AIMTo investigate the effects of 1 alpha hydroxylase and serum calcium on the expression of 24-hydroxylase gene in mice kidney.

METHODSMice with targeted deletion of the 25-hydroxyvitamin D 1 alpha hydroxylase gene(1alpha (OH)ase-/-), and the vitamin D receptor gene (VDR-/-) were used. The study of each mutant had two groups which were (1) mutant with high calcium diet, which maintained fertility but left mice hypocalcaemia; (2) mutant with high lactose diet, which normalized calcium in two mutant. Mice in same litter were as control. There were six groups in total and each group had five mice. All mice were killed at 10-week-old. Serum calcium was determined by an autoanalyzer. RNA was isolated from mouse kidney and the express of 1 alpha hydroxylase gene and 24-Hydroxylase gene were studied by RT-PCR.

RESULTSOn the high calcium intake, all mutant animals were hypocalcaemia (1alpha (OH)ase-/- (78 +/- 10.4) mg/L, P < 0.05; VDR-/- (68 +/- 9.8) mg/L, P < 0.05. WT (111 +/- 16.5 mg/L), but when the high lactose diet was administered, serum calcium levels in two mutant mice rose to wild-type levels. The 1 alpha hydroxylase gene was expressed at very higher levels in the vitamin D receptor mutant mice than in wild-type mice when animals received a high calcium intake; This was reduced by eliminating hypocalcaemia with the high lactose diet. Expression of the 24(OH)ase gene was extremely down-regulated in two mutant mice on the high calcium diet but was restored to wild-type levels on the high lactose diet.

CONCLUSIONThe express of 24-hydroxylase gene was directly regulated by serum calcium rather than 1 alpha-hydroxylase. These studies indicate that both the serum calcium and 1 alpha-hydroxylase exert effects on the expression of 24-hydroxylase gene, but 1 alpha-hydroxylase take the effects by elevated the concentration of serum calcium. There are no direct interaction between 1 alpha-hydroxylase gene and 24-hydroxylase gene.