The COPI coatomer regulates several steps of HDL metabolism

Background

Reverse cholesterol transport by high density lipoproteins (HDL) is considered as an anti-atherogenic metabolic pathway. Hepatocytes determine the efficacy of this pathway by the production of apolipoprotein A-I (apoA-I) and its lipidation by ATP binding cassette transporter A1 (ABCA1), selective uptake of cholesterol via scavenger receptor BI (SR-BI) and uptake of entire HDL particles. The molecular determinants of the latter step are not well understood.

Methods

We performed a genome-wide RNA interference screen for genes limiting the uptake of fluorescent HDL 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 cholesterol levels in the databases of the Global Lipids Genetics Consortium and UK Biobank as well as inborn errors of metabolism and their respective mouse models.

Results

The knockdown of 128 genes significantly inhibited HDL uptake. Six of them encode for components of the COPI 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 as well as APOA1 expression and apoA-I secretion but increased the cell surface abundance of ABCA1 as well as cholesterol efflux. Single nucleotide polymorphisms of COPB1, ARCN1, and COPZ2 were associated with significantly higher HDL-cholesterol (HDL-C) levels in the population while rare COPA and COPG1 variants causing immunopathies in humans and mice were associated with low levels of HDL cholesterol.

Conclusions

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.

Highlights

• By genome-wide RNA interference screening and replication experiments we found six components of the COPI coatomer, namely COPA, COPB1, COPB2, COPG1, ARCN1, and COPZ1 to limit the uptake of HDL holoparticles into Huh-7 hepatocarcinoma cells, possibly by a mechanism that involves ATP binding cassette transporter ABCA1

• Loss of any of expression of any of these six COPI genes decreases HDL lipid uptake by interfering with the glycosylation and cell surface abundance of scavenger receptor SR-BI

• Loss of any of expression of any of these six COPI genes decreases HDL secretion from Huh7 cells, possibly by decreasing gene expression of APOA1 and ABCA1, but despite increasing cell surface abundance of ABCA1 and ABCA1-mediated cholesterol efflux

• Single nucleotide polymorphisms of ARCN1 are associated with significantly higher HDL-cholesterol (HDL-C) levels in the population while rare COPA and COPG1 variants causing immunopathies in humans and mice were associated with rather low levels of HDL cholesterol.