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Inhibitors of Ubiquitin-Activating Enzyme (E1), a New Class of Potential Cancer Therapeutics

¹ Laboratory of Protein Dynamics and Signaling, ² Laboratory of Cancer Prevention, and ³ Molecular Targets Development Program, Center for Cancer Research, National Cancer Institute at Frederick, NIH, Frederick, Maryland; ⁴ The Beatson Institute for Cancer Research, Glasgow, United Kingdom; and ⁵ Meso Scale Discovery, Gaithersburg, Maryland

Requests for reprints: Allan M. Weissman or Yili Yang, Laboratory of Protein Dynamics and Signaling, Building 560 Room 22-103, 560 Boyles Street, Frederick, MD 21702. Phone: 301-846-1222; Fax: 301-846-1666; E-mail: amw{at}nih.gov or yangyili{at}ncifcrf.gov.

The conjugation of proteins with ubiquitin plays numerous regulatory roles through both proteasomal-dependent and nonproteasomal-dependent functions. Alterations in ubiquitylation are observed in a wide range of pathologic conditions, including numerous malignancies. For this reason, there is great interest in targeting the ubiquitin-proteasome system in cancer. Several classes of proteasome inhibitors, which block degradation of ubiquitylated proteins, are widely used in research, and one, Bortezomib, is now in clinical use. Despite the well-defined and central role of the ubiquitin-activating enzyme (E1), no cell permeable inhibitors of E1 have been identified. Such inhibitors should, in principle, block all functions of ubiquitylation. We now report 4[4-(5-nitro-furan-2-ylmethylene)-3,5-dioxo-pyrazolidin-1-yl]-benzoic acid ethyl ester (PYR-41) as the first such inhibitor. Unexpectedly, in addition to blocking ubiquitylation, PYR-41 increased total sumoylation in cells. The molecular basis for this is unknown; however, increased sumoylation was also observed in cells harboring temperature-sensitive E1. Functionally, PYR-41 attenuates cytokine-mediated nuclear factor-B activation. This correlates with inhibition of nonproteasomal (Lys-63) ubiquitylation of TRAF6, which is essential to IB kinase activation. PYR-41 also prevents the downstream ubiquitylation and proteasomal degradation of IB. Furthermore, PYR-41 inhibits degradation of p53 and activates the transcriptional activity of this tumor suppressor. Consistent with this, it differentially kills transformed p53-expressing cells. Thus, PYR-41 and related pyrazones provide proof of principle for the capacity to differentially kill transformed cells, suggesting the potential for E1 inhibitors as therapeutics in cancer. These inhibitors can also be valuable tools for studying ubiquitylation. [Cancer Res 2007;67(19):9472–81]