BCL6 Regulates Brown Adipocyte Dormancy to Maintain Thermogenic Reserve and Fitness
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BCL6 regulates brown adipocyte dormancy to maintain thermogenic reserve and fitness Vassily I. Kutyavina and Ajay Chawlaa,b,c,1,2 aCardiovascular Research Institute, University of California, San Francisco, CA 94143-0795; bDepartment of Physiology, University of California, San Francisco, CA 94143-0795; and cDepartment of Medicine, University of California, San Francisco, CA 94143-0795 Edited by Steven A. Kliewer, The University of Texas Southwestern Medical Center, Dallas, TX, and approved July 1, 2019 (received for review April 30, 2019) Brown adipocytes provide a metabolic defense against environ- in metabolic tissues, such as adipocytes and liver, where its mental cold but become dormant as mammals habituate to warm functions are just beginning to be investigated (18–22). For ex- environments. Although dormancy is a regulated response in brown ample, a recent report found that deletion of Bcl6 in all adipo- adipocytes to environmental warmth, its transcriptional mecha- cytes increased de novo lipogenesis, resulting in selective nisms and functional importance are unknown. Here, we identify B expansion of subcutaneous white adipose tissue (WAT) and cell leukemia/lymphoma 6 (BCL6) as a critical regulator of dormancy protection from obesity-induced insulin resistance (21). Al- in brown adipocytes but not for their commitment, differentiation, though these studies suggest a potential role for BCL6 in fatty or cold-induced activation. In a temperature-dependent manner, acid metabolism, the role of BCL6 in brown adipocyte biology BCL6 suppresses apoptosis, fatty acid storage, and coupled respi- has not been explored. ration to maintain thermogenic fitness during dormancy. Mecha- Here, we investigated the regulatory functions of BCL6 in nistically, BCL6 remodels the epigenome of brown adipocytes to both metabolically active and dormant brown adipocytes. We enforce brown and oppose white adipocyte cellular identity. Thus, found that loss of BCL6 did not impair the commitment, dif- unlike other thermogenic regulators, BCL6 is specifically required ferentiation, or adrenergic activation of brown adipocytes, but it for maintaining thermogenic fitness when mammals acclimate to environmental warmth. selectively impaired their thermogenic capacity during dor- mancy. Thus, unlike other thermogenic regulators that act PHYSIOLOGY metabolism | transcription | thermogenesis | brown fat | acclimation downstream of the sympathetic nervous system, our data suggest that BCL6 acts in parallel with the sympathetic nervous system to reinforce brown adipocyte cellular identity during dormancy. n mammals, brown and beige adipocytes generate heat (ther- Imogenesis) through fuel oxidation and uncoupled respiration Results to defend core temperature in cold environments (1–4). As BCL6 Maintains Thermogenic Capacity of BAT during Dormancy. BCL6 mammals acclimate to warm environments, their need for ther- is expressed in mature BAT and differentiating brown adipocytes mogenesis decreases, resulting in the entry of brown and beige (SI Appendix, Fig. S1 A and B), where its functions are not well adipocytes into a dormant state to conserve energy and prevent hyperthermia (1, 4). For example, depots of brown adipose tissue (BAT) in adult humans are largely dormant but can be activated Significance by prolonged exposure to environmental cold or adrenergic ag- onists (5–9). Although entry of thermogenic adipocytes into During exposure to environmental cold, brown adipocytes pro- dormancy is a well-appreciated adaptation to environmental tect against hypothermia by generating heat (thermogenesis). In warmth, the factors and mechanisms that regulate dormancy in warm environments, brown adipocytes become inactive or dor- BAT remain unknown. mant but still maintain their identity and thermogenic capacity, While both brown and beige adipocytes undergo dormancy allowing rapid reactivation of thermogenesis upon subsequent during acclimation to environmental warmth, their molecular, cold exposure. Our understanding of the dormant state and its metabolic, and epigenetic states are quite distinct. For example, regulation is very limited. Here, we show that the transcription beige adipocytes lose virtually all of their thermogenic capacity factor B cell leukemia/lymphoma 6 (BCL6) is specifically required during acclimation to environmental warmth, whereas brown ad- for maintenance of thermogenic capacity during dormancy in ipocytes retain a significant portion of their thermogenic capacity brown adipocytes. Mechanistically, BCL6 drives a gene expression even after remaining dormant for many weeks (10, 11). This dis- program that promotes survival, fatty acid oxidation, and tinction between beige and brown adipocytes is also observed at uncoupled respiration. Thus, unlike other transcription factors the level of the epigenome. For example, the enhancer landscape that regulate cold-induced thermogenesis, BCL6 is specifically of beige adipocytes undergoes profound remodeling during ad- required for maintaining thermogenic fitness during adaptation aptation to warmth, becoming almost indistinguishable from that to environmental warmth. of white adipocytes (10). In contrast, brown adipocytes largely Author contributions: V.I.K. and A.C. designed research; V.I.K. performed research; V.I.K. maintain their chromatin state during dormancy, which allows and A.C. analyzed data; and V.I.K. and A.C. wrote the paper. them to maintain their thermogenic capacity (10). However, it is The authors declare no conflict of interest. not known what factors contribute to maintenance of a stable This article is a PNAS Direct Submission. epigenetic state during brown adipocyte dormancy. This open access article is distributed under Creative Commons Attribution-NonCommercial- B cell leukemia/lymphoma 6 (BCL6), a sequence-specific NoDerivatives License 4.0 (CC BY-NC-ND). transcriptional repressor, is expressed in many cell types but Data deposition: The data reported in this paper have been deposited in the Gene Ex- has been best studied in the immune system. In germinal center pression Omnibus (GEO) database, https://www.ncbi.nlm.nih.gov/geo (accession no. B cells, it plays an essential role in the germinal center reaction GSE122746). by repressing genes involved in DNA damage response, apo- 1To whom correspondence may be addressed. Email: [email protected]. ptosis, cell activation, and plasma cell differentiation (12, 13). 2Present address: Merck Research Labs, SAF-803, South San Francisco, CA 94080. BCL6 is also required for the differentiation of follicular helper This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. T cells and modulates the inflammatory activation of macro- 1073/pnas.1907308116/-/DCSupplemental. phages (14–17). Outside the immune system, BCL6 is expressed Published online August 2, 2019. www.pnas.org/cgi/doi/10.1073/pnas.1907308116 PNAS | August 20, 2019 | vol. 116 | no. 34 | 17071–17080 Downloaded by guest on September 24, 2021 understood. To investigate the role of BCL6 in brown adipocyte pothesis that BCL6 is important for maintenance of thermogenic biology, we crossed Bcl6f/f mice on C57BL6/J background with capacity in dormant BAT. Ucp1Cre, AdipoqCre,andMyf5Cre to generate conditional knockouts To investigate the role of BCL6 in BAT dormancy, we bred of BCL6 in BAT (Bcl6f/fUcp1Cre), pan-adipose (Bcl6f/fAdipoqCre), and reared mice at thermoneutrality (30 °C), a warm environ- + and Myf5 somitic brown adipocyte precursors (Bcl6f/fMyf5Cre), ment that minimizes thermogenesis and promotes entry of BAT respectively (Fig. 1A). In contrast to a previous study that impli- into its dormant state. Although thermoneutral housing did not cated BCL6 in commitment and differentiation of adipocyte alter expression of the BCL6 protein in BAT (SI Appendix, Fig. + precursors (20), we found that BCL6 was not required in Myf5 S1D), loss of BCL6 had a profound effect on the thermogenic precursors for the development of the brown adipocyte lineage competence of dormant BAT. For example, during a cold chal- (Fig. 1B). Furthermore, in mice raised at the normal vivarium lenge from 30 °C to 10 °C, core temperature rapidly declined in temperature of 22 °C, BCL6 was not required for cold-induced Bcl6f/fUcp1Cre and Bcl6f/fMyf5Cre mice, resulting in severe hypo- thermogenesis, as evidenced by normal core body temperature thermia (Fig. 1E and SI Appendix, Fig. S1E). In both conditional and survival of Bcl6f/fUcp1Cre and Bcl6f/fAdipoqCre mice upon acute knockouts, the inability to maintain core temperature was uni- exposure to 4 °C (Fig. 1 C and D and SI Appendix,Fig.S1C). Since formly lethal, whereas the majority of the control animals were BCL6 was not required for BAT thermogenesis in cold-adapted able to survive the cold challenge after adaptation to thermo- mice (i.e., raised at 22 °C), we considered the alternative hy- neutrality (Fig. 1F and SI Appendix, Fig. S1F). Bcl6f/f Bcl6f/fUcp1Cre f/f f/f Cre Bcl6f/f Bcl6f/fUcp1Cre A Bcl6f/f Bcl6f/f B C Bcl6 Bcl6 Ucp1 D Cre Cre 40 Bcl6f/f Adipoq Ucp1 100 C) 38 BCL6 36 Bcl6f/fAdipoqCre Bcl6f/fMyf5Cre 50 Survival (%) 34 Lamin B1 T : 22 C 4 C Ta: 22 C 4 C Core temperature ( a 32 0 02468 02468 Time (hours) Time (hours) Bcl6f/f Bcl6f/fUcp1Cre EFf/f f/f Cre G H Bcl6 Bcl6 Ucp1 Bcl6f/f Bcl6f/fUcp1Cre p 0.0001 40 T : 30 C 10 C 80 p 0.0001 a f/f 150 100 Bcl6 C) NE p=0.0004 Bcl6f/fUcp1Cre 35 60 NE 100 p=0.0003 p=0.0004 30 (ml/hr) 40 (ml/hr) 2 50 2 VO VO 50 25 Survival (%) 20 Core temperature ( T : 30 C 10 C a Ta=30 C Ta=22 C 20 0 0 0 02468 02468 0 20406080 02550 Time (hours) Time (hours) Time (min) Time (min) Veh CL-316,243 Cre Cre f/f f/f f/f f/f 1 f/f 1 p Bcl6 p I f/f J K L Bcl6 f/f Cre Bcl6 Bcl6 Uc Bcl6 Ucp1 Bcl6 Bcl6 Uc 200 Bcl6f/fUcp1Cre C) 40 p 0.0001 p0.0001 f/f 150 Bcl6 NE p=0.0002 35 100 p=0.0068 (ml/hr) 2 Cre by PKA 1 p VO 50 Phosphorylation Uc p=0.0189 f/f 30 Ta: 22 C 30 C (6 wks) 10 C 0 ADRB3 Bcl6 0 20406080 surface temperature ( iBAT 015 HSP90 Time (min) Time at 4 C (mins) Fig.