1,25-dihydroxyvitamin D (1,25D) is known to inhibit osteoblast proliferation, enhance mineralisation and induce RANKL expression. In vitro mineralisation is also enhanced by increasing medium calcium. We now report the interaction between osteoblast maturation, the level of extracellular calcium and the effect of 1,25D on in vitro mineralisation.
Primary calvarial cells extracted from 1-3 day old C57BL/6 mouse calvariae (Calvarial cells) and primary cortical osteoblast-like cells grown from 4 week-old C57BL/6 femoral cortical bone (Cortical cells) were used for experimentation. The effects of 1,25D (10-9M) with 1.8 and 2.8mM extracellular Ca2+ on mineralisation and gene expression were assessed.
Calvarial cells exhibited less mature properties compared to Cortical cells including a 36% (p<0.01) reduction of mineral deposition and the induction of RANKL mRNA by 1,25D by Day 24 (3-fold, p<0.01) which occurred only in Calvarial cells. In Calvarial cultures 1,25D inhibited mineral deposition at Day 24 only when media contained 1.8mM Ca2+ (decreased 28%, p<0.01) and not with 2.8mM Ca2+. Consistent with this, 1,25D treatment in 1.8mM Ca2+-medium increased MEPE (600%, p<0.01) and SOST (1300%, p<0.01) mRNA levels. In contrast, 1,25D enhanced mineralisation by Cortical cells only in 2.8mM Ca2+-medium at Day 21 (120%, p<0.01), with no effect on MEPE and SOST mRNA expression.
Our data indicate that 1,25D influences mineralisation distinctly according to the stage of osteoblast differentiation and possibly skeletal origin; the effect of 1,25D is to inhibit mineral deposition by immature, calvaria-derived cells, and enhance mineral deposition by mature, cortical bone-derived cells, especially in response to increased extracellular calcium.