Adipose mTORC1 Suppresses Prostaglandin Signaling and Beige Adipogenesis via the CRTC2-COX-2 Pathway

Abstract

Beige adipocytes are present in white adipose tissue (WAT) and have thermogenic capacity to orchestrate substantial energy metabolism and counteract obesity. However, adipocyte-derived signals that act on progenitor cells to control beige adipogenesis remain poorly defined. Here, we show that adipose-specific depletion of Raptor, a key component of mTORC1, promoted beige adipogenesis through prostaglandins (PGs) synthesized by cyclooxygenase-2 (COX-2). Moreover, Raptor-deficient mice were resistant to diet-induced obesity and COX-2 downregulation. Mechanistically, mTORC1 suppressed COX-2 by phosphorylation of CREB-regulated transcription coactivator 2 (CRTC2) and subsequent dissociation of CREB to cox-2 promoter in adipocytes. PG treatment stimulated PKA and promoted differentiation of progenitor cells to beige adipocytes in culture. Ultimately, we show that pharmacological inhibition or suppression of COX-2 attenuated mTORC1 inhibition-induced thermogenic gene expression in inguinal WAT in vivo and in vitro. Our study identifies adipocyte-derived PGs as key regulators of white adipocyte browning, which occurs through mTORC1 and CRTC2.

Keywords: COX-2; CRTC2; UCP1; adipose tissue; beige adipocyte; mTORC1; prostaglandin; thermogenesis.

Figure 1.. Raptor Deficiency Has Differential Effects on Thermogenesis in WAT and BAT

(A) The expression levels of the thermogenic andadipogenic markers UCP1, C/EBPβ, PGC1α, PPARγ, and adiponectin were suppressed by Raptor deficiency in interscapular BAT at room temperature.

ADPN, adiponectin; TH, tyrosine hydroxylase. (B) The expression levels of the thermogenic andadipogenic markers UCP1, C/EBPβ, but not PPARγ and PGC1α were induced in inguinal fat (iWAT) of Raptor-deficient mice. (C) H&E staining of eWAT, iWAT, BAT, and liver from 10-week-old male Raptor-deficient and control mice. (D) Quantification of adipocyte size in iWAT andeWAT of Raptor KO and control mice. Size of adipocytes were determined with ImageJ analysis of H&E-stained tissues. (E) Immunohistochemistry (IHC) staining of UCP1 iniWAT and BAT from 10-week-old Raptor-deficient and control mice. (F) The mRNA levels of thermogenic, beige, andbrown markers in inguinal fat of Raptor-deficient and control mice. (G) O₂ consumption of inguinal and brown fat from 10-week-old Raptor-deficient and control mice. Volume of O₂/min/μg tissue was used as the unit for O₂ consumption. The data are presented as mean ± SEM. (H) The increase of UCP1 levels by Raptor deficiency was greater in the stromal vascular fraction than in adipocyte fraction in iWAT compared to the control samples. Adip, adipocyte fraction; AT, adipose tissue; SVF, stromal vascular fraction. The data in (D)–(F) are presented as the mean ± SEM. *p < 0.05, **p < 0.01.

Figure 2.. Raptor Deficiency-Induced WAT Browning Does Not Require the Sympathetic Nervous System

(A) Surgical denervation led to significant downregulation of tyrosine hydroxylase (TH), while slightly decreasing Raptor deficiency-induced UCP1 expression in iWAT. n = 6/group. 10-weekold male Raptor KO and control mice were used for the following studies. For cold stress and thermoneutrality studies, animals were housed in an environmental chamber at 22_C for 72 hr followed by 30_C, or 6_C for an additional 48 hr. (B–E) Raptor deficiency upregulated expression levels of thermogenic markers UCP1 and C/EBPb in iWAT (B), while slightly downregulating thermogenic gene expression in BAT (C) under thermoneutrality as well as room temperature. n = 5–7/group in (B) and (C). The Raptor deficiency-induced expression of thermogenic genes ucp1 and c/ebpb was blunted under cold exposure conditions (6_C, 2 days) in iWAT (D) despite persistent reduction of thermogenic gene expression in BAT of Raptor KO mice (E). n = 8/group in (D) and (E). (F) Raptor mice displayed decreased O2 consumption under cold stress but not room temperature conditions. O2 consumption was normalized by body mass and analyzed using Student’s t test. The data in (A) and (F) are presented as the mean ± SEM. *p < 0.05, **p < 0.01.

Figure 3.. Raptor Deficiency in Adipocytes Induces the Expression of COX-2 and Production of Prostaglandins In Vivo and In Vitro

RNA-sequencing gene expression signatures of iWAT from 10-week-old male Raptor KO and control mice. n = 2–3/group. (A) Heatmap summarizing differential gene expression in adenoid cystic carcinoma (ACC) tumors versus normal samples and illustrating gene expression signatures for ACC tumors. Blue and red shading indicates downregulation and upregulation, respectively, as shown in the color key at upper right. A larger version of the heatmap with genes labeled is provided in Supplemental Experimental Procedures (Figure S2E). (B) The fold change of differential genes involved ininflammation in iWAT between 2-month-old Raptor KO and control mice. (C) RT-PCR analysis for part of differential genes inRNA sequencing (RNA-seq) of the iWAT in Raptor KO and control mice. (D) The protein levels of COX-2 in various fat depots. n = 4/group. (E) The mRNA levels of COX-2 in various fat depots. n = 3/group. The data are presented as mean ± SEM. (F and G) The expression of COX-2 was induced in iWAT (F) and eWAT (G) of Raptor KO mice. (H) The secretion of PGE2, PGD2, and PGI2 was elevated by Raptor deficiency in iWAT. The levels of PGs were determined by ELISA with the kits. (I) The expression levels of COX2 but not COX-1 were suppressed by Raptor depletion in primary differentiated adipocytes from iWAT. (J) mRNA levels of COX-2 were induced by Raptordeficiency in primary adipocytes from iWAT. n = 4/ group. (K) Secretion of PGE2, PGD2, and PGI2 in primaryRaptor KO and control adipocytes. n = 4/group. The data in (I) was the representative data from three individual experiments. The data in (C), (E), (H), (J), and (K) are presented as the mean ± SEM. *p < 0.05, **p < 0.01.

Figure 4.. Inhibiting COX-2 Suppresses Raptor Deficiency-Induced Development of Beige Adipocytes in White Adipose Tissue

10-week-old male Raptor-deficient and control mice (n = 8/group) were treated with or without COX-2 inhibitor celecoxib by i.p. injection for 10 days (15 μg/kg). (A) Administration of celecoxib blunted the inducing effect of Raptor deficiency on UCP1 and C/EBPβ in iWAT. n = 4/group. (B) The increase in mRNA levels of thermogenic markers UCP1 was diminished by treatment of celecoxib. (C) Representative image for H&E staining of iWAT,eWAT, interscapular BAT, and liver. (D) Quantification of adipocyte size in iWAT andeWAT of Raptor KO and control mice from (C). Size of adipocytes were determined with ImageJ analysis of H&E-stained tissues. n = 4/group. (E) O₂ consumption of inguinal and brown fat from Raptor-deficient and control mice treated with or without celecoxib. Volume of O₂/min/mg tissue was used as the unit for O₂ consumption. (F) The media from Raptor KO primary differentiated adipocytes induced expression of UCP1 and C/EBPβ during differentiation of primary preadipocytes from iWAT, while suppressing COX-2 using RNAi attenuated the promoting effect of Raptor deficiency on thermogenic gene expression in vitro. n = 4/group. The data in (B) and (D)–(F) are presented as the mean ± SEM. *p < 0.05, **p < 0.01. For (D), *p < 0.05, **p < 0.01, control versus KO with vehicle; ^#p < 0.05, ^##p < 0.01, KO with vehicle versus KO with celecoxib.

Figure 5.. Blocking PG Signaling Alleviates Raptor Deficiency-Induced Beige Adipogenesis in WAT

(A) Administration of PGD2, PGE2, and PGI2 by subcutaneous injection (0.1 μg/kg body weight) for 2 days induced the expression of UCP1 and C/EBPβ in iWAT. D2, PGD2; E2, PGE2; I2, PGI2. (B) The mRNA levels of thermogenic markers UCP1 and C/EBPβ were enhanced by treatment of PGD2, PGE2, and PGI2. n = 6/group. (C) The expression levels of UCP1 and C/EBPβ but not PPARγ and PGC1α were induced by PGI2 treatment at different dose during the differentiation of primary preadipocytes. (D) The mRNA levels of UCP1 were analyzed by RTPCR in the differentiated adipocytes treated with or without PGD2, PGE2, or PGI2. n = 4/group. (E and F) The intracellular cAMP levels (E) were determined by ELISA, and phosphorylation of PKA substrates (F) were analyzed by western blot analysis in primary preadipocytes that were starved for 3 hr followed by treatment of PGE2, PGD2, or PGI2 for 2 hr and 20 min, respectively. n = 3/group. (G) The induced expression levels of UCP1 and C/EBPβ by PGI2 were suppressed by PKA inhibition with H89 treatment during the differentiation of primary adipocytes from iWAT. n = 3/group. (H) Administration of PGI antagonist CAY10441 partially reversed the promoting effect of Raptor deficiency on UCP1 in iWAT. (I) The media from Raptor KO primary differentiatedadipocytes induced the expression of UCP1 and C/EBPβduringthedifferentiationofpreadipocytefrom iWAT, while this inducing effect by Raptor deficiency was diminished by the treatment of PGI antagonist CAY10441. n = 3/group. (A), (C), (F), and (H) were the representative data from three individual experiments. The data in (B), (D), (E), (G), and (I) are presented as the mean ± SEM. *p < 0.05, control versus treatment or two particular groups as indicated.

Figure 6.. Adipocyte mTORC1 Inhibition Protects Mice against HFD-Induced Downregulation of COX-2 and Energy Expenditure

(A–D) 6-week-old male C57BL/6 mice were fed with normal chow diet (NCD) or HFD for 4, 16, or 24 weeks. Protein levels of COX-2 and COX-1 along with phosphorylation of S6K (A) and mRNA level of COX-2 and COX-1 (B) in iWAT after 4-week-HFD feeding. Phosphorylation of S6K at Thr³⁸⁹ in iWAT (C) were stimulated by HFD feeding for 4, 16, or 24 weeks. Conversely, the expression levels of COX-2 in iWAT (D) were suppressed by 4 or 16 weeks of HFD feeding, while not significantly altered by 24 weeks of HFD feeding. n = 4–8/group in (C) and (D). (E) Phosphorylation of S6K at Thr³⁸⁹ was increased, and expression levels of COX-2 were suppressed in iWAT of ob/ob mice compared to lean mice. n = 5/group. (F) Rapamycin treatment induced expression of COX-2 in human primary adipocytes from neck adipose tissue. n = 3/group. (G)RaptordeficiencyprotectedagainstthesuppressionofCOX-2inducedby4-weekHFDandupregulatedbasalandHFD-inducedUCP1iniWAT in vivo.n=5–7/group. (H) O₂ consumption in Raptor KO and control mice throughout light and dark cycles on the day before HFD feeding and day 7 and day 30 after HFD feeding. The average O₂ consumption was normalized to whole-body mass and analyzed using Student’s t test. (F) was the representative data from three individual experiments. The data in (B)–(H) are presented as the mean ± SEM. *p < 0.05.

Figure 7.. mTORC1 Signaling Inhibits CRTC2 Phosphorylation at Ser¹³⁶ and Subsequently Suppresses COX-2 Transcription in Adipocytes

(A) Raptor deficiency induced COX-2 expression, which was diminished by treatment of PKA inhibitor H89 in primary preadipocytes. n = 3/group. (B) PKA inhibition attenuated the inducing effect of rapamycin treatment on COX-2 expression. (C) Immunofluorescence staining of CRTC2 after treatment of 100 mM cAMP homolog dibutyryl cyclic AMP, 10 nM rapamycin, or co-treatment in primary preadipocytes. (D) Leucine treatment stimulated the phosphorylation of CRTC2 at Ser¹³⁶ in primary preadipocytes. (E) Rapamycin treatment inhibited CRTC2 phosphorylation at Ser¹³⁶ in primary preadipocytes. (F) Raptor deficiency suppressed rapamycin-stimulated phosphorylation of CRTC2 at Ser¹³⁶ in primary preadipocytes. (G) (G) The effect of rapamycin treatment on COX-2 expression in CRTC2 suppressed and scrambled primary adipocytes. (H–K) Overexpression of CRTC2 markedly upregulated expression levels of COX-2 (H), increased the interaction between CRTC2 and CREB (I), promoted CREB binding to the cox-2 promoter (J), and enhanced luciferase activity of the cox-2 promoter (K), while diminishing the inducing effects of rapamycin in 3T3-L1 preadipocytes. n = 3/group in (J) and (K). Meanwhile, the ability of CRTC2 to induce expression of COX-2, increase CREB binding to the cox-2 promoter, and activate cox-2 promoter was attenuated by phosphor-mimic (S136D) but not phosphor-defective (S136A) mutation of CRTC2 in 3T3-L1 preadipocytes. The interaction between CREB and CRTC2 was determined using immunoprecipitation. The binding of CREB to the cox-2 promoter was determined using ChIP assay. The data in (B)–(I) are representative from at least three individual experiments. The quantified data for (D) and (E) are presented in Figures S5D and S5E. The data in (A), (J), and (K) are presented as the mean ± SEM. *p < 0.05.