The role of protein kinase C (PKC) in the regulation of the cytoskeleton of epithelial cells with tightly sealed contacts, poor contacts, and without contacts were investigated by incubating them with a protein kinase C activator phorbol myristoyl acetate (PMA). The morphology and organization of the membrane skeleton and stress fibers as well as the localization of an actin-bundling PKC substrate MARCKS in confluent MDCK cells originating from the distal tubulus of dog kidney, LLC-PK1 cells originating from the proximal tubulus of pig kidney, src-transformed MDCK cells, epidermoid carcinoma A431 cells, and MDCK cells grown in low calcium medium (LC medium) in low density were visualized with phase contrast and immunofluorescence microscopy. Four different responses to the PMA-treatment in actin-based structures of cultured epithelial cells were observed: 1) disintegration of the membrane skeleton in confluent MDCK cells; 2) depolymerization of the stress fibers in confluent MDCK and LLC-PK1 cells; 3) formation of the membrane skeleton in A431 cells, and 4) formation of the stress fibers and membrane skeleton in LC-MDCK cells. Thus, it seems that in fully confluent tightly sealed epithelium, activation of PKC has a deleterious effect on actin-based structures, whereas in cells without contacts or loose contacts, activation of PKC by PMA results in improvement of actin-based cytoskeletal structures. The main difference between the two kidney cell lines used is their selectivity to ion transport: the monolayer of LLC-PK1 cells is anion selective and MDCK cells cation selective. We propose a model where alterations in the ionic milieu within the MDCK cells by means of cation channels affect the disintegration of the membrane skeleton. The distribution of MARCKS followed the distribution of fodrin in both cell lines upon PMA-treatment, suggesting that phosphorylation of MARCKS by PKC may contribute in the regulation of the integrity of the membrane skeleton.
Copyright 1999 Wiley-Liss, Inc.