Regulation of Protein Kinase B Tyrosine Phosphorylation by Thyroid-Specific Oncogenic RET/PTC Kinases

GDNF/RET Signaling Pathway Activation Eliminates Lewy Body Pathology in Midbrain Dopamine Neurons

BACKGROUND

Parkinson's disease (PD) is associated with proteostasis disturbances and accumulation of misfolded α-synuclein (α-syn), a cytosolic protein present in high concentrations at pre-synaptic neuronal terminals. It is a primary constituent of intracellular protein aggregates known as Lewy neurites or Lewy bodies. Progression of Lewy pathology caused by the prion-like self-templating properties of misfolded α-syn is a characteristic feature in the brains of PD patients. Glial cell line-derived neurotrophic factor (GDNF) promotes survival of mature dopamine (DA) neurons in vitro and in vivo. However, the data on its effect on Lewy pathology is controversial.

OBJECTIVES

We studied the effects of GDNF on misfolded α-syn accumulation in DA neurons.

METHODS

Lewy pathology progression was modeled by the application of α-syn preformed fibrils in cultured DA neurons and in the adult mice.

RESULTS

We discovered that GDNF prevented accumulation of misfolded α-syn in DA neurons in culture and in vivo. These effects were abolished by deletion of receptor tyrosine kinase rearranged during transfection (RET) or by inhibitors of corresponding signaling pathway. Expression of constitutively active RET protected DA neurons from fibril-induced α-syn accumulation.

CONCLUSIONS

For the first time, we have shown the neurotrophic factor-mediated protection against the misfolded α-syn propagation in DA neurons, uncovered underlying receptors, and investigated the involved signaling pathways. These results demonstrate that activation of GDNF/RET signaling can be an effective therapeutic approach to prevent Lewy pathology spread at early stages of PD. © 2020 International Parkinson and Movement Disorder Society.

Oncogenes, mitochondrial metabolism, and quality control in differentiated thyroid cancer

Thyroid cancer is one of the most common malignancies of endocrine organs, and its incidence rate has increased steadily over the past several decades. Most differentiated thyroid tumors derived from thyroid epithelial cells exhibit slow-growing cancers, and patients with these tumors can achieve a good prognosis with surgical removal and radioiodine treatment. However, a small proportion of patients present with advanced thyroid cancer and are unusually resistant to current drug treatment modalities. Thyroid tumorigenesis is a complex process that is regulated by the activation of oncogenes, inactivation of tumor suppressors, and alterations in programmed cell death. Mitochondria play an essential role during tumor formation, progression, and metastasis of thyroid cancer. Recent studies have successfully observed the mitochondrial etiology of thyroid carcinogenesis. This review focuses on the recent progress in understanding the molecular mechanisms of thyroid cancer relating to altered mitochondrial metabolism.

Differential regulation of PKD isoforms in oxidative stress conditions through phosphorylation of a conserved Tyr in the P+1 loop

Protein kinases are essential molecules in life and their crucial function requires tight regulation. Many kinases are regulated via phosphorylation within their activation loop. This loop is embedded in the activation segment, which additionally contains the Mg²⁺ binding loop and a P + 1 loop that is important in substrate binding. In this report, we identify Abl-mediated phosphorylation of a highly conserved Tyr residue in the P + 1 loop of protein kinase D2 (PKD2) during oxidative stress. Remarkably, we observed that the three human PKD isoforms display very different degrees of P + 1 loop Tyr phosphorylation and we identify one of the molecular determinants for this divergence. This is paralleled by a different activation mechanism of PKD1 and PKD2 during oxidative stress. Tyr phosphorylation in the P + 1 loop of PKD2 increases turnover for Syntide-2, while substrate specificity and the role of PKD2 in NF-κB signaling remain unaffected. Importantly, Tyr to Phe substitution renders the kinase inactive, jeopardizing its use as a non-phosphorylatable mutant. Since large-scale proteomics studies identified P + 1 loop Tyr phosphorylation in more than 70 Ser/Thr kinases in multiple conditions, our results do not only demonstrate differential regulation/function of PKD isoforms under oxidative stress, but also have implications for kinase regulation in general.

Classifying kinase conformations using a machine learning approach

Background

Signaling proteins such as protein kinases adopt a diverse array of conformations to respond to regulatory signals in signaling pathways. Perhaps the most fundamental conformational change of a kinase is the transition between active and inactive states, and defining the conformational features associated with kinase activation is critical for selectively targeting abnormally regulated kinases in diseases. While manual examination of crystal structures have led to the identification of key structural features associated with kinase activation, the large number of kinase crystal structures (~3,500) and extensive conformational diversity displayed by the protein kinase superfamily poses unique challenges in fully defining the conformational features associated with kinase activation. Although some computational approaches have been proposed, they are typically based on a small subset of crystal structures using measurements biased towards the active site geometry.

Results

We utilize an unbiased informatics based machine learning approach to classify all eukaryotic protein kinase conformations deposited in the PDB. We show that the orientation of the activation segment, measured by φ, ψ, χ1, and pseudo-dihedral angles more accurately classify kinase crystal conformations than existing methods. We show that the formation of the K-E salt bridge is statistically dependent upon the activation segment orientation and identify evolutionary differences between the activation segment conformation of tyrosine and serine/threonine kinases. We provide evidence that our method can identify conformational changes associated with the binding of allosteric regulatory proteins, and show that the greatest variation in inactive structures comes from kinase group and family specific side chain orientations.

Conclusion

We have provided the first comprehensive machine learning based classification of protein kinase active/inactive conformations, taking into account more structures and measurements than any previous classification effort. Further, our unbiased classification of inactive structures reveals residues associated with kinase functional specificity. To enable classification of new crystal structures, we have made our classifier publicly accessible through a stand-alone program housed at https://github.com/esbg/kinconform [DOI:10.5281/zenodo.249090].

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-017-1506-2) contains supplementary material, which is available to authorized users.

KinView: a visual comparative sequence analysis tool for integrated kinome research

Multiple sequence alignments (MSAs) are a fundamental analysis tool used throughout biology to investigate relationships between protein sequence, structure, function, evolutionary history, and patterns of disease-associated variants. However, their widespread application in systems biology research is currently hindered by the lack of user-friendly tools to simultaneously visualize, manipulate and query the information conceptualized in large sequence alignments, and the challenges in integrating MSAs with multiple orthogonal data such as cancer variants and post-translational modifications, which are often stored in heterogeneous data sources and formats. Here, we present the Multiple Sequence Alignment Ontology (MSAOnt), which represents a profile or consensus alignment in an ontological format. Subsets of the alignment are easily selected through the SPARQL Protocol and RDF Query Language for downstream statistical analysis or visualization. We have also created the Kinome Viewer (KinView), an interactive integrative visualization that places eukaryotic protein kinase cancer variants in the context of natural sequence variation and experimentally determined post-translational modifications, which play central roles in the regulation of cellular signaling pathways. Using KinView, we identified differential phosphorylation patterns between tyrosine and serine/threonine kinases in the activation segment, a major kinase regulatory region that is often mutated in proliferative diseases. We discuss cancer variants that disrupt phosphorylation sites in the activation segment, and show how KinView can be used as a comparative tool to identify differences and similarities in natural variation, cancer variants and post-translational modifications between kinase groups, families and subfamilies. Based on KinView comparisons, we identify and experimentally characterize a regulatory tyrosine (Y177PLK4) in the PLK4 C-terminal activation segment region termed the P+1 loop. To further demonstrate the application of KinView in hypothesis generation and testing, we formulate and validate a hypothesis explaining a novel predicted loss-of-function variant (D523NPKCβ) in the regulatory spine of PKCβ, a recently identified tumor suppressor kinase. KinView provides a novel, extensible interface for performing comparative analyses between subsets of kinases and for integrating multiple types of residue specific annotations in user friendly formats.

Regulatory roles of conserved phosphorylation sites in the activation T-loop of the MAP kinase ERK1

The catalytic domains of most eukaryotic protein kinases are highly conserved in their primary structures. Their phosphorylation within the well-known activation T-loop, a variable region between protein kinase catalytic subdomains VII and VIII, is a common mechanism for stimulation of their phosphotransferase activities. Extracellular signal-regulated kinase 1 (ERK1), a member of the extensively studied mitogen-activated protein kinase (MAPK) family, serves as a paradigm for regulation of protein kinases in signaling modules. In addition to the well-documented T202 and Y204 stimulatory phosphorylation sites in the activation T-loop of ERK1 and its closest relative, ERK2, three additional flanking phosphosites have been confirmed (T198, T207, and Y210 from ERK1) by high-throughput mass spectrometry. In vitro kinase assays revealed the functional importance of T207 and Y210, but not T198, in negatively regulating ERK1 catalytic activity. The Y210 site could be important for proper conformational arrangement of the active site, and a Y210F mutant could not be recognized by MEK1 for phosphorylation of T202 and Y204 in vitro. Autophosphorylation of T207 reduces the catalytic activity and stability of activated ERK1. We propose that after the activation of ERK1 by MEK1, subsequent slower phosphorylation of the flanking sites results in inhibition of the kinase. Because the T207 and Y210 phosphosites of ERK1 are highly conserved within the eukaryotic protein kinase family, hyperphosphorylation within the kinase activation T-loop may serve as a general mechanism for protein kinase down-regulation after initial activation by their upstream kinases.

Retrospective Study of Sirolimus and Cyclophosphamide in Patients with Advanced Differentiated Thyroid Cancers

BACKGROUND

We hypothesize that the combination of an mTOR inhibitor, sirolimus, with a well-known cytotoxic agent, cyclophosphamide, provides a well-tolerated and promising alternative treatment for advanced, differentiated thyroid cancers (DTC).

METHODS

This retrospective review extracted data from patients treated for advanced DTC at the University of Michigan Comprehensive Cancer Center from 1995 through 2013. Fifteen patients treated with combination sirolimus and cyclophosphamide were identified as the sirolimus+cyp group. Seventeen patients treated with standard of care and enrolled in clinical trials were identified as the comparison group.

RESULTS

The one-year progression free survival rate (PFS) was 0.45, 95% CI [0.26, 0.80] in the sirolimus+cyp population and 0.30, 95% CI [0.13, 0.67] in the comparison population. The hazard ratio for PFS from initiation of treatment was 1.47, 95% CI [0.57, and 3.78]. In patients treated as first line, one-year PFS rate was 0.57, 95% CI [0.30, 1.00] in the sirolimus+cyp group and relatively unchanged at 0.29, 95% CI [0.11, 0.74] in the comparison group. The hazard ratio for PFS for first line patients was 1.10, 95% CI[ 0.4, and 3.5]. In patients with 3 or fewer sites of metastases, the one year PFS was 0.58, 95% CI [0.33, 1.00] in the sirolimus+cyp group, and 0.37, 95% CI [0.17, 0.80] in the comparison group. The average number of toxicities was 0.87 in the sirolimus+cyp patients and 1.71 in the comparison group.

CONCLUSIONS

The combination of sirolimus and cyclophosphamide was generally well tolerated with similar progression free survival, highlighting its applicability in patients with limited options.

BRAF (V600E) associates with cytoplasmatic localization of p27kip1 and higher cytokeratin 19 expression in papillary thyroid carcinoma

The genetic alterations are responsible for the altered protein expression in tumors. The knowledge of the link between the altered protein expression and genetic alterations may provide potentially important biological and clinical information. In this study, the expression of some protein markers (Gal-3, p21Kip1, CK19) known to be associated to the papillary thyroid carcinoma (PTC) was assessed in a series of surgical samples by immunohistochemistry, and the association between expression of these markers and the BRAF (V600E) mutation was investigated. Gal-3 positive staining was evident in 26 % of benign nodules. The BRAF (V600E) mutation and Gal-3 expression, were found in 55.5 and 87 % of PTC respectively, and were unlinked. The expression of CK19 in benign nodules was weak and limited to scattered follicular cells. Diffuse cytoplasmatic expression of CK19 was present in malignant tumors in a variable percentage of cells. A higher percentage of CK19 expressing cells was associated with BRAF (V600E) (P ≤ 0.001). All benign nodules displayed nuclear p27kip1 in more than 15 % of the cells. Twenty-nine PTC showed a cytoplasmatic staining with negative nuclei. PTC with cytoplasmatic or 0-5 % of cells with nuclear staining, 6-15 % or >15 % of cells with nuclear staining were 72 (66.7 %), 24 (22.2 %), and 12 (11.1 %) respectively. In BRAF (V600E) positive tumors, the cytoplasmatic localization of p27kip1 was significantly more frequent (P = 0.024). In conclusion, we provide evidences that BRAF (V600E) is non-associated with Gal-3 expression, whereas it is associated with cytoplasmatic localization of p27kip1 and higher CK19 expression in PTC.

mTOR pathway overactivation in BRAF mutated papillary thyroid carcinoma

CONTEXT

There are several genetic and molecular evidences suggesting dysregulation of the mammalian target of rapamycin (mTOR) pathway in thyroid neoplasia. Activation of the phosphatidylinositol-3-kinase/AKT pathway by RET/PTC and mutant RAS has already been demonstrated, but no data have been reported for the BRAF(V600E) mutation.

OBJECTIVE

The aim of this study was to evaluate the activation pattern of the mTOR pathway in malignant thyroid lesions and whether it may be correlated with known genetic alterations, as well as to explore the mechanisms underlying mTOR pathway activation in these neoplasias.

RESULTS

We observed, by immunohistochemical evaluation, an up-regulation/activation of the mTOR pathway proteins in thyroid cancer, particularly in conventional papillary thyroid carcinoma (cPTC). Overactivation of the mTOR signaling was particularly evident in cPTC samples harboring the BRAF(V600E) mutation. Transfection assays with BRAF expression vectors as well as BRAF knockdown by small interfering RNA revealed a positive association between BRAF expression and mTOR pathway activation, which appears to be mediated by pLKB1 Ser428, and emerged as a possible mechanism contributing to the association between BRAF mutation and mTOR pathway up-regulation. When we evaluated the rapamycin in the growth of thyroid cancer cell lines, we detected that cell lines with activating mutations in the MAPK pathway show a higher sensitivity to this drug.

CONCLUSIONS

We determined that the AKT/mTOR pathway is particularly overactivated in human cPTC harboring the BRAF(V600E) mutation. Moreover, our results suggest that the mTOR pathway could be a good target to enhance therapy effects in certain types of thyroid carcinoma, namely in those harboring the BRAF(V600E) mutation.

FOXO3 is inhibited by oncogenic PI3K/Akt signaling but can be reactivated by the NSAID sulindac sulfide

BACKGROUND

Overactivation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway has emerged as a pivotal trigger of thyroid carcinogenesis. Recent findings from other tumor entities revealed that PI3K/Akt-driven carcinogenesis critically involves the inactivation of the tumor-suppressive transcription factor Forkhead box O (FOXO)-3. However, little is known about FOXO3 in the thyroid context.

AIMS

We sought to investigate the influence of the thyroid oncogenes BRAFV600E, H-RASV12, and p110α (H1074R) on the regulation of the PI3K downstream target FOXO3 in vitro. Furthermore, the impact of the expression of phosphatase and tensin homolog deleted from chromosome 10 (PTEN) on the transcriptional activity of FOXO3 was assessed in a panel of 44 human thyroid tumors. Finally, we tried to modulate FOXO3 activity pharmacologically with help of the nonsteroidal antiinflammatory drug sulindac sulfide (SS).

RESULTS

We found that the overexpression of p110α H1074R results in the inactivation of FOXO3 via its nuclear exclusion. In vivo, we observed a direct correlation between PTEN expression and the transcriptional activation of FOXO3. In vitro, we found that stimulation with SS reversed PI3K/Akt-driven inactivation of FOXO3, resulting in its nuclear relocation and a combined induction of the antiproliferative FOXO target genes Gadd45α and p27(kip1) and the proapoptotic FOXO target gene Bim in benign (FRTL-5) and malignant human thyrocytes (FTC-133). In agreement with this, SS promoted the cell cycle arrest and apoptosis in thyroid cells, which could be amplified by the transfection of exogenous FOXO3.

CONCLUSION

Our data suggest that deregulation of proapoptotic FOXO3 represents a central step in PI3K/Akt-mediated thyroid carcinogenesis. Thus, SS might represent an attractive pharmacological tool for targeting thyroid neoplasia with aberrant PI3K/Akt/FOXO3 signaling.

The Akt-specific inhibitor MK2206 selectively inhibits thyroid cancer cells harboring mutations that can activate the PI3K/Akt pathway

CONTEXT

The phosphoinositide 3-kinase (PI3K)/Akt pathway is widely postulated to be an effective therapeutic target in thyroid cancer.

OBJECTIVE

The aim of the study was to test the therapeutic potential of the novel Akt inhibitor MK2206 for thyroid cancer.

DESIGN

We examined the effects of MK2206 on thyroid cancer cells with respect to the genotypes of the PI3K/Akt pathway.

RESULTS

Proliferation of thyroid cancer cells OCUT1, K1, FTC133, C643, Hth7, and TPC1, which harbored PIK3CA, PTEN, Ras, or RET/PTC mutations that could activate the PI3K/Akt pathway, was potently inhibited by MK2206 with IC(50) values mostly below or around 0.5 μm. In contrast, no potent inhibition by MK2206 was seen in most of the Hth74, KAT18, SW1736, WRO, and TAD2 cells that did not harbor mutations in the PI3K/Akt pathway. The inhibition efficacy was also genetic-selective. Specifically, the average inhibition efficacies were 59.2 ± 11.3 vs. 36.4 ± 8.8% (P = 0.005) at 1 μm MK2206 and 64.4 ± 11.5 vs. 38.5 ± 18.9% (P = 0.02) at 3 μm MK2206 for cells with mutations vs. cells without. The SW1736 cell, lacking mutations in the PI3K/Akt pathway, had minimal response to MK2206, but transfection with exogenous PIK3CA mutants, PIK3CA H1047R and E545K, significantly increased its sensitivity to MK2206. MK2206 also completely overcame the feedback activation of Akt from temsirolimus-induced mammalian target of rapamycin suppression, and the two inhibitors synergistically inhibited thyroid cancer cell growth.

CONCLUSIONS

Our study demonstrates a genetic selectivity of MK2206 in inhibiting thyroid cancer cells by targeting the PI3K/Akt pathway, supporting a clinical trial in thyroid cancer.

Cross-regulation between oncogenic BRAF(V600E) kinase and the MST1 pathway in papillary thyroid carcinoma

BACKGROUND

The BRAF(V600E) mutation leading to constitutive signaling of MEK-ERK pathways causes papillary thyroid cancer (PTC). Ras association domain family 1A (RASSF1A), which is an important regulator of MST1 tumor suppressor pathways, is inactivated by hypermethylation of its promoter region in 20 to 32% of PTC. However, in PTC without RASSF1A methylation, the regulatory mechanisms of RASSF1A-MST1 pathways remain to be elucidated, and the functional cooperation or cross regulation between BRAF(V600E) and MST1,which activates Foxo3,has not been investigated.

METHODOLOGY/PRINCIPAL FINDINGS

The negative regulators of the cell cycle, p21 and p27, are strongly induced by transcriptional activation of FoxO3 in BRAF(V600E) positive thyroid cancer cells. The FoxO3 transactivation is augmented by RASSF1A and the MST1 signaling pathway. Interestingly, introduction of BRAF(V600E)markedly abolished FoxO3 transactivation and resulted in the suppression of p21 and p27 expression. The suppression of FoxO3 transactivation by BRAF(V600E)is strongly increased by coexpression of MST1 but it is not observed in the cells in which MST1, but not MST2,is silenced. Mechanistically, BRAF(V600E)was able to bind to the C-terminal region of MST1 and resulted in the suppression of MST1 kinase activities. The induction of the G1-checkpoint CDK inhibitors, p21 and p27,by the RASSF1A-MST1-FoxO3 pathway facilitates cellular apoptosis, whereas addition of BRAF(V600E) inhibits the apoptotic processes through the inactivation of MST1. Transgenic induction of BRAF(V600E)in the thyroid gland results in cancers resembling human papillary thyroid cancers. The development of BRAF(V600E)transgenic mice with the MST1 knockout background showed that these mice had abundant foci of poorly differentiated carcinomas and large areas without follicular architecture or colloid formation.

CONCLUSIONS/SIGNIFICANCE

The results of this study revealed that the oncogenic effect of BRAF(V600E) is associated with the inhibition of MST1 tumor suppressor pathways, and that the activity of RASSF1A-MST1-FoxO3 pathways determines the phenotypes of BRAF(V600E) tumors.

Novel insights into FOXOlogy: FOXOs and their putative role in thyroid carcinogenesis

FOXO transcription factors regulate genes directly involved in the control of cell cycle arrest, apoptosis, DNA damage repair and antioxidative defense mechanisms. Genetic FOXO alterations and inactivation of FOXOs through oncogenic signaling cascades have been identified in several human cancers and contribute to uncoordinated cellular proliferation. To date, little is known about FOXOs in the thyroid context. In this article we will first provide an introduction into the topic of forkhead transcription factors by explaining the principles of FOXO function and regulation. We will then address specific aspects of FOXO3 function in the thyroid and possible consequences of FOXO3 deregulation in thyroid malignancy. Finally, we discuss the potential role of the PI3K/Akt/FOXO3 axis for a targeted drug therapy of advanced thyroid carcinoma.

Multiple implications of 3-phosphoinositide-dependent protein kinase 1 in human cancer

3-phosphoinositide-dependent protein kinase-1 (PDK1) is a central mediator of cellular signaling between phosphoinositide-3 kinase and various intracellular serine/threonine kinases, including protein kinase B, p70 ribosomal S6 kinase, serum and glucocorticoid-inducible kinase, and protein kinase C. PDK1 activates members of the AGC family of protein kinases by phosphorylating serine/threonine residues in the activation loop. Here, we review the regulatory mechanisms of PDK1 and its roles in cancer. PDK1 is activated by autophosphorylation in the activation loop and other serine residues, as well as by phosphorylation of Tyr-9 and Tyr-373/376. Src appears to recognize PDK1 following tyrosine phosphorylation. The role of heat shock protein 90 in regulating PDK1 stability and PDK1-Src complex formation are also discussed. Furthermore, we summarize the subcellular distribution of PDK1. Finally, an important role for PDK1 in cancer chemotherapy is proposed. In conclusion, a better understanding of its molecular regulatory mechanisms in various signaling pathways will help to explain how PDK1 acts as an oncogenic kinase in various cancers, and will contribute to the development of novel cancer chemotherapies.

Protein tyrosine kinase 6 directly phosphorylates AKT and promotes AKT activation in response to epidermal growth factor

Protein tyrosine kinase 6 (PTK6) is a nonmyristoylated Src-related intracellular tyrosine kinase. Although not expressed in the normal mammary gland, PTK6 is expressed in a majority of human breast tumors examined, and it has been linked to ErbB receptor signaling and AKT activation. Here we demonstrate that AKT is a direct substrate of PTK6 and that AKT tyrosine residues 315 and 326 are phosphorylated by PTK6. Association of PTK6 with AKT occurs through the SH3 domain of PTK6 and is enhanced through SH2 domain-mediated interactions following tyrosine phosphorylation of AKT. Using Src, Yes, and Fyn null mouse embryonic fibroblasts (SYF cells), we show that PTK6 phosphorylates AKT in a Src family kinase-independent manner. Introduction of PTK6 into SYF cells sensitized these cells to physiological levels of epidermal growth factor (EGF) and increased AKT activation. Stable introduction of active PTK6 into SYF cells also resulted in increased proliferation. Knockdown of PTK6 in the BPH-1 human prostate epithelial cell line led to decreased AKT activation in response to EGF. Our data indicate that in addition to promoting growth factor receptor-mediated activation of AKT, PTK6 can directly activate AKT to promote oncogenic signaling.

Genetic alterations in the phosphatidylinositol-3 kinase/Akt pathway in thyroid cancer

BACKGROUND

Aberrant activation of the phosphatidylinositol-3 kinase (PI3K)/Akt pathway plays a fundamental role in thyroid tumorigenesis, particularly in follicular thyroid cancer (FTC) and aggressive thyroid cancer, such as anaplastic thyroid cancer (ATC). As the drivers of this process, many genetic alterations activating the PI3K/Akt pathway have been identified in thyroid cancer in recent years.

SUMMARY

This review summarizes the current knowledge on major genetic alterations in the PI3K/Akt pathway. These include PIK3CA mutations and genomic amplification/copy gain, Ras mutations, PTEN mutations, RET/PTC and PPARgamma/Pax8 rearrangements, as well as amplification/copy gain of PIK3CB, PDK1, Akt, and various receptor tyrosine kinase genes. Most of these genetic alterations are particularly common in FTC and many of them are even more common in ATC; they are generally less common in papillary thyroid cancer (PTC), in which the MAP kinase (MAPK) pathway activated by the BRAF mutation instead plays a major role. Methylation and, thus, epigenetic silencing of PTEN, a major negative regulator of the PI3K/Akt pathway, occurs in close association with activating genetic alterations of the PI3K/Akt pathway, constituting a unique self-enhancement mechanism for this pathway. Many of these genetic alterations are mutually exclusive in differentiated thyroid tumors, but with increasing concurrence from benign tumors to FTC to ATC. RET/PTC, Ras, and receptor tyrosine kinase could dually activate the PI3K/Akt and MAPK pathways. Most cases of ATC harbor genetic alterations in these genes or other genetic combinations that can activate both pathways. It is proposed that genetic alterations in the PI3K/Akt pathway promote thyroid cell transformation to FTC and that genetic alterations in the MAPK pathway promote cell transformation to PTC; accumulation of multiple genetic alterations that can activate both pathways promotes thyroid cancer aggressiveness and progression to ATC.

CONCLUSIONS

Genetic alterations are common in the PI3K/Akt pathway in thyroid cancer and play a fundamental role in the tumorigenesis and progression of this cancer. This provides a strong basis for the emerging development of novel genetic-based diagnostic, prognostic, and therapeutic strategies for thyroid cancer.

New targeted molecular therapies for dedifferentiated thyroid cancer

Dedifferentiated thyroid cancer (DeTC) derived from follicular epithelium is often incurable because it does not respond to radioiodine, radiotherapy, or chemotherapy. In cases, RET/PTC rearrangements are found in 30%–40%, RAS mutations in about 10%, and BRAF mutations in around 40%–50%, with no overlap between these mutations results in papillary thyroid cancer, while a higher prevalence of BRAF mutations (up to 70%) has been observed in DeTC. The identification of these activating mutations in DeTC makes this malignancy an excellent model to examine the effect of tyrosine kinase inhibitors (TKIs). Clinical trials with several TKIs targeting RET, and to a lesser extent BRAF, and other TKRs have shown positive results, with about one-third of DeTC showing a reduction in tumor size up to 50%, with the longest treatment duration of approximately three-four years. Angiogenesis inhibitors have also shown promising activity in DeTC. Progress is being made toward effective targeted DeTC therapy. The possibility of testing the sensitivity of primary DeTC cells from each subject to different TKIs could increase the effectiveness of the treatment.

Phosphorylation site mapping of endogenous proteins: a combined MS and bioinformatics approach

We present a novel approach that combines MALDI-TOF profile analysis and bioinformatics-based inclusion criteria to comprehensively predict phosphorylation sites on a single protein of interest from limiting sample. It is technologically difficult to unambiguously identify phosphorylated residues, as many physiologically important phosphorylation sites are of too low abundance in vivo to be unambiguously assigned by mass spectrometry. Conversely, phosphorylation site prediction algorithms, while increasingly accurate, nevertheless overestimate the number of phosphorylation sites. In this study, we show that MODICAS, an MS data management and analysis tool, can be effectively merged with the bioinformatics attributes of residue conservation and phosphosite prediction to generate a short list of putative phosphorylation sites that can be subsequently verified by additional methodologies such as phosphospecific antibodies or mutational analysis. Therefore, the combination of MODICAS driven MS data analysis with bioinformatics-based filtering represents a substantial increase in the ability to putatively identify physiologically relevant phosphosites from limited starting material.

XB130, a tissue-specific adaptor protein that couples the RET/PTC oncogenic kinase to PI 3-kinase pathway

XB130 is a recently cloned 130 kDa-adaptor protein and Src kinase substrate, structurally similar to actin-filament-associated protein. Here we show that XB130 is predominantly expressed in the thyroid. Given that XB130 is a thyroid-specific tyrosine kinase substrate, we asked whether it is targeted by RET/PTC, a genetically rearranged, constitutively active, thyroid-specific tyrosine kinase that plays a pathogenic role in papillary thyroid cancer. RET/PTC induced robust tyrosine phosphorylation of XB130, which promoted its subsequent association with the p85alpha subunit of phosphatidylinositol 3-kinase (PI 3-kinase). We identified tyrosine 54 of XB130 as the major target of RET/PTC-mediated phosphorylation and a critical binding site for the SH2 domains of p85alpha. Importantly, downregulation of XB130 in TPC1 papillary thyroid cancer cells, harboring the RET/PTC1 kinase, strongly reduced Akt activity without altering ERK1/2 phosphorylation, and concomitantly inhibited cell-cycle progression and survival in suspension. In conclusion, XB130 is a novel substrate of the RET/PTC kinase that links RET/PTC signaling to PI 3-kinase activation, and thereby plays an important role in sustaining proliferation and survival of thyroid tumor cells.

Nuclear factor-kB in thyroid carcinogenesis and progression: a novel therapeutic target for advanced thyroid cancer

Apoptosis is an essential physiological process of elimination of destined cells during the development and differentiation or after damage from external stresses such as ionizing radiation or chemotherapeutic agents. Disruption of apoptosis is proved to cause various diseases including cancer. Among numerous molecules involved in diverse anti- or pro-apoptotic signaling pathways, NF-kappaB is one of the key factors controlling anti-apoptotic responses. Its anti-apoptotic effect is thought to be mediated through not only transcriptional activation of dependent genes but also by crosstalking with the JNK pathway. Oncogenic proteins such as Ret/PTC, Ras and BRAF can induce NF-kappaB activation making it an important change in thyroid cancer. A number of specific or non-specific NF-kappaB inhibitors have been tried to take over the cascade in in vitro and in vivo experiments. These agents can induce massive apoptosis especially in combination with radio- or chemotherapy. Current results suggest that the inhibition of the NF-kappaB may be a promising strategy for advanced thyroid cancer treatment but further investigations are warranted to develop specific and clinically effective NF-kappaB inhibitors in future.

Dysregulated RET signaling in thyroid cancer

Numerous biologic processes and such diseases as cancer depend on activation of tyrosine kinase receptors. The RET tyrosine kinase receptor was discovered two decades ago as a transforming gene and was subsequently implicated in the formation of papillary and medullary thyroid carcinoma. This article examines the data about the mechanism of activation of downstream signal transduction pathways by RET oncoproteins. Collectively, these findings have advanced the understanding of the processes underlying thyroid carcinoma formation.

Dysregulation of the phosphatidylinositol 3-kinase pathway in thyroid neoplasia

The phosphatidylinositol 3-kinase (PI3K) signaling pathway is an important regulator of many cellular events, including apoptosis, proliferation, and motility. Enhanced activation of this pathway can occur through several mechanisms, such as inactivation of its negative regulator, phosphatase and tensin homolog deleted on chromosome ten (PTEN), and activating mutations and gene amplification of the gene encoding the catalytic subunit of PI3K (PIK3CA). These genetic abnormalities have been particularly associated with follicular thyroid neoplasia and anaplastic thyroid cancer, suggesting an important role for PI3K signaling in these disorders. In this article, the role of PI3K pathway activation in thyroid cancer is discussed, with a focus on recent advances.

RET/PTC rearrangements and BRAF mutations in thyroid tumorigenesis

Thyroid papillary carcinoma is the most common type of endocrine cancer. It is frequently associated with genetic alterations leading to activation of the MAPK signaling pathway. The two most frequently affected genes, BRAF and RET, are activated by either point mutation or as a result of chromosomal rearrangement. These mutations are tumorigenic in thyroid follicular cells and correlate with specific phonotypical features and biological properties of papillary carcinomas, including tumor aggressiveness and response to radioiodine therapy. Molecular inhibitors that block RET/PTC or BRAF kinase activity have shown substantial therapeutic effects in the experimental systems and are currently being tested in clinical trials.

The antiapoptotic protein BAG3 is expressed in thyroid carcinomas and modulates apoptosis mediated by tumor necrosis factor-related apoptosis-inducing ligand

CONTEXT

We previously showed that BAG3 protein, a member of the BAG (Bcl-2-associated athanogene) co-chaperone family, modulates apoptosis in human leukemias. The expression of BAG3 in other tumor types has not been extensively investigated so far.

OBJECTIVE

The objective of this study was to analyze BAG3 expression in thyroid neoplastic cells and investigate its influence in cell apoptotic response to TNF-related apoptosis-inducing ligand (TRAIL).

DESIGN, SETTING, AND PATIENTS

We investigated BAG3 expression in human thyroid carcinoma cell lines, including NPA, and the effect of BAG3-specific small interfering RNA on TRAIL-induced apoptosis in NPA cells. Subsequently, we analyzed BAG3 expression in 30 benign lesions and 56 carcinomas from patients of the Naples Tumor Institute Fondazione Senatore Pascale.

MAIN OUTCOME MEASURES

The main outcome measures were: analysis of BAG3 protein in NPA cells by Western blot and immunocytochemistry; analysis of apoptosis in TRAIL-stimulated NPA cells by flow cytometry; and evaluation of BAG3 expression in specimens from thyroid lesions by immunohistochemistry.

RESULTS

BAG3 was expressed in human thyroid carcinoma cell lines; small interfering RNA-mediated downmodulation of its levels significantly (P < 0.0195) enhanced NPA cell apoptotic response to TRAIL. The protein was not detectable in 19 of 20 specimens of normal thyroid or goiters, whereas 54 of 56 analyzed carcinomas (15 follicular, 28 papillary, and 13 anaplastic) were clearly positive for BAG3 expression.

CONCLUSIONS

BAG3 downmodulates the apoptotic response to TRAIL in human neoplastic thyroid cells. The protein is specifically expressed in thyroid carcinomas and not in normal thyroid tissue or goiter.

An orally administered multitarget tyrosine kinase inhibitor, SU11248, is a novel potent inhibitor of thyroid oncogenic RET/papillary thyroid cancer kinases

CONTEXT

The oncogenic RET/PTC tyrosine kinase causes papillary thyroid cancer (PTC). The use of inhibitors specific for RET/PTC may be useful for targeted therapy of PTC.

OBJECTIVE

The objective of the study was to evaluate the efficacies of the recently developed kinase inhibitors SU11248, SU5416, and SU6668 in inhibition of RET/PTC.

DESIGN

SU11248, SU5416, and SU6668 were synthesized, and their inhibitory potencies were evaluated using an in vitro RET/PTC kinase assay. The inhibitory effects of the compounds on RET/PTC were evaluated by quantifying the autophosphorylation of RET/PTC, signal transducer and activator of transcription (STAT)-3 activation, and the morphological reversal of RET/PTC-transformed cells.

RESULTS

An in vitro kinase assay revealed that SU5416, SU6668, and SU11248 inhibited phosphorylation of the synthetic tyrosine kinase substrate peptide E4Y by RET/PTC3 in a dose-dependent manner with IC(50) of approximately 944 nm for SU5416, 562 nm for SU6668, and 224 nm for SU11248. Thus, SU11248 effectively inhibits the kinase activity of RET/PTC3. RET/PTC-mediated Y705 phosphorylation of STAT3 was inhibited by addition of SU11248, and the inhibitory effects of SU11248 on the tyrosine phosphorylation and transcriptional activation of STAT3 were very closely correlated with decreased autophosphorylation of RET/PTC. SU11248 caused a complete morphological reversion of transformed NIH-RET/PTC3 cells and inhibited the growth of TPC-1 cells that have an endogenous RET/PTC1.

CONCLUSION

SU11248 is a highly effective tyrosine kinase inhibitor of the RET/PTC oncogenic kinase.

RET as a diagnostic and therapeutic target in sporadic and hereditary endocrine tumors

The RET gene encodes a receptor tyrosine kinase that is expressed in neural crest-derived cell lineages. The RET receptor plays a crucial role in regulating cell proliferation, migration, differentiation, and survival through embryogenesis. Activating mutations in RET lead to the development of several inherited and noninherited diseases. Germline point mutations are found in the cancer syndromes multiple endocrine neoplasia (MEN) type 2, including MEN 2A and 2B, and familial medullary thyroid carcinoma. These syndromes are autosomal dominantly inherited. The identification of mutations associated with these syndromes has led to genetic testing to identify patients at risk for MEN 2 and familial medullary thyroid carcinoma and subsequent implementation of prophylactic thyroidectomy in mutation carriers. In addition, more than 10 somatic rearrangements of RET have been identified from papillary thyroid carcinomas. These mutations, as those found in MEN 2, induce oncogenic activation of the RET tyrosine kinase domain via different mechanisms, making RET an excellent candidate for the design of molecular targeted therapy. Recently, various kinds of therapeutic approaches, such as tyrosine kinase inhibition, gene therapy with dominant negative RET mutants, monoclonal antibodies against oncogene products, and nuclease-resistant aptamers that recognize and inhibit RET have been developed. The use of these strategies in preclinical models has provided evidence that RET is indeed a potential target for selective cancer therapy. However, a clinically useful therapeutic option for treating patients with RET-associated cancer is still not available.