TRANSFORMING GROWTH-FACTOR-ALPHA - RECEPTOR-BINDING AND ACTION ON DNA-SYNTHESIS IN THE SHEEP MAMMARY-GLAND

Microsome fractions prepared from the mammary glands of non-pregnant, pregnant and lactating sheep have been used to study binding of I-125- Iabelled transforming growth factor-alpha (TGF-alpha). Binding was dep endent on microsomal protein concentration, time and temperature. It s howed the characteristics of an epidermal growth factor (EGF) receptor , being displaced by TGF-alpha and EGF, but not by insulin or IGF-I. T he non-linear curve fitting program LIGAND was used to determine affin ity and number of binding sites. A single class of high-affinity bindi ng sites was found. The apparent dissociation constant (K-d) was simil ar in all physiological states (2.43+/-0.27 mol/l x 10(-10), n=23). Nu mbers of binding sites were lower in late-pregnant (20 weeks) and lact ating sheep (14.07+/- 2.45 fmol/mg protein, n=10) than in non-pregnant , 10- or 15-week pregnant sheep (43.04+/-5.93 fmol/mg protein, n=13). DNA synthesis by mammary alveolar epithelial cells cultured on collage n gels was increased twofold by TGF-alpha (maximum response at 10 mu g /l; 1.8 nmol/l) but not by EGF. Cells derived from 15- to 20-week preg nant sheep responded significantly to TGF-alpha on day 3 of culture, b ut the response was delayed to day 4-5 of culture in cells from other physiological states. Dose-response was not significantly affected. TG F-alpha and IGF-I produced an additive effect on DNA synthesis. Oestra diol (10(-12) to 10(-9) M), a potential stimulator of the TGF-alpha ge ne, did not stimulate DNA synthesis alone, or in combination with IGF- I. It is concluded that growth factors acting via the EGF receptor pla y a role in ruminant mammary development, but whether they mediate oes tradiol effects remains unresolved.