Function and differential regulation of the alpha 6 integrin isoforms during parietal endoderm differentiation

F9 embryonal carcinoma cells treated with retinoic acid differentiate in monolayer into parietal endoderm (PE) or in suspension into embryoid bodies with an outer layer of visceral endoderm (VE) surrounding a core of largely undifferentiated cells. Previous reports have shown that cell-extracellular matrix interactions mediated by the beta 1 integrins play a critical role in the differentiation and migration of PE. In the present study we investigated the pattern of expression and function of the integrin alpha 6 beta 1 during the differentiation of F9 cells into VE and PE. F9 cells express integrin subunits alpha 3, alpha 5, alpha 6, and beta 1. Cell adhesion and migration assays demonstrate that alpha 6 beta 1 is the major laminin receptor in undifferentiated F9 cells as well as F9-derived PE cells. However, the amount of alpha 6 protein decreases significantly upon F9 cell differentiation into either VE or PE, as revealed by immunofluorescent staining and immunoprecipitation analysis. In contrast, the amount of steady-state alpha 6 message stays constant before and after F9 cell differentiation, suggesting that the down-regulation of alpha 6 beta 1 occurs post-transcriptionally. In view of previous reports of two alpha 6 isoforms generated by alternative RNA processing, we carried out reverse transcription-PCR analysis and show that, while alpha 6B is the major mRNA isoform before and after F9 cell differentiation, alpha 6A mRNA is weakly expressed in undifferentiated F9 cells and is substantially increased following F9 differentiation into PE. Immunoprecipitations using the isoform-specific antibodies show an increase in alpha 6A and a dramatic decrease in alpha 6B protein following PE differentiation. Pulse-chase experiments indicate that, whereas the stability of alpha 6B protein is unaltered, synthesis of alpha 6B protein is decreased at least threefold following PE differentiation. Further experiments demonstrate that alpha 6A localizes to focal contacts in PE cells. The switch from alpha 6B to alpha 6A and the localization of alpha 6A at focal contacts correlate with the acquisition of PE cell motility, which suggests distinct functions for the two alpha 6 isoforms.