Smoothened as a Novel Signal Stabilization Scaffold in Doxorubicin Resistance

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Smoothened as a Novel “Signal Stabilization Scaffold” in Doxorubicin Resistance Doxorubicin (DXR) is among the most widely used therapeutics in cancer treatment. However, combined with its cardiotoxicity, the need for higher dosing due to drug resistance is a major challenge in treatment. Diffuse B- Cell Lymphoma (DLBCL) is the most common lymphoma in adults, and DXR resistance is the predominant limiting factor for the successful use of current anthracycline based standard therapy (CHOP). Enhanced AKT signaling is known to be associated with loss of DXR sensitivity, and traditionally, mechanisms that directly act on its phosphorylation/dephosphorylation have been the main focus of research. More recently, competing AKT ubiquitination modes have emerged as a critical factor that balances degradation (UB-K48 linkage) versus membrane recruitment, Ser/Thr phosphorylation and activation (UB-K63 linkage). Membrane localized pAKT is represents the most potent oncogenic signal. We found that TRAF6, a (K63)E3-ligase, is responsible for regulating pAKT stability and function in DLBCL. Moreover, we observed for the first time that this mechanism is dependent on the seven transmembrane spanning receptor “Smoothened” (SMO). SMO is localized in raft microdomains, recruits TRAF6, initiates TRAF6 auto stabilization and activation, and thereby enhances signaling that is needed for DXR resistance. This novel role of SMO goes beyond its canonical role in Hedgehog signaling and is reflected in elevated SMO expression in samples from patients with DXR resistant DLBCL. Ectopic SMO expression or SMO knockdown decrease or increase DXR sensitivity respectively in lymphoma cell lines. In this proposal, we will dissect the regions of SMO that are needed for this novel function and identify the interaction partners that are assembled for this raft-localized signal stabilization function. In addition, we will use a patient derived xenograft mouse model to evaluate the contribution of SMO to chemoresistance and evaluate the utility of adding existing SMO inhibitors to the current chemotherapy regimens to reverse resistance.