BETA-4 INTEGRIN TRANSFECTION OF UM-UC-2 (HUMAN BLADDER-CARCINOMA) CELLS - STABLE EXPRESSION OF A SPONTANEOUS CYTOPLASMIC TRUNCATION MUTANT WITH RAPID LOSS OF CLONES EXPRESSING INTACT BETA-4

The alpha 6 beta 4 integrin is a component of the hemidesmosome, the a nchoring structure in the basal membrane of epithelial cells, alpha 6 beta 4 expression is frequently altered in neoplastic cells. It is som etimes lost and sometimes overexpressed, which suggests that disruptio n of normal function is involved in neoplastic transformation. To exam ine the effect of this integrin on the growth and behavior of malignan t cells that have lost beta 4, we transfected a full-length beta 4 cDN A into the UM-UC-2 cell line that expresses alpha 6 but not beta 4. Al though large numbers of clones were obtained when a control vector was used in the transfection, only 12 clones could be isolated that expre ssed beta 4. Of these, only two beta 4-positive clones, clones 8 and 1 1, persisted long enough for further study. Clone 8 cells initially ex pressed beta 4, but within 2 weeks, all positive cells were lost from the culture. Clone 11 persisted in culture and retained strong surface expression of alpha 6 beta 4. Biochemical analysis and Western blotti ng revealed that this clone contained a truncated form of beta 4 that had lost the distal cytoplasmic domain. We conclude that expression of wild-type beta 4 in UM-UC-2 inhibits cell growth, presumably by an in tegrin-mediated signaling pathway. Clone 11 escaped from normal signal ing because the cytoplasmic domain, a region essential for basal polar localization, was lost. The alpha 6 beta 4 integrin appears to have t umor suppressor activity in epithelial tumors.