1,25(OH)2D3 blunts hormone-elevated cytosolic Ca2+ in osteoblast-like cells

Abstract

Cytosolic free calcium ([Ca2+]i) is an important regulator of bone cell physiology. We studied the interaction of vitamin D metabolites on the hormonal-activated Ca message system in the osteoblastic cell line UMR-106. The acute rise in [Ca2+]i induced by different calciotropic hormones [parathyroid hormone, prostaglandin E2 (PGE2)] was dose dependently blunted by 1,25-dihydroxyvitamin D [1,25(OH)2D3; half-maximal inhibitory concn approximately 5 x 10(-11) M] and was initially observed after 8 h of preincubation. The 1,25(OH)2D3 metabolite of vitamin D was two orders of magnitude more potent than 24,25(OH)2D3 and 25(OH)D3. To discern between an effect of 1,25(OH)2D3 on hormonal-induced Ca2+ entry through the plasma membrane channel vs. release of Ca2+ from internal stores, we suspended fura-2-loaded cells in Mn2+ rather than Ca2+ buffers. In cells preincubated with 1,25(OH)2D3, [Ca2+]i release (indicated by [Ca2+]i transient) was significantly blunted, whereas Mn2+ influx (indicating Ca2+ flux across the plasma membrane) was unaltered, suggesting a selective effect of 1,25(OH)2D3 on hormonal-activated release of Ca2+ from intracellular stores. 1,25(OH)2D3 also inhibited the PGE2-induced production of inositol 1,4,5-trisphosphate. We conclude that, in osteoblasts, chronic (hours) incubation with 1,25(OH)2D3 leads to attenuated stimulation of the [Ca2+]i transduction pathway by calciotropic hormones. This effect of 1,25(OH)2D3 may provide a cellular basis for the synergism between the effects of vitamin D and calciotropic hormones at the bone level.

Hormonal regulation of [Ca2+]i in periosteal-derived osteoblasts: effects of parathyroid hormone, 1,25(OH)2D3 and prostaglandin E2

“…The acute rise in [Ca 2ϩ ] i induced by 1,25(OH) 2 D 3 has been attributed to the activation of phospholipase C and consequent generation of IP 3 (Civitelli et al, 1988) and to activation of dihydropyridinesensitive Ca 2ϩ channels (Caffrey and Carson, 1989). In contrast, 1,25(OH) 2 D 3 had no measurable effect on rapid calcium flux in UMR 106-01 cells and rat calvarial-derived bone cells (Dziak, 1978;Green et al, 1992). In this way, our results for femur-derived cells show differences with calvarial-derived bone cells in their responsiveness to 1,25(OH) 2 D 3 .…”

“…Because the fluorescence was obtained from whole cells, localized elevation of [Ca 2ϩ ] i may have been considerably greater. Values for concentrations of [Ca] i for osteoblastic cells have been reported and range between 100 and 200 nM (Yamaguchi et al, 1987;Donahue et al, 1988;Schofl et al, 1991;Green et al, 1992;Li et al, 1997). Due to the variability in absolute [Ca] i that has been recorded and the potential inaccuracies in these measurements, we have reported percentage changes in fluorescence with time in response to our hormonal treatments.…”

“…Prostaglandin E 2 (PGE 2 ) has been shown to promote new bone formation when infused in vivo into rodents and dogs (Mori et al, 1992;Kaplan et al, 1995). Treatment of the osteosarcoma cell line UMR-106 with PGE 2 has been shown to mobilize Ca 2ϩ , changes in [Ca 2ϩ ] i showing dynamics similar to those after treatment with PTH (Yamaguchi et al, 1988;Green et al, 1992). However, the source of Ca 2ϩ responsible for the initial transient is probably intracellular, whereas the response of the same cell line to PTH seems to reflect Ca 2ϩ influx (Yamaguchi et al, 1987(Yamaguchi et al, , 1988.…”

Down-regulation of L-type Ca2+ Channel Transcript Levels by 1,25-Dihyroxyvitamin D3

“…In this study, we report that the ␣ 1C Ca 2ϩ channel expressed by osteoblastic cells is down-regulated at the mRNA level following exposure to 1 nM 1,25(OH) 2 D 3 for 48 h. This finding is one of a handful of genes negatively regulated by 1,25(OH) 2 D 3 which include aggrecan proteoglycan (33), Id (34), atrial natriuretic peptide (35), and collagen type I (36). In previous studies, it was shown that UMR-106 cells treated with 1,25(OH) 2 D 3 for 8 -24 h lost responsiveness to parathyroid hormone-induced increases in intracellular Ca 2ϩ (37). Although the contribution of L-type Ca 2ϩ channels was not directly tested in their study using dihydropyridine channel blockers, it can be inferred that the channel down-regulation seen here after long term exposure to 1,25(OH) 2 D 3 may contribute to the diminished response.…”

Calcium-mediated Stress Kinase Activation by DMP1 Promotes Osteoblast Differentiation

“…Among these, calcium signaling is essential for the proliferation and differentiation of osteoblasts. Earlier studies have shown that treating osteoblasts with parathyroid hormone or vitamin D 3 induces an increase in intracellular calcium ([Ca 2ϩ ] i ) by increasing the release of Ca 2ϩ from the intracellular stores (1)(2)(3)(4)(5). Store-operated Ca 2ϩ channels, which are activated in response to Ca 2ϩ store depletion, control homeostasis between the extracellular Ca 2ϩ reservoir and intracellular Ca 2ϩ storage and control a wide range of cellular functions.…”