RET/PTC (rearranged in transformation/papillary thyroid carcinomas) tyrosine kinase phosphorylates and activates phosphoinositide-dependent kinase 1 (PDK1): an alternative phosphatidylinositol 3-kinase-independent pathway to activate PDK1

Metabolic Reprogramming in Thyroid Cancer

Thyroid cancer is a common endocrine malignancy with increasing incidence globally. Although most cases can be treated effectively, some cases are more aggressive and have a higher risk of mortality. Inhibiting RET and BRAF kinases has emerged as a potential therapeutic strategy for the treatment of thyroid cancer, particularly in cases of advanced or aggressive disease. However, the development of resistance mechanisms may limit the efficacy of these kinase inhibitors. Therefore, developing precise strategies to target thyroid cancer cell metabolism and overcome resistance is a critical area of research for advancing thyroid cancer treatment. In the field of cancer therapeutics, researchers have explored combinatorial strategies involving dual metabolic inhibition and metabolic inhibitors in combination with targeted therapy, chemotherapy, and immunotherapy to overcome the challenge of metabolic plasticity. This review highlights the need for new therapeutic approaches for thyroid cancer and discusses promising metabolic inhibitors targeting thyroid cancer. It also discusses the challenges posed by metabolic plasticity in the development of effective strategies for targeting cancer cell metabolism and explores the potential advantages of combined metabolic targeting.

Master kinase PDK1 in tumorigenesis

3-phosphoinositide-dependent protein kinase 1 (PDK1) is considered as master kinase regulating AGC kinase family members such as AKT, SGK, PLK, S6K and RSK. Although autophosphorylation regulates PDK1 activity, accumulating evidence suggests that PDK1 is manipulated by many other mechanisms, including S6K-mediated phosphorylation, and the E3 ligase SPOP-mediated ubiquitination and degradation. Dysregulation of these upstream regulators or downstream signals involves in cancer development, as PDK1 regulating cell growth, metastasis, invasion, apoptosis and survival time. Meanwhile, overexpression of PDK1 is also exposed in a plethora of cancers, whereas inhibition of PDK1 reduces cell size and inhibits tumor growth and progression. More importantly, PDK1 also modulates the tumor microenvironments and markedly influences tumor immunotherapies. In summary, we comprehensively summarize the downstream signals, upstream regulators, mouse models, inhibitors, tumor microenvironment and clinical treatments for PDK1, and highlight PDK1 as a potential cancer therapeutic target.

Emerging Role of Oxidative Stress on EGFR and OGG1-BER Cross-Regulation: Implications in Thyroid Physiopathology

Thyroid diseases have a complex and multifactorial aetiology. Despite the numerous studies on the signals referable to the malignant transition, the molecular mechanisms concerning the role of oxidative stress remain elusive. Based on its strong oxidative power, H2O2 could be responsible for the high level of oxidative DNA damage observed in cancerous thyroid tissue and hyperactivation of mitogen-activated protein kinase (MAPK) and PI3K/Akt, which mediate ErbB signaling. Increased levels of 8-oxoG DNA adducts have been detected in the early stages of thyroid cancer. These DNA lesions are efficiently recognized and removed by the base excision repair (BER) pathway initiated by 8-oxoG glycosylase1 (OGG1). This study investigated the relationships between the EGFR and OGG1-BER pathways and their mutual regulation following oxidative stress stimulus by H2O2 in human thyrocytes. We clarified the modulation of ErbB receptors and their downstream pathways (PI3K/Akt and MAPK/ERK) under oxidative stress (from H2O2) at the level of gene and protein expression, according to the mechanism defined in a human non-pathological cell system, Nthy-ori 3-1. Later, on the basis of the results obtained by gene expression cluster analysis in normal cells, we assessed the dysregulation of the relationships in a model of papillary thyroid cancer with RET/PTC rearrangement (TPC-1). Our observations demonstrated that a H2O2 stress may induce a physiological cross-regulation between ErbB and OGG1-BER pathways in normal thyroid cells (while this is dysregulated in the TPC-1 cells). Gene expression data also delineated that MUTYH gene could play a physiological role in crosstalk between ErbB and BER pathways and this function is instead lost in cancer cells. Overall, our data on OGG1 protein expression suggest that it was physiologically regulated in response to oxidative modulation of ErbB, and that these might be dysregulated in the signaling pathway involving AKT in the progression of thyroid malignancies with RET/PTC rearrangements.

Development of Metabolic Synthetic Lethality and Its Implications for Thyroid Cancer

Cancer therapies targeting genetic alterations are a topic of great interest in the field of thyroid cancer, which frequently harbors mutations in the RAS, RAF, and RET genes. Unfortunately, U.S. Food and Drug Administration-approved BRAF inhibitors have relatively low therapeutic efficacy against BRAF-mutant thyroid cancer; in addition, the cancer often acquires drug resistance, which prevents effective treatment. Recent advances in genomics and transcriptomics are leading to a more complete picture of the range of mutations, both driver and messenger, present in thyroid cancer. Furthermore, our understanding of cancer suggests that oncogenic mutations drive tumorigenesis and induce rewiring of cancer cell metabolism, which promotes survival of mutated cells. Synthetic lethality (SL) is a method of neutralizing mutated genes that were previously considered untargetable by traditional genotype-targeted treatments. Because these metabolic events are specific to cancer cells, we have the opportunity to develop new therapies that target tumor cells specifically without affecting healthy tissue. Here, we describe developments in metabolism-based cancer therapy, focusing on the concept of metabolic SL in thyroid cancer. Finally, we discuss the essential implications of metabolic reprogramming and its role in the future direction of SL for thyroid cancer.

Molecular aberrations and signaling cascades implicated in the pathogenesis of anaplastic thyroid cancer

Anaplastic Thyroid Cancer (ATC) accounts for >40% thyroid cancer-related deaths and has a dismal prognosis. In the past decade, significant efforts have been made towards understanding the pathogenesis of this disease and developing novel therapeutics. Unfortunately, effective treatment is still lacking and a more thorough understanding of ATC pathogenesis may provide new opportunities to improve ATC therapeutics. This review provides insights into ATC clinical presentation and pathology, and the putative role of genetic aberrations and alterations in molecular signaling pathways in ATC pathogenesis. We reviewed prevalent mutations, chromosomal abnormalities and fusions, epigenetic alterations and dysregulations in ATC, and highlighted several signaling cascades which appeared to be integral to ATC pathogenesis. Moreover, these features offer insights into de-differentiated, aggressive and drug-resistant phenotype of ATC, and thus may help in exploring potential new molecular targets for developing novel therapeutics.

Epigenetic chromatin conformation changes in peripheral blood can detect thyroid cancer

BACKGROUND

Fine needle aspiration has been the traditional method for diagnosing thyroid cancer. Epigenetic chromatin conformation changes offer an alternative method of diagnosing cancer. The purpose of this study is to evaluate an EpiSwitch assay of epigenetic markers that can be used to diagnose thyroid cancer in blood samples.

METHODS

From 2014 to 2016, adult patients with thyroid nodules having thyroidectomy were recruited and grouped based on benign, malignant, and atypia of undetermined significance or follicular lesions of undetermined significance fine needle aspiration cytology. Blood samples were collected before surgery. Final pathologic diagnosis was made from the thyroid specimens. Patients' blood samples were analyzed using the EpiSwitch assay, (Oxford Biodynamics, Oxford, UK), and the results were compared with surgical pathology to determine assay performance.

RESULTS

In total, 58 patients were recruited: 20 benign, 20 malignant, and 18 atypia or follicular lesions of undetermined significance. An analysis of the malignant and benign fine needle aspiration groups found 6 epigenetic markers for thyroid. A total of 28 (48%) patients had thyroid cancer. The assay was able to correctly identify 25 of the 28 malignant nodules, showing sensitivity of 89.3% and specificity of 66.7%. The positive predictive value for the assay was 71.4%, whereas the negative predictive value was 87.0%.

CONCLUSION

An epigenetic assay of peripheral blood shows high sensitivity in detecting thyroid cancer and provides an additional method for its diagnosis.

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.

Selective inhibition of RET mediated cell proliferation in vitro by the kinase inhibitor SPP86

BACKGROUND

The RET tyrosine kinase receptor has emerged as a target in thyroid and endocrine resistant breast cancer. We previously reported the synthesis of kinase inhibitors with potent activity against RET. Herein, we have further investigated the effect of the lead compound SPP86 on RET mediated signaling and proliferation. Based on these observations, we hypothesized that SPP86 may be useful for studying the cellular activity of RET.

METHODS

We compared the effects of SPP86 on RET-induced signaling and proliferation in thyroid cancer cell lines expressing RET-PTC1 (TPC1), or the activating mutations BRAFV600E (8505C) and RASG13R (C643). The effect of SPP86 on RET- induced phosphatidylinositide 3-kinases (PI3K)/Akt and MAPK pathway signaling and cell proliferation in MCF7 breast cancer cells was also investigated.

RESULTS

SPP86 inhibited MAPK signaling and proliferation in RET/PTC1 expressing TPC1 but not 8505C or C643 cells. In TPC1 cells, the inhibition of RET phosphorylation required co-exposure to SPP86 and the focal adhesion kinase (FAK) inhibitor PF573228. In MCF7 cells, SPP86 inhibited RET- induced phosphatidylinositide 3-kinases (PI3K)/Akt and MAPK signaling and estrogen receptorα (ERα) phosphorylation, and inhibited proliferation to a similar degree as tamoxifen. Interestingly, SPP86 and PF573228 inhibited RET/PTC1 and GDNF- RET induced activation of Akt and MAPK signaling to a similar degree.

CONCLUSION

SPP86 selectively inhibits RET downstream signaling in RET/PTC1 but not BRAFV600E or RASG13R expressing cells, indicating that downstream kinases were not affected. SPP86 also inhibited RET signaling in MCF7 breast cancer cells. Additionally, RET- FAK crosstalk may play a key role in facilitating PTC1/RET and GDNF- RET induced activation of Akt and MAPK signaling in TPC1 and MCF7 cells.

Acute regulation of PDK1 by a complex interplay of molecular switches

Phosphoinositide-dependent kinase 1 (PDK1) is the master regulator of at least 23 other AGC kinases whose downstream signalling has often been implicated in various diseases and in particular in cancer. Therefore there has been great interest in determining how PDK1 is controlled and how it regulates its substrates spatially and temporally. The understanding of these mechanisms could offer new possibilities for therapeutic intervention. Over the years, a more comprehensive view of the mechanisms involved in the regulation of PDK1 has emerged and these comprise serine/threonine as well as tyrosine phosphorylation, subcellular localization, regulator binding and conformation status. In the present review, we discuss how various molecular mechanisms are together responsible for the conformational regulation behind the activation of PDK1 in cells.

Receptor tyrosine kinases in osteosarcoma: not just the usual suspects

Despite aggressive surgical and chemotherapy protocols, survival rates for osteosarcoma patients have not improved over the last 30 years. Therefore, novel therapeutic agents are needed. Receptor tyrosine kinases have emerged as targets for the development of new cancer therapies since their activation leads to enhanced proliferation, survival, and metastasis. In fact, aberrant expression and activation of RTKs have been associated with the progression of many cancers. Studies from our lab using phosphoproteomic screening identified RTKs that are activated and thus may contribute to the signaling within metastatic human osteosarcoma cells. Functional genomic screening using siRNA was performed to distinguish which of the activated RTKs contribute to in vitro phenotypes associated with metastatic potential (motility, invasion, colony formation, and cell growth). The resulting RTK hits were then validated using independent validation experiments. From these results, we identified four RTKs (Axl, EphB2, FGFR2, and Ret) that have not been previously studied in osteosarcoma and provide targets for the development of novel therapeutics.

Astrocytic phosphorylation of PDK1 on Tyr9 following an excitotoxic lesion in the mouse hippocampus

3-phosphoinositide-dependent kinase-1 (PDK1) is suggested to play important roles in the regulation of synaptic plasticity and neuronal cell survival in the mature CNS. Although few studies have investigated the roles of PDK1, little is known about PDK1 changes in glial cells under neuropathological conditions. In current report, phosphorylation of PDK1 was monitored specially on tyrosine residues, following the induction of an excitotoxic lesion in rat brain by using kainic acid administration. In injured hippocampal CA3 region, Tyr9 phosphorylation of PDK1 was increased from 4h until 3 day post-injection. Double immunohistochemistry further evaluated that these phosphorylated forms of PDK1 were localized in astrocytes not other cells. Overexpression of unphosphorylatable mutant, PDK1-Y9F leads to inhibit Protein kinase B (PKB/Akt) activation and cAMP responsive element binding protein (CREB) phosphorylation. In conclusion, our results suggested for the first time that tyrosine phosphorylation of PDK1 is required for PKB and CREB activation in KA-mediated excitotoxic lesion in mouse brain.

Mullerian inhibiting substance expression in papillary thyroid cancer

OBJECTIVE

To examine the expression of Mullerian inhibiting substance (MIS) in papillary thyroid cancer.

MATERIALS AND METHODS

The MIS expression was examined by studying the immunohistochemistry in deparafinized sections prepared from tissue blocks of patients who were diagnosed with papillary thyroid cancer, as given in the pathology archive records (n = 23).

RESULTS

In all the cases studied, 50% (n = 10) showed strong staining and 50% showed moderate staining. The percentage of staining was found to be 94.2 ± 3.1% in strongly stained cases and 92.2 ± 2.1% in moderately stained cases. Normal thyroid tissues neighboring the tumor did not display any staining.

CONCLUSION

The MIS expression can be used as a significant tool in differential diagnosis of papillary thyroid cancer and also to shed light on its etiopathogenesis.

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.

PDK1 protein phosphorylation at Thr354 by murine protein serine-threonine kinase 38 contributes to negative regulation of PDK1 protein activity

Murine protein serine-threonine kinase 38 (MPK38) is a member of the AMP-activated protein kinase-related serine/threonine kinase family, which acts as cellular energy sensors. In this study, MPK38-induced PDK1 phosphorylation was examined to elucidate the biochemical mechanisms underlying phosphorylation-dependent regulation of 3-phosphoinositide-dependent protein kinase-1 (PDK1) activity. The results showed that MPK38 interacted with and inhibited PDK1 activity via Thr(354) phosphorylation. MPK38-PDK1 complex formation was mediated by the amino-terminal catalytic kinase domain of MPK38 and the pleckstrin homology domain of PDK1. This activity was dependent on insulin, a PI3K/PDK1 stimulator, as well as various apoptotic stimuli, including TNF-α, H(2)O(2), thapsigargin, and ionomycin. MPK38 inhibited PDK1 activity in a kinase-dependent manner and alleviated PDK1-mediated suppression of TGF-β (or ASK1) signaling, probably via the phosphorylation of PDK1 at Thr(354). In addition, MPK38-mediated inhibition of PDK1 activity was accompanied by the modulation of PDK1 binding to its positive and negative regulators, serine/threonine kinase receptor-associated protein and 14-3-3, respectively. Together, these findings suggest an important role for MPK38-mediated phosphorylation of PDK1 in the negative regulation of PDK1 activity.

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.

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.

Beyond RET: potential therapeutic approaches for advanced and metastatic medullary thyroid carcinoma

Medullary thyroid carcinoma (MTC) is a rare calcitonin-producing neuroendocrine tumour that originates from the parafollicular C-cells of the thyroid gland. The RET proto-oncogene encodes the RET receptor tyrosine kinase, which has essential roles in cell survival, differentiation and proliferation. Activating mutations of RET are associated with the pathogenesis of MTC and have been demonstrated in nearly all hereditary and in 30-50% of sporadic MTC cases, making this receptor an excellent target for small-molecule inhibitors for this tumour. Clinical trials of small organic inhibitors of tyrosine kinase receptors (TKIs) targeting the RET receptor have shown efficacy for treatment of metastatic MTC with 30-50% of patients responding to these agents. Despite the importance of the RET receptor in MTC, it is clear that other signal transduction pathways, tyrosine kinase receptors, and tumour suppressor genes are involved in MTC tumourigenesis and progression. A better understanding of molecular cross-talk between these signal pathways and the RET receptor may lead to combinatorial therapy that will improve outcomes beyond what is currently possible with RET-directed TKIs. Finally, there is evidence that immunological-based therapy using dendritic cell vaccination strategies have been effective for reducing tumour mass in a small number of patients. The identification of additional MTC-specific tumour antigens and a better understanding of specific epitopes in these tumour antigens may lead to improvement of response rates.

TUSC4/NPRL2, a novel PDK1-interacting protein, inhibits PDK1 tyrosine phosphorylation and its downstream signaling

3-Phosphoinositide-dependent protein kinase-1 (PDK1) is a key regulator of cell proliferation and survival signal transduction. PDK1 is known to be constitutively active and is further activated by Src-mediated phosphorylation at the tyrosine-9, -373, and -376 residues. To identify novel regulators of PDK1, we performed E. coli-based two-hybrid screening and revealed that tumor suppressor candidate 4 (TUSC4), also known as nitrogen permease regulator-like 2 (NPRL2), formed a complex with PDK1 and suppressed Src-dependent tyrosine phosphorylation and activation of PDK1 in vitro and in cells. The NH(2)-terminal 133 amino acid residues of TUSC4 were involved in binding to PDK1. The deletion mutant of TUSC4 that lacked the NH(2)-terminal domain showed no inhibitory effects on PDK1 tyrosine phosphorylation or activation. Thus, complex formation is indispensable for TUSC4-mediated PDK1 inactivation. The siRNA-mediated down-regulation of TUSC4 induced cell proliferation, while ectopic TUSC4 expression inactivated the PDK1 downstream signaling pathway, including Akt and p70 ribosomal protein S6 kinase, and increased cancer cell sensitivity to several anticancer drugs. Our results suggest that TUSC4/NPRL2, a novel PDK1-interacting protein, plays a role in regulating the Src/PDK1 signaling pathway and cell sensitivity to multiple cancer chemotherapeutic drugs.

Nox4 NAD(P)H oxidase mediates Src-dependent tyrosine phosphorylation of PDK-1 in response to angiotensin II: role in mesangial cell hypertrophy and fibronectin expression

Activation of glomerular mesangial cells (MCs) by angiotensin II (Ang II) leads to hypertrophy and extracellular matrix accumulation. Here, we demonstrate that, in MCs, Ang II induces an increase in PDK-1 (3-phosphoinositide-dependent protein kinase-1) kinase activity that required its phosphorylation on tyrosine 9 and 373/376. Introduction into the cells of PDK-1, mutated on these tyrosine residues or kinase-inactive, attenuates Ang II-induced hypertrophy and fibronectin accumulation. Ang II-mediated PDK-1 activation and tyrosine phosphorylation (total and on residues 9 and 373/376) are inhibited in cells transfected with small interfering RNA for Src, indicating that Src is upstream of PDK-1. In cells expressing oxidation-resistant Src mutant C487A, Ang II-induced hypertrophy and fibronectin expression are prevented, suggesting that the pathway is redox-sensitive. Ang II also up-regulates Nox4 protein, and siNox4 abrogates the Ang II-induced increase in intracellular reactive oxygen species (ROS) generation. Small interfering RNA for Nox4 also inhibits Ang II-induced activation of Src and PDK-1 tyrosine phosphorylation (total and on residues 9 and 373/376), demonstrating that Nox4 functions upstream of Src and PDK-1. Importantly, inhibition of Nox4, Src, or PDK-1 prevents the stimulatory effect of Ang II on fibronectin accumulation and cell hypertrophy. This work provides the first evidence that Nox4-derived ROS are responsible for Ang II-induced PDK-1 tyrosine phosphorylation and activation through stimulation of Src. Importantly, this pathway contributes to Ang II-induced MC hypertrophy and fibronectin accumulation. These data shed light on molecular processes underlying the oxidative signaling cascade engaged by Ang II and identify potential targets for intervention to prevent renal hypertrophy and fibrosis.

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.

Creation and characterization of a doxycycline-inducible mouse model of thyroid-targeted RET/PTC1 oncogene and luciferase reporter gene coexpression

BACKGROUND

RET/PTC1 chromosomal rearrangement is associated with papillary thyroid carcinoma formation in children exposed to ionizing radiation. We previously created a transgenic mouse model with thyroid-targeted constitutive RET/PTC1 expression and demonstrated papillary thyroid carcinoma formation.

OBJECTIVE

In this study, we aimed to create a doxycycline-inducible mouse model of thyroid RET/PTC1 and luciferase reporter gene coexpression to allow for noninvasive monitoring of transgene expression in mice of various ages and timepoints after induction.

DESIGN

Transgenic mice carrying the rtTA gene driven by the thyroglobulin promoter were generated, and crossed with responder mice carrying RET/PTC1 and firefly luciferase genes under control of a bidirectional tetracycline response element.

MAIN OUTCOMES

Most bitransgenic mice had thyroid-targeted, doxycycline-independent transgene expression. Only one line had thyroid-targeted, doxycycline-regulated RET/PTC1 and luciferase coexpression, in which doxycycline induction of RET/PTC1 led to Erk phosphorylation and reduced expression of the sodium/iodide symporter (NIS). However, thyroid lesions were not found in any bitransgenic mice examined.

CONCLUSIONS

We found that acute RET/PTC1 expression can activate the MEK/Erk pathway and downregulate NIS expression in the mouse thyroid gland. However, a higher level of RET/PTC1 is likely necessary for tumor formation. Thyroid luciferase induction was detectable noninvasively using IVIS in vivo imaging.

Tumor Suppressor LKB1 Inhibits Activation of Signal Transducer and Activator of Transcription 3 (STAT3) by Thyroid Oncogenic Tyrosine Kinase Rearranged in Transformation (RET)/Papillary Thyroid Carcinoma (PTC)

The tumor suppressor LKB1 (STK11) is a cytoplasmic/nuclear serine/threonine kinase, defects in which cause Peutz-Jeghers syndrome (PJS) in humans and animals. Recent studies showed that loss of function of LKB1 is associated with sporadic forms of lung, pancreatic, and ovarian cancer. In cancer cells, LKB1 is inactivated by two mechanisms: mutations in its central kinase domain or complete loss of LKB1 expression. Inactivation of LKB1 is associated with progression of PJS and transformation of benign polyps into malignant tumors. This study examines the effect of LKB1 on regulation of STAT3 and expression of transcriptional targets of STAT3. The results show that LKB1 inhibits rearranged in transformation (RET)/papillary thyroid carcinoma (PTC)-dependent activation of signal transducer and activator of transcription 3 (STAT3), which is mediated by phosphorylation of STAT3 tyrosine 705 by RET/PTC. Suppression of STAT3 transactivation by LKB1 requires the kinase domain but not the kinase activity of LKB1. The centrally located kinase domain of LKB1 is an approximately 260-amino-acid region that binds to the linker domain of STAT3. Chromatin immunoprecipitation studies indicate that expression of LKB1 reduces the binding of STAT3 to its target promoters and suppresses STAT3-mediated expression of Cyclin D1, VEGF, and Bcl-xL. Knockdown of LKB1 by specific small interfering RNA led to an increase in STAT3 transactivation activity and promoted cell proliferation in the presence of RET/PTC. Thus, this study suggests that LKB1 suppresses tumor growth by inhibiting RET/PTC-dependent activation of oncogenic STAT3.

Regulation of 3-phosphoinositide-dependent protein kinase-1 (PDK1) by Src involves tyrosine phosphorylation of PDK1 and Src homology 2 domain binding

3-Phosphoinositide-dependent protein kinase-1 (PDK1) appears to play a central regulatory role in many cell signalings between phosphoinositide-3 kinase and various intracellular serine/threonine kinases. In resting cells, PDK1 is known to be constitutively active and is further activated by tyrosine phosphorylation (Tyr(9) and Tyr(373/376)) following the treatment of the cell with insulin or pervanadate. However, little is known about the mechanisms for this additional activation of PDK1. Here, we report that the SH2 domain of Src, Crk, and GAP recognized tyrosine-phosphorylated PDK1 in vitro. Destabilization of PDK1 induced by geldanamycin (a Hsp90 inhibitor) was partially blocked in HEK 293 cells expressing PDK1-Y9F. Co-expression of Hsp90 enhanced PDK1-Src complex formation and led to further increased PDK1 activity toward PKB and SGK. Immunohistochemical analysis with anti-phospho-Tyr(9) antibodies showed that the level of Tyr(9) phosphorylation was markedly increased in tumor samples compared with normal. Taken together, these data suggest that phosphorylation of PDK1 on Tyr(9), distinct from Tyr(373/376), is important for PDK1/Src complex formation, leading to PDK1 activation. Furthermore, Tyr(9) phosphorylation is critical for the stabilization of both PDK1 and the PDK1/Src complex via Hsp90-mediated protection of PDK1 degradation.

Fine-needle aspiration molecular analysis for the diagnosis of papillary thyroid carcinoma through BRAF V600E mutation and RET/PTC rearrangement

OBJECTIVE

To evaluate BRAF(V600E) mutation on consecutive fine-needle aspiration biopsy (FNAB) specimens in order to assess FNAB's usefulness in preoperative papillary thyroid carcinoma (PTC) diagnosis with the contemporaneous analysis of RET/PTC1 and RET/PTC3 rearrangements obtained from ex vivo thyroid nodules.

DESIGN

Thyroid FNABs from 156 subjects with nodules and 49 corresponding surgical samples were examined for the presence of BRAF mutation by real-time allele-specific polymerase chain reaction, confirmed with the use of a laser pressure catapulting system. Samples were also examined for RET/PTC rearrangements. The results were compared with the cytological diagnosis and histopathology.

MAIN OUTCOMES

13/156 cytological examinations were diagnostic for PTC and 19/156 showed suspicious/indeterminate FNAB (12.2%). FNAB-BRAF(V600E) mutation was detected in 11/16 (69%) cases with histological confirmation of PTC. In our series, RET/PTC rearrangement was detected in only one case of PTC, whereas it was not present in any case of adenoma, goiter, or Hashimoto's thyroiditis. No PTC case was found positive at the same time for BRAF mutation and RET/PTC rearrangements.

CONCLUSION

BRAF(V600E) mutation detected on FNAB specimens, more than RET/PTC rearrangements, is highly specific for PTC and its routine research might well be an adjunctive and integrative diagnostic tool for the preoperative diagnostic iter.

Fused protein of deltaPKC activation loop and PDK1-interacting fragment (deltaAL-PIF) functions as a pseudosubstrate and an inhibitory molecule for PDK1 when expressed in cells

To elucidate the role of 3-phosphoinositide-dependent protein kinase-1 (PDK1) in cellular signaling, we constructed and expressed a pseudosubstrate of PDK1, designated as deltaAL-PIF, and characterized its properties in cultured cells. deltaAL-PIF consists of two fused proteins of the protein kinase Cdelta (deltaPKC) activation loop (deltaAL) and PDK1-interacting fragment (PIF). The phosphorylation of deltaAL-PIF was detected with anti-deltaPKC phospho-Thr505-specific antibody and was increased in proportion to the expression level of co-expressed GST-PDK1, indicating that it acts as a pseudosubstrate of PDK1. In cells expressing deltaAL-PIF, basal phosphorylation level at the activation loop of PKBalpha, deltaPKC and gammaPKC was reduced, compared with that in control cells, suggesting that deltaAL-PIF functions as an inhibitory molecule for PDK1. deltaAL-PIF affected the stability, translocation and endogenous activity of PKCs. These effects of deltaAL-PIF on gammaPKC properties were confirmed by investigation using conditioned PDK1 knockout cells. Furthermore, apoptosis frequently occurred in cells expressing deltaAL-PIF for 3 days. These findings revealed that deltaAL-PIF served as an effective pseudosubstrate and an inhibitory molecule for PDK1, suggesting that this molecule can be used as a tool for investigating PDK-mediated cellular functions as well as being applicable for anti-cancer therapy.

Investigation of interleukin 1beta-mediated regulation of NF-kappaB activation in colonic cells reveals divergence between PKB and PDK-transduced events

Recent work has highlighted a role for PDK1 in adaptive immunity, however its contribution to innate immunity has not been addressed. We have investigated the role of PKB and PDK1 in IL-1beta-induced NF-kappaB activation. Over-expression of either in HCT 116 and HEK 293T cells, effected a reproducible NF-kappaB activation. This was validated in a one-hybrid assay utilizing Gal4-RelA and Gal4-luciferase assay. N-tosyl phenylalanyl chloromethyl ketone (TPCK), wortmannin and Ly294002 inhibited IL-1beta-induced NF-kappaB activation in both systems indicating involvement of the PI3K axis in this response. p65 (Rel A) Ser536 phosphorylation was not affected by the PI3K inhibitors but was dose-dependently attenuated by TPCK. Evaluation of IKK-associated activity using GST-p65 substrate phosphorylation in immune complex assays, revealed that whilst TPCK attenuated this, neither of the PI3K inhibitors had any effect. Furthermore whilst TPCK inhibited IL-1beta-induced p65 DNA binding, this was not apparent with either of wortmannin or Ly294002. Similarly, over-expression of PDK1 but not PKB resulted in promotion of p65 DNA binding. Using a p65-S536A reporter construct, we found inhibition of only PDK1 over-expression-induced, but not PKB over-expression-induced NF-kappaB activation. This was supported using biochemical analysis in which immunoprecipitated IKKgamma from IL-1beta-activated cells was unable to phosphorylate a p65-S536A substrate, confirming this as the dominant IKK-dependent site. In further support of a dissociated response, we observed an attenuation of the Ser177/181 IKK phosphorylation by TPCK but not in response to PI3K inhibition. Our data reveals for the first time that PDK1 and PKB may differentially activate NF-kappaB, and that TPCK may subserve a useful anti-inflammatory function by inhibiting IKKbeta.

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.

Regulation of Akt/PKB by phosphatidylinositol 3-kinase-dependent and -independent pathways in B-cell chronic lymphocytic leukemia cells: role of protein kinase Cβ

Apoptosis of B cell chronic lymphocytic leukemia (B-CLL) cells is regulated by the PI-3K-Akt pathway. In the present work, we have analyzed the mechanisms of Akt phosphorylation in B-CLL cells. Freshly isolated cells present basal Akt phosphorylation, which is PI-3K-dependent, as incubation with the PI-3K inhibitor LY294002 decreased Ser-473 and Thr-308 phosphorylation in most samples analyzed (seven out of 10). In three out of 10 cases, inhibition of protein kinase C (PKC) inhibited basal Akt phosphorylation. Stromal cell-derived factor-1alpha, IL-4, and B cell receptor activation induced PI-3K-dependent Akt phosphorylation. PMA induced the phosphorylation of Akt at Ser-473 and Thr-308 and the phosphorylation of Akt substrates, independently of PI-3K in B-CLL cells. In contrast, PKC-mediated phosphorylation of Akt was PI-3K-dependent in normal B cells. Finally, a specific inhibitor of PKCbeta blocked the phosphorylation and activation of Akt by PMA in B-CLL cells. Taken together, these results suggest a model in which Akt could be activated by two different pathways (PI-3K and PKCbeta) in B-CLL cells.

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.

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

Papillary thyroid carcinoma (PTC) is a heterogenous disorder characterized by unique gene rearrangements and gene mutations that activate signaling pathways responsible for cellular transformation, survival, and antiapoptosis. Activation of protein kinase B (PKB) and its downstream signaling pathways appears to be an important event in thyroid tumorigenesis. In this study, we found that the thyroid-specific oncogenic RET/PTC tyrosine kinase is able to phosphorylate PKB in vitro and in vivo. RET/PTC-transfected cells showed tyrosine phosphorylation of endogenous and exogenous PKB, which was independent of phosphorylation of T308 and S473 regulated by the upstream kinases phosphoinositide-dependent kinase-1 and -2, respectively. The PKB Y315 residue, which is known to be phosphorylated by Src tyrosine kinase, was also a major site of phosphorylation by RET/PTC. RET/PTC-mediated tyrosine phosphorylation results in the activation of PKB kinase activity. The activation of PKB by RET/PTC blocked the activity of the forkhead transcription factor, FKHRL1, but a Y315F mutant of PKB failed to inhibit FKHRL1 activity. In summary, these observations suggest that RET/PTC is able to phosphorylate the Y315 residue of PKB, an event that results in maximal activation of PKB for RET/PTC-induced thyroid tumorigenesis.

RET tyrosine kinase signaling in development and cancer

The variety of diseases caused by mutations in RET receptor tyrosine kinase provides a classic example of phenotypic heterogeneity. Gain-of-function mutations of RET are associated with human cancer. Gene rearrangements juxtaposing the tyrosine kinase domain to heterologous gene partners have been found in sporadic papillary carcinomas of the thyroid (PTC). These rearrangements generate chimeric RET/PTC oncogenes. In the germline, point mutations of RET are responsible for multiple endocrine neoplasia type 2 (MEN 2A and 2B) and familial medullary thyroid carcinoma (FMTC). Both MEN 2 mutations and PTC gene rearrangements potentiate the intrinsic tyrosine kinase activity of RET and, ultimately, activate the RET downstream targets. Loss-of-function mutations of RET cause Hirschsprung's disease (HSCR) or colonic aganglionosis. A deeper understanding of the molecular signaling of normal versus abnormal RET activity in cancer will enable the development of potential new treatments for patients with sporadic and inherited thyroid cancer or MEN 2 syndrome. We now review the role and mechanisms of RET signaling in development and carcinogenesis.

Multiple signaling pathways converge on beta-catenin in thyroid cancer

The beta-catenin pathway has been conclusively demonstrated to regulate differentiation and patterning in multiple model systems. In thyroid cancer, alterations are often seen in proteins that regulate beta-catenin, including those of the RAS, PI3K/AKT, and peroxisome proliferation activated receptor-gamma (PPARgamma) pathways, and evidence from the literature suggests that beta-catenin may play a direct role in the dedifferentiation commonly observed in late-stage disease. RET/PTC rearrangements are frequent in thyroid cancer and appear to be exclusive from mutational events in RAS and BRAF. Activation of AKT by phosphatidylinositide-3 kinase (PI3K), a RAS effector, results in GSK3beta phosphorylation and deactivation and subsequent beta-catenin upregulation in thyroid cancer. Activating mutations in beta-catenin, which have been demonstrated in late-stage thyroid tumors, correlate with beta-catenin nuclear localization and poor prognosis. We hypothesize that activation of the RAS, PI3K/AKT, and PPARgamma pathways ultimately impinges upon beta-catenin. We further propose that if mutations in BRAF, RAS, and RET/PTC rearrangements are mutually exclusive in certain thyroid tumors or tumor types, as has already been shown for papillary thyroid cancer, then these interconnected pathways may cooperate in the initiation and promotion of the disease. We believe that clinical benefit for thyroid cancer patients could be derived from disrupting the middle or distal pathway effectors of these pathways, such as AKT or beta-catenin.

CR6-Interacting Factor 1 Interacts with Orphan Nuclear Receptor Nur77 and Inhibits Its Transactivation

CR6-interacting factor 1 (CRIF1) was recently identified as a nuclear protein that interacts with the Gadd45 (growth arrest and DNA damage inducible 45) family of proteins and participates in the regulation of the G1/S phase of the cell cycle. However, the nuclear action of CRIF1 is largely unknown. In this study, we demonstrate that CRIF1 acts as a novel coregulator of transactivation of the orphan nuclear receptor Nur77. Both in vitro and in vivo studies show that CRIF1 interacts with Nur77 via the Nur77 AB domain and that it dramatically inhibits the AB domain-mediated transactivation of Nur77. Transient transfection assays demonstrate that CRIF1 inhibits steroid receptor coactivator-2-mediated Nur77 transactivation, and silencing of endogenous CRIF1 by small interfering RNA relieves this repression. CRIF1 possesses intrinsic repressor activities that are not affected by the histone deacetylase inhibitor Trichostatin A. In addition, overexpression of CRIF1 inhibits TSH/protein kinase A-induced Nur-responsive element promoter activity. CRIF1 inhibited Nur77-dependent induction of E2F1 promoter activity, mRNA expression, and Nur77-mediated G1/S progression in cell cycle. These results suggest that CRIF1 acts as a repressor of the orphan nuclear receptor Nur77 by inhibiting AB domain-mediated transcriptional activity.

The major target of the endogenously generated reactive oxygen species in response to insulin stimulation is phosphatase and tensin homolog and not phosphoinositide-3 kinase (PI-3 kinase) in the PI-3 kinase/Akt pathway

Phosphoinositide-3 kinase (PI-3 kinase) and its downstream signaling molecules PDK-1 and Akt were analyzed in SK-N-SH and SK-N-BE(2) human neuroblastoma cell lines. When cells were stimulated with insulin, PI-3 kinase was activated in both cell lines, whereas the translocation of PDK-1 to the membrane fraction and phosphorylated Akt were observed only in SK-N-SH cells. Analyses of the insulin-mediated reactive oxygen species (ROS) generation and Phosphatase and Tensin homolog (PTEN) oxidation indicate that PTEN oxidation occurred in SK-N-SH cells, which can produce ROS, but not in SK-N-BE(2) cells, which cannot increase ROS in response to insulin stimulation. When SK-N-SH cells were pretreated with the NADPH oxidase inhibitor diphenyleneiodonium chloride before insulin stimulation, insulin-mediated translocation of PDK-1 to the membrane fraction and phosphorylation of Akt were remarkably reduced, whereas PI-3 kinase activity was not changed significantly. These results indicate that not only PI-3 kinase activation but also inhibition of PTEN by ROS is needed to increase cellular level of phosphatidylinositol 3,4,5-trisphosphate for recruiting downstream signaling molecules such as PDK-1 and Akt in insulin-mediated signaling. Moreover, the ROS generated by insulin stimulation mainly contributes to the inactivation of PTEN and not to the activation of PI-3 kinase in the PI-3 kinase/Akt pathway.

Regulation of Signal Transducer and Activator of Transcription 1 (STAT1) and STAT1-Dependent Genes by RET/PTC (Rearranged in Transformation/Papillary Thyroid Carcinoma) Oncogenic Tyrosine Kinases

Chimeric RET/PTC (rearranged in transformation/papillary thyroid carcinoma) oncoproteins are constitutively active tyrosine kinases found in thyroid papillary carcinoma and nonneoplastic Hashimoto’s thyroiditis. Although several proteins have been identified to be substrates of RET/PTC kinases, the pathogenic roles played by RET/PTC in malignant and benign thyroid diseases and the molecular mechanisms that are involved are not fully understood. We found that RET/PTC expression phosphorylates the Y701 residue of STAT1, a type II interferon (IFN)-responsive protein. RET/PTC-mediated signal transducer and activator of transcription 1 (STAT1) phosphorylation requires RET/PTC kinase activity to be intact but other tyrosine kinases, such as Janus kinases or c-Src, are not involved. RET/PTC-induced STAT1 transcriptional activation was not inhibited by suppressor of cytokine signaling-1 or -3, or protein inhibitors of activated STAT3 [(protein inhibitor of activated STAT (PIAS3)], but PIAS1 strongly repressed the RET/PTC-induced transcriptional activity of STAT1. RET/PTC-induced STAT1 activation caused IFN regulatory factor-1 expression. We found that STAT1 and IFN regulatory factor-1 cooperated to significantly increase transcription from type IV IFN-γresponsive promoters of class II transactivator genes. Significantly, cells stably expressing RET/PTC expressed class II transactivator and showed enhanced de novo membrane expression of major histocompatibility complex (MHC) class II proteins. Furthermore, RET/PTC1-bearing papillary thyroid carcinoma cells strongly expressed MHC class II (human leukocyte-associated antigen-DRα) genes, whereas the surrounding normal tissues did not. Thus, RET/PTC is able to phosphorylate and activate STAT1. This may lead to enhanced MHC class II expression, which may explain why the tissues surrounding RET/PTC-positive cancers are infiltrated with lymphocytes. Such immune response-promoting activity of RET/PTC may also relate to the development of Hashimoto’s thyroiditis.