SALIVARY MESENCHYME CELLS THAT INDUCE MAMMARY EPITHELIAL HYPERPLASIA UP-REGULATE EGF RECEPTORS IN PRIMARY CULTURES OF MAMMARY EPITHELIUM WITHIN COLLAGEN GELS

Salivary mesenchyme is a potent stimulator of mammary epithelial hyper plasia and carcinogen-induced tumor formation in vivo. We have utilize d a three-dimensional collagen gel culture system, which mimics the in vivo growth environment, to identify growth stimulatory molecules pro duced by salivary mesenchyme cells. In this report we describe the dev elopment and characteristics of salivary mesenchyme cell lines, and we present further evidence that these cells produce growth factor(s) wh ich could account for the effect by interacting with epidermal growth factor (EGF) receptors on primary mouse mammary epithelial cells isola ted from midpregnant mice. Using a receptor assay with isolated cell m embranes, we characterized [I-125]-EGF binding to mammary epithelial c ells cultured within collagen gels. Scatchard analysis revealed one cl ass of high affinity EGF receptors with a Kd ranging from 8.3 x 10(-11 ) M on day one to 5.1 x 10(-11) M on day 10 of the culture period. Add ition of 10 ng/ml purified EGF to the culture medium progressively up- regulated the expression of EGF receptors during a 10-day culture peri od. Scatchard analysis showed that the increase in specific [I-125]-EG F binding was due predominantly to an increase in EGF receptor number. We also demonstrated that conditioned medium collected from salivary mesenchyme cells competed effectively for EGF receptor sites on mammar y epithelial cells, and chronic exposure to conditioned medium up-regu lated EGF receptor expression. Thus, EGF-related growth factor(s) rele ased by salivary mesenchyme cells may induce hyperplasia of adjacent m ammary epithelium in vivo, both by directly activating EGF receptors, and by provoking long term up-regulation of EGF receptors.