Snail transcription factor NLS and importin β1 regulate the subcellular localization of Cathepsin L and Cux1

Abstract

Several recent studies have highlighted an additional unexpected localization and site of action for Cathepsin L (Cat L) protease within the nucleus in breast, colon and prostate cancer, however, its role in the nucleus was unclear. It was proposed to mediate proteolytic processing of the transcription factor CCAAT-displacement protein/cut homeobox transcription factor (Cux1) from the full-length p200 isoform to generate the p110 and p90 isoforms, of which the p110 isoform was shown to act as a cell cycle regulator to accelerate entry into the S phase. The p110 isoform has also been shown to bind to the promoter regions of Snail and E-cadherin to activate Snail and inactivate E-cadherin transcription, thus promoting epithelial mesenchymal transition (EMT). Mechanistic studies on what drives Cat L nuclear localization have not been reported. Our hypothesis is that Snail shuttles into the nucleus with Cat L through binding to importin-β. Snail knockdown with siRNA in MDA-MB-468 breast cancer cells led to nuclear to cytoplasmic shuttling of Cat L and decreased levels of Cux1, while overexpression of Snail in MCF-7 breast cancer cells or HEK-293 human embryonic kidney cells led to increased nuclear expression of both Cat L and Cux1. Additionally, transient transfection of Snail NLS mutants not only abrogated Snail nuclear localization but also nuclear localization of Cat L and Cux1. Interestingly, importin β1 knockdown with siRNA decreased Snail and Cux1 levels, as well as nuclear localization of Cat L. Therefore, we show for the first time that the nuclear localization of Cat L and its substrate Cux1can be positively regulated by Snail NLS and importin β1, suggesting that Snail, Cat L and Cux1 all utilize importin β1 for nuclear import.

Cathepsin L secretion by host and neoplastic cells potentiates invasion

“…However, inhibition of both lysosomal and extracellular cathepsin L are not likely to cause a decrease in the expression of M2 markers. A lesser known function of cathepsin L is its role in regulating the transcriptional activator, CCAAT-displacement protein/cut homeobox (CDP/Cux) in the nucleus [39, 40]. Nuclear cathepsin L cleaves Cux, resulting in the activation of the Cux transcription factor [39].…”

‘Patchiness’ and basic cancer research: unravelling the proteases

“…Analysis of HSP90-Cathepsin D cleavage-null mutants, partially prevented apoptosis of U937 and Jurkat cells highlighting a central role for Cathepsin D in HSP90 level regulation in LL, LMP and apoptosis. Meanwhile, Burton et al (2017) linked nuclear Cathepsin L with CUX1 proteolytic processing and thus linked Cathepsin L with EMT (and MET) progression in BC and PC cells [142]. Interestingly, in this study subcellular Cathepsin L was also observed to translocate from the nucleus to the cytoplasm, highlighting a novel protein trafficking-function for Cathepsin L. Kim et al (2018) showed that Cathepsin S could bind BRCA1 and facilitate it's ubiquitination and breakdown, thus suppressing DNA repair in BC development, while knocked-down expression of Cathepsin S stabilized BRCA1 levels, thus collectively linking Cathepsin S to BRCA1 signaling transduction [143].…”

Nuclear Import of Epstein-Barr virus BLLF2 is Mediated by an Importin β1-Dependent Mechanism

“…The classical importin-dependent mechanism for nuclear transport is well known for importin α/β/NLScargo complex, of which importin α discriminates the NLS, and importin β executes the association with small regulatory Ran-GTP to ship the complex into the nucleus [5,22,[50][51][52]. Moreover, proteins imported into the nucleus also can directly attach to importin β beyond the engagement of importin α-like adaptor [53]. Up to now, some herpesvirus-encoded proteins are reported to be transported into the nucleus by diverse mechanisms.…”