Mechanisms of diethylstilbestrol-induced calcium movement in MG63 human osteosarcoma cells

17β-Estradiol inhibits outward voltage-gated K⁺ currents in human osteoblast-like MG63 cells

Previous studies have shown that 17β-estradiol has a pivotal function by blocking voltage-gated K⁺ (Kv) channels in several different types of cells such as cardiac myocytes and neurons. Outward Kv currents can also be measured in osteoblasts, although little is known about the effects of 17β-estradiol on these currents. In human osteoblast-like MG63 cells, we found that 17β-estradiol inhibits peak and end Kv currents, with IC₅₀ values of 480 and 325 nM, respectively. To elucidate the mechanism of inhibition, the kinetics of Kv currents were investigated. The half-maximum activation potential (V(½)) was 1.3 mV and was shifted left to -4.4 mV after application of 500 nM 17β-estradiol. For steady-state inactivation, the V(½) was -55.0 mV and weakly shifted left to -58.2 mV. To identify the molecular basis of outward Kv currents in MG63 cells, we performed RT-PCR analyses. The expression of Kv2.1 channels appeared to dominate over that of other Kv channels in MG63 cells. In COS-7 cells with heterologously expressed Kv2.1 channels, 17β-estradiol also inhibits macroscopic currents of Kv2.1. Our data indicate that 17β-estradiol inhibits Kv currents in human osteoblast-like MG63 cells and that Kv2.1 is a potential molecular correlate of outward Kv currents in these cells.

Safrole induces cell death in human tongue squamous cancer SCC-4 cells through mitochondria-dependent caspase activation cascade apoptotic signaling pathways

Safrole is one of important food-borne phytotoxin that exhibits in many natural products such as oil of sassafras and spices such as anise, basil, nutmeg, and pepper. This study was performed to elucidate safrole-induced apoptosis in human tongue squamous carcinoma SCC-4 cells. The effect of safrole on apoptosis was measured by flow cytometry and DAPI staining and its regulatory molecules were studied by Western blotting analysis. Safrole-induced apoptosis was accompanied with up-regulation of the protein expression of Bax and Bid and down-regulation of the protein levels of Bcl-2 (up-regulation of the ratio of Bax/Bcl-2), resulting in cytochrome c release, promoted Apaf-1 level and sequential activation of caspase-9 and caspase-3 in a time-dependent manner. We also used real-time PCR to show safrole promoted the mRNA expressions of caspase-3, -8, and -9 in SCC-4 cells. These findings indicate that safrole has a cytotoxic effect in human tongue squamous carcinoma SCC-4 cells by inducing apoptosis. The induction of apoptosis of SCC-4 cells by safrole is involved in mitochondria- and caspase-dependent signal pathways.

KCl Depolarization Increases HIF-1 Transcriptional Activity via the Calcium-Independent Pathway in SGC7901 Gastric Cancer Cells

BACKGROUND

Hypoxia-inducible factor 1alpha (HIF-1alpha) has been reported to be expressed aberrantly in gastric cancer cells. Stability and transactivation of HIF-1 were associated with the change of intracellular calcium. We hypothesized that KCl depolarization may modulate HIF-1 activity in gastric cancer cells through calcium involvement.

METHODS

HIF-1alpha expression and its transcriptional activity were determined in SGC7901 gastric cancer cells treated with KCl and/or CoCl2 under normoxia. KCl induced change in the intracellular free calcium concentration and its effect on HIF-1 activity was investigated subsequently.

RESULTS

Exposure of SGC7901 cells to KCl (50 mM) could induce HIF-1alpha expression and its nucleus accumulation under normoxic conditions, reaching the peak at 8 and 2 h, respectively. KCl could also induce transactivation of the HIF-1 reporter gene and its target gene VEGF secretion at 8 h. Further experiments confirmed that depolarization of SGC7901 cells with KCl caused an increase in intracellular free calcium concentration. Chelation of intracellular calcium by BAPTA [1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid] induced HIF-1alpha accumulation and HIF-1 activity. However, elevation of cytosolic calcium level by ionomycin, a calcium ionophore, failed to induce HIF-1 transcriptional activity.

CONCLUSIONS

KCl depolarization would act through the calcium-independent pathway leading to enhanced HIF-1 transcriptional activity in gastric cancer cells.

Arachidonic acid influences intracellular calcium handling in human osteoblasts

The effect of arachidonic acid (AA) on intracellular Ca(2+) concentration ([Ca(2+)]i) in human osteoblasts MG63 was studied. AA caused a concentration-dependent increase in [Ca(2+)]i, mainly due to inward Ca(2+) transport from extracellular environment. Moreover, AA in Ca(2+) -free medium produced a small, transient increase of [Ca(2+)]i, indicating that AA may also trigger Ca(2+) release from intracellular stores. Because the [Ca(2+)]i response to AA was inhibited by the cyclooxygenase (COX) inhibitor indomethacin, we tested the effect of prostaglandins (PGs), products of COX pathway. PGs E1 and E2 caused an increase in [Ca(2+)]i, which, however, was far lower than that obtained with AA. The [Ca(2+)]i response to AA was not inhibited by nifedipine, suggesting that AA did not activate a voltage-dependent Ca(2+) channel. Our results indicate that AA could modulate [Ca(2+)]i in MG63 human osteoblasts, where it may influence Ca(2+) transport across both plasma and endoplasmic membranes. Furthermore, they suggest that osteoblast activity may be modulated by AA.

Diethylstilbestrol increases intracellular calcium in lens epithelial cells

The effects of diethylstilbestrol (DES) on steady-state intracellular calcium concentration ([Ca(2+)](i)) and resting Ca(2+) influx were examined in primary cultures of bovine lens epithelial cells using conventional fluorometric techniques (Fura-2). At low concentrations (10 microM), DES usually induced relatively rapid increases in [Ca(2+)](i) that occurred over an interval of 10-50 s and that persisted for several minutes in the continued presence of the drug. In about 10% of the cells, cyclic oscillations in [Ca(2+)](i) were seen after adding 10 microM DES. At higher concentrations (100 microM), the drug induced more prolonged increases in [Ca(2+)](i) lasting several minutes. DES did not affect Mn(2+) quench determinations of resting Ca(2+) influx, and neither 100 microM GdCl(3), which blocked resting Ca(2+) influx, nor low [Ca(2+)](o) solutions substantially diminished the influence of DES on [Ca(2+)](i). Pretreatment of cells with the smooth endoplasmic reticulum Ca(2+) ATPase (SERCA) inhibitors cyclopiazonic acid (CPA) or thapsigargin completely abolished the effect of 10 microM DES on [Ca(2+)](i), while the IP(3) receptor blocker 2-aminoethoxydiphenyl borane (2-APB) had no effect. These results indicate that DES releases CPA-sensitive stores of intracellular Ca(2+), perhaps by inhibiting SERCA-dependent Ca(2+) sequestration.

Novel effect of N-palmitoyl-L-serine phosphoric acid on cytosolic Ca2+ levels in human osteoblasts

The effect of N-palmitoyl-L-serine phosphoric acid (L-NASPA), which has been used as an inhibitor of lysophosphatidic acid receptors, on intracellular Ca2+ concentration ([Ca2+]i) in human osteosarcoma MG63 cells was measured by using fura-2. L-NASPA (0.1-10 microM) caused a rapid and transient plateau [Ca2+]i rise in a concentration-dependent manner (EC50=0.5 microM). The L-NASPA-induced [Ca2+]i rise was partly reduced by removal of extracellular Ca2+ but was not altered by L-type voltage-gated Ca2+ channel blockers. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, induced a [Ca2+]i rise, after which the increasing effect of L-NASPA on [Ca2+]i was completely inhibited; also, pretreatment with L-NASPA partly reduced thapsigargin-induced [Ca2+]i rise. U73122, an inhibitor of phospholipase C, abolished histamine (but not L-NASPA)-induced [Ca2+]i rise. Overnight incubation with 1 microM L-NASPA did not affect cell proliferation, but 10-20 microM L-NASPA exerted 4% and 15% inhibition, respectively. Collectively, L-NASPA rapidly increased [Ca2+]i in MG63 cells by evoking both extracellular Ca2+ influx and intracellular Ca2+ release, and is cytotoxic at higher concentrations.

Effect of the anti-breast cancer drug tamoxifen on Ca(2+) movement in human osteosarcoma cells

The anti-breast cancer drug tamoxifen has recently been shown to cause an increase in [Ca(2+)]i in renal tubular cells, breast cells and bladder cells. Because tamoxifen is known to interact with oestrogens leading to modulation of bone metabolism, the present study was aimed at exploring whether tamoxifen could alter Ca(2+) signaling in human osteoblast-like MG63 cells. Cytosolic free Ca(2+) levels were recorded by using the Ca(2+)-sensitive dye fura-2. Tamoxifen induced a sustained [Ca(2+)]i increase at concentrations above 1 microM with an EC(50) of 8 microM. Removal of extracellular Ca(2+) reduced the response by 40%, suggesting that tamoxifen induced both Ca(2+) influx and store Ca(2+) release. Tamoxifen-induced Ca(2+) influx was confirmed as tamoxifen caused Mn(2+) influx-induced quench of fura-2 fluorescence. In Ca(2+)-free medium, pretreatment with 10 microM tamoxifen abolished the [Ca(2+)]i increase induced by 1 microM thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor), and by 2 microM carbonylcyanide m-chlorophenylhydrazone (a mitochondrial uncoupler). Conversely, pretreatment with thapsigargin and carbonylcyanide m-chlorophenylhydrazone only reduced 64% of tamoxifen-induced [Ca(2+)]i increases. Addition of 2 microM U73122 to inhibit phospholipase C activity abolished the [Ca(2+)]i increase induced by 1 microM histamine, a phospholipase C-dependent Ca(2+) mobilizer, without affecting 10 microM tamoxifen-induced Ca(2+) release. The [Ca(2+)]i increase induced by 10 microM tamoxifen was not altered by 10 microM of nifedipine, verapamil and diltiazem. Together, the data show that tamoxifen induced a lasting increase in [Ca(2+)]i in human osteoblast-like cells by causing Ca(2+) influx and releasing Ca(2+) from multiple stores in a phospholipase C-independent manner.

Effect of lignans isolated from Hernandia nymphaeifolia on estrogenic compounds-induced calcium mobilization in human neutrophils

The effect of five lignans isolated from Hernandia nymphaeifolia on estrogenic compounds (17beta-estradiol, tamoxifen and clomiphene)-induced Ca(2+) mobilization in human neutrophils was investigated. The five lignans were epi-yangambin, epi-magnolin, epi-aschantin, deoxypodophyllotoxin and yatein. In Ca(2+)-containing medium, the lignans (50-100 microM) inhibited 10 microM 17beta-estradiol- and 5 microM tamoxifen-induced increases in intracellular free Ca(2+) levels ([Ca(2+)](i)) without changing 25 microM clomiphene-induced [Ca(2+)](i) increase. 17beta-estradiol and tamoxifen increased [Ca(2+)](i) by causing Ca(2+) influx and Ca(2+) release because their responses were partly reduced by removing extracellular Ca(2+). In contrast, clomiphene solely induced Ca(2+) release. The effect of the lignans on these two Ca(2+) movement pathways underlying 17beta-estradiol- and tamoxifen-induced [Ca(2+)](i) increases was explored. All the lignans (50-100 microM) inhibited 10 microM 17beta-estradiol-and 5 microM tamoxifen-induced Ca(2+) release, and 17beta-estradiol-induced Ca(2+) influx. However, only 100 microM epi-aschantin was able to reduce tamoxifen-induced Ca(2+) influx while the other lignans had no effect. Collectively, this study shows that the lignans altered estrogenic compounds-induced Ca(2+) signaling in human neutrophils in a multiple manner.