A team of researchers led by siectists at Queen’s University Belfast have found a new way through which the p53* gene, also known as TP53 (tumor protein 53) and the ‘Guardian of the Genome,’ the most frequently mutated and well-studied tumor-suppressor gene in cancer, can prevent cell death in colorectal cancer. 
Based on this discovery, the scientists identified new treatments that may improve the effectiveness of chemotherapy given to patients with colorectal cancer.
Colorectal cancer is the third most common cancer in men and the second most common cancer in women. According to estimares from the American Cancer Society (Surveillance Research), there will be an 104,610 new cases of colon cancer and 43,340 cases of rectal cancer diagnosed in the US in 2020. And while the majority of cases of colorectal cancer will be diagnosed in adults ages 50 and older, 17,930 (12%) will be diagnosed in individuals younger than age 50, the equivalent of 49 new cases per day.
Furthermore, the American Cancer Society (ACS) estimates that in the United States 53,200 people will die from colorectal cancer in 2020, including 3,640 men and women younger than age 50.
Results from the study, conducted by a team of sientists led by Simon S. McDade, Ph.D. and Professor Daniel B. Longley BA, Ph.D. from the Patrick G Johnson Centre for Cancer Research at Queen’s University, has been published in the journal Proceedings of the National Academics of Science (PNAS).
The tumour suppressor gene p53 normally instructs cells to stop growing or the to die in response to cancer-causing stresses, including DNA damaging UV light from the sun. However, unitil now, the molecular basis on how and why ~50% of colorectal cancers in which p53 is not mutated, suppress its ability to induce cell-cycle arrest leading to apoptosis, was poorly understood.
The sientists found that when colorectal cancer cells in which p53 is not mutated are treated with chemotherapy, they become highly reliant on increased levels of a protein called FLIP for survival.
FLIP, or FLICE-inhibitory protein, is a caspase-8 analogue modulating death receptor-mediated apoptosis by preventing caspase-8 activation. They bind to the cytoplasmic protein complex of death receptors to modulate/inhibit the recruitment/activation of caspases. FLIPs are either inactive caspase-like proteins (lacking enzyme active sites) or small bridging molecules that mimic the prodomains of caspases.
The team, unexpectedly, found that in colorectal cancer cells, FLIP levels are increased by p53 itself, paradoxically blocking p53-induced cancer cell-death. They identified FLIP as a direct p53 transcriptional target gene that is rapidly up-regulated in response to Nutlin-3A, an MDM2 inhibitor that potently activates p53.
In addition, the scientists found that this reliance on FLIP can be exploited with drugs called HDAC inhibitors, a class of compounds that increase acetylation of lysine residues on histone proteins as well as other, nonhistone, proteins by inhibiting the activity of HDAC enzymes, that block p53-induced expression of FLIP but maintain activation of cell-death. This enhances the ability of chemotherapy to kill cancer cells.
The scientists noted that entinostat (Syndax), a clinically relevant selective, class-I HDAC inhibitor that has been studied in multiple solid tumors, efficiently promoted apoptosis in colorectal cancer cells in response to Nutlin-3A, which otherwise predominantly induced cell-cycle arrest. They further observed enhanced apoptosis when entinostat was combined with clinically relevant, p53-activating chemotherapy in vitro, and this translated into enhanced in vivo efficacy.
Future clinical trials
Based on these findings, this research provides evidence for future clinical trials combining p53 activating therapies, such as chemotherapy, with HDAC inhibitors to treat colorectal cancers with non-mutated p53.
“We believe that we have uncovered an important evolutionarily conserved physiological role for p53 in preventing cell-death that normally functions to protect critical epithelial barriers in our skin, gastrointestinal tract and airways in the face of environmental damage, such as UV light and environmental toxins, to which they are exposed on a daily basis,” explained the study’s lead, Simon S. McDade, Ph.D., Senior Lecturer and Functional Genomics Group Leader at Queen’s University.
“We also believe that this mechanism has been exploited by colorectal cancers and other cancer types with non-mutated p53 to prevent them from dying in response to standard-of-care chemotherapy and that this is something we can exploit to increase effectiveness of treatments,” he added.
“Importantly, this mechanism of resistance to chemotherapy caused by p53’s upregulation of FLIP can be overcome by HDAC inhibitors and novel agents currently in development in our lab that directly target FLIP, thereby paving the way for improving treatment of colorectal tumours with non-mutated p53,” added Professor Daniel Longley, Chair of Cancer Biology at Queen’s University.
Further insight and development
Ongoing studies, related to these findings recently funded by the Biotechnology and Biological Sciences Research Council (BBSRC) are expected to provide further insights into the maintenance of skin and intestinal health that could inform new ways of protecting these vital barriers and combating diseases in which these barriers become compromised; these include inflammatory diseases of the skin (e.g. atopic dermatitis and psoriasis) and gut (e.g. inflammatory bowel disease). In addition, funding from The Wellcome Trust and MRC is supporting the development of novel FLIP inhibitors for the treatment of colorectal cancer and other cancers.
* p53 is a gene that codes for a protein that regulates the cell cycle and, and, as a result, functions as a tumor suppression. A the ‘Guardian of the Genome,’ p53 plays a major role as a ‘central tumor suppressor’ in conserving stability by preventing genome mutation, guarding the genome by orchestrating a variety of DNA-damage-response (DDR) mechanisms. The name comes from its molecular mass: p53 is in the 53 kilodalton fraction of cell proteins.
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