Antitumor Effects of Proteasome Inhibition in Anaplastic Thyroid Carcinoma

Abstract

The ubiquitin–proteasome pathway has been identified as a potential molecular target for cancer therapy. In this study, we investigated the effect of the proteasome inhibitor bortezomib on anaplastic thyroid carcinoma (ATC) characterized by complete refractoriness to multimodal therapeutic approaches. Methods: The ATC cell lines C643 and SW1736 were treated with bortezomib (1 nM to 1 μM) for 12–72 h. Thereafter, growth inhibition was analyzed by thymidine uptake experiments and determination of the viable cell number. Apoptosis was measured and a cell cycle analysis was done. Using gene chip analysis and the real-time quantitative PCR system, we measured transcriptional changes. The activity of the nuclear factor (NF)-κB and p53 signal transduction pathways was monitored using the reporter constructs pNF-κB-TA-Luc and pp53-TA-Luc in the luciferase activity assay. Uptake measurements using ³H-FDG, ¹⁴C-aminoisobutyric acid, and Na¹²⁵iodide were performed to investigate metabolic changes and iodide symporter activity in vitro. Moreover, the ¹⁸F-FDG uptake was evaluated in ATC tumor–bearing nude mice 1 or 2 d after treatment with bortezomib. Results: Bortezomib induced growth inhibition, apoptosis, and G₂–M cell cycle arrest associated with upregulation of p21^CIP1/WAF1 expression in SW1736 and C643 cells. Moreover, the glucose metabolism and aminoisobutyric acid uptake significantly decreased in vitro in both of the ATC cell lines in vivo only in SW1736 tumors at 2 d after the bortezomib treatment. The transcriptional profile in bortezomib-treated SW1736 and C643 cells revealed increased expression of genes involved in stress response, apoptosis, regulation of the cell cycle, and differentiation. Using real-time quantitative PCR for the quantification of gene expression, we additionally noticed upregulation of the tumor necrosis factor–related apoptosis-inducing ligand and the thyroid-specific transcription factors Pax8 and TTF-1, leading to expression of the thyroid-specific target genes thyroglobulin, sodium iodide symporter, thyroperoxidase, and thyroid-stimulating hormone receptor and to a moderate accumulation of iodide in ATC cells. Conclusion: On the basis of our data, bortezomib represents a promising antineoplastic agent for the treatment of ATC. To improve the clinical outcome, further investigation into the potential of bortezomib therapy of thyroid cancer is clearly warranted.