Normal cell growth and motility are controlled by complex signalling pathways that respond to diverse environmental cues including nutrients, growth factors, hormones and components of the extracellular matrix. Understanding how intracellular signalling networks function in normal cells, and how they are altered in cancer cells, is a major challenge of cancer biology research. Playing a key role in the regulation of gene expression, cellular growth and survival, the mitogen-activated protein kinase (MAPK) is one signalling element that’s of particular interest in cancer research. Aberrant MAPK signalling is known to play a critical role in the development and progression of cancer and determining response to treatment.
The role of the ERK, p38 and JNK pathways in cancer
There are three main groups of MAPKs in mammalian species; the extracellular signal-regulated protein kinases (ERK1/2), the p38 MAP kinases and the c-Jun NH2-terminal kinases (JNK1/2/3). These pathways can be activated by various cell stimuli, including growth factors, cytokines and cellular stress, which lead to a cascade of sequential signalling events. The cellular response of the MAPK pathway appears to depend on the nature of the stimulus involved and the duration of the signal. In general, the ERK pathway is mainly involved in growth, differentiation, and development, while the JNK and p38 MAPK pathways are typically involved in inflammation, apoptosis, growth, and differentiation.
Although the ERK1/2 pathway has often been linked to cell growth, recent studies have implicated this pathway in other aspects of the phenotype of tumours including promoting cancer cell survival by inhibiting apoptosis. Dysregulation of this pathway has been found in a variety of malignancies, including hepatocellular carcinoma, gastric adenocarcinoma, and renal cell carcinoma. As such, aberrant ERK1/2 activation has now been linked to one-third of all human cancers, making it a valuable therapeutic target.
The roles of both JNK and p38 in cancer are complex and in some cases, heavily disputed. Increased levels of phosphorylated p38 have been found in various malignancies, including follicular lymphoma, lung, thyroid and breast carcinomas, as well as glioma and head and neck squamous cell carcinomas. Conversely, studies using mice with disruption of the genes that encode the p38 kinases have demonstrated enhanced transforming potential of fibroblasts, indicating a potential role of the p38 MAPK pathway in tumour suppression. Following on from this discovery, the p38 pathway has now been implicated in the activation of p53 and p53-mediated apoptosis, further affirming its role in suppressing tumour proliferation.
JNK signalling is also said to have two faces. Its functions in cancer are very difficult to pick apart because of the seemingly contradictory roles it plays in promoting cell survival and proliferation on one hand and cell death on the other. The ability of JNK1 to suppress the expression of anti-apoptotic genes, while JNK2 negatively regulates the activity of genes related to tumour suppression and the induction of cell differentiation, apoptosis, and cell growth reflects this dual role. Recent research has highlighted how JNK signalling can be modulated by an RNAi-based inhibitor for therapeutic benefit. One standout study found that the JNK inhibitor, SP600125, is able to significantly inhibit the proliferation of hepatocellular carcinoma cells by blocking the activity of JNK. To date, no therapy of this kind has made it to clinical trials, but significant progress has been made.
Developing drugs to target the MAPK signalling pathway
Drugs targeting the MAPK pathway have already generated striking clinical responses, most notably in melanoma. From the discovery of the BRAF mutation in melanoma in 2002, to the approval of the first BRAF inhibitor vemurafenib for melanoma treatment by the US FDA in 2011, therapies targeting the MAPK pathway were proven to be effective in less than a decade. The success of vemurafenib naturally stimulated more intensive investigation aimed at developing new treatments targeting specific molecules in the MAPK pathway.
One recent study led by Dr Simon Crabb, Associate Professor, University of Southampton, focused on targeting MAPK signalling in bladder cancer. A common cancer with high aggressiveness and relatively high mortality due to a lack of precise biomarkers, bladder cancer treatment has also suffered issues with drug resistance. The researchers exposed various bladder cancer cell lines to erlotinib, an EGFR inhibitor, and lapatinib, a dual EGFR/HER2 inhibitor. Utilising phosphokinase assays and immunoblotting they then analysed the impact these medications had on downstream signalling pathways.
When the cell lines were exposed to erlotinib, analysis revealed altered expression of 10 protein targets, including the overexpression of p38 MAPK. After exposure to lapatinib only three of the previously identified targets had altered expression, again p38 MAPK was affected. Finally, the two treatments were combined. In this case, the only protein to be affected was p38 MAPK. Based on this finding the authors hypothesise that cancer cells use increased p38 MAPK phosphorylation as a resistance mechanism in response to EGFR/HER2 inhibition.
Finally, to further validate their findings, the team sought to trial a combination treatment of lapatinib and a p38 MAPK inhibitor, known to have an anti-proliferative effect on cancer. They found that the anti-cancer effect was significantly greater when the two treatments were combined and are now calling for increased research into p38 as a potential target in bladder cancer.
Tumour drug addiction
Despite the success of MAPK-targeted therapies in melanoma, not all patients have benefitted equally. Even when treatments are initially effective the benefits often diminish over time as tumours develop resistance to treatment. Building on previous research in this area UCLA professor, Dr Roger Lo, and a team of collaborators have recently published findings that highlight a novel approach to overcoming this resistance. His team discovered that treatment-resistant melanoma tumours develop a dependency on MAPK-targeted therapy to retain their fitness. Thus, when treatment is discontinued, withdrawal occurs, and the tumour weakens.
Recent clinical studies and case reports indicate that, for patients who have relapsed on MAPK-targeted inhibitors, re-introduction of MAPK therapy following an intentional break from the drug can lead to a secondary response. Suggesting that drug-resistant tumour cells might be replaced by drug-sensitive tumour cells whilst the drug is not present.
The team found that withdrawal of MAPK therapies triggered a large build-up of MAPK signals to levels that proved stressful to the tumour cells. When this level was high enough, stress signals lead to DNA damage and trigger a form of cell death called parthanatos. Further boosting the MAPK signals and DNA damage in the tumour cells intensified tumour cell death. These findings indicate that DNA damage repair inhibitors, which have already been used in gynaecological cancers, may prove to be useful against treatment-resistant melanoma.
In a press statement, co-author of the study, Dr Gatien Moriceau said “We were very encouraged to see that we could shrink multiple types of treatment-resistant melanoma in mouse models”.
The team is now looking at designing rotational therapy regimes to leverage this “drug addiction” effect. They also hope that their work prompts research into innovative therapies and new clinical studies that capitalise on the mechanisms of MAPK inhibitor addiction. As more than 87,000 new cases of melanoma will be diagnosed this year in the USA alone, the potential impact is huge.