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Identification of c-Cbl as a New Ligase for Insulin-like Growth Factor-I Receptor with Distinct Roles from Mdm2 in Receptor Ubiquitination and Endocytosis

Department of Oncology and Pathology, Karolinska Institutet, Cancer Center Karolinska, Karolinska University Hospital, Stockholm, Sweden

Requests for reprints: Leonard Girnita, CCK R08:04, Karolinska University Hospital, 17176 Stockholm, Sweden. Phone: 46-8-51775242; Fax: 46-8-321047; E-mail: Leonard.Girnita{at}ki.se.

Key Words: IGF-IR • ubiquitination • c-Cbl • Mdm2

The insulin-like growth factor receptor (IGF-IR) plays several pivotal roles in cancer. Although most studies on the function of the IGF-IR have been attributed to kinase-dependent signaling, recent findings by our group and others have implicated biological roles mediated by ubiquitination of the receptor. As previously reported, the E3 ligases Mdm2 and Nedd4 mediate IGF-IR ubiquitination. Here we show that c-Cbl is a novel E3 ligase for IGF-IR. On ligand stimulation, both Mdm2 and c-Cbl associate with IGF-IR and mediate receptor polyubiquitination. Whereas Mdm2 catalyzed lysine 63 (K63) chain ubiquitination, c-Cbl modified IGF-IR through K48 chains. Mdm2-mediated ubiquitination occurred when cells were stimulated with a low concentration (5 ng/mL) of IGF-I, whereas c-Cbl required high concentrations (50–100 ng/mL). Mdm2-ubiquitinated IGF-IR was internalized through the clathrin endocytic pathway whereas c-Cbl–ubiquitinated receptors were endocytosed via the caveolin route. Taken together, our results show that c-Cbl constitutes a new ligase responsible for the ubiquitination of IGF-IR and that it complements the action of Mdm2 on ubiquitin lysine residue specificity, responsiveness to IGF-I, and type of endocytic pathway used. The actions and interactions of Mdm2 and c-Cbl in the ubiquitination and endocytosis of IGF-IR may have implications in cancer. In addition, identification and functional characterization of new E3 ligases are important in itself because therapeutic targeting of substrate-specific E3 ligases is likely to represent a critical strategy in future cancer treatment. [Cancer Res 2008;68(14):5669–77]

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