The discovery of a second estrogen receptor (ER beta) has significant implications for our understanding of the molecular basis for the diverse actions of estrogen. Here we report the differential activation by natural and xenobiotic estrogens of ER alpha and ER beta when linked to different response elements. Receptor mediated activation of reporter constructs containing either the estrogen response element (ERE) from the vitellogenin (Vit) gene or from the luteinizing hormone beta (LH) gene were examined in transiently transfected Cos-1 cells. ER beta preferentially activated the consensus Vit ERE whereas ER alpha showed greater activation at the divergent LH ERE. This differential activation was observed for a number of ligands including estradiol, estrone, bisphenol A, octylphenol and diethystilbestrol. These findings show that the nature of the ERE, as well as the ratio of ER subtypes in a particular cell/tissue, will influence whether particular estrogen responsive genes are activated in the presence of natural or xenobiotic estrogens.
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WD Pennie, TC Aldridge, and AN Brooks
MT Rae, SM Rhind, CE Kyle, DW Miller, and AN Brooks
The aims of this study were to determine which hormones may have a role in the expression of maternal undernutrition effects on reproductive function, in both the developing fetus and the adult offspring. This was undertaken by measuring the effects of long-term maternal undernutrition on metabolic hormone profiles and pituitary responses to single doses of GnRH and GH-releasing factor (GRF) in fetal sheep. From mating, groups of ewes were fed rations providing either 100% (HIGH) or 50% (LOW) of estimated metabolisable energy requirements for pregnancy throughout the experiment until slaughter at approximately 119 days of gestation. Fetal and maternal blood samples were collected from 113 until 119 days of gestation, via carotid and jugular catheters respectively, and assayed for insulin, IGF-I, GH, thyroxine and triiodothyronine (T(3)). Undernutrition had no effects on fetal weight, fetal gonad weight of either sex, fetal insulin or IGF-I concentrations. Male LOW fetuses exhibited a significantly attenuated response (P<0.05) to a bolus challenge of GnRH compared with HIGH fetuses. Basal fetal GH concentrations and the response to exogenous GRF were similar in both treatment groups, although LOW fetuses exhibited more secretory episodes (P<0.01). Mean T(3) concentrations were significantly lower in both the maternal (P<0.01) and fetal (P<0.05) plasma of LOW animals compared with HIGH animals. It is concluded that pituitary function was altered in fetal males and could influence male reproductive development. On the other hand, in female sheep, fetal gonadal abnormalities and reductions in reproductive capacity in adult life which are associated with fetal undernutrition are unlikely to be attributable to altered pituitary function. Additionally, these studies raise the possibility that thyroid hormones may have a role in the expression of maternal undernutrition effects on fetal development.
B Williams, AN Brooks, TC Aldridge, WD Pennie, R Stephenson, and CA McArdle
GnRH acts via phospholipase C (PLC) activating G-protein coupled receptors to stimulate secretion of gonadotrophins from gonadotrophs. These cells are also regulated by gonadal steroids, which act centrally to influence GnRH secretion, and peripherally to modulate GnRH action. We have shown that oestradiol can stimulate proliferation and modulate GnRH-stimulated [(3)H]inositol phosphate ([(3)H]IP(x)) accumulation (used as a measure of PLC activity) in a gonadotroph-derived cell line (alphaT3-1). Here we show that when alphaT3-1 cells were incubated in medium with 2% foetal calf serum (FCS), [(3)H]thymidine incorporation was not stimulated by oestradiol but was reduced to <2% of control by the oestrogen antagonist, raloxifene. The inhibitory effect of 10 or 1000 nM raloxifene was reversed competitively by oestradiol. A similar pattern of effects was seen when effects of oestradiol and raloxifene on the proportion of cells in the S-phase of the cell cycle (as measured by flow cytometry of propidium iodide-labelled cells) and on oestrogen receptor activity (as measured by trans-activation of the oestrogen-response elements in the vitellogenin promoter) were quantified. In addition, RT-PCR revealed expression of alpha and beta (but not beta2) subtypes of oestrogen receptors. Thus, oestrogen is an essential mitogen for alphaT3-1 cells, its mitogenic effect is oestrogen receptor mediated and is associated with a marked alteration of cell cycle distribution, and the full extent of these effects are best revealed in the presence of raloxifene. Using this strategy, we found that cells cultured for 4 days with 10 nM raloxifene expressed GnRH receptors (K(d) for (125)I-buserelin 4.33 nM) and that their activation by GnRH caused a concentration-dependent increase in [(3)H]IP(x) (in cells labelled with [(3)H]inositol) and inositol 1,4,5 trisphophate (in unlabelled cells). Addition of 10 nM oestradiol (to overcome receptor blockade by raloxifene) reduced GnRH receptor number by 31% but increased maximal effects on [(3)H]IP(x) and Ins(1,4,5)P(3) approximately 4-fold. The effects of oestradiol on GnRH receptor number and signalling were not, however, mimicked by culture for 2 days in medium with 10% FCS and the S-phase blocker, thymidine (15 mM). This treatment increased the proportion of cells in the S-phase 2- to 3-fold but did not alter GnRH receptor number or signalling. Other treatments which altered cell cycle transition (hydroxyurea, colcemid, methotrexate) also failed to alter GnRH receptor number or signalling and no correlation was seen between GnRH receptor number or GnRH-stimulated [(3)H]IP(x) accumulation and the proportion of cells in the S-phase or G2/M-phases of the cell cycle. Thus, oestradiol has pronounced effects on GnRH signalling, proliferation and cell cycle distribution in alphaT3-1 cells, but these trophic effects do not underlie the modulation of GnRH signalling.