Impact of environmental endocrine disrupting chemicals on the development of obesity

Developmental effects of diethylstilbestrol DES on obesity

2010 Review Abstracts

Environmental chemicals with hormone-like activity can disrupt programming of endocrine signaling pathways during development and result in adverse effects, some of which may not be apparent until much later in life. Recent reports link exposure to environmental endocrine disrupting chemicals during development with adverse health consequences, including obesity and diabetes. These particular diseases are quickly becoming significant public health problems and are fast reaching epidemic proportions worldwide. This review summarizes data from experimental animals and humans which support an association of endocrine disrupting chemicals, such as diethylstilbestrol, bisphenol A, phytoestrogens, phthalates, and organotins, with the development of obesity. Potential mechanisms are summarized and future research needs are discussed.

Also, the developing fetus and neonate have increased metabolic rates as compared to adults which in some cases may make them more vulnerable to chemical toxicity. It is now well established in the fields of nutrition and endocrine disruption that exposure to environmental chemicals during development can interfere with complex differentiating endocrine signaling pathways and cause adverse consequences later in life; the well known reproductive tract toxicity of diethylstilbestrol (DES) is one of the best examples of adverse consequences of endocrine disrupting chemicals. The concept of the “developmental origins of adult disease”, as the term implies, suggests that there is a time lag between exposure and manifestation of disease. In other words, the effects of exposure during development may not be readily apparent until much later in life.

DES, a potent synthetic estrogen, was widely prescribed to pregnant women from the 1940s through the 1970s in the mistaken belief that it could prevent threatened miscarriages. It was estimated that a range of 2 to 8 million pregnancies worldwide were exposed to DES. Today, it is well known that prenatal DES exposure resulted in a low but significant increase in neoplastic lesions and a high incidence of benign lesions in both the male and female offspring. The DES paradigm was a clear example that prenatal exposure could lead to adult-onset disease. To study the mechanisms involved in DES toxicity, we developed experimental mouse models of perinatal (prenatal or neonatal) DES exposure in which outbred mice were treated with DES on days 9-16 of gestation (the period of major organogenesis in the mouse) or days 1-5 of neonatal life (a period of cellular differentiation of the reproductive tract and a critical period of immune, behavioral, and adipocyte differentiation). These perinatal DES animal models have successfully duplicated, and in some cases predicted, many of the alterations (structural, functional, cellular, and molecular) observed in similarly DES-exposed humans. Further, these models have also shown multigenerational transmission of disease patterns implicating epigenetic mechanisms in the transmission of these effects.

Although our initial focus was on reproductive tract abnormalities and subfertility/infertility, we subsequently examined the relationship of perinatal DES treatment with the development of obesity later in life. We wanted to determine if DES was an “obesogen” as well as a reproductive toxicant and, if so, what were its molecular targets and the mechanisms through which it might act. For our obesity experiments, mice were treated with DES on days 1-5 of neonatal life using a low dose of 0.001 mg/day (1 µg/kg/day); this dose did not affect body weight during treatment but was associated with a significant increase in body weight in the adult animal by 4 to 6 months of age; male mice treated as neonates did not have an increase in body weight.

Unlike the low dose of DES (0.001 mg/day = 1 µg/kg/day), a higher dose (1000 µg/kg/day =1 mg/kg/day) caused a significant decrease in body weight during treatment but it was followed by a “catch-up” period around puberty and then finally resulted in an increase in body weight of the DES-treated mice compared to controls after ~2 months of age. This “catch-up” in weight between treated and control animals is reminiscent of the thrifty phenotype which is a well known phenomenon in the field of nutrition and was described in human infants who received poor nutrition during fetal life but later had “catch-up” growth that finally resulted in overweight and obesity later in life. Further studies indicated that the increase in body weight in DES-exposed mice was associated with an increase in the percent of body fat as determined by mouse densitometry.

Increased body weight in both low and high DES-treated mice compared to controls was observed throughout adulthood; however, by 18 months of age, statistical differences in body weight between DES-treated mice and controls were difficult to show because individual animal variability within groups increased as they aged due to the altered health status of the DES animals. We concluded that since various doses of DES resulted in obesity whether or not pups were underweight during treatment, multiple pathways might be involved in the programming for obesity related to environmental estrogens.
Since densitometry images of DES-treated mice suggested excessive abdominal fat, specific fat pads were weighed to see if particular fat pads were affected by DES treatment or whether there was a generalized change throughout the mouse, since it is well known that increased abdominal fat is associated with cardiovascular disease and diabetes in humans. Weights of inguinal, parametrial, gonadal, and retroperitoneal fat pads were all increased in DES treated mice as compared to controls at 6-8 months of age, suggesting a potential impact on cardiovascular disease following developmental exposure to DES. Brown fat weights were not significantly different in these animals.

Examination of DES-treated mice (1000 µg/kg/day =1 mg/kg/day) and controls at 2 months of age, prior to the treated mice becoming overweight and obese, showed elevated serum levels of leptin, adiponectin, IL-6, and triglycerides, suggesting that these endpoints may be important early markers of subsequent adult disease. Elevated levels of leptin are not surprising considering the increase number and size of the adipocytes in the DES-treated mice, but the increase in adiponectin was not expected since low levels usually correlate with obesity and diabetes in humans. This may indicate insensitivity to these hormones and/or a loss of the negative feedback mechanisms that regulate adipogenesis. Nevertheless, additional studies are needed to determine the mechanisms involved. At 6 months of age, insulin and all of the serum markers except triglycerides were found to be significantly elevated as compared to controls.

Glucose levels were also measured in DES (1000 µg/kg/day =1 mg/kg/day) and control mice at 2 months of age prior to the development of obesity and excessive weight gain. Interestingly, 25% of the DES-treated mice had significantly higher glucose levels than controls; these mice also showed a slower clearance rate of glucose from the blood since higher levels were seen throughout the experiment. Additional glucose measurements in older mice may help determine if a higher percentage of mice are affected with age and if higher and sustained levels of glucose can be demonstrated. To date, our data suggest that overweight and obesity observed in perinatal DES-treated mice will be associated with the development of diabetes, similar to the association of obesity with diabetes in humans. Further studies from our laboratory support a role for altered glucose metabolism as we have shown a high prevalence of islet cell hyperplasia in mice exposed to DES or other environmental estrogens including BPA and genistein.

Since the imbalance of activity levels and food intake are known contributors to obesity, ambulatory activity was measured in DES (1000 µg/kg/day =1 mg/kg/day) and control mice at 2 months of age before a difference in body weight could be detected. Overall, no statistical difference could be shown in activity between groups, although the DES group showed slightly less movement as compared to controls. This slight difference was not sufficient to explain the enhanced weight gain in DES mice as they aged.

Food consumption was also assessed; DES-treated mice consumed more than controls over the course of the experiment (~3 grams more), but the amounts were not statistically different between the groups. Taking into account both the marginal decrease in activity and the increase in food intake in DES-treated mice as compared to controls, it is unlikely that these two parameters can solely explain the development of obesity in DES-treated mice.

A recent study indicated a role for developmental genes in the origins of obesity and body fat distribution in mice and humans. We examined whether exposure to environmental chemicals which exerted hormonal activity would alter expression of genes involved in programming adipocytes during development. Several genes were found to be implicated in altered adipocyte differentiation and function (Hoxa5, Gpc4, and Tbx15) as well as fat cell distribution (Thbd, Nr2f1, and Sfrp2). We investigated changes in gene expression by microarray analysis in uterine samples from DES-treated mice (1000 µg/kg/day =1 mg/kg/day) compared to controls at 19 days of age. Genes involved in adipocyte differentiation were not different in the uterus following neonatal DES exposure. However, genes involved in fat distribution were altered; Thbd and Nr2f1 were significantly down-regulated and Sfrp2 was significantly up-regulated in DES-treated uteri compared to controls. These findings support the idea that environmental estrogens may play a role in regulating the expression of obesity-related genes in development. The identification of genes and molecular mechanisms that may be associated with EDCs and obesity is an exciting area of new research.

Although only neonatal exposure to DES has been discussed thus far in this review, exposure during prenatal life has also been shown to be associated with obesity later in life. Interestingly, high prenatal DES doses caused lower birth weight compared to controls, followed by a “catch-up period”, finally resulting in obesity; low prenatal DES doses had no effect on birth weight but it still resulted in obesity later in life. Thus, it appears that the effects of DES on adipocytes may depend on the time of exposure and the dose and that multiple mechanisms may be altered and result in the same obesity phenotype. Other investigators have also reported analogous findings with DES and other estrogenic chemicals.

References

  • Full study (free access) : Impact of environmental endocrine disrupting chemicals on the development of obesity, Hormones, PMID: 20688618, 2010 Jul-Sep.
  • Featured image PMC1931509/figure/F6.
DES DIETHYLSTILBESTROL RESOURCES

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