Characterization of 11 beta-hydroxysteroid dehydrogenase activity and corticosteroid receptor expression in human osteosarcoma cell lines

Studies in vitro and in vivo have shown that corticosteroids play an import ant role in bone physiology and pathophysiology. It is now established that corticosteroid hormone action is regulated, in part, at the pre-receptor l evel through the expression of isozymes of 11 beta-hydroxysteroid dehydroge nase (11 beta-HSD), which are responsible for the interconversion of hormon ally active cortisol to cortisone. In this report we demonstrate 11 beta-HS D activity in human osteoblast (OB) cells. Osteosarcoma-derived OB cell lin es TE-85, MG-63 and SaOS-2 and fibrosarcoma Hs913T cells express the type 2 isoform of 11 beta-HSD, as determined by reverse transcription polymerase chain reaction (RT-PCR) and specific enzyme assays. Enzyme activity was sho wn to be strictly NAD dependent with a K-m of approximately 71 nM; 11 beta- HSD type 1 mRNA expression and enzyme activity were not detected. All four cell lines expressed mRNA for the glucocorticoid receptor (GR) and mineralo corticoid receptor, but specific binding was only detectable with radiolabe lled dexamethasone (K-d=10 nM) and not aldosterone. MG-63 cells had two to three times more GR than the other OB cells, which correlated with the high er levels of 11 beta-HSD 2 activity in these cells. In contrast to the oste osarcoma cell studies, RT-PCR analysis of primary cultures of human OB cell s revealed the presence of mRNA for 11 beta-HSD 1 as well as 11 beta-HSD 2. However, enzyme activity in these cells remained predominantly oxidative, i.e. inactivation of cortisol to cortisone (147 pmol/h per mg protein at 50 0 nM cortisol) was greater than cortisone to cortisol (10.3 pmol/h per mg p rotein at 250 nM cortisone). Data from normal human OB and osteosarcoma cel ls demonstrate the presence of an endogenous mechanism for inactivation of glucocorticoids in OB cells. We postulate that expression of the type 1 and type 2 isoforms of 11 beta-HSD in human bone plays an important role in no rmal bone homeostasis, and may be implicated in the pathogenesis of steroid -induced osteoporosis.