Potentiation by stannous chloride of calcium entry into osteoblastic MC3T3-E1 cells through voltage-dependent L-type calcium channels

Microdamage induced calcium efflux from bone matrix activates intracellular calcium signaling in osteoblasts via L-type and T-type voltage-gated calcium channels

Mechanisms by which bone microdamage triggers repair response are not completely understood. It has been shown that calcium efflux ([Ca(2+)]E) occurs from regions of bone undergoing microdamage. Such efflux has also been shown to trigger intracellular calcium signaling ([Ca(2+)]I) in MC3T3-E1 cells local to damaged regions. Voltage-gated calcium channels (VGCCs) are implicated in the entry of [Ca(2+)]E to the cytoplasm. We investigated the involvement of VGCC in the extracellular calcium induced intracellular calcium response (ECIICR). MC3T3-E1 cells were subjected to one dimensional calcium efflux from their basal aspect which results in an increase in [Ca(2+)]I. This increase was concomitant with membrane depolarization and it was significantly reduced in the presence of Bepridil, a non-selective VGCC inhibitor. To identify specific type(s) of VGCC in ECIICR, the cells were treated with selective inhibitors for different types of VGCC. Significant changes in the peak intensity and the number of [Ca(2+)]I oscillations were observed when L-type and T-type specific VGCC inhibitors (Verapamil and NNC55-0396, respectively) were used. So as to confirm the involvement of L- and T-type VGCC in the context of microdamage, cells were seeded on devitalized notched bone specimen, which were loaded to induce microdamage in the presence and absence of Verapamil and NNC55-0396. The results showed significant decrease in [Ca(2+)]I activity of cells in the microdamaged regions of bone when L- and T-type blockers were applied. This study demonstrated that extracellular calcium increase in association with damage depolarizes the cell membrane and the calcium ions enter the cell cytoplasm by L- and T-type VGCCs.

Elevation of extracellular Ca2+ induces store-operated calcium entry via calcium-sensing receptors: a pathway contributes to the proliferation of osteoblasts

Aims

The local concentration of extracellular Ca²⁺ ([Ca²⁺]_o) in bone microenvironment is accumulated during bone remodeling. In the present study we investigated whether elevating [Ca²⁺]_o induced store-operated calcium entry (SOCE) in primary rat calvarial osteoblasts and further examined the contribution of elevating [Ca²⁺]_o to osteoblastic proliferation.

Methods

Cytosolic Ca²⁺ concentration ([Ca²⁺]_c) of primary cultured rat osteoblasts was detected by fluorescence imaging using calcium-sensitive probe fura-2/AM. Osteoblastic proliferation was estimated by cell counting, MTS assay and ATP assay. Agonists and antagonists of calcium-sensing receptors (CaSR) as well as inhibitors of phospholipase C (PLC), SOCE and voltage-gated calcium (Cav) channels were applied to study the mechanism in detail.

Results

Our data showed that elevating [Ca²⁺]_o evoked a sustained increase of [Ca²⁺]_c in a dose-dependent manner. This [Ca²⁺]_c increase was blocked by TMB-8 (Ca²⁺ release inhibitor), 2-APB and BTP-2 (both SOCE blockers), respectively, whereas not affected by Cav channels blockers nifedipine and verapamil. Furthermore, NPS2143 (a CaSR antagonist) or U73122 (a PLC inhibitor) strongly reduced the [Ca²⁺]_o-induced [Ca²⁺]_c increase. The similar responses were observed when cells were stimulated with CaSR agonist spermine. These data indicated that elevating [Ca²⁺]_o resulted in SOCE depending on the activation of CaSR and PLC in osteoblasts. In addition, high [Ca²⁺]_o significantly promoted osteoblastic proliferation, which was notably reversed by BAPTA-AM (an intracellular calcium chelator), 2-APB, BTP-2, TMB-8, NPS2143 and U73122, respectively, but not affected by Cav channels antagonists.

Conclusions

Elevating [Ca²⁺]_o induced SOCE by triggering the activation of CaSR and PLC. This process was involved in osteoblastic proliferation induced by high level of extracellular Ca²⁺ concentration.

Evaluation of deoxyribonucleic acid toxicity induced by the radiopharmaceutical 99mTechnetium-Methylenediphosphonic acid and by stannous chloride in Wistar rats

Radiopharmaceuticals are employed in patient diagnostics and disease treatments. Concerning the diagnosis aspect, technetium-99m (^99mTc) is utilized to label radiopharmaceuticals for single photon computed emission tomography (SPECT) due to its physical and chemical characteristics. ^99mTc fixation on pharmaceuticals depends on a reducing agent, stannous chloride (SnCl₂) being the most widely-utilized. The genotoxic, clastogenic and anegenic properties of the ^99mTc-MDP(methylene diphosphonate used for bone SPECT) and SnCl₂ were evaluated in Wistar rat blood cells using the Comet assay and micronucleus test. The experimental approach was to endovenously administer NaCl 0.9% (negative control), cyclophosphamide 50 mg/kg b.w. (positive control), SnCl₂ 500 μg/mL or ^99mTc-MDP to animals and blood samples taken immediately before the injection, 3, and 24 h after (in the Comet assay) and 36 h after, for micronucleus test. The data showed that both SnCl₂ and ^99mTc-MDP-induced deoxyribonucleic acid (DNA) strand breaks in rat total blood cells, suggesting genotoxic potential. The ^99mTc-MDP was not able to induce a significant DNA strand breaks increase in in vivo assays. Taken together, the data presented here points to the formation of a complex between SnCl₂ in the radiopharmaceutical ^99mTc-MDP, responsible for the decrease in cell damage, compared to both isolated chemical agents. These findings are important for the practice of nuclear medicine.

A large-conductance (BK) potassium channel subtype affects both growth and mineralization of human osteoblasts

The pharmacology of the large-conductance K(+) (BK) channel in human osteoblasts is not well defined, and its role in bone is speculative. Here we assess BK channel properties in MG63 cells and primary human osteoblasts and determine whether pharmacological modulation affects cell function. We used RT-PCR and patch-clamp methods to determine the expression of BK channel subunits and cell number assays in the absence and presence of BK channel modulators. RT-PCR showed the presence of KCNMA1, KCNMB1, KCNMB2, KCNMB3, and KCNMB4 subunits. The BK channel was voltage dependent, with a mean unitary conductance of 228.8 pS (n = 10) in cell-attached patches (140 mM K(+)/140 mM K(+)) and a conductance of 142.5 pS (n = 16) in excised outside-out and 155 pS (n = 6) in inside-out patches in 3 mM K(+)/140 mM K(+). The selectivity ratio (ratio of K(+) to Na(+) permeability) was 15:1. The channel was blocked by tetraethylammonium (TEA, 0.3 mM), iberiotoxin (5-60 nM), tetrandrine (5-30 microM), and paxilline (10 microM) and activated by isopimaric acid (20 microM). BK channel modulators affected MG63 cell numbers: TEA and tetrandrine significantly increased cell numbers at low concentrations (3 mM and 3 microM, respectively) and reduced cell numbers at higher concentrations (>10 mM and >10 microM, respectively). Neither iberiotoxin (20-300 nM) nor slotoxin (300 nM) affected cell numbers. The increase in cell numbers by TEA was blocked by isopimaric acid. TEA (0.1-3.0 mM) significantly increased mineralization in primary osteoblasts. In conclusion, the BK channel has a distinctive pharmacology and is thus a target for therapeutic strategies aimed at modulating osteoblast proliferation and function.

Calcium signalling and calcium transport in bone disease

Calcium transport and calcium signalling mechanisms in bone cells have, in many cases, been discovered by study of diseases with disordered bone metabolism. Calcium matrix deposition is driven primarily by phosphate production, and disorders in bone deposition include abnormalities in membrane phosphate transport such as in chondrocalcinosis, and defects in phosphate-producing enzymes such as in hypophosphatasia. Matrix removal is driven by acidification, which dissolves the mineral. Disorders in calcium removal from bone matrix by osteoclasts cause osteopetrosis. On the other hand, although bone is central to management of extracellular calcium, bone is not a major calcium sensing organ, although calcium sensing proteins are expressed in both osteoblasts and osteoclasts. Intracellular calcium signals are involved in secondary control including cellular motility and survival, but the relationship of these findings to specific diseases is not clear. Intracellular calcium signals may regulate the balance of cell survival versus proliferation or anabolic functional response as part of signalling cascades that integrate the response to primary signals via cell stretch, estrogen, tyrosine kinase, and tumor necrosis factor receptors.

Cytotoxic and genotoxic effects induced by stannous chloride associated to nuclear medicine kits

At present, more than 75% of routine nuclear medicine diagnostic procedures use technetium-99m (99mTc). The binding between 99mTc and the drug to obtain the radiopharmaceutical needs a reducing agent, with stannous chloride (SnCl2) being one of the most used. There are controversies about the cytotoxic, genotoxic and mutagenic effects of SnCl2 in the literature. Thus, the approaches below were used to better understand the biological effects of this salt and its association in nuclear medicine kits [methylenediphosphonate (MDP) bone scintigraphy and diethylenetriaminepentaacetic acid (DTPA) kidney and brain scintigraphy]: (i) bacterial inactivation experiments; (ii) agarose gel electrophoresis of supercoiled and linear plasmid DNA and (iii) bacterial transformation assay. The Escherichia coli strains used here were AB1157 (wild type) and BW9091 (xthA mutant). Data obtained showed that both MDP and SnCl2 presented a high toxicity, but this was not observed when they were assayed together in the kit, thereby displaying a mutual protect effect. DTPA salt showed a moderate toxicity, and once more, the DTPA kit provided protection, compared to the SnCl2 effect alone. The results suggest a possible complex formation, either MDP-SnCl2 or DTPA-SnCl2, originating an atoxic compound. On the other hand, SnCl2-induced cell inactivation and the decrease in bacterial transformation generated by DTPA found in XthA mutant strain suggest that the lack of this enzyme could be responsible for the effects observed, being necessary to induce DNA damage repair.

Sensitivity to Sn2+ of the yeast Saccharomyces cerevisiae depends on general energy metabolism, metal transport, anti-oxidative defences, and DNA repair

Resistance to stannous chloride (SnCl(2)) of the yeast Saccharomyces cerevisiae is a product of several metabolic pathways of this unicellular eukaryote. Sensitivity testing of different null mutants of yeast to SnCl(2) revealed that DNA repair contributes to resistance, mainly via recombinational (Rad52p) and error-prone (Rev3p) steps. Independently, the membrane transporter Atr1p/Snq1p (facilitated transport) contributed significantly to Sn(2+)-resistance whereas absence of ABC export permease Snq2p did not enhance sensitivity. Sensitivity of the superoxide dismutase mutants sod1 and sod2 revealed the importance of these anti-oxidative defence enzymes against Sn(2+)-imposed DNA damage while a catalase-deficient mutant (ctt1) showed wild type (WT) resistance. Lack of transcription factor Yap1, responsible for the oxidative stress response in yeast, led to 3-fold increase in Sn(2+)-sensitivity. While loss of mitochondrial DNA did not change the Sn(2+)-resistance phenotype in any yeast strain, cells with defect cytochrome c oxidase (CcO mutants) showed gradually enhanced sensitivities to Sn(2+) and different spontaneous mutation rates. Highest sensitivity to Sn(2+) was observed when yeast was in exponential growth phase under glucose repression. During diauxic shift (release from glucose repression) Sn(2+)-resistance increased several hundred-fold and fully respiring and resting cells were sensitive only at more than 1000-fold exposure dose, i.e. they survived better at 25 mM than exponentially growing cells at 25 microM Sn(2+). This phenomenon was observed not only in WT but also in already Sn(2+)-sensitive rad52 as well as in sod1, sod2 and CcO mutant strains. The impact of metabolic steps in contribution to Sn(2+)-resistance had the following ranking: Resting WT cells > membrane transporter Snq1p > superoxide dismutases > transcription factor Yap1p >or= DNA repair >> exponentially growing WT cells.