Oncogenic Ras dominates overexpression of E-cadherin in malignant transformation of intestinal epithelial cells.

Abstract

Loss of the adherens junction protein E-cadherin is a critical event during Ras-mediated transformation of intestinal epithelial cells. The purpose of our study was to determine if overexpression of E-cadherin prevents Ras-induced malignant transformation and suppresses cell growth. Rat intestinal epithelial cells were constructed with a mutated human Ha-RasVal12 cDNA. In these cells, Ras is constitutively expressed or induced by addition of isopropyl-1-thio-B-D-galactopyranoside. Cells were transfected with a bicistronic retroviral system that expressed green fluorescent protein alone or this protein and human E-cadherin. E-cadherin expression was measured by Western blot analysis, and localization by immunofluorescence. Anchorage-independent growth in soft agar was examined as well as tumor growth in nude mice. After Ras induction, endogenous E-cadherin was downregulated, whereas overexpression of human E-cadherin was sustained. Oncogenic Ras dominated overexpression of E-cadherin by causing malignant transformation and E-cadherin mislocalization. Ras also promoted growth in soft agar and tumors in nude mice despite E-cadherin overexpression. Oncogenic Ras subverts the tumor suppressor activity of E-cadherin in Ras-transformed intestinal epithelial cells by downregulating endogenous E-cadherin and mislocalizing transfected E-cadherin. The role of E-cadherin as a tumor suppressor in intestinal malignancies may be restricted by mutated or overactive Ras.