No abstract

PC12 cells, which differentiate morphologically and biochemically into sympathetic neuronlike cells in response to nerve growth factor, also respond to epidermal growth factor . The response to epidermal growth factor is similar in certain respects to the response to nerve growth factor . Both peptides produce rapid increases in cellular adhesion and 2-deoxyglucose uptake and both induce ornithine decarboxylase . But nerve growth factor causes a decreased cell proliferation and a marked hypertrophy of the cells. In contrast, epidermal growth factor enhances cell proliferation and does not cause hypertrophy . Nerve growth factor induces the formation of neurites ; epidermal growth factor does not.When both factors are presented simultaneously, the cells form neurites . Furthermore, the biological response to epidermal growth factor, as exemplified by the induction of ornithine decarboxylase, is attenuated by prior treatment of the cells with nerve growth factor . PC12 cells have epidermal growth factor receptors. The binding of epidermal growth factor to these receptors is rapid and specific, and exhibits an equilibrium constant of 1 .9 x 10 -9 M. Approximately 80,000 receptors are present per cell, and this number is independent of cell density.Treatment of the cells with nerve growth factor reduces the amount of epidermal growth factor binding by at least 809'0. The decrease in receptor binding begins after -12-18 h of nerve growth factor treatment and is complete within 3 d . Scatchard plots indicate that the number of binding sites decreases, not the affinity of the binding sites for epidermal growth factor .The rat pheochromocytoma clone PC 12 has been used extensively as a model of neuronal differentiation . This clone, developed from an adrenal tumor by Greene and his colleagues (12, 18-20, 22, 45), is of adrenergic neural crest derivation . The cells grow readily under standard conditions and exhibit many of the properties of adrenal medullary chromaffin cells . In the presence of nerve growth factor in nanogram or even subnanogram amounts, the cells differentiate into a phenotype resembling sympathetic neurons . The changes occur within a few days and include process formation, electrical excitability, amine uptake and storage, and a cessation of cell division . The cells contain increased levels of certain transmitter-synthesizing enzymes (13,20,41) and will make synapses with appropriate target tissues (41). The differentiation appears to be reversible, but except for this, it resembles, as far as has been explored, the changes occurring in normal cells differentiating into sympathetic neurons .

MCF-7 human breast cancer cells have been studied for hormonal regulation of secretion of an insulin growth factor-I (IGF-I)-related growth factor. 17 beta-Estradiol, which is required for tumorigenesis of the cell line in the nude mouse and which stimulates proliferation in vitro, was able to significantly induce IGF-I secretion at 10(-13) M, with maximal induction at 10(-11) M. Under optimal conditions IGF-I could be induced 4-fold after 4 days. Demonstration of estrogenic stimulations required removal of phenol red, a weak estrogen, from the cell culture medium. In addition to estrogen, insulin, epidermal growth factor, and transforming growth factor alpha induce both cellular proliferation and IGF-I secretion, while growth inhibitory antiestrogens, transforming growth factor beta, and glucocorticoids have the opposite effect. In each case, modulations in IGF-I secretion preceeded effects on cellular proliferation. IGF-I was not regulated by human GH, basic fibroblast growth factor, platelet-derived growth factor, or PRL, none of which affected proliferation rate. Thus, regulation of IGF-I secretion in human breast cancer is controlled by different hormones from those previously reported in human fibroblasts. Regulation of IGF-I by neither estrogen nor antiestrogen was associated with changes in steady-state mRNA levels; thus regulation may occur at a step beyond mRNA. We conclude that IGF-I production is tightly coupled to growth regulation by estrogens, antiestrogens, and other hormones and may contribute to autocrine and/or paracrine growth regulation by these agents in breast cancer.