TENASCIN-C INHIBITS EXTRACELLULAR MATRIX-DEPENDENT GENE-EXPRESSION INMAMMARY EPITHELIAL-CELLS - LOCALIZATION OF ACTIVE REGIONS USING RECOMBINANT TENASCIN FRAGMENTS

The physiological role of tenascin in vivo has remained obscure. Altho ugh tenascin is regulated in a stage and tissue-dependent manner, knoc k-out mice appear normal, When tenascin expression was examined in the normal adult mouse mammary gland, little or none was present during l actation, when epithelial cells actively synthesize and secrete milk p roteins in an extracellular matrix/lactogenic hormone-dependent manner . In contrast, tenascin was prominently expressed during involution, a stage characterized by the degradation of the extracellular matrix an d the subsequent loss of milk production, Studies with mammary cell li nes indicated that tenascin expression was high on plastic, but was su ppressed in the presence of the laminin-rich, Engelbreth-Holm-Swarm (E HS) tumour biomatrix. When exogenous tenascin was added together with EHS to mammary epithelial cells. beta-casein protein synthesis and ste ady-state mRNA levels were inhibited in a concentration-dependent mann er. Moreover, this inhibition by tenascin could be segregated from its effects on cell morphology. Using two beta-casein promoter constructs attached to the chloramphenicol acetyltransferase reporter gene we sh owed that tenascin selectively suppressed extracellular matrix/prolact in-dependent transcription of the beta-casein gene in three-dimensiona l cultures, Finally, we mapped the active regions within the fibronect in type III repeat region of the tenascin molecule that are capable of inhibiting beta-casein protein synthesis, Our data are consistent wit h a model where both the loss of a laminin-rich basement membrane by e xtracellular matrix-degrading enzymes and the induction of tenascin co ntribute to the loss of tissue-specific gene expression and thus the i nvoluting process.