Obesity is characterized by abnormal or excessive fat accumulation and is a serious public health concern worldwide. In the USA, 78 million individuals are obese while 11 million are severely obese. Current treatment strategies of obesity start with proper diet, regular exercise, and lifestyle changes [1], which have not shown long-term efficacy, especially in cases of moderate to severe obesity. Several anti-obesity medications, such as enzyme inhibitors (dipeptidyl peptidase inhibitors) [2], incretin and glucagon-like peptide-1 (GLP-1) receptor agonists [3], angiopoietin-like proteins [4], have been researched. A number of these Food and Drug Administration (FDA)-approved drugs were withdrawn from the market owing to adverse side-effects 1, 5. For patients with severe obesity and type 2 diabetes, bariatric surgery proved beneficial as marked improvements in the cardiovascular disease risk factor profile were noted along with prevention of new onset of diabetes. However, bariatric surgery is suitable for only a small section of carefully selected patients with obesity, and after undergoing surgery the patients need regular monitoring [6]. Hence, for proper management of obesity, new approaches need to be developed.

Multipotent adipose stem cells (ASCs) have the potential to differentiate into adipocytes, myocytes, chondrocytes, and osteocytes [7]. Sex steroids influence the differentiation of ASCs to preadipocytes and eventually to adipocytes, promoting differential accumulation of body fat in women and men 8, 9, 10. Women typically show a “pear form” of body type in obesity due to the gluteo-femoral region fat accumulation, compared to the typical “apple form” of body type in obesity for men due accumulation of subcutaneous abdominal and visceral adipose [11]. The correlation between obesity and metabolic complications is critically dependent on body fat distribution [12]. Reports suggest that visceral obesity is strongly associated with metabolic diseases like insulin resistance, hypertension, chronic low-grade inflammation, and prothrombotic state, which accelerates the risk of type 2 diabetes and cardiovascular disease and is prevalent in both sexes.

Testosterone and estradiol are the primary sex hormones produced in males and females, respectively, that play key role in reproductive tissue development and promote secondary sexual characteristics. Testosterone inhibits lipid uptake and stimulates lipolysis by enhancing the number of lipolytic beta-adrenergic receptors [13]. Testosterone also inhibits both lipoprotein lipase activity in adipocytes and preadipocyte differentiation to adipocytes [14]. It is reported that obesity causes reduction in testosterone level, wherein 40% of obese men show below normal testosterone levels [15]. Severe obesity additionally reduces free and bioavailable testosterone 16, 17. Testosterone therapy in men with testosterone deficiency (hypogonadism) has proven to be beneficial resulting in increased lean body mass, increased fat loss and reduction in body weight, waist circumference, and Body-Mass Index (BMI) 18, 19, 20, 21. These effects of testosterone therapy are dose dependent [22] and are observed when androgen level remains within physiological range. It concomitantly improves glucose-insulin homeostasis [23] without affecting the lipid profile [24]. Supra-physiological androgen levels caused by over-administration enhance cardiovascular risk owing to changes of lipid-lipoprotein profile [25]. Thus, the administered dose of hormones is critically important and needs to be explored.

Estradiol interacts with vascular smooth muscle [26], influences vascular reactivity [27], and has antioxidant effects [28]. Estradiol plays significant role in regulating adipose tissue metabolism and thereby reduces the level of circulating cholesterol and free fatty acids (FFAs) 29, 30. Estrogens contribute to cholesterol uptake, reverse cholesterol transport, and cholesterol biosynthesis in the liver. For example, estrogens protect against atherosclerosis via removing the cholesterol from peripheral tissues, either by direct excretion into bile or conversion to bile acids and subsequent secretion into bile [31]. Evidence have shown that postmenopausal women who lack estrogen tend to have higher levels of plasma FFAs compared to women who receive estrogen treatment. This suggests that adipose tissue triglyceride fatty acid stores may be more frequently mobilized or that there is a decrease in FFA clearance. If estrogen deficiency causes an increase in FFA release, this could result in insulin resistance and may signify a more significant mobilization of lower body fat [32]. Studies reported that postmenopausal women have 3.3-fold higher prevalence of obesity and metabolic syndrome as compared to premenopausal women [33]. This increase in central adiposity is correlated to loss of circulating estradiol and 17β-estradiol replacement therapy can help in the reduction of characteristic abdominal weight gain pattern usually associated with menopause [34]. It has been reported that exogenous administration of 17β-estradiol promotes fat loss in ovariectomized rodents. However, exogenous hormone therapy may lead to reduced level of androgens which may lead to adversity.

Sex hormones exert their biological effects through specific receptors. Both estrogen and androgen receptors are present in preadipocytes and adipocytes of both rats 35, 36 and humans 37, 38 the number of these receptors being variable and depending on the cells' anatomical origin. Considering the opposite effects of sex steroids in men and women, more experiments are needed to explore the role of sex hormones on human adipocytes. In the present study, we examined the dose-dependent effect of 17β-estradiol and testosterone on the adipogenic differentiation and maturation of human ASCs (hASCs) obtained from female and male patients. We isolated hASCs from male and female adipose tissue and seeded on top of a positively-charged elastin-based matrix coating to develop the 3-D tissue model [39]. Using this 3-D model system, we hypothesized that while the exposure of hASC spheroids to the sex steroids at the beginning of the adipogenic maturation period will inhibit fat accumulation, the exposure of adipogenically matured hASC spheroids to the sex steroids will reduce the intracellular triglyceride accumulation. We found estradiol, but not testosterone, leads to a dose dependent reduction in intracellular triglyceride accumulation in female hASCs, while both 17β-estradiol and testosterone had only a modest effect on the male hASCs.