Dual effect of tamoxifen, an anti-breast-cancer drug, on intracellular Ca(2+) and cytotoxicity in intact cells

Anti-Cancer Agents in Proliferation and Cell Death: The Calcium Connection

Calcium (Ca2+) signaling and the modulation of intracellular calcium ([Ca2+]i) levels play critical roles in several key processes that regulate cellular survival, growth, differentiation, metabolism, and death in normal cells. On the other hand, aberrant Ca2+-signaling and loss of [Ca2+]i homeostasis contributes to tumor initiation proliferation, angiogenesis, and other key processes that support tumor progression in several different cancers. Currently, chemically and functionally distinct drugs are used as chemotherapeutic agents in the treatment and management of cancer among which certain anti-cancer drugs reportedly suppress pro-survival signals and activate pro-apoptotic signaling through modulation of Ca2+-signaling-dependent mechanisms. Most importantly, the modulation of [Ca2+]i levels via the endoplasmic reticulum-mitochondrial axis and corresponding action of channels and pumps within the plasma membrane play an important role in the survival and death of cancer cells. The endoplasmic reticulum-mitochondrial axis is of prime importance when considering Ca2+-signaling-dependent anti-cancer drug targets. This review discusses how calcium signaling is targeted by anti-cancer drugs and highlights the role of calcium signaling in epigenetic modification and the Warburg effect in tumorigenesis.

Tamoxifen inhibits the proliferation of non‑melanoma skin cancer cells by increasing intracellular calcium concentration

Tamoxifen is an estrogen receptor (ER) antagonist used as first-line chemotherapy in breast cancer. Recent studies suggest that tamoxifen may be effective not only for ER‑positive but also for ER‑negative cancer cases. The aim of the present study was to investigate the antiproliferative effect of tamoxifen against human non‑melanoma skin cancer cells. Tamoxifen inhibited the proliferation of the skin squamous cell carcinoma (SCC) cell lines A431, DJM‑1 and HSC‑1. A431 cells did not express ER‑α or -β, suggesting that tamoxifen may exert antiproliferative effects on skin SCC cells via a non‑ER‑mediated pathway. Tamoxifen increased the intracellular calcium concentration of skin SCC cells, and this increase in intracellular calcium concentration by calcium ionophore A23187 suppressed the proliferation of skin SCC cells. These data indicate that tamoxifen inhibited the proliferation of human skin SCC cells via increasing intracellular calcium concentration. Voltage-gated calcium channels and non‑selective cation channels are involved in the increase in intracellular calcium concentration induced by tamoxifen. The broad-spectrum protein kinase C (PKC) inhibitor phloretin significantly attenuated the antiproliferative effect of tamoxifen on skin SCC cells. From these data, it may be concluded that tamoxifen inhibits the proliferation of skin SCC cells by induction of extracellular calcium influx via calcium channels in the plasma membrane and by subsequent activation of PKC.

Chemotherapeutic effect of tamoxifen on temozolomide-resistant gliomas

Tamoxifen, a selective estrogen receptor modulator, is widely used in the chemotherapy of estrogen receptor-positive breast cancer. Recent studies have indicated that tamoxifen might have a potential chemotherapeutic effect on glioma. In the present study, we determined the chemotherapeutic action of tamoxifen on human glioma cell lines. Methylation of 06-methylguanine-DNA methyltransferase was identified in A172, U251, and BT325 glioma cell lines, but not in the U87 cell line. Consistently, A172, U251, and BT325 cell lines are resistant to temozolomide. Tamoxifen induced significant cytotoxic action in A172, U251, BT325, and U87 cell lines. Further, Hoechst 33342 staining and apoptosis flow cytometric analysis demonstrated that tamoxifen induced apoptosis in the BT325 cell line. Mitochondrial complex analysis indicated that tamoxifen, but not other estrogen receptor modulators, dose-dependently inhibits complex I activity. In summary, our study suggests that tamoxifen might have a chemotherapeutic effect on temozolomide-resistant glioma through its direct action on mitochondrial complex I inhibition and could provide further evidence to support future clinical trials of tamoxifen for the treatment of glioblastoma.

In silico elucidation of the molecular mechanism defining the adverse effect of selective estrogen receptor modulators. PLoS Comput Biol. 2007;3:e217. [PMID: 18052534 PMCID: PMC2098847 DOI: 10.1371/journal.pcbi.0030217]

Early identification of adverse effect of preclinical and commercial drugs is crucial in developing highly efficient therapeutics, since unexpected adverse drug effects account for one-third of all drug failures in drug development. To correlate protein–drug interactions at the molecule level with their clinical outcomes at the organism level, we have developed an integrated approach to studying protein–ligand interactions on a structural proteome-wide scale by combining protein functional site similarity search, small molecule screening, and protein–ligand binding affinity profile analysis. By applying this methodology, we have elucidated a possible molecular mechanism for the previously observed, but molecularly uncharacterized, side effect of selective estrogen receptor modulators (SERMs). The side effect involves the inhibition of the Sacroplasmic Reticulum Ca2+ ion channel ATPase protein (SERCA) transmembrane domain. The prediction provides molecular insight into reducing the adverse effect of SERMs and is supported by clinical and in vitro observations. The strategy used in this case study is being applied to discover off-targets for other commercially available pharmaceuticals. The process can be included in a drug discovery pipeline in an effort to optimize drug leads and reduce unwanted side effects.

Early identification of the side effects of preclinical and commercial drugs is crucial in developing highly efficient therapeutics, as unexpected side effects account for one-third of all drug failures in drug development and lead to drugs being withdrawn from the market. Compared with the experimental identification of off-target proteins that cause side effects, computational approaches not only save time and costs by providing a candidate list of potential off-targets, but also provide insight into understanding the molecular mechanisms of protein–drug interactions. In this paper we describe an integrated approach to identifying similar drug binding pockets across protein families that have different global shapes. In a case study, we elucidate a possible molecular mechanism for the observed side effects of selective estrogen receptor modulators (SERMs), which are widely used to treat and prevent breast cancer and other diseases. The prediction provides molecular insight into reducing the side effects of SERMs and is supported by clinical and biochemical observations. The strategy used in this case study is being applied to discover off-targets for other commercially available pharmaceuticals and to repurpose existing safe pharmaceuticals to treat different diseases. The process can be included in a drug discovery pipeline in an effort to optimize drug leads, reduce unwanted side effects, and accelerate development of new drugs.

Exploring the Mode-of-Action of Bioactive Compounds by Chemical-Genetic Profiling in Yeast

Discovering target and off-target effects of specific compounds is critical to drug discovery and development. We generated a compendium of "chemical-genetic interaction" profiles by testing the collection of viable yeast haploid deletion mutants for hypersensitivity to 82 compounds and natural product extracts. To cluster compounds with a similar mode-of-action and to reveal insights into the cellular pathways and proteins affected, we applied both a hierarchical clustering and a factorgram method, which allows a gene or compound to be associated with more than one group. In particular, tamoxifen, a breast cancer therapeutic, was found to disrupt calcium homeostasis and phosphatidylserine (PS) was recognized as a target for papuamide B, a cytotoxic lipopeptide with anti-HIV activity. Further, the profile of crude extracts resembled that of its constituent purified natural product, enabling detailed classification of extract activity prior to purification. This compendium should serve as a valuable key for interpreting cellular effects of novel compounds with similar activities.

Effect of Y-24180 on Ca2+ movement and proliferation in renal tubular cells

In canine renal tubular cells, the effect of Y-24180, a presumed specific platelet activating factor (PAF) receptor antagonist, on intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured by using fura-2 as a Ca(2+)-sensitive fluorescent probe. Y-24180 (0.1-10 microM) caused a rapid and sustained [Ca(2+)](i) rise in a concentration-dependent manner. The [Ca(2+)](i) rise was prevented by 30% by removal of extracellular Ca(2+), but was not changed by dihydropyridines, verapamil and diltiazem. Y-24180-induced Ca(2+) influx was confirmed by Mn(2+)-influx induced quench of fura-2 fluorescence. In Ca(2+)-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a monophasic [Ca(2+)](i) rise, after which the increasing effect of 5 microM Y-24180 on [Ca(2+)](i) was abolished; conversely, depletion of Ca(2+) stores with 5 microM Y-24180 abolished thapsigargin-induced [Ca(2+)](i) rise. U73122, an inhibitor of phoispholipase C, inhibited ATP-, but not Y-24180-induced [Ca(2+)](i) rise. Overnight treatment with Y-24180 did not alter cell proliferation rate. Collectively, these results suggest that Y-24180 acts as a potent, but not cytotoxic, Ca(2+) mobilizer in renal tubular cells, by stimulating both extracellular Ca(2+) influx and intracellular Ca(2+) release. Since alterations in Ca(2+) movement may interfere many cellular signaling processes unrelated to modulation of PAF receptors, caution must be applied in using this chemical as a selective PAF receptor antagonist.

The anti-breast cancer drug tamoxifen alters Ca2+ movement in Chinese hamster ovary (CHO-K1) cells

The anti-breast cancer drug tamoxifen has recently been shown to cause an increase in intracellular free-Ca(2+) concentrations ([Ca(2+)](i)) in renal tubular cells, breast cells and bladder cells. Because tamoxifen is known to alter ovary function in human patients and in rats, the present study was aimed at exploring whether tamoxifen could alter Ca(2+) movement in Chinese hamster ovary (CHO-K1) cells. Cytosolic free-Ca(2+) levels in populations of cells have been explored by using fura-2 as a fluorescent Ca(2+) indicator. Tamoxifen at concentrations above 1 micro M increased [Ca(2+)](i) in a concentration-dependent manner with an EC(50) value of 8 micro M. The Ca(2+) signal was reduced by removing extracellular Ca(2+), but was not affected by nifedipine, verapamil, diltiazem or ICI 182,780 (an estrogen receptor antagonist). Pretreatment with 1 micro M thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor) to deplete the endoplasmic reticulum Ca(2+) abolished 10 micro M tamoxifen-induced Ca(2+) release. Neither inhibition of phospholipase C with 2 micro M U73122 nor depletion of ryanodine-sensitive Ca(2+) stores with 50 micro M ryanodine affected tamoxifen-induced Ca(2+) release. Cell proliferation assays using ELISA revealed that overnight incubation with 5-10 micro M tamoxifen inhibited cell proliferation by 20%, and 20 micro M tamoxifen killed all cells. Together, the results suggest that, in CHO-K1 cells, tamoxifen induced a [Ca(2+)](i) increase by causing store-Ca(2+) release from the endoplasmic reticulum in an phospholipase C-independent manner, and by inducing Ca(2+) influx. The action of tamoxifen appears to be dissociated from estrogen receptor activation. Longer incubation with tamoxifen (>5 micro M) was cytotoxic.

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.

Tamoxifen-induced increases in cytoplasmic free Ca2+ levels in human breast cancer cells

Tamoxifen has been shown to increase cytoplasmic free Ca2+ levels [Ca2+]i in renal tubular cells and bladder cancer cells, and to after Ca2+ signaling in MCF-7 breast cancer cells. The present study examined the effect of tamoxifen on [Ca2+], in ZR-75-1 human breast cancer cells using fura-2 as an indicator. Tamoxifen increased [Ca2+]i at a concentration above 2 microM with an EC50 of 5 microM. Removing extracellular Ca2+ reduced the response by 48+/-2%. In Ca2+-free medium, after tamoxifen-induced [Ca2+]i increased had returned to baseline, adding 3 mM Ca2+ increased [Ca2+]i in a concentration-dependent manner. Further, pretreatment with 10 microM tamoxifen abolished the [Ca2+]i increase induced by 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor); and conversely, pretreatment with thapsigargin prevented tamoxifen from releasing more Ca2+. Tamoxifen (10 microM)-induced Ca2+ release was not changed by inhibiting phospholipase C activity with 2 microM U73122. Trypan blue exclusion assay revealed that tamoxifen (1-10 microM) did not alter viability after 1 min of incubation, but killed 10% of cells after 3-10 min of incubation. Together, this study shows that tamoxifen (>2 microM) induced a significant, immediate increase in [Ca2+]i in ZR-75-1 breast cancer cells. Tamoxifen acted by releasing Ca2+ from the endoplasmic reticulum Ca2+ stores in a manner independent of phospholipase C activity, and by inducing Ca2+ entry from extracellular medium. Tamoxifen may be of mild cytotoxicity after acute exposure.