Differential control of mRNA levels for Thy-1 antigen and laminin in rat mammary epithelial and myoepithelial-like cells.

Thy-1 antigen and laminin are two components often associated with the basement membrane of the rat mammary gland and are thought to be synthesized, at least in part, by the adjacent myoepithelial cells in vivo. The relative levels of Thy-1 mRNA and laminin mRNA are compared in a rat mammary cuboidal epithelial cell line and a derivative elongated myoepithelial-like cell line by hybridizing cloned cDNAs to cellular mRNA isolated from these cell types. Although the elongated myoepithelial-like cells synthesize four times as much laminin protein as the cuboidal epithelial cells, there is only a 1.7-fold increase in laminin mRNA between the two cell types. In contrast the 17-fold increase in Thy-1 antigen between the elongated cells and the cuboidal cells can be accounted for completely by a 14-18-fold increase in Thy-1 mRNA, suggesting that changes in the steady-state levels of Thy-1 mRNA in these cell lines are modulated at either a transcriptional or a post-transcriptional level. Run-off transcription by nuclei isolated from the cell lines does not distinguish between these two possibilities. The comparative results on Thy-1 antigen and laminin show that the enhanced production of two proteins often associated with the basement membrane of the rat mammary gland can be controlled at different levels in the elongated myoepithelial-like cells.