Recent advances in molecular biology of thyroid cancer and their clinical implications

Standard and emerging therapies for metastatic differentiated thyroid cancer

Differentiated thyroid cancer accounts for >90% of cases of thyroid cancer, with most patients having an excellent prognosis. Distant metastases occur in 10%-15% of patients, decreasing the overall 10-year survival rate in this group to 40%. Radioactive iodine has been the mainstay of treatment for distant metastases, with good results when lesions retain the ability to take up iodine. For patients with metastatic disease resistant to radioactive iodine, treatment options are few and survival is poor. Chemotherapy and external beam radiotherapy have been used in these patients, but with disappointing results. In recent years, our understanding of the molecular pathways involved in thyroid cancer has increased and a number of molecular targets have been identified. These targets include the proto-oncogenes BRAF and RET, known to be common mutations in thyroid cancer; vascular endothelial growth factor receptor and platelet-derived growth factor receptor, associated with angiogenesis; and the sodium-iodide symporter, with the aim of restoring its expression and hence radioactive iodine uptake. There are now multiple trials of tyrosine kinase inhibitors, angiogenesis inhibitors, and other novel agents available to patients with metastatic thyroid cancer. This review discusses both traditional and novel treatments for metastatic differentiated thyroid cancer with a particular focus on emerging treatments for patients with radioactive iodine-refractory disease.

Induction of thyroid gene expression and radioiodine uptake in thyroid cancer cells by targeting major signaling pathways

CONTEXT

Radioiodine ablation is commonly used to treat thyroid cancer, but a major challenge is often the loss of radioiodine avidity of the cancer caused by aberrant silencing of iodide-handling genes.

OBJECTIVES

This study was conducted to test the therapeutic potential of targeting the aberrantly activated MAPK and PI3K/Akt/mTOR pathways and histone deacetylase to restore radioiodine avidity in thyroid cancer cells.

EXPERIMENTAL DESIGN

We tested the effects of specific inhibitors targeting these pathways/molecules that had established clinical applicability, including the MAPK kinase inhibitor RDEA119, mTOR inhibitor temsirolimus, Akt inhibitor perifosine, and histone deacetylase inhibitor SAHA, individually or in combinations, on the expression of iodide-handling genes and radioiodide uptake in a large panel of thyroid cancer cell lines.

RESULTS

The expression of a large number of iodide-handling genes could be restored, particularly the sodium/iodide symporter, TSH receptor, and thyroperoxidase, by treating cells with these inhibitors. The effect was particularly robust and synergistic when combinations of inhibitors containing SAHA were used. Robust expression of sodium/iodide symporter in the cell membrane, which plays the most important role in iodide uptake in thyroid cells, was confirmed by immunofluorescent microscopy. Radioiodide uptake by cells was correspondingly induced under these conditions. Thyroid gene expression and radioiodide uptake could both be further enhanced by TSH.

CONCLUSIONS

Targeting major signaling pathways could restore thyroid gene expression and radioiodide uptake in thyroid cancer cells. Further studies are warranted to test this therapeutic potential in restoring radioiodine avidity of thyroid cancer cells for effective ablation treatment.

Targeting vascular endothelial growth factor receptor in thyroid cancer: the intracellular and extracellular implications

Our understanding of the molecular pathophysiology of differentiated thyroid cancer (DTC) has developed considerably over the last 10 years. Aberrant signaling through B-Raf and Akt has been implicated in the tumorigenesis of DTC. Moreover, these highly vascular tumors have proven to be sensitive to the inhibition of vascular endothelial growth factor receptor (VEGFR-2). It is likely that the multikinase inhibitors, sorafenib, sunitinib, axitinib, and motesanib, whose targets include VEGFR-2, exert their effects primarily through inhibition of endothelial cells. However, as VEGFR-2 is expressed on DTC cells, these compounds may have direct antitumor action. This review will discuss the key signaling pathways involved in thyroid cancer and their implications for targeted therapy.

Genetic alterations in the phosphoinositide 3-kinase/Akt signaling pathway confer sensitivity of thyroid cancer cells to therapeutic targeting of Akt and mammalian target of rapamycin

We investigated the genotype-dependent therapeutic potential of targeting the phosphoinositide 3-kinase (PI3K)/Akt pathway for thyroid cancer. Proliferation of TPC1, Hth7, FTC133, OCUT1, K1, and BCPAP cells that harbored PI3K/Akt-activating genetic alterations was potently inhibited by the Akt inhibitor perifosine, whereas SW1736, Hth74, WRO, KAT18, and TAD2 cells that harbored no genetic alterations had no or only modest responses. Inhibition of Akt phosphorylation by perifosine was seen in these cells. Genetic-dependent apoptosis was induced by perifosine in cells selectively tested. Similarly, potent inhibition of cell proliferation by the mammalian target of rapamycin (mTOR) inhibitor temsirolimus occurred in virtually all the cells harboring genetic alterations, whereas modest inhibition was seen in some of the cells not harboring genetic alterations. Temsirolimus inhibited the phosphorylation of p70S6K, a substrate of mTOR. Knockdown of Akt1/2 or mTOR by shRNA approach inhibited the proliferation and colony formation of FTC133 and OCUT1 cells that harbored genetic alterations in the PI3K/Akt pathway but had no effect on SW1736 and KAT18 cells that did not. Transfection with PIK3CA mutants greatly sensitized SW1736 cells to perifosine and temsirolimus. Growth of xenograft tumors derived from FTC133 cells but not SW1736 cells in nude mice was dramatically inhibited by perifosine. Thus, this work for the first time shows that genetic alterations in the PI3K/Akt pathway confer thyroid cancer cells addiction to this pathway and their sensitivity to inhibition by targeting Akt and mTOR. This genotype-based targeting of the PI3K/Akt pathway using Akt and mTOR inhibitors may offer an effective therapeutic strategy for thyroid cancer and warrants further studies.

Distinct genetic alterations in the mitogen-activated protein kinase pathway dictate sensitivity of thyroid cancer cells to mitogen-activated protein kinase kinase 1/2 inhibition

BACKGROUND

The mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway plays an important role in papillary and anaplastic thyroid cancer (PTC and ATC) due to activating mutations in BRAF, RAS, or rearrangements in RET/PTC1. The objective of this study was to thoroughly test whether the BRAF V600E mutation predicts response to mitogen-activated protein kinase kinase 1/2 (MKK1/2) inhibition, as shown in other tumor types, using an authenticated panel of thyroid cancer cell lines.

METHODS

PTC and ATC cells harboring distinct mutations in the MAPK pathway were treated with two different inhibitors selective for MKK1/2 (CI-1040 or U0126). The consequences of MKK1/2 inhibition on cell growth, survival, invasion, and MAPK signaling was determined.

RESULTS

Inhibition of MKK1/2 using CI-1040 or U0126 differentially inhibits the growth of a panel of PTC and ATC cell lines in two-dimensional culture, with those harboring the BRAF V600E mutation (SW1736) or BRAF-V600E/PI3K-E542K mutations (K1) being the most sensitive, the RET/PTC1 rearrangement (TPC1) and BRAF V600E mutant (BCPAP), intermediate, and the HRAS-G13R mutant (C643), the least sensitive. Growth of these cells is more sensitive to MKK1/2 inhibition when grown in 2% versus 10% serum. Baseline levels of phospho-ERK1/2 were similar in all of the cell lines, and inhibition phospho-ERK1/2 did not predict sensitivity to MKK1/2 inhibition. When cells are grown in three-dimensional culture, MKK1/2 inhibition of growth correlates with mutational status (BRAF > RET/PTC1 > RAS). Finally, PTC and ATC invasiveness is differentially inhibited by CI-1040, which is independent of tumor type or mutation present.

CONCLUSIONS

Different mutations in the MAPK pathway play distinct roles in the growth and invasion of thyroid cancer cells. These results indicate that MKK1/2 inhibitors have the potential to inhibit thyroid cancer growth and invasion, but that responses differ based on mutation status and growth conditions.