LRP1 Controls Intracellular Cholesterol Storage and Fatty Acid Synthesis through Modulation of Wnt Signaling

Abstract

The low-density lipoprotein receptor-related protein LRP1 is a cell surface receptor with functions in diverse physiological pathways, including lipid metabolism. Here we show that LRP1-deficient fibroblasts accumulate high levels of intracellular cholesterol and cholesteryl-ester when stimulated for adipocyte differentiation. We demonstrate that LRP1 stimulates a canonical Wnt5a signaling pathway that prevents cholesterol accumulation. Moreover, we show that LRP1 is required for lipolysis and stimulates fatty acid synthesis independently of the noradrenergic pathway, through inhibition of GSK3␤ and its previously unknown target acetyl-CoA carboxylase (ACC). As a result of ACC inhibition, mature LRP1-deficient adipocytes of adult mice are hypotrophic, and lower uptake of fatty acids into adipose tissue leads to their redistribution to the liver. These results establish LRP1 as a novel integrator of adipogenic differentiation and fat storage signals.The number of adipocytes in an organism is determined by a tightly regulated differentiation process of fibroblast-like preadipocytes (1, 2). Fat cell differentiation (adipogenesis) is controlled by hormonal-induced coordinated expression and activation of two main groups of transcription factors, the CCAAT/enhancer-binding protein (C/EBP) family and peroxisome proliferator-activated receptor ␥ (PPAR␥) 4 (2). PPAR␥, a member of the nuclear hormone receptors superfamily, is a crucial component of this cascade, as adipogenesis is impaired in PPAR␥-deficient mesenchymal stem cells (3). Activation of PPAR␥ induces the expression of lipogenic genes, such as adipocyte fatty acid-binding protein (422/aP2) (4), CD36 and lipoprotein lipase (LPL) (5). Accumulation of intracellular triglyceride (TG) droplets ultimately gives rise to the morphologically distinct fat cell (2). During periods of caloric restriction, TGs stored in adipocytes are catabolized into glycerol and fatty acids, to provide energy. Mobilization of lipids involves the sequential activation of hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) (6), two lipolytic enzymes responsible for more than 95% of the TG hydrolase activity in the adipose tissues of mammals (7). The low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor. Two NPXY motifs in the intracellular domain (ICD) serve as docking sites for several cytoplasmic adaptor proteins including Shc, Disabled-1, JIP1, PSD-95, CED-6/GULP, ARH, and Fe65, which control intracellular trafficking, as well as signaling events (8). LRP1 interacts with and mediates endocytosis of more than 40 unrelated ligands ranging from viruses to protease/protease inhibitor complexes, cytokines, and growth factors (9). In the liver, LRP1 and the LDL receptor (LDLr) share the endocytosis and subsequent degradation of TG-rich very-low-density lipoproteins and chylomicron remnants. However, endocytosis and clearance of macromolecules is only one function of LRP1. There is now substantial evidence that LRP1 also serves ...

Apolipoprotein E and Apolipoprotein E Receptors: Normal Biology and Roles in Alzheimer Disease

“…Loss of LRP1 also results in increased GSK3b activity, at least in fibroblasts and adipocytes, as a result of a loss of autocrine Wnt5a expression (Terrand et al 2009). Conditional LRP1 knockout mice, lacking LRP1 expression exclusively in postmitotic neurons, also display severe locomotor abnormalities , raising the possibility that these dysfunctions could also be caused in part by defective regulation of tau phosphorylation, although this has not been explored at the time of this writing.…”

Convergent Signaling Pathways Controlled by LRP1 (Receptor-related Protein 1) Cytoplasmic and Extracellular Domains Limit Cellular Cholesterol Accumulation

“…Adipocyte differentiation was induced using a mixture of insulin, dexamethasone, isobutylmethylxanthine (Sigma), and rosiglitazone (Applied Biochemical Technology) (11). Medium was changed every 2 days, and after 10 days cells were fixed in 10% formaldehyde, and neutrals lipids were stained with Oil Red O (Sigma).…”

“…3C, lane 2 and 8). In a complementary approach, we treated the HEK 293 overexpressing SREBP-2 with a Wnt5a-enriched conditioned medium (11). Cell fractionation experiments revealed that SREBP-2 nuclear translocation was decreased after 6 h (Fig.…”

“…Several mechanisms have been proposed. LRP1 has been shown to promote cholesterol export in vSMCs through induction of ATP binding cassette transporter A1 (ABCA1) levels (5) and to induce a Wnt5a/␤-catenin pathway to limit cholesterol overload in mouse embryonic fibroblasts (11). Moreover, smLRP1 mice express very low levels of Wnt5a in vSMCs (12).…”

Proteomic Analysis of GLUT4 Storage Vesicles Reveals LRP1 to Be an Important Vesicle Component and Target of Insulin Signaling

“…This whole animal study did not, however, assess fat cell glucose transport in vitro nor did a more recent study of adipocyte differentiation from Lrp1 Ϫ/Ϫ fibroblasts (49). Thus, to assess the direct effects of LRP1 deficiency in these mice on GSV components, we determined GLUT4 expression by Western blot as shown in Fig.…”