Human CEACAM1-LF regulates lipid storage in HepG2 cells via fatty acid transporter CD36.

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is expressed in liver and secreted as biliary glycoprotein 1 (BGP1) via bile canaliculi (BCs). CEACAM1-LF is a 72 amino acid cytoplasmic domain mRNA splice isoform with two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Ceacam1-/- or Ser503Ala transgenic mice have been shown to develop insulin resistance and non-alcoholic fatty liver disease; however, the role of the human equivalent residue, Ser508, in lipid dysregulation is unknown. Human HepG2 hepatocytes that express CEACAM1 and form BC in vitro were compared to CEACAM1-/- cells and to CEACAM1-/- cells expressing Ser508Ala null or Ser508Asp phosphorylation mimic mutations, or to phosphorylation null mutations in the tyrosine ITIMs known to be phosphorylated by the tyrosine kinase Src. CEACAM1-/- cells and the Ser508Asp and Tyr520Phe mutants strongly retained lipids, while Ser508Ala and Tyr493Phe mutants had low lipid levels compared to wild type cells, indicating the ITIM mutants phenocopied the Ser508 mutants. We found that the fatty acid transporter CD36, was up-regulated in the S508A mutant, co-expressed in BCs with CEACAM1, co-IPed with CEACAM1 and Src, and when down-regulated via RNAi, an increase in lipid droplet content was observed. Nuclear translocation of CD36 associated kinase LKB1 was increased 7-fold in the S508A mutant vs CEACAM1-/- cells and correlated with increased activation of CD36-associated kinase AMPK in CEACAM1-/- cells. Thus, while CEACAM1-/- HepG2 cells upregulate lipid storage similar to Ceacam1-/- in murine liver, the null mutation Ser508Ala led to decreased lipid storage, emphasizing evolutionary changes between the CEACAM1 genes in mouse and man.