Blocking the processes that drive cancer cell growth is at the heart of many new anti-cancer therapies. Unfortunately, after initial success, cancer cells are generally able to develop workarounds to reactivate the pathways that promote growth. Work by researchers at the Babraham Institute in partnership with the global biopharmaceutical company AstraZeneca shows that this workaround strategy proves fatal for the cancer cells once the treatment compound is withdrawn. The research is published today in the journal Nature Communications.
One of the major cell signalling pathways controlling cell growth and division is the RAS-BRAF-MEK-ERK pathway. Consisting of three enzymes working in a linear cascade, the pathway leads to the activation of ERK to promote cell division. Many cancers, including most melanomas and some colon cancers, arise due to mutations in the BRAF kinase, which results in an unprompted growth signal and inappropriate cell division.
Dr Simon Cook and his team are specialists in the RAS-BRAF-MEK-ERK cell signalling pathway and through a long-standing partnership with collaborators at AstraZeneca had early access to a MEK inhibitor drug undergoing clinical testing to investigate how resistance to this drug develops.
“Tumours cells have shown themselves to be remarkably adaptable when treated with inhibitors of RAS-ERK signalling such as MEK or RAF inhibitors. Even in cases where these inhibitors are effective, invariably tumour cells adapt and acquire resistance to these drugs and their mechanisms of action,” says Dr Paul Smith, lead researcher at AstraZeneca. “Working with Simon Cook and his team at the Babraham Institute allowed us to apply their expert knowledge in cell signalling to better understand the mechanism of acquired resistance and excitingly, it has led to results that might change how MEK and other RAS-ERK pathway inhibitors, could be used for the treatment of cancer.”
Simon Cook and his team used an AstraZeneca-developed MEK inhibitor called selumetinib (AZD6244/ARRY-142886) to study how resistance arises. After exposure of human cancer cell lines to selumetinib over several weeks, resistant cells arose through a process called gene amplification where the cells multiply their copies of their addicted oncogene, the mutated BRAF gene. This amplification of BRAF maintains growth signalling through ERK by overwhelming the presence of the drug working to shut down this pathway in much the same way as a tidal surge can overwhelm flood defences.
The researchers found that if they stopped treating the selumetinib resistant cells with the drug, their resistance to it was rapidly lost so that the tumour cells reverted back to being fully drug sensitive (by 5-10 weeks of growth in the absence of selumetinib depending on cell type).
The researchers were intrigued to find out why as this might tell us something important about how cell growth is controlled and might have relevance to the way new anti-cancer drugs are administered to patients.
They found that BRAF gene amplification becomes an impediment for cells in a drug-free environment because the cells are locked into abnormally high ERK activation. As a result of high ERK activity, a cellular ageing pathway is activated and the cells enter a permanent arrest of cell growth. Cells showing only modest levels of BRAF and ERK activity survive in this pool of cells and can divide but reacquire drug sensitivity so can be eliminated by a second wave of the drug.
“We know that cells need to maintain a level of activated ERK within a very specific range,” explains Dr Cook. “Rather than promoting more growth, too much ERK actually halts cell growth.”
“In the presence of the inhibitor drug, the resistance mechanism sustains the cell’s ERK level and cell growth,” continues lead researcher Dr Mathew Sale, a member of Simon Cook’s group. “Once the inhibitor drug is gone however, the mechanism devised to maintain ERK signalling actually pushes the levels into the ‘too high’ zone – where the cell can no longer divide and often enters a phase of sustained growth inhibition and sometimes death.”
In cell culture at least, this is a mechanism whereby the withdrawal of the anti-cancer drug can be used to eliminate resistant cells, clearing them from the population.
A previous study in laboratory mice has shown that intermittent dosing of similar drugs may elicit a more prolonged inhibition of tumour growth but the mechanism underlying this effect was not clear. This new study clearly defines the ERK ‘sweet spot’ as the determinant of reversibility, suggesting that in the case of cancers involving BRAF mutations, future clinical trials should consider intermittent dosing regimens to forestall the development of drug resistance.
Publication referenceMEK1/2 inhibitor withdrawal reverses acquired resistance driven by BRAF amplification but promotes EMT/chemoresistance when KRAS is amplified. Sale et al. Nature Communications. DOI: 10.1038/s41467-019-09438-w
This study was funded by grants to the Babraham Institute from Cancer Research UK, an AstraZeneca-Cambridge Cancer Centre Collaborative Award, Institute Strategic Programme Grants from the BBSRC, and AstraZeneca, and including two PhD studentships from the Cambridge Cancer Centre and the BBSRC. Work in David Adam’s lab was supported by Cancer Research UK whilst work in Mark Arend’s lab was supported by awards from Cancer Research UK and the Wellcome Trust.
Dr Louisa Wood, Babraham Institute Communications Manager, email@example.com, 01223 496230
Chromosomes in colon cancer cells showing multiple copies of the BRAF gene (green). Gene amplification has been use by the cells as a mechanism to overcome MEK1/2 inhibition of growth. Red dots show the location of each chromosome’s centromere. The clusters of green dots represent copies of the BRAF gene within a chromosome (green dots clustering around a single red dot). Credit: Matthew Sale, Babraham Institute, Cambridge; Karen Howarth, Hutchison MRC Research Centre, Cambridge.
Affiliated authors (in author order):
Matthew Sale, Cook group, Signalling research programme
Kathy Balmanno, Cook group, Signalling research programme
Jayeta Saxena, Cook group, Signalling research programme
Eiko Ozono, Cook group, Signalling research programme
Katarzyna Wojdyla, Mass spectrometry facility, Babraham Institute
Becky Gilley, Cook group, Signalling research programme
Anna Woroniuk, Cook group, Signalling research programme
Pilar Caro, Cook group, Signalling research programmeDavid Oxley, facility head, Mass spectrometry facility, Babraham InstituteSimon Cook, group leader, Signalling research programme
Animal research statement:
As a publicly funded research institute, the Babraham Institute is committed to engagement and transparency in all aspects of its research. The research presented here used mice where parental and drug resistant tumour cells were implanted into immune-compromised (nude) mice to see if the cells were able to grow as tumours and to assess whether this growth was dependent upon the mice receiving the drug (selumetinib). These experiments were performed at AstraZeneca or the Sanger Institute under strict Home Office guidelines.
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About the Babraham Institute
The Babraham Institute undertakes world-class life sciences research to generate new knowledge of biological mechanisms underpinning ageing, development and the maintenance of health. Our research focuses on cellular signalling, gene regulation and the impact of epigenetic regulation at different stages of life. By determining how the body reacts to dietary and environmental stimuli and manages microbial and viral interactions, we aim to improve wellbeing and support healthier ageing. The Institute is strategically funded by the Biotechnology and Biological Sciences Research Council (BBSRC), part of UK Research and Innovation, through an Institute Core Capability Grant and also receives funding from other UK research councils, charitable foundations, the EU and medical charities.
02 May 2019