(-)- Epigallocatechin-3-gallate induces GRP78 accumulation in the ER and shifts mesothelioma constitutive UPR into proapoptotic ER stress

Our plain-English summary of (-)- Epigallocatechin-3-gallate induces GRP78 accumulation in the ER and shifts mesothelioma constitutive UPR into proapoptotic ER stress, by S. Martinotti, E. Ranzato, B. Burlando (Journal of Cellular Physiology, 2018).

The study

• Authors: S. Martinotti, E. Ranzato, B. Burlando
• Journal: Journal of Cellular Physiology
• Published: 2018;233(10):7082-7090
• Cited by: 33 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

GRP78 is a molecular chaperone of the endoplasmic reticulum (ER) that aids proper folding of nascent polypeptides. When unfolded proteins accumulate, GRP78 triggers unfolded protein response (UPR), involving activation of transcription factors like XBP1 and CHOP that may restore cell homeostasis. Increased expression of GRP78 and mild UPR can be constitutive in cancer cells, hindering apoptosis, and promoting cell survival, for example, by GRP78 relocation to the plasma membrane that activates MAPK and PI3 K/AKT pathways. These processes are thought to favor the insurgence of chemoresistance and worsen patient outcome. We have previously shown that (-)-epigallocatechin-3-gallate (EGCG) enhances ROS production and alters Ca2+ homeostasis in cell lines deriving from therapy-recalcitrant malignant mesothelioma (MMe). We consider here the EGCG impact on GRP78 and downstream factors by using qRT-PCR, Western blot, immunofluorescence, caspase assays, GRP78 siRNA silencing, and cell surface ELISA. MMe cells were found to display mild constitutive UPR, as shown by increased levels of GRP78, and presence of the protein at the cell surface, linked to AKT activation. Exposure to EGCG further increased GRP78 in the ER, and induced ATF4, spliced XBP1, CHOP, and EDEM expressions, combined with a reduction of cell surface GRP78 and a rise in caspase 3 and 8 activities. We propose that GRP78 accumulation in the ER, caused by EGCG, converts constitutive UPR of MMe cells into proapoptotic ER stress. This argues for a possible therapeutic use of EGCG in the treatment of MMe as a co-drug able to abolish chemoresistance to conventional drugs at tolerable doses.

© 2018 Wiley Periodicals, Inc.

In plain English

This was a laboratory study of cells grown in dishes, not of people. The authors tested EGCG, a compound from green tea, on two malignant mesothelioma cell lines, REN (epithelioid) and MM98 (sarcomatous), and on normal mesothelial cells taken from biopsies. Asbestos exposure was not studied. The mesothelioma cells had more GRP78 than normal mesothelial cells. GRP78 is a protein that helps cells cope with stress in a cell compartment called the endoplasmic reticulum. In REN cells, EGCG raised the activity of caspase 3, an enzyme linked to programmed cell death. The increase was significant at doses as low as 1 µM after 1 hour, peaked at 50 µM and returned to control values at 100 µM. EGCG further increased GRP78 inside the mesothelioma cells and induced several stress-response genes. In REN cells, GRP78 on the cell surface fell by about 50% after 24 hours at 50 µM. The authors proposed that EGCG turns the cells' ongoing mild stress response into a stress that leads to cell death. They argued for possible use of EGCG alongside conventional drugs, while noting that its bioavailability and cell penetration could be low.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) study on cultured cell lines
• Who was studied: Two human malignant mesothelioma cell lines, REN (epithelioid) and MM98 (sarcomatous), plus primary normal mesothelial cells derived from biopsies; no patients were studied
• Exposure: (−)-epigallocatechin-3-gallate (EGCG), a green tea compound, at doses from 1 to 100 µM, mostly for 1 and 6 hours (24 hours in the cell-surface test); asbestos exposure was not studied
• Main results:
  • Untreated REN and MM98 mesothelioma cells showed higher GRP78 levels than normal mesothelial cells
  • In REN cells after 1 hour, caspase 3 activity rose significantly at doses as low as 1 µM EGCG, peaked at 50 µM, and returned to control values at 100 µM
  • In REN cells, caspase 8 activity peaked at 50 µM EGCG after 1 hour and fell sharply at 100 µM. Induction was weaker at 6 hours but still significantly higher than controls
  • After 6 hours, EGCG further increased GRP78 protein in both mesothelioma cell lines
  • In normal mesothelial cells, GRP78 increased significantly only at the highest EGCG concentrations of 50 and 100 µM
  • GRP78 gene (mRNA) induction by EGCG was limited, while EDEM, ATF4 and CHOP genes showed strong dose-dependent induction
  • EGCG significantly increased the ratio of spliced to total XBP1
  • Silencing the GRP78 gene in REN cells showed a positive correlation between GRP78 levels and AKT activation (phosphorylation)
  • In REN cells, GRP78 at the cell surface fell by about 50% after 24 hours of exposure to 50 µM EGCG
  • Catalase and BAPTA-AM did not stop the EGCG-induced rise in GRP78. The authors interpreted this as showing the effect is independent of the oxidative stress and calcium changes they had seen in earlier work
• The authors' conclusion: The authors proposed that EGCG disrupts GRP78 function and so converts the constitutive mild stress response (UPR) of mesothelioma cells into irreversible, cell-death-promoting ER stress. They argued for a possible use of EGCG as a co-drug combined with conventional treatments
• Limitations the authors state:
  • The authors noted that EGCG bioavailability and cell penetration could be low
  • The authors noted that the measured GRP78 gene induction does not seem to fully account for the large rise in GRP78 protein

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Mesothelioma, Apoptosis.

Related studies

• Epigallocatechin-3-gallate induces mesothelioma cell death via H2 O2 -dependent T-type Ca2+ channel opening — E. Ranzato, S. Martinotti, et al. (2012)
• Negative modulation of mitochondrial oxidative phosphorylation by epigallocatechin-3 gallate leads to growth arrest and apoptosis in human malignant pleural mesothelioma cells — D. Valenti, L. de Bari, et al. (2013)
• Asbestos-induced alveolar epithelial cell apoptosis. The role of endoplasmic reticulum stress response — D.W. Kamp, G. Liu, et al. (2013)
• GRP78, HSP72/73, and cJun stress protein levels in lung epithelial cells exposed to asbestos, cadmium, or H2O2 — C.R. Timblin, Y.M. Janssen, et al. (1998)
• Asbestos-induced disruption of calcium homeostasis induces endoplasmic reticulum stress in macrophages — A.J. Ryan, J.L. Larson-Casey, et al. (2014)
• Repression of GADD153/CHOP by NF-kappaB: a possible cellular defense against endoplasmic reticulum stress-induced cell death — S. Nozaki, G.W. Sledge, et al. (2001)

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