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Targeted Destruction of c-Myc by an Engineered Ubiquitin Ligase Suppresses Cell Transformation and Tumor Formation

¹ Department of Molecular Biochemistry, Hokkaido University Graduate School of Medicine, Sapporo, Japan; ² Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka, Japan; and ³ Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Kawaguchi, Saitama, Japan

Requests for reprints: Keiichi I. Nakayama, Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, Fukuoka 812-8582, Japan. Phone: 81-92-642-6815; Fax: 81-92-642-6819; E-mail: nakayak1{at}bioreg.kyushu-u.ac.jp.

Given that expression of c-Myc is up-regulated in many human malignancies, targeted inactivation of this oncoprotein is a potentially effective strategy for cancer treatment. The ubiquitin-proteasome pathway of protein degradation is highly specific and can be engineered to achieve the elimination of undesirable proteins such as oncogene products. We have now generated a fusion protein (designated Max-U) that is composed both of Max, which forms a heterodimer with c-Myc, and of CHIP, which is a U box-type ubiquitin ligase (E3). Max-U physically interacted with c-Myc in transfected cells and promoted the ubiquitylation of c-Myc in vitro. It also reduced the stability of c-Myc in vivo, resulting in suppression of transcriptional activity dependent on c-Myc. Expression of Max-U reduced both the abundance of endogenous c-Myc in and the proliferation rate of a Burkitt lymphoma cell line. Furthermore, expression of Max-U but not that of a catalytically inactive mutant thereof markedly inhibited both the anchorage-independent growth in vitro of NIH 3T3 cells that overexpress c-Myc as well as tumor formation by these cells in nude mice. These findings indicate that the targeted destruction of c-Myc by an artificial E3 may represent an effective therapeutic strategy for certain human malignancies.