Expression and functional consequences of 11β-hydroxysteroid dehydrogenase activity in human bone

Glucocorticoids have an essential role in skeletal development and function but are detrimental in excess. In several tissues, glucocorticoid action is dependent upon the expression of 11β-hydroxysteroid dehydrogenase (11β-HSD) isozymes, which interconvert active cortisol (F) and inactive cortisone (E). We previously demonstrated the expression of 11β-HSD isozymes in human osteosarcoma cell lines, osteoblast cultures, and fetal bone. We now characterize 11β-HSD expression in adult human bone using specific antihuman 11β-HSD antibodies, riboprobes, and enzyme activity studies. In addition, the effect of 11β-HSD on bone metabolism in vivo was assessed using the 11β-HSD inhibitor carbenoxolone in eight normal male volunteers. In fresh normal human bone tissue, both 11β-dehydrogenase (cortisol-to-cortisone conversion) and reductase (cortisone-to-cortisol conversion) activities were demonstrated. There was considerable interindividual variation in the dehydrogenase, but not reductase, activity. In bone homogenates, activity was NADP-dependent with a Km for F of 4.8 ± 1.2 μmol/L, suggesting the presence of 11β-HSD1. This was confirmed by reverse transcription-polymerase chain reaction (RT-PCR) analysis. Immunohistochemistry and in situ hybridization studies demonstrated 11β-HSD1 isozyme expression in cells of the osteoblast lineage and in osteoclasts. The 11β-HSD2 isozyme was expressed, but only in osteoblasts and at a low level. Ingestion of 300 mg of carbenoxolone by eight normal volunteers for 7 days resulted in a significant decrease in the bone resorption markers, pyridinoline (Pyr) and deoxypyridinoline (DPyr) (change in urinary Pyr/creatinine −1.55 ± 0.55 [mean ± SE], for DPyr/creatinine −0.4 ± 0.14 nmol/mmol; p< 0.05 for both), with no overall change in the bone formation markers C- and N-terminal propeptides of type I collagen (PICP and PINP). These data suggest that local tissue metabolism of glucocorticoids is likely to be important in determining the sensitivity of both osteoblasts and osteoclasts to glucocorticoids. In particular, variation in 11β-HSD isozyme expression and activity may explain individual variation in susceptibility to glucocorticoid-induced osteoporosis.