Hepatic nonvesicular cholesterol transport is critical for systemic lipid homeostasis

In cell models, changes in the “accessible” pool of plasma membrane (PM) cholesterol are linked with the regulation of ER sterol synthesis and metabolism by the Aster family of nonvesicular transporters. However, the relevance of such nonvesicular transport mechanisms for lipid homeostasis in vivo has not been defined. Here we reveal two physiological contexts that generate accessible PM cholesterol and engage the Aster pathway in liver: fasting and reverse cholesterol transport (RCT). During fasting, adipose tissue–derived fatty acids activate hepatocyte sphingomyelinase to liberate sequestered PM cholesterol. Aster-dependent cholesterol transport during fasting facilitates cholesteryl ester (CE) formation, cholesterol movement into bile, and VLDL production. During RCT, HDL delivers excess cholesterol to the hepatocyte PM through SR-BI. Loss of hepatic Asters impairs cholesterol movement into feces, raises plasma cholesterol levels, and causes cholesterol accumulation in peripheral tissues. These results reveal fundamental mechanisms by which Aster cholesterol flux contributes to hepatic and systemic lipid homeostasis. Overall design: RNA-seq was performed on liver from male C57BL/6J mice fasted for 4 hours or 16 hours (n = 3, 3). Total RNA was extracted using TRIzol and the RNeasy Mini Kit with on-column DNase I digestion (Qiagen). Library preparation and sequencing was done at GENEWIZ. Adapter and quality trimming of raw FASTQ files was performed using Trimmomatic. FastQC was used to analyze FASTQ files before and after trimming. Trimmed FASTQ files were aligned to GRCm38/mm10 using STAR. HTSeq-count was used to extract gene counts and differential gene expression analysis was performed using DESeq2.