FOXO3, with the characteristic fork head DNA-binding domain is part of the O subclass of the forkhead family of transcription factors. These transcription factors have important roles in mammalian cells in regards to regulating cell homeostasis, differentiation, longevity and steer cell death. The activity of FOXO3 in particular contributes to therapy-resistance programs that protect cancer cells during chemo and radiotherapy. Recent studies have also found the DNA-binding domain (DBD) of FOXO aid protein-protein interactions with other key regulators of longevity and death, and drug resistance. A reversible inhibition of FOXO3 activity by small compounds thereby might boost anti-tumor immune responses.
In order to inhibit the FOXO3 activity, it was first necessary to identify small molecule compounds that could block the interaction between FOXO3 and DNA. Using the structural data of FOXO3 DBD and FOXO4 DBD, the researchers developed six different pharmacophore models that were used for in silico screening of small molecule compound databases. Selected compounds were then tested for their ability to inhibit FOXO3 function both in vitro, and in cancer cell lines. The interactions of these compounds with FOXO3 DBD were assessed using NMR spectroscopy and docking studies.
The teams of Dr. V. Obšilová from the Institute of Physiology CAS in BIOCEV and prof. T. Obšil from the Faculty of Science, Charles University with the team of prof. M. J. Ausserlechner from the Medical University of Innsbruck, Austria identified the compounds S9 and its oxalate salt S9OX as compounds able to inhibit the FOXO3 activity in cancer cells. They also proposed that due to their mode of binding to FOXO3 DBD, these compounds may also interfere with protein-protein interactions of FOXO3. The advantage of these compounds is the strict control of application-dose and -time and the fact that they are not immunogenic allowing repeated applications – so dose- and application time can be adjusted to damage cancer cells or boost anti-cancer immunity, but also limit unwanted side effects of FOXO-inhibition on stem cells and other somatic tissues. Future research will be focused on investigation whether or not S9 can be used as a chemical foundation for developing FOXO-regulatory compounds that would control the functions and target gene subsets of FOXO transcription factors.
Original paper: Hagenbuchner J*, Obsilova V*, Kaserer T*, Kaiser N, Rass B, Psenakova K, Docekal V, Alblova M, Kohoutova K, Schuster D, Aneichyk T, Vesely J, Obexer P, Obsil T+, Ausserlechner MJ+. Modulating FOXO3 transcriptional activity by small, DBD-binding molecules. eLife. 2019, 8(Dec 4), e48876. doi: 10.7554/eLife.48876. IF = 7.551
Compounds S9 blocks the DNA binding surface of Forkhead transcription factor FOXO3. The figure shows the structural model of the DNA-binding domain of FOXO3 with bound compound S9 based on data from NMR measurements and docking simulations.