New research suggests that when cancer cells are under stress, cellular chaperone protein GRP78 alters its behavior and migrates to the cell's nucleus, where it modifies gene activities and allows cancer cells to become more mobile and invasive.
A groundbreaking new finding regarding the 78-kDa glucose–regulated protein (GRP78) has shed light on the intricate process of cancer cell metastasis and offers new therapeutic options to reduce their spread.
According to the study published in Proceedings of the National Academy of Sciences, when cancer cells are under stress, cellular chaperone protein GRP78 alters its behavior and migrates to the cell's nucleus, where it modifies gene activities and allows cancer cells to become more mobile and invasive.
The study was conducted by researchers at the Keck School of Medicine of the University of Southern California (USC). Previous research by the same team demonstrated how GRP78 becomes hijacked during times of cellular stress due to cancer or SARS-CoV-2 infection, allowing for the replication of viral invaders, as well as the progression and increasing treatment resistance of cancer.
Before the current study, it was known that GRP78 typically resides in the endoplasmic reticulum (ER) of the cell. According to the researchers, the new finding that the protein can translocate to the nucleus when overexpressed and gain new functions regulating homeostasis and tumorigenesis represents a paradigm shift for cell biology.
“This study uncovers a molecular mechanism by which cancer cells respond to stress through nuclear translocation of GRP78/BiP, which assumes a role as a transcriptional regulator, allowing cells to adopt an invasive phenotype and impacting other pathways,” they wrote. “Our study further suggests that GRP78/BiP inhibitors may offer a therapeutic approach to suppress EGFR in various human lung cancer cells without the limitations of targeting specific mutations.”
The study further highlighted several more key findings that expand on unconventional roles of cell chaperone proteins in health and disease.
First, it showed that suppressing GRP78 in various human lung cancer cell lines—regardless of their EGFR mutational or amplification status—consistently led to reduced levels of EGFR protein expression both on and within the cell. Since EGFR plays diverse roles in tumorigenesis, proliferation, survival, and metastasis in lung cancer and other malignancies, this finding suggests that targeting GRP78 could be a promising approach to suppress EGFR expression, potentially overcoming the limitations of current EGFR-targeted therapies.
Another major finding was that, in the analysis of gene expression in 207 human lung cancer cell lines, there was a significant positive correlation between GRP78 and EGFR transcript levels. After confirming that reducing GRP78 also decreased EGFR mRNA levels in various cell lines, the researchers discovered that GRP78 regulates EGFR promoter activity without affecting its mRNA stability. Additionally, the knockdown of GRP78 did not lead to a reduction in the mRNA level of KRAS, another well-known oncogene. According to the study, this suggests that the effect of GRP78 depletion on gene transcription is specific and not a result of a widespread global transcriptional shutdown.
The USC researchers also investigated the mechanism of GRP78 nuclear translocation and identified a strong nuclear localization signal (NLS) sequence near the carboxyl end of the ATPase domain that contained positively charged amino acids, such as lysine and arginine. In the study, mutating these charged residues to neutral alanine prevented GRP78 from translocating into the nucleus, while the chaperone function remained unaffected. Additionally, the NLS mutant failed to rescue EGFR transcriptional activity, indicating the importance of the NLS in stimulating gene transcription. The study also suggested that posttranslational modifications—which can alter the charged residues—and the possible route or routes of GRP78 entry into the nucleus remain interests for future research.
The study also showed that GRP78 binds to ID2, another cellular protein that typically suppresses genes such as EGFR, and many of which allow for the migration of cells. However, ID2 cannot do this when bound to GRP78, and without that suppression, cancer cells become more invasive.
Finally, the study's RNA-Seq and bioinformatics analysis unveiled pathways controlled by nuclear GRP78. Notably, genes elevated by nuclear GRP78, but not the NLS mutant, were linked to cell migration and invasion, particularly associated with the extracellular matrix and adhesion phenotype.
Future investigations are warranted to explore other interacting partners and functions of nuclear GRP78, the authors said.
Liu Z, Liu G, Ha DP, Wang, J, Xiong M, Lee, AS. ER chaperone GRP78/BiP translocates to the nucleus under stress and acts as a transcriptional regulator. PNAS. Published online July 24, 2023. doi:10.1073/pnas.2303448120