Retinoic acid stimulation of the sodium/iodide symporter in MCF-7 breast cancer cells is mediated by the insulin growth factor-I/phosphatidylinositol 3-kinase and p38 mitogen-activated protein kinase signaling pathways

Going Nuclear with Amino Acids and Proteins - Basic Biochemistry and Molecular Biology Primer for the Technologist

In recent years, there has been an influx of new tracers into the field of nuclear medicine and molecular imaging. Most of these tracers that have been FDA approved for clinical imaging exploit various mechanisms of protein biochemistry and molecular biology to bring about their actions, such as amino acid metabolism, protein folding, receptor-ligand interactions, and surface transport mechanisms. In this review, we attempt to paint a clear picture of the basic biochemistry and molecular biology of protein structure, translation, transcription, post-translational modifications, and protein targeting, in the context of the various radiopharmaceuticals currently used clinically, all in an easy-to-understand language for entry level technologists in the field. Tracer characteristics, including indications, dosage, injection-to-imaging time, and the logic behind the normal and pathophysiologic biodistribution of these newer molecular tracers, are also discussed.

Analysis of NIS Plasma Membrane Interactors Discloses Key Regulation by a SRC/RAC1/PAK1/PIP5K/EZRIN Pathway with Potential Implications for Radioiodine Re-Sensitization Therapy in Thyroid Cancer

The functional expression of the sodium-iodide symporter (NIS) at the membrane of differentiated thyroid cancer (DTC) cells is the cornerstone for the use of radioiodine (RAI) therapy in these malignancies. However, NIS gene expression is frequently downregulated in malignant thyroid tissue, and 30% to 50% of metastatic DTCs become refractory to RAI treatment, which dramatically decreases patient survival. Several strategies have been attempted to increase the NIS mRNA levels in refractory DTC cells, so as to re-sensitize refractory tumors to RAI. However, there are many RAI-refractory DTCs in which the NIS mRNA and protein levels are relatively abundant but only reduced levels of iodide uptake are detected, suggesting a posttranslational failure in the delivery of NIS to the plasma membrane (PM), or an impaired residency at the PM. Because little is known about the molecules and pathways regulating NIS delivery to, and residency at, the PM of thyroid cells, we here employed an intact-cell labeling/immunoprecipitation methodology to selectively purify NIS-containing macromolecular complexes from the PM. Using mass spectrometry, we characterized and compared the composition of NIS PM complexes to that of NIS complexes isolated from whole cell (WC) lysates. Applying gene ontology analysis to the obtained MS data, we found that while both the PM-NIS and WC-NIS datasets had in common a considerable number of proteins involved in vesicle transport and protein trafficking, the NIS PM complexes were particularly enriched in proteins associated with the regulation of the actin cytoskeleton. Through a systematic validation of the detected interactions by co-immunoprecipitation and Western blot, followed by the biochemical and functional characterization of the contribution of each interactor to NIS PM residency and iodide uptake, we were able to identify a pathway by which the PM localization and function of NIS depends on its binding to SRC kinase, which leads to the recruitment and activation of the small GTPase RAC1. RAC1 signals through PAK1 and PIP5K to promote ARP2/3-mediated actin polymerization, and the recruitment and binding of the actin anchoring protein EZRIN to NIS, promoting its residency and function at the PM of normal and TC cells. Besides providing novel insights into the regulation of NIS localization and function at the PM of TC cells, our results open new venues for therapeutic intervention in TC, namely the possibility of modulating abnormal SRC signaling in refractory TC from a proliferative/invasive effect to the re-sensitization of these tumors to RAI therapy by inducing NIS retention at the PM.

RA and ω-3 PUFA co-treatment activates autophagy in cancer cells

Retinoic acid (RA), is a promising therapeutic agent for the treatment of breast cancer. However, metabolic disorders and drug resistance reduce the efficacy of RA. In this study, we found that RA and ω-3 polyunsaturated fatty acids (ω-3 PUFAs) synergistically induced cell death in vitro and in vivo and autophagy activation. Moreover, RA-induced hypercholesterolemia was completely corrected by ω-3 PUFA supplementation. In addition, we demonstrated that the effects of this combination on the autophagic flux were independent of the two major canonic regulatory complexes controlling autophagic vesicle formation. The treatment activated Gαq-p38 MAPK signaling pathways, which resulted in autophagy of breast cancer cells. Knockdown of Gαq or P38 expression prevented RA and ω-3 PUFAs from inducing autophagy. Data indicated that Gαq-p38activation was mediated by the co-activation of GPR40 and RARα in lipid rafts, rather than by the activation of GPR120, RARβ, or RARγ. The results of this study suggest that hyperlipidemic drug side effects may be ameliorated by the administration of ω-3 PUFAs. Thus, the therapeutic indexes of the corresponding drugs may be increased.

RARβ2-dependent signaling represses neuronal differentiation in mouse ES cells

Embryonic Stem (ES) cells are pluripotent cells that can be induced to differentiate into cells of all three lineages: mesoderm, endoderm, and ectoderm. In culture, ES cells can be differentiated into mature neurons by treatment with Retinoic Acid (RA) and this effect is mediated mainly through the activation of the RA nuclear receptors (RAR α, β, and γ), and their isoforms. However, little is known about the role played by specific RAR types on ES cell differentiation. Here, we found that treatment of ES cells with AC55649, an RARβ2 agonist, increased endodermal marker gene expression. On the other hand, we found that the inhibition of RARβ with 5μM LE135, together with RA treatment, increased the efficiency of mouse ES cell differentiation into neurons by more than 4-fold as compared to cells treated with RA only. Finally, we performed proteomic analyses on ES cells treated with RA vs RA plus AC55649 in order to identify the signaling pathways activated by the RARβ agonist. Our proteomic analyses using antibody microarrays indicated that proteins such as p38 and AKT were upregulated in cells treated with RA plus the agonist, as compared to cells treated with RA alone. Our results indicate that RARβ may function as a repressor of neuronal differentiation through the activation of major cell signaling pathways, and that the pharmacological inhibition of this nuclear receptor may constitute a novel method to increase the efficiency of ES to neuronal differentiation in culture.

Resveratrol has anti-thyroid effects both in vitro and in vivo

Resveratrol is a natural polyphenol with antioxidant, anti-inflammatory, and antiproliferative properties. We have shown previously that resveratrol decreases sodium/iodide symporter expression and iodide uptake in thyrocytes, both in vitro and in vivo. In the present study, we further investigated the effects of resveratrol, with evaluation of the expression of additional thyroid-specific genes in the FRTL-5 rat thyroid cell line: thyroglobulin, thyroid peroxidase, TSH receptor, Nkx2-1, Foxe1 and Pax8. We observed decreased expression of these genes in FRTL-5 cells treated with 10 μM resveratrol. The effects of resveratrol was further evaluated in vivo using Sprague-Dawley rats treated with resveratrol 25 mg/kg body weight intraperitoneally, for 60 days. No clinical signs of hypothyroidism were seen, although the treated rats showed significant increase in thyroid size. Serum TSH and thyroid hormone levels were in the normal range, with significantly higher TSH seen in resveratrol-treated rats, compared with control rats. Histological and immunohistochemical analyses confirmed increased proliferative activity in the thyroid from resveratrol-treated rats. These data suggest that resveratrol acts as a thyroid disruptor and a goitrogen, which indicates the need for caution as a supplement and for therapeutic uses.

Thyrotropin and Insulin-Like Growth Factor 1 Receptor Crosstalk Upregulates Sodium-Iodide Symporter Expression in Primary Cultures of Human Thyrocytes

BACKGROUND

Major regulation of thyroid gland function is mediated by thyrotropin (TSH) activating the TSH receptor (TSHR) and inducing upregulation of genes involved in thyroid hormone synthesis. Evidence suggests that the insulin-like growth factor 1 (IGF-1) receptor (IGF-1R) may play a role in regulating TSHR functional effects. This study examined the potential role of TSHR/IGF-1R crosstalk in primary cultures of human thyrocytes.

RESULTS

TSH/IGF-1 co-treatment elicited additive effects on thyroglobulin (TG), thyroperoxidase (TPO), and deiodinase type 2 (DIO2) mRNA levels but synergistic effects on sodium-iodide symporter (NIS) mRNA. Similar cooperativity was seen on the level of TG protein secretion (additive) and NIS protein expression (synergistic). The IGF-1R tyrosine kinase inhibitor linsitinib inhibited TSH-stimulated upregulation of NIS but not TG, indicating that NIS regulation is in part IGF-1R dependent and occurs via receptor crosstalk. Cooperativity was not seen at the level of cAMP/protein kinase A (PKA) signaling, IGF-1R phosphorylation, or Akt activation. However, TSH and IGF-1 synergistically activated ERK1/2. Pharmacological inhibition of ERK1/2 by the MEK1/2 inhibitor U0126 and of Akt by MK-2206 virtually abolished NIS stimulation by TSH and the synergistic effect of IGF-1.

CONCLUSION

As linsitinib inhibited upregulation of NIS stimulated by TSH alone, it is concluded that crosstalk between TSHR and IGF-1R, without agonist activation of IGF-1R, plays a role in NIS regulation in human thyrocytes via a mechanism involving ERK1/2 and/or Akt. Fully understanding the nature of this crosstalk has clinical implications for the treatment of thyroid diseases, including thyroid cancer.

miRNA-106a directly targeting RARB associates with the expression of Na(+)/I(-) symporter in thyroid cancer by regulating MAPK signaling pathway

BACKGROUND

Serum miRNAs profiles between papillary thyroid carcinoma (PTC) patients with non-(131)I and (131)I-avid lung metastases are differentially expressed. These miRNAs have to be further validated and the role of these miRNAs in the molecular function level of thyroid cancer cell lines has not been investigated.

METHODS

Expression levels of six identified miRNAs were assessed via quantitative real-time PCR (qRT-PCR) in the serum of eligible patients. Dual-luciferase reporter assay was used to determine the potential target of miR-106a. Cell viability and apoptosis were evaluated by MTT assay and flow cytometry analysis, respectively. The change of gene expression was detected by qRT-PCR and western blotting analysis. In vitro iodine uptake assay was conducted by a γ-counter.

RESULTS

Compared to PTC patients with (131)I-avid lung metastases, miR-106a was up-regulated in the serum of patients with non-(131)I-avid lung metastases. The results of dual-luciferase reporter assay demonstrated that miR-106a directly targeted retinoic acid receptor beta (RARB) 3'-UTR. miR-106a-RARB promoted viability of thyroid cancer cells by regulating MEKK2-ERK1/2 and MEKK2-ERK5 pathway. miR-106a-RARB inhibited apoptosis of thyroid cancer cells by regulating ASK1-p38 pathway. Moreover, miR-106a-RARB could regulate the expression of sodium iodide symporter, TSH receptor and alter the iodine uptake function of thyroid cancer cells.

CONCLUSIONS

miRNA-106a, directly targeting RARB, associates with the viability, apoptosis, differentiation and the iodine uptake function of thyroid cancer cell lines by regulating MAPK signaling pathway in vitro. These findings in the present study may provide new strategies for the diagnosis and treatment in radioiodine-refractory differentiated thyroid carcinoma.

Iodide transport and breast cancer

Breast cancer is the second most common cancer worldwide and the leading cause of cancer death in women, with incidence rates that continue to rise. The heterogeneity of the disease makes breast cancer exceptionally difficult to treat, particularly for those patients with triple-negative disease. To address the therapeutic complexity of these tumours, new strategies for diagnosis and treatment are urgently required. The ability of lactating and malignant breast cells to uptake and transport iodide has led to the hypothesis that radioiodide therapy could be a potentially viable treatment for many breast cancer patients. Understanding how iodide is transported, and the factors regulating the expression and function of the proteins responsible for iodide transport, is critical for translating this hypothesis into reality. This review covers the three known iodide transporters - the sodium iodide symporter, pendrin and the sodium-coupled monocarboxylate transporter - and their role in iodide transport in breast cells, along with efforts to manipulate them to increase the potential for radioiodide therapy as a treatment for breast cancer.

Apigenin in combination with Akt inhibition significantly enhances thyrotropin-stimulated radioiodide accumulation in thyroid cells

BACKGROUND

Selectively increased radioiodine accumulation in thyroid cells by thyrotropin (TSH) allows targeted treatment of thyroid cancer. However, the extent of TSH-stimulated radioiodine accumulation in some thyroid tumors is not sufficient to confer therapeutic efficacy. Hence, it is of clinical importance to identify novel strategies to selectively further enhance TSH-stimulated thyroidal radioiodine accumulation.

METHODS

PCCl3 rat thyroid cells, PCCl3 cells overexpressing BRAF(V600E), or primary cultured tumor cells from a thyroid cancer mouse model, under TSH stimulation were treated with various reagents for 24 hours. Cells were then subjected to radioactive iodide uptake, kinetics, efflux assays, and protein extraction followed by Western blotting against selected antibodies.

RESULTS

We previously reported that Akt inhibition increased radioiodine accumulation in thyroid cells under chronic TSH stimulation. Here, we identified Apigenin, a plant-derived flavonoid, as a reagent to further enhance the iodide influx rate increased by Akt inhibition in thyroid cells under acute TSH stimulation. Akt inhibition is permissive for Apigenin's action, as Apigenin alone had little effect. This action of Apigenin requires p38 MAPK activity but not PKC-δ. The increase in radioiodide accumulation by Apigenin with Akt inhibition was also observed in thyroid cells expressing BRAF(V600E) and in primary cultured thyroid tumor cells from TRβ(PV/PV) mice.

CONCLUSION

Taken together, Apigenin may serve as a dietary supplement in combination with Akt inhibitors to enhance therapeutic efficacy of radioiodine for thyroid cancer.

The sodium iodide symporter (NIS): regulation and approaches to targeting for cancer therapeutics

Expression of the sodium iodide symporter (NIS) is required for efficient iodide uptake in thyroid and lactating breast. Since most differentiated thyroid cancer expresses NIS, β-emitting radioactive iodide is routinely utilized to target remnant thyroid cancer and metastasis after total thyroidectomy. Stimulation of NIS expression by high levels of thyroid-stimulating hormone is necessary to achieve radioiodide uptake into thyroid cancer that is sufficient for therapy. The majority of breast cancer also expresses NIS, but at a low level insufficient for radioiodine therapy. Retinoic acid is a potent NIS inducer in some breast cancer cells. NIS is also modestly expressed in some non-thyroidal tissues, including salivary glands, lacrimal glands and stomach. Selective induction of iodide uptake is required to target tumors with radioiodide. Iodide uptake in mammalian cells is dependent on the level of NIS gene expression, but also successful translocation of NIS to the cell membrane and correct insertion. The regulatory mechanisms of NIS expression and membrane insertion are regulated by signal transduction pathways that differ by tissue. Differential regulation of NIS confers selective induction of functional NIS in thyroid cancer cells, as well as some breast cancer cells, leading to more efficient radioiodide therapy for thyroid cancer and a new strategy for breast cancer therapy. The potential for systemic radioiodide treatment of a range of other cancers, that do not express endogenous NIS, has been demonstrated in models with tumor-selective introduction of exogenous NIS.

Modulation of sodium iodide symporter expression and function by LY294002, Akti-1/2 and Rapamycin in thyroid cells

The selective increase of Na(+)/I(-) symporter (NIS)-mediated active iodide uptake in thyroid cells allows the use of radioiodine I(131) for diagnosis and targeted treatment of thyroid cancers. However, NIS-mediated radioiodine accumulation is often reduced in thyroid cancers due to decreased NIS expression/function. As PI3K signaling is overactivated in many thyroid tumors, we investigated the effects of inhibitors for PI3K, Akt, or mTORC1 as well as their interplay on NIS modulation in thyroid cells under chronic TSH stimulation. PI3K inhibition by LY294002 increased NIS-mediated radioiodide uptake (RAIU) mainly through upregulation of NIS expression, however, mTORC1 inhibition by Rapamycin did not increase NIS-mediated RAIU despite increased NIS protein levels. In comparison, Akt inhibition by Akti-1/2 did not increase NIS protein levels, yet markedly increased NIS-mediated RAIU by decreasing iodide efflux rate and increasing iodide transport rate and iodide affinity of NIS. The effects of Akti-1/2 on NIS-mediated RAIU are not detected in nonthyroid cells, implying that Akti-1/2 or its derivatives may represent potential pharmacological reagents to selectively increase thyroidal radioiodine accumulation and therapeutic efficacy.

Autophagy inhibition synergistically enhances anticancer efficacy of RAMBA, VN/12-1 in SKBR-3 cells, and tumor xenografts

VN/12-1 is a novel retinoic acid metabolism blocking agent discovered in our laboratory. The purpose of the study was to elucidate the molecular mechanism of anticancer activity of VN/12-1 in breast cancer cell lines and in tumor xenografts. We investigated the effects of VN/12-1 on induction of autophagy and apoptosis in SKBR-3 cells. Furthermore, we also examined the impact of pharmacologic and genomic inhibition of autophagy on anticancer activity of VN/12-1. Finally, the antitumor activity of VN/12-1 was evaluated as a single agent and in combination with autophagy inhibitor chloroquine in an SKBR-3 mouse xenograft model. Short exposure of low dose (<10 μmol/L) of VN/12-1 induced endoplasmic reticulum stress, autophagy, and inhibited G(1)-S phase transition and caused a protective response. However, a higher dose of VN/12-1 initiated apoptosis in vitro. Inhibition of autophagy using either pharmacologic inhibitors or RNA interference of Beclin-1 enhanced anticancer activity induced by VN/12-1 in SKBR-3 cells by triggering apoptosis. Importantly, VN/12-1 (5 mg/kg twice weekly) and the combination of VN/12-1 (5 mg/kg twice weekly) + chloroquine (50 mg/kg twice weekly) significantly suppressed established SKBR-3 tumor growth by 81.4% (P < 0.001 vs. control) and 96.2% (P < 0.001 vs. control), respectively. Our novel findings suggest that VN/12-1 may be useful as a single agent or in combination with autophagy inhibitors for treating human breast cancers. Our data provides a strong rationale for clinical evaluation of VN/12-1 as single agent or in combination with autophagy inhibitors.

Regulation of sodium iodide symporter gene expression by Rac1/p38β mitogen-activated protein kinase signaling pathway in MCF-7 breast cancer cells

Activation of p38 MAPK is a key pathway for cell proliferation and differentiation in breast cancer and thyroid cells. The sodium/iodide symporter (NIS) concentrates iodide in the thyroid and lactating breast. All-trans-retinoic acid (tRA) markedly induces NIS activity in some breast cancer cell lines and promotes uptake of β-emitting radioiodide (131)I sufficient for targeted cytotoxicity. To identify a signal transduction pathway that selectively stimulates NIS expression, we investigated regulation by the Rac1-p38 signaling pathway in MCF-7 breast cancer cells and compared it with regulation in FRTL-5 rat thyroid cells. Loss of function experiments with pharmacologic inhibitors and small interfering RNA, as well as RT-PCR analysis of p38 isoforms, demonstrated the requirement of Rac1, MAPK kinase 3B, and p38β for the full expression of NIS in MCF-7 cells. In contrast, p38α was critical for NIS expression in FRTL-5 cells. Treatment with tRA or overexpression of Rac1 induced the phosphorylation of p38 isoforms, including p38β. A dominant negative mutant of Rac1 abolished tRA-induced phosphorylation in MCF-7 cells. Overexpression of p38β or Rac1 significantly enhanced (1.9- and 3.9-fold, respectively), the tRA-stimulated NIS expression in MCF-7 cells. This study demonstrates differential regulation of NIS by distinct p38 isoforms in breast cancer cells and thyroid cells. Targeting isoform-selective activation of p38 may enhance NIS induction, resulting in higher efficacy of (131)I concentration and treatment of breast cancer.

Reduction of stimulated sodium iodide symporter expression by estrogen receptor ligands in breast cancer cells

PURPOSE

The sodium iodide symporter (NIS) mediates active iodide uptake in lactating breast tissue, and when its levels are enhanced by all-trans retinoic acid (atRA), NIS has been proposed as a target for the imaging and radiotherapy of breast cancer. Importantly, the estrogen receptor α (ERα) is an important regulator of atRA induced NIS gene expression in breast cancer cells. In this study, we investigated the effect of an ER agonist (17β-estradiol, E(2)) or antagonist [trans-hydroxytamoxifen (TOT) or raloxifene (RAL)] treatment on the regulation of NIS gene expression and iodide uptake in an ERα-positive breast cancer (MCF-7) model.

METHODS

NIS functional activity was measured in vitro by (125)I uptake assay after incubation with E(2) (from 10(-15) to 10(-5) M), TOT (from 5×10(-8) to 5×10(-6) M), or RAL (from 5×10(-8) to 5×10(-6) M) in the presence or absence of atRA (10(-7) M). Under the same conditions, NIS mRNA expression was examined by reverse transcriptase polymerase chain reaction. Athymic mice with MCF-7 xenograft tumors were treated with atRA alone or atRA together with E(2) to evaluate the change of (125)I uptake in tumor tissues in vivo.

RESULTS

In the iodide uptake study in cells, E(2), TOT, or RAL treatment alone did not stimulate (125)I uptake. However, when iodide uptake was stimulated by atRA, cotreatment with E(2), TOT or RAL decreased (125)I uptake in a concentration-dependent manner. The hormone effects on NIS mRNA expression levels in MCF-7 cells were similar. The results of the in vivo biodistribution study showed that (125)I uptake was reduced 50% in tumor tissues of mice treated with atRA/E(2) as compared to tumors treated only with atRA.

CONCLUSION

Our results suggest that combination treatment of atRA and ER ligands could limit the functional activity of the NIS gene induced by atRA, thereby compromising its use as a target for diagnosis or radiotherapy in breast cancer.

KT5823 differentially modulates sodium iodide symporter expression, activity, and glycosylation between thyroid and breast cancer cells

Na(+)/I(-) symporter (NIS)-mediated iodide uptake into thyroid follicular cells serves as the basis of radioiodine therapy for thyroid cancer. NIS protein is also expressed in the majority of breast tumors, raising potential for radionuclide therapy of breast cancer. KT5823, a staurosporine-related protein kinase inhibitor, has been shown to increase thyroid-stimulating hormone-induced NIS expression, and thus iodide uptake, in thyroid cells. In this study, we found that KT5823 does not increase but decreases iodide uptake within 0.5 h of treatment in trans-retinoic acid and hydrocortisone-treated MCF-7 breast cancer cells. Moreover, KT5823 accumulates hypoglycosylated NIS, and this effect is much more evident in breast cancer cells than thyroid cells. The hypoglycosylated NIS is core glycosylated, has not been processed through the Golgi apparatus, but is capable of trafficking to the cell surface. KT5823 impedes complex NIS glycosylation at a regulatory point similar to brefeldin A along the N-linked glycosylation pathway, rather than targeting a specific N-glycosylated site of NIS. KT5823-mediated effects on NIS activity and glycosylation are also observed in other breast cancer cells as well as human embryonic kidney cells expressing exogenous NIS. Taken together, KT5823 will serve as a valuable pharmacological reagent to uncover mechanisms underlying differential NIS regulation between thyroid and breast cancer cells at multiple levels.

Retinoic acid induces expression of the thyroid hormone transporter, monocarboxylate transporter 8 (Mct8)

Retinoic acid (RA) and thyroid hormone are critical for differentiation and organogenesis in the embryo. Mct8 (monocarboxylate transporter 8), expressed predominantly in the brain and placenta, mediates thyroid hormone uptake from the circulation and is required for normal neural development. RA induces differentiation of F9 mouse teratocarcinoma cells toward neurons as well as extraembryonal endoderm. We hypothesized that Mct8 is functionally expressed in F9 cells and induced by RA. All-trans-RA (tRA) and other RA receptor (RAR) agonists dramatically (>300-fold) induced Mct8. tRA treatment significantly increased uptake of triiodothyronine and thyroxine (4.1- and 4.3-fold, respectively), which was abolished by a selective Mct8 inhibitor, bromosulfophthalein. Sequence inspection of the Mct8 promoter region and 5'-rapid amplification of cDNA ends PCR analysis in F9 cells identified 11 transcription start sites and a proximal Sp1 site but no TATA box. tRA significantly enhanced Mct8 promoter activity through a consensus RA-responsive element located 6.6 kilobases upstream of the coding region. A chromatin immunoprecipitation assay demonstrated binding of RAR and retinoid X receptor to the RA response element. The promotion of thyroid hormone uptake through the transcriptional up-regulation of Mct8 by RAR is likely to be important for extraembryonic endoderm development and neural differentiation. This finding demonstrates cross-talk between RA signaling and thyroid hormone signaling in early development at the level of the thyroid hormone transporter.

Stimulation of retinoic acid-induced functional sodium iodide symporter (NIS) expression and cytotoxicity of ¹³¹I by carbamazepine in breast cancer cells

The sodium iodide symporter (NIS) mediates the active iodide uptake in the thyroid gland as well as lactating breast tissue. Recently, we reported significant stimulation of all-trans retinoic acid (atRA)-induced NIS expression in the estrogen-receptor positive human breast cancer cell line MCF-7 by dexamethasone (Dex) in vitro and in vivo, which might offer the potential to image and treat breast cancer with radioiodine. In this study, based on its known interaction with the pregnane-X-receptor (PXR) forming a heterodimer with the retinoid-X-receptor (RXR), we examined the effect of carbamazepine (CBZ), a potent activator of PXR, on atRA-induced NIS expression and therapeutic efficacy of (131)I in MCF-7 cells. For this purpose, functional NIS expression in MCF-7 cells was examined by iodide uptake assay, quantitative real-time PCR as well as Western blot analysis, followed by investigation of (131)I cytotoxicity in vitro after incubation with CBZ (4, 25, 100 μM) in the presence of atRA (1 μM) with or without Dex (100 nM). Incubation with CBZ stimulated atRA-induced iodide accumulation up to twofold in a concentration-dependent manner, while atRA/Dex-stimulated iodide uptake was further stimulated up to 1.5-fold by additional CBZ treatment based on significantly increased NIS mRNA and protein levels. This stimulatory effect of CBZ was shown to be dependent on the PI3K-Akt pathway without involvement of mTOR. In contrast, treatment with CBZ alone had no effect on functional NIS expression. Moreover, selective cytotoxicity of (131)I was significantly increased from approximately 20% in MCF-7 cells treated with atRA alone to 50% after treatment with CBZ in the presence of atRA, which was further enhanced to 90% after combined treatment with atRA/Dex/CBZ. In conclusion, CBZ represents another potent stimulator of atRA-induced functional NIS expression resulting in an enhanced selective killing effect of (131)I in MCF-7 breast cancer cells.

The pan-DAC inhibitor LBH589 is a multi-functional agent in breast cancer cells: cytotoxic drug and inducer of sodium-iodide symporter (NIS)

New drugs with anti-tumor activity, also able to modify the expression of selected molecules, are under evaluation in breast cancer which is becoming resistant to conventional treatment, or in metastatic disease. The sodium-iodide symporter (NIS), which mediates iodide uptake into thyroid cells, and is the molecular basis of radioiodine imaging and therapy in thyroid cancer, is also expressed in a large portion of breast tumors. Since NIS expression in breast cancer is not sufficient for a significant iodide uptake, drugs able to induce its expression and correct function are under evaluation. In the present study, we report for the first time that the pan-deacetylase (DAC) inhibitor LBH589 (panobinostat) significantly induced NIS, both as mRNA and as protein, through the increase of NIS promoter activity, with the final consequence of obtaining a significant up-take of iodide in MCF7, T47D, and MDA-MB231 breast cancer cells. Moreover, we observed that LBH589 causes a significant reduction in cell viability of estrogen-sensitive and -insensitive breast cancer cells within nanomolar range. The anti-tumor effect of LBH589 is sustained by apoptosis induction and cell cycle arrest in G(2)/M. In conclusion, our data suggest that LBH589 might be a powerful tool in the management of breast cancer due to its multiple effects and support a potential application of LBH589 in the diagnosis and treatment of this disease.

Intronic elements in the Na+/I- symporter gene (NIS) interact with retinoic acid receptors and mediate initiation of transcription

Activity of the sodium/iodide symporter (NIS) in lactating breast is essential for iodide (I(-)) accumulation in milk. Significant NIS upregulation was also reported in breast cancer, indicating a potential use of radioiodide treatment. All-trans-retinoic acid (tRA) is a potent ligand that enhances NIS expression in a subset of breast cancer cell lines and in experimental breast cancer models. Indirect tRA stimulation of NIS in breast cancer cells is very well documented; however, direct upregulation by tRA-activated nuclear receptors has not been identified yet. Aiming to uncover cis-acting elements directly regulating NIS expression, we screened evolutionary-conserved non-coding genomic sequences for responsiveness to tRA in MCF-7. Here, we report that a potent enhancer in the first intron of NIS mediates direct regulation by tRA-stimulated nuclear receptors. In vitro as well as in vivo DNA-protein interaction assays revealed direct association between retinoic acid receptor-alpha (RARalpha) and retinoid-X-receptor (RXR) with this enhancer. Moreover, using chromatin immunoprecipitation (ChIP) we uncovered early events of NIS transcription in response to tRA, which require the interaction of several novel intronic tRA responsive elements. These findings indicate a complex interplay between nuclear receptors, RNA Pol-II and multiple intronic RAREs in NIS gene, and they establish a novel mechanistic model for tRA-induced gene transcription.

Activation of the PI3 kinase pathway by retinoic acid mediates sodium/iodide symporter induction and iodide transport in MCF-7 breast cancer cells

Iodide uptake in the thyroid and breast is mediated by the sodium/iodide symporter (NIS). NIS activation is used for radioiodide imaging and therapeutic ablation of thyroid carcinoma. NIS is expressed in >70% of breast cancers but at a level insufficient for radioiodine treatment. All-trans retinoic acid (tRA) induces NIS gene expression and functional iodide uptake in human breast cancer cell lines and mouse breast cancer models. tRA usually regulates gene expression by direct interaction of RA receptor (RAR) with a target gene, but it can also act through nongenomic pathways. We report a direct influence of tRA treatment on the phosphoinositide 3-kinase (PI3K) signal transduction pathway that mediates tRA-induced NIS expression in MCF-7 breast cancer cells. MCF-7 cells express all three RAR isoforms, alpha, beta, and gamma, and RXRalpha. We previously identified RARbeta and RXRalpha as important for NIS induction by tRA. Treatment with LY294002, the PI3K inhibitor, or p85alpha knockdown with siRNA abolished tRA-induced NIS expression. Immunoprecipitation experiments and glutathione S-transferase pull-down assay showed a direct interaction between RARbeta2, RXRalpha, and p85alpha. RA also induced rapid activation of Akt in MCF-7 cells. Treatment with an Akt inhibitor or Akt knockdown with siRNA reduced NIS expression. These findings indicate that RA induction of NIS in MCF-7 cells is mediated by rapid activation of the PI3K pathway and involves direct interaction with RAR and retinoid X receptor. Defining these mechanisms should lead to methods to further enhance NIS expression, as well as retinoid targets that influence growth and differentiation of breast cancer.