Adipose-specific lipoprotein lipase deficiency more profoundly affects brown than white fat biology

Abstract

Adipose fat storage is thought to require uptake of circulating triglyceride (TG)-derived fatty acids via lipoprotein lipase (LpL). To determine how LpL affects the biology of adipose tissue, we created adipose-specific LpL knock-out (ATLO) mice, and we compared them with whole body LpL knock-out mice rescued with muscle LpL expression (MCK/L0) and wild type (WT) mice. ATLO LpL mRNA and activity were reduced, respectively, 75 and 70% in gonadal adipose tissue (GAT), 90 and 80% in subcutaneous tissue, and 84 and 85% in brown adipose tissue (BAT). ATLO mice had increased plasma TG levels associated with reduced chylomicron TG uptake into BAT and lung. ATLO BAT, but not GAT, had altered TG composition. GAT from MCK/L0 was smaller and contained less polyunsaturated fatty acids in TG, although GAT from ATLO was normal unless LpL was overexpressed in muscle. High fat diet feeding led to less adipose in MCK/L0 mice but TG acyl composition in subcutaneous tissue and BAT reverted to that of WT. Therefore, adipocyte LpL in BAT modulates plasma lipoprotein clearance, and the greater metabolic activity of this depot makes its lipid composition more dependent on LpL-mediated uptake. Loss of adipose LpL reduces fat accumulation only if accompanied by greater LpL activity in muscle. These data support the role of LpL as the "gatekeeper" for tissue lipid distribution.

Keywords: Adipocyte; Fatty Acid; Hypertriglyceridemia; Lipids; Macrophages; Obesity.

FIGURE 1.

Characterization of ATLO mice. Ap2-cre mice were crossed with Lpl floxed mice. A, genotyping strategy. A PCR with two different reverse primers was used. In the absence of Cre recombinase, primers A and B generate a 700-bp PCR product, and primers A and C generate a 1925-bp product. In the presence of Cre recombinase, deletion of the first exon of Lpl results in a 400-bp PCR product. A representative gel of this PCR in adipose tissue is shown. Heart DNA obtained from a heart-specific LpL knock-out was used a positive control in the gel. B, Lpl mRNA analysis by quantitative PCR in GAT, SCAT, BAT, and SVC isolated from GAT. C, Immunofluorescence of LpL and CD11b in GAT and colocalization quantification in GAT, SCAT, and BAT. D, LpL activity was measured in homogenized whole tissues and in postheparin plasma using a radiolabeled triglyceride substrate. E, LpL activity and mRNA measured in lungs from WT, ATLO, and MCK/L0 mice. Mean ± S.E. is shown. PHP, postheparin plasma. Statistical significance: *, p < 0.05; **, p < 0.01; ***, p < 0.001.

FIGURE 2.

Body weight and fat morphology in ATLO and MCK/L0 mice in chow and HFD. A, body weight was monitored on chow and high fat diet from weaning until 16 weeks of age. B, adipose tissue mass from WT, ATLO, and MCK/L0 mice on chow and on HFD. C, macroscopic and microscopic appearance of the adipose tissues on chow diet. D, percentage of body fat and lean mass in WT, ATLO, and MCK/L0 mice on chow and HFD. Representative DEXA scans are shown. E, liver TG levels in WT, ATLO, and MCK/L0 mice on chow and after HFD. Statistical significance: ATLO or MCK/L0 versus control: *, p < 0.05; **, p < 0.01; ***, p < 0.001. Chow versus HFD: #, p < 0.05; ##, p < 0.01; ###, p < 0.001.

FIGURE 3.

Assessment of liver TG secretion and tissue uptake. A, mice were intraperitoneally injected with tyloxapol. Blood was collected at different time points, and accumulated TG was determined. B and C, mice were injected with endogenously labeled [¹⁴C]TG and [³H]retinol, and after 15 min tissues were harvested. Radioactivity was measured in the homogenized tissues. Statistical significance: *, p < 0.05.

FIGURE 4.

Lipidomic analysis of GAT, SCAT, and BAT. Adipose tissue lipids were extracted as described under “Experimental Procedures,” and fatty acids were identified and quantified by LC/MS. Percentage of saturated, monounsaturated, and polyunsaturated fatty acids were calculated for WT, ATLO, and MCK/L0 adipose tissues. A, TG acyl composition on chow diet. B, acyl-CoA composition on chow diet. C, uptake of 2-deoxy-d-[1-³H]glucose in adipose tissues. Statistical significance: ATLO or MCK/L0 versus WT: *, p < 0.05; **, p < 0.01; ***, p < 0.001.

FIGURE 5.

Lipidomic analysis of GAT, SCAT, and BAT. Adipose tissue lipids were extracted as described under “Experimental Procedures,” and fatty acids were identified and quantified by LC/MS. Percentage of saturated, monounsaturated, and polyunsaturated fatty acids were calculated for WT, ATLO, and MCK/L0 adipose tissues. A, TG acyl composition on HFD. B, acyl-CoA composition on HFD. C, Western blot and densitometric analysis for VLDL receptor (VLDLR), LDL receptor (LDLR), and LRP1 in GAT on chow and high fat-fed WT, ATLO, and MCK/L0 mice.

FIGURE 6.

Macrophage LpL replenishment in MCK/L0 GAT by bone marrow transplant. Bone marrow from WT or from MCK/L0 mice was transplanted into MCK/L0 recipients. After an 8-week repopulation period, mice were injected directly in their GAT with an MCP-1-expressing adenovirus. A, Lpl mRNA was analyzed in GAT. B, LpL activity. C, mRNA expression for Angptl4. D, GAT TG acyl composition. E, mRNA for Fasn. F, mRNA for Pparg1, Pparg2, and Ppard was measured. G, Lpl mRNA in BAT. H, BAT TG acyl composition in the transplanted mice. I, Lpl mRNA in SCAT. J, SCAT TG acyl composition. Statistical significance: in comparisons versus MCK/L0→MCK/L0 (D and H) or versus WT (rest of panels): *, p < 0.05; **, p < 0.01.

FIGURE 7.

MCK/ATLO mice phenotype. A, GAT mass of MCK/ATLO mice compared with WT and with ATLO mice. B, mRNA expression for Fasn and Scd1 in GAT of WT, ATLO, and MCK/ATLO mice. C, GAT TG acyl composition. Statistical significance: ATLO or MCK/ATLO versus WT: *, p < 0.05; **, p < 0.01; ***, p < 0.001.