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 1alpha,25-dihydroxyvitamin D3 (1,25[OH]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.
Kurahashi
Izuru
I
Department of Biochemistry, Ohu University School of Dentistry, Koriyama, Japan.
Matsunuma
Ayako
A
Kawane
Tetsuya
T
Abe
Masatoshi
M
Horiuchi
Noboru
N
D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D003513:Cycloheximide / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology*; D004100:Dihydroxycholecalciferols / Q000378:metabolism*; D005938:Glucocorticoids / Q000494:pharmacology*; D007668:Kidney / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D018374:LLC-PK1 Cells; D010006:Osteoblasts / Q000187:drug effects* / Q000378:metabolism; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D016760:Proto-Oncogene Proteins c-fos / Q000096:biosynthesis / Q000235:genetics; D016755:Proto-Oncogene Proteins c-jun / Q000096:biosynthesis / Q000235:genetics; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D051381:Rats; D013250:Steroid Hydroxylases / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D013552:Swine; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D065668:Vitamin D3 24-Hydroxylase
D004100:Dihydroxycholecalciferols; D005938:Glucocorticoids; D011500:Protein Synthesis Inhibitors; D016760:Proto-Oncogene Proteins c-fos; D016755:Proto-Oncogene Proteins c-jun; D012333:RNA, Messenger; D003907:Dexamethasone; D003577:Cytochrome P-450 Enzyme System; D003513:Cycloheximide; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D013755:Tetradecanoylphorbol Acetate