THE ALPHA-2-BETA-1 INTEGRIN REGULATES COLLAGEN-MEDIATED MDCK EPITHELIAL MEMBRANE REMODELING AND TUBULE FORMATION

Previous studies have demonstrated that incubation of MDCK cell epithe lial cysts in collagen gel induced a reversal in cell surface polarity that was regulated by beta 1 integrins. Further experiments were done to identify the specific collagen binding integrin involved by applyi ng collagen gel overlays to the apical membrane of subconfluent MDCK m onolayers. Cell surface levels of the apical membrane glycoprotein gp1 35 were monitored by ELISA to quantitate the extent of collagen-mediat ed membrane remodeling. After an 8 hour incubation with collagen, ther e was a 35% reduction in gp135 while the cell surface levels of the al pha 2, alpha 3 and beta 1 integrin subunits were not affected. Immunof luorescence microscopy confirmed the loss of gp135 from selected regio ns of the apical cell surface while the alpha 2 and beta 1 integrin su bunits were distributed in small clusters over the entire apical membr ane in both control and collagen-treated monolayers. Collagen-mediated loss of gp135 was inhibited by monoclonal antibodies which recognize either the alpha 2 or beta 1 integrin subunits but not by a monoclonal antibody against the alpha 6 beta 1 integrin. These results demonstra ted that remodeling of the apical membrane had occurred, allowing the selective retention of beta 1 integrins but not gp135. They were suppo rted by the observation that collagen-mediated loss of apical membrane microvilli was inhibited by the monoclonal antibody against the alpha 2 integrin subunit. Incubation of conflueut monolayers with collagen gel induced the formation of polarized epithelial tubules within 16 ho urs, Epithelial tubule biogenesis was completely inhibited by monoclon al antibodies against either the alpha 2 or beta 1 integrin subunits, providing strong evidence that the alpha 2 beta 1 integrin is essentia l for collagen-mediated epithelial membrane remodeling and tubule form ation.