2005 Progress Report: Low Dose Effects of In Utero Exposure to Cadmium on PubertyEPA Grant Number: R832136
Title: Low Dose Effects of In Utero Exposure to Cadmium on Puberty
Investigators: Martin, Mary Beth , Hilakivi-Clarke, Leena , Lumpkin, Michael
Institution: Georgetown University
EPA Project Officer: Hahn, Intaek
Project Period: December 1, 2004 through November 30, 2007 (Extended to November 30, 2009)
Project Period Covered by this Report: December 1, 2004 through November 30, 2005
Project Amount: $738,798
RFA: Development and Characterization of Biological Systems for Studying Low Dose Effects of Endocrine Disrupting Chemicals (2004) RFA Text | Recipients Lists
Research Category: Endocrine Disruptors , Economics and Decision Sciences , Health , Safer Chemicals , Health Effects
During the last century, the onset of puberty and menarche in girls has occurred at significantly younger ages. Although this trend has ceased in European populations, puberty onset continues to advance in the U.S. population. It has been suggested that the high incidence of hormone related diseases such as the early onset of puberty is caused by the presence of environmental estrogens. Studies from our laboratory show that the heavy metal cadmium has potent estrogen-like activity in vitro and in vivo and that in utero exposure to environmentally relevant amounts of cadmium advances the onset of puberty, increases weight gain, and alters the development of the mammary gland in female rat offspring. This project is designed to test the hypothesis that in utero exposure to low doses of cadmium alters the hypothalamic-pituitary-gonadal axis and consequently alters the onset of puberty, predisposes to obesity, and accelerates the development of the mammary gland. This project has the following four objectives:
Objective 1: Determine the Dose-Response Effects of In Utero Exposure to Cadmium on the Onset of Puberty, Weight Gain, and Mammary Gland Development
More specifically, Objective 1 will determine whether there is a linear relationship between the in utero exposure to low doses of cadmium (0.05, 0.5, 5, 50, and 500 ng/kgbw) and the estrogen like effects in female offspring.
Objective 2: Determine the Mechanism by Which In Utero Exposure to Cadmium Alters the Onset of Puberty
Two hypotheses have been proposed for the regulation of the onset of puberty. The gonadostat hypothesis proposes that the onset of puberty is caused by a decreased sensitivity of the hypothalamus to the negative feedback of gonadol steroids, whereas the intrinsic restraint hypothesis proposes that the onset of puberty is regulated by the production of stimulatory signals (glutamate, neuropeptide Y [NPY], and transforming growth factor-alpha) that override the inhibitory signal (gamma-aminobutyric acid [GABA]) in the hypothalamus. Objective 2 will determine whether in utero exposure to cadmium alters either or both of these pathways.
Objective 3: Determine the Mechanism by Which In Utero Exposure to Cadmium Alters Weight Gain
Energy homeostasis is controlled by the hypothalamus in response to endocrine (gherlin, PYY3-36, insulin, and leptin) and neuronal (NPY and agouti-related peptide) signals. Objective 3 will determine whether in utero exposure to cadmium alters the development and/or the responsiveness of the hypothalamus to these signals.
Objective 4: Determine the Mechanism by Which In Utero Exposure to Cadmium Accelerates the Development of the Mammary Gland
More specifically, Objective 4 will determine whether cadmium alters the development of the mammary gland in utero or whether the metal alters the onset of puberty and consequently the development of the gland. Alternatively, exposure to cadmium may not alter the architecture of the gland in utero but may alter the pattern of gene expression.
In a preliminary study, we asked whether treatment of prepubertal, ovariectomized female rats with estrogen altered expression of key receptors in the hypothalamus. The receptors tested included receptors for hormones involved in pubertal on-set (GABA, glutamate, and leptin) and appetite regulation (leptin). Animals were ovariectomized at day 21, allowed to rest for 2 weeks, and were either untreated (control) or treated with 3 mg/kg estradiol for 3 days. Receptor expression in the hypothalamus was measured using real-time reverse-transcriptase polymerase chain reaction and the data were analyzed by One-Way ANOVA analysis (p < 0.05). Receptors that were expressed in higher amounts included Ob-Rb (leptin), GABAA (GABA), and NMDA-R (glutamate). Expression of the EGF-1 receptor was not altered significantly. To ask whether cadmium altered weight gain, animals in a second preliminary study were placed on control or cadmium containing diet on postnatal day 21 and body weights were monitored until day 218. The average weight of animals on the cadmium diet (309 grams) was significantly greater (p < 0.046) than the average weights of animals on the control diet (289.89 grams).
We currently are investigating whether in utero exposure to cadmium has similar effects on expression of key genes in the hypothalamus. These studies are designed as outlined in Objectives 2 through 4.