Rearranged During Transfection Protein Research Articles

Discovery and Synthesis of Heterobifunctional Degraders of Rearranged during Transfection (RET) Kinase.

We describe the design, synthesis, and structure-activity relationship (SAR) of heterobifunctional RET ligand-directed degraders (LDDs) derived from three different second-generation RET inhibitors. These LDDs are composed of a target binding motif (TBM) that binds to the RET protein, a linker, and a cereblon binding motif (CBM) as the E3 ligase recognition unit. This led to the identification of a series of pyrazolopyridine-based heterobifunctional LDDs, as exemplified by compound 39. LDD 39 demonstrated high in vitro inhibitory and degradation potency against both RET wild-type and the two representative mutants, V804M and G810R. Importantly, in PK/PD studies, 39 exhibited a differentiated and favorable in vivo profile compared to the corresponding tyrosine kinase inhibitor (TKI), compound 3. Robust and sustained degradation of total-RET (tRET) protein and inhibition of phospho-RET (pRET) signaling were observed in TPC-1 xenograft tumors driven by RET and the RET/G810R mutant following a single dose of LDD 39 at 15 and 75 mg/kg, respectively.

Loss of tumor suppressor TMEM127 drives RET-mediated transformation through disrupted membrane dynamics

Internalization from the cell membrane and endosomal trafficking of receptor tyrosine kinases (RTKs) are important regulators of signaling in normal cells that can frequently be disrupted in cancer. The adrenal tumor pheochromocytoma (PCC) can be caused by activating mutations of the rearranged during transfection (RET) receptor tyrosine kinase, or inactivation of TMEM127, a transmembrane tumor suppressor implicated in trafficking of endosomal cargos. However, the role of aberrant receptor trafficking in PCC is not well understood. Here, we show that loss of TMEM127 causes wildtype RET protein accumulation on the cell surface, where increased receptor density facilitates constitutive ligand-independent activity and downstream signaling, driving cell proliferation. Loss of TMEM127 altered normal cell membrane organization and recruitment and stabilization of membrane protein complexes, impaired assembly, and maturation of clathrin-coated pits, and reduced internalization and degradation of cell surface RET. In addition to RTKs, TMEM127 depletion also promoted surface accumulation of several other transmembrane proteins, suggesting it may cause global defects in surface protein activity and function. Together, our data identify TMEM127 as an important determinant of membrane organization including membrane protein diffusability and protein complex assembly and provide a novel paradigm for oncogenesis in PCC where altered membrane dynamics promotes cell surface accumulation and constitutive activity of growth factor receptors to drive aberrant signaling and promote transformation.

Loss of tumor suppressor TMEM127 drives RET-mediated transformation through disrupted membrane dynamics.

Internalization from the cell membrane and endosomal trafficking of receptor tyrosine kinases (RTKs) are important regulators of signaling in normal cells that can frequently be disrupted in cancer. The adrenal tumor pheochromocytoma (PCC) can be caused by activating mutations of the rearranged during transfection (RET) receptor tyrosine kinase, or inactivation of TMEM127, a transmembrane tumor suppressor implicated in trafficking of endosomal cargos. However, the role of aberrant receptor trafficking in PCC is not well understood. Here, we show that loss of TMEM127 causes wildtype RET protein accumulation on the cell surface, where increased receptor density facilitates constitutive ligand-independent activity and downstream signaling, driving cell proliferation. Loss of TMEM127 altered normal cell membrane organization and recruitment and stabilization of membrane protein complexes, impaired assembly, and maturation of clathrin-coated pits, and reduced internalization and degradation of cell surface RET. In addition to RTKs, TMEM127 depletion also promoted surface accumulation of several other transmembrane proteins, suggesting it may cause global defects in surface protein activity and function. Together, our data identify TMEM127 as an important determinant of membrane organization including membrane protein diffusability and protein complex assembly and provide a novel paradigm for oncogenesis in PCC where altered membrane dynamics promotes cell surface accumulation and constitutive activity of growth factor receptors to drive aberrant signaling and promote transformation.

Abstract 4039: RET overexpression is frequent in lung neuroendocrine tumors (NET) and associates with response to RET tyrosine kinase inhibitors (RET TKIs) in NET cell lines

Abstract Background: RET (rearranged during transfection) is a proto-oncogene encoding for a tyrosine kinase membrane receptor that promotes protein synthesis and cell proliferation. A small percentage of human tumors harbor RET fusions or mutations as oncogenic drivers and RET-TKIs have been developed. Two of them, selpercatinib and pralsetinib, have recently been approved. However, their effect on tumors overexpressing wild-type RET (wt-RET) has not been investigated. A large-scale mRNA analysis performed by our group revealed that ~2% of lung tumors present high expression of wt-RET in absence of fusions. Interestingly, ~60% of high wt-RET tumors presented neuroendocrine characteristics. Methods: Twenty large cell neuroendocrine (LCNEC) and 19 small cell (SCLC) lung carcinomas from two additional institutions were analyzed by nCounter® to validate the previous results. For in vitro experiments, a panel of cell lines derived from lung NETs (DMS-53, NCI-H82, NCI-H727) was used, together with a lung adenocarcinoma cell line (LC2/ad, a positive control harboring the CDCC6-RET translocation). RET fusions and mRNA expression were analyzed by nCounter®, RET protein expression by Western blotting and immunohistochemistry (IHC) and RET mutations by next generation sequencing. The inhibitory effects of selpercatinib and pralsetinib were analyzed in vitro by MTT in 2D and spheroid/3D cultures. Finally, in vivo experiments implanting tumor cells into athymic nude mice are ongoing. Results: Among the 20 LCNECs and 19 SCLCs analyzed, 3 (15%) and 3 (15%) presented very high levels of wt-RET mRNA by nCounter, respectively. Fusions were absent in all cases. The NCI-H82 and NCI-H727 NET cell lines showed very low RET mRNA and protein expression by nCounter, Western blotting and IHC. In contrast, DMS-53 cells presented levels of RET mRNA and protein higher than the observed in the RET-fusion positive LC2/ad. In 2D models, the three NET cell lines tested showed moderate resistance to pralsetinib and selpercatinib, with IC50s of 1.5 - 9.7 µM for pralsetinib and 4.6 - 6.1 µM for selpercatinib; while the RET-fusion positive LC2/ad cells showed an IC50 in the nM range, as expected. In 3D cultures, DMS-53 was found to be sensitive to RET TKIs, with IC50s of and 0.3 µM for praseltinib and 2.5 µM for selpercatinib. In contrast, spheroids of the two RET low cell lines showed complete resistance to both drugs (IC50>10 µM). Experiments in animal models are currently ongoing, final results will be presented at the meeting. Conclusions: Overexpression of RET is frequent in lung tumors with neuroendocrine features (LCNEC, SCLC). Selpercatinib and Pralsetinib show antitumor activity in lung NETs cells expressing high levels of wt-RET. Citation Format: Jaume Roca-Arias, Jordi Bertran-Alamillo, Ruth Román, Cristina Aguado, Laura López, Leticia Ferro-Leal, Beatriz Garcia-Pelaez, Rodrigo de Oliveira Cavagna, Silvia Teixeira, Rafael Rosell, Rui Manuel Reis, Miguel A. Molina-Vila, Ivana Sullivan. RET overexpression is frequent in lung neuroendocrine tumors (NET) and associates with response to RET tyrosine kinase inhibitors (RET TKIs) in NET cell lines. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4039.

Progress and challenges in RET-targeted cancer therapy.

The rearranged during transfection (RET) is a receptor protein tyrosine kinase. Oncogenic RET fusions or mutations are found most often in non-small cell lung cancer (NSCLC) and in thyroid cancer, but also increasingly in various types of cancers at low rates. In the last few years, two potent and selective RET protein tyrosine kinase inhibitors (TKIs), pralsetinib (BLU-667) and selpercatinib (LOXO-292, LY3527723) were developed and received regulatory approval. Although pralsetinib and selpercatinib gave high overall response rates (ORRs), < 10% of patients achieved a complete response (CR). The RET TKI-tolerated residual tumors inevitably develop resistance by secondary target mutations, acquired alternative oncogenes, or MET amplification. RET G810 mutations located at the kinase solvent front site were identified as the major on-target mechanism of acquired resistance to both selpercatinib and pralsetinib. Several next-generation of RET TKIs capable of inhibiting the selpercatinib/pralsetinib-resistant RET mutants have progressed to clinical trials. However, it is likely that new TKI-adapted RET mutations will emerge to cause resistance to these next-generation of RET TKIs. Solving the problem requires a better understanding of the multiple mechanisms that support the RET TKI-tolerated persisters to identify a converging point of vulnerability to devise an effective co-treatment to eliminate the residual tumors.

Designing Novel Compounds for the Treatment and Management of RET-Positive Non-Small Cell Lung Cancer-Fragment Based Drug Design Strategy.

Rearranged during transfection (RET) is an oncogenic driver receptor that is overexpressed in several cancer types, including non-small cell lung cancer. To date, only multiple kinase inhibitors are widely used to treat RET-positive cancer patients. These inhibitors exhibit high toxicity, less efficacy, and specificity against RET. The development of drug-resistant mutations in RET protein further deteriorates this situation. Hence, in the present study, we aimed to design novel drug-like compounds using a fragment-based drug designing strategy to overcome these issues. About 18 known inhibitors from diverse chemical classes were fragmented and bred to form novel compounds against RET proteins. The inhibitory activity of the resultant 115 hybrid molecules was evaluated using molecular docking and RF-Score analysis. The binding free energy and chemical reactivity of the compounds were computed using MM-GBSA and density functional theory analysis, respectively. The results from our study revealed that the developed hybrid molecules except for LF21 and LF27 showed higher reactivity and stability than Pralsetinib. Ultimately, the process resulted in three hybrid molecules namely LF1, LF2, and LF88 having potent inhibitory activity against RET proteins. The scrutinized molecules were then subjected to molecular dynamics simulation for 200 ns and MM-PBSA analysis to eliminate a false positive design. The results from our analysis hypothesized that the designed compounds exhibited significant inhibitory activity against multiple RET variants. Thus, these could be considered as potential leads for further experimental studies.

Discovery of a Potent Candidate for RET-Specific Non-Small-Cell Lung Cancer-A Combined In Silico and In Vitro Strategy.

Rearranged during transfection (RET) is a tyrosine kinase oncogenic receptor, activated in several cancers including non-small-cell lung cancer (NSCLC). Multiple kinase inhibitors vandetanib and cabozantinib are commonly used in the treatment of RET-positive NSCLC. However, specificity, toxicity, and reduced efficacy limit the usage of multiple kinase inhibitors in targeting RET protein. Thus, in the present investigation, we aimed to figure out novel and potent candidates for the inhibition of RET protein using combined in silico and in vitro strategies. In the present study, screening of 11,808 compounds from the DrugBank repository was accomplished by different hypotheses such as pharmacophore, e-pharmacophore, and receptor cavity-based models in the initial stage. The results from the different hypotheses were then integrated to eliminate the false positive prediction. The inhibitory activities of the screened compounds were tested by the glide docking algorithm. Moreover, RF score, Tanimoto coefficient, prime-MM/GBSA, and density functional theory calculations were utilized to re-score the binding free energy of the docked complexes with high precision. This procedure resulted in three lead molecules, namely DB07194, DB03496, and DB11982, against the RET protein. The screened lead molecules together with reference compounds were then subjected to a long molecular dynamics simulation with a 200 ns time duration to validate the inhibitory activity. Further analysis of compounds using MM-PBSA and mutation studies resulted in the identification of potent compound DB07194. In essence, a cell viability assay with RET-specific lung cancer cell line LC-2/ad was also carried out to confirm the in vitro biological activity of the resultant compound, DB07194. Indeed, the results from our study conclude that DB07194 can be effectively translated for this new therapeutic purpose, in contrast to the properties for which it was originally designed and synthesized.

RET Protein Expression in Colorectal Cancer; An Immunohistochemical Assessment.

Background:RET (rearranged during transfection) is a transmembrane receptor tyrosine kinase and a receptor for the GDNF-family ligands. It plays the role of a tumor suppressor in colorectal cancer. Therefore, it is expected that RET gene becomes downregulated in colorectal cancer (CRC). In this study, we evaluated immuno-histochemical expression of RET in CRC and assessed its correlation with some of the clinicopathological features to study the prognostic value in CRC. Materials and Methods:In total, 60 cases of colorectal cancer (CRC) from the patients who underwent surgical gastroenterology operations were randomly selected. The samples included one tumor-rich section per case and one adjacent tumor-free section as the normal control for that case. Then, immunohistochemistry (ICH) was performed for RET on all the samples and the expression of RET was analyzed. Furthermore, the correlation of RET with clinicopathological features including age, gender, location of the tumor, grade, and stage was evaluated. Results:The expression of RET caused significant downregulation in cancer samples compared to the normal control ones (P = 0.002). This downregulation increased in correlation to both grade and metastasis to lymph nodes (P = 0.03 & 0.02 respectively). However, no correlation was found between the expression of RET and gender as well as location of the tumor. Conclusion: RET may be considered as a protein marker in CRC detection and prognosis.

Characterization of LDD-2633 as a Novel RET Kinase Inhibitor with Anti-Tumor Effects in Thyroid Cancer.

Rearranged during transfection (RET), a receptor tyrosine kinase, is activated by glial cell line-derived neurotrophic factor family ligands. Chromosomal rearrangement or point mutations in RET are observed in patients with papillary thyroid and medullary thyroid carcinomas. Oncogenic alteration of RET results in constitutive activation of RET activity. Therefore, inhibiting RET activity has become a target in thyroid cancer therapy. Here, the anti-tumor activity of a novel RET inhibitor was characterized in medullary thyroid carcinoma cells. The indirubin derivative LDD-2633 was tested for RET kinase inhibitory activity. In vitro, LDD-2633 showed potent inhibition of RET kinase activity, with an IC50 of 4.42 nM. The growth of TT thyroid carcinoma cells harboring an RET mutation was suppressed by LDD-2633 treatment via the proliferation suppression and the induction of apoptosis. The effects of LDD-2633 on the RET signaling pathway were examined; LDD-2633 inhibited the phosphorylation of the RET protein and the downstream molecules Shc and ERK1/2. Oral administration of 20 or 40 mg/kg of LDD-2633 induced dose-dependent suppression of TT cell xenograft tumor growth. The in vivo and in vitro experimental results supported the potential use of LDD-2633 as an anticancer drug for thyroid cancers.

Expression of RET is associated with Oestrogen receptor expression but lacks prognostic significance in breast cancer

BackgroundThe Rearranged during Transfection (RET) protein is overexpressed in a subset of Estrogen Receptor (ER) positive breast cancer, with both signalling pathways functionally interacting. This cross-talk plays a pivotal role in the resistance of breast cancer cells to anti-endocrine therapies, and RET expression is assumed to correlate with poor prognosis based on findings in small patient cohorts. The aim of our study was to investigate the impact of RET expression on patient outcome in human breast cancer.MethodsWe performed an immunohistochemical analysis of RET protein expression on a tissue microarray encompassing 990 breast cancer patients and correlated its expression with clinicopathological parameters and survival data.ResultsExpression of RET was detected in 409 out of 990 cases (41.3%). RET and ER expression significantly correlated (p < 0.0001). The Luminal B HER2-positive subtype showed the highest expression rate (48.9%). In univariate and multivariate survival analyses, RET expression had no impact on overall survival.ConclusionWe confirmed the co-expression of RET and ER, but we did not find RET expression to be an independent prognostic factor in human breast cancer. Clinical trials with newly developed RET inhibitors are needed to evaluate if RET inhibition has a beneficial impact on patient survival in ER positive breast cancer.

Abstract 1842: Novel KIF5B-RET+ NSCLC cell lines demonstrate differential responses to RET inhibitors

Abstract Chromosomal rearrangements involving the rearranged during transfection (RET) tyrosine kinase occur in 1-2% of NSCLCs. This results in the expression of a constitutively active fusion kinase that promotes cancer cell growth, proliferation and survival. In lung cancer, RET is most commonly fused to KIF5B, but can have many other 5' partners such as CCDC6, NCOA, TRIM33 and TRIM24. Clinical trials with several multi-kinase inhibitors, which potently inhibit RET, have had low response rates between 18-53%. Novel, more highly specific RET inhibitors are now in clinical development. Collectively, the clinical trial data suggest that patients with KIF5B-RET rearrangements have exceptionally low responses to RET inhibitors in relation to non-KIF5B-RET fusions. In order to understand the role of RET 5' partners in regulating response to RET inhibitors we have generated two novel KIF5B-RET+ patient-derived cell lines, CUTO22 (K23;R12) and CUTO32 (K15;R12). We have confirmed that each of these cell lines contain the KIF5B-RET fusion and express the RET fusion protein. MTS proliferation assays have revealed that the CUTO22 cell line is very sensitive to the highly specific RET inhibitor, CEP-32496/RXDX-105, and the multi-kinase inhibitor ponatinib. Conversely, the CUTO32 cell line is dramatically resistant to both RXDX-105 and ponatinib. Phospho-RET (pRET) is inhibited by RXDX-105 and ponatinib in a dose-dependent manner in both the CUTO22 and CUTO32 cell line. Interestingly, despite successful RET inhibition, downstream signaling in pERK and pAKT remains active. In a time-course experiment, pRET is inhibited for up to 24 hours while pERK and pAKT is sustained. Additionally, knockdown of RET with siRNA did not reduce downstream signaling activation in CUTO22 or CUTO32 cells. Combined inhibition of possible bypass pathways, EGFR or mTOR, did not significantly enhance sensitivity to RET inhibitors in CUTO22 and CUTO32 cell lines. To isolate the role of the 5' partners in response to RET inhibitors we have utilized CRISPR/Cas9 technology to generate Kif5b-Ret and Trim24-Ret rearrangements in Ba/F3 cells. We have confirmed with PCR or RT-PCR that these cell lines harbor the intended Ret fusions and express the RET protein. Both the Kif5b-Ret+ and Trim24-Ret+ Ba/F3 cell lines are highly sensitive to multiple RET inhibitors. In conclusion, these data suggest that RET inhibitors successfully inhibit the target protein, but not downstream signaling, suggesting that KIF5B-RET may have unique co-alterations or reliance on alternative pathways to promote growth and proliferation. Citation Format: Laura Schubert, Anh T. Le, Andrea E. Doak, Robert C. Doebele. Novel KIF5B-RET+ NSCLC cell lines demonstrate differential responses to RET inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1842.

Abstract 4783: Selective antitumor activity of NSC311152 toward medullary thyroid cancer by downregulating the transcription of RET oncogene

Abstract The RET (REarranged during Transfection) proto-oncogene encodes a tyrosine-kinase receptor specifically expressed in tissues originated from neuroectodermal tissues. Dominant-activating mutations in this RET proto-oncogene is often associated with Multiple Endocrine Neoplasia type 2A and 2B and Familial Medullary Thyroid Carcinoma syndromes, characterized by the occurrence of medullary thyroid carcinomas. Human medullary thyroid carcinoma cell line TT cell line harbours a MEN2A-type mutation, so this cell line could be a useful cell system to investigate the antitumor activity of new agents interfering with the function of RET oncogene in medually thyroid cancer. Recently, ellipticine and its derivatives were identified as excellent RET G-quadruplex stabilizing agents. Furthermore, these compounds also interfered with the transcriptional mechanism of the RET gene in TT cells, subsequently decreasing the endogenous RET protein expression. In present study, we demonstrated the specificity of NSC311152 by using papillary thyroid carcinoma (PTC) cells, the K1, which carry BRAF and PIK3CA mutations but lacks the dominant-activating mutations in this RET proto-oncogene. NSC311152 selectively suppresses cell proliferation by inhibiting the intracellular Raf/MEK/ERK and PI3K/Akt/mTOR signaling pathways in the TT cells. The systemic administration of NSC311152 in a mouse MTC xenograft model significantly inhibited the tumor growth through RET downregulation, whereas NSC31152 only showed a moderate antitumor activity against K1 xenograft tumors. Citation Format: Daekyu Sun, Vishnu Muthuraj Kumarasamy, Tariq Alqahtani. Selective antitumor activity of NSC311152 toward medullary thyroid cancer by downregulating the transcription of RET oncogene [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4783.

Demonstration of a potent RET transcriptional inhibitor for the treatment of medullary thyroid carcinoma based on an ellipticine derivative.

Dominant-activating mutations in the RET (rearranged during transfection) proto-oncogene, which encodes a receptor tyrosine kinase, is often associated with the development of medullary thyroid carcinoma (MTC). The proximal promoter region of the RET gene consists of a guanine-rich sequence containing five runs of three consecutive guanine residues that serve as the binding site for transcriptional factors. As we have recently shown, this stretch of nucleotides in the promoter region is highly dynamic in nature and tend to form non-B DNA secondary structures called G-quadruplexes, which suppress the transcription of the RET gene. In the present study, ellipticine and its derivatives were identified as excellent RET G-quadruplex stabilizing agents. Circular dichroism (CD) spectroscopic studies revealed that the incorporation of a piperidine ring in an ellipticine derivative, NSC311153 improves its binding with the G-quadruplex structure and the stability induced by this compound is more potent than ellipticine. Furthermore, this compound also interfered with the transcriptional mechanism of the RET gene in an MTC derived cell line, TT cells and significantly decreased the endogenous RET protein expression. We demonstrated the specificity of NSC311153 by using papillary thyroid carcinoma (PTC) cells, the TPC1 cell line which lacks the G-quadruplex forming sequence in the promoter region due to chromosomal rearrangement. The RET downregulation selectively suppresses cell proliferation by inhibiting the intracellular Raf/MEK/ERK and PI3K/Akt/mTOR signaling pathways in the TT cells. In the present study, we also showed that the systemic administration of a water soluble NSC311153 analog in a mouse MTC xenograft model inhibited the tumor growth through RET downregulation.

Exon Skipping in the RET Gene Encodes Novel Isoforms That Differentially Regulate RET Protein Signal Transduction

Rearranged during transfection (RET), a receptor tyrosine kinase that is activated by the glial cell line-derived neurotrophic factor family ligands (GFLs), plays a crucial role in the development and function of the nervous system and additionally is required for kidney development and spermatogenesis. RET encodes a transmembrane receptor that is 20 exons long and produces two known protein isoforms differing in C-terminal amino acid composition, referred to as RET9 and RET51. Studies of human pheochromocytomas identified two additional novel transcripts involving the skipping of exon 3 or exons 3, 4, and 5 and are referred to as RET(Δ) (E3) and RET(Δ) (E345), respectively. Here we report the presence of Ret(Δ) (E3) and Ret(Δ) (E345) in zebrafish, mice, and rats and show that these transcripts are dynamically expressed throughout development of the CNS, peripheral nervous system, and kidneys. We further explore the biochemical properties of these isoforms, demonstrating that, like full-length RET, RET(ΔE3) and RET(ΔE345) are trafficked to the cell surface, interact with all four GFRα co-receptors, and have the ability to heterodimerize with full-length RET. Signaling experiments indicate that RET(ΔE3) is phosphorylated in a similar manner to full-length RET. RET(ΔE345), in contrast, displays higher baseline autophosphorylation, specifically on the catalytic tyrosine, Tyr(905), and also on one of the most important signaling residues, Tyr(1062) These data provide the first evidence for a physiologic role of these isoforms in RET pathway function.

Selective Mitochondrial Uptake of MKT-077 Can Suppress Medullary Thyroid Carcinoma Cell Survival In Vitro and In Vivo.

BackgroundMedullary thyroid carcinoma (MTC) is a neuroendocrine tumor mainly caused by mutations in the rearranged during transfection (RET) proto-oncogene. Not all patients with progressive MTC respond to current therapy inhibiting RET, demanding additional therapeutic strategies. We recently demonstrated that disrupting mitochondrial metabolism using a mitochondria-targeted agent or by depleting a mitochondrial chaperone effectively suppressed human MTC cells in culture and in mouse xenografts by inducing apoptosis and RET downregulation. These observations led us to hypothesize that mitochondria are potential therapeutic targets for MTC. This study further tests this hypothesis using1-ethyl-2-[[3-ethyl-5-(3-methylbenzothiazolin-2-yliden)]-4-oxothiazolidin-2-ylidenemethyl] pyridinium chloride (MKT-077), a water-soluble rhodocyanine dye analogue, which can selectively accumulate in mitochondria.MethodsThe effects of MKT-077 on cell proliferation, survival, expression of RET and tumor protein 53 (TP53), and mitochondrial activity were determined in the human MTC lines in culture and in mouse xenografts.ResultsMKT-077 induced cell cycle arrest in TT and MZ-CRC-1. Intriguingly, MKT-077 also induced RET downregulation and strong cell death responses in TT cells, but not in MZ-CRC-1 cells. This discrepancy was mainly due to the difference between the capacities of these cell lines to retain MKT-077 in mitochondria. The cytotoxicity of MKT-077 in TT cells was mainly attributed to oxidative stress while being independent of TP53. MKT-077 also effectively suppressed tumor growth of TT xenografts.ConclusionMKT-077 can suppress cell survival of certain MTC subtypes by accumulating in mitochondria and interfering with mitochondrial activity although it can also suppress cell proliferation via other mechanisms. These results consistently support the hypothesis that mitochondrial targeting has therapeutic potential for MTC.

Involvement of G-quadruplex structures in regulation of human RET gene expression by small molecules in human medullary thyroid carcinoma TT cells

The RET (rearranged during transfection) proto-oncogene encodes a receptor tyrosine kinase for members of the glial cell line-derived neurotrophic factor family of extracellular signaling molecules. The activating germline point mutations in the RET, which are known to induce oncogenic activation of RET tyrosine kinase, are associated with the development of medullary thyroid carcinoma (MTC) and pathogenesis of multiple endocrine neoplasia type 2 (MEN2). The polypurine/polypyrimidine tract in the proximal promoter region of the human RET gene (-51 to -33 relative to transcription start site) is essential for basal transcriptional activity of this gene. This tract consists of a guanine-rich sequence containing five runs of at least three contiguous guanines separated by one or more bases, conforming to a general motif capable of forming an intramolecular G-quadruplex. Here, we show that specific G-quadruplex structures formed in the RET promoter region act to repress the transcription of this gene, and transcription of this gene can be controlled by ligand-mediated G-quadruplex stabilization. In this study, NSC194598, a derivative of indeno[1,2,3-de]quinazoline, was found to be a novel G-quadruplex interactive agent that interfered with transcriptional activation of mutated RET gene in human medullary thyroid carcinoma TT cells. This compound significantly reduced endogenous RET protein levels and increased apoptosis in these cells. Our results provide further support for the idea that G-quadruplex structures may have a critical role in transcriptional regulation of the RET gene in vivo, providing insight into a novel strategy for transcriptional repression of this gene by small molecules.

An extended family with familial medullary thyroid carcinoma and Hirschsprung's disease.

Familial medullary thyroid carcinoma (FMTC) is an autosomal dominant inherited disease that has highly characteristic clinical features, including medullary thyroid carcinoma (MTC). Mutation of the RET proto-oncogene is known to be responsible for development of FMTC and for multiple endocrine neoplasia types 2A and 2B. Hirschsprung's disease is the most common form of structural intestinal obstructive disease in human newborns. Hirschsprung's disease is defined by the absence of neural crest-derived enteric ganglia along a variable length of the bowel that invariably involves the rectoanal junction. Co-segregation of FMTC and Hirschsprung's disease is uncommon; nevertheless, in 3 generations of 1 family, we observed 5 patients with FMTC, 2 patients with Hirschsprung's disease, and 1 patient with characteristics of both FMTC and Hirschsprung's disease. Moreover, a Cys620Ser mutation in RET was identified in 4 of the 8 patients. This mutation had both activating and inactivating effects on the RET (REarranged during Transfection) protein. There were individual differences in the penetrance of Hirschsprung's disease due to the RET mutation, but the penetrance of MTC was uniform and high. Genetic testing is important for making decisions about treatment and follow-up in families of this kind.

In silico profiling and structural insights of missense mutations in RET protein kinase domain by molecular dynamics and docking approach

A major challenge remaining in drug design efforts towards protein kinase is due to the development of drug resistance initiated by the missense mutations in the kinase catalytic domain. Gain or loss of function mutations in the REarranged during Transfection (RET) tyrosine kinase gene have been associated with the development of a wide range of human associated cancers and Hirschsprung's disease. However, to what extent these mutations might affect bio-molecular functions remains unclear. In this article, the functionally significant mutations in RET were screened with the aid of various sequence and structure based in silico prediction methods. We mapped the deleterious mutants, modelled mutant proteins and deciphered the impact of mutations on drug binding mechanisms in the RET crystal structure of PDB ID: with the potential inhibitor vandetanib by docking analysis. Furthermore, molecular dynamics simulations were undertaken to understand the mechanistic action of cancer associated mutations in altering the protein kinase structure, dynamics, and stability. According to our results, the overall effect of V804M, M918T and S922Y were destabilizing and mostly alter the electrostatic component of the binding energy. Specifically, the mutation of gatekeeper residue valine 804 present in the ATP binding pocket affects the protein stability and confers resistance to the drug vandetanib, which was consistent with previously published experimental results. Overall, our findings may provide useful structural insights for in-depth understanding of the molecular mechanism underlying RET mutation and developing effective drugs.

Central Role of RET in Thyroid Cancer

RET (rearranged during transfection) is a receptor tyrosine kinase involved in the development of neural crest derived cell lineages, kidney, and male germ cells. Different human cancers, including papillary and medullary thyroid carcinomas, lung adenocarcinomas, and myeloproliferative disorders display gain-of-function mutations in RET. Accordingly, RET protein has become a promising molecular target for cancer treatment.

MK-2206 Causes Growth Suppression and Reduces Neuroendocrine Tumor Marker Production in Medullary Thyroid Cancer Through Akt Inhibition

Development of targeted therapies for medullary thyroid cancer (MTC) has focused on inhibition of the rearranged during transfection (RET) proto-oncogene. Akt has been demonstrated to be a downstream target of RET via the key mediator phosphoinositide-3-kinase. MK-2206 is an orally administered allosteric Akt inhibitor that has exhibited minimal toxicity in phase I trials. We explored the antitumor effects of this compound in MTC. Human MTC-TT cells were treated with MK-2206 (0-20 μM) for 8 days. Assays for cell viability were performed at multiple time points with MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide). The mechanism of action, mechanism of growth inhibition, and production of neuroendocrine tumor markers were assessed with Western blot analysis. MK-2206 suppressed MTC cell proliferation in a dose-dependent manner (p ≤ 0.001). Levels of Akt phosphorylated at serine 473 declined with increasing doses of MK-2206, indicating successful Akt inhibition. The apoptotic proteins cleaved poly (ADP-ribose) polymerase and cleaved caspase-3 increased in a dose-dependent manner with MK-2206, while the apoptosis inhibitor survivin was markedly reduced. Importantly, the antitumor effects of MK-2206 were independent of RET inhibition, as the levels of RET protein were not blocked. MK-2206 significantly suppresses MTC proliferation without RET inhibition. Given its high oral bioavailability and low toxicity profile, phase II studies with this drug alone or in combination with RET inhibitors are warranted.