Scavenger receptor class B, type 1 facilitates cellular fatty acid uptake

doi:10.1016/j.bbalip.2019.158554

Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids

Volume 1865, Issue 2, February 2020, 158554

https://doi.org/10.1016/j.bbalip.2019.158554 Get rights and content

Highlights

•
SR-B1 can facilitate fatty acid uptake.
•
BLT-1, a specific inhibitor of SR-B1-mediated HDL-CE uptake, does not block SR-B1-mediated FA uptake.
•
Primary adipocytes from SR-B1 knockout mice have attenuated FA uptake.
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SSO, an inhibitor of CD36-mediated FA uptake, does not block SR-B1-mediated FA uptake.

Abstract

SR-B1 belongs to the class B scavenger receptor, or CD36 super family. SR-B1 and CD36 share an affinity for a wide array of ligands. Although they exhibit similar ligand binding specificity, SR-B1 and CD36 have some very specific lipid transport functions. Whereas SR-B1 primarily facilitates the selective delivery of cholesteryl esters (CEs) and cholesterol from HDL particles to the liver and non-placental steroidogenic tissues, as well as participating in cholesterol efflux from cells, CD36 primarily mediates the uptake of long-chain fatty acids in high fatty acid-requiring organs such as the heart, skeletal muscle and adipose tissue. However, CD36 also mediates cholesterol efflux and facilitates selective lipoprotein-CE delivery, although less efficiently than SR-B1. Interestingly, the ability or efficiency of SR-B1 to mediate fatty acid uptake has not been reported. In this paper, using overexpression and siRNA-mediated knockdown of SR-B1, we show that SR-B1 possesses the ability to facilitate fatty acid uptake. Moreover, this function is not blocked by BLT-1, a specific chemical inhibitor of HDL-CE uptake activity of SR-B1, nor by sulfo-N-succinimidyl oleate, which inhibits fatty acid uptake by CD36. Attenuated fatty acid uptake was also observed in primary adipocytes isolated from SR-B1 knockout mice. In conclusion, facilitation of fatty acid uptake is an additional function that is mediated by SR-B1.

Introduction

The class B scavenger receptor family is also known as the CD36 super family and consists of three principal family members and their splice variants. The principal members are 1) SR-B1 (scavenger receptor class B, type 1, also known as SCARB1 or its human homologue CLA-1 [CD36 and LIMP2 Analogous-1]), 2) cluster determinant 36 (CD36), also known as fatty acid translocase (FAT), and 3) lysosomal integral membrane protein type 2 (LIMP-2), also known as SCARB2 [[1], [2], [3], [4], [5], [6]]. In addition, there are splice variants of SR-B1 termed SR-BII and CLA-2 in lower species and humans, respectively. Proteins in this family are located in plasma or lysosomal membranes and function to transport lipids, enzymes and viral particles through the membrane of the cells or organelles (lysosomes) and, therefore, play pivotal roles in maintaining lipid homeostasis.

SR-B1 (82-kDa) and CD36 (88-kDa) share 30% sequence identity and both proteins are structurally composed of a large heavily glycosylated extracellular domain, two transmembrane domains and two cytoplasmic N- and C-terminal tails [7,8]. Organs with the highest expression of CD36 include white and brown adipose tissues, lung, heart, mammary gland, and macrophages, whereas a relatively high expression has been demonstrated in the ovary, adrenal cortex, testicular Leydig cells, kidney and skeletal muscle [9]. Likewise, SR-B1 is expressed in a variety of tissues and cells including adipose tissue, vascular endothelial cells, smooth muscle cells, macrophages, phagocytes, and intestinal cells, but is predominantly expressed in hepatocytes, adrenocortical cells, and ovarian granulosa/luteal cells, as well as hormonal stimulated testicular Leydig cells [10].

While LIMP-2 is located within lysosomal membranes and mediates lysosomal delivery of β-glucocerebrosidase and serves as a receptor for enterovirus 71 and coxsackieviruses, both SR-B1 and CD36 are located within the plasma membrane. SR-B1 and CD36 share an affinity for a wide array of ligands, including native and modified lipoproteins, advanced glycation end products, and anionic phospholipids [[11], [12], [13]]. Although they exhibit similar ligand binding specificity, SR-B1 and CD36 have some very specific lipid transport functions. Whereas SR-B1 primarily facilitates the selective delivery of cholesteryl esters (CEs) and cholesterol from HDL particles to the liver and non-placental steroidogenic tissues [[13], [14], [15], [16]], as well as participating in cholesterol efflux from cells [17,18], CD36 primarily mediates the uptake of long-chain fatty acids by high fatty acid-requiring organs such as the heart, skeletal muscle and adipose tissue [7]. However, CD36 also mediates cholesterol efflux and facilitates selective lipoprotein-CE delivery [7,19]. Based on the use of various CD36/SR-B1 chimeras and in vitro transient expression experiments, it has been suggested that CD36 is less efficient than SR-B1 in mediating selective HDL-CE uptake [12,[20], [21], [22]]. Interestingly, the ability or efficiency of SR-B1 to mediate fatty acid uptake has not been reported. We addressed this issue in the current paper and show that SR-B1 possesses the ability to facilitate fatty acid uptake. Moreover, this function is not blocked by BLT-1 (block lipid transport-1), a specific chemical inhibitor of HDL-CE uptake activity of SR-B1, nor by SSO (sulfo-N-succinimidyl oleate), an inhibitor of fatty acid uptake by CD36.

Section snippets

Chemicals and reagents

Reagents were obtained from the following sources: Cholesteryl BODIPY FLC12 (cholesteryl 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacence-3-dodecanoate) was obtained from Molecular Probes (Life Technologies, Grand Island, NY). Bicinchoninic acid assay protein kit was from Pierce Biotechnology, Inc. (Rockford, IL); organic solvents were from J. T. Baker (Phillipsburg, NJ); TRIzol reagent and SuperScript II were from Invitrogen (Carlsbad, CA); RNeasy kit was from QIAGEN (Valencia, CA);

Increased fatty acid uptake in HEK293 cells with overexpression of SR-B1

To examine whether SR-B1 can facilitate fatty acid uptake, plasmid vectors with rat SR-B1 cDNA, CD36 cDNA as a positive control, or empty vector were transfected into HEK293 cells. Subsequently cells were assayed for fatty acid uptake. As shown in Fig. 1, cells with overexpression of SR-B1 displayed increased fatty acid uptake compared to empty vector and to a degree that was comparable to cells overexpressing CD36. Due to the rapid fatty acid uptake into the cells, the accumulative uptake over

Discussion

Fatty acids are essential building blocks of biological membranes and also serve as an energy source of almost all living cells. In addition, fatty acids play important roles in signal-transduction pathways, cellular fuel sources, composition of hormones and lipids, the modification of proteins, and energy storage within adipose tissue in the form of triglycerides. Apart from adipose tissue, most other cell types have limited storage of fatty acids in the form of triglycerides in lipid

Declaration of competing interest

Authors declare no conflict of interests.

Acknowledgements

This work was supported by Merit Review Award # I01BX001923 (SA), and # I01BX000398 (FBK) and Senior Research Career Scientist Award # IK6B004200 (SA) from the U.S. Department of Veterans Affairs, Biomedical Laboratory Research Development Program and NIH grant P30 DK116074 (FBK).

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¹: These authors contributed equally to the work.

²: Current address: PCET Division, Aragen Bioscience, 260 Cochrane Circle, Morgan Hill, CA 94307, United States of America.

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Published by Elsevier B.V.

Scavenger receptor class B, type 1 facilitates cellular fatty acid uptake

Highlights

Abstract

Introduction

Section snippets

Chemicals and reagents

Increased fatty acid uptake in HEK293 cells with overexpression of SR-B1

Discussion

Declaration of competing interest

Acknowledgements

J. Biol. Chem.

Genomics

J. Biol. Chem.

Cell

J. Lipid Res.

Metabolism

J. Biol. Chem.

J. Lipid Res.

J. Biol. Chem.

Mol. Cell. Endocrinol.

J. Lipid Res.

J. Biol. Chem.

Int. J. Biochem. Cell Biol.

J. Biol. Chem.

J. Lipid Res.

J. Biol. Chem.

Methods

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Metabolism

Biochem. Pharmacol.