Mammalian target of rapamycin is the key effector of phosphatidylinositol-3-OH-initiated proliferative signals in the thyroid follicular epithelium

Molecular Alterations in Thyroid Cancer: From Bench to Clinical Practice

Thyroid cancer comprises different clinical and histological entities. Whereas differentiated (DTCs) malignancies are sensitive to radioiodine therapy, anaplastic (ATCs) and medullary (MTCs) tumors do not uptake radioactive iodine and display aggressive features associated with a poor prognosis. Moreover, in a majority of DTCs, disease evolution leads to the progressive loss of iodine sensitivity. Hence, iodine-refractory DTCs, along with ATCs and MTCs, require alternative treatments reflective of their different tumor biology. In the last decade, the molecular mechanisms promoting thyroid cancer development and progression have been extensively studied. This has led to a better understanding of the genomic landscape, displayed by thyroid malignancies, and to the identification of novel therapeutic targets. Indeed, several pharmacological compounds have been developed for iodine-refractory tumors, with four multi-target tyrosine kinase inhibitors already available for DTCs (sorafenib and lenvatinib) and MTCs (cabozantib and vandetanib), and a plethora of drugs currently being evaluated in clinical trials. In this review, we will describe the genomic alterations and biological processes intertwined with thyroid cancer development, also providing a thorough overview of targeted drugs already tested or under investigation for these tumors. Furthermore, given the existing preclinical evidence, we will briefly discuss the potential role of immunotherapy as an additional therapeutic strategy for the treatment of thyroid cancer.

Synergistic repression of thyroid hyperplasia by cyclin C and Pten

The cyclin C-Cdk8 kinase has been identified as both a tumor suppressor and an oncogene depending on the cell type. The genomic locus encoding cyclin C (Ccnc) is often deleted in aggressive anaplastic thyroid tumors. To test for a potential tumor suppressor role for cyclin C, Ccnc alone, or Ccnc in combination with a previously described thyroid tumor suppressor Pten, was deleted late in thyroid development. Although mice harboring individual Pten or Ccnc deletions exhibited modest thyroid hyperplasia, the double mutant demonstrated dramatic thyroid expansion resulting in animal death by 22 weeks. Further analysis revealed that Ccnc^thyr-/- tissues exhibited a reduction in signal transducer and activator of transcription 3 (Stat3) phosphorylation at Ser727. Further analysis uncovered a post-transcriptional requirement of both Pten and cyclin C in maintaining the levels of the p21 and p53 tumor suppressors (also known as CDKN1A and TP53, respectively) in thyroid tissue. In conclusion, these data reveal the first tumor suppressor role for cyclin C in a solid tumor model. In addition, this study uncovers new synergistic activities of Pten and cyclin C to promote quiescence through maintenance of p21 and p53.

PI3K/mTOR inhibition potentiates and extends palbociclib activity in anaplastic thyroid cancer

Anaplastic thyroid carcinoma (ATC) is the most aggressive form of thyroid cancer. Despite its low incidence, it accounts for a disproportionate number of thyroid cancer-related deaths, because of its resistance to current therapeutic approaches. Novel actionable targets are urgently needed to prolong patient survival and increase their quality of life. Loss and mutation of the RB1 tumor suppressor are rare events in ATC, which suggests that therapies directed at inhibiting the cyclin D/CDK4 complexes, responsible for RB phosphorylation and inactivation, might be effective in this tumor type. In fact, we found that the CDK4/6 inhibitor, palbociclib, strongly inhibits proliferation in all the RB1 wild type ATC cell lines tested. Efficacy was also observed in vivo, in a xenograft model. However, ATC cells rapidly developed resistance to palbociclib. Resistance was associated with increased levels of cyclin D1 and D3. To counter cyclin D overexpression, we tested the effect of combining palbociclib with the PI3K/mTOR dual inhibitor, omipalisib. Combined treatment synergistically reduced cell proliferation, even in cell lines that do not carry PI3K-activating mutations. More importantly, low-dose combination was dramatically effective in inhibiting tumor growth in a xenograft model. Thus, combined PI3K/mTOR and CDK4/6 inhibition is a highly promising novel approach for the treatment of aggressive, therapy-resistant thyroid cancer.

Evaluation of preclinical efficacy of everolimus and pasireotide in thyroid cancer cell lines and xenograft models

BACKGROUND

Signaling through mTOR and somatostatin pathway is implicated in thyroid cancer development.

METHOD

We evaluated everolimus, an mTOR inhibitor and pasireotide, a multi receptor somatostatin analogue as potential therapy of thyroid cancer focusing on the in vitro and in vivo efficacy, as well as possible mechanism to explain any observed interaction.

RESULTS

Both everolimus and pasireotide inhibit the growth of thyroid cancer cell lines in vitro with varied efficacy that correlates with tumor origin and somatostatin receptor (SSTR) expression profile of the cell lines. In vitro activity of everolimus show positive correlation with the expression of SSTR types 1, 4 and 5 (CC: 0.9; 0.85, 0.87) while pasireotide activity show negative correlation with SSTR2 (CC: -0.87). Although there is greater modulation of pS6 when pasireotide is combined with everolimus, there is no significant abrogation of the expected feedback upregulation of AKT induced by everolimus. Also, the combination is not significantly better than each agent alone in short and long term in vitro assays. Continuous administration of everolimus at a low dose as opposed to high intermittent dosing schedule has greater antitumor efficacy against thyroid cancer xenografts in vivo. Pasireotide LAR has modest in vivo efficacy and the combination of everolimus and pasireotide LAR achieve greater tumor growth inhibition than each agent alone in TPC-1 xenograft model of thyroid cancer (p = 0.048).

CONCLUSION

Our findings provide support for the clinical evaluation of everolimus and pasireotide in thyroid cancer and other neuroendocrine tumors.

A comprehensive review of regulatory test methods for endocrine adverse health effects

Development of new endocrine disruption-relevant test methods has been the subject of intensive research efforts for the past several decades, prompted in part by mandates in the 1996 Food Quality Protection Act (FQPA). While scientific understanding and test methods have advanced, questions remain on whether current scientific methods are capable of adequately addressing the complexities of the endocrine system for regulatory health and ecological risk assessments. The specific objective of this article is to perform a comprehensive, detailed evaluation of the adequacy of current test methods to inform regulatory risk assessments of whether a substance has the potential to perturb endocrine-related pathways resulting in human adverse effects. To that end,  approximately 42 existing test guidelines (TGs) were considered in the evaluation of coverage for endocrine-related adverse effects. In addition to evaluations of whether test methods are adequate to capture endocrine-related effects, considerations of further enhancements to current test methods, along with the need to develop novel test methods to address existing test method gaps are described. From this specific evaluation, up to 35 test methods are capable of informing whether a chemical substance perturbs known endocrine related biological pathways. Based on these findings, it can be concluded that current validated test methods are adequate to discern substances that may perturb the endocrine system, resulting in an adverse health effect. Together, these test methods predominantly form the core data requirements of a typical food-use pesticide registration submission. It is recognized, however, that the current state of science is rapidly advancing and there is a need to update current test methods to include added enhancements to ensure continued coverage and public health and environmental protection.

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.

PI3K/Akt/mTOR: A promising therapeutic target for non-medullary thyroid carcinoma

Thyroid carcinoma (TC) is the most common endocrine malignancy. The pathogenesis of TC is complex and involves multiple genetic events that lead to activation of oncogenic pathways such as the MAP kinase (MAPK) pathway and the PI3K/Akt/mTOR pathway. The PI3K/Akt pathway has emerged as an important player in the pathogenesis of TC, particularly in follicular and advanced anaplastic or poorly differentiated TC. Because these patients have a poor prognosis, particularly when their tumors become resistant to the conventional treatment with radioactive iodine, efforts have been made to identify possible targets for therapy within these pathways. Orally available drugs targeting the PI3K/Akt/mTOR pathway are being used with success in treatment of several types of malignant tumors. There is an increasing amount of preclinical and clinical data supporting that this pathway may represent a promising target for systemic therapy in TC. The present review focuses on the most recent developments on the role of the PI3K/Akt pathway in the pathogenesis of non-medullary TC and will provide insight into how this pathway can be targeted either alone or in the context of multimodal therapeutic strategies for treatment of advanced TC.

New therapies for dedifferentiated papillary thyroid cancer

The number of thyroid cancers is increasing. Standard treatment usually includes primary surgery, thyroid-stimulating hormone suppressive therapy, and ablation of the thyroid remnant with radioactive iodine (RAI). Despite the generally good prognosis of thyroid carcinoma, about 5% of patients will develop metastatic disease, which fails to respond to RAI, exhibiting a more aggressive behavior. The lack of specific, effective and well-tolerated drugs, the scarcity of data about the association of multi-targeting drugs, and the limited role of radioiodine for dedifferentiated thyroid cancer, call for further efforts in the field of new drugs development. Rearranged during transfection (RET)/papillary thyroid carcinoma gene rearrangements, BRAF (B-RAF proto-oncogene, serine/threonine kinase) gene mutations, RAS (rat sarcoma) mutations, and vascular endothelial growth factor receptor 2 angiogenesis pathways are some of the known pathways playing a crucial role in the development of thyroid cancer. Targeted novel compounds have been demonstrated to induce clinical responses and stabilization of disease. Sorafenib has been approved for differentiated thyroid cancer refractory to RAI.

SIN1, a critical component of the mTOR-Rictor complex, is overexpressed and associated with AKT activation in medullary and aggressive papillary thyroid carcinomas

BACKGROUND

Mammalian target of rapamycin (mTOR) forms 2 active complexes in the cell: the rapamycin-sensitive mTOR-Raptor (mTORC1) and the rapamycin-insensitive mTOR-Rictor (mTORC2). The latter activates AKT kinase, which promotes tumor cell survival and proliferation by multiple downstream targets. Mammalian stress-activated protein kinase interacting protein 1 (SIN1), an essential subunit of the mTORC2 complex, maintains the integrity of the complex and substrate specificity and regulates Akt activation. The role of mTOR-Rictor complex activation in thyroid carcinogenesis remains unknown. Therefore, we investigated expression patterns of Sin1 in the cells lines of thyroid carcinoma and tumors and their association with AKT activation, histologic type, and tumor aggressiveness.

METHODS

Tissue specimens from 42 patients with thyroid cancer, including follicular (5), papillary (18), medullary (16), and poorly differentiated (3) carcinomas were analyzed via immunohistochemistry for SIN1 expression and AKT phosphorylation at Ser473 residue (Ser473-p-AKT). Eight of 18 papillary carcinomas were aggressive histologic variants. In addition, expression of Sin1 and activation of AKT kinase were analyzed in fresh-frozen tissue samples (normal/tumor), primary cell cultures (papillary thyroid carcinoma [PTC]), and an established thyroid cancer cell line (medullary thyroid carcinoma) by Western blotting.

RESULTS

With immunohistochemistry, we found that Sin1 was overexpressed in medullary thyroid carcinomas and aggressive variants of papillary thyroid carcinoma compared with conventional papillary and follicular carcinomas (P < .001). Sin1 expression correlated with AKT activation in the entire study group (P = .002). Using Western blot analysis, we found that Sin1 and p-AKT were detected at a greater level in cultured primary cells from aggressive PTC compared with conventional PTC as well as in cell lines of medullary and anaplastic thyroid carcinoma. High expression levels of SIN1 were detected in papillary thyroid carcinomas compared with benign nodules in immunoblots in which we used fresh-frozen patient samples. Two of the Sin1 protein isoforms, p76 and p55, were detected predominantly in PTC samples.

CONCLUSION

Sin1, a critical factor of the mTORC2 complex is overexpressed in clinically aggressive thyroid cancer types and is associated strongly with activation of AKT kinase. Sin1-dependent AKT activation might represent a target for experimental therapy.

Switch in signaling control of mTORC1 activity after oncoprotein expression in thyroid cancer cell lines

CONTEXT

Thyroid growth is regulated by TSH and requires mammalian target of rapamycin (mTOR). Thyroid cancers frequently exhibit mutations in MAPK and/or phosphoinositol-3-kinase-related kinase effectors.

OBJECTIVE

The objective of the study was to explore the contribution of RET/PTC, RAS, and BRAF to mTOR regulation and response to mTOR inhibitors.

METHODS

PCCL3 cells conditionally expressing RET/PTC3, HRAS(G12V), or BRAF(V600E) and human thyroid cancer cells harboring mutations of these genes were used to test pathways controlling mTOR and its requirement for growth.

RESULTS

TSH/cAMP-induced growth of PCCL3 cells requires mTOR, which is stimulated via protein kinase A in a MAPK kinase (MEK)- and AKT-independent manner. Expression of RET/PTC3, HRAS(G12V), or BRAF(V600E) in PCCL3 cells induces mTOR but does not entirely abrogate the cAMP-mediated control of its activity. Acute oncoprotein-induced mTOR activity is regulated by MEK and AKT, albeit to differing degrees. By contrast, mTOR was not activated by TSH/cAMP in human thyroid cancer cells. Tumor genotype did not predict the effects of rapamycin or the mTOR kinase inhibitor AZD8055 on growth, with the exception of a PTEN-null cell line. Selective blockade of MEK did not influence mTOR activity of BRAF or RAS mutant cells. Combined MEK and mTOR kinase inhibition was synergistic on growth of BRAF- and RAS-mutant thyroid cancer cells in vitro and in vivo.

CONCLUSION

Thyroid cancer cells lose TSH/cAMP dependency of mTOR signaling and cell growth. mTOR activity is not decreased by the MEK or AKT inhibitors in the RAS or BRAF human thyroid cancer cell lines. This may account for the augmented effects of combining the mTOR inhibitors with selective antagonists of these oncogenic drivers.

Evaluation of PTEN, PI3K, MTOR, and KRAS expression and their clinical and prognostic relevance to differentiated thyroid carcinoma

Aim of the study

Important signalling pathways play fundamental roles in the pathogenesis of thyroid carcinoma (TC). PTEN, mTOR, PI3K-p85 and K-Ras are the principal factors involved in these signalling pathways. To immunohistochemically examine the expressions of PI3K, mTOR and PTEN in patients suffering from follicular TC, papillary TC or variants thereof, as well as to investigate KRAS mutations via PCR to determine their clinical and prognostic relevance to differentiated thyroid cancer.

Material and methods

The expression of PTEN, PI3K-p85 and mTOR was immunohistochemically examined, and the mutation of K-Ras was examined via PCR. The results obtained were compared to the clinico-pathologic characteristics of the patients.

Results

A significant correlation was found between p85 expression and lymphovascular invasions and between PTEN expression and multifocality (p = 0.048 and p = 0.04, respectively), and a correlation between p85 and capsular invasion was found, with a borderline statistical significance (p = 0.056). No expression of PTEN, p85 or Mtor was detected in normal tissue. K-Ras mutation was examined in 66 of the 101 patients (57.4%), and the percentage of patients exhibiting a K-Ras mutation was 17.4%. All of the patients exhibiting a K-Ras mutation were women (p = 0.047). The disease-free survival was 44.6 months (95% CI: 37.9–51.3) and was statistically significantly higher in the group that displayed level 1 or lower expression of p85 (p = 0.043).

Conclusions

The expression levels of the aforementioned markers were significantly higher in TC cells than in normal tissue. A significant correlation was detected between K-Ras mutation and gender. This study demonstrates that p85 and PTEN are markers that should be evaluated in further studies of TC.

Follicular thyroid cancers demonstrate dual activation of PKA and mTOR as modeled by thyroid-specific deletion of Prkar1a and Pten in mice

CONTEXT

Thyroid cancer is the most common form of endocrine cancer, and it is a disease whose incidence is rapidly rising. Well-differentiated epithelial thyroid cancer can be divided into papillary thyroid cancer (PTC) and follicular thyroid cancer (FTC). Although FTC is less common, patients with this condition have more frequent metastasis and a poorer prognosis than those with PTC.

OBJECTIVE

The objective of this study was to characterize the molecular mechanisms contributing to the development and metastasis of FTC.

DESIGN

We developed and characterized mice carrying thyroid-specific double knockout of the Prkar1a and Pten tumor suppressor genes and compared signaling alterations observed in the mouse FTC to the corresponding human tumors.

SETTING

The study was conducted at an academic research laboratory. Human samples were obtained from academic hospitals.

PATIENTS

Deidentified, formalin-fixed, paraffin-embedded (FFPE) samples were analyzed from 10 control thyroids, 30 PTC cases, five follicular variant PTC cases, and 10 FTC cases.

INTERVENTIONS

There were no interventions.

MAIN OUTCOME MEASURES

Mouse and patient samples were analyzed for expression of activated cAMP response element binding protein, AKT, ERK, and mammalian target of rapamycin (mTOR). Murine FTCs were analyzed for differential gene expression to identify genes associated with metastatic progression.

RESULTS

Double Prkar1a-Pten thyroid knockout mice develop FTC and recapitulate the histology and metastatic phenotype of the human disease. Analysis of signaling pathways in FTC showed that both human and mouse tumors exhibited strong activation of protein kinase A and mTOR. The development of metastatic disease was associated with the overexpression of genes required for cell movement.

CONCLUSIONS

These data imply that the protein kinase A and mTOR signaling cascades are important for the development of follicular thyroid carcinogenesis and may suggest new targets for therapeutic intervention. Mouse models paralleling the development of the stages of human FTC should provide important new tools for understanding the mechanisms of FTC development and progression and for evaluating new therapeutics.

Inhibition of AMPK and Krebs cycle gene expression drives metabolic remodeling of Pten-deficient preneoplastic thyroid cells

Rapidly proliferating and neoplastically transformed cells generate the energy required to support rapid cell division by increasing glycolysis and decreasing flux through the oxidative phosphorylation (OXPHOS) pathway, usually without alterations in mitochondrial function. In contrast, little is known of the metabolic alterations, if any, which occur in cells harboring mutations that prime their neoplastic transformation. To address this question, we used a Pten-deficient mouse model to examine thyroid cells where a mild hyperplasia progresses slowly to follicular thyroid carcinoma. Using this model, we report that constitutive phosphoinositide 3-kinase (PI3K) activation caused by PTEN deficiency in nontransformed thyrocytes results in a global downregulation of Krebs cycle and OXPHOS gene expression, defective mitochondria, reduced respiration, and an enhancement in compensatory glycolysis. We found that this process does not involve any of the pathways classically associated with the Warburg effect. Moreover, this process was independent of proliferation but contributed directly to thyroid hyperplasia. Our findings define a novel metabolic switch to glycolysis driven by PI3K-dependent AMPK inactivation with a consequent repression in the expression of key metabolic transcription regulators.

An inducible knockout mouse to model the cell-autonomous role of PTEN in initiating endometrial, prostate and thyroid neoplasias

PTEN is one of the most frequently mutated tumor suppressor genes in human cancers. The role of PTEN in carcinogenesis has been validated by knockout mouse models. PTEN heterozygous mice develop neoplasms in multiple organs. Unfortunately, the embryonic lethality of biallelic excision of PTEN has inhibited the study of complete PTEN deletion in the development and progression of cancer. By crossing PTEN conditional knockout mice with transgenic mice expressing a tamoxifen-inducible Cre-ER^T under the control of a chicken actin promoter, we have generated a tamoxifen-inducible mouse model that allows temporal control of PTEN deletion. Interestingly, administration of a single dose of tamoxifen resulted in PTEN deletion mainly in epithelial cells, but not in stromal, mesenchymal or hematopoietic cells. Using the mT/mG double-fluorescent Cre reporter mice, we demonstrate that epithelial-specific PTEN excision was caused by differential Cre activity among tissues and cells types. Tamoxifen-induced deletion of PTEN resulted in extremely rapid and consistent formation of endometrial in situ adenocarcinoma, prostate intraepithelial neoplasia and thyroid hyperplasia. We also analyzed the role of PTEN ablation in other epithelial cells, such as the tubular cells of the kidney, hepatocytes, colonic epithelial cells or bronchiolar epithelium, but those tissues did not exhibit neoplastic growth. Finally, to validate this model as a tool to assay the efficacy of anti-tumor drugs in PTEN deficiency, we administered the mTOR inhibitor everolimus to mice with induced PTEN deletion. Everolimus dramatically reduced the progression of endometrial proliferations and significantly reduced thyroid hyperplasia. This model could be a valuable tool to study the cell-autonomous mechanisms involved in PTEN-loss-induced carcinogenesis and provides a good platform to study the effect of anti-neoplastic drugs on PTEN-negative tumors.

Activation of mTOR signaling in medullary and aggressive papillary thyroid carcinomas

BACKGROUND

Because mammalian target of rapamycin (mTOR) may be involved in thyroid carcinogenesis, we investigated the expression and activation patterns of mTOR signaling proteins in thyroid carcinoma cells and tumors and their association with tumor histology and aggressiveness.

METHODS

Tissue specimens from 50 patients with thyroid cancer were analyzed for eIF4E, a critical downstream target of the mTOR pathway, using immunohistochemistry. In addition, fresh-frozen samples from patients, and primary tumor cell cultures were analyzed for expression and activation of mTOR signaling proteins by Western blot. Moreover, pharmacologic studies with rapamycin were performed.

RESULTS

High expression of eIF4E was observed in medullary thyroid carcinomas (MTC) and in aggressive variants of papillary thyroid carcinomas (PTC) as compared with conventional PTC and follicular thyroid carcinomas (P < .0001). The level of eIF4E expression also correlated with tumor stage (P = .002). Using Western blot analysis, p-rpS6, p-4EBP1, 4EBP1, and eIF4E were detected at higher levels in aggressive PTC and MTC cells. Treatment of MTC cells with increasing concentrations of rapamycin resulted in significant cell death and in decreased cell growth associated with deactivation of the mTOR pathway.

CONCLUSION

mTOR signaling, which controls protein synthesis through regulation of translation initiation, is activated in aggressive PTC and MTC and represents a promising target for investigational therapies in these patients.

The mTOR protein as a target in thyroid cancer

INTRODUCTION

The mammalian target of rapamycin (mTOR) protein is a downstream effector of the phosphatidilinositol-3 kinase (PI3K)/Akt pathway, which regulates not only cell proliferation and viability, but also iodide uptake in thyroid cells. Genetic alterations in the PI3K/Akt/mTOR pathway are common during thyroid cancer progression, and thus, these proteins are attractive targets for cancer therapy. So far, specific mTOR inhibitors, such as rapamycin analogs, have been developed and studied as anti-cancer agents.

AREAS COVERED

This review discusses evidence that justifies the potential use of mTOR signaling pathway inhibitors as therapeutic agents for thyroid cancer.

EXPERT OPINION

In the near future, mTOR-targeted drugs might represent a new approach for the therapy of thyroid cancer patients; rapamycin analogs have already been developed and are currently being clinically tested. Besides the antiproliferative action of mTOR inhibition, the stimulatory effect on thyroid iodide uptake can also be useful in the treatment of recurrent thyroid cancer. Therefore, if rapamycin analogs are able to increase iodide uptake in thyroid cancer, either alone or in combination with other agents, this will represent a new approach for the treatment of thyroid cancer, which may possibly improve the treatment of patients in which radioiodine therapy is not effective.

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.

Mouse models of follicular and papillary thyroid cancer progression

A significant number of well-differentiated thyroid cancers progress or recur, becoming resistant to current therapeutic options. Mouse models recapitulating the genetic and histological features of advanced thyroid cancer have been an invaluable tool to dissect the mechanisms involved in the progression from indolent, well differentiated tumors to aggressive, poorly differentiated carcinomas, and to identify novel therapeutic targets. In this review, we focus on the lessons learned from models of epithelial cell-derived thyroid cancer showing progression from hyperplastic lesions to locally invasive and metastatic carcinomas.

Cross-talk between PI3K and estrogen in the mouse thyroid predisposes to the development of follicular carcinomas with a higher incidence in females

It is well known that thyroid disease is more frequent in women than in men, however the molecular basis for this gender difference is still poorly understood. PI3K activation, through different mechanisms including loss of the PTEN tumor suppressor, is being increasingly recognized as a major player in the development of thyroid neoplastic lesions. Loss of Pten in the mouse thyroid results in a significant increase in the thyrocyte proliferative index, which is more prominent in the female mice. Here we show that 52% of the Pten^−/− female mice, but only 12% of the males, develop follicular adenomas by one year of age. In addition, 50% of female mutants, but only 35% of males older than one year of age develop invasive, and often metastatic, follicular carcinomas. Mutant females have a significantly shorter overall survival compared to male mutants. Hormonal manipulation experiments established a direct role of estrogens in controlling the increased thyrocyte proliferation index in mutant females. Furthermore, while genetic ablation of one Cdkn1b allele accelerated the development of neoplastic lesions, it also abolished the gender differences in survival and reduced the difference in neoplastic lesion development rate, underlining a key role of p27 in mediating estrogen action in the thyroid follicular cells.

These data, based on a clinically relevant model of thyroid follicular carcinoma, provide for the first time in vivo evidence that circulating estrogens are directly responsible for the increased female susceptibility to thyroid disease, at least upon activation of the PI3K pathway, and provide novel insights into the gender differences characterizing thyroid neoplastic disorders.

The PI3K-Akt-mTOR pathway in initiation and progression of thyroid tumors

The phosphoinositide-3 (OH) kinase (PI3K) signaling cascade is involved in regulating glucose uptake and metabolism, growth, motility, and other essential functions for cell survival. Unregulated activation of this pathway commonly occurs in cancer through a variety of mechanisms, including genetic mutations of kinases and regulatory proteins, epigenetic alterations that alter gene expression and translation, and posttranslational modifications. In thyroid cancer, constitutive activation of PI3K signaling has been shown to play a role in the genetic predisposition for thyroid neoplasia in Cowden's syndrome, and is recognized to be frequently overactivated in sporadic forms of thyroid cancer including those with aggressive clinical behaviors. In this review, the key signaling molecules in the PI3K signaling cascade, the abnormalities known to occur in thyroid cancer, and the potential for therapeutic targeting of PI3K pathway members will be discussed.

cAMP-dependent activation of mammalian target of rapamycin (mTOR) in thyroid cells. Implication in mitogenesis and activation of CDK4

How cAMP-dependent protein kinases [protein kinase A (PKA)] transduce the mitogenic stimulus elicited by TSH in thyroid cells to late activation of cyclin D3-cyclin-dependent kinase 4 (CDK4) remains enigmatic. Here we show in PC Cl3 rat thyroid cells that TSH/cAMP, like insulin, activates the mammalian target of rapamycin (mTOR)-raptor complex (mTORC1) leading to phosphorylation of S6K1 and 4E-BP1. mTORC1-dependent S6K1 phosphorylation in response to both insulin and cAMP required amino acids, whereas inhibition of AMP-activated protein kinase and glycogen synthase kinase 3 enhanced insulin but not cAMP effects. Unlike insulin, TSH/cAMP did not activate protein kinase B or induce tuberous sclerosis complex 2 phosphorylation at T1462 and Y1571. However, like insulin, TSH/cAMP produced a stable increase in mTORC1 kinase activity that was associated with augmented 4E-BP1 binding to raptor. This could be caused in part by T246 phosphorylation of PRAS40, which was found as an in vitro substrate of PKA. Both in PC Cl3 cells and primary dog thyrocytes, rapamycin inhibited DNA synthesis and retinoblastoma protein phosphorylation induced by TSH and insulin. Although rapamycin reduced cyclin D3 accumulation, the abundance of cyclin D3-CDK4 complexes was not affected. However, rapamycin inhibited the activity of these complexes by decreasing the TSH and insulin-mediated stimulation of activating T172 phosphorylation of CDK4. We propose that mTORC1 activation by TSH, at least in part through PKA-dependent phosphorylation of PRAS40, crucially contributes to mediate cAMP-dependent mitogenesis by regulating CDK4 T172-phosphorylation.

Biomarker Development for the Clinical Activity of the mTOR Inhibitor Everolimus (RAD001): Processes, Limitations, and Further Proposals

The mTOR inhibitor everolimus (RAD001, Afinitor) is an orally active anticancer agent. Everolimus demonstrates growth-inhibitory activity against a broad range of tumor cell histotypes in vitro and has the capacity to retard tumor growth in preclinical tumor models in vivo through mechanisms directed against both the tumor cell and the solid tumor stroma components. These properties have rendered it to be a clinically active drug, with subsequent registration in renal cell carcinoma (Motzer et al. [2008]. Lancet372, 449-456) as well as showing strong potential as a combination partner (André F et al. [2008]. J Clin Oncol26. Abstract 1003). Although everolimus has a high specificity for its molecular target, the ubiquitous nature of mTOR and the multifactorial influence that mTOR signaling has on cell physiology have made studies difficult on the identification and validation of a biomarker set to predict and monitor drug sensitivity for clinical use. In this review, a summary of the preclinical and clinical data relevant to biomarker development for everolimus is presented, and the advantages and problems of current biomarkers are reviewed. In addition, alternative approaches to biomarker development are proposed on the basis of examples of a combination of markers and functional noninvasive imaging. In particular, we show how basal levels of pAKT and pS6 together could, in principle, be used to stratify patients for likely response to an mTOR inhibitor.

Inhibition of mTORC1 signaling reduces tumor growth but does not prevent cancer progression in a mouse model of thyroid cancer

Selective drugs targeting dysregulated oncogenic pathways are promising cancer therapies. Because the mammalian target of rapamycin complex 1 (mTORC1) pathway is hyperactivated in human follicular thyroid cancer (FTC), we hypothesized that its inhibition could block cancer development and progression. We, therefore, analyzed the effect of a treatment with a specific mTORC1 inhibitor (RAD001) in a faithful mouse model of FTC with constitutive mTORC1 activation (TRbeta(PV/PV)Pten(+/-) mice). The treatment did not prevent capsular and vascular invasion of the thyroid and the occurrence of lung metastasis. However, it substantially decelerated thyroid tumor growth, thereby prolonging TRbeta(PV/PV)Pten(+/-) mouse life span. RAD001 efficiently inhibited mTORC1 activity, as shown by the reduced phosphorylation of its downstream targets involved in the activity of the translation machinery, such as ribosomal S6 kinase (p70(S6K)), eukaryotic translation initiation factor 4E binding protein (4E-BP1) and the eukaryotic translation initiation factors eIF-4B and eIF-4G. Whereas mTORC1 signaling inhibition did not alter cell apoptosis, it induced a significant decrease in cell proliferation that was associated with the reduced abundance and altered activity of key regulators of cell cycle progression. Altogether, our data indicate that mTORC1 signaling plays a major role in the integration of the mitogenic signal in FTC. Therefore, our preclinical study with a relevant mouse model of FTC demonstrates for the first time that RAD001 efficaciously stabilizes cancer growth although it does not prevent its fatal outcome. In conclusion, our work underscores that in the treatment of FTC patients, RAD001 can only be used in combination with drugs and therapies inducing tumor shrinkage and blocking metastasis.

Protein kinase A and B-Raf mediate extracellular signal-regulated kinase activation by thyrotropin

Thyrotropin (TSH) regulates thyroid cell proliferation and function through cAMP-mediated signaling pathways that activate protein kinase A (PKA) and Epac/Rap1. The respective roles of PKA versus Epac/Rap1 in TSH signaling remain unclear. We set out to determine whether PKA and/or Rap1 mediate extracellular signal-regulated kinase (ERK) activation by TSH. Neither blocking Rap1 activity nor silencing the expression of Rap1 impaired TSH or forskolin-induced ERK activation in Wistar rat thyroid cells. Direct activation of Epac1 failed to stimulate ERK activity in starved cells, suggesting that Epac-induced Rap1 activity is not coupled to ERK activation in rat thyroid cells. By contrast, PKA activity was required for cAMP-stimulated ERK phosphorylation and was sufficient to increase ERK phosphorylation in starved cells. Expression of dominant-negative Ras inhibited ERK activation by TSH, forskolin, and N(6)-monobutyryl (6MB)-cAMP, a selective activator of PKA. Silencing the expression of B-Raf also inhibited ERK activation by TSH, forskolin, and 6MB-cAMP, but not that stimulated by insulin or serum. Depletion of B-Raf impaired TSH-induced DNA synthesis, indicating a functional role for B-Raf in TSH-regulated proliferation. Collectively, these results position PKA, Ras, and B-Raf as upstream regulators of ERK activation and identify B-Raf as a selective target of cAMP-elevating agents in thyroid cells. These data provide the first evidence for a functional role for B-Raf in TSH signaling.

Dual inhibition of mitogen-activated protein kinase kinase and mammalian target of rapamycin in differentiated and anaplastic thyroid cancer

CONTEXT

Differentiated thyroid cancer and anaplastic thyroid cancer tumors frequently have activation of the ras/raf /MAPK kinase (MEK)/ERK and phosphatidylinositol 3-kinase (PI-3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathways.

OBJECTIVE

The objective of the study was to investigate the efficacy of MEK and mTOR inhibitors in preclinical thyroid cancer treatment models with defined mutation status.

EXPERIMENTAL DESIGN

The MEK inhibitor AZD6244 (ARRY-142886) and mTOR inhibitor rapamycin were tested separately and in combination in 10 differentiated thyroid cancer and anaplastic thyroid cancer cell lines and in a xenograft model for evidence of pathway inhibition, growth inhibition, apoptosis, and long-range adaptation and resistance.

RESULTS

Seven of 10 tested lines had evidence of significant basal activity of the PI-3K/AKT/mTOR pathway, with elevated phosphorylated AKT and phosphorylated p70 S6 kinase. Activation of ras/RAF/MEK/ERK was equally common in this panel. All 10 lines exhibited better than 60% growth inhibition with combined MEK and mTOR inhibition, including lines with BRAF, Ret-PTC, ras, and PTEN mutations. Rapamycin or AZD6244 alone achieved this threshold in six and two lines, respectively. Dual-pathway inhibition in the Ret-PTC mutant cell line TPC1 caused an intense G(1) arrest in cell culture and reversible cytostatic inhibition in a xenograft model. We did not observe significant feedback up-regulation of AKT activation in either acute or prolonged exposures.

CONCLUSION

These preclinical results support the inclusion of thyroid cancer patients in early-phase clinical trials combining RAS/RAF/MEK/ERK and PI-3K/AKT/mTOR pathway inhibition.

Intracellular parasitism with Toxoplasma gondii stimulates mammalian-target-of-rapamycin-dependent host cell growth despite impaired signalling to S6K1 and 4E-BP1

The Ser/Thr kinase mammalian-target-of-rapamycin (mTOR) is a central regulator of anabolism, growth and proliferation. We investigated the effects of Toxoplasma gondii on host mTOR signalling. Toxoplasma invasion of multiple cell types rapidly induced sustained mTOR activation that was restricted to infected cells, as determined by rapamycin-sensitive phosphorylation of ribosomal protein S6; however, phosphorylation of the growth-associated mTOR substrates 4E-BP1 and S6K1 was not detected. Infected cells still phosphorylated S6K1 and 4E-BP1 in response to insulin, although the S6K1 response was blunted. Parasite-induced S6 phosphorylation was independent of S6K1 and did not require activation of canonical mTOR-inducing pathways mediated by phosphatidylinositol 3-kinase-Akt and ERK. Host mTOR was localized in a vesicular pattern surrounding the parasitophorous vacuole, suggesting potential activation by phosphatidic acid in the vacuolar membrane. In spite of a failure to phosphorylate 4E-BP1 and S6K1, intracellular T. gondii triggered host cell cycle progression in an mTOR-dependent manner and progression of infected cells displayed increased sensitivity to rapamycin. Moreover, normal cell growth was maintained during parasite-induced cell cycle progression, as indicated by total cellular S6 levels. The Toxoplasma-infected cell provides a unique example of non-canonical mTOR activation supporting growth that is independent of signalling through either S6K1 or 4E-BP1.

DeltaNp73alpha inhibits PTEN expression in thyroid cancer cells

DeltaNp73 is a N-terminally truncated p53 family member with a dominant negative function, which is upregulated in cancer. PTEN is a lipid phosphatase, which is involved in the attenuation of tyrosine kinase signaling. PTEN expression is increased by p53, and its function is blunted in several malignancies. Because in most of the thyroid carcinomas, DeltaNp73alpha is upregulated, whereas PTEN expression down regulated, we investigated whether DeltaNp73alpha may influence PTEN expression in this cell model. We found that DeltaNp73alpha overexpression in thyroid cancer cells reduces PTEN expression, whereas DeltaNp73alpha down-regulation by siRNA increases PTEN expression. Real-time PCR indicated that overexpression of DeltaNp73alpha is able to reduce PTEN mRNA levels. Moreover, chromatin immunoprecipitation (ChIP) and luciferase assays indicated that DeltaNp73alpha binds to -1031-779 region of the PTEN promoter, which is a different site than that for p53, thereby inhibiting promoter activity. Interestingly, also the transcriptionally active p73 isoforms (TAp73alpha and TAp73beta) bound to this DNA sequence and, at variance with DeltaNp73alpha, stimulated PTEN promoter activity to an extent similar to that of p53. In accordance with its effect on PTEN protein levels, DeltaNp73alpha increased phospho-Akt protein content and, as a consequence, Mdm2-mediated p53 degradation. This effect of DeltaNp73alpha resulted in increased thyroid cancer cell proliferation and reduced apoptosis and was reverted by the PI3-kinase inhibitor LY294002, indicating the role of Akt pathway in this effect. Taken together, these results indicate a novel p73 regulated mechanism for PTEN expression in thyroid cancer cells, and that, also through this mechanism, DeltaNp73alpha exerts its protumorigenic effect.

Oncogenic Kras requires simultaneous PI3K signaling to induce ERK activation and transform thyroid epithelial cells in vivo

Thyroid tumors arising from the follicular cells often harbor mutations leading to the constitutive activation of the PI3K and Ras signaling cascades. However, it is still unclear what their respective contribution to the neoplastic process is, as well as to what extent they interact. We have used mice harboring a Kras oncogenic mutation and a Pten deletion targeted to the thyroid epithelium to address in vivo these questions. Here, we show that although each of these two pathways, alone, is unable to transform thyroid follicular cells, their simultaneous activation is highly oncogenic, leading to invasive and metastatic follicular carcinomas. In particular, phosphatidylinositol-3-kinase (PI3K) activation suppressed Kras-initiated feedback signals that uncouple mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) and ERK activation, thus stunting MAPK activity; in addition, PI3K and Kras cooperated to drastically up-regulate cyclin D1 mRNA levels. Finally, combined pharmacologic inhibition of PI3K and MAPK completely inhibited the growth of double-mutant cancer cell lines, providing a compelling rationale for the dual targeting of these pathways in thyroid cancer.

New agents in the treatment for malignancies of the salivary and thyroid glands

The treatment of relatively rare malignancies, such as those of the salivary glands and iodine refractory thyroid cancer, has been invigorated by the development of novel molecular targeting agents. Accrual to clinical trials for these disease sites continues to be limited by their relatively low incidence. Nonetheless, multicenter collaborations have contributed greatly to the development of a number of emerging systemic therapies. This article briefly summarizes the epidemiology and pathogenesis of salivary gland and thyroid cancer, and then describes some of the new drugs under evaluation for these malignancies.

Androgen-induced programs for prostate epithelial growth and invasion arise in embryogenesis and are reactivated in cancer

Cancer cells differentiate along specific lineages that largely determine their clinical and biologic behavior. Distinct cancer phenotypes from different cells and organs likely result from unique gene expression repertoires established in the embryo and maintained after malignant transformation. We used comprehensive gene expression analysis to examine this concept in the prostate, an organ with a tractable developmental program and a high propensity for cancer. We focused on gene expression in the murine prostate rudiment at three time points during the first 48 h of exposure to androgen, which initiates proliferation and invasion of prostate epithelial buds into surrounding urogenital sinus mesenchyme. Here, we show that androgen exposure regulates genes previously implicated in prostate carcinogenesis comprising pathways for the phosphatase and tensin homolog (PTEN), fibroblast growth factor (FGF)/mitogen-activated protein kinase (MAPK), and Wnt signaling along with cellular programs regulating such 'hallmarks' of cancer as angiogenesis, apoptosis, migration and proliferation. We found statistically significant evidence for novel androgen-induced gene regulation events that establish and/or maintain prostate cell fate. These include modulation of gene expression through microRNAs, expression of specific transcription factors, and regulation of their predicted targets. By querying public gene expression databases from other tissues, we found that rather than generally characterizing androgen exposure or epithelial budding, the early prostate development program more closely resembles the program for human prostate cancer. Most importantly, early androgen-regulated genes and functional themes associated with prostate development were highly enriched in contrasts between increasingly lethal forms of prostate cancer, confirming a 'reactivation' of embryonic pathways for proliferation and invasion in prostate cancer progression. Among the genes with the most significant links to the development and cancer, we highlight coordinate induction of the transcription factor Sox9 and suppression of the proapoptotic phospholipid-binding protein Annexin A1 that link early prostate development to early prostate carcinogenesis. These results credential early prostate development as a reliable and valid model system for the investigation of genes and pathways that drive prostate cancer.

Thyroid-stimulating hormone/cAMP-mediated proliferation in thyrocytes

Current research on thyrotropin-activated proliferation in the thyrocyte needs to be aimed at a better understanding of crosstalk and negative-feedback mechanisms with other proliferative pathways, especially the insulin/IGF-1-induced phosphoinositol-3 kinase pathway and the serum-induced MAPK or Wnt pathways. Convergence of proliferative pathways in mTOR is a hotspot of current research, and combined treatment using double class inhibitors for thyroid cancer may bring some success. New thyroid-stimulating hormone receptor (TSHR)-interacting proteins, a better picture of cAMP targets, a deeper knowledge of the action of the protein kinase A regulatory subunits, especially their interactions with the replication machinery, and a further understanding of mechanisms that lead to cell cycle progression through G₁/S and G₂/M checkpoints are areas that need further elucidation. Finally, massive information coming from microarray data analysis will prompt our understanding of thyroid-stimulating hormone-promoted thyrocyte proliferation in health and disease.

On the action of cyclosporine A, rapamycin and tacrolimus on M. avium including subspecies paratuberculosis

Background

Mycobacterium avium subspecies paratuberculosis (MAP) may be zoonotic. Recently the “immuno-modulators” methotrexate, azathioprine and 6-MP and the “anti-inflammatory” 5-ASA have been shown to inhibit MAP growth in vitro. We concluded that their most plausible mechanism of action is as antiMAP antibiotics. The “immunosuppressants” Cyclosporine A, Rapamycin and Tacrolimus (FK 506) treat a variety of “autoimmune” and “inflammatory” diseases. Rapamycin and Tacrolimus are macrolides. We hypothesized that their mode of action may simply be to inhibit MAP growth.

Methodology

The effect on radiometric MAP ¹⁴CO₂ growth kinetics of Cyclosporine A, Rapamycin and Tacrolimus on MAP cultured from humans (Dominic & UCF 4) or ruminants (ATCC 19698 & 303) and M. avium subspecies avium (ATCC 25291 & 101) are presented as “percent decrease in cumulative GI” (%-ΔcGI.)

Principal Findings

The positive control clofazimine has 99%-ΔcGI at 0.5 µg/ml (Dominic). Phthalimide, a negative control has no dose dependent inhibition on any strain. Against MAP there is dose dependent inhibition by the immunosuppressants. Cyclosporine has 97%-ΔcGI by 32 µg/ml (Dominic), Rapamycin has 74%-ΔcGI by 64 µg/ml (UCF 4) and Tacrolimus 43%-ΔcGI by 64 µg/ml (UCF 4)

Conclusions

We show heretofore-undescribed inhibition of MAP growth in vitro by “immunosuppressants;” the cyclic undecapeptide Cyclosporine A, and the macrolides Rapamycin and Tacrolimus. These data are compatible with our thesis that, unknowingly, the medical profession has been treating MAP infections since 1942 when 5-ASA and subsequently azathioprine, 6-MP and methotrexate were introduced in the therapy of some “autoimmune” and “inflammatory” diseases.