Effects of vitamin D 3 on signaling by prostaglandin E 2 in osteoblast‐like cells

We investigated the effects of vitamin D 3 on the signaling pathways by prostaglandin E 2 (PGE 2 ) in osteoblast‐like MC3T3‐E1 cells. The pretreatment with 1,25‐dihydroxyvitamin D 3 (1,25‐(OH) 2 D 3 ), an active form of vitamin D 3 , significantly inhibited cAMP accumulation induced by 10 μM PGE 2 in a dose‐dependent manner in the range between 1 pM and 1 nM. This effect of 1,25‐(OH) 2 D 3 was dependent on the time of pretreatment up to 8 h. 1,25‐(OH) 2 D 3 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) 2 D 3 significantly inhibited PGE 2 ‐induced IP 3 formation in a dose‐dependent manner between 10 pM and 1 nM. However, 1,25‐(OH) 2 D 3 had little effect on NaF‐induced IP 3 formation. The pretreatment with 24,25‐dihydroxyvitamin D 3 , an inactive form of vitamin D 3 , affected neither cAMP accumulation nor IP 3 formation induced by PGE 2 . These results strongly suggest that 1,25‐(OH) 2 D 3 modulates the signaling by PGE 2 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 PGE 2 receptor and GTP‐binding protein, probably G i2 .