Abstract

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: 5α-reductase, 3α-hydroxysteroid oxidoreductase (3α-HSO), 3β-HSO, 20α-HSO, and 6α-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, 3α-hydroxy-4-pregnen-20-one (3α-dihydroprogesterone; 3αHP) and 20α-dihydroprogesterone (20αHP), by the actions of 3α-HSO and 20α-HSO respectively and to the 5α-pregnane, 5α-pregnane-3,20-dione(5α-dihydroprogesterone; 5αP), by the irreversible action of 5α-reductase. In vitro studies on a number of breast cell lines indicate that 3αHP promotes normalcy by downregulating cell proliferation and detachment, whereas 5αP 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:5α-pregnanes is high because of high P 3α- and 20α-HSO activities/expression and low P 5α-reductase activity/expression. In breast tumor tissue and tumorigenic cell lines, the ratio is reversed in favor of the 5α-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 cancer-inhibiting 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 estrogen-insensitive and have lacked endocrine explanations. As the P metabolites, 5αP and 3αHP, 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.