Interplay of genes

The biological process of cell division is essential for the growth and reproduction of all living organisms. Certain proteins can control this division process through their activity and control the proliferation of cells. Loss of this control is therefore an important feature of carcinogenesis. Proteins that promote cell division thus also often contribute to the development of cancer (oncoproteins). At the same time, however, proteins can also inhibit cell division and, as so-called tumor suppressors, counteract the development of cancer. Probably the best-known representative of this is the transcription factor p53. This protein is able to specifically interrupt the process of cell division in order to stop the proliferation of stressed and degenerated cells. This buys the cell time to repair existing damage by internal repair mechanisms. However, if these are too severe and not fully repairable, then an internal "self-destruction" program is initiated and the cell stops proliferating.

Transcription factors very often play an important role in the development of cancer, as they usually regulate many genes. The mutation of a transcription factor thus has far-reaching consequences for a cell and, as in the case of p53, can strongly promote cancer development: p53 is mutated in about half of all tumors or is deactivated in various ways in the remaining tumors. Researchers at the Leibniz Institute for Aging Research - Fritz Lipmann Institute (FLI) in Jena, Germany, have now studied in more detail parts of the network of genes regulated by p53: Through p53, the protein RFX7 is activated, another transcription factor and tumor suppressor, which in turn activates its own network of genes.

Novel tumor suppressor RFX7.

"Like p53, the RFX7 protein is a transcription factor whose increased mutation rate has only recently been detected in lymphoma thanks to state-of-the-art analytical methods, but which, in contrast to p53, has hardly been researched yet," explains PD Dr. Martin Fischer, laboratory manager in the Hoffmann research group at the FLI. In addition to cancer, metabolic and neurological diseases have previously been linked to RFX7 - major obstacles to healthy aging.

The researchers now found that as a result of stress, the protein RFX7 is turned on by p53, and that many of the genes regulated by RFX7 are also known tumor suppressors. "The fact that RFX7 appears to exert its function as a tumor suppressor by controlling other tumor suppressors, thereby indirectly enabling p53 to regulate these proteins, is extremely exciting for us," reports Steve Hoffmann, head of the Bioinformatics Research Group at FLI. "By identifying RFX7 target genes, we were able to systematically search tumor databases for changes in gene regulation. In cancer tissues, RFX7 often appears to be inactive, even when it is not mutated."

Control p53 and RFX7 - Inhibition of mTORC1 and AKT

Cell growth is an essential preparation for cell division. Particularly important in this process are the kinases AKT and mTORC1; both enzymes that are activated by growth factors and other external signals and can promote both tumor development and cell growth. In cancer, these two kinases are often overactive; in the aging process, mTORC1 plays an important role. It has been known for some time that p53 can inhibit both AKT and mTORC1 kinases to counteract cancer development.

"The control by p53 of kinases that are important for survival and therefore critical is therefore enormously important for cell fitness," explains Dr. Fischer. However, deciphering the underlying mechanisms has so far proved very difficult. So far, only a few proteins have been identified that play a role in the p53-mediated inhibition of AKT and mTORC1. The Jena research team led by Dr. Fischer and Prof. Hoffmann now found that p53 absolutely needs the transcription factor RFX7 to inhibit both kinases, once again highlighting the importance of RFX7 for future cancer research.

The now successful mapping of RFX7-controlled genes has identified, among others, the metabolic regulator DDIT4, which is activated by p53 indirectly via the tumor suppressor RFX7. The findings, published in the journal Oncogene, suggest that p53/RFX7-dependent inhibition of AKT is mediated via DDIT4. "Investigating additional target genes of RFX7 may help us uncover how exactly the regulation of mTORC1 signaling is mediated in a p53 and RFX7-dependent manner," Dr. Fischer says, providing an outlook for the future.

Nutrient supply influences mTORC1 regulation

Cancer research usually uses tumor samples from patients whose cells are cultured in Petri dishes in the laboratory. However, the cell culture medium used and the nutrients it contains do not correspond one-to-one to the real conditions in a normal organism. Such cell lines with a non-physiological supply of nutrients have, however, been used in previous and current studies to regulate mTORC1. "Nutrient availability, however, appears to be an extremely critical factor for the regulation of mTORC1," Steve Hoffmann emphasizes, "which requires careful consideration of culture conditions in experimental trials."

In cell culture medium under conventional, non-physiological nutrient conditions, mTORC1 is particularly active and can be inhibited by activation of p53 and RFX7. In contrast, when physiological nutrient conditions are used, the activity of mTORC1 is much lower and cannot be further reduced by activation of p53 and RFX7. Under these conditions, loss of p53 or RFX7 results in increased activity of mTORC1. "The results under physiological nutrient conditions reflect much better the real conditions in the organism, i.e. also in a diseased person. If p53 or RFX7 is mutated, the activity of the mTORC1 kinase is probably increased, so that tumor development is favored as a result," Dr. Fischer emphasizes.

Reactivation of RFX7 useful for cancer therapy?

"Our results show how RFX7 functions as a novel tumor suppressor: it activates other tumor suppressors, such as the metabolic regulator DDIT4, and inhibits important growth regulators, such as the kinases AKT and mTORC1," Dr. Fischer summarizes. "It is still an open question through which further functions RFX7 controls the development of cancer as well as regulates metabolic and neurological processes. In the future, we therefore want to find out how the RFX7 transcription factor itself is regulated. This is important to know because, unlike p53, RFX7 is deactivated in many tumors but does not appear to be mutated. Reactivation of RFX7 could therefore be useful for cancer therapy."