Wortmannin, an Inhibitor of Phosphoinositide 3-Kinase, Inhibits Transcytosis in Polarized Epithelial Cells

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits both basolateral to apical and apical to basolateral transcytosis of ricin in Fisher rat thyroid (FRT) cells by 50% at 100 nM in a continuous transcytosis assay. In MDCK cells, a similar effect of wortmannin on basolateral to apical transcytosis of ricin was found, whereas apical to basolateral transcytosis was inhibited to a lesser degree. Transcytosis of dimeric IgA in MDCK cells expressing the polymeric immunoglobulin receptor was also reduced to 50% of controls, suggesting that wortmannin inhibits membrane translocation rather than sorting of specific proteins in the transcytotic pathway. This effect of wortmannin is selective, however, in that endocytosis at the basolateral domain and recycling at both the basolateral and apical membrane domains are unaffected, and apical endocytosis and apical secretion are only moderately reduced. We have shown previously that cAMP stimulates a late stage in basolateral to apical transcytosis in MDCK cells through activation of protein kinase A (Hansen, S. H., and Casanova, J.E. (1994) J. Cell Biol. 126, 677-687). Elevation of cellular cAMP still induced a 100% increase in transcytosis in wortmannin-treated cells, but transcytosis was no longer increased when compared to cells which received no drugs. In contrast, in experiments using a 17 degrees C block to accumulate ricin internalized from the basolateral surface in the apical compartment of MDCK cells, wortmannin had little effect on the stimulation of transcytosis by activators of protein kinase A observed under these conditions. The data thus suggest the existence of a wortmannin-sensitive step in the transcytotic pathway, positioned after endocytosis but prior to translocation into the protein kinase A-sensitive apical compartment, implying a role for phosphoinositide 3-kinase in an intermediate step in transcytosis in polarized epithelial cells.