Human studies have suggested that local glucocorticoid (GC) generation within osteoblasts plays a critical role in bone loss seen during aging, in response to inflammation and treatment with GCs. Human osteoblasts express the enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) that converts inactive GCs (cortisone, dehydrocorticosterone, prednisone) to their active counterparts (cortisol, corticosterone, prednisolone). Enzyme expression increases with age, in response to inflammatory cytokines and with therapeutic GCs. Reports on GC metabolism in mouse osteoblasts have been conflicting however with the expression of the GC inactivating enzyme 11β-HSD2 suggested. To establish whether local GC metabolism differs between humans and mice we have characterised expression, activity and regulation of GC metabolising enzymes in osteoblasts derived from C57BL/6 mice and examined regulation by inflammatory cytokines.
Primary cultures of mouse osteoblasts were derived from calvaria (n=8 mice) and long bones (n=8) of 1424 week old mice by outgrowth of collagenase treated bone chips. The osteoblastic character of these cells was confirmed by high basal and GC-inducible alkaline phosphatase activity (2-fold induction with dexamethasone) and expression of bone restricted genes (Cbfa1 and osteocalcin) by RT-PCR. 11β-HSD1 but not 11β-HSD2 mRNA was detected. Enzyme activity studies revealed predominant reductase activity (cortisone to cortisol conversion 3.8±2.1; dehydrocorticosterone to corticosterone 1.7±0.8 pmol/mg/hr) further indicating exclusive 11β-HSD1 expression. Enzyme activity was equivalent to that seen in human osteoblasts. As in human osteoblasts 11β-HSD1 expression increased with IL-1β treatment (8.9±2.5 fold increase with 10 ng/ml IL-1β). The cofactor generating enzyme hexose-6-phosphate dehydrogenase was also expressed.
These data indicate that GC metabolising enzyme expression in osteoblasts of C57BL/6 mice is similar to that in humans and is regulated in a similar fashion. This supports the use of these mice as a model for the impact of local GC metabolism on bone.