Dexamethasone enhances vitamin D-24-hydroxylase expression in osteoblastic (UMR-106) and renal (LLC-PK1) cells treated with 1alpha,25-dihydroxyvitamin D3

Chronic glucocorticoid therapy causes rapid bone loss and clinical osteoporosis. We previously found that dexamethasone, a potent glucocorticoid, increased renal expression of vitamin D-24-hydroxylase, which degrades such vitamin D metabolites as 25-hydroxyvitamin D₃ and 1α,25-dihydroxyvitamin D₃ (1,25OH]₂D₃). We therefore investigated the mechanisms of this increase in UMR-106 osteoblast-like cells and LLC-PK₁ kidney cells. To induce 24-hydroxylase expression, 1,25(OH)₂D₃ (10⁻⁷ M) and dexamethasone were added simultaneously to the medium of LLC-PK₁ cells, and 24 h before dexamethasone treatment, 1,25(OH)₂D₃ was added to the medium of UMR-106 cells. Dexamethasone dose dependently increased 24-hydroxylase mRNA and enzymatic activity in 1,25(OH)₂D₃-treated LLC-PK₁ and UMR-106 cells. Maximal stimulation was observed with 10⁻⁶ M dexamethasone in both cell lines. The addition of 10⁻⁶ M dexamethasone significantly increased the abundance of 24-hydroxylase mRNA by 24 and 8 h in 1,25(OH)₂D₃-treated LLC-PK₁ and UMR-106 cells, respectively. Stimulation for dexamethasone in UMR-106 cells persisted for up to 48 h. Dexamethasone stimulation of 24-hydroxylase mRNA expression in UMR-106 cells was abolished by pretreatment with cycloheximide, an inhibitor of protein synthesis. Northern and Western analyses indicated that 10⁻⁶ M dexamethasone markedly increased the abundance of c-fos mRNA at 20 min and c-fos protein concentration at 60 min in 1,25(OH)₂D₃-treated UMR-106 cells but only slightly induced the abundance of c-jun mRNA. The addition of phorbol 12-myristate 13-acetate increased mRNA expression for both c-fos and 24-hydroxylase in 1,25(OH)₂D₃-treated UMR-106 cells. The effect of dexamethasone on 24-hydroxylase mRNA expression was blocked by RO31-8220, a specific inhibitor of protein kinase C. Thus, dexamethasone in the presence of 1,25(OH)₂D₃ enhances expression of 24-hydroxylase in UMR-106 osteoblastic cells via new protein synthesis. The mechanism of this effect appears to involve activation of the AP-1 site by increased c-fos protein.