Estrogen regulates the amount of white adipose tissue (WAT) in females, but its role in males and whether WAT effects involve estrogen receptor-␣ (ER␣) or ER␤ were unclear. We analyzed the role of ER␣ in WAT and brown adipose tissue by comparing these tissues in wild-type (WT) and ER␣-knockout (␣ERKO) male and female mice. Brown adipose tissue weight was similar in ␣ERKO and WT males at all ages. Progressive increases in WAT were seen in ␣ERKO males with advancing age. Epididymal, perirenal, and inguinal WAT weighed 139 -185% more in ␣ERKO than in WT males by 270 -360 days of age. Epididymal and perirenal adipocyte size was increased 20% in ␣ERKO males. Adipocyte number was 82-168% greater in fat pads of ␣ERKO vs. WT males. Compared with WT, 90-day-old ␣ERKO females had increases in fat pad weights (54 -103%), adipocyte size, and number. Both ␣ERKO males and females had insulin resistance and impaired glucose tolerance, similar to humans lacking ER␣ or aromatase. Energy intake was equal in WT and ␣ERKO males, indicating that obesity was not induced by hyperphagia. In contrast, energy expenditure was reduced by 11% in ␣ERKO compared with WT males, indicating that altered energy expenditure may be important for the observed obesity. In summary, ER␣ absence causes adipocyte hyperplasia and hypertrophy, insulin resistance, and glucose intolerance in both sexes. These results are evidence that estrogen͞ER␣ signaling is critical in female and male WAT; obesity in ␣ERKO males involves a mechanism of reduced energy expenditure rather than increased energy intake. O besity is a significant human health problem whose incidence is reaching epidemic proportions in some Western countries. For example, obesity in Americans has risen dramatically in the past 40 years, from 12.8% in 1962 to 22.5% in 1998, and 55% of the population is considered overweight (1). Obesity is associated with increased type II diabetes, heart disease, certain cancers, and other health problems, and obesity is estimated to be responsible for 300,000 deaths͞year in the U.S. (2). Because of these human health concerns, there is intense interest in factors that regulate development and function of white adipose tissue (WAT). In addition, factors regulating WAT in food animals are important because of concerns over excess fat consumption in Western diets, which may contribute to adverse health effects.Evidence from both humans and laboratory animals suggests that estrogen plays an important role in WAT regulation. Ovariectomy of rodents increases WAT, and estrogen replacement decreases WAT (3). Similarly, postmenopausal women have increased WAT, and estrogen therapy decreases WAT levels compared with untreated postmenopausal women (4).Female WAT expresses the classical estrogen receptor, estrogen receptor-␣ (ER␣), as well as the recently described ER␤ (5-8). Although the relative role of ER␣ and ER␤ and the mechanism by which estrogen regulates WAT are unclear, estrogen effects on glucose homeostasis in females may be involved (9). For example, glucose to...

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Estradiol-17␤ (E 2 ) acts through the estrogen receptor (ER) to regulate uterine growth and functional differentiation. To determine whether E 2 elicits epithelial mitogenesis through epithelial ER versus indirectly via ERpositive stromal cells, uteri from adult ER-deficient ER knockout (ko) mice and neonatal ER-positive wild-type (wt) BALB͞c mice were used to produce the following tissue recombinants containing ER in epithelium (E) and͞or stroma (S), or lacking ER altogether: wt-S ؉ wt-E, wt-S ؉ ko-E, ko-S ؉ ko-E, and ko-S ؉ wt-E. Tissue recombinants were grown for 4 weeks as subrenal capsule grafts in intact female nude mice, then the hosts were treated with either E 2 or oil a week after ovariectomy. Epithelial labeling index and ER expression were determined by [ 3 H]thymidine autoradiography and immunohistochemistry, respectively. In tissue recombinants containing wt-S (wt-S ؉ wt-E, wt-S ؉ ko-E), E 2 induced a similar large increase in epithelial labeling index compared with oil-treated controls in both types of tissue recombinants despite the absence of epithelial ER in wt-S ؉ ko-E tissue recombinants. This proliferative effect was blocked by an ER antagonist, indicating it was mediated through ER. In contrast, in tissue recombinants prepared with ko-S (ko-S ؉ ko-E and ko-S ؉ wt-E), epithelial labeling index was low and not stimulated by E 2 despite epithelial ER expression in ko-S ؉ wt-E grafts. In conclusion, these data demonstrate that epithelial ER is neither necessary nor sufficient for E 2 -induced uterine epithelial proliferation. Instead, E 2 induction of epithelial proliferation appears to be a paracrine event mediated by ER-positive stroma. These data in the uterus and similar studies in the prostate suggest that epithelial mitogenesis in both estrogen and androgen target organs are stromally mediated events.