Altered Ribosome-associated protein quality control (RQC) factors and their roles in Glioblastoma
Introduction
Ribosome-associated protein quality control (RQC) is a surveillance mechanism to recognize and solve the ribosome stalling and collision in protein translation. Sequential actions of several RQC factors lead to dissociation of stalled ribosomes and elimination of potential toxic products, proteins with a C-terminal extension (CTE). Disrupted translation has been found in cancer cells. However, the involvement of the RQC pathway in cancer genesis and growth is complex. Glioblastoma (GBM) is the most malignant brain tumor characterized by highly acquired resistance to chemotherapy. It is interesting to study how RQC factors are regulated and how aberrant RQC products affect proteostasis in GBM, which could lead to potential new therapeutic methods. Our preliminary data in several patient-derived glioblastoma stem cell (GSC) lines showed that the RQC pathway has been disturbed and the mitochondrial ATP5A protein, as a possible substrate, has been decorated with CTE. CTEs cont aining proteins are the results of defective RQC mechanism that may contribute to the pathogenesis and chemotherapy resistance in GSC.
Methods
Two non-cancerous human astrocytes lines, SVG and NHA, a GBM cell line SF268, and a patient-derived GSC line GSC827 respectively, were cultured as previously reported. GEPIA database was used to evaluate differences at the transcriptomic level of RQC genes in GBM and normal brain tissue. CTE-modified ATP5A proteins (ATP5A-AT3-Flg and ATP5A-AT20-flag) and RQC genes were introduced in cell lines to study the effects of CTE on cancer cell proliferation and programmed cell death (staurosporine (STS)-induced apoptosis).
Results
Our data showed that multiple RQC proteins were differently regulated in GBM cells. For example, compared to normal astrocytes, NEMF, the CTE syntheizer, was modified in GSC, while ANKZF1, which hydrolyzes tRNA from the NEMF-ribosome complex, decreased in GSC cells, indicating the activation of RQC and accumulation of CTE modified peptides. Moreover, in the transcriptomic analysis of RQC genes, several RQC genes were found upregulated in GBM compared to normal brain tissues. In functional analysis, SVG cells were more sensitive to STS-induced apoptosis, while tumor cells (SF268 and GSC) exhibit obvious resistance to both apoptosis and autophagy. In addition, SF268 cells with stable expression of ATP5A-AT3 and AT20 became more resistant to both apoptosis and autophagy. Our study suggests that ATP5A-CTE may protect cancer cells from programmed cell death pathways. Future studies are required to investigate how altered RQC proteins affect CTEs formation in GBM cell lines, how proteins with CTEs contribute to GBM tumorigenesis, and how manipulation of RQC factors affects cancer cell behavior.
Conclusion
GBMs, especially GSCs, sustain major RQC dysregulation and accumulate CTE-containing proteins in cells. The RQC pathway is highly relevant to cancer survival and fitness in the face of chemically-induced cell death.