MODULATION OF EPIDERMAL GROWTH-FACTOR BINDING AND RECEPTOR GENE-EXPRESSION BY HORMONES AND GROWTH-FACTORS IN SHEEP PITUITARY-CELLS

Epidermal growth factor (EGF) is a potent mitogen for sheep pituitary cells but the factors controlling the binding and expression of EGF and its receptor (EGFR) in the pituitary are poorly understood. Regulation of EGF binding and EGFR gene expression may determine cellular responsiveness to EGF and could play a role in neoplastic development. Scatchard analysis of I-125-EGF binding in cultured sheep pituitary cells revealed two receptor binding sites (high affinity class of 2.5+/-0.5 x 10(3) receptors/ cell with a dissociation affinity constant (K-d) of 3.2+/-0.7 x 10(-10) M and low affinity class of 3.3+/-1.0 x 10(4) receptors/cell with a K-d of 7.1+/-1.3 x 10(-9) M). Exposure of the cultured cells to some target gland hormones of the pituitary (oestrogen, tri-iodothyronine and hydrocortisone), pituitary growth factors (EGF, basic fibroblast growth factor, transforming growth factor-beta) and a tumour-promoting phorbol ester (TPA) resulted in an increase in the binding affinity of the high affinity receptors while reducing the receptor number and also a reduction of EGFR mRNA levels, shown by Northern blot analysis. In contrast, forskolin, an activator of adenylate cyclase, showed no significant effect on EGF binding and receptor gene expression. We conclude that the EGFR in normal pituitary cells can be modulated by several hormones and other growth factors at both receptor binding and mRNA levels. Transmodulation of EGFR by hormones and growth factors in the pituitary may be one of the regulatory mechanisms controlling the balance of normal pituitary growth and function. Defects in this regulatory system could have a role in the multistep process of pituitary tumourigenesis.