EFFECTS OF NH4CL AND NOCODAZOLE ON POLARIZED FIBRONECTIN SECRETION VARY AMONGST DIFFERENT EPITHELIAL-CELL TYPES

The extracellular matrix protein fibronectin was found to be secreted by three polarized epithelial cell lines Madin-Darby canine kidney (MDCK), Caco-2 and LLC-PK1. About 54 and 46% of fibronectin was secreted from the apical and basolateral cell surfaces, respectively, in MDCK cells. in Caco-2 and LLC-PK1 cells, the majority (about 92-93%) of fibronectin secretion occurs from the basolateral cell surface, with the remaining 7-8% from the apical surface. In all three cell types, NH4Cl was found to inhibit basolateral secretion (resulting in enhanced apical secretion), white total fibronection secretion was not significantly affected (although a delay in secretion was observed). Nocodazole reduced total fibronectin secretion to about 70% of control levels in MDCK and Caco-2 cells, with significant inhibition on secretion from both surfaces. In contrast, total fibronectin secretion was enhanced by nocodazole in LLC-PK1 cells. Furthermore, the majority of fibronectin secretion was redirected to the apical cell surface in LLC-PK1 cells. These observations demonstrate that the nature as well as the extent of the effects of NH4Cl and nocodazole on polarized fibronectin secretion varies amongst different epithelial cell types.