Research into c-JUN oncoprotein could pave way for novel colon cancer therapies

A new collaborative study looks at how multiple phosphorylations of a protein can regulate the transcription of genes linked to colon cancer.

The multidisciplinary study, led by scientists in Dr Anastasia Mylona’s group at Imperial College, and in collaboration with a UCL team led by Professor John Christodoulou, and the CR-UK Convergence Science Centre, led by Professor Axel Behrens, was published in Nature Communications. 

Phosphorylation is an important cellular process where a phosphate group that is a type of chemical modification is added to a protein, changing its function. Proteins, like transcription regulators of genes, often get phosphorylated at multiple sites by a single kinase enzyme in response to a cellular signal.

Deregulation of phosphorylation in cell signalling commonly underlies cancer aetiology. Understanding the mechanism of modification of multiple phosphorylation sites of a protein by a kinase is a vital task, and research on that area will likely be benefiting cancer treatment. 

The study, led by Dr Anastasia Mylona from the Department of Surgery and Cancer, looked at a protein called c-JUN, which is an oncogenic transcription factor that plays an important role in turning the genes on when a cell receives a signal to proliferate. Such signals activate a kinase called JNK, a member of a bigger kinase family the MAP Kinases, which puts phosphate groups at four sites on c-JUN. These phosphorylations regulate the transcription of genes linked to colon cancer.

The collaborative research found that phosphorylation of c-JUN by JNK occurred at four sites but with different modification rates. Two sites were phosphorylated faster and two more slowly thus defining three phosphorylated states of c-Jun: the unphosphorylated, the doubly phosphorylated and the fully phosphorylated state. These phosphorylation states directly recruit or release binding of other protein co-factors so that c-JUN function switches from transcriptionally inactive to active and finally inactive. Thus from a single kinase signal c-JUN can activate and subsequently limit transcriptional levels without the need of secondary enzymes, such as phosphatases that remove the phosphate groups from proteins deactivating a protein function.

This mechanism contrasts the dogma of the classic ‘on and off switch’ of the transcriptional response via the dual role of kinases and phosphatases in MAPK signalling and perhaps more general in signalling pathways, revealing that the temporal dimension of multisite phosphorylation of a protein domain allows for more elaborate signalling responses.

It is hoped that the study can have important implications for paving the way for more specific and efficient cancer therapies. The JNK kinase has been a desirable drug target for years, but so far JNK inhibitors have not been translated into clinical use. The main reason is the lack of specificity of the current inhibitors. Inhibitors targeting specific JNK-mediated downstream substrates and cellular events, such as temporal phospho-c-JUN states or specific interactions of phospho-c-JUN states with binding partners, may thus show increased tumour specificity and efficacy.

Although further research will be required, this type of fundamental discovery research is important for opening avenues for novel and more efficient cancer treatments.  

An intrinsic temporal order of c-JUN N-terminal phosphorylation regulates its activity by orchestrating co-factor recruitment
Christopher A. Waudby, Saul Alvarez-Teijeiro, E. Josue Ruiz, Simon Suppinger, Nikos Pinotsis, Paul R. Brown, Axel Behrens, John Christodoulou & Anastasia Mylona
Nature Communications, volume 13, Article number: 6133 (2022)