Thyroid Carcinoma Cell Line TPC-1 Research Articles

1,25-Dihydroxy vitamin D3 inhibits the Ras-MEK-ERK pathway and regulates proliferation and apoptosis of papillary thyroid carcinoma

ObjectiveTo explore the effects of 1,25-dihydroxy vitamin D3 [1,25-(OH)2D3] on the proliferation and apoptosis of papillary thyroid carcinoma and to investigate its possible mechanism. Materials and methodsThe papillary thyroid carcinoma cell line TPC-1 was cultured, and the cells were divided into control group, the 1,25-(OH)2D3 group, and the 1,25-(OH)2D3 + ML-098 (Ras agonist) group. Cell proliferation was observed by MTT. The colony formation viability of cells was detected by the plate cloning assay. Cell migration was observed by the scratch assay. Apoptosis was detected by flow cytometry. The expression of Ki67 and Caspase-3, and the activity of Ras-MEK-ERK pathway were detected by western blot. ResultsCompared with the Control group, the proliferation, colony formation and migration ability of cells in the drug group were significantly decreased. The number of apoptotic cells was significantly increased, the expression of Ki67 protein was decreased, and the expression of Caspase-3 protein was upregulated. The phosphorylation levels of Ras, p-ERK1/2, and p-MEK were decreased. Compared with the drug group, the cloning and migration biological activity of cells in the 1,25-(OH)2D3 + ML-098 group was significantly enhanced (p < 0.05). The number of apoptotic cells was decreased, while the Ki67 protein level was increased. In addition, the Caspase-3 protein level was decreased, and the Ras-MEK-ERK level was also enhanced. Furthermore, the antitumor activity of 1,25-(OH)2D3 was reversed by the Ras agonist ML-098. Conclusion1,25-(OH)2D3 can inhibit the activity and promote apoptosis of the papillary thyroid carcinoma cell line TPC-1, and its mechanism may be related to the inhibition of the Ras-MEK-ERK pathway activity, thus affecting the proliferation and expression of apoptosis-related proteins.

Effect of miR‑449a‑mediated Notch signaling pathway on the proliferation, apoptosis and invasion of papillary thyroid carcinoma cells.

The present study aimed to investigate the effect of miR-449a-mediated Notch signaling pathway on the proliferation, apoptosis and invasion of papillary thyroid carcinoma cells. Human papillary thyroid carcinoma cell line TPC-1 was selected, and cells were grouped and transfected: Control group (without any treatment), negative control (NC) group (transfection with NC plasmid), miR-449a mimic group (transfection with miR-449a mimic), miR-449a inhibitor group (transfection with miR-449a inhibitor), DAPT group (addition of γ-secretase inhibitor DAPT to inhibit the Notch signaling pathway), and miR-449a inhibitor + DAPT group (transfection with miR-449a inhibitor and addition of DAPT). The target relationship between miR-449a and Notch1 was detected by dual-luciferase reporter assay. qRT-PCR and western blotting were used to assess the expression of miR-449a, Notch1 and Jagged1 in cells. Cell proliferation was detected using EdU; the cell cycle and apoptosis were detected by flow cytometry; cell invasion ability was detected by Transwell assay. PCNA, MMP-2, MMP-9, Bcl-2 and Bax mRNA and protein expression were assessed by qRT-PCR and western blotting. The results revealed that miR-449a negatively regulated Notch1. Compared with the control group, there was significantly increased miR-449a expression in the miR-449a mimic group, and there was significantly decreased expression of Notch1, Jagged1, PCNA, MMP-2, MMP-9 and Bcl-2, increased Bax, reduced cell proliferation, increased G1-phase cell fraction, decreased S-phase cell fraction, an increased apoptosis rate, and decreased invasion ability in the miR-449a mimic group and DAPT group (all P<0.05). However, the results in the miR-449a inhibitor group were the opposite of those in miR-449a mimic group (all P<0.05). There was no significant difference in cell proliferation, apoptosis and invasion in the NC group and miR-449a inhibitor + DAPT group compared to the control group (all P>0.05). miR-449a overexpression can inhibit Notch signaling pathway, thereby inhibiting the proliferation and invasion of papillary thyroid carcinoma cells and promoting cell apoptosis.

MicroRNA-92b suppresses proliferation of papillary thyroid cancer cells by regulating ilent information regulator 1 gene expression

Objective To investigate the regulating effects of microRNA (miRNA, miR)-122 on the expression of ilent information regulator 1 (SIRT1), and the proliferation abilities of papillary thyroid cancer cells. Methods Real-time quantitative polymerase chain reaction (Real-time PCR) was used to observe the expression of miR-122 in papillary thyroid carcinoma and adjacent tissues and to analyze its correlation with SIRT1 expression. After transfecting miR-122 mimics into human papillary thyroid carcinoma cell line TPC-1, the dual luciferase reporter gene assay was used to observe the regulation of miR-122 on SIRT1 gene. The effect of miR-122 on the proliferation of thyroid cancer cells was observed by CCK-8 assay. At the same time, the overexpressed SIRT1 plasmid was co-transfected into TPC-1 cells, and then the effect of miR-122 and SIRT1 expression on the proliferation of papillary thyroid carcinoma cells was detected by CCK-8 assay. Analysis was performed using GraphPad Prism 7.0 statistical software. Correlation analysis was performed using Spearman rank correlation analysis. Student’s t test was used to compare the two sample means. Results Compared with adjacent tissues (1.20±0.15), the expression level of miR-122 (3.06±0.26) in papillary thyroid carcinoma was significantly down-regulated, and the difference was statistically significant (t=5.972, P<0.01). Correlation analysis showed that miR-122 was negatively correlated with SIRT1 mRNA expression (r=-0.621, P<0.01). The relative luciferase activity of SIRT1 in miR-122 mimics transfection group was significantly lower than that in the control group, and the difference was statistically significant (t=17.730, P<0.01). The cell proliferation ability of miR-122 mimics group was significantly lower than that of the control group, and the difference was statistically significant (t=3.991, P<0.01). After co-transfection of the SIRT1 overexpression plasmid, the inhibition of the proliferation of thyroid papillary carcinoma cells by miR-122 mimics was reversed. Conclusion miR-122 can inhibit the proliferation of papillary thyroid cancer cells, and its mechanism may be related to the targeted inhibition of SIRT1 expression. Key words: MicroRNA-122; Ilent information regulator 1; Papillary thyroid cancer; Proliferation

Mechanism of microRNA-1243 on proliferation and migration of human papillary thyroid carcinoma TPC-1 cells

Objective To observe the biological effects of microRNA (miRNA, miR)-1243 overexpression on proliferation and migration of human papillary thyroid carcinoma cell line TPC-1. Methods The expression of miR-1243 in 2×105 TPC-1 cells was detected by real-time quantitative polymerase chain reaction (RT-qPCR). The capacities of migration and the migration in TPC-1 cells were tested by water soluble methacon compounds (MTS) or Transwell assays, respectively. The candidate targets for miR-1243 were investigated using predication algorithm provided by miRanda and RNAhybrid. Luciferase activity was measured by dual luciferase reporter assay. Results The expression levels of miR-1243 in blank control and negative control groups were 0.97±0.31 and 1.208±0.40 respectively, significantly lower than 932.84±52.34 in miR-1243 mimic transfected group (P=0.000). The relative survival rates of the blank control group and the negative control group were 4.82±0.10 and 4.91±0.09, respectively, which were significantly higher than that of 4.41±0.06 in miR-1243 mimic transfected group (P=0.032). The proliferation ability of TPC-1 cells transfected with miR-1243 was significantly inhibited. The number of migratory cells in the miR-1243 mimic group was 15.5±6.5, which was significantly less than that in the blank control group (33.5±9.5) and the negative control group (31.75±10.75) (P=0.016). The bioinformatics analysis revealed that the AKT serine/threonine kinase 1 (AKT1) mRNA may be the target of miR-1243. The results of dual luciferase assay showed that WT 3’ untranslated region (UTR) of AKT1 cotransfected with miR-1243 mimics significantly decreased luciferase activity (45.92±10.91) as compared with empty vector group (98.97±9.96), mutant group (85.02±11.04) and negative control mimic group (97.38±10.29) (P=0.005). Conclusion Overexpression of miR-1243 can inhibit the proliferation and migration of TPC-1 cells. The miR-1243 may be involved in the regulation of thyroid papillary carcinoma by direct targeting of AKT1. Key words: Thyroid papillary carcinoma; MicroRNA-124; Cell proliferation; Cell migration; AKT serine / threonine kinase 1

Steroid receptor coactivator-1 interacts with NF-κB to increase VEGFC levels in human thyroid cancer.

Thyroid cancer is the most common endocrine cancer, and has a high incidence of lymphatic metastasis. Vascular endothelial growth factor C (VEGFC) is essential for development of lymphatic vessels and lymphatic metastases during carcinogenesis. Steroid receptor coactivator-1 (SRC-1) interacts with nuclear receptors and transcription factors to promote tumor proliferation and metastasis. However, the correlation between SRC-1 and VEGFC levels in the lymphatic metastases of thyroid cancer remains unclear. We analyzed 20-paired specimens of thyroid cancer tissue and normal thyroid tissue and found increased levels of SRC-1 and VEGFC proteins in 13/20 and 15/20 thyroid cancer specimens, respectively, when compared with those levels in specimens of normal thyroid tissue. A high level of SRC-1 expression was positively correlated with VEGFC and lymphatic endothelial cell marker LYVE-1 expression. Papillary thyroid carcinoma cell line TPC-1 displayed high levels of SRC-1 and VEGFC expression and was selected for stable knockdown of SRC-1 in vitro. Inhibition of SRC-1 significantly reduced the VEGFC levels in TPC-1 cells. We found that SRC-1 binds to transcription factor NF-kB (p50/p65), and that this coactivation complex directly promoted VEGFC transcription, which could be abrogated by SRC-1 knockdown. Up-regulated NF-kB signaling was also confirmed in thyroid cancer tissues. In vivo studies showed that SRC-1 knockdown restricted tumor growth, reduced the numbers of LYVE-1-positive lymphatic vessels, and decreased the levels of VEGFC in tumor tissues. These results suggest a tumorigenic role for SRC-1 in thyroid cancer via its ability to regulate VEGFC expression.

The expression and significance of epithelial cellular adhesion molecule and Claudin-7 in thyroid cancer

Objective To investigate the expression and significance of epithelial cellular adhesion molecule (EpCAM) and Claudin-7 in thyroid cancer cells and tissues. Methods Papillary thyroid carcinoma cell line TPC-1, follicular thyroid carcinoma cell line FTC-133, and undifferentiated thyroid cancer cell lines FRO and ACT-1 were inoculated and incubated for 24 h. The mRNA levels of EpCAM and Claudin-7 in thyroid cancer cell lines were detected by reverse transcriptase-polymerase chain reaction (RT-PCR). Western blotting was used to investigate the protein levels of EpCAM and Claudin-7 in the thyroid cancer cells. The expressions of EpCAM and Claudin-7 in thyroid cancer cells were examined by fluorescence microscope. The expression levels of EpCAM and Claudin-7 in thyroid cancer tissues were tested by immunohistochemistry. Results The mRNA and protein expression levels of EpCAM and Claudin-7 were up-regulated in FRO and ACT-1 cells (Respectively, 18.7%, 29.6% and 81.5%, 70.0%), but there were few expression in TPC-1 and FTC-133 cells. Immunohistochemstry showed the results from fluorescence microscope was in accordance with those from RT-PCR and Western blotting. The postive expression rates of EpCAM and Claudin-7 in the other thyroid cancer cells were 2.6% and 5.2% sperately, and were 64.9% and 81.8% in undifferential thyroid cancer tissue. Conclusion EpCAM and Claudin-7 are highly expressed in undifferentiated thyroid carcinoma, which promotes the occurrence and development of thyroid carcinoma. Key words: Epithelial cellular adhesion molecule; Claudin-7; Thyroid cancer

Effect of human leukocyte antigen-F associated transcript 10 on the proliferation and apoptosis of thyroid carcinoma cells

Objective To observe the effects of human leukocyte antigen (HLA)-F associated transcript 10 (FAT10)ΔGG, a carboxyl-terminal diglycine deficient mutant, on the proliferation and apoptosis of thyroid carcinoma cell line TPC-1. Methods Specimens of cervical carcinoma in situ and normal thyroid tissue, 10 each, were collected. The expression levels of FAT10 protein in these specimens were detected by Western blotting. Site-directed mutagenesis was applied to construct the mutant pcDNA3.0-flag-FAT10ΔGG plasmid. The TPC-1 cells were then transiently transfected with wild-type FAT10, FAT10ΔGG and empty vector (used as negative control), and the wild-type TPC-1 cells served as blank control group. The transfection efficiency of FAT10 or FAT10ΔGG was detected by Western blotting, and cell proliferation was determined by cell counting kit-8 (CCK-8) assay. Cisplatin was used to induce cells after cells were transfected for 24 h, and the cell apoptotic rate was examined by flow cytometry. Results Western blotting showed a significantly increased expression of FAT10 protein in cervical carcinoma tissues as compared with that in normal cervical tissues. Over-expression of wild FAT10 in TPC-1 cells obviously promoted cell proliferation, but this promotion was significantly inhibited in cells transfected with its diglycine mutant. As compared with blank control group [(22.7±4.2)%] and negative control group [(24.1±3.8)%], the apoptotic rate was significantly reduced in wild FAT10 group [(10.9±2.0)%, P 0.05]. Conclusion FAT10 can promote cell proliferation and inhibit cell apoptosis through its carboxyl-terminal diglycine motif, and it may play an essential role in carcinogenesis and development of cancer. Key words: Thyroid carcinoma; Human leukocyte antigen-F associated transcript 10; Apoptosis

Ribonucleotide reductase large subunit M1 plays a different role in the invasion and metastasis of papillary thyroid carcinoma and undifferentiated thyroid carcinoma.

Ribonucleotide reductase (RR) has been reported to be associated with several types of cancer while the expression and role of RR in thyroid carcinoma (TC) has not been investigated. Here, we first examined the expression level of three RR subunit proteins (RRM1, RRM2, and RRM2B) in papillary thyroid carcinoma (PTC) and undifferentiated thyroid carcinoma (UTC) patient samples by immunohistochemistry. The results showed that RRM1 was higher expressed in 95.2% cancer tissues compared with their adjacent normal tissues in 146 PTC samples. The expression level of RRM1 was positively correlated with T stage, lymph node metastasis (LNM), extrathyroidal invasion (ETI), and TNM stage in PTC patients. However, in 12 UTC samples, RRM1 expression was negatively expressed in six cases. To further determine the biological role of RRM1 in TC, ectopic expression or siRNA-mediated knockdown of RRM1 were carried out in the high-differentiated thyroid carcinoma cell line TPC-1 and the poor-differentiated thyroid carcinoma cell line SW579, respectively. In TPC-1 and SW579 cells, overexpression and siRNA knockdown of RRM1 demonstrated that RRM1 promoted DNA synthesis and proliferation in both cell lines as shown by EdU incorporation and cell viability assays. However, RRM1 enhanced cell migration and invasion in TPC-1 cells but inhibited that in SW579 cells as shown by wound healing and transwell assays. Moreover, we also found that RRM1 promoted PTEN expression and reduced Akt phosphorylation in a RR-activity-independent manner in the low-differentiated TC cells but not in the high-differentiated TC cells. In contrast, RRM2 expression was higher expressed in both PTC and UTC patient samples, consisting with its oncogenic role in other cancers. Therefore, we suggest that RRM1 promotes thyroid carcinoma proliferation as a component of RR but may play a different role in the invasion and metastasis of differently differentiated thyroid carcinomas through a non-RR pathway, which could be meaningful to precision treatment of thyroid carcinoma with RR inhibitors.

Effects of iodine intake on the orthotopic nude mouse model of human papillary thyroid carcinoma cell line TPC-1

Objective To observe the effects of different iodine intake on the thyroid tumor growth and thyroid function in the orthotopic nude mice model of human papillary thyroid carcinoma (PTC) cell line TPC-1.Methods Human PTC cell line TPC-1 (2 × 105) was injected into the left thyroid gland of nude mice.After the operation,the nude mice were randomly divided into three groups:low iodine group (LI),normal iodine group (NI),and high iodine group(HI,50 folds of normal iodine) based on the iodine levels contained in their diet.4 and 12 weeks later,the nude mice were executed,then their thyroid tumors were removed and weighted.The levels of urinary iodine were measured with As3+-Ce4+ catalytic spectrophotometry using ammonium persulfate digestion method.The thyroid hormone level was detected using chemiluminescent immunoassay.The morphology and structure of thyroid tumor tissue was observed by microscope.Results The iodine intervention feeding was successful according to urinary iodine level of LI,NI,and HI groups,paralleled to their iodine intakes.However,the difference of the weight of thyroid tumor in three groups had no statistical significance(P＞0.05).At 4 weeks,compared with control group,the levels of thyroid hormones were normal in NI group,while lower T4 and normal T3 were found in LI group.However,T4 was higher and T3 was lower in HI group (P＜0.05).At 12 weeks,the levels of thyroid hormones all were decreased due to the enlargement of thyroid gland tumor in NI,LI and HI groups.T4 and T3 in LI group were the lowest among three groups,even T4 was below detection limit.T4 was normal and T3 was lower in HI group as compared to control group.Conclusion Iodine intake may not significantly affect tumor growth in the orthotopic nude mice model of human PTC cell line TPC-1,but it has a significant effect on the synthesis of thyroid hormones. Key words: Iodine ; Papillary thyroid carcinoma ; Orthotopic model

Glutathione peroxidase 3 promoter region hypermethylation and loss of expression in papillary thyroid carcinomas

Objective To explore the station of glutathione peroxidase 3 (GPX3) promoter region methylation in papillary thyroid carcinoma (PTC) and analyze the relationship between the expression of GPX3 and its methylation.Methods Methylation-specific polymerase chain reaction (MSP) was employed to detect GPX3 gene promoter region methylation in 47 papillary thyroid carcinoma samples,10 non-cancerous tissues and the papillary thyroid carcinoma cell line TPC-1.By using reverse transcription polymerase chain reaction (RT-PCR) the mRNA expression of GPX3 wad detected in TPC-1 cells.Results 48.9％ (23/47) of PTC was methylated,while none of the 10 non-cancerous tissues was methylated.The promoter of GPX3 was hypermethylated in TPC-1 cells,and after treatment of TPC-1 with 5-aza-2' -deoxycytidine for 96 h,the mRNA of GPX3 was re-expressed.Conclusion GPX3 promoter region methylation probably become PTC diagnostic marker.GPX3 promoter hypermethylation is an important epigenetic regulation mechanism of its mRNA expression. Key words: DNA methylation; Glutathione peroxidase 3; Thyroid carcinoma

Activated Leukocyte Cell Adhesion Molecule Expression and Shedding in Thyroid Tumors

Activated leukocyte cell adhesion molecule (ALCAM, CD166) is expressed in various tissues, cancers, and cancer-initiating cells. Alterations in expression of ALCAM have been reported in several human tumors, and cell adhesion functions have been proposed to explain its association with cancer. Here we documented high levels of ALCAM expression in human thyroid tumors and cell lines. Through proteomic characterization of ALCAM expression in the human papillary thyroid carcinoma cell line TPC-1, we identified the presence of a full-length membrane-associated isoform in cell lysate and of soluble ALCAM isoforms in conditioned medium. This finding is consistent with proteolytically shed ALCAM ectodomains. Nonspecific agents, such as phorbol myristate acetate (PMA) or ionomycin, provoked increased ectodomain shedding. Epidermal growth factor receptor stimulation also enhanced ALCAM secretion through an ADAM17/TACE-dependent pathway. ADAM17/TACE was expressed in the TPC-1 cell line, and ADAM17/TACE silencing by specific small interfering RNAs reduced ALCAM shedding. In addition, the CGS27023A inhibitor of ADAM17/TACE function reduced ALCAM release in a dose-dependent manner and inhibited cell migration in a wound-healing assay. We also provide evidence for the existence of novel O-glycosylated forms and of a novel 60-kDa soluble form of ALCAM, which is particularly abundant following cell stimulation by PMA. ALCAM expression in papillary and medullary thyroid cancer specimens and in the surrounding non-tumoral component was studied by western blot and immunohistochemistry, with results demonstrating that tumor cells overexpress ALCAM. These findings strongly suggest the possibility that ALCAM may have an important role in thyroid tumor biology.

RET-Mediated Cell Adhesion and Migration Require Multiple Integrin Subunits

The RET receptor tyrosine kinase is an important mediator of several human diseases, most notably of neuroendocrine cancers. These diseases are characterized by aberrant cell migration, a process tightly regulated by integrins. Our goals were to investigate the role of integrins in RET-mediated migration in two neoplastic cell models: the neural-derived cell line SH-SY5Y, and the papillary thyroid carcinoma cell line TPC-1. We also evaluated whether multiple integrin subunits have a role in RET-mediated cell migration. We evaluated the expression and activation of integrins in response to RET activation using standard cell adhesion and migration (wound-healing) assays. We examined focal adhesion formation, using integrin-paxillin coimmunoprecipitations and immunofluorescence, as an indicator of integrin activity. Our data indicate that β1 integrin (ITGB1) is expressed in both SH-SY5Y and TPC-1 cell lines and that these cells adhere strongly to matrices preferentially associated with ITGB1. We showed that RET can activate ITGB1, and that RET-induced cell adhesion and migration require ITGB1. Furthermore, we showed that β3 integrin (ITGB3) also plays a role in RET-mediated cell adhesion and migration in vitro and ITGB3 expression correlates with RET-mediated invasion in a mouse tumor xenograft model, suggesting that RET mediates the activity of multiple integrin subunits. Our data are the first to show that multiple integrin subunits contribute to cell adhesion and migration downstream of RET, suggesting that coordinated signaling through these pathways is important for cell interactions with the microenvironment during tumor invasion and progression.

Role of COX-2, thromboxane A2 synthase, and prostaglandin I2 synthase in papillary thyroid carcinoma growth

The development of papillary thyroid carcinoma is influenced by many factors including genetic alterations, growth factors, and physical agents such as radiation. Arachidonic acid and its derivatives including prostaglandins (PG) and thromboxane along with the enzymes involved in their synthesis have been shown to influence the growth of various tumors. We analyzed the immunoreactivity for cyclooxygenase-2 (COX-2) and mRNA expression levels of the enzymes COX-2, thromboxane A2 (TXA2) synthase, and PGI2 synthase by RT-PCR in papillary carcinomas and matching normal tissues to determine the role of these enzymes in the development of papillary thyroid carcinomas. A papillary thyroid carcinoma cell line TPC-1 was also studied in vitro to determine the role of the specific COX-2 inhibitor NS-398 on COX-2 and vascular endothelial growth factor-A, since COX-2 also has a role in regulating tumor angiogenesis. RT-PCR analysis showed significant increases in TXA2 synthase mRNA levels in papillary thyroid carcinomas compared to normal thyroid tissues. Although COX-2 mRNA levels were generally increased in papillary carcinomas, the differences were not statistically significant. There were no significant differences in PGI2 synthase mRNA levels. COX-2 protein expression was greater in papillary carcinoma compared to normal thyroid tissues; however, the levels were quite variable. In vitro studies with a COX-2 inhibitor, NS-398, showed inhibition of tumor growth along with increased levels of COX-2 and vascular endothelial growth factor-A mRNA expression. These results indicate that specific enzyme levels in the PG synthesis pathway such as TXA2 synthase are increased in papillary thyroid carcinomas. COX-2 also has a role in papillary thyroid growth, since a specific inhibitor of COX-2 regulates papillary thyroid carcinoma cell proliferation. These results implicate several enzymes in the synthesis of prostanoids as regulators of thyroid papillary carcinoma proliferation and suggest that increased levels of expression of these enzymes may play a role in the pathogenesis of these tumors.

Inactivation of Ret/Ptc1 oncoprotein and inhibition of papillary thyroid carcinoma cell proliferation by indolinone RPI-1.

Genetic alterations causing oncogenic activation of the RET gene are recognized as pathogenic events in papillary and medullary thyroid carcinomas. Inhibition of Ret oncoprotein functions could thereby represent a specific therapeutic approach. We previously described the inhibitory activity of the 2-indolinone derivative RPI-1 (formerly Cpdl) on the tyrosine kinase activity and transforming ability of the products of the RET/PTC1 oncogene exogenously expressed in murine cells. In the present study, we investigated the effects of RPI-1 in the human papillary thyroid carcinoma cell line TPC-1 spontaneously harboring the RET/PTC1 rearrangement. Treatment with RPI-1 inhibited cell proliferation and induced accumulation of cells at the G2 cell cycle phase. In treated cells, Ret/Ptc1 tyrosine phosphorylation was abolished along with its binding to Shc and phospholipase C(gamma), thereby indicating abrogation of constitutive signaling mediated by the oncoprotein. Activation of JNK2 and AKT was abolished, thus supporting the drug inhibitory efficacy on downstream pathways. In addition, cell growth inhibition was associated with a reduction in telomerase activity by nearly 85%. These findings in a cellular context relevant to the pathological function of RET oncogenes support the role of Ret oncoproteins as useful targets for therapeutic intervention, and suggest RPI-1 as a promising candidate for preclinical development in the treatment of thyroid tumors expressing RET oncogenes.