A window into acquired resistance to targeted therapies – through the eyes of a MEK inhibitor.

Hi!

Cancer cells, like all other cells in multicellular organisms, are often dependent on inputs from the environment outside the cell for signals that drive growth and survival, among other things. Since abnormal growth and a failure to die like normal cells do are hallmarks of cancer, it makes sense to try and block signalling pathways that contribute to these features using drugs specific to the proteins in these pathways. This is the fundamental premise behind targeted therapies.

The work I’m going to focus on in this article happens to do with inhibition of MEK, which connects external signals to a set of transcription factors that promote the expression of genes related to cell growth and survival.

The Map-kinase signalling pathway. External growth factor receptor kinases are coupled to transcription of genes promoting cell survival and proliferation by means of the Ras-Raf-MEK-ERK signalling cascade. Kinases are proteins that add an inorganic phosphate group to other proteins or in some cases, lipids. 

This pathway is of interest because b-Raf is found to have a very particular mutation, V600E, in a majority of malignant melanomas – something so characteristic of the disease that there is a drug that specifically targets the mutant version of this protein, but responses are often short-lived because cells learn to get round the blockade of the protein. Interestingly, this pathway is also involved in colorectal cancer, often involving the same mutations or a mutation in the protein that comes before b-Raf, called k-ras, which is a very potent oncogene.

Simon Cook and his group at the Babraham Institute tried to figure out how cancer cells that depended on this pathway would come to acquire resistance to a MEK inhibitor (which is downstream of B-raf and k-ras). To do this, they took two colorectal cancer cell lines with a b-Raf mutation and two with a k-ras mutation and cultured them in the presence of ever increasing concentrations of a MEK inhibitor : AZD6244. They fundamentally found that resistant cells seemed to acquire amplifications in the number of copies of b-Raf or k-Ras, thus serving to maintain the same signal intensity downstream of MEK in the presence of the drug as they would have in the absence thereof.

Resistance to a MEK inhibitor in a B-raf mutant cell line is explained by amplification of B-raf at the DNA level, which is reflected at the protein level and increased activation of ERK1 (P-ERK1/2). The graph at the bottom right shows that knocking down b-Raf levels using RNA interference reverses resistance to the MEK inhibitor.

They also found that in Ras mutant cell lines, amplification of k-Ras was to blame for the phenotype. The problem with that is Ras is a pain in the rear to develop drugs against, but with b-Raf there are inhibitors available and it should be possible to resensitize resistant cells to the MEK inhibitor by hitting it at both points in the pathway.

The trouble with this MEK inhibitor is that it leads to cell cycle arrest being the major response as opposed to cell death, so it would be sensible to see if, for already dampened levels of ERK activation through MEK inhibition, it should be possible to increase the proportion of cancer cells that actually fuck off and die instead of just waiting for the drug to wear off.

Apoptosis is a process mediated by a combination of pro-apoptotic proteins and anti-apoptotic proteins and when a threshold is reached in terms of dominance of pro-apoptotic proteins it sets of a cascade of signalling events that leads to the destruction of the cells. One of the reasons cell cycle arrest is favoured over cell death is the hyperactivity of anti-apoptotic proteins such as Bcl2. There is a protein called BH3 which can bind to and disable Bcl2, rendering cells much more susceptible to apoptosis if the MEK pathway is hit, so they looked at combining a drug that mimics the structure of BH3 with the MEK inhibitor and promptly found that apoptosis was greatly enhanced and the emergence of resistance delayed .

Finally, of course, it is worth considering the fact that in malignant melanoma, drug holidays, where treatment is not administered for a while, has been shown to reverse resistance well in line with what we’d expect – the overexpression of oncogenes is associated with oncogene induced senescence and what might maintain the activity of the pathway in the presence of the drug might activate the pathway too much when the drug is taken away – like how a car might crash if you suddenly took the brakes off while the pedal was still pressed to the same extent as when driving with the brakes on. This means that drug resistance is favoured only because of a selective pressure imposed by the drug and might actually be a detriment in competition with drug sensitive cells when the drug is absent. Take the drug away when the cells that are most dependent form the bulk of the tumour, and they really do crash dramatically

That’s all from me until next time.

Cheers,
Ankur.

PS – additional papers…

[1] http://www.nature.com/onc/journal/v32/n10/full/onc2012160a.html
[2] http://www.nature.com/jid/journal/v130/n6/full/jid201026a.html

 

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One response to “A window into acquired resistance to targeted therapies – through the eyes of a MEK inhibitor.

  1. Pingback: A Bird’s Eye View of Cancer Research… | Exploreable

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