MEK signaling modulates sodium iodide symporter at multiple levels and in a paradoxical manner

Children's Oncology Group's 2023 blueprint for research: Rare tumors

The Children's Oncology Group (COG) Rare Tumor Committee includes the Infrequent Tumor and Retinoblastoma subcommittees, encompassing a wide range of extracranial solid tumors that do not fall within another COG disease committee. Current therapeutic trial development focuses on nasopharyngeal carcinoma, adrenocortical carcinoma, pleuropulmonary blastoma, colorectal carcinoma, melanoma, and thyroid carcinoma. Given the rarity of these tumors, novel strategies and international collaborative efforts are necessary to advance research and improve outcomes.

[Therapy concepts for thyroid carcinoma]

Theranostics via the sodium iodide symporter (NIS) offer a unique option in differentiated thyroid carcinoma. The diagnostic and therapeutic nuclides have similar uptake and kinetics, making the NIS the most important theranostic target in this disease. Radioiodine refractory thyroid carcinomas (RRTC) are characterised by reduced/absent NIS expression, thus eliminating this structure as a theranostic target. Also due to limited therapeutic options, there are approaches to generate new theranostic targets in RRTC, via the expression of somatostatin receptors (SSTR) or the prostate-specific membrane antigen (PSMA), but the current evidence does not yet allow a final evaluation of the prospects of success.

[Therapy concepts for thyroid carcinoma]

Theranostics via the sodium iodide symporter (NIS) offer a unique option in differentiated thyroid carcinoma. The diagnostic and therapeutic nuclides have similar uptake and kinetics, making the NIS the most important theranostic target in this disease. Radioiodine refractory thyroid carcinomas (RRTC) are characterised by reduced/absent NIS expression, thus eliminating this structure as a theranostic target. Also due to limited therapeutic options, there are approaches to generate new theranostic targets in RRTC, via the expression of somatostatin receptors (SSTR) or the prostate-specific membrane antigen (PSMA), but the current evidence does not yet allow a final evaluation of the prospects of success.

Molecular mechanisms of radioactive iodine refractoriness in differentiated thyroid cancer: Impaired sodium iodide symporter (NIS) expression owing to altered signaling pathway activity and intracellular localization of NIS

The advanced, metastatic differentiated thyroid cancers (DTCs) have a poor prognosis mainly owing to radioactive iodine (RAI) refractoriness caused by decreased expression of sodium iodide symporter (NIS), diminished targeting of NIS to the cell membrane, or both, thereby decreasing the efficacy of RAI therapy. Genetic aberrations (such as BRAF, RAS, and RET/PTC rearrangements) have been reported to be prominently responsible for the onset, progression, and dedifferentiation of DTCs, mainly through the activation of mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K)/AKT signaling pathways. Eventually, these alterations result in a lack of NIS and disabling of RAI uptake, leading to the development of resistance to RAI therapy. Over the past decade, promising approaches with various targets have been reported to restore NIS expression and RAI uptake in preclinical studies. In this review, we summarized comprehensive molecular mechanisms underlying the dedifferentiation in RAI-refractory DTCs and reviews strategies for restoring RAI avidity by tackling the mechanisms.

Tumor microenvironment affects exogenous sodium/iodide symporter expression

For decades, sodium/iodide symporter NIS-mediated iodide uptake has played a crucial role in the radioactive ablation of thyroid cancer cells. NIS-based gene therapy has also become a promising tool for the treatment of tumors of extrathyroidal origin. But its applicability has been hampered by reduced expression of NIS, resulting in a moderated capacity to accumulate 131I and in inefficient ablation. Despite numerous preclinical enhancement strategies, the understanding of NIS expression within tumors remains limited. This study aims at a better understanding of the functional behavior of exogenous NIS expression in the context of malignant solid tumors that are characterized by rapid growth with an insufficient vasculature, leading to hypoxia and quiescence. Using subcutaneous HT29NIS and K7M2NIS tumors, we show that NIS-mediated uptake and NIS expression at the plasma membrane of cancer cells are impaired in the intratumoral regions. For a better understanding of the underlying molecular mechanisms induced by hypoxia and quiescence (separately and in combination), we performed experiments on HT29NIS cancer cells. Hypoxia and quiescence were both found to impair NIS-mediated uptake through mechanisms including NIS mis-localization. Modifications in the expression of proteins and metabolites involved in plasma membrane localization and in energy metabolism were found using untargeted proteomics and metabolomics approaches. In conclusion, our results provide evidence that hypoxia and quiescence impair NIS expression at the plasma membrane, and iodide uptake. Our study also shows that the tumor microenvironment is an important parameter for successful NIS-based cancer treatment.

Rutin Scavenges Reactive Oxygen Species, Inactivates 5'-Adenosine Monophosphate-Activated Protein Kinase, and Increases Sodium-Iodide Symporter Expression in Thyroid PCCL3 Cells

BACKGROUND

Thyroid iodide uptake, mediated by the sodium-iodide symporter (NIS), is essential for thyroid hormone synthesis and also for treatment of thyroid diseases, such as thyroid cancer, through radioiodine therapy. Therefore, compounds able to increase thyroid iodide uptake could be clinically useful, and it is of great importance to unravel the mechanisms underlying such an effect. It has been shown previously that the flavonoid rutin increases thyroid radioiodide uptake in vivo in rats. This study aimed to investigate the mechanisms involved in the stimulatory effect of rutin on iodide uptake.

METHODS

This study evaluated iodide uptake, NIS expression and its subcellular distribution, iodide efflux, reactive oxygen species levels, and the intracellular pathways involved in NIS regulation in a rat thyroid PCCL3 cell line treated with rutin.

RESULTS

Similar to previous results found in vivo, rutin increased radioiodide uptake in PCCL3 cells, which was accompanied by increased NIS expression (at both the mRNA and protein levels) and a reduction of radioiodide efflux. Moreover, the results suggest that rutin could regulate NIS subcellular distribution, leading to higher levels of NIS at the cell membrane. In addition, rutin decreased the levels of intracellular reactive oxygen species and phospho-5'-adenosine monophosphate-activated protein kinase.

CONCLUSIONS

The flavonoid rutin seems to be an important stimulator of radioiodide uptake, acting at multiple levels, an effect that can be due to decreased oxidative stress, reduced 5'-adenosine monophosphate-activated protein kinase activation, or both. Since thyroid iodide uptake is crucial for effective radioiodine therapy, the results suggest that rutin could be useful as an adjuvant in radioiodine therapy.

Flavonoids, Thyroid Iodide Uptake and Thyroid Cancer-A Review

Thyroid cancer is the most common malignant tumor of the endocrine system and the incidence has been increasing in recent years. In a great part of the differentiated carcinomas, thyrocytes are capable of uptaking iodide. In these cases, the main therapeutic approach includes thyroidectomy followed by ablative therapy with radioiodine. However, in part of the patients, the capacity to concentrate iodide is lost due to down-regulation of the sodium-iodide symporter (NIS), the protein responsible for transporting iodide into the thyrocytes. Thus, therapy with radioiodide becomes ineffective, limiting therapeutic options and reducing the life expectancy of the patient. Excessive ingestion of some flavonoids has been associated with thyroid dysfunction and goiter. Nevertheless, studies have shown that some flavonoids can be beneficial for thyroid cancer, by reducing cell proliferation and increasing cell death, besides increasing NIS mRNA levels and iodide uptake. Recent data show that the flavonoids apingenin and rutin are capable of increasing NIS function and expression in vivo. Herein we review literature data regarding the effect of flavonoids on thyroid cancer, besides the effect of these compounds on the expression and function of the sodium-iodide symporter. We will also discuss the possibility of using flavonoids as adjuvants for therapy of thyroid cancer.

Modulation of thyroidal radioiodide uptake by oncological pipeline inhibitors and Apigenin

Targeted radioiodine therapy for thyroid cancer is based on selective stimulation of Na+/I- Symporter (NIS)-mediated radioactive iodide uptake (RAIU) in thyroid cells by thyrotropin. Patients with advanced thyroid cancer do not benefit from radioiodine therapy due to reduced or absent NIS expression. To identify inhibitors that can be readily translated into clinical care, we examined oncological pipeline inhibitors targeting Akt, MEK, PI3K, Hsp90 or BRAF in their ability to increase RAIU in thyroid cells expressing BRAFV600E or RET/PTC3 oncogene. Our data showed that (1) PI3K inhibitor GDC-0941 outperformed other inhibitors in RAIU increase mainly by decreasing iodide efflux rate to a great extent; (2) RAIU increase by all inhibitors was extensively reduced by TGF-β, a cytokine secreted in the invasive fronts of thyroid cancers; (3) RAIU reduction by TGF-β was mainly mediated by NIS reduction and could be reversed by Apigenin, a plant-derived flavonoid; and (4) In the presence of TGF-β, GDC-0941 with Apigenin co-treatment had the highest RAIU level in both BRAFV600E expressing cells and RET/PTC3 expressing cells. Taken together, Apigenin may serve as a dietary supplement along with small molecule inhibitors to improve radioiodine therapeutic efficacy on invasive tumor margins thereby minimizing future metastatic events.

Enhancement of human sodium iodide symporter gene therapy for breast cancer by HDAC inhibitor mediated transcriptional modulation

The aberrant expression of human sodium iodide symporter (NIS) in breast cancer (BC) has raised the possibility of using targeted radioiodide therapy. Here we investigate modulation of endogenous, functional NIS expression by histone deacetylase inhibitors (HDACi) in vitro and in vivo. Luciferase reporter based initial screening of six different HDACi shows 2–10 fold enhancement of NIS promoter activity in majority of the cell types tested. As a result of drug treatment, endogenous NIS transcript and protein shows profound induction in BC cells. To get an insight on the mechanism of such transcriptional activation, role of Stat4, CREB and other transcription factors are revealed by transcription factor profiling array. Further, NIS-mediated intracellular iodide uptake also enhances substantially (p < 0.05) signifying functional relevance of the transcriptional modulation strategy. Gamma camera imaging confirms 30% higher uptake in VPA or NaB treated BC tumor xenograft. Corroborating with such functional impact of NIS, significant reduction in cell survival (p < 0.005) is observed in VPA, NaB or CI994 drug and ¹³¹I combination treatment in vivo indicating effective radioablation. Thus, for the first time this study reveals the mechanistic basis and demonstrates functional relevance of HDACi pre-treatment strategy in elevating NIS gene therapy approach for BC management in clinic.

microRNA-339-5p modulates Na+/I- symporter-mediated radioiodide uptake

Na(+)/I(-) symporter (NIS)-mediated radioiodide uptake (RAIU) serves as the basis for targeted ablation of thyroid cancer remnants. However, many patients with thyroid cancer have reduced NIS expression/function and hence do not benefit from radioiodine therapy. microRNA (miR) has emerged as a promising therapeutic target in many diseases; yet, the role of miRs in NIS-mediated RAIU has not been investigated. In silico analysis was used to identify miRs that may bind to the 3'UTR of human NIS (hNIS). The top candidate miR-339-5p directly bound to the 3'UTR of hNIS. miR-339-5p overexpression decreased NIS-mediated RAIU in HEK293 cells expressing exogenous hNIS, decreased the levels of NIS mRNA, and RAIU in transretinoic acid/hydrocortisone (tRA/H)-treated MCF-7 human breast cancer cells as well as thyrotropin-stimulated PCCl3 rat thyroid cells. Nanostring nCounter rat miR expression assay was conducted to identify miRs deregulated by TGFβ, Akti-1/2, or 17-AAG known to modulate RAIU in PCCl3 cells. Among 38 miRs identified, 18 were conserved in humans. One of the 18 miRs, miR-195, was predicted to bind to the 3'UTR of hNIS and its overexpression decreased RAIU in tRA/H-treated MCF-7 cells. miR-339-5p was modestly increased in most papillary thyroid carcinomas (PTCs), yet miR-195 was significantly decreased in PTCs. Interestingly, the expression profiles of 18 miRs could be used to distinguish most PTCs from nonmalignant thyroid tissues. This is the first report, to our knowledge, demonstrating that hNIS-mediated RAIU can be modulated by miRs, and that the same miRs may also play roles in the development or maintenance of thyroid malignancy. Accordingly, miRs may serve as emerging targets to halt the progression of thyroid cancer and to enhance the efficacy of radioiodine therapy.

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: state of the art of its molecular characterization

The sodium/iodide symporter (NIS or SLC5A5) is an intrinsic membrane protein implicated in iodide uptake into thyroid follicular cells. It plays a crucial role in iodine metabolism and thyroid regulation and its function is widely exploited in the diagnosis and treatment of benign and malignant thyroid diseases. A great effort is currently being made to develop a NIS-based gene therapy also allowing the radiotreatment of nonthyroidal tumors. NIS is also expressed in other tissues, such as salivary gland, stomach and mammary gland during lactation, where its physiological role remains unclear. The molecular identity of the thyroid iodide transporter was elucidated approximately fifteen years ago. It belongs to the superfamily of sodium/solute symporters, SSS (and to the human transporter family, SLC5), and is composed of 13 transmembrane helices and 643 amino acid residues in humans. Knowledge concerning NIS structure/function relationship has been obtained by taking advantage of the high resolution structure of one member of the SSS family, the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT), and from studies of gene mutations leading to congenital iodine transport defects (ITD). This review will summarize current knowledge regarding the molecular characterization of NIS.

MEK inhibition leads to lysosome-mediated Na+/I- symporter protein degradation in human breast cancer cells

The Na(+)/I(-) symporter (NIS (SLC5A5)) is a transmembrane glycoprotein that mediates active iodide uptake into thyroid follicular cells. NIS-mediated iodide uptake in thyroid cells is the basis for targeted radionuclide imaging and treatment of differentiated thyroid carcinomas and their metastases. Furthermore, NIS is expressed in many human breast tumors but not in normal non-lactating breast tissue, suggesting that NIS-mediated radionuclide uptake may also allow the imaging and targeted therapy of breast cancer. However, functional cell surface NIS expression is often low in breast cancer, making it important to uncover signaling pathways that modulate NIS expression at multiple levels, from gene transcription to posttranslational processing and cell surface trafficking. In this study, we investigated NIS regulation in breast cancer by MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK) signaling, an important cell signaling pathway involved in oncogenic transformation. We found that MEK inhibition decreased NIS protein levels in all-trans retinoic acid/hydrocortisone-treated MCF-7 cells as well as human breast cancer cells expressing exogenous NIS. The decrease in NIS protein levels by MEK inhibition was not accompanied by a decrease in NIS mRNA or a decrease in NIS mRNA export from the nucleus to the cytoplasm. NIS protein degradation upon MEK inhibition was prevented by lysosome inhibitors but not by proteasome inhibitors. Interestingly, NIS protein level was correlated with MEK/ERK activation in human breast tumors from a tissue microarray. Taken together, MEK activation appears to play an important role in maintaining NIS protein stability in human breast cancers.

Molecular pathogenesis and mechanisms of thyroid cancer

Thyroid cancer is a common endocrine malignancy. There has been exciting progress in understanding its molecular pathogenesis in recent years, as best exemplified by the elucidation of the fundamental role of several major signalling pathways and related molecular derangements. Central to these mechanisms are the genetic and epigenetic alterations in these pathways, such as mutation, gene copy-number gain and aberrant gene methylation. Many of these molecular alterations represent novel diagnostic and prognostic molecular markers and therapeutic targets for thyroid cancer, which provide unprecedented opportunities for further research and clinical development of novel treatment strategies for this 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.

Update on the molecular signature of differentiated thyroid cancer: clinical implications and potential opportunities

With the development and maturation of new technologies, there has been a steady incorporation of powerful new tools into the evaluation and management of thyroid nodules and thyroid cancer. An increasing number of reports on oncogene testing and molecular screening in fine-needle aspiration biopsy samples have been published. However, there remains a paucity of data and consensus on combining both conventional and molecular technologies to determine the diagnosis and/or prognosis of disease. All patients with differentiated thyroid cancer stand to benefit from the identification and incorporation of reliable molecular markers into clinical practice. Identification of reliable markers would allow for stratification of treatment, affording the medical and surgical teams an ability to individually tailor evaluation and treatment, applying aggressive therapy and monitoring only when clinically warranted. For the majority of patients with thyroid cancer, the incorporation of a validated, multifaceted molecular profiling system may not improve survival; however, there is great opportunity for these efforts to decrease the morbidity associated with our current approach.

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.

Single photon emission computed tomography imaging for temporal dynamics of thyroidal and salivary radionuclide accumulation in 17-allyamino-17-demothoxygeldanamycin-treated thyroid cancer mouse model

Selective iodide uptake and prolonged iodine retention in the thyroid is the basis for targeted radioiodine therapy for thyroid cancer patients; however, salivary gland dysfunction is the most frequent nonthyroidal complications. In this study, we have used noninvasive single photon emission computed tomography functional imaging to quantify the temporal dynamics of thyroidal and salivary radioiodine accumulation in mice. At 60  min post radionuclide injection, radionuclide accumulation in the salivary gland was generally higher than that in thyroid due to much larger volume of the salivary gland. However, radionuclide accumulation per anatomic unit in the salivary gland was lower than that in thyroid and was comparable among mice of different age and gender. Differently, radionuclide accumulation per anatomic unit in thyroid varied greatly among mice. The extent of thyroidal radioiodine accumulation stimulated by a single dose of exogenous bovine TSH (bTSH) in triiodothyronine (T₃)-supplemented mice was much less than that in mice received neither bTSH nor T₃ (nontreated mice), suggesting that the duration of elevated serum TSH level is important to maximize thyroidal radioiodine accumulation. Furthermore, the extent and duration of radioiodine accumulation stimulated by bTSH was less in the thyroids of the thyroid-targeted RET/PTC1 (thyroglobulin (Tg)-PTC1) mice bearing thyroid tumors compared with the thyroids in wild-type (WT) mice. Finally, the effect of 17-allyamino-17-demothoxygeldanamycin on increasing thyroidal, but not salivary, radioiodine accumulation was validated in both WT mice and Tg-PTC1 preclinical thyroid cancer mouse model.

Sorafenib: rays of hope in thyroid cancer

BACKGROUND

Sorafenib (BAY 43-9006) is an inhibitor of multiple-receptor tyrosine kinases involved in tumor growth and angiogenesis, which can be advantageously administered orally. Initially used as monotherapy in advanced renal cell carcinoma, sorafenib was proven to increase progression-free survival while enhancing disease control. Clinical trials on sorafenib are at present ongoing for the treatment of various malignancies, including thyroid cancer (TC).

SUMMARY

Specifically, in two phase II studies recently conducted on papillary TC, although the respective results were not entirely compatible as regard partial response rate and progression-free survival, sorafenib demonstrated a relatively favorable benefit/risk profile. In another more recent phase II study, whose primary endpoint was the reinduction of radioactive iodine uptake at 26 weeks, although no reinduction of radioactive iodine uptake was observed, 59% had a beneficial response and 34% had stable disease. Sorafenib hence appears to be a valid alternative to conventional treatment of metastatic papillary TC refractory to radioiodine therapy.

CONCLUSIONS

Further prospective investigations are required to define the characteristics of tumor response to the drug and the factors inducing resistance to treatment. A major issue demanding immediate attention involves optimization of sorafenib treatment: this concerns multidrug combination with different tyrosine kinase inhibitors or immunomodulating agents with the aim of reducing doses and thereby improving drug tolerability and antineoplastic capability.

Characterisation of the purified human sodium/iodide symporter reveals that the protein is mainly present in a dimeric form and permits the detailed study of a native C-terminal fragment

The sodium/iodide symporter is an intrinsic membrane protein that actively transports iodide into thyroid follicular cells. It is a key element in thyroid hormone biosynthesis and in the radiotherapy of thyroid tumours and their metastases. Sodium/iodide symporter is a very hydrophobic protein that belongs to the family of sodium/solute symporters. As for many other membrane proteins, particularly mammalian ones, little is known about its biochemistry and structure. It is predicted to contain 13 transmembrane helices, with an N-terminus oriented extracellularly. The C-terminal, cytosolic domain contains approximately one hundred amino acid residues and bears most of the transporter's putative regulatory sites (phosphorylation, sumoylation, di-acide, di-leucine or PDZ-binding motifs). In this study, we report the establishment of eukaryotic cell lines stably expressing various human sodium/iodide symporter recombinant proteins, and the development of a purification protocol which allowed us to purify milligram quantities of the human transporter. The quaternary structure of membrane transporters is considered to be essential for their function and regulation. Here, the oligomeric state of human sodium/iodide symporter was analysed for the first time using purified protein, by size exclusion chromatography and light scattering spectroscopy, revealing that the protein exists mainly as a dimer which is stabilised by a disulfide bridge. In addition, the existence of a sodium/iodide symporter C-terminal fragment interacting with the protein was also highlighted. We have shown that this fragment exists in various species and cell types, and demonstrated that it contains the amino-acids [512-643] from the human sodium/iodide symporter protein and, therefore, the last predicted transmembrane helix. Expression of either the [1-512] truncated domain or the [512-643] domain alone, as well as co-expression of the two fragments, was performed, and revealed that co-expression of [1-512] with [512-643] allowed the reconstitution of a functional protein. These findings constitute an important step towards an understanding of some of the post-translational mechanisms that finely tune iodide accumulation through human sodium/iodide symporter regulation.

Identification of in vivo phosphorylation sites and their functional significance in the sodium iodide symporter

The Na+/I- symporter (NIS)-mediated iodide uptake activity is the basis for targeted radioiodide ablation of thyroid cancers. Although it has been shown that NIS protein is phosphorylated, neither the in vivo phosphorylation sites nor their functional significance has been reported. In this study, Ser-43, Thr-49, Ser-227, Thr-577, and Ser-581 were identified as in vivo NIS phosphorylation sites by mass spectrometry. Kinetic analysis of NIS mutants of the corresponding phosphorylated amino acid residue indicated that the velocity of iodide transport of NIS is modulated by the phosphorylation status of Ser-43 and Ser-581. We also found that the phosphorylation status of Thr-577 may be important for NIS protein stability and that the phosphorylation status of Ser-227 is functionally silent. Thr-49 appears to be critical for proper local structure/conformation of NIS because mutation of Thr-49 to alanine, aspartic acid, or serine results in reduced NIS activity without alterations in total or cell surface NIS protein levels. Taken together, we showed that NIS protein levels and functional activity could be modulated by phosphorylation through distinct mechanisms.