ITCH facilitates proteasomal degradation of TXNIP in hypoxia- induced lung cancer cells

Background

Lung cancer (LC) is one of the most common cancers and a leading cause of cancer‐related deaths worldwide. In many pathological conditions, particularly in the tumor microenvironment, cells and tissues frequently exist in a hypoxic state.

Here, we evaluated Itchy E3 ubiquitin protein ligase (ITCH) expression in LC cells following hypoxia treatment.

Methods

LC cell lines were treated with hypoxic condition. Cell migration, invasion, inflammation, reactive oxygen species (ROS) production, and apoptosis of LC cells were determined by wound healing assay, Transwell invasive assay, ELISA, DCFH‐DA staining, and flow cytometry, respectively. qPCR and WB were used to determine the expression of ITCH and TXNIP. Co‐IP was performed to assess the interaction between ITCH and TXNIP.

Results

ITCH expression was downregulated in LC cells under hypoxic conditions. Next, LC cells were subjected to hypoxic conditions and changes in cell viability and metastasis were determined. Hypoxic conditions resulted in increased migration and invasion abilities of LC cells. Intracellular reactive oxygen species (ROS) production, inflammation, and apoptosis were also promoted by hypoxia. We found that ITCH overexpression led to the proteasomal degradation of thioredoxin‐interacting protein (TXNIP), whereas the expression of the ITCH C830A mutant did not affect TXNIP levels in LC cells. The gain‐of‐function experiment demonstrated that migration, invasion, ROS generation, inflammation, and apoptosis of hypoxia‐conditioned LC cells were ameliorated by ITCH overexpression, whereas the ITCH C830A mutant did not cause any changes in these phenotypes. Furthermore, the contribution of TXNIP knockdown and ITCH overexpression to the hypoxia‐induced features in LC cells with ITCH C830A was found to be similar.

Conclusion

Our results suggest a novel mechanism underlying the changes in ITCH‐mediated malignant phenotypes of hypoxia‐conditioned LC cells via TXNIP.

Predicting PY motif-mediated protein-protein interactions in the Nedd4 family of ubiquitin ligases

The Nedd4 family contains several structurally related but functionally distinct HECT-type ubiquitin ligases. The members of the Nedd4 family are known to recognize substrates through their multiple WW domains, which recognize PY motifs (PPxY, LPxY) or phospho-threonine or phospho-serine residues. To better understand protein interactor recognition mechanisms across the Nedd4 family, we report the development and implementation of a python-based tool, PxYFinder, to identify PY motifs in the primary sequences of previously identified interactors of Nedd4 and related ligases. Using PxYFinder, we find that, on average, half of Nedd4 family interactions are likely PY-motif mediated. Further, we find that PPxY motifs are more prevalent than LPxY motifs and are more likely to occur in proline-rich regions and that PPxY regions are more disordered on average relative to LPxY-containing regions. Informed by consensus sequences for PY motifs across the Nedd4 interactome, we rationally designed a focused peptide library and employed a computational screen, revealing sequence- and biomolecular interaction-dependent determinants of WW-domain/PY-motif interactions. Cumulatively, our efforts provide a new bioinformatic tool and expand our understanding of sequence and structural factors that contribute to PY-motif mediated interactor recognition across the Nedd4 family.

Tobramycin suppresses HUWE1 degradation to control MCL-1 stability during tumour development

HUWE1 is an E3 ubiquitin ligase that is involved in cancer cell proliferation by regulating MCL-1 stability. The HECT domain has been shown to be required for the ubiquitin ligase activity of HUWE1. To identify efficient drugs that impair the activity of HUWE1, and thus decrease MCL-1 accumulation, we screened 2000 candidate compounds that might suppress HUWE1 activity. To evaluate these 2000 candidates, the HECT domain of HUWE1, which is the catalytic domain responsible for MCL1 ubiquitination, was selected as a conjugation site, and putative binding candidates were filtrated. Tobramycin emerged as one of the compounds that show efficient binding ability with the HECT domain of HUWE1. The surface plasmon resonance (SPR) results validated the specific binding of Tobramycin with the HECT domain. Subsequent analyses demonstrated its potential to inhibit cancer cell proliferation by binding to the HECT domain of HUWE1 and impeding the HUWE1-mediated ubiquitination of MCL-1. Consequently, the accumulation of MCL-1 inhibited the proliferation of tumour cells, while the apoptosis rates were not significantly altered after Tobramycin treatment. In vitro experiments showed that Tobramycin could inhibit cell proliferation by regulating the G2/M transition in cancer cell models, including A549 and HeLaCaco2 cell lines. Our results indicated that Tobramycin could be a potential new probe to develop targeted therapies for the prevention or treatment of HUWE1-overexpressing cancers.