Progestin Regulation of Cellular Proliferation*

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Abstract
I. Introduction THE control of cellular proliferation is known to involve a delicate balance between the effects of different regulatory molecules including hormones and growth factors which provide, in the immediate environment of the cell, signals to stimulate or inhibit cellular replication (1). The past decade has seen significant progress in the identification and characterization of novel regulatory molecules, generally referred to as growth factors, and definition of their molecular mechanisms of action (2, 3). With the recent emphasis on peptide growth factors it is perhaps less well appreciated that some of the largest physiological changes (both positive and negative) in rates of growth and cell proliferation are mediated by steroid hormones. Recent studies on breast cancer cells (4) and immature rat uterus (5, 6), illustrating estrogenic control of autocrine and paracrine growth factor production, support the view that some of these steroid-induced responses are intimately linked to growth factor-mediated pathways although debate continues as to whether the growth factor response is the cause, or the consequence, of the estrogenic effect on cellular proliferation (7, 8). Generally speaking, the molecular mechanisms by which steroid hormones control proliferation are not well defined, with the most developed knowledge being in the area of estrogen action where a number of recent reviews are available (4, 9–11).

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