1α,25(OH)2 Vitamin D3 Induction of ATP Secretion in Osteoblasts

In the absence of mechanical stimulation, brief exposure of osteoblasts to 1α,25(OH)₂vitamin D₃ (1,25D) triggers plasma membrane electrical responses that couple to exocytosis. Here we describe for the first time 1,25D induction of exocytotic ATP release in static ROS 17/2.8 and SAOS‐2 cells and primary calvarial osteoblasts expressing a vitamin D receptor (VDR). We found that 10 nM 1,25D optimally induced 45 ± 1% and 40 ± 1% of partial and complete exocytotic events, respectively, from a 1,25D‐sensitive pool of ATP‐containing secretory vesicles within 60 s. We measured a dose‐dependent 1,25D induction of ATP secretion, with maximal response of ～6.2‐fold (16.93 ± 1.82 nM for SAOS‐2) and 3.1‐fold (18.89 ± 1.39 nM for ROS 17/2.8) obtained with 10 nM 1,25D compared with basal ATP levels (2.75 ± 0.39 nM, SAOS‐2; 6.09 ± 0.58 nM, ROS 17/2.8 cells). The natural metabolite 25(OH)vitamin D₃ (25D, 10 nM) induced a significant 3.6‐fold increase of ATP release in ROS 17/2.8 cells, but there was no induction with the antagonist 1β,25(OH)₂vitamin D₃ (1β,25D, 10 nM) or the steroid 17β‐estradiol (10 nM). 1,25D‐induced ATP secretion was abolished when cells were preincubated with inhibitors of vesicular exocytosis. siRNA VDR silencing prevented 1,25D stimulation of ATP exocytosis in ROS 17/2.8 and SAOS‐2 cells. Similarly, 1,25D failed to stimulate ATP exocytosis in primary osteoblasts from a VDR knockout mouse. ATP secretion coupled to 1,25D induction of cytosolic calcium and chloride channel potentiation. Rapid 1,25D stimulation of ATP secretion involving nontranscriptional VDR functions in osteoblasts may help explain 1,25D bone anabolic properties.