Retinoic acid-induced stimulation of sodium iodide symporter expression and cytotoxicity of radioiodine in prostate cancer cells

We reported recently the induction of androgen-dependent iodide uptake activity in the human prostatic adenocarcinoma cell line LNCaP using a prostate-specific antigen (PSA) promoter-directed expression of the sodium iodide symporter (NIS) gene. This offers the potential to treat prostate cancer with radioiodine. In the current study, we examined the regulation of PSA promoter-directed NIS expression and therapeutic effectiveness of (131)I in LNCaP cells by all-trans-retinoic acid (atRA). For this purpose, NIS mRNA and protein expression levels in the NIS-transfected LNCaP cell line NP-1 were examined by Northern and Western blot analysis following incubation with atRA (10 (-9) to 10(-6) M) in the presence of 10(-9) M mibolerone (mib). In addition, NIS functional activity was measured by iodide uptake assay, and in vitro cytotoxicity of (131)I was examined by in vitro clonogenic assay. Following incubation with atRA, NIS mRNA levels in NP-1 cells were stimulated 3-fold in a concentration-dependent manner, whereas NIS protein levels increased 2.3-fold and iodide accumulation was stimulated 1.45-fold. This stimulatory effect of atRA, which has been shown to be retinoic acid receptor mediated, was completely blocked by the pure androgen receptor antagonist casodex (10(-6) M), indicating that it is androgen receptor dependent. The selective killing effect of (131)I in NP-1 cells was 50% in NP-1 cells incubated with 10(-9) M mib. This was increased to 90% in NP-1 cells treated with atRA (10(-7) M) plus 10(-9) M mib. In conclusion, treatment with atRA increases NIS expression levels and selective killing effect of (131)I in prostate cancer cells stably expressing NIS under the control of the PSA promoter. Therefore atRA may be used to enhance the therapeutic response to radioiodine in prostate cancer cells following PSA promoter-directed NIS gene delivery.     

Dexamethasone stimulation of retinoic Acid-induced sodium iodide symporter expression and cytotoxicity of 131-I in breast cancer cells

CONTEXT: The sodium iodide symporter (NIS) mediates the active iodide uptake in the thyroid gland as well as lactating breast tissue. Recently induction of functional NIS expression was reported in the estrogen receptor-positive human breast cancer cell line MCF-7 by all-trans retinoic acid (atRA) treatment in vitro and in vivo, which might offer the potential to treat breast cancer with radioiodine. OBJECTIVE: In the current study, we examined the effect of dexamethasone (Dex) on atRA-induced NIS expression and therapeutic efficacy of 131-I in MCF-7 cells. DESIGN: For this purpose, NIS mRNA and protein expression levels in MCF-7 cells were examined by Northern and Western blot analysis after incubation with Dex (10(-9) to 10(-7) m) in the presence of atRA (10(-6) m) as well as immunostaining using a mouse monoclonal human NIS-specific antibody. In addition, NIS functional activity was measured by iodide uptake and efflux assay, and in vitro cytotoxicity of 131-I was examined by in vitro clonogenic assay. RESULTS: After incubation with Dex in the presence of atRA, NIS mRNA levels in MCF-7 cells were stimulated up to 11-fold in a concentration-dependent manner, whereas NIS protein levels increased up to 16-fold and iodide accumulation was stimulated up to 3- to 4-fold. Furthermore, iodide efflux was modestly decreased after stimulation with Dex in the presence of atRA. Furthermore, in the in vitro clonogenic assay, selective cytotoxicity of 131-I was significantly increased from approximately 17% in MCF-7 cells treated with atRA alone to 80% in MCF-7 cells treated with Dex in the presence of atRA. CONCLUSION: Treatment with Dex in the presence of atRA significantly increases functional NIS expression levels in addition to inhibiting iodide efflux, resulting in an enhanced selective killing effect of 131-I in MCF-7 breast cancer cells.     

Application of 188rhenium as an alternative radionuclide for treatment of prostate cancer after tumor-specific sodium iodide symporter gene expression

CONTEXT: We reported recently the induction of iodide accumulation in prostate cancer cells (LNCaP) by prostate-specific antigen promoter-directed sodium iodide symporter (NIS) expression that allowed a significant therapeutic effect of (131)iodine ((131)I). These data demonstrated the potential of the NIS gene as a novel therapeutic gene, although in some extrathyroidal tumors, therapeutic efficacy may be limited by rapid iodide efflux due to a lack of iodide organification. OBJECTIVE: In the current study, we therefore studied the potential of (188)rhenium ((188)Re), as an alternative radionuclide, also transported by NIS, with a shorter half-life and higher energy beta-particles than (131)I. RESULTS: NIS-transfected LNCaP cells (NP-1) concentrated 8% of the total applied activity of (188)Re as compared with 16% of (125)I, which was sufficient for a therapeutic effect in an in vitro clonogenic assay. gamma-Camera imaging of NP-1 cell xenografts in nude mice revealed accumulation of 8-16% injected dose (ID)/g (188)Re (biological half-life 12.9 h), which resulted in a 4.7-fold increased tumor absorbed dose (450 mGy/MBq) for (188)Re as compared with (131)I. After application of 55.5 MBq (131)I or (188)Re, smaller tumors showed a similar average volume reduction of 86%, whereas in larger tumors volume reduction was significantly increased from 73% after (131)I treatment to 85% after application of (188)Re. CONCLUSION: Although in smaller prostate cancer xenografts both radionuclides seemed to be equally effective after prostate-specific antigen promoter-mediated NIS gene delivery, a superior therapeutic effect has been demonstrated for (188)Re in larger tumors.     

A novel therapeutic strategy for medullary thyroid cancer based on radioiodine therapy following tissue-specific sodium iodide symporter gene expression

CONTEXT: In contrast to papillary and follicular thyroid cancer, medullary thyroid cancer (MTC) remains difficult to treat due to its unresponsiveness to radioiodine therapy and its limited responsiveness to chemo- and radiotherapy. OBJECTIVE: To investigate an alternative therapeutic approach, we examined the feasibility of radioiodine therapy of MTC after human sodium iodide symporter (hNIS) gene transfer using the calcitonin promoter to target hNIS gene expression to MTC cells (TT). DESIGN: TT cells were stably transfected with an expression vector, in which hNIS cDNA was coupled to the calcitonin promoter. Functional hNIS expression was confirmed by iodide accumulation assays, Northern and Western blot analysis, immunostaining, and in vitro clonogenic assay. RESULTS: hNIS-transfected TT cells showed perchlorate-sensitive iodide uptake, accumulating 125-I about 12-fold in vitro with organification of 4% of accumulated iodide resulting in a significant decrease in iodide efflux. NIS protein expression was confirmed by Western blot analysis using a monoclonal hNIS-specific antibody, which revealed a major band of a molecular mass of 80-90 kDa. In addition, immunostaining of hNIS-transfected TT cells revealed hNIS-specific immunoreactivity, which was primarily membrane associated. In an in vitro clonogenic assay, 84% of NIS-transfected TT cells were killed by exposure to 131-I, whereas only about 0.6% of control cells were killed. CONCLUSIONS: A therapeutic effect of 131-I has been demonstrated in MTC cells after induction of tissue-specific iodide uptake activity by calcitonin promoter-directed hNIS expression. This study demonstrates the potential of NIS as a therapeutic gene, allowing radioiodine therapy of MTC after tissue-specific NIS gene transfer.     

Approaches to gene therapy with sodium/iodide symporter.

Since cloning and characterization of the sodium iodide symporter (NIS) gene, several investigators explored the possibility of a novel cytoreductive gene therapy strategy based on NIS gene transfer into non-thyroidal tumor cells followed by radioiodine therapy. NIS gene transfer has been shown to be capable of inducing radioiodine accumulation in vitro and in vivo in several non-thyroidal cancer cell lines. Following PSA promoter-mediated NIS gene delivery we were able to demonstrate prostate-specific iodide accumulation in prostate cancer cells that was high enough to elicit a therapeutic response of 131-I in vitro and in vivo. This study clearly demonstrates the potential of NIS as a novel therapeutic gene for non-thyroidal cancers, in particular prostate cancer.     

Differential regulation of sodium/iodide symporter gene expression by nuclear receptor ligands in MCF-7 breast cancer cells

The sodium/iodide symporter (NIS) mediates iodide uptake in lactating breast tissue and is expressed in some breast cancers. We have previously demonstrated that all-trans retinoic acid (tRA) stimulates NIS gene expression and the selective cytotoxic effect of beta-emitting radioiodide-131 ((131)I) in both in vitro and in vivo MCF-7 breast cancer cell systems. We studied the ability of natural and synthetic retinoids, in combination with other nuclear receptor ligands, to achieve greater and more sustained induction of NIS in MCF-7 cells and enhance (131)I-mediated cytotoxicity. Selective stimulation of retinoic acid receptor (RAR) beta/gamma produced marked NIS induction; and selective stimulation of RARalpha, RARgamma, or retinoid X receptor produced more modest induction. Maximal NIS induction was seen with 9-cis retinoic acid and AGN190168, a RAR beta/gamma-agonist. Dexamethasone (Dex), but not the other nuclear receptor ligands, in combination with tRA synergistically induced iodide uptake and NIS mRNA expression, predominantly by prolonging NIS mRNA half-life. The addition of Dex reduced the EC(50) of tRA for NIS stimulation to approximately 7%, such that 10(-7) m tRA with addition of Dex enhanced iodide uptake and selective cytotoxicity of (131)I greater than 10(-6) m tRA alone. AGN190168 combined with Dex synergistically increased iodide uptake and significantly prolonged induction (5 d) of iodide uptake compared with that induced by the combination of tRA/Dex or 9-cis retinoic acid/Dex. The addition of Dex reduced the effective dose of retinoid and prolonged the induction of NIS, especially with AGN190168, suggesting higher efficacy of (131)I after combination treatment.     

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.     

Regulation of sodium iodide symporter gene expression in FRTL-5 rat thyroid cells.

The sodium iodide symporter (NIS), first identified in FRTL-5 cells, plays a critical role in iodide transport in the thyroid gland and in the production of the iodine-containing thyroid hormones. The aim of our study was to examine the regulation of NIS RNA steady-state levels and protein expression as well as functional activity in FRTL-5 cells. FRTL-5 cells cycling in media containing thyrotropin (TSH) were incubated for 48 hours with dexamethasone (10(-8)-10(-5) M), triiodothyronine (T3; 10(-9)-10(-6) M), methimazole (100 microM), propylthiouracil (PTU; 100 microM), perchlorate (10 microM) and potassium iodide (40 microM). In other experiments, cells were treated for 48 hours with various cytokines including interleukin-6 (IL-6) (100 U/mL), interferon-gamma (IFN-gamma) (100 U/mL), tumor necrosis factor-alpha (TNF-alpha) (10 ng/ml), IL-1alpha (100 U/mL), and IL-1beta (100 U/mL). Northern blot analysis using a 32P-labeled rat NIS-specific cDNA probe (nucleotides 1397-1937) revealed NIS mRNA as a single species of approximately 3 kb. When normalized for beta-actin mRNA signal intensities, NIS RNA steady-state levels in viable FRTL-5 cells were suppressed by approximately 80% after incubation with dexamethasone and T3 in a concentration-dependent manner. Iodide accumulation was decreased by up to 40% after incubation with dexamethasone and T3, respectively, in a concentration-dependent manner. Using a rabbit polyclonal rNIS-specific antibody, Western blot analysis of FRTL-5 cell membranes revealed a 60% and 70% suppression of NIS protein expression after treatment with T3 (0.1 microM) and dexamethasone (1 microM), respectively. In additon, NIS RNA steady-state levels were decreased by approximately 50% after treatment of monolayers with methimazole, PTU, and potassium iodide, respectively. Incubation with methimazole and PTU resulted in a 20% and 25% decrease of iodide accumulation, respectively, whereas potassium iodide suppressed iodide accumulation by approximately 50%. Treatment of FRTL-5 cells with IL-6 and IL-1beta resulted in a 30% decrease of NIS RNA steady-state levels. IL-6 did not alter NIS functional activity, but IL-1beta suppressed iodide accumulation by approximately 25%. IFN-gamma and perchlorate failed to alter NIS RNA steady-state levels. In contrast to IFN-gamma that had no effect on iodide accumulation, perchlorate almost completely suppressed iodide accumulation. TNF-alpha and IL-1alpha failed to alter NIS RNA steady-state levels in higher passage numbers of FRTL-5 cells, whereas treatment with TNF-alpha and IL-1alpha of early passages of FRTL-5 cells (<20 cell passages) resulted in a 70% and 40% decrease of NIS RNA steady-state levels, respectively, and in a 20% suppression of NIS functional activity. In conclusion, our data suggest that various agents known to affect iodide transport are capable of differentially altering NIS gene expression and function in cultured thyroid cells. Suppression of NIS gene expression and function by certain cytokines may be responsible, at least in part, for the impaired radioiodine uptake by thyroid tissue in certain forms of thyroiditis.     

PI3K对分化型甲状腺癌中NIS表达调节的研究进展

钠碘转运体（NIS）介导甲状腺滤泡细胞的碘浓聚,从而成为多种甲状腺良恶性疾病诊断和治疗的分子生物学基础。促甲状腺激素（TSH）是调节NIS表达的主要因子,TSH同受体结合,通过相应的信号级联反应来调节NIS的表达,磷脂酰肌醇-3-激酶（PI3K）信号转导途径介导TSH对NIS mRNA和蛋白表达的抑制。分化型甲状腺癌（DTC）中PI3K途径的激活极为常见,PI3K抑制剂促进DTC功能性NIS的表达,提高其对放射碘的摄取,对提高DTC患者的疗效、改善预后有重要意义。     

Gene therapy for thyroid cancer: current status and future prospects.

Despite multimodality treatment for thyroid cancer, including surgical resection, radioiodine therapy, thyrotropin (TSH)-suppressive thyroxine treatment, and chemotherapy/radiotherapy, survival rates have not improved over the last decades. Therefore, development and evaluation of novel treatment strategies, including gene therapy, are urgently needed. A variety of gene therapy approaches have been evaluated for the treatment of follicular cell-derived and medullary thyroid cancer, including corrective gene therapy (p53 restoration, expression of a dominant negative RET mutant), cytoreductive gene therapy (suicide gene/prodrug strategy herpes simplex virus-thymidine kinase [HSV-tk]/ganciclovir, antiangiogenic therapy with endostatin) and immunomodulatory gene therapy (expression of interleukin (IL)-2 and IL-12). Furthermore, cloning of the sodium iodide symporter (NIS) gene has paved the way for the development of a novel cytoreductive gene therapy strategy based on NIS gene transfer followed by the application of radioiodine therapy ((131)I). NIS gene delivery into medullary and follicular cell-derived thyroid cancer cells has been shown to be capable of establishing or restoring radioiodine accumulation and might therefore represent an effective therapy for medullary and dedifferentiated thyroid tumors that lack iodide accumulating activity. The data summarized in this review article clearly demonstrate that the currently available strategies represent potentially curative novel therapeutic approaches for future gene therapy of thyroid cancer. The combination of different therapeutic genes has been demonstrated to be very useful to enhance therapeutic efficacy and seems to have a promising role at least as part of a multimodality approach for advanced thyroid cancer.     

The sodium iodide symporter: its pathophysiological and therapeutic implications

The sodium iodide symporter (NIS) is an intrinsic plasma membrane protein that mediates the active transport of iodide in the thyroid gland and a number of extrathyroidal tissues, in particular lactating mammary gland. Because of its crucial role in the ability of thyroid follicular cells to trap iodide, cloning of NIS opened an exciting and extensive new field of thyroid-related research. Cloning and molecular characterization of NIS allowed investigation of its expression and regulation in thyroidal and nonthyroidal tissues, and its potential pathophysiological and therapeutic implications in benign and malignant thyroid disease. In addition to its key function in thyroid physiology, NIS-mediated iodide accumulation allows diagnostic thyroid scintigraphy as well as effective therapeutic application of radioiodine in benign and malignant thyroid disease. Characterization and application of NIS as a novel therapeutic gene and the presence of high native NIS expression in the majority of breast cancers further suggest a promising role of NIS in diagnosis and therapy of cancer outside the thyroid gland.     

Der Natrium-Jodid-Symporter (NIS)

The sodium iodide symporter (NIS) is an intrinsic plasma membrane protein that mediates the active transport of iodide in the thyroid gland and a number of extrathyroidal tissues, in particular lactating mammary gland. Because of its crucial role in the ability of thyroid follicular cells to trap iodide, cloning of NIS opened an exciting and extensive new field of thyroid-related research. Cloning and molecular characterization of NIS allowed investigation of its expression and regulation in thyroidal and nonthyroidal tissues, and its potential pathophysiological and therapeutic implications in benign and malignant thyroid diseases. In addition, NIS-mediated iodide accumulation allows diagnostic thyroid scintigraphy as well as effective therapeutic application of radioiodine in benign and malignant thyroid disease. Characterization and application of NIS as a novel therapeutic gene for cytoreductive gene therapy of extrathyroidal tumors, and the presence of high endogenous NIS expression in the majority of breast cancers further suggest a promising role of NIS in diagnosis and therapy of cancer outside the thyroid gland.     

Treatment with drugs able to reduce iodine efflux significantly increases the intracellular retention time in thyroid cancer cells stably transfected with sodium iodide symporter complementary deoxyribonucleic acid

CONTEXT: One of the major limits of gene therapy with sodium iodide symporter (NIS), which enables cells to be subjected to radioiodine therapy, is that NIS-transfected cells rapidly release the intracellular iodine. METHODS: We transfected human anaplastic (FRO) and medullary (TT) thyroid cancer-derived cell lines that were unable to take up iodine with human NIS cDNA. The possibility of increasing the iodine retention time by treating the transfected clones with myricetin, lithium, 17-(allylamino)-17-demethoxygeldanamycin (17-AAG), and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) was explored. RESULTS: We obtained 19 FRO and 16 TT clones stably transfected with NIS. Twelve of 19 FRO and nine of 16 TT clones expressed the full-length NIS mRNA; 11 of 12 FRO and four of nine TT clones were able to take up radioiodine and correctly expressed NIS protein on the plasma membrane. Kinetic analysis of iodide uptake in the two clones (FRO-19 and TT-2) with the highest uptaking activity revealed that the plateau was reached after 30 min by FRO-19 and after 60 min by TT-2. The t(1/2) of the iodide efflux was 9 min in FRO-19 and 20 min in TT-2. The treatment of the two cell lines with four different drugs revealed that DIDS and 17-AAG, but not myricetin and lithium, significantly increased the intracellular iodide retention time in FRO-19, but not in TT-2. CONCLUSIONS: We showed that 17-AAG and DIDS prolong the retention time of (131)I in NIS-transfected thyroid tumoral cells, thus reinforcing the hope of using this approach for future clinical application, especially in patients with thyroid carcinoma who are no longer responsive to conventional therapy.     

Regulation of growth hormone receptors in human prostate cancer cell lines

We previously demonstrated the gene expression of two growth hormone (GH) receptor (GHR) isoforms in prostate cancer (PCa) patient tissues and human PCa cell lines. In that initial study, we characterized LNCaP cell GH binding characteristics to GHR and its activation of relevant signal transduction pathways. We now show that GH binding to GHR and GHR mRNA expression in the cell lines studied are hormonally regulated. In the androgen-dependent LNCaP cells, the potent, specific and stable androgen analogue, mibolerone, caused a time- and biphasic dose-dependent, stimulation of ¹²⁵I-hGH specific binding to cells cultured in serum-free medium (SFM); however, when LNCaP cells were grown in chemically defined Gc full medium, long-term mibolerone-induced inhibition was observed. This effect of Gc on the androgen response was mimicked by the triiodothyronine (T₃) contained in GC. In contrast, oestradiol (E₂), cortisol, and insulin-like growth factor (IGF)-I and -II all caused stimulation of GH binding. Furthermore, we also observed homologous and heterologous, isoform- and cell-type-specific regulation of GHR mRNA expression in all three cell lines. In LNCaP cells, GH caused stimulation of both GHR mRNA and of its exon 9-truncated isoform, GHR_tr; however, mibolerone, E₂ and T₃ all stimulated GHR_tr mRNA more potently than they did GHR. In androgen-independent PC3 cells, GH stimulated GHR_tr expression, but almost not GHR, while in contrast, in androgen-independent DU145 cells, GH caused a clear reduction in GHR and less so in GHR_tr. The differential regulation of GHR isoform gene expression in human PCa cell lines and of GHR functional capacity (GH binding), by hormones and growth factors relevant to disease progression, suggests that GHR may prove to be an additional therapeutic target to slow down/prevent progression of human prostate cancer.     

Sodium iodide symporter expression and radioiodine distribution in extrathyroidal tissues

The functional role of the sodium iodide symporter (NIS) in extrathyroidal tissues was investigated by examining its mRNA and protein expression, together with the evidence of radioiodine (131)I uptake in 302 patients who underwent (131)I total body scanning, following the administration of high doses of (131)I for a papillary or follicular thyroid carcinoma. By using a real-time kinetic quantitative RT-PCR and immunohistochemistry, the expression of NIS protein was detected mainly in secretory tissues. In parallel, 1311 uptake was evidenced in the majority of patients in the salivary glands (in 39%) and stomach (in 78%), but was found in breast in only 4 young female patients. These data demonstrate a strong correlation between the organ radioactivity distribution, as observed in vivo, and NIS protein expression. Interestingly, (131)I is rarely concentrated by mammary glands, even when large doses are administered. Moreover, a (131)I transfer in secretion fluids may represent a potential source of contamination responsible for false positive images and diagnostic pitfalls.     

Amiodarone inhibits thyroidal iodide transport in vitro by a cyclic adenosine 5'-monophosphate- and iodine-independent mechanism

Thyroid side effects are common in patients treated for cardiac arrhythmias with amiodarone (AM). A major disturbance is inhibited thyroidal radioiodine uptake in AM-induced thyrotoxicosis, which makes 131I therapy ineffective. On the other hand, failure to escape from the Wolff-Chaikoff effect by down-regulation of the sodium/iodide symporter (NIS) is proposed to explain AM-induced hypothyroidism. However, previously no experimental studies on the possible mechanisms have been conducted. We therefore investigated the early effects of AM on thyroidal iodide transport using bicameral chamber cultures of primary pig thyrocytes that reproduce the three tissue compartments (epithelium, lumen, and extrafollicular space) of the gland. AM dose-dependently (1-50 microm) inhibited the TSH-stimulated transepithelial (basal to apical) transport of 125I- by up to 90%. The inhibitory effect was noticed already after 8 h and was further pronounced after 1-4 d, depending on the AM concentration. The intracellularly accumulated 125I- was reduced by perchlorate but not AM, and quantitative real-time RT-PCR revealed no change in the NIS expression in AM-treated cells. Blocking of cAMP degradation with 3-isobutyl-1-methylxanthine or withdrawal of AM reversed AM-induced changes in electrolyte transport but were unable to recover the suppressed 125I- transport. The iodine-free AM analog dronedarone also inhibited 125I- transport to the same extent as AM. The findings indicate that AM blocks thyroidal iodide uptake by reducing the iodide permeability of the apical plasma membrane of the thyroid epithelial cells. The effect is iodine independent and long-lasting and does not involve impaired function of NIS or the TSH receptor/cAMP signaling pathway.     

Enhancement of sodium/iodide symporter expression in thyroid and breast cancer

The sodium/iodide symporter (NIS) mediates iodide uptake in the thyroid gland and lactating breast. NIS mRNA and protein expression are detected in most thyroid cancer specimens, although functional iodide uptake is usually reduced resulting in the characteristic finding of a 'cold' or non-functioning lesion on a radioiodine image. Iodide uptake after thyroid stimulating hormone (TSH) stimulation, however, is sufficient in most differentiated thyroid cancer to utilize beta-emitting radioactive iodide for the treatment of residual and metastatic disease. Elevated serum TSH, achieved by thyroid hormone withdrawal in athyreotic patients or after recombinant human thyrotropin administration, directly stimulates NIS gene expression and/or NIS trafficking to the plasma membrane, increasing radioiodide uptake. Approximately 10-20% differentiated thyroid cancers, however, do not express the NIS gene despite TSH stimulation. These tumors are generally associated with a poor prognosis. Reduced NIS gene expression in thyroid cancer is likely due in part, to impaired trans-activation at the proximal promoter and/or the upstream enhancer. Basal NIS gene expression is detected in about 80% breast cancer specimens, but the fraction with functional iodide transport is relatively low. Lactogenic hormones and various nuclear hormone receptor ligands increase iodide uptake in breast cancer cells in vitro, but TSH has no effect. A wide range of 'differentiation' agents have been utilized to stimulate NIS expression in thyroid and breast cancer using in vitro and in vivo models, and a few have been used in clinical studies. Retinoic acid has been used to stimulate NIS expression in both thyroid and breast cancer. There are similarities and differences in NIS gene regulation and expression in thyroid and breast cancer. The various agents used to enhance NIS expression in thyroid and breast cancer will be reviewed with a focus on the mechanism of action. Agents that promote tumor differentiation, or directly stimulate NIS gene expression, may result in iodine concentration in 'scan-negative' thyroid cancer and some breast cancer.