Regulation of neuropeptide gene expression by steroid hormones

Steroid hormones modify several brain functions, at least in part by altering expression of particular genes. Of interest are those genes that are involved in cell-cell communication in the brain, for instance neuropeptide genes and genes that code for enzymes involved in synthesis of neurotransmitters. Steroid regulation of mRNA levels for several genes has been reported, including the genes coding for the neuropeptides vasopressin, corticotropin releasing factor, luteinizing hormone-releasing factor, pro-opiomelanocortin; somatostatin, preproenkephalin, and the enzyme tyrosine hydroxylase. Steroid control of releasing factor genes is consistent with classical neuroendocrine concepts of negative feedback. Steroid-induced plasticity of gene expression is sometimes in evidence, with the presence or absence of a particular steroid inducing expression of a neuropeptide gene in neurons that under other conditions do not express the gene. As a means of gaining some insight into the mechanism of action of steroid hormones, several groups have determined some of the neuropeptide profiles of neurons that contain receptors for steroid hormones. Marked heterogeneity is found, in that often only a subpopulation of phenotypically-similar neurons, even within a single brain area, contains receptors for a given steroid.     

Regulation of apoptosis by steroid hormones

Steroid hormones play major roles in regulation of growth, development, homeostasis, and cell death. Together with other hormones and growth factors, steroids regulate both the function and cellular composition of organs throughout the body. In this article we will discuss the mechanisms of steroid hormone regulated apoptosis. Emphasis will be placed on the effect of glucocorticoids on lymphoid cells.     

Regulation of leptin gene expression and secretion by steroid hormones.

Previous work has shown that 17 beta-estradiol is the primary ovarian signal regulating body weight and adiposity, although its mechanisms of action remain unclear. We hypothesized that 17 beta-estradiol could enhance leptin levels as a mechanism of its anorectic effects. Administration of 5 microg 17 beta-estradiol subcutaneously (s.c.) for 2 days significantly elevated leptin mRNA levels in adipose tissue as compared to vehicle controls (P < 0.003). A time-course administration of estrogen showed increased mRNA levels in adipose tissue between 6 and 12 h after estrogen injection as compared to vehicle controls (P < 0.03). Corresponding to the increased leptin mRNA levels at 6 and 12 h, elevated plasma leptin levels were observed at 12 h after estrogen administration as compared to controls (P < 0.05). Administration of progesterone (1 mg/rat) after estradiol injection did not enhance the elevated leptin mRNA levels in adipose tissue. Serum leptin levels from cycling rats did not differ significantly between metestrous and proestrous animals. In conclusion, the present studies demonstrate that 17 beta-estradiol can regulate leptin gene expression and secretion in the female rat, thus providing a better understanding of the possible anorectic effect of estrogens.     

Demonstration of adrenergic catecholamine receptors in rat myometrium and their regulation by sex steroid hormones.

The molecular basis of sex steroid hormone-modulation of catecholamine-regulated smooth muscle cell contraction in the uterus was investigated at the level of the catecholamine receptor in rat myometrium. Myometrial membrane binding sites for ³H)-dihydroergocryptine bound α-but not β-adrenergic antagonists and stereospecifically bound the α-agonists (?)-norepinephrine > (?)-epinephrine > phenylephrine. Binding sites for (?) (³H)-dihydroalprenolol were specific for β-adrenergic antagonists and stereospecifically bound (?)-isoproterenol > epinephrine ? norepinephrine. These results were consistent with the expected properties of the myometrial α- and β-adrenergic catecholamine receptors. Myometrial content of β- but not α-adrenergic catecholamine receptors was significantly elevated during proestrus and estrus, suggesting a role for sex steroid hormones in the regulation of these receptors. This posibility was substantiated in ovariectomized rats where castration resulted in a reduction in myometrial β-receptor content which was restored in a dose-dependent manner by estrodiol administration. We conclude: 1) rat uterus contains a substantial concentration of α- and β-adrenergic catecholamine receptors, 2) sex steroid hormones may modulated uterine contractility by regulation of these cell surface receptors; 3) modulation of cell responses to surface active hormones and agents by regulation of their cell surface receptors may be a major way in which sex steroids regulate target organ function.     

Regulation of brain adrenergic receptors during aging

Recent studies in our laboratory suggest that the synthesis of alpha- and beta-adrenergic receptors in certain tissues from brain and pineal gland may be impaired with age. This decreased ability of the aged brain to synthesize adrenergic receptors may explain the loss of these receptors in selected brain regions during the aging process, as well as the reduced capacity of aged brain tissue to increase or up-regulate the density of these receptors in response to reduced noradrenergic activation of the tissues or to reduced estrogen levels. The reduced adaptability of brain adrenergic receptors, in turn, may account for the decreased ability of aged individuals to adjust their physiological responses to a changing environment.     

Regulation of leptin by steroid hormones in rat adipose tissue.

We investigated if steroid hormones regulate the secretion and the expression of leptin in female and male rat adipose tissue fragments in vitro. Dexamethasone time and dose-dependently increased the secretion and mRNA expression of leptin with a half-maximal stimulation of approximately 1 nM. A time-course revealed a maximal stimulatory effect of 17 beta-estradiol after 24 hours. In male adipose tissue 17 beta-estradiol increased leptin secretion (32% by 50 nM 17 beta-estradiol, P = 0.07 and 34% by 500 nM 17 beta-estradiol, P < 1780.05) after 24 hours. An additional effect of estrogen was seen in the dexamethasone (50 nM) stimulated cells (38% with 50 nM 17 beta-estradiol, P < 0.05 and 48% by 500 nM 17 beta-estradiol, P < 0.05). Basal secretion of leptin was equal in female and male adipose tissue, whereas the effects of 17 beta-estradiol (50 nM) and dexamethasone were significantly increased in female as compared with male adipose tissue. Progesterone, testosterone, dihydrotestosterone and dehydroepiandrostendione-sulfate neither affected leptin secretion in male nor female adipose tissue in vitro. Furthermore, to investigate the effect of estrogen female rats were ovariectomized (OVX) and the adipose tissue was incubated in vitro and compared with adipose tissue leptin secretion from sham operated rats (SHAM), and with ovariectomized rats treated with 17 beta-estradiol (EST). A decreased basal and dexamethasone-stimulated leptin secretion from OVX rats compared with SHAM rats was found (P < 0.005) whereas 17 beta-estradiol treatment of ovariectomized rats maintained a normal leptin secretion. However, the dexamethasone stimulation was equally increased above basal levels in SHAM, OVX and EST rats (3.7 +/- 1.2, 2.9 +/- 0.8, 4.2 +/- 1.4, NS, ANOVA) respectively.     

Regulation of calcitonin gene-related peptide expression in dorsal root ganglia of rats by female sex steroid hormones

Calcitonin gene-related peptide (CGRP), a potent vasodilator primarily synthesized in dorsal root ganglia (DRG) neurons, has been shown to decrease vascular resistance and thus regulate blood flow to a variety of organs in rats. Serum CGRP levels in the human have been reported to increase with pregnancy and decrease postpartum. It has been suggested that female sex steroid hormones play a role in cardiovascular function, but the mechanisms are unknown. In this study, we examined the effects of estradiol-17beta (E(2)) and progesterone (P(4)) on the expression of CGRP in DRG in adult rats both in vivo and in vitro. Ovariectomized (ovx) animals were injected s.c. with 5 microg E(2), 4 mg P(4), or 5.0 microg E(2) + 4 mg P(4) in 0.5 ml sesame oil or with oil only, and groups of 4 rats were killed at 0, 24, or 48 h. DRGs were then removed and analyzed for CGRP mRNA and immunoreactive (i-)CGRP content by Northern blotting and RIA, respectively. Primary cultures of DRG neurons from adult female rats were used to assess the effects of varying doses of E(2) (1, 10, 100 nM), P(4) (10, 100, 1000 nM), or E(2) (10 nM) + P(4) (100 nM) in the absence or presence of nerve growth factor (NGF; 20 ng/ml); and CGRP mRNA content in the cells and i-CGRP in the medium were quantitated at 24 or 48 h after incubation. Results of in vivo studies showed that E(2) caused a significant increase in CGRP mRNA at 24 h (1.8-fold) and in i-CGRP levels both at 24 h (2. 8-fold) and at 48 h (3.4-fold) in DRG of ovx rats. P(4) also stimulated expression of both CGRP mRNA and i-CGRP. In the in vitro studies, either E(2) or P(4) alone or the two in combination were without effect on CGRP expression in cultured DRG neurons at all the doses tested. However, in the presence of NGF, both CGRP mRNA and peptide levels were significantly enhanced by E(2), P(4), and E(2)+P(4) in a time-dependent (2.0- to 2.8-fold at 24 h, 3.0- to 5. 0-fold at 48 h) and dose-dependent manner, with maximal effects achieved at 1.0 nM (E(2)) and 100 nM (P(4)) at 24 h of incubation. In summary, both E(2) and P(4), either alone or in combination, stimulate CGRP peptide synthesis in DRG neurons through increasing CGRP mRNA. The effects of these steroid hormones are mediated through amplifying the NGF-induced synthesis of CGRP in these neurons. Thus, we propose that the cardiovascular functions of female sex steroid hormones may be mediated, at least in part, by the up-regulation of neuronal CGRP synthesis, via NGF-mediated mechanisms.     

Negative regulation of parathyroid hormone-related protein expression by steroid hormones

Elevated parathyroid hormone-related protein (PTHrP) is responsible for humoral hypercalcemia of malignancy (HHM), which is of clinical significance in treatment of terminal patients with malignancies. Steroid hormones were known to cause suppression of PTHrP expression. However, detailed studies linking multiple steroid hormones to PTHrP expression are lacking. Here we studied PTHrP expression in response to steroid hormones in four cell lines with excessive PTHrP production. Our study established that steroid hormones negatively regulate PTHrP expression. Vitamin D receptor, estrogen receptor α, glucocorticoid receptor, and progesterone receptor, were required for repression of PTHrP expression by the cognate ligands. A notable exception was the androgen receptor, which was dispensable for suppression of PTHrP expression in androgen-treated cells. We propose a pathway(s) involving nuclear receptors to suppress PTHrP expression.     

Regulation of proliferation of rat cartilage and bone by sex steroid hormones

We have demonstrated previously that 17β-estradiol (E₂) stimulates proliferation of skeletal tissues, both in vivo and in vitro, as measured by increased DNA synthesis and creatine kinase (CK) specific activity. The effect of E₂ on bone is sex specific. E₂ is active only in females and androgens only in males. By contrast, in cartilage of both sexes, dihydrotestosterone (DHT) as well as E₂ stimulates CK specific activity and DNA synthesis. In bone, we find that sex steroids stimulate skeletal cell proliferation in gonadectomized as well as in immature rats. Ovariectomized (OVX) rats, between 1 and 4 weeks after surgery, show stimulation of CK by E₂. The basal activity and response of CK changes with the varying endogenous levels of E₂ in cycling rats, in which the highest basal activity is at proestrus and estrus and the highest response is in diestrus. In rats of all ages tested, both the basal and stimulated specific activity of CK is higher in diaphysis and epiphysis than in the uterus, or in the adipose tissue adjacent to the uterus, which has a response similar to that of the uterus itself. The effect of E₂ in vivo, and in chrondroblasts and osteoblasts in vitro, is inhibited by high levels of the antiestrogen tamoxifen which, by itself, in similar high concentrations, shows stimulatory effects. In addition to the sex steroids, skeletal cells are also stimulated by secosteroid and peptide calciotrophic hormones. The interactions of the sex steroids and the other calciotrophic hormones. These results provide the first steps towards understanding the regulation of bone cell proliferation and growth by the concerted action of a variety of hormones and growth factors.