TSGΔ154-1054 splice variant increases TSG101 oncogenicity by inhibiting its E3-ligase-mediated proteasomal degradation

Abstract

Tumor susceptibility gene 101 (TSG101) elicits an array of cellular functions, including promoting cytokinesis, cell cycle progression and proliferation, as well as facilitating endosomal trafficking and viral budding. TSG101 protein is highly and aberrantly expressed in various human cancers. Specifically, a TSG101 splicing variant missing nucleotides 154 to 1054 (TSGΔ154-1054), which is linked to progressive tumor-stage and metastasis, has puzzled investigators for more than a decade. TSG101-associated E3 ligase (Tal)- and MDM2-mediated proteasomal degradation are the two major routes for posttranslational regulation of the total amount of TSG101. We reveal that overabundance of TSG101 results from TSGΔ154-1054 stabilizing the TSG101 protein by competitively binding to Tal, but not MDM2, thereby perturbing the Tal interaction with TSG101 and impeding subsequent polyubiquitination and proteasomal degradation of TSG101. TSGΔ154-1054 therefore specifically enhances TSG101-stimulated cell proliferation, clonogenicity, and tumor growth in nude mice. This finding shows the functional significance of TSGΔ154-1054 in preventing the ubiquitin-proteasome proteolysis of TSG101, which increases tumor malignancy and hints at its potential as a therapeutic target in cancer treatment.

Cancer-Specifically Re-Spliced TSG101 mRNA Promotes Invasion and Metastasis of Nasopharyngeal Carcinoma

“…We conclude that the over-expression of TSG∆154-1054 prohibits the degradation of the full-length TSG101 protein, which is fully consistent with our recent discovery of the TSG∆154-1054 protein function as a competitive inhibitor of the ubiquitin-proteasome degradation of TSG101 protein [31].…”

“…This discrepancy is likely attributable to the presence of TSG∆154-1054 in these cancer tissues. We previously demonstrated that the protein product of TSG∆154-1054 competitively binds to Tal, but not MDM2, thereby impeding the proper Tal binding to TSG101, leading to the protection of TSG101 from the subsequent polyubiquitination and following proteasomal degradation [31]. Our cycloheximide chase analysis indeed substantiated this TSG∆154-1054 triggered mechanism to protect the integrity of TSG101.…”

“…All of the procedures were conducted in accordance with the ethical standards of the committee on human experimentation of the College of Medicine, National Taiwan University (Institutional Review Board approval number: 201311004RIND). TSG∆154-1054 expressing TW01 stable lines were previously established in our laboratory [31].…”

“…In addition, the TSG101∆154-1054 transcript is often present in late-stage breast cancer and it correlates significantly with advanced axillary lymph node metastasis [30]. Importantly, we have recently demonstrated the function of the truncated TSG101∆154-1054 protein generated via re-splicing of TSG101 mRNA, i.e., the protection of full-length TSG101 protein from its ubiquitin-mediated proteasomal degradation [31]. Because of the common occurrence of increased TSG101 protein and its splice variant TSG101∆154-1054 in breast tumor progression, here we investigated their potential involvement in the tumorigenesis of NPC.…”

The Exon Junction Complex Core Represses Cancer-Specific Mature mRNA Re-splicing: A Potential Key Role in Terminating Splicing

“…Using these two substrates, we discovered that the mRNA re-splicing event occurs on mature spliced mRNA, in which the normal splicing removes all authentic splice sites and brings the weak re-splice sites into close proximity [ 9 ]. The re-spliced TSG101 mRNA, TSG101Δ154-1054 mRNA ( Figure 1 a), produces an aberrant protein product that up-regulates the full-size TSG101 by interfering with the ubiquitin-dependent degradation of TSG101 protein [ 10 ]. Importantly, overexpression of the TSG101 protein enhances cell proliferation and promotes malignant tumor formation in nude mice [ 10 , 11 ].…”