Phosphorylation of human CEACAM1-LF by PKA and GSK3ß promotes its interaction with ß-catenin.

CEACAM1-LF, a homotypic cell adhesion adhesion molecule, transduces intracellular signals via a 72 amino acid cytoplasmic domain that contains two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and a binding site for ß-catenin. Phosphorylation of Ser503 by PKC in rodent CEACAM1 was shown to affect bile acid transport or hepatosteatosis via the level of ITIM phosphorylation, but the phosphorylation of the equivalent residue in human CEACAM1 (Ser508) was unclear. Here we studied this analogous phosphorylation by NMR analysis of the 15N labeled cytoplasmic domain peptide. Incubation with a variety of Ser/Thr kinases revealed phosphorylation of Ser508 by GSK3bß but not by PKC. The lack of phosphorylation by PKC is likely due to evolutionary sequence changes between the rodent and human genes. Phosphorylation site assignment by mass spectrometry and NMR revealed phosphorylation of Ser472, Ser461 and Ser512 by PKA, of which Ser512 is part of a conserved consensus site for GSK3ß binding. We showed here that only after phosphorylation of Ser512 by PKA was GSK3ß able to phosphorylate Ser508. Phosphorylation of Ser512 by PKA promoted a tight association with the armadillo repeat domain of ß-catenin at an extended region spanning the ITIMs of CEACAM1. The kinetics of phosphorylation of the ITIMs by Src, as well dephosphorylation by SHP2, were affected by the presence of Ser508/512 phosphorylation, suggesting that PKA and GSK3ß may regulate the signal transduction activity of human CEACAM1-LF. The interaction of CEACAM1-LF with ß-catenin promoted by PKA is suggestive of a tight association between the two ITIMs of CEACAM1-LF.

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 the 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 nonalcoholic 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 with CEACAM1-/- cells and 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 with wild-type cells, indicating that the ITIM mutants phenocopied the Ser508 mutants. We found that the fatty acid transporter CD36 was upregulated in the S508A mutant, coexpressed in BCs with CEACAM1, co-IPed with CEACAM1 and Src, and when downregulated via RNAi, an increase in lipid droplet content was observed. Nuclear translocation of CD36 associated kinase LKB1 was increased sevenfold in the S508A mutant versus 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 humans.