- Email: [email protected]
Brown Fat-Derived Exosomes: Small Vesicles with Big Impact
Cell Metabolism
Previews Brown Fat-Derived Exosomes: Small Vesicles with Big Impact Yong Chen1,2 and Alexander Pfeifer1,* 1Institute
of Pharmacology and Toxicology, University of Bonn, 53127 Bonn, Germany address: Department of Cell and Tissue Biology, UCSF Diabetes Center, University of California, San Francisco, San Francisco, CA 94143-0669, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2017.03.012 2Present
Adipose tissue (AT) not only stores energy, but also secretes hormones and releases small vesicles known as exosomes. Thomou et al. (2017) now show that exosomes secreted by brown fat carry miRNAs that regulate the liver. Thus, AT exosomes might have therapeutic and diagnostic relevance for metabolic disorders. White adipose tissue (WAT) is known for its huge capacity to store energy in the form of lipids. In addition, it is also an endocrine organ (Scherer, 2006). The other major type of fat is brown adipose tissue (BAT), which has the specialized ability to dissipate energy in response to cold and pharmacological stimulation (Rosen and Spiegelman, 2014). BAT-dependent energy expenditure (EE) depends on the uncoupling protein 1 (UCP-1), which uncouples mitochondrial fuel oxidation from ATP production resulting in heat generation (Figure 1). Importantly, BAT mass correlates with leanness in human adults (van Marken Lichtenbelt et al., 2009). Although the major focus of BAT research clearly lies on EE as a potential route to treat metabolic imbalance in diabetic and overweight patients, recent publications demonstrate auto-/paracrine and endocrine functions of BAT (Villarroya et al., 2013). In a recent issue of Nature, Thomou et al. (2017) now further extend our understanding of BAT by identifying a function for exosomal miRNAs. MicroRNAs (miRNAs) are small noncoding RNA molecules containing approximately 20–22 nucleotides, that participate in RNA silencing and regulate the expression of many genes (Chen et al., 2017). Their formation depends on several enzymes including Dicer. miRNAs are of fundamental importance for adipocyte development and identity: knockout of Dicer in adipocytes abrogates miRNA processing resulting in an abnormal fat distribution and a white fat-like phenotype of BAT (BAT ‘‘whitening’’) (Thomou et al., 2017). Moreover, several other groups have identified specific miRNAs that play important roles in BAT (Chen et al., 2017). miRNAs are found not only within cells,
but also in the extracellular space and in the circulation. Most circulating miRNAs are actively secreted by cells via microvesicles like exosomes. Exosomes are about 100 nm in size and contain—apart from miRNAs—other RNAs and proteins that they transport to recipient cells and tissues. White (Ogawa et al., 2010) and brown (Chen et al., 2016) adipocytes have been previously shown to release exosomes containing miRNAs. Importantly, activation of BAT results in significant changes in both the number of exosomes released into circulation, as well as the pattern of exosomal miRNAs. One of these exosomal miRNAs, miR-92a (Figure 1), inversely correlates with BAT activity in mice and humans and could serve as a potential serum biomarker for active BAT (Chen et al., 2016). To determine the extent to which AT contributes to circulating miRNAs, Thomou et al. (2017) analyzed exosomes from serum of mice with an adiposespecific knockout of Dicer (ADicerKO). ADicerKO mice suffer from a severe metabolic phenotype with lipodystrophy (i.e., decreased WAT mass and BAT ‘‘whitening’’) and metabolic syndrome. ADicerKO mice exhibit a huge decrease in serum exosomal miRNAs. Moreover, serum samples from patients with congenital generalized lipodystrophy and patients with HIV-associated lipodystrophy, who have been found to have decreased Dicer levels in AT, also showed a decreased expression of miRNAs in exosomes indicating that AT is a major source of circulating miRNAs also in humans. Transplantation of different fat pads (BAT, subcutaneous and visceral WAT) into ADicerKO mice resulted in restoration
of circulating miRNAs. However, when assessing the physiological effects of fat transplantation, the authors found that only BAT transplants improved the diabetic phenotype of Dicer-deficient mice. To identify the physiological target of BAT-derived exosomes, they focused on fibroblast growth factor 21 (FGF21) and found increased levels of this important metabolic hormone in the circulation of ADicerKO mice. FGF21 is mainly produced in the liver and has multiple beneficial effects on energy homeostasis and whole-body metabolism. Among other functions, FGF21 has been demonstrated to directly stimulate thermogenic capacity of BAT and to induce ‘‘browning’’ of WAT (Fisher et al., 2012). Since BAT transplantation reduced the levels of Fgf21 mRNA in the liver of ADicerKO mice, Thomou et al. (2017) hypothesized that exosomal miRNAs might be responsible. A search of the miRDB database and luciferase reporter assays using liver cells identified miR-99b as the miRNA targeting Fgf21. Next, they showed that wild-type exosomes or ADicerKO exosomes reconstituted with miR-99b reduced mRNA levels of Fgf21 in isolated hepatocytes to a higher extent than ADicerKO exosomes. In addition, they transfected wild-type and ADicerKO mice with a FGF21-luciferase reporter and found higher FGF21 reporter activity in ADicerKO mice, which was suppressed when the mice were injected with wildtype serum exosomes. Similarly, the increased FGF21 reporter activity and hepatic mRNA levels of Fgf21 in ADicerKO mice were suppressed when ADicerKO exosomes reconstituted with miR-99b were injected into the mice (Figure 1).
Cell Metabolism 25, April 4, 2017 ª 2017 Elsevier Inc. 759
Cell Metabolism
Previews
To directly study whether to its capacity to dissipate BAT-derived miRNAs can energy—may regulate metaindeed regulate the liver, the bolism by controlling other orauthors used two elegant apgans via exosomal miRNAs. proaches. (1) They injected mice with a vector expressing REFERENCES human miRNA (miR-302f) in BAT, along with a reporter Chen, Y., Buyel, J.J., Hanssen, M.J., gene for this human miRNA in Siegel, F., Pan, R., Naumann, J., Schell, M., van der Lans, A., Schlein, the liver. 5 days later they C., Froehlich, H., et al. (2016). Nat. found a 95% reduction of the Commun. 7, 11420. miR-302f reporter in the liver Chen, Y., Pan, R., and Pfeifer, A. showing communication be(2017). Pharmacol. Ther. 170, 1–7. tween the miRNA expressed in BAT and the target/reporter Deng, Z.B., Poliakov, A., Hardy, R.W., Clements, R., Liu, C., Liu, Y., expressed in liver. (2) They Wang, J., Xiang, X., Zhang, S., transferred serum exosomes Zhuang, X., et al. (2009). Diabetes 58, 2498–2505. from mice expressing miR302f in BAT into mice carrying Fisher, F.M., Kleiner, S., Douris, N., the miR-302f reporter in the Fox, E.C., Mepani, R.J., Verdeguer, F., Wu, J., Kharitonenkov, A., Flier, liver, which also resulted in Figure 1. Brown Adipocytes Release miRNAs into the Circulation J.S., Maratos-Flier, E., and Spiegelvia Exosomes a 95% reduction in miR-302f man, B.M. (2012). Genes Dev. 26, Exosomes released by brown adipose tissue (BAT) contain miR-99b that reporter activity showing that 271–281. targets Fgf21 in liver (Thomou et al., 2017) and miR-92a, a potential BAT miRNAs can be delivered biomarker for BAT activity (Chen et al., 2016), as well as other miRNAs. Ogawa, R., Tanaka, C., Sato, M., NaOrange rectangle, mitochondria; yellow circle, lipid droplet; violet oval, to the liver by exosomes. gasaki, H., Sugimura, K., Okumura, nucleus; green triangle, exosomal miR-92a; blue circle, exosomal miR-99b. Nevertheless, several quesK., Nakagawa, Y., and Aoki, N. (2010). Biochem. Biophys. Res. tions remain open. Is there a Commun. 398, 723–729. preference of BAT to communicate with the liver? Thomou et al. et al., 2016), one might speculate that Rosen, E.D., and Spiegelman, B.M. (2014). Cell (2017) speculate that AT exosomes are FGF21 levels are further downregulated 156, 20–44. also taken up by other cells and organs. after BAT activation. This leads to the Scherer, P.E. (2006). Diabetes 55, 1537–1545. Indeed, exosomes isolated from explants next question: since FGF21 enhances Thomou, T., Mori, M.A., Dreyfuss, J.M., Konishi, of murine WAT have previously been BAT function and induces ‘‘browning’’ of M., Sakaguchi, M., Wolfrum, C., Rao, T.N., Winnay, shown to be taken up by peripheral blood WAT (Fisher et al., 2012), inhibition of J.N., Garcia-Martin, R., Grinspoon, S.K., et al. monocytes and to regulate their activa- FGF21 expression in liver might constitute (2017). Nature 542, 450–455. tion/differentiation (Deng et al., 2009). a negative feedback loop or brake that van Marken Lichtenbelt, W.D., Vanhommerig, Does activation (e.g., by cold exposure) limits brown adipocyte activation J.W., Smulders, N.M., Drossaerts, J.M., Kemerink, G.J., Bouvy, N.D., Schrauwen, P., and or ‘‘whitening’’ (BAT adopts a white-like (Figure 1). Teule, G.J. (2009). N. Engl. J. Med. 360, Overall, the results by Thomou et al. 1500–1508. phenotype during obesity) of BAT influence BAT-liver communication? Since (2017) shine new light on BAT and its Villarroya, J., Cereijo, R., and Villarroya, F. BAT of cold-exposed mice exhibits a 9- importance for energy homeostasis. (2013). Am. J. Physiol. Endocrinol. Metab. 305, fold increase in exosome release (Chen They demonstrate that BAT—in addition E567–E572.
760 Cell Metabolism 25, April 4, 2017
Previews Brown Fat-Derived Exosomes: Small Vesicles with Big Impact Yong Chen1,2 and Alexander Pfeifer1,* 1Institute
of Pharmacology and Toxicology, University of Bonn, 53127 Bonn, Germany address: Department of Cell and Tissue Biology, UCSF Diabetes Center, University of California, San Francisco, San Francisco, CA 94143-0669, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2017.03.012 2Present
Adipose tissue (AT) not only stores energy, but also secretes hormones and releases small vesicles known as exosomes. Thomou et al. (2017) now show that exosomes secreted by brown fat carry miRNAs that regulate the liver. Thus, AT exosomes might have therapeutic and diagnostic relevance for metabolic disorders. White adipose tissue (WAT) is known for its huge capacity to store energy in the form of lipids. In addition, it is also an endocrine organ (Scherer, 2006). The other major type of fat is brown adipose tissue (BAT), which has the specialized ability to dissipate energy in response to cold and pharmacological stimulation (Rosen and Spiegelman, 2014). BAT-dependent energy expenditure (EE) depends on the uncoupling protein 1 (UCP-1), which uncouples mitochondrial fuel oxidation from ATP production resulting in heat generation (Figure 1). Importantly, BAT mass correlates with leanness in human adults (van Marken Lichtenbelt et al., 2009). Although the major focus of BAT research clearly lies on EE as a potential route to treat metabolic imbalance in diabetic and overweight patients, recent publications demonstrate auto-/paracrine and endocrine functions of BAT (Villarroya et al., 2013). In a recent issue of Nature, Thomou et al. (2017) now further extend our understanding of BAT by identifying a function for exosomal miRNAs. MicroRNAs (miRNAs) are small noncoding RNA molecules containing approximately 20–22 nucleotides, that participate in RNA silencing and regulate the expression of many genes (Chen et al., 2017). Their formation depends on several enzymes including Dicer. miRNAs are of fundamental importance for adipocyte development and identity: knockout of Dicer in adipocytes abrogates miRNA processing resulting in an abnormal fat distribution and a white fat-like phenotype of BAT (BAT ‘‘whitening’’) (Thomou et al., 2017). Moreover, several other groups have identified specific miRNAs that play important roles in BAT (Chen et al., 2017). miRNAs are found not only within cells,
but also in the extracellular space and in the circulation. Most circulating miRNAs are actively secreted by cells via microvesicles like exosomes. Exosomes are about 100 nm in size and contain—apart from miRNAs—other RNAs and proteins that they transport to recipient cells and tissues. White (Ogawa et al., 2010) and brown (Chen et al., 2016) adipocytes have been previously shown to release exosomes containing miRNAs. Importantly, activation of BAT results in significant changes in both the number of exosomes released into circulation, as well as the pattern of exosomal miRNAs. One of these exosomal miRNAs, miR-92a (Figure 1), inversely correlates with BAT activity in mice and humans and could serve as a potential serum biomarker for active BAT (Chen et al., 2016). To determine the extent to which AT contributes to circulating miRNAs, Thomou et al. (2017) analyzed exosomes from serum of mice with an adiposespecific knockout of Dicer (ADicerKO). ADicerKO mice suffer from a severe metabolic phenotype with lipodystrophy (i.e., decreased WAT mass and BAT ‘‘whitening’’) and metabolic syndrome. ADicerKO mice exhibit a huge decrease in serum exosomal miRNAs. Moreover, serum samples from patients with congenital generalized lipodystrophy and patients with HIV-associated lipodystrophy, who have been found to have decreased Dicer levels in AT, also showed a decreased expression of miRNAs in exosomes indicating that AT is a major source of circulating miRNAs also in humans. Transplantation of different fat pads (BAT, subcutaneous and visceral WAT) into ADicerKO mice resulted in restoration
of circulating miRNAs. However, when assessing the physiological effects of fat transplantation, the authors found that only BAT transplants improved the diabetic phenotype of Dicer-deficient mice. To identify the physiological target of BAT-derived exosomes, they focused on fibroblast growth factor 21 (FGF21) and found increased levels of this important metabolic hormone in the circulation of ADicerKO mice. FGF21 is mainly produced in the liver and has multiple beneficial effects on energy homeostasis and whole-body metabolism. Among other functions, FGF21 has been demonstrated to directly stimulate thermogenic capacity of BAT and to induce ‘‘browning’’ of WAT (Fisher et al., 2012). Since BAT transplantation reduced the levels of Fgf21 mRNA in the liver of ADicerKO mice, Thomou et al. (2017) hypothesized that exosomal miRNAs might be responsible. A search of the miRDB database and luciferase reporter assays using liver cells identified miR-99b as the miRNA targeting Fgf21. Next, they showed that wild-type exosomes or ADicerKO exosomes reconstituted with miR-99b reduced mRNA levels of Fgf21 in isolated hepatocytes to a higher extent than ADicerKO exosomes. In addition, they transfected wild-type and ADicerKO mice with a FGF21-luciferase reporter and found higher FGF21 reporter activity in ADicerKO mice, which was suppressed when the mice were injected with wildtype serum exosomes. Similarly, the increased FGF21 reporter activity and hepatic mRNA levels of Fgf21 in ADicerKO mice were suppressed when ADicerKO exosomes reconstituted with miR-99b were injected into the mice (Figure 1).
Cell Metabolism 25, April 4, 2017 ª 2017 Elsevier Inc. 759
Cell Metabolism
Previews
To directly study whether to its capacity to dissipate BAT-derived miRNAs can energy—may regulate metaindeed regulate the liver, the bolism by controlling other orauthors used two elegant apgans via exosomal miRNAs. proaches. (1) They injected mice with a vector expressing REFERENCES human miRNA (miR-302f) in BAT, along with a reporter Chen, Y., Buyel, J.J., Hanssen, M.J., gene for this human miRNA in Siegel, F., Pan, R., Naumann, J., Schell, M., van der Lans, A., Schlein, the liver. 5 days later they C., Froehlich, H., et al. (2016). Nat. found a 95% reduction of the Commun. 7, 11420. miR-302f reporter in the liver Chen, Y., Pan, R., and Pfeifer, A. showing communication be(2017). Pharmacol. Ther. 170, 1–7. tween the miRNA expressed in BAT and the target/reporter Deng, Z.B., Poliakov, A., Hardy, R.W., Clements, R., Liu, C., Liu, Y., expressed in liver. (2) They Wang, J., Xiang, X., Zhang, S., transferred serum exosomes Zhuang, X., et al. (2009). Diabetes 58, 2498–2505. from mice expressing miR302f in BAT into mice carrying Fisher, F.M., Kleiner, S., Douris, N., the miR-302f reporter in the Fox, E.C., Mepani, R.J., Verdeguer, F., Wu, J., Kharitonenkov, A., Flier, liver, which also resulted in Figure 1. Brown Adipocytes Release miRNAs into the Circulation J.S., Maratos-Flier, E., and Spiegelvia Exosomes a 95% reduction in miR-302f man, B.M. (2012). Genes Dev. 26, Exosomes released by brown adipose tissue (BAT) contain miR-99b that reporter activity showing that 271–281. targets Fgf21 in liver (Thomou et al., 2017) and miR-92a, a potential BAT miRNAs can be delivered biomarker for BAT activity (Chen et al., 2016), as well as other miRNAs. Ogawa, R., Tanaka, C., Sato, M., NaOrange rectangle, mitochondria; yellow circle, lipid droplet; violet oval, to the liver by exosomes. gasaki, H., Sugimura, K., Okumura, nucleus; green triangle, exosomal miR-92a; blue circle, exosomal miR-99b. Nevertheless, several quesK., Nakagawa, Y., and Aoki, N. (2010). Biochem. Biophys. Res. tions remain open. Is there a Commun. 398, 723–729. preference of BAT to communicate with the liver? Thomou et al. et al., 2016), one might speculate that Rosen, E.D., and Spiegelman, B.M. (2014). Cell (2017) speculate that AT exosomes are FGF21 levels are further downregulated 156, 20–44. also taken up by other cells and organs. after BAT activation. This leads to the Scherer, P.E. (2006). Diabetes 55, 1537–1545. Indeed, exosomes isolated from explants next question: since FGF21 enhances Thomou, T., Mori, M.A., Dreyfuss, J.M., Konishi, of murine WAT have previously been BAT function and induces ‘‘browning’’ of M., Sakaguchi, M., Wolfrum, C., Rao, T.N., Winnay, shown to be taken up by peripheral blood WAT (Fisher et al., 2012), inhibition of J.N., Garcia-Martin, R., Grinspoon, S.K., et al. monocytes and to regulate their activa- FGF21 expression in liver might constitute (2017). Nature 542, 450–455. tion/differentiation (Deng et al., 2009). a negative feedback loop or brake that van Marken Lichtenbelt, W.D., Vanhommerig, Does activation (e.g., by cold exposure) limits brown adipocyte activation J.W., Smulders, N.M., Drossaerts, J.M., Kemerink, G.J., Bouvy, N.D., Schrauwen, P., and or ‘‘whitening’’ (BAT adopts a white-like (Figure 1). Teule, G.J. (2009). N. Engl. J. Med. 360, Overall, the results by Thomou et al. 1500–1508. phenotype during obesity) of BAT influence BAT-liver communication? Since (2017) shine new light on BAT and its Villarroya, J., Cereijo, R., and Villarroya, F. BAT of cold-exposed mice exhibits a 9- importance for energy homeostasis. (2013). Am. J. Physiol. Endocrinol. Metab. 305, fold increase in exosome release (Chen They demonstrate that BAT—in addition E567–E572.
760 Cell Metabolism 25, April 4, 2017