Expression of 11 beta-hydroxysteroid dehydrogenase in rat osteoblastic cells: Pre-receptor regulation of glucocorticoid responses in bone

11 beta -hydroxysteroid dehydrogenase (11 beta -HSD) acts as a pre-receptor signaling mechanism for corticosteroids by regulating the access of active glucocorticoids to both glucocorticoid (GR) and mineralocorticoid receptor s (MR). To examine the relationship between endogenous glucocorticoid metab olism and osteoblast function, we have characterized the expression of 11 b eta -HSD isozymes in rat osteosarcoma cells. Analysis of mRNA from ROS 25/1 > UMR 106 and ROS 17/2.8 cells revealed transcripts for both 11 beta -HSD t ype 1 (11 beta -HSD1) and type 2 (11 beta -HSD2) in all three fell lines. H owever, enzyme activity studies showed only high affinity dehydrogenase act ivity (inactivation of corticosterone (B) to 11-dehydrocorticosterone (A)), characteristic of 11 beta -HSD2; conversion of B to A was higher in ROS 25 /1>UMR 106 cells>ROS 17/2.8. Although al I three cell lines had similar num bers of GR (50,000/cell), glucocorticoid modulation of alkaline phosphatase activity and cell proliferation was only detectable in ROS 17/2.8 cells. F urther studies showed that 11 beta -HSD2 activity in each of the cel Is was potently stimulated by both A and B, but not by synthetic dexamethasone. T his effect was blocked by the 11 beta -HSD inhibitor, 18 beta -glycyrrhetin ic acid (but not by GR or MR antagonists) suggesting direct, allosteric reg ulation of 11 beta -HSD2 activity. These data indicate that in osteosarcoma cells 11 beta -HSD2 plays a key role in controlling GR-mediated responses; cells with relatively high levels of 11 beta -HSD2 activity were insensiti ve to glucocorticoids, whilst cells with low levels showed functional respo nses to both dexamethasone and B. In addition to the established effects of 11 beta -HSD2 in protecting MR in the kidney and colon, our data suggest t hat 11 beta -HSD2 in bone represents an important pre-receptor mechanism in determining ligand availability to GR. J. Cell. Biochem. 81:453-462, 2001. (C) 2001 Wiley-Liss, Inc.