Molecular mechanism of steroid hormone action during aging. A review

Soy isoflavones and bone health: a double-edged sword?

Numerous publications and research studies on isoflavones have prompted a nationwide increase in the consumption of soy-based foods and supplements in the United States. Isoflavones are natural endocrine active compounds generally considered to promote health and prevent or slow the onset of certain chronic diseases such as osteoporosis. The beneficial effects of soy isoflavones on bone may, however, be life-stage specific and dependent on the estrogen receptor number and endogenous hormone milieu. Perimenopausal and early menopausal women may therefore be more receptive to the therapeutic effects of isoflavones on bone loss prior to the diminution of estrogen receptors that occurs in the postmenopausal years, whereas laboratory studies in developmental age range animals have demonstrated the potential for adverse effects following exposure to high levels of soy isoflavones. Clinical studies in developing humans that either support or refute findings in animal studies are lacking. The effects of chronic consumption of high levels of soy isoflavones at each life stage to assess risk-benefit ratios should be a high priority of research.

Age- and sex-related expression of norbin in the brain cortex of mice

Norbin is a novel neuron specific protein that extends the neurites of neuronal cells. It is expressed in neural tissues like brain cortex, hippocampus, spinal cord and cerebellum. In this paper, we have studied the expression of norbin mRNA and protein in the brain cortex of male and female mice of different ages. Northern blot analysis showed that the level of norbin mRNA increased in both sexes during aging. However, Western blotting revealed that the protein increased in male but decreased in female with advancing age. These findings suggest that norbin is involved in brain function which is dependent on age and sex.

Sex steroids modulate the synthesis and phosphorylation of proteins in the brain cortex of aging mice

We have analysed the synthesis and phosphorylation of total cellular proteins and their modulation by sex steroids (testosterone and 17beta-estradiol) in the brain cortex of adult (25-28 weeks) and old (54-58 weeks) male and female AKR mice. The level of (35S) methionine incorporation in total proteins is comparatively higher in males than females. It declines significantly in older males but shows no difference with age in females. After gonadectomy, the extent of (35S) methionine incorporation decreases in adults but not in the old. The incorporation is induced remarkably by estradiol in males and by both sex steroids in females. Further analysis by fluorography shows several proteins, but only a few (66, 45 and 29 kDa) vary in levels significantly with age, sex and hormonal treatment. The data on phosphorylation of total cellular proteins by (32P) orthophosphate incorporation exhibit no age-dependent variation. However, it is reduced drastically by gonadectomy in adults. After the addition of testosterone, the extent of phosphorylation is enhanced significantly in adults but remains the same in the old of both sexes. Estradiol also increases this modification remarkably in males of both ages and adult females, but shows no effect in old females. These results suggest that both testosterone and estradiol modulate the synthesis and phosphorylation of brain cortex proteins in age- and sex-dependent manner. This leads to alterations in physiological activities of the animal.

Expression of 112-kDa estrogen receptor in mouse brain cortex and its autoregulation with age

The mouse brain cortex expresses ER whose size is larger than uterus and level changes with age and hormonal treatment. Western blot analysis shows the presence of 66 kDa ER in uterus but 112 and 116 kDa ER in the brain. Whereas 116 kDa ER is phosphorylated and predominantly present in nuclei, 112 kDa ER is unphosphorylated and present in both cytosol as well as nuclei. Similar to uterus, the level of ER decreases drastically in the brain cortex of old mice. Hormonal regulation of ER shows age-specific pattern. Ovariectomy reduces the level of ER in adult but increases it significantly in old. While testosterone does not alter the level of ER in either age, estradiol decreases it considerably in old but shows no detectable difference in adult. Our results provide evidence for the expression of a novel form of ER in mouse brain cortex and its regulation by sex steroid hormones in age-specific manner.

Estrogen-induced synthesis of uterine proteins declines during aging

The synthesis of total cellular as well as acid-soluble nuclear proteins and estrogen receptor is high in the uteri of young (22 weeks) and decreases to half in old (104 weeks) rats. Administration of estrogen induces the synthesis of these proteins significantly in young but shows no remarkable effect in old rats. Interestingly, a specific cytosolic protein of 45 kDa is stimulated about two-fold after estrogen injection in young but not in old rats. These findings further establish the reduced responsiveness of uterus to estrogen in old age.

Analysis in vitro of uterine estrogen receptor conformation of young and old rats

The conformation of estrogen receptor (ER) and its in vitro transformation by RNase, Urea and ATP were analysed using the uteri of young (16 weeks) and old (92 weeks) rats. Following the digestion of ER with proteolytic enzymes like trypsin and chymotrypsin and the analysis of cleaved fragments by SDS-PAGE, similar pattern is observed in both ages. In vitro transformation of ER by RNase, Urea and ATP shows that the degree of transformation is lower in old than young. Furthermore, the transformed ER from old is less capable of binding to DNA than that from young. Thus our results show that the conformation of ER probably does not change with age, but the degree of transformation and the ability of transformed receptor to bind to DNA decrease with age.

Homoeostatic imbalance during cellular ageing: altered responsiveness

The inability of normal cells to maintain themselves for ever is a reflection of homoeostatic imbalance and a progressive failure of maintenance. Ageing cells respond less to growth stimulants whereas they show increased sensitivity to toxic agents including antibiotics, phorbol esters, radiation and other physical stresses. No major quantitative and qualitative defects in the receptor systems have been detected that could explain the reasons for altered responsiveness during ageing. Random metabolic defects in the processes involved in maintaining homoeostasis may be critical for causing homoeostatic imbalance, cellular ageing and death.

Effect of age on physico-chemical properties of the uterine nuclear estrogen receptors of albino rats

Estrogen receptors (ER) were extracted with high ionic strength buffer from the nuclei of uteri of young (21 weeks) and old (89 weeks) rats. Following the analysis of these receptors on a sucrose gradient and a Sephadex column, two peaks representing the two forms of receptors were obtained. The minor peak sedimented at 6.8 S with Stokes radius (RS) = 7.2 nm, molecular weight (Mr) = 204 K and frictional ratio (f/f0) = 1.71. On the other hand, the major peak sedimented at 4.1 S with RS = 3.3 nm, Mr = 57 K and f/f0 = 1.18. These properties of nuclear ER were similar in both ages. Also, the half life of ER complexes from both ages at 25 degrees C and 37 degrees C were 135 min and 30 min, respectively. However, these complexes were retained for longer periods in the nuclei of young than old rats. Furthermore, the dissociation constant of the binding of nuclear receptors to estrogen remained constant, but the number of binding sites decreased from 1.56 in young to 1.05 pmol/mg DNA in the old. In young rats, about 61% of nuclear receptors bound to DNA-cellulose. Out of this 2/5 was eluted with 0.15 M and the remaining 3/5 with 0.5 M KCl. On contrary, only 37% of total receptors bound to DNA-cellulose in the old. Out of this 3/5 was eluted with 0.15 M and the remaining 2/5 with 0.5 M KCl. These data suggested that despite the similarity in different physicochemical properties, the number of estrogen binding sites and the retention time of ER complexes in nuclei and the ability of these complexes to bind to DNA decrease in the uterus of old age.

Effect of post-synthetic modifications of proteins on the binding of estrogen-receptor complex to uterine nuclei of aging rats

The binding of estrogen-receptor (ER) complex to nuclei following post-synthetic modifications of proteins was examined in the uteri of young (18 weeks) and old (96 weeks) rats. Acetylation decreases the binding of ER complex to nuclei but methylation shows no effect on the extent of binding in both ages. On the other hand, phosphorylation enhances the binding of ER complex by two-fold in nuclei from young rats but reduces this to half in nuclei from old rats. The pattern of binding in salt-resistant nuclear fractions is similar to that in total nuclei except in methylation where old rats show about 20% higher binding as compared to the respective control. These findings suggest that post-synthetic modifications of proteins modulate the binding of ER complex to uterine nuclei in an age-specific manner.