UCH-L1, a Potent Oncogene That Drives the Development of B-Cell Lymphoma, Promotes Akt Signaling by Regulating Mtor Complex Assembly

Abstract 4182

De-ubiquitinating enzymes (DUBs) are important regulators of many cellular processes through their ability to reverse the modifications installed by ubiquitin ligases. Several members of the DUB family are implicated in human cancers, where they may act as oncogenes or tumor suppressors. There is little direct in vivo evidence, however, for their roles in cancer. We recently reported that the DUB UCH-L1 is a potent oncogene that drives the development of spontaneous B-cell lymphoma in transgenic mice, and is required for the survival of malignant B-cells in vitro. The mechanism of these effects involves the strong induction of Akt signaling that is observed in culture and in lymphoid tissues of Uchl1 transgenic mice. Akt is phosphorylated on two distinct sites by the kinases PDK1 and mTOR, with both phosphorylation events required for maximal signaling. The activity of mTOR towards its substrates is dependent on the binding partners to which it is bound. When mTOR binds to raptor, it forms mTORC1, an important regulator of nutrient sensitive protein translation, whereas when complexed with rictor and SIN1, it forms mTORC2, a critical regulator of Akt signaling. It is unclear how the cell regulates the formation of mTORC1/2. Here we show a novel mechanism by which UCH-L1 regulates mTOR-dependent Akt signaling. By co-immunoprecipitating mTOR and its binding partners, we find that UCH-L1 expression promotes the binding of mTOR with rictor at the expense of raptor, without changing the level of either protein. These data suggest a novel ubiquitin-dependent regulatory step in mTOR complex assembly. In addition, UCH-L1 expression leads to a reduction in the levels of the Akt antagonist phosphatase PHLPP1 in vitro and in vivo. This decline is completely dependent on UCH-L1 DUB activity as expression of a catalytic mutant does not affect levels of PHLPP1. We find that the mechanism of this decline is not due to proteasome degradation, nor transcriptional control. We therefore hypothesize that the UCH-L1 associated shift in cellular mTOR activities leads to reduced mTORC1-dependent translation of PHLPP1. To test this, we inhibited mTORC1 activity by depleting raptor using shRNA and found a dramatic decline in PHLPP1 levels, whereas depletion of rictor had no effect on PHLPP1. We conclude that UCH-L1 boosts Akt signaling through a novel two-step mechanism by which it promotes the generation of mTORC2, reduces the levels of mTORC1, and thereby impairs the translation of PHLPP1. These data have important implications for understanding the roles of ubiquitin, mTOR, and Akt, in lymphoma biology and may lead to more intelligent use of next generation mTOR inhibitors in the treatment of lymphoma.