Transcriptional induction of matrix metalloproteinase-13 (collagenase-3) by 1 alpha,25-dihydroxyvitamin D-3 in mouse osteoblastic MC3T3-E1 cells

The removal of unmineralized matrix from the bone surface is essential for the initiation of osteoclastic bone resorption because osteoclasts cannot a ttach to the unmineralized osteoid. Matrix metalloproteinases (MMPs) are kn own to digest bone matrix. We recently reported that among the MMPs express ed in mouse osteoblastic cells, MMP-13 (collagenase-3) was the one most pre dominantly up-regulated by bone resorbing factors including 1 alpha ,25-dih ydroxyvitamin D-3 [1 alpha ,25(OH)(2)D-3]. In this study, we examined the m echanism of regulation of MMP-13 expression by 1 alpha ,25(OH)(2)D-3 in mou se osteoblastic MC3T3-E1 cells. 1 alpha ,25(OH)(2)D-3 increased steady-stat e messenger RNA (mRNA) and protein levels of MMP-13. De novo protein synthe sis was essential for the induction because cycloheximide (CHX) decreased t he effect of 1 alpha ,25(OH)(2)D-3 on the MMP-13 mRNA level, 1 alpha ,25(OH )(2)D-3 did not alter the decay of MMP-13 mRNA in transcriptionally arreste d MC3T3-E1 cells; however, it increased the MMP-13 heterogeneous nuclear RN A (hnRNA) level and MMP-13 transcriptional rate. The binding activity of nu clear extracts to the AP-1 binding site, but not to the Cbfa1 binding site, in the MMP-13 promoter region was up-regulated by 1 alpha ,25(OH)(2)D-3, s uggesting the mediation of AP-1 in this transcriptional induction. To deter mine the contribution of MMPs to bone resorption by 1 alpha ,25(OH)(2)D-3, the inhibitory effect of BB94, an MMP inhibitor, on resorbed pit formation by mouse crude osteoclastic cells was examined on either an uncoated or col lagen-coated dentine slice. BB94 did not prevent resorbed pit formation on uncoated dentine whereas it did on collagen-coated dentine. We therefore pr opose that the transcriptional induction of MMP-13 in osteoblastic cells ma y contribute to the degradation of unmineralized matrix on the bone surface as an early step of bone resorption by 1 alpha ,25(OH)(2)D-3.