9603779 Soto The mechanisms that control cell proliferation in multicellular organisms are poorly understood; little is known about the signals mediating these events, except for certain hormones. Among them, sex steroids control the cell number and proliferative activity of their target organs. Animal studies revealed that administration of estrogens to ovariectomized mammals results in 1) inhibition of cell death, 2) a swift but transient proliferative response and later 3) inhibition of cell proliferation. The mechanisms underlying these phenomena have yet to be elucidated. The establishment of estrogen-sensitive cell lines from several mammalian species during the last two decades provide models whereby the proliferative effect of estrogens may be studied. Three working hypotheses have been proposed to explain estrogen (E) induction of cell proliferation. The first proposes that E acts directly; in the second E acts through the induction of growth factors, and the third proposes that E neutralizes the action of an inhibitor. The third hypothesis is based on the following evidence: 1) estrogen-sensitive cell lines proliferate maximally in serumless medium regardless of the presence of estrogens. 2) Homologous serum from ovariectomized donors, or serum from intact donors rendered estrogenless by charcoaldextran stripping results in a dose-dependent inhibition of cell proliferation. 3) Estrogens induce cell proliferation by neutralizing the inhibitory effect of serum. The nature of this inhibitor was unclear until recently (the inhibitor is either serum albumin, or a protein of native molecular weight and pI similar to that of albumin); this laboratory explored both hypotheses, and concluded that the inhibitor (estrocolyone-I, EK-I) function is coded within the albumin molecule. Serum albumins purified from hamster, bovine rat and human serum have EK-I activity; this activity is intrinsic to the albumin molecule, since it is present in recombinant human serum albumin and certain re combinant truncated albumin peptides. In order to facilitate the understanding of the serum-mediated inhibition, serum-resistant variants of estrogen-sensitive cells were developed. The objectives of this research are i) to test whether the findings summarized above, obtained using established cell lines, apply to normal target cells in organ explants and primary cultures, ii) to determine whether the EK-I activity in analbuminemic rat and human serum is due to truncated albumin peptides, iii) to assess whether EK-I interacts with target cells via membrane receptors, and iv) to assess the role of estrogens, growth factors and EK-I on the control of proliferation of estrogen-target cells. Methods to be used include: FPLC/HPLC, affinity chromatography, ID and 2D electrophoresis, receptor binding assays, cell and tissue culture. The fulfillment of these objectives will provide mechanistic understanding of the control of cell proliferation of estrogen-target cells. This evidence will finally validate the physiological relevance of the important finding that serum albumin is the long-sought after EK-I. ***
