METASTASIS-ASSOCIATED 5T4 ONCOFETAL ANTIGEN IS CONCENTRATED AT MICROVILLUS PROJECTIONS OF THE PLASMA-MEMBRANE

The 5T4 oncofoetal antigen is a 72 kDa glycoprotein defined by a monoc lonal antibody raised against human placental trophoblast and is expre ssed in many different carcinomas but detected only at low levels in s ome normal epithelia. Immunohistochemical analysis of the patterns of expression in colorectal carcinomas has indicated a significant associ ation between the presence of the antigen in tumour cells and metastat ic spread. A cDNA encoding the 5T4 molecules has been isolated and the extracellular portion contains several leucine-rich repeats which hav e been implicated in cellular interactions, To study the cell biologic al role of 5T4 molecules, murine L cells (A9 derivative) were stably t ransfected with 5T4 cDNA under the control of the CMV immediate-early promoter. The 5T4-expressing cells exhibited a more spindle-shaped mor phology compared to the vector alone transfected cells, Confocal immun ofluorescence microscopy revealed 'polkadot' pattern of 5T4 antigen ex pression, heterogeneous in intensity between cells, but distributed ov er the entire cell surface. Transmission and scanning electron microsc opy showed that the 5T4 antigen is concentrated at microvillus project ions of the plasma membrane both in the transfected A9 cells and in va rious carcinoma cell lines, Such projections express an array of surfa ce molecules which function in cell adhesion and motility. This associ ation of 5T4 antigen with microvillus projections was also observed in various carcinoma cell lines, 5T4 expression in A9 cells was also ass ociated with an altered pattern of cell division, decreased cell-subst ratum adhesion and increased cellular motility. These results support the hypothesis that 5T4 molecules may have a direct function in tropho blast and tumour cell invasion processes.