LACTOGENIC HORMONES AND EXTRACELLULAR-MATRIX REGULATE EXPRESSION OF IGF-1 LINKED TO MMTV-LTR IN MAMMARY EPITHELIAL-CELLS

The cell line MD-IGF-1, containing an ovine IGF-1 cDNA driven by the m ouse mammary tumor virus-long terminal repeat (MMTV-LTR) promoter, was used to study expression of IGF-1 linked to the MMTV-LTR in bovine ma mmary epithelial cells in response to various hormonal and substratum stimuli. Acute sensitivity of the MMTV-LTR promoter to glucocorticoids and sex steroids was ascertained by transient transfection of parenta l MAC-T cells with an MMTV-CAT construct. Specifically, CAT activity w as induced by glucocorticoids, but not by 17beta-estradiol or progeste rone. Induction of MD-IGF-1 cells with dexamethasone (DEX) alone trigg ered a 29.5-fold increase in secretion of recombinant IGF-1 (348.9 vs 11.8 pg/mug DNA), and stimulated a 1.7-fold increase in total DNA with in 72 h. Growth of MD-IGF-1 cells was enhanced by exogenous IGF-1, ins ulin, and TGF-alpha. In contrast, TGF-beta inhibited cell proliferatio n, while epidermal growth factor, estrogen, progesterone, and testoste rone had no effect. Extracellular matrix from the Engelbreth-Holm-Swar m (EHS) tumor, in the presence of DEX, prolactin (PRL), and insulin st imulated a 29.4-fold increase in secretion of IGF-1 (591.9 pg/mug DNA) , compared with cells in absence of hormones (20.1 pg/mug DNA). EHS an d DEX plus PRL triggered a 63.2-fold increase in IGF-1 secretion (689. 1 pg/mug DNA), compared with MD-IGF-I cells cultured on plastic (10.9 pg/mug DNA), in the absence of hormones. These data indicate that the MMTV-LTR is regulated by both lactogenic hormones and extracellular ma trix in MD-IGF-1 cells and that the MMTV-LTR may be a useful regulator y element for targeting expression of foreign proteins in bovine mamma ry epithelial cells.