A TSG101/MDM2 regulatory loop modulates MDM2 degradation and MDM2/p53 feedback control

Abstract

The p53 tumor suppressor protein and the MDM2 oncoprotein form a feedback-control loop that up-regulates cellular MDM2 production, blocks p53 activity, and promotes p53 decay. tsg101 was discovered as a gene whose deficiency results in neoplastic transformation of NIH 3T3 cells and the ability to generate metastatic tumors in nude mice. Its protein product contains a domain, Ubc, characteristic of the catalytic domain of ubiquitin conjugase (E2) enzymes but lacking an active-site cysteine crucial for ubiquitin conjugase activity. Here we report that TSG101 participates with MDM2 in an autoregulatory loop that modulates the cellular levels of both proteins, and also of p53, by affecting protein decay. We show that the Ubc domain of TSG101 interferes with ubiquitination of MDM2, that TSG101 inhibits MDM2 decay and elevates its steady-state level, and that these events are associated with down-regulation of p53 protein. Conversely, pulse-chase and Western blot experiments in wild-type and mutant fibroblasts indicate that elevation of MDM2 by overexpression of wild-type p53, by amplification of the endogenous MDM2 gene, or by transfection of MDM2-expressing constructs promotes TSG101 loss, which we show occurs by 26S proteasome-dependent decay. Our results identify TSG101 as both a regulator of, and target of, MDM2͞p53 circuitry.tumorigenesis ͉ ubiquitination ͉ proteolysis

When cell biology meets development: endocytic regulation of signaling pathways

“…Indeed, the human homolog of the class E protein Vps23p, Tsg101, reduces EGF recycling when compared to wild-type cells (Babst et al 2000). However, the suppressive effect of wild-type Tsg101/Vps23 on proliferation may be attributable to effects on the p53/MDM2 pathway in addition to lysosomal degradation of EGFR (Li et al 2001).…”

Negative regulation of cell growth and differentiation by TSG101 through association with p21 Cip1/WAF1

“…Based on sequence analysis, it has been suggested that TSG101 is a ubiquitin-conjugating enzyme E2 variant (14), and the UBC-like domain has been implicated in TSG101-mediated stabilization of the MDM2 protein (18) as well as in other cellular functions involving ubiquitination (16,17). Stabilization of the p21 protein by TSG101 is accompanied by only limited interference with p21 ubiquitination, consistent with the fact that p21, although a substrate for ubiquitination, also can undergo proteasome-mediated degradation by a ubiquitinindependent mechanism (23).…”

“…Recent experiments have shown that TSG101 has an important role in ubiquitin-mediated endosomal sorting pathways (15)(16)(17). There also is evidence implicating the TSG101 UBC domain on regulation of the p53͞MDM2 feedback control loop in human tumor cells (18). However, the mechanism(s) by which TSG101 acts to affect proliferation of normal cells has not been elucidated.…”

β-Arrestin Is Crucial for Ubiquitination and Down-regulation of the Insulin-like Growth Factor-1 Receptor by Acting as Adaptor for the MDM2 E3 Ligase

“…Although MDM2 is mostly known for its role in interaction with p53, it also binds to and interacts with other molecules (13)(14)(15)(16)(17). An interesting example is its interaction with the ␤-arrestins, which function as adaptor proteins for agonistoccupied G protein-coupled receptors (GPCR) that are phosphorylated by G protein-coupled receptor kinases (13).…”