Proteasomal regulation of the NF1 tumor supressor and Ras signaling

The Ras signaling pathway plays an important role in transducing signals from activated growth factor receptors and regulates a diverse set of biological responses, including cell proliferation, differentiation and survival in nearly every tissue of the body. The specificity of these responses depends on mechanisms that regulate the amplitude and duration of Ras activity. One critical regulator of Ras is neurofibromin, which acts as a RasGAP protein to restrain Ras signaling in the absence of extracellular cues and properly downregulates Ras activation in order to maintain normal cell proliferation. Upon growth factor receptor activation, neurofibromin is degraded by the ubiquitin-proteasome pathway and this event is critical for proper activation of Ras signaling and cell cycle entry. However, the factor(s) that direct the degradation of neurofibromin are not known. Here, we have investigated the mechanisms by which the ubiquitin-proteasome pathway regulates neurofibromin stability. Specifically, we have identified the E3 ubiquitin ligase that mediates the ubiquitination and degradation of neurofibromin at the onset of the cell cycle. Critically, this enzyme is required for the proper activation of Ras and the Raf/MEK/ERK effector pathway in a manner dependent on the stability of neurofibromin. Furthermore, we have identified the adaptor protein that likely promotes the specific recognition of neurofibromin by its E3 ubiquitin ligase and demonstrate that the ectopic expression of this protein destabilizes neurofibromin and causes the upregulation of ERK signaling. As a critical negative regulator of Ras, neurofibromin acts as a tumor suppressor, whose genetic inactivation plays an important role in the deregulation of Ras signaling that drives tumor formation in the familial cancer syndrome neurofibromatosis type I (NF1), and also in sporadic tumors such as glioblastoma. We further investigated whether the proteasomal-mediated deregulation of neurofibromin also contributes to its functional inactivation in glioblastoma. We have found that neurofibromin is aberrantly destabilized by the proteasome in glioblastoma-derived cells, primary tumor cultures and tumor tissue, and have determined that this destabilization critically contributes to their transformed phenotype. Collectively, our results reveal important insights into how degradation of neurofibromin functions during normal cellular proliferation, and also illustrate that its proteasomal destabilization contributes to tumorigenesis.