Smoothened stabilizes and protects TRAF6 from degradation: A novel non-canonical role of smoothened with implications in lymphoma biology

Abstract

Tumor necrosis factor receptor-associated factor 6 (TRAF6), an (K63) E3-ligase, plays a role in many biological processes and its activity is relevant in diffuse large B cell lymphoma (DLBCL) biology. Although molecules that trigger TRAF6 activation have been defined, those that stabilize TRAF6 and/or enhance TRAF6 function remain largely unclear. We found that TRAF6 amplifies pAKT signaling in DLBCL. Moreover, TRAF6 activation and stabilization of its ubiquitination profile are facilitated by smoothened (SMO), signal transducer of canonical Hedgehog signaling. Here, we report that SMO is needed to facilitate and maintain TRAF6-dependent elevated pAKT levels, and that the SMO/TRAF6 axis contributes to doxorubicin resistance in DLBCL. Mechanistically, we found that SMO, through its C-terminal tail, stabilizes and protects TRAF6 from degradation, an effect mediated by ubiquitin-specific protease-8. Moreover, this functional link between SMO and TRAF6 is reflected in DLBCL patients where high expression of both molecules correlates with poor prognosis. In summary, our study reveals a novel cell survival mechanism in which SMO stabilizes and protects TRAF6 from degradation. The axis SMO/TRAF6/AKT is highly relevant in the biology of DLBCL and is involved in doxorubicin resistance.