Caloric restriction perturbs the pituitary-ovarian axis and inhibits mouse mammary tumor virus production in a high-spontaneous-mammary-tumor-incidence mouse strain (C3H/SHN).
Dietary restriction (DR) retards aging and extends maximum life span. It is also known to decrease the incidence of hormone-dependent tumors. In the present investigation, we focussed primarily on the influence of DR on the pituitary-ovarian axis, and subsequently on gene expression of the mouse mammary tumor virus. F1 females from the mating of SHN female and C3H male mice were used in this study, since these hybrids display a very high incidence of mouse mammary tumors. The mice weaned at 3 weeks were raised on either a calorically-restricted diet (DR: 50 kcal/week; N = 5) or on a control diet (C: 95 kcal/week N = 5) for 5 weeks. Three C57BL/6J Jcl ad libitum-fed female mice, 8 weeks of age, were used as reference animals since this strain has a very low incidence of mammary tumors. The mean cellular contents of prolactin (PRL) and growth hormone (GH) in the pituitary, as determined by immunohistochemistry, were found to be reduced in mice raised on the DR diet. The decrease in the mean cellular content of PRL (50% of the mean control value) was accompanied by a decrease in the number of lactotrophs (17% of the mean value of control diet mice). However, the decrease in cellular content of GH (53% of the mean control value) was not accompanied by a decrease in the number of somatotrophs (no. of somatotrophs in DR = 103% of mean control value). Histologically, ovaries from DR mice showed many growing and atretic follicles, with few corpora lutea. In contrast, both control-diet and reference (C57BL/6J Jcl) mice showed two or three corpora lutea per ovarian section. In accordance with this finding, DR mice had not established stable estrus cycles by 8 weeks of age, in contrast to both control-diet and reference mice. Since caloric restriction has been shown to decrease mammary tumor virus (MMTV) gene expression, MMTV production was investigated by electron microscopy to confirm the validity of our experimental conditions. In DR or reference C57BL/6J Jcl mice, MMTV particles were rarely found in the mammary gland samples, but were always found in samples from control mice. The development of mammary glands, as indicated by the number of villi or the development of the rough endoplasmic reticulum, was delayed in DR mice. Thus, it was concluded that caloric restriction decreases the number of lactrotrophs, inhibits ovulation and delays mammary gland development. This immature status is considered to be due to perturbations in the pituitary-ovarian axis by caloric restriction.