Surface expression, polarization, and functional significance of CD73 in human intestinal epithelia

Abstract

During active intestinal inflammation polymorphonuclear leukocytes (PMN) transmigrate into the lumen and release 5'-AMP (J. Clin. Invest. 1993. 91:2320-2325). 5'-AMP is converted to adenosine by the apical epithelial surface with subsequent activation of electrogenic Cl- secretion (the basis of secretory diarrhea) via apical A(2b) adenosine receptors (J. Biol. Chem. 1995.270:2387-2394). Using a polarized human intestinal epithelial monolayer (T84), we now characterize the basis of the observed conversion of 5'-AMP to adenosine required for this paracrine signaling pathway. An inhibitor of the ecto-5'-nucleotidase CD73, α, β-methylene ADP (AOPCP), inhibited epithelial Cl- secretory responses to 5'-AMP, but not to authentic adenosine. Confocal immunofluorescent microscopy revealed CD73 to be surface expressed on both model and natural human intestinal epithelia. Expression was about sixfold greater on the apical cell surface as assessed biochemically by selective cell surface biotinylation, and morphologically by immunofluorescence. Treatment with phosphotidylinositol specific-phospholipase C (PI-PLC) released 95% of apical CD73, indicating that the intestinal CD73 possesses a glycosylphosphatidylinositol (GPI) anchor. Neither adenosine nor 5'-AMP stimulation induced intact T84 cells to shed surface CD73. The bulk of apical CD73 (~60%) was released from the cell surface by treatment with 1% Triton X-100 (TX-100) at 4°C, but such release was not affected by pretreatment with ligand or by prior, antibody-mediated cross-linking of CD73. Subsequent analyses showed that the subpool of CD73 released by TX-100 at 4°C was not truly solubilized, but rather represented TX-100-induced release of CD73- containing membrane fragments. These membrane fragments displayed light density on sucrose gradients characteristic of detergent insoluble glycosphingolipid-rich membrane domains (DIGs)/caveolae, were solubilized by n-octyl glucoside (NOG, 1%) at 4°C, and contained caveolin. These data indicate that human intestinal epithelia express CD73, which is apically polarized and targeted to microdomains with DIGs/caveolae characteristics. CD73 likely participates in translating paracrine, PMN-derived 5'-AMP signals to the authentic effector adenosine. These studies define CD73 as central to PMN-mediated intestinal Cl- secretion, the major direct-acting mechanism by which PMN induce intestinal epithelial Cl- secretion.