Characterization of transforming growth factor-alpha receptors in the avian ovary: alterations in ligand binding to granulosa cells during follicular maturation.

The presence of epidermal growth factor receptors (EGF-R) and the ligands epidermal growth factor/transforming growth factor-alpha (EGF/TGF alpha) have been reported in mammalian ovaries where they are implicated in folliculogenesis and steroidogenesis. Evidence is presented to show that authentic EGF/TGF alpha receptors are expressed by the avian granulosa cells. The TGF alpha receptors (TGF alpha-R) from chicken granulosa cells were characterized by specific binding of 125I-human TGF alpha. In this study, competition with human EGF, human TGF alpha, human IGF-I, human basic fibroblast growth factor (bFGF) and insulin for 125I-human TGF alpha binding demonstrated that the avian granulosa cell TGF alpha-R binds human EGF with 300-fold lower affinity than human TGF alpha. IGF-I, bFGF and insulin did not displace bound 125I-TGF alpha. Scatchard analysis showed that a single class of high-affinity binding sites is present on the granulosa cells (Kd 0.23 +/- 0.009 nM). However, the number of binding sites altered during follicular maturation with a significant decline in the most mature follicle. These results go some way to explaining the basis for the changing sensitivity of avian granulosa cells to EGF/TGF alpha stimulation as they mature. In addition, the gonadotrophins, LH and FSH, increased the number of receptors in cultured granulosa cells and may therefore partially influence folliculogenesis and steroidogenesis through this route.