Oncogenic addiction to high 26S proteasome level

Peter Tsvetkov,Yosef Shaul,Julia Adler,Nina Reuven,Assaf Biran,Nadav Myers

doi:10.1038/s41419-018-0806-4

Peter Tsvetkov, Yosef Shaul + Show 4 more

Open Access

https://doi.org/10.1038/s41419-018-0806-4

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Abstract

Proteasomes are large intracellular complexes responsible for the degradation of cellular proteins. The altered protein homeostasis of cancer cells results in increased dependency on proteasome function. The cellular proteasome composition comprises the 20S catalytic complex that is frequently capped with the 19S regulatory particle in forming the 26S proteasome. Proteasome inhibitors target the catalytic barrel (20S) and thus this inhibition does not allow the deconvolution of the distinct roles of 20S versus 26S proteasomes in cancer progression. We examined the degree of dependency of cancer cells specifically to the level of the 26S proteasome complex. Oncogenic transformation of human and mouse immortalized cells with mutant Ras induced a strong posttranscriptional increase of the 26S proteasome subunits, giving rise to high 26S complex levels. Depletion of a single subunit of the 19S RP was sufficient to reduce the 26S proteasome level and lower the cellular 26S/20S ratio. Under this condition the viability of the Ras-transformed MCF10A cells was severely compromised. This observation led us to hypothesize that cancer cell survival is dependent on maximal utilization of its 26S proteasomes. We validated this possibility in a large number of cancer cell lines and found that partial reduction of the 26S proteasome level impairs viability in all cancer cells examined and was not correlated with cell doubling time or reduction efficiency. Interstingly, normal human fibroblasts are refractory to the same type of 26S proteasome reduction. The suppression of 26S proteasomes in cancer cells activated the UPR and caspase-3 and cells stained positive with Annexin V. In addition, suppression of the 26S proteasome resulted in cellular proteasome redistribution, cytoplasm shrinkage, and nuclear deformation, the hallmarks of apoptosis. The observed tumor cell-specific addiction to the 26S proteasome levels sets the stage for future strategies in exploiting this dependency in cancer therapy.

Highlights

Proteasomal protein degradation is crucial in maintaining cellular integrity, regulating cell cycle, proliferation, and cell death[1]
Proteasomal degradation of proteins is mediated by two distinct proteasome complexes—the 26S and the 20S proteasomes
The 26S proteasome consists of the 20S catalytic domain assembled with either one or two 19S regulatory particles (RP)[2,3]

Summary

Introduction

Proteasomal protein degradation is crucial in maintaining cellular integrity, regulating cell cycle, proliferation, and cell death[1]. Proteasome inhibitors were shown to be effective in various screens of solid and hematologic tumors[19,21,22] Proteasome inhibitors such as bortezomib, MG132, and carfilzomib inhibit the catalytic activity within the 20S proteasome particle that is essentially responsible for the activity of both the 20S and the 26S proteasomes[23]. These drugs cannot be utilized to characterize or distinguish between any unique functions that either the 20S or the 26S proteasome complex play in the cell

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