Endothelial LRP1 regulates metabolic responses by acting as a co-activator of PPARγ

Abstract

Low-density lipoprotein receptor-related protein 1 (LRP1) regulates lipid and glucose metabolism in liver and adipose tissue. It is also involved in central nervous system regulation of food intake and leptin signalling. Here we demonstrate that endothelial Lrp1 regulates systemic energy homeostasis. Mice with endothelial-specific Lrp1 deletion display improved glucose sensitivity and lipid profiles combined with increased oxygen consumption during high-fat-diet-induced obesity. We show that the intracellular domain of Lrp1 interacts with the nuclear receptor Pparγ, a central regulator of lipid and glucose metabolism, acting as its transcriptional co-activator in endothelial cells. Therefore, Lrp1 not only acts as an endocytic receptor but also directly participates in gene transcription. Our findings indicate an underappreciated functional role of endothelium in maintaining systemic energy homeostasis.

Introduction

Low-density lipoprotein (LDL)-related protein 1 (LRP1), a multifunctional member of LDL receptor family, is involved in a variety of biological processes, such as lipid metabolism, endocytosis and signal transduction^1,2,3. Global deletion of Lrp1 gene in mice leads to embryonic lethality⁴. Depletion of Lrp1 in the livers of LDL receptor-deficient mice results in the accumulation of cholesterol-rich remnant lipoproteins in the circulation, suggesting the critical role of Lrp1 in the clearance of cholesterol-rich remnant lipoproteins⁵. Mice with hepatocyte-specific deletion of Lrp1 also display defects in cholesterol efflux and high-density lipoprotein (HDL) secretion⁶. On the other hand, inactivation of adipocyte Lrp1 results in delayed postprandial lipid clearance, reduced body weight, alterations in adipocyte tissue metabolism and resistance of high-fat-induced glucose tolerance and obesity⁷. The studies of Lrp1 forebrain knockout mice suggest that neuronal Lrp1 regulates food intake and energy homeostasis by directly affecting leptin signalling⁸. Although these tissue-specific knockout studies have provided us enormous information of Lrp1 functions and linked it to lipid metabolism, glucose homeostasis and obesity, the underlying mechanisms remain elusive.

Recently, we and others have demonstrated that Lrp1 is also expressed in endothelial cells (ECs) and can be induced by hypoxia and statins^{9,10,siRNA design and transient transfection}

The stealth siRNA duplexes were obtained from Life Technologies. The siRNA against mouse Pparγ is a duplex of 5′-ucaagggugccaguuucgauccgua-3′. The control siRNA is the Stealth RNAi negative control duplex (Cat. No. 12935-300) and was purchased from Life Technologies. The siRNAs were transfected into isolated Wt ECs according to our previous published protocol11. Briefly, for each sample, 2 × 10⁵ ECs were transfected with 100 pmol siRNA. The experiments with siRNA-transfected ECs were performed 2 days later.

Structure prediction

The retrieved sequence of human LRP1β was used for the prediction of its secondary structure by SABLE server ( http://sable.cchmc.org/)³⁸.

Statistical analysis

No statistical methods were used to predetermine the sample size. No randomization was used as all mice used were genetically defined, inbred mice. Data analysis for metabolic phenotype was performed in a blinded fashion. Statistical data were drawn from normally distributed group with similar variance between groups. All data presented in this study are representative results of at least three independent experiments. Data are shown as the mean±s.e.m. ‘n’ represents the number of biological replicates. Differences were analysed with two-way analysis of variance and followed by a Fisher’s least significant difference test unless otherwise specifically stated. Values of P≤0.05 were considered statistically significant.

Data availability

The data that support the findings of this study are available within the article, its Supplementary Information files and from the corresponding author upon reasonable request.