In the 70 years since progesterone (P) was identified in corpus luteum extracts, its metabolism has been examined extensively in many tissues and cell lines from numerous species. In addition to the reproductive tissues and adrenals, every other tissue that has been investigated appears to have one or more P-metabolizing enzyme, each of which is specific for a particular site on the P molecule. In the past, the actions of the P metabolizing enzymes generally have been equated to a means of reducing the P concentration in the tissue microenvironment, and the products have been dismissed as inactive waste metabolites. In human breast tissues and cell lines, the following P-metabolizing enzymes have been identified: 5a-reductase, 3a-hydroxysteroid oxidoreductase (3a-HSO), 3b-HSO, 20a-HSO, and 6a-hydroxylase. Rather than providing diverse pathways for inactivating and controlling the concentration of P in breast tissue microenvironments, it is proposed that the enzymes act directly on P to produce two types of autocrines/paracrines with opposing regulatory roles in breast cancer. Evidence is reviewed which shows that P is directly converted to the 4-pregnenes, 3a-hydroxy-4-pregnen-20-one (3a-dihydroprogesterone; 3aHP) and 20a-dihydro-progesterone (20aHP), by the actions of 3a-HSO and 20a-HSO respectively and to the 5a-pregnane, 5a-pregnane-3,20-dione(5a-dihydroprogesterone; 5aP), by the irreversible action of 5a-reductase. In vitro studies on a number of breast cell lines indicate that 3aHP promotes normalcy by downregulating cell proliferation and detachment, whereas 5aP promotes mitogenesis and metastasis by stimulating cell proliferation and detachment. The hormones bind to novel, separate, and specific plasma membrane-based receptors and influence opposing actions on mitosis, apoptosis, and cytoskeletal and adhesion plaque molecules via cell signaling pathways. In normal tissue, the ratio of 4-pregnenes:5a-pregnanes is high because of high P 3a-and 20a-HSO activities/expression and low P 5a-reductase activity/expression. In breast tumor tissue and tumorigenic cell lines, the ratio is reversed in favor of the 5a-pregnanes because of altered P-metabolizing enzyme activities/expression. The evidence suggests that the promotion of breast cancer is related to changes in in situ concentrations of cancerinhibiting and -promoting P metabolites. Current estrogen-based theories and therapies apply to only a fraction of all breast cancers; the majority (about two-thirds) of breast cancer cases are estrogeninsensitive and have lacked endocrine explanations. As the P metabolites, 5aP and 3aHP, have been shown to act with equal efficacy on all breast cell lines tested, regardless of their tumorigenicity, estrogen sensitivity, and estrogen receptor/progesterone receptor status, it is proposed that they offer a new hormonal basis for all forms of breast cancer. New diagnostic and therapeutic possibilities for breast cancer progression, control, regression, and prevention are suggested.
