Downregulation of 26S proteasome catalytic activity promotes epithelial-mesenchymal transition

Abstract

The epithelial-mesenchymal transition (EMT) endows carcinoma cells with phenotypic plasticity that can facilitate the formation of cancer stem cells (CSCs) and contribute to the metastatic cascade. While there is substantial support for the role of EMT in driving cancer cell dissemination, less is known about the intracellular molecular mechanisms that govern formation of CSCs via EMT. Here we show that β2 and β5 proteasome subunit activity is downregulated during EMT in immortalized human mammary epithelial cells. Moreover, selective proteasome inhibition enabled mammary epithelial cells to acquire certain morphologic and functional characteristics reminiscent of cancer stem cells, including CD44 expression, self-renewal, and tumor formation. Transcriptomic analyses suggested that proteasome-inhibited cells share gene expression signatures with cells that have undergone EMT, in part, through modulation of the TGF-β signaling pathway. These findings suggest that selective downregulation of proteasome activity in mammary epithelial cells can initiate the EMT program and acquisition of a cancer stem cell-like phenotype. As proteasome inhibitors become increasingly used in cancer treatment, our findings highlight a potential risk of these therapeutic strategies and suggest a possible mechanism by which carcinoma cells may escape from proteasome inhibitor-based therapy.

Proteasome activity regulates CD8+ T lymphocyte metabolism and fate specification

“…Pluripotent stem cells exhibit higher levels of proteasome activity compared with their differentiated progeny (18,19). Moreover, regulation of proteasome activity has been shown to influence the specification and function of cancer stem cells, a subset of cancer cells that are characterized by self-renewal capacity and the ability to initiate tumors (20,21). Our data provide evidence supporting a role for proteasome activity in another important biologic context -CD8 + T lymphocyte fate specification in response to microbial infection.…”

“…Proteasome activity assays. Cells were incubated with 5 μM activitybased proteasome probe (21,28) for 2 hours at 37°C, then washed and analyzed with flow cytometry. To analyze probed cells using SDS-PAGE, cells were lysed with cold lysis buffer (50 mM Tris-HCl, pH 8.5, 150 mM NaCl, 1% Triton X-100) for 30 minutes at 4°C with vortexing.…”

“…We therefore asked whether asymmetric localization of the proteasome during mitosis might lead to distinct proteasome activity levels in the nascent daughter cells following division. Using a model system that we have previously employed to examine T cells undergoing their first division in vivo in response to microbial infection (9), we assessed the total activity of the proteasome catalytic subunits using irreversible, covalent activity-based probes that have been previously described (Supplemental Figure 1; supplemental material available online with this article; https://doi.org/10.1172/ JCI90895DS1) (21,27,28).…”

“…Regulation of endogenous proteasome activity has been previously shown to control cell fate decisions of human embryonic stem cells (18,19) and cancer stem cells (20,21). Moreover, recent studies in lymphocytes have shown that proteasome activity can also influence their differentiation and function (22)(23)(24)(25)(26).…”

Proteasome expression and activity in cancer and cancer stem cells

“…In addition, patients with breast cancer with low PSMB2 and PSMB5 units expression in their tumor tissue had a worse survival than high-expression counterparts. 35 Of note, though, the PSMB2 unit checked in this study through the genomic Oncomine platform would correspond to a non-enzymatic unit of the β ring (β4) and not the intended β2 unit.…”