Grb10/Nedd4-mediated multiubiquitination of the insulin-like growth factor receptor regulates receptor internalization. 
The adaptor protein Grb10 is an interacting partner of the IGF-I receptor (IGF-IR) and the insulin receptor (IR). Previous work from our laboratory has established the role of Grb10 as a negative regulator of IGF-IR-dependent cell proliferation. We have shown that Grb10 binds the E3 ubiquitin ligase Nedd4 and promotes IGF-I-stimulated ubiquitination, internalization, and degradation of the IGF-IR, thereby giving rise to long-term attenuation of signaling. Recent biochemical evidence suggests that tyrosine-kinase receptors (RTK) may not be polyubiquitinated but monoubiquitinated at multiple sites (multiubiquitinated). However, the type of ubiquitination of the IGF-IR is still not defined. Here we show that the Grb10/Nedd4 complex upon ligand stimulation mediates multiubiquitination of the IGF-IR, which is required for receptor internalization. Moreover, Nedd4 by promoting IGF-IR ubiquitination and internalization contributes with Grb10 to negatively regulate IGF-IR-dependent cell proliferation. We also demonstrate that the IGF-IR is internalized through clathrin-dependent and-independent pathways. Grb10 and Nedd4 remain associated with the IGF-IR in early endosomes and caveosomes, where they may participate in sorting internalized receptors. Grb10 and Nedd4, unlike the IGF-IR, which is targeted for lysosomal degradation are not degraded and likely directed into recycling endosomes. These results indicate that Grb10 and Nedd4 play a critical role in mediating IGF-IR down-regulation by promoting ligand-dependent multiubiquitination of the IGF-IR, which is required for receptor internalization and regulates mitogenesis.
