COP1 Acts as a Ubiquitin Ligase for PCDH9 Ubiquitination and Degradation in Human Glioma

Ubiquitin ligase RFWD2 promotes dendritic spine and synapse formation by activating the ERK/PEA3/c-Jun pathway in rat cerebral cortical neurons

The regulation of protein degradation through the ubiquitin-proteasome system is essential for normal brain development, axon growth, synaptic growth and plasticity. The E3 ubiquitin ligase RFWD2 plays a key role in the onset and development of neurological diseases, including the pathogenesis of Alzheimer's disease (AD), but the mechanisms controlling the homeostasis of neuronal synaptic proteins are still poorly understood. Here, we showed that the expression level of RFWD2 gradually decreased with the age of the rats and was negatively correlated with the development of cerebral cortical neurons and dendrites in vivo. RFWD2 was shown to localize to presynaptic terminals and some postsynaptic sides of both excitatory synapses and inhibitory synapses via colocalization with neuronal synaptic proteins (SYN, PSD95, Vglut1 and GAD67). Overexpression of RFWD2 promoted dendrite development and dendritic spine formation and markedly decreased the expression of synaptophysin and PSD95 by reducing the expression of ETV1, ETV4, ETV5 and c-JUN in vitro. Furthermore, the whole-cell membrane slice clamp results showed that RFWD2 overexpression resulted in greater membrane capacitance in neuronal cells, inadequate cell repolarization, and a longer time course for neurons to emit action potentials with decreased excitability. RFWD2 regulates dendritic development and plasticity, dendritic spine formation and synaptic function in rat cerebral cortex neurons by activating the ERK/PEA3/c-Jun pathway via a posttranslational regulatory mechanism and can be used as an efficient treatment target for neurological diseases.

Deficiency of SIAH1 promotes the formation of filopodia by increasing the accumulation of FASN in liver cancer

It has been shown that the formation of filopodia is a key step in tumor cell metastasis, but there is limited research regarding its mechanism. In this study, we demonstrated that fatty acid synthase (FASN) promoted filopodia formation in liver cancer cells by regulating fascin actin-bundling protein 1 (FSCN1), a marker protein for filopodia. Mechanistically, on the one hand, the accumulation of FASN is caused by the enhanced deubiquitination of FASN mediated by UCHL5 (ubiquitin c-terminal hydrolase L5). In this pathway, low expression of SIAH1 (Seven in absentia homolog 1) can decrease the ubiquitination and degradation of ADRM1 (adhesion regulating molecule 1) thereby increasing its protein level, which will recruit and activate the deubiquitination enzyme UCHL5, leading to FASN undergo deubiquitination and escape from proteasomal degradation. On the other hand, the accumulation of FASN is related to its weakened ubiquitination, where SIAH1 directly acts as a ubiquitin ligase toward FASN, and low expression of SIAH1 reduces the ubiquitination and degradation of FASN. Both the two pathways are involved in the regulation of FASN in liver cancer. Our results reveal a novel mechanism for FASN accumulation due to the low expression of SIAH1 in human liver cancer and suggest an important role of FASN in filopodia formation in liver cancer cells.

Constitutive photomorphogenic protein 1 ubiquitinates interleukin-1 receptor accessory protein in human liver cancer

BACKGROUND

Constitutive photomorphogenic protein 1 (COP1) plays a pivotal role in the development and progression of several human cancers and is reported to be upregulated in liver cancer. However, the role of COP1 in human liver cancer is unclear.

METHODS

We analyzed the COP1 expression in normal liver and liver cancer tissue samples using western blot and immunohistochemical analysis. We overexpressed and silenced COP1 in HepG2 and Huh7 cells and analyzed the effect on liver cancer cell proliferation. Additionally, COP1 was used as a bait to screen COP1-interacting proteins in a human cDNA library in a yeast two-hybrid screen and the results were confirmed with co-immunoprecipitation (co-IP) assays. Moreover, immunofluorescence staining was performed to assess co-localization. The protein levels of COP1 and mIL1RAcP were determined in clinical samples.

RESULTS

COP1 was upregulated in liver cancer samples compared to that in normal tissue samples. COP1 overexpression promoted proliferation of liver cancer cells, while COP1 knockdown exerted the opposite effect. Yeast two-hybrid screen identified interleukin-1 receptor accessory protein (IL1RAP) as a potential COP1-interacting protein. Co-IP assays further confirmed that COP1 interacts with both preIL1RAP and membrane-bound form of IL1RAP (mIL1RAP). Furthermore, COP1 upregulated mIL1RAP protein levels and promoted nuclear translocation and activation of the nuclear factor kappa B (NF-κB) (p50/p65) dimer. Additionally, we demonstrated that COP1 regulated mIL1RAP expression through K63-linked polyubiquitination, suggesting that COP1 plays a role in stabilizing mIL1RAP. Finally, the protein levels of COP1 and mIL1RAcP were found to be positively correlated in clinical samples.

CONCLUSION

COP1 regulates IL1RAP, which in turn results in activation of the NF-κB signaling. Our findings suggest that the COP1/IL1RAP/NF-κB axis promotes proliferation of liver cancer cells and is a potential target for the treatment of liver cancer.