COPI Coatomer Regulates Several Steps of HDL Metabolism

Reverse cholesterol transport by HDLs (high-density lipoproteins) is considered an antiatherogenic metabolic pathway. Hepatocytes are the main contributors to the efficacy of this pathway by the production of apoA-I (apolipoprotein A1) and its lipidation by ABCA1 (ATP-binding cassette transporter A1), selective uptake of cholesterol via SR-BI (scavenger receptor class B type 1), and uptake of entire HDL particles. The molecular determinants of the latter step are not well understood.

We performed a genome-wide RNA interference screen for genes limiting the uptake of HDL fluorescently labeled at its protein moiety into Huh-7 hepatocarcinoma cells. Top hit genes were validated by targeted in vitro experiments and the analysis of associations between their variants and HDL-C (HDL-cholesterol) levels in the databases of the Global Lipids Genetics Consortium and the UK Biobank, as well as inborn errors of metabolism and their respective mouse models.

The knockdown of 128 genes significantly inhibited HDL uptake. Six of them encode components of the COPI (coat protein I) coatomer, namely, COPA, COPB1, COPB2, COPG1, ARCN1, and COPZ1. Knocking down any of them decreased the uptake of both fluorescently labeled proteins and lipids of HDL, the cell surface abundance of SR-BI, and APOA1 expression and apoA-I secretion but increased the cell surface abundance of ABCA1. Common single-nucleotide polymorphisms of ARCN1 and COPB1 were associated with significantly higher HDL-C levels in the population, while rare COPA and COPG1 variants causing immunopathies were associated with rather lower levels of HDL-C in both affected patients and the corresponding genetically modified mice.

In hepatocytes, the COPI coatomer regulates HDL holoparticle uptake, selective lipid uptake, apoA-I secretion, and cholesterol efflux, and thereby, it influences plasma levels of HDL-C.