Caloric restriction increases ratio of estrogen to androgen receptors expression in murine ovaries

Estrogens and androgens are involved in the development and normal functioning of
the ovaries. Estrogen, which is mainly synthesized in the ovary, is also involved
in the development and functioning of other components of the female reproductive
system. It is primarily expressed by preovulatory follicles under the influence of
FSH 1]. In healthy premenopausal women, the main estrogen, 17?-estradiol, is produced in
the ovaries. In postmenopausal women and in men, 17?-estradiol is produced in extra
gonadal sites, mainly by aromatization of circulating testosterone in adipose tissue
2], 3]. The main function of estrogen in females is to control the estrous cycle and regulate
development and maturation of the reproductive tract. Ovarian-produced estrogens are
essential for cell proliferation, follicular development, and growth as well as follicular
atresia. Estrogens also augment the action of follicle-stimulating hormone (FSH),
increase responsiveness of ovaries to gonadotropins, increase aromatase activity,
and estrogen synthesis 1]–5].

The biological effects of estrogen are mediated by two nuclear estrogen receptors
(ERs): ER? and ER?. In the ovary, ER? expression is predominant. In immature and adult
animals, ER? is found in the nuclei of granulosa cells of primary, secondary, and
mature follicles, and a weak immunoreactive response is also detected in atretic follicle
granulosa cells. Immunoreactivity for ER? was also found in theca cells, luteal cells,
interstitial cells, ovarian surface epithelium, and oocytes. By contrast, ER? is present
in theca cells, interstitial cells, and in ovarian surface epithelium; low expression
is noted in granulosa cells 1], 4], 6], 7]. It is interesting that the number of ER? receptors is significantly reduced in animals
in the nonbreeding season 1]. It has also been observed that, while ER? knockout female mice are infertile and
do not ovulate, ER? knockout female mice display impaired fertility, with reduced
litter numbers and smaller litter size compared with wild type mice 5].

The androgens play an important regulatory role in the ovaries during follicular development
and are responsible for follicle initiation and early growth 1], 8]. The main androgen, androstendione, is synthesized from progestagens in theca cells
under the influence of luteinizing hormone (LH) 9]–11]. The androgens appear to be capable of improving the early stages of folliculogenesis
in ovaries. In mammals, including in human, androgens are converted into estrogen,
which regulates follicular development via binding to ERs during the reproductive
life span. In aged mammals, only a small portion of androgens is converted into estrogens,
and the androgens predominantly bind to the androgen receptor (AR) and may affect
follicular development 8]–13].

The ovarian AR is localized mainly in granulosa cells but is also found in oocytes,
theca, and interstitial cells 7], 14]–16]. The follicles in the early stages of development express a larger number of AR proteins
than those in more advanced stages. It is significant that expression of AR mRNA increases
as the follicles make the transition from primordial to preantral follicles, and the
expression of AR is highest in preantral follicles, gradually decreasing as the follicles
mature. The AR and its mRNA are developmentally regulated and are down-regulated during
FSH-stimulated preovulatory follicular development 7], 8], 14]–17]. Granulosa cells in preantral and antral follicles constitute a crucial site for
AR-mediated action involved in maintaining follicle and embryo survival and optimal
female fertility. The expression of the AR is higher in the ovaries of old female
animals and postmenopousal women than in reproductive age women and animals in nonbreeding
season 1], 7], 8], 12]–19]. In seasonally breeding animals, androgens play a crucial role in the transition
from the breeding to the nonbreeding season and regulate follicular atresia. Animals
in the breeding season exhibit all stages of follicular development, but in the nonbreeding
season only preantral follicles are present 1].

It is well known that nutrition and energy metabolism influence reproductive function
20]. In human and other mammals, malnutrition delays the onset of puberty and leads to
ovulation problems and embryonic mortality. Nutrients and metabolic hormones, including
insulin and insulin-like growth factor 1 (IGF-1), are needed to maintain normal function
of the hypothalmus–pituitary axis and also to maintain the reproductive cells in the
gonads 20]–23].

Caloric restriction (CR) is one of the factors that extends life span, and it minimizes
the age-related dysfunction of many organs, including those of the reproductive system
24], 25]. It is known that decreased food intake leads to slower growth but also increased
longevity 21], 24], 26]. Selesniemi et al. 22] showed that CR delays sexual maturation, extends female fertile lifespan, and leads
to maintenance of the ovarian follicle reserve. Gutierrez et al. 27] showed that an increase in dietary intake leads to increased recruitment of small
follicles during the first follicular wave of the estrous cycle but not to follicle
selection and dominance. Armstrong et al. 28] observed a dietary-induced increase in aromatase activity in small follicles. Nelson
et al. 29] reported that CR delays the age-related loss of cycling capacity, the age-related
increase in cycle length, and the age-related loss of primordial follicles 29].

Our recent morphological study 30] showed that the ovaries of 2.5-year-old wild type (WT) mice on CR appeared to be
younger than 2.5-year-old wild type mice fed ad libitum. While ovaries of WT mice on CR still contained some follicles at different stages
of development, ovaries in 2.5-year-old WT mice fed ad libitum were depleted of follicles. Similarly, we observed that the ovaries of 10-month-old
female mice on CR had higher numbers of primordial, primary, and preantral follicles
than the ovaries of mice fed ad libitum25]. We also examined male mice on CR and we observed a ~25 % reduction in body weight
in male mice on CR 25].

To better explain these observations, we examined the expression of ER and AR receptors
in the ovaries of mice on CR compared with mice fed ad libitum.