1,25-Dihydroxyvitamin D₃ inhibition of Col1a1 promoter expression in calvariae from neonatal transgenic mice

We studied the effect of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) on organ cultures of transgenic mouse calvariae containing segments of the Col1a1 promoter extending to -3518, -2297, -1997, -1794, -1763, and -1719 bp upstream of the transcription start site fused to the chloramphenicol acetyltransferase (CAT) reporter gene. 1,25(OH)₂D₃ had a dose-dependent inhibitory effect on the expression of the -3518 bp promoter construct (ColCAT3.6), with maximal inhibition of about 50% at 10 nM. This level of inhibition was consistent with the previously observed effect on the endogenous Col1a1 gene in bone cell models. All of the shorter constructs were also inhibited by 10 nM 1,25(OH)₂D₃, suggesting that the sequences required for 1,25(OH)₂D₃ inhibition are downstream of -1719 bp. The inhibitory effect of 1,25(OH)₂D₃ on transgene mRNA was maintained in the presence of the protein synthesis inhibitor cycloheximide, suggesting that the inhibitory effect on Col1a1 gene transcription does not require de novo protein synthesis. We also examined the in vivo effect of 1,25(OH)₂D₃ treatment of transgenic mice on ColCAT activity, and found that 48 h treatment caused a dose-dependent inhibition of CAT activity in calvariae comparable to that observed in organ cultures. In conclusion, we demonstrated that 1,25(OH)₂D₃ inhibits Col1A1 promoter activity in transgenic mouse calvariae, both in vivo and in vitro. The results indicate that there is a 1,25(OH)₂D₃ responsive element downstream of -1719 bp. The inhibitory effect does not require new protein synthesis.