The adipocyte-inducible secreted phospholipases PLA2G5 and PLA2G2E play distinct roles in obesity

Abstract

Metabolic disorders, including obesity and insulin resistance, have their basis in dysregulated lipid metabolism and low-grade inflammation. In a microarray search of unique lipase-related genes whose expressions are associated with obesity, we found that two secreted phospholipase A2s (sPLA2s), PLA2G5 and PLA2G2E, were robustly induced in adipocytes of obese mice. Analyses of Pla2g5(-/-) and Pla2g2e(-/-) mice revealed distinct roles of these sPLA2s in diet-induced obesity. PLA2G5 hydrolyzed phosphatidylcholine in fat-overladen low-density lipoprotein to release unsaturated fatty acids, which prevented palmitate-induced M1 macrophage polarization. As such, PLA2G5 tipped the immune balance toward an M2 state, thereby counteracting adipose tissue inflammation, insulin resistance, hyperlipidemia, and obesity. PLA2G2E altered minor lipoprotein phospholipids, phosphatidylserine and phosphatidylethanolamine, and moderately facilitated lipid accumulation in adipose tissue and liver. Collectively, the identification of "metabolic sPLA2s" adds this gene family to a growing list of lipolytic enzymes that act as metabolic coordinators.

Figure 1. Obesity-Induced Expression of Pla2g5 and Pla2g2e in Hypertrophic Adipocytes

(A) Microarray of lipase-related genes in the WAT of C57BL/6 mice fed a LFD or HFD for 18 weeks. Values indicate fold-increases in the HFD group relative to the LFD group (n = 4, mean ± SD). (B) Expression of sPLA₂ mRNAs relative to Rn18s in the WAT of C57BL/6 mice fed a LFD (n = 6) or HFD (n = 9). (C) Expression of sPLA₂ mRNAs relative to RN18S in human visceral WAT (n = 18). (D, E) Pla2g5 (D) and Pla2g2e (E) expression in various tissues and cells of C57BL/6 mice (n = 5). (F) Expression of Pla2g5 and Pla2g2e in WAT of 12-wk-old Lep^ob/ob mice, the expression in age-matched C57BL/6 mice being regarded as 1 (n = 3). (G) Pla2g5 and Pla2g2e expression in adipocytes and SVF from WAT of LFD- or HFD-fed mice, the expression in adipocytes from LFD-fed mice being regarded as 1 (n = 8). In (B, D, E, G), mice were fed a LFD or HFD for 26 weeks. Data are compiled from one (A, C, F) or two (B, D, E, G) experiments. Mean ± SEM (B–I), *p < 0.05, **p < 0.01.

Figure 2. Increased Diet-Induced Obesity in Pla2g5^−/− Mice

(A) Representative photos of Pla2g5^+/+ and Pla2g5^−/− mice fed a LFD or HFD for 14 weeks. (B) Body weights of Pla2g5^+/+ and Pla2g5^−/− mice placed on a LFD or HFD for the indicated periods. (C) CT analysis of fat volumes in Pla2g5^+/+ and Pla2g5^−/− mice. Yellow areas indicate fat deposition in HFD-fed mice (Inset). (D, E) Fasting plasma leptin (D) and insulin (E) levels in Pla2g5^+/+ and Pla2g5^−/− mice. (F) ITT using 6-h fasted Pla2g5^+/+ and Pla2g5^−/− mice. (G) Immunoblotting of phosphorylated (P−) and total Akt in WAT of HFD-fed Pla2g5^+/+ and Pla2g5^−/− mice with (+) or without (−) 5-min treatment with insulin. The ratios of P-Akt to Akt were determined by densitometry, with the value of insulin-untreated Pla2g5^+/+ mice being regarded as 1 (n = 4). (H) GTT using 16-h fasted Pla2g5^+/+ and Pla2g5^−/− mice. (I) Glucose-stimulated insulin secretion under the conditions in (H). (J) Hematoxylin-eosin staining of WAT in Pla2g5^+/+ and Pla2g5^−/− mice (Scale bar, 50 μm). (K) Expression of adipogenic, lipogenic and lipolytic genes normalized by Gapdh in WAT of HFD-fed Pla2g5^+/+ and Pla2g5^−/− mice, with the expression in Pla2g5^+/+ mice being regarded as 1. (L, M) Plasma ALT (L) and hepatic TG (M) levels in Pla2g5^+/+ and Pla2g5^−/− mice. (N) Hematoxylin-eosin staining of liver from Pla2g5^+/+ and Pla2g5^−/− mice (Scale bar, 50 μm). In (C-N), mice were fed a LFD or HFD for 26 weeks. Data are compiled from two (C, G, I) or three (B, D–F, H, K-M) experiments. Images in (A, J, N) are representative of two experiments. Mean ± SEM, *p < 0.05, **p < 0.01.

Figure 3. Altered Lipoprotein Profiles in HFD-Fed Pla2g5^−/− Mice

(A) Plasma phospholipids (PL) and cholesterol (Chol) levels in Pla2g5^+/+ and Pla2g5^−/− mice (n = 4). (B) PL and Chol levels in LDL and HDL from Pla2g5^+/+ and Pla2g5^−/− mice (n = 4). (C) TG levels in individual lipoprotein particles from HFD-fed Pla2g5^+/+ and Pla2g5^−/− mice (n = 4). (D) HPLC profiles of PL, Chol and TG in plasma lipoproteins from HFD-fed Pla2g5^+/+ and Pla2g5^−/− mice. Two examples from each genotype are shown. (E) ESI-MS of PC, PE and PS in LDL from HFD-fed Pla2g5^+/+ and Pla2g5^−/− mice. (F) ESI-MS of unsaturated fatty acids in WAT of Pla2g5^+/+ and Pla2g5^−/− mice. (G) Correlation of PLA2G5 expression levels in human visceral WAT with LDL or HDL cholesterol [n = 128 (64 from mesentric WAT and 58 from colorectal fat appendices)]. In (A-F), mice were fed a LFD or HFD for 26 weeks. Data are representative of (A-C) or compiled from (D-G) two experiments. Mean ± SEM, *p < 0.05, **p < 0.01.

Figure 4. Exacerbated Adipose Tissue Inflammation in HFD-Fed Pla2g5^−/− Mice

(A, B) Expression of M1 or M2 macrophage markers normalized by Gapdh in WAT of Pla2g5^+/+ and Pla2g5^−/− mice, with their expression in Pla2g5^+/+ mice on a LFD being regarded as 1 (A). Ratios of M1 (Tnf) to M2 (Cd163) genes (B). (C, D) FACS analysis of adipose tissue macrophages (ATM) from Pla2g5^+/+ and Pla2g5^−/− mice. Counts of F4/80⁺CD11b⁺ ATM and proportions of CD11c^hi (M1) and CD206^hi (M2) macrophages in SVF from WAT (n = 4) (C). Representative FACS profiles of CD11c⁺ cells in the SVF (D). (E, F) TUNEL staining of WAT in Pla2g5^+/+ and Pla2g5^−/− mice. Average scores of TUNEL-positive cells (E) and representative images (Scale bar, 50 μm) (F). Mice were fed a LFD or HFD for 26 weeks. Data are compiled from two experiments (A, B, E) or representative of two experiments (C, D, F). Mean ± SEM, *p < 0.05, **p < 0.01.

Figure 5. PLA2G5 Promotes M2 Macrophage Polarization

(A, B) Expression of M1 and M2 macrophage markers in WT BMDMs cultured for 24 h with (+) or without (−) LPS + IFN-γ (A) or 200 μM PA (B) in the presence or absence of 70 nM PLA2G5 (n = 7). (C) Expression of M1 and M2 macrophage markers in WT BMDMs after culture for 24 h with various concentrations of PA (n = 6). (D) Expression of Pla2g5 in Pla2g5^+/+ and Pla2g5^−/− BMDMs after culture for 24 h with 200 μM PA (n = 6). (E, F) Expression of M1 and M2 markers in WT BMDMs cultured for 24 h with (+) or without (−) LPS + IFN-γ (D) or 200 μM PA (E) in the presence or absence of 1 μM lipid mediators (n = 6). (G) PGE₂ generation by WT BMDMs treated for 6 h with PLA2G5 in the presence (+) or absence (−) of 200 μM PA (n = 3). (H) Effects of OA or LA on the expression of M1 or M2 macrophage markers in WT BMDMs treated for 24 h with 200 μM PA (n = 3). (I) Expression of Il33 relative to Gapdh in WAT of Pla2g5^+/+ and Pla2g5^−/− mice fed a LFD or HFD for 26 weeks. Data are compiled from two experiments (A-F, I) or representative of two experiments (G, H). Mean ± SEM, *p < 0.05, **p < 0.01.

Figure 6. Altered Diet-Induced Adiposity, Fatty Liver and Lipoproteins in Pla2g2e^−/− Mice

(A) Representative photos of Pla2g2e^+/+ and Pla2g2e^−/− mice fed a LFD or HFD. (B) Body weights of Pla2g2e^+/+ and Pla2g2e^−/− mice fed a LFD or HFD for the indicated periods. (C) CT analysis of fat volumes in Pla2g2e^+/+ and Pla2g2e^−/− mice. Pink and yellow areas indicate visceral and subcutaneous fats in HFD-fed mice, respectively (Inset). (D, E) Hematoxylin-eosin staining of WAT (D) or liver (E) from Pla2g2e^+/+ and Pla2g2e^−/− mice (Scale bar, 50 μm). (F, G) Hepatic TG (F) and plasma ALT and AST (G) levels in Pla2g2e^+/+ and Pla2g2e^−/− mice. (H) Plasma levels of phospholipids (PL) and cholesterol (Chol) in Pla2g2e^+/+ and Pla2g2e^−/− mice. (I) PL, TG and Chol levels in lipoproteins from HFD-fed Pla2g2e^+/+ and Pla2g2e^−/− mice (n = 5). (J) ESI-MS of PC, PE and PS in LDL from HFD-fed Pla2g2e^+/+ and Pla2g2e^−/− mice. Mice were fed a LFD or HFD for 16 (A, C) or 18 (D-J) weeks. Data are compiled from two (H-J) or three (B, C, F, G) experiments. Representative images of two experiments are shown (A, D, E). Mean ± SEM, *p < 0.05, **p < 0.01.

Figure 7. Schematic Diagram of the Roles of Metabolic sPLA₂s

In obesity, PLA2G2E is induced in adipocytes in accordance with adipogenesis and hydrolyzes PE and PS in VLDL, LDL and HDL, eventually promoting fat storage in WAT and liver. Subsequently, obesity-associated stress induces PLA2G5 in hypertrophic adipocytes. PLA2G5 hydrolyzes PC in hyperlipidemic LDL and facilitates the skewing of macrophages from M1 to M2 subsets, thereby playing protective roles against adipose tissue inflammation, insulin resistance, obesity, fatty liver and hyperlipidemia. Saturated fatty acids (e.g. palmitate) supplied abundantly from adipocytes trigger M1 polarization of macrophages, which is ameliorated by PLA2G5-driven unsaturated fatty acids (oleate > linoleate) from LDL or possibly some lipid mediators from macrophages. It remains unknown whether particular PLA2G2E-produced lipid mediator(s) would participate in this porcess.