Multipoint targeting of the PI3K/mTOR pathway in mesothelioma

Our plain-English summary of Multipoint targeting of the PI3K/mTOR pathway in mesothelioma, by S. Zhou, L. Liu, H. Li, et al (British Journal of Cancer, 2014).

The study

• Authors: S. Zhou, L. Liu, H. Li, G. Eilers, Y. Kuang, S. Shi, Z. Yan, X. Li, J.M. Corson, F. Meng, H. Zhou, Q. Sheng, J.A. Fletcher, W.B. Ou
• Journal: British Journal of Cancer
• Published: 2014;110(10):2479-88
• Cited by: 79 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Mesothelioma is a notoriously chemotherapy-resistant neoplasm, as is evident in the dismal overall survival for patients with those of asbestos-associated disease. We previously demonstrated co-activation of multiple receptor tyrosine kinases (RTKs), including epidermal growth factor receptor (EGFR), MET, and AXL in mesothelioma cell lines, suggesting that these kinases could serve as novel therapeutic targets. Although clinical trials have not shown activity for EGFR inhibitors in mesothelioma, concurrent inhibition of various activated RTKs has pro-apoptotic and anti-proliferative effects in mesothelioma cell lines. Thus, we hypothesised that a coordinated network of multi-RTK activation contributes to mesothelioma tumorigenesis.

Methods: Activation of PI3K/AKT/mTOR, Raf/MAPK, and co-activation of RTKs were evaluated in mesotheliomas. Effects of RTK and downstream inhibitors/shRNAs were assessed by measuring mesothelioma cell viability/growth, apoptosis, activation of signalling intermediates, expression of cell-cycle checkpoints, and cell-cycle alterations.

Results: We demonstrate activation of the PI3K/AKT/p70S6K and RAF/MEK/MAPK pathways in mesothelioma, but not in non-neoplastic mesothelial cells. The AKT activation, but not MAPK activation, was dependent on coordinated activation of RTKs EGFR, MET, and AXL. In addition, PI3K/AKT/mTOR pathway inhibition recapitulated the anti-proliferative effects of concurrent inhibition of EGFR, MET, and AXL. Dual targeting of PI3K/mTOR by BEZ235 or a combination of RAD001 and AKT knockdown had a greater effect on mesothelioma proliferation and viability than inhibition of individual activated RTKs or downstream signalling intermediates. Inhibition of PI3K/AKT was also associated with MDM2-p53 cell-cycle regulation.

Conclusions: These findings show that PI3K/AKT/mTOR is a crucial survival pathway downstream of multiple activated RTKs in mesothelioma, underscoring that PI3K/mTOR is a compelling target for therapeutic intervention.

In plain English

This laboratory study looked at cancer cells grown in dishes. It did not study people or asbestos exposure. The authors used five mesothelioma cell lines and, for comparison, non-cancerous mesothelial cells. Compared with the normal cells, a growth-signalling chain called PI3K/AKT/mTOR was strongly switched on in all five cancer lines. Blocking three cell-surface receptors (EGFR, MET and AXL) together cut AKT phosphorylation, a sign of AKT activation, by 69% in line MESO924 and 61% in MESO428 versus controls. The drug BEZ235, which blocks PI3K and mTOR together, reduced cell proliferation by 74%, 78%, 64% and 60% in four lines. That was more than drugs aimed at single points in the pathway. In one normal-cell culture, the PI3K-blocking drugs had only mild effects. The authors judged that BEZ235 mostly slowed cell growth rather than killing cells. In two lines, BEZ235 alone, or an mTOR drug combined with switching off AKT genes, cut viability by 60% at day 3 and 80% at day 6. The authors concluded that this pathway is a crucial survival pathway in mesothelioma and a compelling treatment target.

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 of cultured mesothelioma cell lines treated with drugs and gene-silencing (shRNA)
• Who was studied: Five mesothelioma cell lines: MESO924, MESO257 and JMN1B from epithelial-type mesotheliomas, MESO296 from a mixed-histology mesothelioma, and MESO428 from a spindle-cell mesothelioma. Comparison cells were normal mesothelial cells 97–510 (from a non-neoplastic pleural effusion) and HM3 (from non-neoplastic peritoneum). No human subjects were studied.
• Exposure: No asbestos or fiber exposure was studied. Cells were treated with inhibitor drugs (BEZ235, GDC0941, LY294002, RAD001/everolimus, U0126, gefitinib, PHA665752) and with shRNAs silencing AXL, AKT1, AKT2 or AKT3.
• Main results:
  • Compared with normal mesothelial cells, AKT and S6 were strongly activated in all five mesothelioma cell lines. MAPK was activated in all lines except JMN1B.
  • BEZ235 (50 nM) inhibited cell proliferation by 74% in MESO924, 78% in MESO257, 64% in MESO296 and 60% in MESO428.
  • In the mesothelioma lines, GDC0941 gave 40–50% inhibition and the MEK inhibitor U0126 gave 5–25% reduction in proliferation.
  • In normal mesothelial cells 97–510, the PI3K inhibitors BEZ235, GDC0941 and LY294002 had only mild anti-proliferative effects.
  • Combined inhibition of EGFR, MET and AXL gave the largest reduction in AKT phosphorylation, compared with DMSO and empty-vector controls: 69% in MESO924 and 61% in MESO428.
  • Inhibiting EGFR, MET and AXL, alone or together, had little effect on MAPK and S6 activation.
  • Combined EGFR, MET and AXL inhibition decreased cell viability by 55–60%, which the authors described as comparable to BEZ235.
  • BEZ235 cell proliferation IC50s at day 3 were 11.8 nM (MESO924), 11.4 nM (MESO257), 12 nM (MESO296) and 10.8 nM (MESO428).
  • Apoptosis was mild. Fragmented-apoptotic cells were 9% (control) vs 18% (50 nM BEZ235) in MESO296, and 12% vs 20% in MESO428.
  • The authors interpreted the absence of cleaved PARP and caspases as showing that PI3K inhibition mostly inhibited growth rather than causing apoptosis.
  • AKT inhibition by GDC0941 abolished MDM2 phosphorylation and induced p53 and p21 expression.
  • In MESO924 and MESO428, the largest reductions in cell viability (60% at day 3 and 80% at day 6) followed BEZ235 or RAD001 combined with AKT1 or AKT2 knockdown.
• The authors' conclusion: The authors concluded that PI3K/AKT/mTOR is a crucial survival pathway downstream of multiple activated receptor tyrosine kinases in mesothelioma, and that PI3K/mTOR is a compelling target for therapeutic intervention.
• Limitations the authors state:
  • Small-molecule AKT inhibitors were not evaluated; AKT was targeted only by gene silencing.

Topics

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

Medical terms in this study, explained in our Lexicon: Mesothelioma, Chemotherapy, Neoplasm, Overall Survival, EGFR, Clinical Trial, Tumorigenesis, Apoptosis.

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