**2. Adipose tissue**

Adipose tissue has a vital role in the lives of mammals. The primary function of adipose tissue is to store excess energy within the body in the form of free fatty acids (FFAs) and heat production. However, adipocytes are currently regarded as an independent endocrine organ since the metabolic and endocrine actions have been revealed.

In mammals, two types of adipose tissue exist (white adipose tissue [WAT] and brown adipose tissue [BAT]) [13]. WAT represents the major component of adipose tissue and provides most of the total body fat [13]. Moreover, WAT is the main source of FFAs that are available as energy substrates for generation through oxidative phosphorylation of adenosine triphosphate (ATP) high-energy bonds [13, 14]. Excess WAT in the upper parts of the body (android type obesity) represents a strong risk factor for some inflammatory pathologies [15]. In contrast, accumulation of WAT in lower body parts (gynecoid type obesity) is not associated with metabolic complications [13, 15].

In contrast to WAT, BAT participates in energy expenditure from non-oxidative phosphorylation in the form of heat for cold adaption [16]. The uncoupling of phosphorylation in BAT is attributed to the activity of uncoupling protein-1, which is expressed on the mitochondrial membrane, by creating a proton leak that depletes the electrochemical gradient needed for oxidative phosphorylation [14, 16]. BAT represents a smaller number of fat cells that have a rich vascular supply, which can respond more rapidly to sympathetic nervous system stimulation, and elicits heat production, rather than ATP production from nonshivering cold adaptive thermogenesis [14, 16]. In humans, BAT helps to maintain body temperature in newborns, but BAT regresses with increasing age and is completely lost in adulthood [17, 18]. Recently, Virtanen et al. studied BAT deposition in healthy adults using the glucose analogue, 18F-flurodeoxyglucose, uptake by PET and computed tomography [18]. Metabolically-active BAT depots in paracervical and supraclavicular adipose tissue, which can be induced in response to cold and sympathetic nervous system activation, has been reported [14, 16, 18]. The presence of BAT in human adults is of potential interest in understanding the mechanism of obesity, and may provide a rationale for pharmacologic and gene expression manipulation to combat human obesity [14, 18, 19].

## **3. Deposition and distribution of adipose tissue**

Deposition of the fat mass has a different pattern between the genders. Young males have little subcutaneous fat and do not show a central-peripheral difference [20], whereas women of reproductive age have more subcutaneous fat than men at all measured subcutaneous regions [21]. In addition, with an increasing severity of obesity, adipose tissue thickness is higher in the central regions in men, but women exhibit a peripheral deposition [20]. This pattern of fat deposition is altered in women across the menopausal transition; specifically, the total amount of body fat increases, and the peripheral fat shifts around the abdomen. Furthermore, the change in visceral adipose tissue has been ascribed to both chronologic aging and menopause [22, 23]. The results from a clinical study with 156 healthy women

involved in obesity. In addition we summarize evidence for the effect of postmenopausal

Adipose tissue has a vital role in the lives of mammals. The primary function of adipose tissue is to store excess energy within the body in the form of free fatty acids (FFAs) and heat production. However, adipocytes are currently regarded as an independent endocrine

In mammals, two types of adipose tissue exist (white adipose tissue [WAT] and brown adipose tissue [BAT]) [13]. WAT represents the major component of adipose tissue and provides most of the total body fat [13]. Moreover, WAT is the main source of FFAs that are available as energy substrates for generation through oxidative phosphorylation of adenosine triphosphate (ATP) high-energy bonds [13, 14]. Excess WAT in the upper parts of the body (android type obesity) represents a strong risk factor for some inflammatory pathologies [15]. In contrast, accumulation of WAT in lower body parts (gynecoid type

In contrast to WAT, BAT participates in energy expenditure from non-oxidative phosphorylation in the form of heat for cold adaption [16]. The uncoupling of phosphorylation in BAT is attributed to the activity of uncoupling protein-1, which is expressed on the mitochondrial membrane, by creating a proton leak that depletes the electrochemical gradient needed for oxidative phosphorylation [14, 16]. BAT represents a smaller number of fat cells that have a rich vascular supply, which can respond more rapidly to sympathetic nervous system stimulation, and elicits heat production, rather than ATP production from nonshivering cold adaptive thermogenesis [14, 16]. In humans, BAT helps to maintain body temperature in newborns, but BAT regresses with increasing age and is completely lost in adulthood [17, 18]. Recently, Virtanen et al. studied BAT deposition in healthy adults using the glucose analogue, 18F-flurodeoxyglucose, uptake by PET and computed tomography [18]. Metabolically-active BAT depots in paracervical and supraclavicular adipose tissue, which can be induced in response to cold and sympathetic nervous system activation, has been reported [14, 16, 18]. The presence of BAT in human adults is of potential interest in understanding the mechanism of obesity, and may provide a rationale for pharmacologic and gene expression manipulation to combat human obesity

Deposition of the fat mass has a different pattern between the genders. Young males have little subcutaneous fat and do not show a central-peripheral difference [20], whereas women of reproductive age have more subcutaneous fat than men at all measured subcutaneous regions [21]. In addition, with an increasing severity of obesity, adipose tissue thickness is higher in the central regions in men, but women exhibit a peripheral deposition [20]. This pattern of fat deposition is altered in women across the menopausal transition; specifically, the total amount of body fat increases, and the peripheral fat shifts around the abdomen. Furthermore, the change in visceral adipose tissue has been ascribed to both chronologic aging and menopause [22, 23]. The results from a clinical study with 156 healthy women

hormone therapy (HT) on changes in body composition.

organ since the metabolic and endocrine actions have been revealed.

obesity) is not associated with metabolic complications [13, 15].

**3. Deposition and distribution of adipose tissue** 

**2. Adipose tissue** 

[14, 18, 19].

during 4 years of follow-up showed an increase in visceral adipose tissue and total body fat, and a 32% reduction in fat oxidation during the menopausal transition [4]. In the study, the subcutaneous adipose tissue increased in accordance with age independent of menopausal status, while the findings of increased visceral adipose tissue and total body fat were noted only in postmenopausal women. This distinctive physiologic change in amount and distribution of fat in women is noteworthy because the central fat deposition has a more deleterious effect on the development of cardiovascular and metabolic disease [24, 25].

Although the exact mechanism regarding fat redistribution after menopause remains unclear, the phenomenon with declining estrogen level may be due to alterations in adipose tissue metabolism [4]. Several studies have shown that estrogen directly promotes subcutaneous fat accumulation [26], and the loss of estrogen by menopause is associated with an increase in central fat. Several longitudinal studies lend support in suggesting that estrogen plays an important role in regulating body fat distribution. Postmenopausal women who receive estrogen replacement have significantly lower waist-to-hip ratios and less visceral adipose tissue than women who have never received estrogen replacement therapy [27, 28].

In experimental studies, 17β-estradiol (E2) has been shown to regulate adipose tissue by increasing the number of adipocytes through effects on proliferation and differentiation [29, 30]. The number and size of adipocytes is a determining factor for adiposity, and the adipocyte size is balanced by lipogenesis and the lipolysis pathway. Palin et al. have found a direct regulatory effect of E2 on the expression of lipoprotein lipase (LPL) and hormonesensitive lipase (HSL) in human subcutaneous abdominal adipose tissue [31]. LPL is a major modulator of lipid deposition as triglycerides into adipocytes, and HSL is the rating-limiting enzyme involved in the process of lipolysis. Therefore, the direct effect of estrogen on these enzymes might lead to fat redistribution in postmenopausal women.

As other mechanisms suggest, the role of the estrogen receptor (ER) is focused on estrogenrelated action on the regulation of adiposity. Adipocytes express two main subtypes of ERs (alpha [ERα] and beta [ERβ]). ERα was discovered first, and the biological effects of ERα on adiposity have been thoroughly described. ERα is considered to be essential for genomic actions of E2 on the regulation of body fat [32]. Because the hypothalamic nuclei that regulate energy homeostasis express ERα, E2 action could affect adiposity [33-35]. An animal study conducted by Heine et al. demonstrated that glucose intolerance, hypertrophy, and hyperplasia of adipocytes are induced in ERα knockout mice [36], thus supporting a critical role of ERα in determining adiposity.

In contrast, the biological implications of the more recently discovered ERβ have been less revealing than ERα. The binding affinity of estrogen to ERα and ERβ is known to be similar, but the two subtypes of ER only have 56% identity in the ligand binding domain [26, 37, 38]. Therefore, different or competitive roles between both ERs have been repeatedly suggested. Naaz et al. studied the role of ERβ in adipose tissue [39] in mice with ERαKO. When compared to the results generated in ERαKO mice, it was shown that removing E2/ERβ signaling induced a decrease in body weight, the amount of fat, and adipocyte size. Therefore, the authors suggested a potential role for ERβ in regulating adiposity, as well as ERα, but with opposing actions. Thus, the roles for ERs in adipocytes might be an interesting target to further elucidate the estrogen effects on regulating adiposity.

Adipose Tissue Metabolism and Effect of

affect the diverse obesity phenotypes.

resistin expression is discordant [64, 75].

adipocytes [64, 70].

Postmenopausal Hormone Therapy on Change of Body Composition 295

recent data derived from healthy pre- and post-menopausal women, postmenopausal women had increased levels of tissue plasminogen activator antigen (tPA), MCP-1, and adiponectin [24]. Furthermore, an increase in intraabdominal fat was correlated with Creactive protein, tPA, and leptin, and negatively with adiponectin levels. The results imply that during the menopausal transition, women have adverse changes in inflammatory

Several animal studies have indicated a stimulatory effect of estrogen on leptin expression and secretion in rat adipose tissues [59, 62]. Machinal et al. reported that there are regional differences in leptin expression between subcutaneous and deep fat tissues in rats, and that leptin secretion increased in the deep fat tissue tissues [63]. Based on one study involving human adipocytes, estrogen is likely to stimulate leptin expression [53]. In a previous study, the association between ER and adipokine expression in 3T3-L1 adipocytes was investigated [64]. The results showed that ERα has a stimulatory effect on leptin expression, while the expression of ERβ is inversely correlated with leptin expression. Therefore, discordant findings regarding the estrogen effect on leptin expression/secretion from other studies can be explained by the different expression of the two ER subtypes, which have opposite actions on the expression of leptin. In addition, if there are regulating factors (genetic or environmental) for ERα or ERβ expression in adipocytes, this may explain how the different expression of ER in adipose tissue can

Adiponectin is a 30 kDa protein secreted abundantly from adipocytes [65, 66], and functions to exert anti-diabetic and anti-atherogenic properties. The serum adiponectin levels are decreased in obese individuals, metabolic syndrome, and type 2 DM [67, 68]. Similar to leptin, circulating adiponectin levels show a sexual dimorphism with higher levels in women than men [69]. Although the estrogen effects for adiponectin expression are limited, some *in vitro* studies have reported no direct regulatory effect in humans and mouse

Resistin is a cysteine-rich protein that was originally described as an adipose-derived protein in rodents that links obesity to insulin resistance [71]. However, in human data, the relationship of resistin with obesity and insulin metabolism is still debated. A widely reported biological role of resistin is the regulatory effect involved in inflammatory processes. In addition to adipocytes, resistin is induced by lipopolysaccharides and TNF-α in macrophages [72]. The other data showed that resistin induces or is induced by IL-6 and TNF-α via NF-κB in human monocytes [73, 74]. Data on estrogen effects for resistin expression are limited. In a mouse adipocyte model, the regulatory effect of 17β-E2 on

Recently described novel adipokines, such as visfatin, retinol binding protein-4, and omentin, have been shown to exert some metabolic properties, but their biological actions

Hormone therapy (HT) is widely used for the treatment of menopausal symptoms and preventing bone loss in postmenopausal women. Estrogen replacement, when combined with various progestogens for endometrial protection, has been established as a conventional formulation. The benefit of HT is known to reduce vasomotor symptoms,

**6. Postmenopausal hormone therapy and change in body composition** 

linked to obesity and interactions with estrogen need to be elucidated.

markers and adipokines which correlate with increased visceral obesity.
