1α,25-Dihydroxyvitamin D3 rapidly increases cytosolic calcium in clonal rat osteosarcoma cells lacking the vitamin D Receptor

Abstract

1α,25-Dihydroxyvitamin D₃ [1α,25-(OH)₂D₃] rapidly increases cytosolic calcium in a variety of cell types. Although these rapid effects do not appear to directly involve genome activation, the requirement for the classic vitamin D receptor is unclear. Clonal rat osteosarcoma cells, ROS 17/2.8, respond to 1α,25-(OH)₂D₃ with an increase in osteocalcin message but ROS 24/1 cells do not. The lack of the receptor for vitamin D in the ROS 24/1 cells has been confirmed by the absence of any detectable vitamin D-receptor complex binding to the vitamin D-responsive element (VDRE) of the osteocalcin gene and the absence of vitamin D receptor mRNA in the cells. Quin-2-loaded ROS 17/2.8 and ROS 24/1 cells were treated with 1α,25-(OH)₂D₃ in the presence and absence of extracellular calcium and with the inactive epimer, 1β,25-dihydroxyvitamin D₃ [1β,25-(OH)₂D₃]. The 1α,25-(OH)₂D₃ increased cytosolic calcium in the ROS 17/2.8 and 24/1 cells after 5 minutes in a dose-responsive manner and in the presence and absence of extracellular calcium. Pretreatment of both cell lines with 1β,25-(OH)₂D₃ for 30 s blocked the hormone-induced rise in cytosolic calcium. The rapid effects of 1α,25-(OH)₂D₃ on ROS cells with and without the vitamin D receptor and the ability of the inactive epimer to inhibit these effects indicate that the signaling system mediating the hormone's rapid actions is not the classic vitamin D receptor.