Extracellular NAD(+) induces calcium signaling and apoptosis in human osteoblastic cells.

Abstract ADP-ribosyl cyclase/CD38 is a bifunctional enzyme that catalyzes at its ectocellular domain the synthesis from NAD + (cyclase) and the hydrolysis (hydrolase) of the calcium-mobilizing second messenger cyclic ADP ribose (cADPR). Furthermore, CD38 mediates cADPR influx inside a number of cells, thereby inducing Ca 2+ mobilization. Intracellularly, cADPR releases Ca 2+ from ryanodine-sensitive pools, thus activating several Ca 2+ -dependent functions. Among these, the inhibition of osteoclastic-mediated bone resorption has been demonstrated. We found that HOBIT human osteoblastic cells display ADP-ribosyl cyclase activity and we examined the effects of CD38 stimulation on osteoblasts function. Extracellular NAD + induced elevation of cytosolic calcium due to both Ca 2+ influx from the extracellular medium and Ca 2+ release from ryanodine-sensitive intracellular stores. Culturing these cells in the presence of NAD + caused a complete growth arrest with a time-dependent decrease of cell number and the appearance of apoptotic nuclei. The first changes could be observed after 24 h of treatment and became fully evident after 72–96 h. We propose a role of extracellular NAD + in bone homeostatic control.