Regulating the stability and localization of CDK inhibitor p27^Kip1 via CSN6-COP1 axis

Abstract

The COP9 signalosome subunit 6 (CSN6), which is involved in ubiquitin-mediated protein degradation, is overexpressed in many types of cancer. CSN6 is critical in causing p53 degradation and malignancy, but its target in cell cycle progression is not fully characterized. Constitutive photomorphogenic 1 (COP1) is an E3 ubiquitin ligase associating with COP9 signalosome to regulate important target proteins for cell growth. p27 is a critical G1 CDK inhibitor involved in cell cycle regulation, but its upstream regulators are not fully characterized. Here, we show that the CSN6-COP1 link is regulating p27(Kip1) stability, and that COP1 is a negative regulator of p27(Kip1). Ectopic expression of CSN6 can decrease the expression of p27(Kip1), while CSN6 knockdown leads to p27(Kip1) stabilization. Mechanistic studies show that CSN6 interacts with p27(Kip1) and facilitates ubiquitin-mediated degradation of p27(Kip1). CSN6-mediated p27 degradation depends on the nuclear export of p27(Kip1), which is regulated through COP1 nuclear exporting signal. COP1 overexpression leads to the cytoplasmic distribution of p27, thereby accelerating p27 degradation. Importantly, the negative impact of COP1 on p27 stability contributes to elevating expression of genes that are suppressed through p27 mediation. Kaplan-Meier analysis of tumor samples demonstrates that high COP1 expression was associated with poor overall survival. These data suggest that tumors with CSN6/COP1 deregulation may have growth advantage by regulating p27 degradation and subsequent impact on p27 targeted genes.

CSN6 promotes tumorigenesis of gastric cancer by ubiquitin-independent proteasomal degradation of p16INK4a

“…Thus, we performed ubiquitination assay and found that CSN6 overexpression did not increase the endogenous ubiquitination level of p16 ( Figure 5A ). The COP9 signalosome usually collaborates with an E3 ligase such as SKP2 or COP1 to degrade target proteins through ubiquitination 30,31 . We then examined whether CSN6 reduced the expression of p16 through E3 ubiquitin ligases.…”

“…In the ubiquitination pathway, ubiquitin is activated in a two-step reaction by an E1 ubiquitin-activating enzyme. Then GC cells were treated with PYR-41 (an ubiquitin E1 inhibitor), and the results showed that p27 which is identified to be degraded by ubiquitination 31 , was upregulated with the inhibition of E1 activity. However, p16 did not change when the E1 activity was inhibited ( Figure 5C ).…”

“…Although the ubiquitin-dependent proteasomal degradation process is considered the major route that mediates proteasomal degradation 23 , proteins can also be targeted for degradation by the 20S proteasome, which does not require ubiquitylation or the presence of the 19S regulatory particle 31 . Our study showed that the E3 ligases were not involved in CSN6-mediated regulation of p16 protein degradation ( Figure 5B ).…”

COP9 signalosome subunit CSN5, but not CSN6, is upregulated in lung adenocarcinoma and predicts poor prognosis

“…Statistical analyses were carried out using IBM SPSS statistics 19 and GraphPad Prism 6. The Pearson's Х 2 test was used to determine the correlations between protein expression and clinical variables.…”

“…For instance, both CSN5 and CSN6 was identified as a c-Jun activator (16,17). CSN5 and CSN6 are capable of inducing degradation of cyclin-dependent kinase inhibitor p27Kip1 and p57Kip2 (14,15,18,19). Furthermore, CSN5 and CSN6 facilitate degradation of p53 via stabilization of MDM2 (11,20).…”