Targeting the proteasome as a promising therapeutic strategy in thyroid cancer. 
Targeting the ubiquitin-proteasome system by using proteasome inhibitors represents a novel approach for cancer therapy.  Anaplastic thyroid cancer (ATC), a subtype of thyroid cancer (TC), fails to respond to conventional TC treatment.  Here we investigated the effects of bortezomib on TC in vitro.  Further, the study aimed to evaluate its potential for TC treatment in vivo.  Three anaplastic (Hth74, C643, Kat4), one follicular (FTC133), and one papillary (TPC1) TC cell lines were used.  Antiproliferative, proapoptotic, and transcriptional effects of bortezomib treatment were analyzed in vitro and growth inhibition of ATC xenografts in vivo.  Tumor samples were analyzed by Ki67, CD31, caspase-3, and NF-κB immunohistochemistry.  In vitro, bortezomib inhibited proliferation of TC cells (IC(50) 4-10 nM), increased caspase-3 activity and induced cell cycle arrest.  NF-κB activity was affected differently.  In vivo, bortezomib treatment was effective in reducing tumor volume (up to 74%), accompanied by reduced proliferation (Ki67) and 57% reduced tumor vascularity.  Proteasome inhibition is effective in reducing cell growth and inducing apoptosis of ATC in vitro and inhibiting tumor growth and vascularity in vivo.  However, the impact on nuclear transcription remains controversial.  Clinical evaluation of bortezomib treatment in ATC is warranted.