Effects of vitamin D₃ on signaling by prostaglandin E₂ in osteoblast‐like cells

We investigated the effects of vitamin D₃ on the signaling pathways by prostaglandin E₂ (PGE₂) in osteoblast‐like MC3T3‐E1 cells. The pretreatment with 1,25‐dihydroxyvitamin D₃ (1,25‐(OH)₂D₃), an active form of vitamin D₃, significantly inhibited cAMP accumulation induced by 10 μM PGE₂ in a dose‐dependent manner in the range between 1 pM and 1 nM. This effect of 1,25‐(OH)₂D₃ was dependent on the time of pretreatment up to 8 h. 1,25‐(OH)₂D₃ also inhibited the cAMP accumulation induced by NaF, a GTP‐binding protein activator, or forskolin which directly activates adenylate cyclase. On the other hand, 1,25‐(OH)₂D₃ significantly inhibited PGE₂‐induced IP₃ formation in a dose‐dependent manner between 10 pM and 1 nM. However, 1,25‐(OH)₂D₃ had little effect on NaF‐induced IP₃ formation. The pretreatment with 24,25‐dihydroxyvitamin D₃, an inactive form of vitamin D₃, affected neither cAMP accumulation nor IP₃ formation induced by PGE₂. These results strongly suggest that 1,25‐(OH)₂D₃ modulates the signaling by PGE₂ in osteoblast‐like cells as follows: the inhibitory effect on the cAMP production is exerted at a point downstream from adenylate cyclase and the inhibitory effect on the phosphoinositide hydrolysis is exerted at the point between the PGE₂ receptor and GTP‐binding protein, probably G_i2.