The Sum of All Browning in FGF21 Therapeutics

FGF21 mimetics are a promising therapeutic tool, believed to exert their anti-obesity effect partly through browning of white fat. Véniant et al., 2015Véniant M.M. Sivits G. Helmering J. Komorowski R. Lee J. Fan W. Moyer C. Lloyd D.J. Cell Metab. 2015; 21: 731-738Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar and Samms et al., 2015Samms R.J. Smith D.P. Cheng C.C. Antonellis P.P. Perfield 2nd, J.W. Kharitonenkov A. Gimeno R.E. Adams A.C. Cell Rep. 2015; https://doi.org/10.1016/j.celrep.2015.04.046Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar present evidence arguing against fat browning as the primary mechanism causal to weight loss following FGF21-based treatment in mice. FGF21 mimetics are a promising therapeutic tool, believed to exert their anti-obesity effect partly through browning of white fat. Véniant et al., 2015Véniant M.M. Sivits G. Helmering J. Komorowski R. Lee J. Fan W. Moyer C. Lloyd D.J. Cell Metab. 2015; 21: 731-738Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar and Samms et al., 2015Samms R.J. Smith D.P. Cheng C.C. Antonellis P.P. Perfield 2nd, J.W. Kharitonenkov A. Gimeno R.E. Adams A.C. Cell Rep. 2015; https://doi.org/10.1016/j.celrep.2015.04.046Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar present evidence arguing against fat browning as the primary mechanism causal to weight loss following FGF21-based treatment in mice. Fibroblast growth factor 21 (FGF21) is a master regulator of substrate turnover and orchestrates crosstalk between liver, white adipose tissue (WAT), brown adipose tissue (BAT), skeletal muscle, and pancreas. Pharmacologic FGF21 reverses obesity, diabetes, dyslipidemia, and hepatic steatosis in rodents and monkeys, seeding hopes that FGF21-based therapies could become a powerful weapon to combat the metabolic syndrome. As FGF21 is a potent inducer of uncoupling protein 1 (UCP1) in WAT, WAT browning has been proposed to underlie FGF21-mediated metabolic benefits. Two recent publications in Cell Metabolism (Véniant et al., 2015Véniant M.M. Sivits G. Helmering J. Komorowski R. Lee J. Fan W. Moyer C. Lloyd D.J. Cell Metab. 2015; 21: 731-738Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar) and Cell Reports (Samms et al., 2015Samms R.J. Smith D.P. Cheng C.C. Antonellis P.P. Perfield 2nd, J.W. Kharitonenkov A. Gimeno R.E. Adams A.C. Cell Rep. 2015; https://doi.org/10.1016/j.celrep.2015.04.046Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar) dissect the inter-dependence between WAT browning and therapeutic actions of FGF21-based treatment, concluding that the pharmacological effects of FGF21 are to a large extent independent from WAT browning. Both groups followed the same overall experimental paradigm and first compared interscapular BAT (iBAT) and inguinal WAT (igWAT) in mice at 21°C and/or thermoneutrality before and after FGF21 treatment, either with a long-acting FGF21 analog (FGF21-Fc) (Véniant et al., 2015Véniant M.M. Sivits G. Helmering J. Komorowski R. Lee J. Fan W. Moyer C. Lloyd D.J. Cell Metab. 2015; 21: 731-738Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar) or hFGF21 infusion (Samms et al., 2015Samms R.J. Smith D.P. Cheng C.C. Antonellis P.P. Perfield 2nd, J.W. Kharitonenkov A. Gimeno R.E. Adams A.C. Cell Rep. 2015; https://doi.org/10.1016/j.celrep.2015.04.046Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar). As BAT activity is not required at thermoneutrality, iBAT showed expected signs of BAT whitening in mice at thermoneutrality. Following FGF21 treatment, UCP1 was upregulated in iBAT of 21°C-housed mice, and whitened iBAT of mice at thermoneutrality regained a uniform BAT phenotype. Significant igWAT browning with UCP1 protein induction was only observed in mice at 21°C. Despite differences in igWAT browning extent, FGF21 treatment, however, induced similar increase in energy expenditure without altering food intake, generating similar weight loss and glycemic/lipid improvement at both temperatures. The authors then repeated experiments at thermoneutrality using UCP1-knockout (KO) mice. In mice treated with FGF21-Fc, energy expenditure enhancement was fully preserved without alteration in food intake (Véniant et al., 2015Véniant M.M. Sivits G. Helmering J. Komorowski R. Lee J. Fan W. Moyer C. Lloyd D.J. Cell Metab. 2015; 21: 731-738Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar). In contrast, hFGF21-infused mice manifested a blunted energy expenditure elevation, but food intake was decreased (Samms et al., 2015Samms R.J. Smith D.P. Cheng C.C. Antonellis P.P. Perfield 2nd, J.W. Kharitonenkov A. Gimeno R.E. Adams A.C. Cell Rep. 2015; https://doi.org/10.1016/j.celrep.2015.04.046Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar). Resultant weight loss and glycemic/lipid benefits were similar to wild-type animals. Collectively, these findings led to the proposal that WAT browning and UCP1 are not required for pharmacological effects of FGF21 treatment. These intriguing studies interrogating the mechanisms of FGF21-mediated metabolic benefits raise many questions. First, if WAT browning and/or UCP1 are not involved in weight loss, what are the precise mechanisms of FGF21-induced benefits? Second, given the remarkable efficacy of FGF21 in ameliorating dysmetabolism in animals, why were effects of FGF21 analog only modest in humans (Gaich et al., 2013Gaich G. Chien J.Y. Fu H. Glass L.C. Deeg M.A. Holland W.L. Kharitonenkov A. Bumol T. Schilske H.K. Moller D.E. Cell Metab. 2013; 18: 333-340Abstract Full Text Full Text PDF PubMed Scopus (640) Google Scholar)? Finally, in a broader context, if WAT browning is merely an epiphenomenon of FGF21 treatment, does this smother the excitement over WAT browning as an anti-obesity strategy? Recent studies have unveiled a functional BAT/WAT axis in mice. BAT paucity induces compensatory WAT browning sufficient to maintain resistance to obesity (Schulz et al., 2013Schulz T.J. Huang P. Huang T.L. Xue R. McDougall L.E. Townsend K.L. Cypess A.M. Mishina Y. Gussoni E. Tseng Y.H. Nature. 2013; 495: 379-383Crossref PubMed Scopus (288) Google Scholar), and marked WAT browning emerges in BAT-ablated mice to restore FGF21-mediated glycemic improvement (Emanuelli et al., 2014Emanuelli B. Vienberg S.G. Smyth G. Cheng C. Stanford K.I. Arumugam M. Michael M.D. Adams A.C. Kharitonenkov A. Kahn C.R. J. Clin. Invest. 2014; 124: 515-527Crossref PubMed Scopus (173) Google Scholar). Thus, BAT-WAT crosstalk exerts determinative in vivo influence, indicating the need to consider BAT status when assessing the browning phenomenon. As iBAT was “whitened” in animals at thermoneutrality pre-treatment, FGF21 induced browning of whitened iBAT, despite decreased browning of igWAT (Véniant et al., 2015Véniant M.M. Sivits G. Helmering J. Komorowski R. Lee J. Fan W. Moyer C. Lloyd D.J. Cell Metab. 2015; 21: 731-738Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar, Samms et al., 2015Samms R.J. Smith D.P. Cheng C.C. Antonellis P.P. Perfield 2nd, J.W. Kharitonenkov A. Gimeno R.E. Adams A.C. Cell Rep. 2015; https://doi.org/10.1016/j.celrep.2015.04.046Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar). Thermogenic effects of augmented iBAT could exceed igWAT browning because of greater UCP1 content (Nedergaard and Cannon, 2014Nedergaard J. Cannon B. Cell Metab. 2014; 20: 396-407Abstract Full Text Full Text PDF PubMed Scopus (383) Google Scholar), which implicates a browning hierarchy with browning of whitened iBAT potentially overshadowing igWAT browning. In this regard, the observed similarity in metabolic benefits following FGF21 treatment at 21°C versus thermoneutrality may be interpreted as congruent to “the sum of all browning” (i.e., browning of whitened iBAT + browning of igWAT), and total browning is required for maximum FGF21 pharmacological efficacy (Figure 1). So why were FGF21-mediated benefits preserved in the absence of UCP1? FGF21-Fc recapitulated metabolic benefits primarily through energy expenditure enhancement. This represents UCP1-independent thermogenesis, ascribed to multiorgan mitochondrial PGC-1α enrichment (Véniant et al., 2015Véniant M.M. Sivits G. Helmering J. Komorowski R. Lee J. Fan W. Moyer C. Lloyd D.J. Cell Metab. 2015; 21: 731-738Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar), as distinguished from UCP1-dependent pathways (i.e., WAT browning) in wild-type animals. In contrast, UCP1-independent thermogenesis was blunted in mice treated with hFGF21 infusion, and decreased food intake contributed to total weight loss (Samms et al., 2015Samms R.J. Smith D.P. Cheng C.C. Antonellis P.P. Perfield 2nd, J.W. Kharitonenkov A. Gimeno R.E. Adams A.C. Cell Rep. 2015; https://doi.org/10.1016/j.celrep.2015.04.046Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar). These findings suggest the recruitment of UCP1-independent thermogenesis and appetite suppression may be FGF21 formulation dependent. FGF21 is known to potentiate torpor and fasting centrally. Since hFGF21 can enter the central nervous system (CNS), centrally acting hFGF21 may quench thermogenesis and appetite. These torpid and anorexic signals may have been masked by FGF21-boosted UCP1-dependent thermogenesis and orexigenic compensation in wild-type animals. It remains to be determined whether FGF21-Fc crosses the blood-brain barrier, and it is possible that FGF21 analog design could be trading CNS penetrance for superior peripheral pharmacokinetics at the expense of attenuated appetite-suppressive benefits. Future studies of FGF21 analogs should address their central versus peripheral actions on appetite versus energy expenditure. In a broader context, how do these findings shape the prospect of FGF21-based therapy in humans? UCP1-KO mice represent an extreme phenotype in which alternative thermogenic mechanisms become activated as adaptation to UCP1 deficiency (Feldmann et al., 2009Feldmann H.M. Golozoubova V. Cannon B. Nedergaard J. Cell Metab. 2009; 9: 203-209Abstract Full Text Full Text PDF PubMed Scopus (969) Google Scholar). The recruitability of UCP1-independent thermogenesis by FGF21 in humans is unclear, and human BAT (hBAT) possesses mixed features of classic BAT and brite/beige fat (Jespersen et al., 2013Jespersen N.Z. Larsen T.J. Peijs L. Daugaard S. Homøe P. Loft A. de Jong J. Mathur N. Cannon B. Nedergaard J. et al.Cell Metab. 2013; 17: 798-805Abstract Full Text Full Text PDF PubMed Scopus (419) Google Scholar), indicating that modern humans closely resemble mice at thermoneutrality. hBAT is therefore more similar to brown-able “whitened iBAT” or igWAT. Indeed, WAT browning is inducible by FGF21 in human adipocytes (Lee et al., 2014Lee P. Linderman J.D. Smith S. Brychta R.J. Wang J. Idelson C. Perron R.M. Werner C.D. Phan G.Q. Kammula U.S. et al.Cell Metab. 2014; 19: 302-309Abstract Full Text Full Text PDF PubMed Scopus (556) Google Scholar) and cold acclimation recruits hBAT in vivo (Yoneshiro et al., 2013Yoneshiro T. Aita S. Matsushita M. Kayahara T. Kameya T. Kawai Y. Iwanaga T. Saito M. J. Clin. Invest. 2013; 123: 3404-3408Crossref PubMed Scopus (668) Google Scholar). Taken together, efficacy of FGF21-based therapeutics may ultimately hinge on browning in humans, despite their astonishing versatility in eliciting UCP1-independent thermogenesis in UCP1-KO animals. Since circulating FGF21 increases in humans following cold exposure, and FGF21 rise is greater among those with higher hBAT abundance (Lee et al., 2014Lee P. Linderman J.D. Smith S. Brychta R.J. Wang J. Idelson C. Perron R.M. Werner C.D. Phan G.Q. Kammula U.S. et al.Cell Metab. 2014; 19: 302-309Abstract Full Text Full Text PDF PubMed Scopus (556) Google Scholar), it is tempting to speculate that hBAT abundance/responsiveness could modulate FGF21 effects. A recent clinical trial of FGF21 analog treatment in humans did not show significant glycemic benefits (Gaich et al., 2013Gaich G. Chien J.Y. Fu H. Glass L.C. Deeg M.A. Holland W.L. Kharitonenkov A. Bumol T. Schilske H.K. Moller D.E. Cell Metab. 2013; 18: 333-340Abstract Full Text Full Text PDF PubMed Scopus (640) Google Scholar). Could anti-diabetic effects be more prominent among selected individuals with greater hBAT abundance? It would be interesting to investigate whether hBAT status in humans impacts FGF21 analog therapeutic efficacy in the future. Overall, these new studies challenge the view that WAT browning underlies FGF21-mediated pharmacological effects. This dissociation may have been clouded by “hierarchical browning” of whitened BAT, as the “sum of all browning” may be responsible, at least partly, for the efficacy of FGF21 therapeutics. With multiple FGF21 analogs poised for clinical trials in the era of hBAT renaissance, it will be important to determine the contribution of hBAT activation and/or recruitment to the observed effects. Discrete Aspects of FGF21 In Vivo Pharmacology Do Not Require UCP1Samms et al.Cell ReportsMay 5, 2015In BriefFGF21 is a metabolic regulator whose effects are associated with increases in UCP1 and energy expenditure. To delineate the contribution of uncoupled respiration to the pharmacology of FGF21, Samms et al. examine its effects in UCP1 null mice. They report that the majority of metabolic effects triggered by FGF21 occur independently of UCP1. Full-Text PDF Open AccessPharmacologic Effects of FGF21 Are Independent of the “Browning” of White Adipose TissueVéniant et al.Cell MetabolismMay 05, 2015In BriefPart of the beneficial pharmacological effects of FGF21 therapy is ascribed to “browning” of white adipose tissue. Véniant et al. show here that FGF21-induced browning is temperature dependent and that the effects on body weight reduction and glucose homeostasis can occur in UCP1 KO mice Full-Text PDF Open Archive