MAMMARY EPITHELIAL-CELL DIFFERENTIATION IN-VITRO IS REGULATED BY AN INTERPLAY OF EGF ACTION AND TENASCIN-C DOWN-REGULATION

Expression of the extracellular matrix glycoprotein tenascin-C in the mammary gland is associated with cellular proliferation and cell motil ity during organogenesis and tumorigenesis. Because the source and the regulation of tenascin-C in these tissues are unclear, we have used t enascin-C cDNA, FITC-immunofluorescence and immuno-precipitation to ex amine tenascin-C expression of mammary epithelial cells, Using several mammary epithelial cell lines we could show that tenascin-C can be pr oduced and secreted by epithelial cells, However it was found that ten ascin-C synthesis was inversely correlated with the polarized epitheli al phenotype, Among three mouse mammary epithelial cell clones, tenasc in-C expression was most abundant in HC-11 cells, the least differenti ated cell type, Expression levels were high during the growth phase bu t were nearly abolished when cells were grown to confluence and induce d to express milk proteins, Downregulation of tenascin-C by EGF appare ntly commits HC-11 cells to respond to lactogenic hormones and consequ ently, hormone induced levels of beta-casein mRNA decreased significan tly when HC-11 cells were grown on a tenascin-C substrate. On the othe r hand, TGF-beta, another growth factor involved in coordinated growth and differentiation of the mammary gland in vivo was found to be a ve ry potent inducer of tenascin-C. The generation of fully polarized and tight epithelium affected the levels of tenascin-C expression, In con trast to HC-11 cells, which do not form epithelial domes in vitro, hig hly polarized and dome forming EpH4 and Fos-ER cells nearly lacked ten ascin-C. Similarly, induction of dome formation in the rat mammary ste m cell line Rama 25 by the differentiation inducer dimethylsulfoxide c aused a loss of TN-C-transcripts, The inability of Fos-ER cells to dev elop domes in the presence of soluble tenascin-C also suggests its int erference with induction and maintenance of mammary epithelial cell di fferentiation.