BAG3 down-modulation reduces anaplastic thyroid tumor growth by enhancing proteasome-mediated degradation of BRAF protein

Context: Anaplastic thyroid tumors (ATC) express high levels of BAG3, a member of the BAG family of cochaperone proteins that is involved in regulating cell apoptosis through multiple mechanisms. Objective: The objective of the study was the investigation of the influence of B-cell lymphoma- 2-associated athanogene 3 (BAG3) on ATC growth. Design and Subjects: We investigated the effects of BAG3 down-modulation, obtained by using a specific small interfering RNA, on in vitro and in vivo growth of the human ATC cell line 8505C. Because BRAF protein plays an important role in ATC cell growth, we analyzed the effects of BAG3 down-modulation on BRAF protein levels. Furthermore, by using a proteasome inhibitor, we verified whether BAG3-mediated regulation of BRAF levels involved a proteasome-dependent mechanism. Results: BAG3 down-modulation significantly inhibits ATC growth in vitro and in vivo. BAG3 coimmunoprecipitates with BRAF protein, and its down-modulation results in a significant reduction of BRAF protein levels, which can be reverted by incubation with the proteasome inhibitor MG132. Conclusion: BAG3 protein sustains ATC growth in vitro and in vivo. The underlying molecular mechanism appears to rely on BAG3 binding to BRAF, thus protecting it from proteasome-dependent degradation. These results are in line with the reported ability of BAG3 to interfere with the proteasomal delivery of a number of other client proteins.