Myc protein is stabilized by suppression of a novel E3 ligase complex in cancer cells

Abstract

Rapid Myc protein turnover is critical for maintaining basal levels of Myc activity in normal cells and a prompt response to changing growth signals. We characterize a new Myc-interacting factor, TRPC4AP (transient receptor potential cation channel, subfamily C, member 4-associated protein)/TRUSS (tumor necrosis factor receptor-associated ubiquitous scaffolding and signaling protein), which is the receptor for a DDB1 (damage-specific DNA-binding protein 1)–CUL4 (Cullin 4) E3 ligase complex for selective Myc degradation through the proteasome. TRPC4AP/TRUSS binds specifically to the Myc C terminus and promotes its ubiquitination and destruction through the recognition of evolutionarily conserved domains in the Myc N terminus. TRPC4AP/TRUSS suppresses Myc-mediated transactivation and transformation in a dose-dependent manner. Finally, we found that TRPC4AP/TRUSS expression is strongly down-regulated in most cancer cell lines, leading to Myc protein stabilization. These studies identify a novel pathway targeting Myc degradation that is suppressed in cancer cells.

Regulation of c-Myc protein stability by proteasome activator REGγ

“…20 Several E3 ligases, including FBW7, SKP2, TRUSS, HectH9, β-Trcp1 and Hsc70-interacting protein (CHIP), have been identified as responsible for c-Myc degradation. 19,[22][23][24][25][26][27][28] However, it is still unclear whether c-Myc stability is regulated by ubiquitin-independent pathways. In the present study, we demonstrated that proteasome activator REGγ is a novel negative regulator of c-Myc and provided evidence that this regulation is evolutionarily conserved.…”

The G1 phase E3 ubiquitin ligase TRUSS that gets deregulated in human cancers is a novel substrate of the S-phase E3 ubiquitin ligase Skp2

“…As we observed that overexpression of Skp2 down-regulated TRUSS levels and an earlier report suggested low levels of TRUSS in a number of human cancer cell lines, 12 we next investigated the pattern of TRUSS and Skp2 protein expression in different tumor tissues. Serial sections of different human tumors (one each of HCC, pancreatic adenocarcinoma, colonic adenocarcinoma and breast invasive ductal carcinoma) and their adjacent non-cancerous tissues (ANCT) prepared from the paraffin blocks, were analyzed by immunohistochemistry (IHC) using specific antibodies.…”

“…12 Considering the importance of c-Myc in cell cycle regulation, we analyzed the expression of TRUSS in the synchronized population of human hepatoma Huh-7 cells (Fig. S1A).…”

“…12 Therefore, we measured the levels of TRUSS in hepatoma cells following enforced expression of HBx. There was a marked increase in the level of recombinant HA-TRUSS protein as well as c-Myc in the presence of HBx (Fig.…”

“…[7][8][9][10][11] Tumor necrosis factor receptor-associated ubiquitous scaffolding and signaling protein is one of the DCAFs known to regulate c-Myc are well known to be overexpressed in several hematological and solid tumors. Though, deregulated expression of TRUSS has been observed in many cancers, 12 very little is known about its other cellular targets and functions. Alterations, such as mutations in the myc gene, its overexpression, translocation, gene amplification or even enhanced protein stability contribute to Myc-induced oncogenesis.…”