Calorie restriction in radiation-exposed mice affects the expression of autophagy-related protein p62

Calorie restriction (CR) is known to decrease not only spontaneous but also chemical- and radiation-induced carcinogenesis. However, the mechanism of anti-cancer effect by CR is extremely complex and not fully understood. In this study, we focused on changes in autophagy-related proteins (LC3 and p62) as factors related to changes in nutritional status and the suppression of carcinogenesis by CR. B6C3F1 male mice were exposed to 0 Gy or 3.8 Gy X-rays at 1 week of age, and then both groups were fed either a standard diet (95 kcal/week) or a calorie-restricted diet with approximately 30% fewer calories (65 kcal/week) from 7 weeks of age. Mice were sacrificed at several time points, and their livers were collected and analyzed. CR suppressed the development of liver tumors in the irradiated group during the study period. The expression level of LC3 did not change significantly with or without CR and irradiation. On the other hand, the expression of p62 was significantly and temporarily increased in the irradiated CR group, in which radiation carcinogenesis was suppressed. Using immunofluorescence analysis, we found cells expressed p62 strongly throughout the cytoplasm in the CR group. In some of the p62-strong positive cells, the disappearance of the cell nucleus and a decrease in the nuclear DNA content were observed, and it was not co-localized with the cell proliferation marker, suggesting that cell death had occurred. Long-term CR, which inhibits radiation carcinogenesis, did not significantly affect autophagy activity in the liver as determined by LC3-II levels. On the other hand, increased expression of p62 was observed in the CR group in which the anti-cancer effect was confirmed. In particular, p62 strongly positive cells with smaller or no nuclei were detected exclusively in the CR group, suggesting a novel p62-mediated mechanism in the suppression of radiation-induced liver carcinogenesis by CR.