testicular marker protein genes and prostatic secretory protein genes were quantified by Q-RT-PCR. On Day 3 postexposure, the testicular gene expressions of steroidogenesis-related enzymes, Cyp17a1 and Hsd3b1, decreased in ⁵⁶MnO₂-exposed groups. Germ cell-specific Spag4 and Zpbp mRNA levels were also reduced. On postexposure Day 61, the Cyp11a1 gene expression became significantly reduced in the testes in the group exposed to the highest dose of ⁵⁶MnO₂, while another steroidogenesis-related StAR gene mRNA level reduced in the ⁶⁰Co γ-rays group. There were no differences in Spag4 and Zpbp mRNA levels among groups on Day 61. No histopathological changes were observed in the testes in any group following exposure. Expression in the prostatic protein genes, including CRP1, KS3, and PSP94, significantly decreased in ⁵⁶MnO₂-exposed groups as well as in the ⁶⁰Co γ-rays group on Day 61 postexposure. These data suggest that the internal exposure to ⁵⁶MnO₂ powder, at doses of less than 100 mGy, affected the gene expressions in the testis and the prostate, while 2 Gy of external γ-irradiation was less effective.

Solid cancer mortality risk among a cohort of Hiroshima early entrants after the atomic bombing, 1970-2010: implications regarding health effects of residual radiation

There are two types of exposure to atomic bomb (A-bomb) radiation: exposure to initial radiation released at the time of the detonation of the bomb, and exposure to residual radiation, which remains afterwards. Health hazards caused by exposure from residual radiation have not yet been clarified. The purpose of our study was to reveal the relationships between mortality risk from solid cancer and residual radiation based on data from the early entrants to Hiroshima. It is hard to identify the individual residual radiation doses. However, these are assumed to depend on the date of entry and the entrants' behavior. Individual behavior is thought to be closely related to gender and age at exposure. We investigated a cohort of 45 809 individuals who were living in Hiroshima Prefecture on 1 January 1970 and were registered on the Database of Atomic Bomb Survivors as entrants after the bombing. Poisson regression methods were used to estimate excess relative risks (ERR) with data cross-classified by sex, age at entry, and date of entry. In males in their 20s, 30s, and 40s at entry and in females less than 10 years old and in their 40s at entry, solid cancer mortality risks were significantly higher among persons who entered the city on the day of the bombing than those who entered three or more days later. With adjustments for the age-dependent sensitivities to radiation exposure, it was extrapolated that middle-aged people who entered the city on the day of the bombing were exposed to higher levels of residual radiation than younger people.

Hepatic Gene Expression Changes in Rats Internally Exposed to Radioactive 56MnO2 Particles at Low Doses

We have studied the biological effects of the internal exposure to radioactive manganese-56 dioxide (⁵⁶MnO₂), the major radioisotope dust found in soil after atomic bomb explosions. Our previous study of blood chemistry indicated a possible adverse effect of ⁵⁶MnO₂ on the liver. In the present study, we further examined the effects on the liver by determining changes in hepatic gene expressions. Male Wistar rats were exposed to ⁵⁶MnO₂ particles (three groups with the whole-body doses of 41, 90, and 100 mGy), stable MnO₂ particles, or external ⁶⁰Co γ-rays (2 Gy), and were examined together with the non-treated control group on postexposure day 3 and day 61. No histopathological changes were observed in the liver. The mRNA expression of a p53-related gene, the cyclin-dependent kinase inhibitor 1A, increased in ⁵⁶MnO₂ as well as in γ-ray irradiated groups on postexposure day 3 and day 61. The expression of a stress-responsive gene, nuclear factor κB, was also increased by ⁵⁶MnO₂ and γ-rays on postexposure day 3. However, the expression of cytokine genes (interleukin-6 or chemokine ligand 2) or fibrosis-related TGF-β/Smad genes (Tgfb1, Smad3, or Smad4) was not altered by the exposure. Our data demonstrated that the internal exposure to ⁵⁶MnO₂ particles at less than 0.1 Gy significantly affected the short-term gene expressions in the liver in a similar manner with 2 Gy of external γ-irradiation. These changes may be adaptive responses because no changes occurred in cytokine or TGF-β/Smad gene expressions.

Gene Expression Changes in the Spleen, Lungs, and Liver of Wistar Rats Exposed to β-Emitted 31SiO2 Particles

To understand the biological effects of residual radioactivity after the atomic bomb explosion in Hiroshima and Nagasaki, we previously investigated the effects of 56Mn, a major residual radioisotope. Our rat study demonstrated that inhalation exposure to 56MnO2 microparticles affected gene expression in the lungs, testes, and liver, despite the low radiation doses. Because 56Mn is a β- and γ-emitter, the differential effects between β- and γ-rays should be clarified. In this study, 31Si, a β-emitter with a radioactive half-life similar to that of 56Mn, was used to determine its effects. Male Wistar rats were exposed to sprayed neutron-activated 31SiO2 microparticles, stable SiO2 microparticles, or X-rays. The animals were examined on days 3 and 14 after irradiation. The expression of radiation-inducible marker genes, including Ccng1, Cdkn1a, and Phlda3, was measured in the spleen, lungs, and liver. Furthermore, the expressions of pathophysiological marker genes, including Aqp1, Aqp5, and Smad7 in the lungs and Cth, Ccl2, and Nfkb1 in the liver, were determined. Impacts of 31SiO2 exposure were observed mainly in the liver, where the expression of Cth markedly increased on post-exposure days 3 and 14. Our data suggest that internal exposure to β-emitted microparticles has significant biological effects and its possible roles as residual radiation after atomic bombing.