Cellular localization of the diazepam binding inhibitor (DBI) in the gastrointestinal tract of mice and its coexistence with the fatty acid binding protein (FABP)

The diazepam binding inhibitor (DBI), initially isolated as an endogenous 1 0-kDa polypeptide from the brain, has the ability to displace ligands from benzodiazepine binding sites on gamma -aminobutyric acid (GABA) receptors. However, DBI is widely distribute outside the brain, with the highest expre ssion in the intestine. The present in situ hybridization study revealed th e cellular expression of DBI mRNA throughout the gastrointestinal tract of mice, showing it to be intensely expressed in the spinous layer in the stra tified squamous epithelium of the oral cavity, esophagus and forestomach, i n surface mucous cells in the glandular stomach, and in columnar (absorptiv e) cells of the intestinal villi. A precise identification of DBI-expressin g cell types was confirmed immunohistochemically, although the expressing c ells detectable by the two histochemical methods differed slightly in their extension. Noteworthily, DBI always coexisted with the fatty acid binding protein (FABP), which participates in the uptake and metabolic processing o f long chain fatty acids. In addition to the biochemical finding that DBI i s identical with the acyl-CoA binding protein (ACBP), the distributional pa tterns of DBI and its colocalization with FABPs suggests its involvement in the absorption and metabolism of lipid in the epithelia of the digestive t ract.