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The 4-Pregnene and 5-Pregnane Progesterone Metabolites Formed in Nontumorous and Tumorous Breast Tissue Have Opposite Effects on Breast Cell Proliferation and Adhesion¹

Hormonal Regulatory Mechanisms Laboratory, University of Western Ontario, London, Ontario, N6A 5B7 Canada

Progesterone is required for the full proliferative activity of the breasts and may be directly or indirectly involved in either stimulating or inhibiting breast cancer. To determine whether the effects on breast cancer are attributable to progesterone metabolites, we compared the capacity of nontumorous and tumorous breast tissue to convert progesterone and then tested the effects of these metabolites on breast cell proliferation and anchorage. Tissues from the operated breasts of six patients with infiltrating duct carcinomas were incubated with [¹⁴C]progesterone for 2, 4, and 8 h, and the metabolites were identified and quantified. The identified metabolites (equal to >95% of recovered radioactivity) can be divided into those that retain the double bond of progesterone in the carbon-4 position of ring A (4-pregnenes) and those that are 5-reduced (5-pregnanes). The results show that tumorous breast tissue has elevated 5-reductase activity, which results in significantly higher total levels of 5-pregnanes, especially 5-pregnane-3,20-dione (5P), whereas normal (nontumorous) breast tissue produces more 4-pregnenes, especially 3-hydroxy-4-pregnen-20-one (3HP). 5P and 3HP are each one enzymatic step removed from progesterone, resulting from the action of either 5-reductase or 3-hydroxysteroid oxidoreductase (3-HSO), respectively. The ratio of 5-pregnanes:4-pregnenes is >5-fold greater and the ratio of 5P:3HP is nearly 30-fold greater in tumorous than nontumorous breast tissue incubates. In vitro studies with three breast cell lines (MCF-7, MCF-10A, and ZR-75-1) show that 3HP dose dependently inhibits, whereas 5P significantly stimulates, proliferation. Additional studies with MCF-7 and MCF-10A cells indicate that each of the 4-pregnenes isolated from breast tissue suppresses, whereas each respective 5-reduced product stimulates, cell proliferation. Studies of cell anchorage were conducted using MCF-7 cells and various concentrations of 5P or 3HP. The number of cells attached to the substrate was significantly (P < 0.05) decreased by treatment with 30 nM 5P and increased by treatment with 50 nM 3HP. Conversely, the number of cells detached from the substrate after partial trypsin exposure was significantly increased by treatment with 40 nM 5P and decreased by treatment with 30 nM 3HP. The results suggest that a change in in situ progesterone metabolism, resulting in an increased 5-pregnane:4-pregnene (especially 5P:3HP) ratio, may promote breast cancer by promoting increased cell proliferation and detachment, whereas increases in 4-pregnenes may retard these tumorigenic processes. These studies suggest that endogenous progesterone metabolites may provide a new hormonal basis for breast cancer.

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