Several reports demonstrate estrogen receptor involvement in specific brain functions. In addition, estrogen receptors are expressed at early stages of brain development, suggesting that estrogens or related molecules may play an instructive role in the differentiation of specific brain areas. The lack of model systems in which these phenomena could be studied prompted us to develop a neuroblastoma cell line expressing the estrogen receptor. The cell line expresses the hormone receptor at levels compatible with a physiological activity. The activated estrogen receptor is capable of blocking proliferation of the cells without exerting toxic effects. Following growth arrest, the cells display a neuron-like morphology and express T and synaptophysin, two proteins synthesized in differentiating neurons. The cell line generated will provide a valuable model system for molecular and biochemical studies of the activity of estrogens in neural-derived cells.Estrogens are important for regulation of cell growth and maturation of several tissues (e.g., uterus, liver, and brain).

Insulin is a well known mitotic agent for neuroblastoma cells. Human SK-N-BE neuroblastoma cells stably transfected with the estrogen receptor, however, undergo growth arrest and differentiation when treated with insulin. These effects were shown to be due to an insulin-dependent activation of the unliganded estrogen receptor. Here, we demonstrate that this activation involves the AF-2 COOH-terminal domain of the estrogen receptor and that the communication between estrogen and insulin receptor systems occurs via selected and specific transduction signals. In fact, by the use of dominant negative and dominant positive mutants we demonstrate that p21ras is essential for insulin and estrogen receptor coupling. With pharmacological tools, we prove that PI 3'kinase does not contribute to this cross-talk and that protein kinase C triggers transduction signals that act in synergism with p21ras. These results prove the intricacy of all these intracellular paths of communication. The finding that, in neuroblastoma cells, selected signal transduction systems are involved in the insulin-dependent activation of estrogen receptor is of particular interest considering that estrogen receptor might restrict the role played by insulin during the differentiation of neural cells and interfere with its proliferative potential while allowing its regulation of other functions related to cell survival.