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 anchoring structure in the basal membrane of epithelial cells. alpha 6 beta 4 expression is frequently altered in neoplastic cells. It is sometimes lost and sometimes overexpressed, which suggests that disruption of normal function is involved in neoplastic transformation. To examine the effect of this integrin on the growth and behavior of malignant cells that have lost beta 4, we transfected a full-length beta 4 cDNA into the UM-UC-2 cell line that expresses alpha 6 but not beta 4. Although large numbers of clones were obtained when a control vector was used in the transfection, only 12 clones could be isolated that expressed beta 4. Of these, only two beta 4-positive clones, clones 8 and 11, persisted long enough for further study. Clone 8 cells initially expressed 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 blotting 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 integrin-mediated signaling pathway. Clone 11 escaped from normal signaling because the cytoplasmic domain, a region essential for basal polar localization, was lost. The alpha 6 beta 4 integrin appears to have tumor suppressor activity in epithelial tumors.