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Androgens and metabolic syndrome: Lessons from androgen receptor knock out (ARKO) mice
Journal of Steroid Biochemistry & Molecular Biology 109 (2008) 254–257
Contents lists available at ScienceDirect
Journal of Steroid Biochemistry and Molecular Biology journal homepage: www.elsevier.com/locate/jsbmb
Androgens and metabolic syndrome: Lessons from androgen receptor knock out (ARKO) mice夽 Toshihiko Yanase a,∗ , WuQiang Fan a , Kanako Kyoya a , Liu Min a , Ryoichi Takayanagi a , Shigeaki Kato b , Hajime Nawata c a b c
Department of Medicine and Bioregulatory Science, Kyushu University, Maidashi 3-1-1, Higashi-ku, Fukuoka 812-8582, Japan Institute of Molecular and Cellular Biosciences, Graduate School of Agricultural and Life Sciences, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan Graduate School of Medical Science, Kyushu University, Maidashi 3-1-1, Higashi-ku, Fukuoka 812-8582, Japan
a r t i c l e Keywords: Androgen receptor Obesity Thermogenesis Gene knockout
i n f o
a b s t r a c t Testosterone (T) is an important factor for determining body composition in males. Abdominal obesity is inversely correlated with serum T levels in men, leading to greater mortality. Pathologically hypogonadal men also have a significantly higher fat mass, which is reversed by T administration. However, the mechanism for such anti-obesity effect of androgen has not been well clarified. Androgen receptor (AR) null male mice revealed late-onset obesity. Male ARKO mice were euphagic compared to the wild-type male controls, but also less dynamic and less oxygen consuming. Transcript profiling indicated that male ARKO mice had lower transcripts for the thermogenetic uncoupling protein 1 (UCP1). We also found enhanced secretion of adiponectin, which is insulin-sensitizing, from adipose tissue in comparison to wild type, which might partly explain why the overall insulin sensitivity of male ARKO mice remained almost intact despite their apparent obesity. In addition, decreased lipolysis rather than increased lipid synthesis was observed, which might also account for the increased adiposity in male ARKO mice. The results revealed that AR plays important roles in male metabolism by affecting the energy balance, and is negative to both adiposity and insulin sensitivity. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Obesity is now becoming a big medical problem in the world and even in Japan, it is not exceptional [1]. Body mass index of Japanese men is increasing every year in every generation. On the other hand, that of Japanese women under 50-year-old is rather decreasing every year in recent 30 years [1], probably because women pay much more attention to their own body image. The most important thing is that the probability that a person of each BMI dies 10 years later is much higher in men than in women [2]. So, to tackle with obesity especially in middle-aged men is a very important issue. Now, there is a criterion for clinical diagnosis of metabolic syndrome, which is considered to be a risk factor for cardiovascular disease beyond cholesterol [3]. In these criteria, visceral fat obesity is the most upstream pathogenesis of this syndrome. Our data shows that visceral fat to subcutaneous fat ratio increases with age
in men (Fig. 1) and that free testosterone concentrations in men are inversely correlated with % of body fat (Fig. 2). These data suggest that endogenous testosterone in men may be protective against the increase of fat mass with age. The inverse relationship between endogenous testosterone and fat mass has been also confirmed by many other investigations [4–7]. As one of such investigations, Mauras et al. showed that GnRH administration for 10 weeks to healthy young men caused low free testosterone level, low lipid oxidation and low resting energy expenditure, whereas the treatment brought a increase of % of body fat mass [4]. These results again indicate that physiological level of testosterone has anti-obesity effect in men. In addition, a significantly higher fat mass in pathologically hypogonadal men is reversed by testosterone administration [8,9]. However, the mechanism for the anti-obesity effect of androgens has not been well elucidated. 2. Lessons from androgen receptor knock out mice
夽 Presented at the 12th International Congress on Hormonal Steroids and Hormones & Cancer’ (Athens, Greece, 13–16 September 2006). ∗ Corresponding author. Tel.: +81 92 642 5276; fax: +81 92 642 5287. E-mail address: [email protected] (T. Yanase). 0960-0760/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsbmb.2008.03.017
Androgen receptor knock out (ARKO) mice were independently developed by two groups including one of authors, Kato [10,11]. In addition to several interesting phenotypes of these mice like impaired brain masculinization [12] and osteoporosis [13], the mice
T. Yanase et al. / Journal of Steroid Biochemistry & Molecular Biology 109 (2008) 254–257
255
Fig. 1. V/S ratio in men with age. V/S (visceral to subcutaneous fat) in men increases with age (P < 0.01). V/S ratio was measured by CT-based body composition analysis at the level of navel.
also showed metabolic phenotype. Namely, only male ARKO mice but not female ARKO mice showed a late-onset obesity [10]. After this observation, we analyzed the detailed mechanism how male ARKO mice develop obesity [14]. This male ARKO mouse showed marked increase of visceral and subcutaneous fat. In these ARKO male mice, not only white adipose tissue (WAT) but also brown adipose tissue (BAT) was something wrong in the character. BAT is associated with energy expenditure by heat production using uncoupling protein 1 (UCP-1) [15]. But, BAT from male ARKO mice showed WAT-like appearance. Although BAT usually does not produce leptin, the BAT of these mice produced exceptionally high amount of leptin just like WAT. These data suggest some functional impairment of BAT in these mice. A similar abnormality of BAT has been also reported in -adrenergic receptor KO mice [16]. The concept of energy balance, which comprises both energy intake (feeding) and energy expenditure (physical activity, basal metabolism and adaptive thermogenesis), is the key to understanding obesity [17]. We then examined the energy balance in male ARKO mice, in respect of energy intake or energy expenditure. As a result, food intake in male ARKO mice was almost as same as that of wild type. However, this mouse was less dynamic, namely distance of run in 8 h was very low and the numbers of standingup opportunities were smaller than those of wild type. Actually, energy expenditure was impaired in these mice because oxygen consumption ratio of male ARKO mice was much smaller than that
Fig. 2. Relationship between male-free testosterone concentrations in serum and % of body fat. % of body fat was measured using body composition analyzer, in body. Inverse relationship between both parameters was observed (P < 0.05).
Fig. 3. Comparison of oxygen consumption between male wild type and male ARKO mice (cited from Ref. [14]).
of wild type (Fig. 3). Lowered spontaneous activities of male ARKO mice were also observed even in 8 weeks old age, at which male ARKO mice were not obese, indicating that less dynamic character of male ARKO mice is an intrinsic character but is not the result of obesity. One of the molecular mechanisms for the disturbed energy expenditure could be explained by the dramatic down regulation of UCP-1 in WAT in these mice. UCP-1 in BAT was also down regulated. However, the down regulation of UCP-1 in WAT is probably much more meaningful, because AR was mainly expressed in WAT rather than in BAT. UCP-1 is probably a direct target gene of DHT-AR because DHT-AR activation upregulated UCP-promoter activity in 3T3L-1 cells. Among transcripts involved in lipid homeostasis, hormonesensitive lipase of WAT in this ARKO mice was sharply down regulated, while transcripts of lipogenetic enzymes or protein like fatty acid synthase (FAS), acyl-coA carboxylase and sterol regulatory element-binding protein 1C (SRBP1C) in WAT were unchanged. These data may suggest that WAT accumulation in male ARKO mice may be partly brought by the inhibition of lypolysis. Obesity is usually associated with insulin resistance and glucose intolerance. However, another interesting feature of these mice was relatively intact insulin sensitivity in insulin challenge test and glucose tolerance in iPGTT although basal insulin levels of male ARKO mice tended to be higher than those of wild type. One of the reasons for the relatively intact insulin sensitivity of ARKO mice might be relatively high serum concentration of adiponectin compared to wild type, because adiponectin has recently been clarified to be an important insulin-sensitizing adipocytokine [18]. It should be noted that adiponectin transcript was down regulated as usually observed in obese phenotype, indicating that adiponectin secretion from WAT was probably increased in this ARKO mice. This is very compatible with recent findings that testosterone suppresses adponectin secretion, especially high molecular weight adiponectin, from WAT [19–21], although the mechanism remained unclear. In addition, relatively lower expression of PPAR␥ might also contribute to the intact insulin sensitivity in these mice as proposed by Dr. Kadowaki’s group based on the observation of PPAR␥ hetero knock out mice [22]. Skeletal muscle is an important target tissue of androgen. Relatively intact insulin sensitivity and glucose tolerance of these mice were also supported by the relatively increased expressions of glu-
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T. Yanase et al. / Journal of Steroid Biochemistry & Molecular Biology 109 (2008) 254–257
age, thus leading to easiness of weight gain in middle-aged people. This is because middle-aged obesity is usually associated with the decrease of muscle mass, the major organ in that basal metabolism takes place. Exercise reduces fat mass and increases muscle mass, leading to the increase of basal metabolism. Importantly, androgen exerts a similar effect as observed by exercise [8,9,26]. Actually, testosterone therapy in adult men with androgen deficiency syndromes was guided based on their serum testosterone levels and symptoms [26]. However, a long-term treatment with androgen stimulates prostate and thus may be a risk factor for the development of prostate cancer [26]. In the near future, we may expect to have a kind of drug, so called selective androgen receptor modulator (SARM), which may have anti-obesity effect but may not stimulate prostate. Fig. 4. Summary of metabolic phenotype of male ARKO mice. Male ARKO mice showed late-onset obesity, which was probably caused by decreased energy expenditure and decreased lypolysis. In spite of marked obesity, these mice showed relatively intact insulin sensitivity and glucose tolerance. Increased serum adiponectin level might contribute to the relatively intact glucose metabolism (cited from Ref. [14] and modified).
cose transporter 4, and several glucose oxidation enzymes like hexokinase 1, phosphofluctokinase and pyruvate kinase in skeletal muscle tissues. A summary of metabolic phenotype of these male ARKO mice was shown in Fig. 4. In another study of male ARKO mice [23], male specific late-onset obesity was also noted. However, insulin resistance was more clearly observed in their study. The discrepant observation may result from the difference in the stage of development of insulin resistance in these mice, namely compensated stage or disrupted stage. 3. Testosterone and preadipocyte differentiation Most recently, T or DHT was reported to suppress 3T3-L1 preadipocyte differentiation [24]. This effect was blocked by the anti-androgen, flutamide, indicating AR is involved in this mechanism. The summary of the mechanism proposed in this paper was that dihydrotestosterone (DHT)-dependent AR activation stimulates nuclear translocation of -catenin, which further makes complexes with TCF (T-cell factor 4)/LEF (lymphoid-enhancer factor) and then suppresses preadipocyte differentiation. Since canonical Wnt/-catenin signaling has been already known to suppress preadipocyte differentiation [25], DHT–AR activation signaling bypasses the canonical Wnt/-catenin signaling and directly stimulates -catenin and its downstream pathways. In the time course of transcripts of key molecules that controls differentiation process from 3T3-L1 preadipocyte to mature adipocyte, there is a kind cascade starting from the upstream regulator, like CEBP proteins and PPAR␥ and finally leading to the expressions of aP2 and many other adipocyrokines in mature adipocytes. Very interestingly, AR was expressed from the early stage of differentiation and its expression was increased with the progress of the adipocytes differentiation (unpublished observation). This result suggests that AR is probably working as a kind of brake for adipocyte differentiation. This mechanism also explains well the anti-obesity effect of androgens–AR system. 4. Clinical implications The above findings may provide some clinical implications as to why middle-aged people are prone to develop obesity and what we have to do to protect against obesity. Usually, basal metabolic rate is very high in younger age especially in adolescent ages. They seldom develop obesity because of high-energy expenditure. But after this period, basal metabolic rate decreases gradually with
5. Conclusions Activation of androgen–AR signaling may cause anti-obesity effect by inhibiting preadipocyte differentiation and by stimulating energy expenditure and lypolysis, whereas it may cause exacerbation of insulin sensitivity partly through inhibitory effect on adiponectin secretion from adipose tissue. References [1] N. Yoshiike, F. Kaneda, H. Takimoto, Epidemiology of obesity and public health strategies for its control in Japan, Asia Pac. J. Clin. Nutr. 11 (Suppl. 8) (2002) S727–S731. [2] S. Tsugane, S. Sasaki, Y. Tsubono, Under- and overweight impact on mortality among middle-aged Japanese men and women: a 10-y follow-up pf JPHC study cohort I, Int. J. Obes. Relat. Metab. Disord. 26 (2002) 529–537. [3] Third Report of the National Cholesterol Education Program (NCEP) expert panel on detection, evaluation and treatment of high blood cholesterol in adults (Adult Treatment Panel III), Final Report, Circulation 106 (2002) 3143–3421. [4] N. Mauras, V. Hayes, S. Welch, A. Rini, K. Helgeson, M. Dokler, J.D. Veldhuis, R.J. Urban, Testosterone deficiency in young men: marked alterations in whole body protein kinetics, strength, and adiposity, J. Clin. Endocrinol. Metab. 83 (1998) 1886–1992. [5] R. Pasquali, Obesity and androgens: facts and perspectives, Fertil. Steril. 85 (2006) 1319–1340. [6] A. Vermeulen, Andropause, Maturitas 34 (2000) 5–15. [7] U.G. Kyle, L. Genton, D. Hans, L. Karesgard, D.O. Slosman, C. Pichard, Agerelated differences in fat-free mass, skeletal muscle, body cell mass and fat mass between 19 and 94 years, Eur. J. Clin. Nutr. 55 (2001) 663–672. [8] C. Rolf, S. von Eckardstein, U. Koken, E. Nieschlag, Testosterone substitution of hypogonadal men prevents the age-dependent increases in body mass index, body fat and leptin seen in healthy ageing men: results of a cross-sectional study, Eur. J. Endocrinol. 146 (2002) 505–511. [9] C. Wang, R.S. Swedloff, A. Iranmanesh, A. Dobs, P.J. Snyder, G. Cunningham, A.M. Matsumoto, T. Weber, N. Berman, Transdermal testosterone gel improves sexual function, mood, muscle strength, and body composition parameters in hypogonadal men, J. Clin. Endocrinol. Metab. 85 (2000) 2839–2853. [10] T. Sato, T. Matsumoto, T. Yamada, T. Watanabe, H. Kawano, S. Kato, Late onset of obesity in male androgen receptor-deficient (AR KO) mice, Biochem. Biophys. Res. Commun. 300 (2003) 167–171. [11] S. Yeh, M.Y. Tsai, Q. Xu, X.M. Mu, H. Lardy, K.E. Huang, H. Lin, S.D. Yeh, S. Altuwaijri, X. Zhou, L. Xing, B.F. Boyce, M.C. Hung, S. Zhang, L. Gan, C. Chang, Generation and characterization of androgen receptor knockout (ARKO) mice: an in vivo model for the study of androgen functions in selective tissues, Proc. Natl. Acad. Sci. U.S.A. 99 (2002) 13498–13503. [12] T. Sato, T. Matsumoto, H. Kawano, T. Watanabe, Y. Uematsu, K. Sekine, T. Fukuda, K. Aihara, A. Krust, T. Yamada, S. Kato, Brain masculinization requires androgen receptor function, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 1673–1678. [13] H. Kawano, T. Sato, T. Yamada, T. Matsumoto, K. Sekine, T. Watanabe, T. Nakamura, T. Fukuda, K. Yoshimura, T. Yoshizawa, S. Kato, Suppressive function of androgen receptor in bone resorption, Proc. Natl. Acad. Sci. U.S.A. 100 (2003) 9416–9421. [14] W. Fan, T. Yanase, M. Nomura, T. Okabe, K. Goto, T. Sato, H. Kawano, S. Kato, H. Nawata, Androgen receptor null male mice develop late-onset obesity due to decreased energy expenditure and lypolytic activity but show normal insulin sensitivity with high adiponectin secretion, Diabetes 54 (2005) 1000–1008. [15] G. Argyropoulos, M.E. Harper, Uncoupling proteins and thermoregulation, J. Appl. Physiol. 92 (2002) 2187–2198. [16] E.S. Bachman, H. Dhillon, C.Y. Zhang, S. Cinti, A.C. Bianco, B.K. Kobilka, B.B. Lowell, betaAR signaling required for diet-induced thermogenesis and obesity resistance, Science 297 (2002) 843–845.
T. Yanase et al. / Journal of Steroid Biochemistry & Molecular Biology 109 (2008) 254–257 [17] B.M. Spiegelman, J.S. Flier, Obesity and the regulation of energy balance, Cell 104 (2001) 531–543. [18] N. Maeda, I. Shimomura, K. Kishida, H. Nishizawa, M. Matsuda, H. Nagaretani, N. Furuyama, H. Kondo, M. Takahashi, Y. Arita, R. Komuro, N. Ouchi, S. Kihara, Y. Tochino, K. Okutomi, M. Horie, S. Takeda, T. Aoyama, T. Funahashi, Y. Matsuzawa, Diet-induced insulin resistance in mice lacking adiponectin/ACRP30, Nat. Med. 8 (2002) 731–737. [19] H. Nishizawa, I. Shimomura, K. Kishida, N. Maeda, H. Kuriyama, H. Nagaretani, M. Matsuda, H. Kondo, N. Furuyama, S. Kihara, T. Nakamura, Y. Tochino, T. Funahashi, Y. Matsuzawa, Androgens decrease plasma adiponectin, an insulinsensitizing adipocyte-derived protein, Diabetes 51 (2002) 2734–2741. [20] F. Lanfranco, M. Zitzmann, M. Simoni, E. Nieschlag, Serum adiponectin levels in hypogonadal males: influence of testosterone replacement therapy, Clin. Endocrinol. (Oxf.) 60 (2004) 500–507. [21] A. Xu, K.W. Chan, R.L. Hoo, Y. Wang, K.C. Tan, J. Zhang, B. Chen, M.C. Lam, C. Tse, G.J. Cooper, K.S. Lam, Testosterone selectively reduces the high molecular weight form of adiponectin by inhibiting its secretion from adipocytes, J. Biol. Chem. 280 (2005) 18073–18080. [22] N. Kubota, Y. Terauchi, H. Miki, H. Tamemoto, T. Yamauchi, K. Komeda, S. Satoh, R. Nakano, C. Ishii, T. Sugiyama, K. Eto, Y. Tsubamoto, A. Okuno, K. Murakami,
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Contents lists available at ScienceDirect
Journal of Steroid Biochemistry and Molecular Biology journal homepage: www.elsevier.com/locate/jsbmb
Androgens and metabolic syndrome: Lessons from androgen receptor knock out (ARKO) mice夽 Toshihiko Yanase a,∗ , WuQiang Fan a , Kanako Kyoya a , Liu Min a , Ryoichi Takayanagi a , Shigeaki Kato b , Hajime Nawata c a b c
Department of Medicine and Bioregulatory Science, Kyushu University, Maidashi 3-1-1, Higashi-ku, Fukuoka 812-8582, Japan Institute of Molecular and Cellular Biosciences, Graduate School of Agricultural and Life Sciences, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan Graduate School of Medical Science, Kyushu University, Maidashi 3-1-1, Higashi-ku, Fukuoka 812-8582, Japan
a r t i c l e Keywords: Androgen receptor Obesity Thermogenesis Gene knockout
i n f o
a b s t r a c t Testosterone (T) is an important factor for determining body composition in males. Abdominal obesity is inversely correlated with serum T levels in men, leading to greater mortality. Pathologically hypogonadal men also have a significantly higher fat mass, which is reversed by T administration. However, the mechanism for such anti-obesity effect of androgen has not been well clarified. Androgen receptor (AR) null male mice revealed late-onset obesity. Male ARKO mice were euphagic compared to the wild-type male controls, but also less dynamic and less oxygen consuming. Transcript profiling indicated that male ARKO mice had lower transcripts for the thermogenetic uncoupling protein 1 (UCP1). We also found enhanced secretion of adiponectin, which is insulin-sensitizing, from adipose tissue in comparison to wild type, which might partly explain why the overall insulin sensitivity of male ARKO mice remained almost intact despite their apparent obesity. In addition, decreased lipolysis rather than increased lipid synthesis was observed, which might also account for the increased adiposity in male ARKO mice. The results revealed that AR plays important roles in male metabolism by affecting the energy balance, and is negative to both adiposity and insulin sensitivity. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Obesity is now becoming a big medical problem in the world and even in Japan, it is not exceptional [1]. Body mass index of Japanese men is increasing every year in every generation. On the other hand, that of Japanese women under 50-year-old is rather decreasing every year in recent 30 years [1], probably because women pay much more attention to their own body image. The most important thing is that the probability that a person of each BMI dies 10 years later is much higher in men than in women [2]. So, to tackle with obesity especially in middle-aged men is a very important issue. Now, there is a criterion for clinical diagnosis of metabolic syndrome, which is considered to be a risk factor for cardiovascular disease beyond cholesterol [3]. In these criteria, visceral fat obesity is the most upstream pathogenesis of this syndrome. Our data shows that visceral fat to subcutaneous fat ratio increases with age
in men (Fig. 1) and that free testosterone concentrations in men are inversely correlated with % of body fat (Fig. 2). These data suggest that endogenous testosterone in men may be protective against the increase of fat mass with age. The inverse relationship between endogenous testosterone and fat mass has been also confirmed by many other investigations [4–7]. As one of such investigations, Mauras et al. showed that GnRH administration for 10 weeks to healthy young men caused low free testosterone level, low lipid oxidation and low resting energy expenditure, whereas the treatment brought a increase of % of body fat mass [4]. These results again indicate that physiological level of testosterone has anti-obesity effect in men. In addition, a significantly higher fat mass in pathologically hypogonadal men is reversed by testosterone administration [8,9]. However, the mechanism for the anti-obesity effect of androgens has not been well elucidated. 2. Lessons from androgen receptor knock out mice
夽 Presented at the 12th International Congress on Hormonal Steroids and Hormones & Cancer’ (Athens, Greece, 13–16 September 2006). ∗ Corresponding author. Tel.: +81 92 642 5276; fax: +81 92 642 5287. E-mail address: [email protected] (T. Yanase). 0960-0760/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsbmb.2008.03.017
Androgen receptor knock out (ARKO) mice were independently developed by two groups including one of authors, Kato [10,11]. In addition to several interesting phenotypes of these mice like impaired brain masculinization [12] and osteoporosis [13], the mice
T. Yanase et al. / Journal of Steroid Biochemistry & Molecular Biology 109 (2008) 254–257
255
Fig. 1. V/S ratio in men with age. V/S (visceral to subcutaneous fat) in men increases with age (P < 0.01). V/S ratio was measured by CT-based body composition analysis at the level of navel.
also showed metabolic phenotype. Namely, only male ARKO mice but not female ARKO mice showed a late-onset obesity [10]. After this observation, we analyzed the detailed mechanism how male ARKO mice develop obesity [14]. This male ARKO mouse showed marked increase of visceral and subcutaneous fat. In these ARKO male mice, not only white adipose tissue (WAT) but also brown adipose tissue (BAT) was something wrong in the character. BAT is associated with energy expenditure by heat production using uncoupling protein 1 (UCP-1) [15]. But, BAT from male ARKO mice showed WAT-like appearance. Although BAT usually does not produce leptin, the BAT of these mice produced exceptionally high amount of leptin just like WAT. These data suggest some functional impairment of BAT in these mice. A similar abnormality of BAT has been also reported in -adrenergic receptor KO mice [16]. The concept of energy balance, which comprises both energy intake (feeding) and energy expenditure (physical activity, basal metabolism and adaptive thermogenesis), is the key to understanding obesity [17]. We then examined the energy balance in male ARKO mice, in respect of energy intake or energy expenditure. As a result, food intake in male ARKO mice was almost as same as that of wild type. However, this mouse was less dynamic, namely distance of run in 8 h was very low and the numbers of standingup opportunities were smaller than those of wild type. Actually, energy expenditure was impaired in these mice because oxygen consumption ratio of male ARKO mice was much smaller than that
Fig. 2. Relationship between male-free testosterone concentrations in serum and % of body fat. % of body fat was measured using body composition analyzer, in body. Inverse relationship between both parameters was observed (P < 0.05).
Fig. 3. Comparison of oxygen consumption between male wild type and male ARKO mice (cited from Ref. [14]).
of wild type (Fig. 3). Lowered spontaneous activities of male ARKO mice were also observed even in 8 weeks old age, at which male ARKO mice were not obese, indicating that less dynamic character of male ARKO mice is an intrinsic character but is not the result of obesity. One of the molecular mechanisms for the disturbed energy expenditure could be explained by the dramatic down regulation of UCP-1 in WAT in these mice. UCP-1 in BAT was also down regulated. However, the down regulation of UCP-1 in WAT is probably much more meaningful, because AR was mainly expressed in WAT rather than in BAT. UCP-1 is probably a direct target gene of DHT-AR because DHT-AR activation upregulated UCP-promoter activity in 3T3L-1 cells. Among transcripts involved in lipid homeostasis, hormonesensitive lipase of WAT in this ARKO mice was sharply down regulated, while transcripts of lipogenetic enzymes or protein like fatty acid synthase (FAS), acyl-coA carboxylase and sterol regulatory element-binding protein 1C (SRBP1C) in WAT were unchanged. These data may suggest that WAT accumulation in male ARKO mice may be partly brought by the inhibition of lypolysis. Obesity is usually associated with insulin resistance and glucose intolerance. However, another interesting feature of these mice was relatively intact insulin sensitivity in insulin challenge test and glucose tolerance in iPGTT although basal insulin levels of male ARKO mice tended to be higher than those of wild type. One of the reasons for the relatively intact insulin sensitivity of ARKO mice might be relatively high serum concentration of adiponectin compared to wild type, because adiponectin has recently been clarified to be an important insulin-sensitizing adipocytokine [18]. It should be noted that adiponectin transcript was down regulated as usually observed in obese phenotype, indicating that adiponectin secretion from WAT was probably increased in this ARKO mice. This is very compatible with recent findings that testosterone suppresses adponectin secretion, especially high molecular weight adiponectin, from WAT [19–21], although the mechanism remained unclear. In addition, relatively lower expression of PPAR␥ might also contribute to the intact insulin sensitivity in these mice as proposed by Dr. Kadowaki’s group based on the observation of PPAR␥ hetero knock out mice [22]. Skeletal muscle is an important target tissue of androgen. Relatively intact insulin sensitivity and glucose tolerance of these mice were also supported by the relatively increased expressions of glu-
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T. Yanase et al. / Journal of Steroid Biochemistry & Molecular Biology 109 (2008) 254–257
age, thus leading to easiness of weight gain in middle-aged people. This is because middle-aged obesity is usually associated with the decrease of muscle mass, the major organ in that basal metabolism takes place. Exercise reduces fat mass and increases muscle mass, leading to the increase of basal metabolism. Importantly, androgen exerts a similar effect as observed by exercise [8,9,26]. Actually, testosterone therapy in adult men with androgen deficiency syndromes was guided based on their serum testosterone levels and symptoms [26]. However, a long-term treatment with androgen stimulates prostate and thus may be a risk factor for the development of prostate cancer [26]. In the near future, we may expect to have a kind of drug, so called selective androgen receptor modulator (SARM), which may have anti-obesity effect but may not stimulate prostate. Fig. 4. Summary of metabolic phenotype of male ARKO mice. Male ARKO mice showed late-onset obesity, which was probably caused by decreased energy expenditure and decreased lypolysis. In spite of marked obesity, these mice showed relatively intact insulin sensitivity and glucose tolerance. Increased serum adiponectin level might contribute to the relatively intact glucose metabolism (cited from Ref. [14] and modified).
cose transporter 4, and several glucose oxidation enzymes like hexokinase 1, phosphofluctokinase and pyruvate kinase in skeletal muscle tissues. A summary of metabolic phenotype of these male ARKO mice was shown in Fig. 4. In another study of male ARKO mice [23], male specific late-onset obesity was also noted. However, insulin resistance was more clearly observed in their study. The discrepant observation may result from the difference in the stage of development of insulin resistance in these mice, namely compensated stage or disrupted stage. 3. Testosterone and preadipocyte differentiation Most recently, T or DHT was reported to suppress 3T3-L1 preadipocyte differentiation [24]. This effect was blocked by the anti-androgen, flutamide, indicating AR is involved in this mechanism. The summary of the mechanism proposed in this paper was that dihydrotestosterone (DHT)-dependent AR activation stimulates nuclear translocation of -catenin, which further makes complexes with TCF (T-cell factor 4)/LEF (lymphoid-enhancer factor) and then suppresses preadipocyte differentiation. Since canonical Wnt/-catenin signaling has been already known to suppress preadipocyte differentiation [25], DHT–AR activation signaling bypasses the canonical Wnt/-catenin signaling and directly stimulates -catenin and its downstream pathways. In the time course of transcripts of key molecules that controls differentiation process from 3T3-L1 preadipocyte to mature adipocyte, there is a kind cascade starting from the upstream regulator, like CEBP proteins and PPAR␥ and finally leading to the expressions of aP2 and many other adipocyrokines in mature adipocytes. Very interestingly, AR was expressed from the early stage of differentiation and its expression was increased with the progress of the adipocytes differentiation (unpublished observation). This result suggests that AR is probably working as a kind of brake for adipocyte differentiation. This mechanism also explains well the anti-obesity effect of androgens–AR system. 4. Clinical implications The above findings may provide some clinical implications as to why middle-aged people are prone to develop obesity and what we have to do to protect against obesity. Usually, basal metabolic rate is very high in younger age especially in adolescent ages. They seldom develop obesity because of high-energy expenditure. But after this period, basal metabolic rate decreases gradually with
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