Proteasome Inhibition: Thinking Outside the Box

Abstract

A new era in cancer therapy is emerging with the development of tumor-specific agents exhibiting less toxicity. Since the advent of imatinib, several tumor-directed treatment options have been developed. However, therapies not directed specifically at a tumor target also have potential benefits. The 26S proteasome is a critical regulator of cell homeostasis through the degradation of key signaling molecules including p21, p27, and p53. Additionally, the proteasome degrades I-kB which inhibits the activity of NF-kB, an important promoter of cell proliferation. Blocking function of the proteasome disrupts tumor growth by shifting the balance of the cell from proliferation to apoptosis. In vitro, the proteasome inhibitor, bortezomib, inhibits NF-kB activity and prevents growth of several malignant cell types including multiple myeloma. Given the central role of NF-kB in the pathogenesis of multiple myeloma, bortezomib was a good candidate for use in therapy. Treatment of heavily pre-treated patients with bortezomib led to response rates of 30%–40%. More importantly, bortezomib led to improvements in bone metabolism, a major cause of morbidity in multiple myeloma. This effect was seen independent of the response of the myeloma. This finding correlates with in vitro studies which demonstrate increased BMP2 expression and osteoblast number after exposure to bortezomib. Moreover, bortezomib blocks NF-kB-mediated angiogenesis and tumor cell metastasis. While tumor-targeted treatments have an important role in the future of cancer therapy, these examples show that it is important not to lose sight of the benefits of less-specific agents in the treatment of malignant neoplasms.