Localization of type 7 17β-hydroxysteroid dehydrogenase in mouse tissues

The enzyme type 7 17β-hydroxysteroid dehydrogenase (17β-HSD) selectively catalyzes the conversion of estrone (E1) into estradiol (E2). In order to obtain detailed information about the exact sites of action of type 7 17β-HSD, we have studied the cellular localization of type 7 17β-HSD mRNA in mouse tissues using in situ hybridization (ISH). In parallel studies, we also measured the enzyme mRNA levels by quantitative real time (RT)-PCR. In the ovary, strong hybridization signal was restricted to corpus luteum cells. In the female mammary gland, type 7 17β-HSD mRNA was found to be expressed in stromal cells surrounding the ducts. In the clitoral and preputial glands, specific labeling was observed in the epithelial cells of both acini and small ducts. In the adrenal gland, hybridization signal was observed in the zona fasciculata and reticularis in the cortex. In the liver, hybridization signal was found in all the hepatocytes. In the colon, type 7 17β-HSD mRNA expression was restricted to epithelial cells of the mucosa. From the results obtained with quantitative real time RT-PCR, it appears, with a very few exceptions, that in tissues exhibiting low mRNA expression no ISH signal could be detected. The present data suggest that E2 can be formed through the action of type 7 17β-HSD in specific cell types in the ovary and peripheral tissues, in addition to type 1 17β-HSD, thus providing tissues with an alternative route of formation of E2.