During recent years it has become clear that application of growth factors, hormones and even neurotransmitters onto quiescent cells can lead to long term changes in cell morphology and function. The initial events in this adaptations are the production of second messengers inside the cell and induction of a specific set of genes, named IEG’s (immediate early genes). These genes are transcriptionally activated and their induction is fast, large with respect to their pre-induction level, and transient. The function of that genes is to couple transient and rapid changes in normal metabolism to long-term modifications on gene expression. The liver constitutes an special, physiologically normal system in which to study the mitogenic response of epithelial cells and, of course, the induction of IEG’s and their role in the maintenance of the cellular identity. Study of proto-oncogene induction in hepatic models (like partial hepatectomy, hepatomas and hormonal stimulation of normal hepatocytes, among others), have provided new insights in the understanding of the mechanisms of cellular communication and maintenance of the “temporal” phenotype of a given cell.
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