Direct interactions of CEACAM1 and CD36 with LPS and each other
Micellar lipopolysaccharide (mLPS), a major shed product of gram-negative bacteria in the intestine, may result in TLR4-mediated sepsis in the circulation. CEACAM1 and CD36, expressed on both epithelia and immune cells, regulate TLR4 signaling and inflammation, suggesting a shared role in innate immunity. Furthermore, CEACAM1 associates with CD36 on hepatocytes, regulating lipid storage and bile acids (BAs) in the liver, where LPS detoxification occurs. Using E. coli mLPS-Ra as a model and soluble forms sCEACAM1 or sCD36, we assessed their direct binding to mLPS by SPR, SEC, and TEM. SPR derived steady state affinity constants (KD= 1.47 x 10-6 M and KD= 2.23 x 10-6 M) were obtained for mLPS-Ra binding to sCEACAM1 and sCD36, respectively. mLPS-Ra binding to sCEACAM1 and sCD36 was reduced by bile acids in order of sodium deoxycholate > sodium cholate. mLPS exhibited dose dependent binding to sCEACAM and sCD36 by SEC and the isolated complexes formed well defined micelles on TEM. A KD of 5.28 x 10-8 M was obtained for sCEACAM1 binding to immobilized sCD36. Antibody-based-proximity ligation assays (PLA) demonstrated the association of the ectodomains of CEACAM1 and CD36 on hepatic (P< 0.001) and HEK cells (P< 0.018), while biotin-based PLA demonstrated association of the cytoplasmic domains of CEACAM1 and a CD36-BioID2 fusion protein in HEK cells. Alpha Fold predicted CD36 binding in cis to CEACAM1 in a membrane model, in agreement with the results of the PLAs. Based on the SPR, SEC and TEM binding studies, CEACAM1 and CD36 share a cooperative mLPS binding function distinct from their regulation of TLR4 binding.