New insights into Bcl-2 family interactions from biophysics to function

Bcl-2 pro-survival Bcl-2 family proteins preserve mitochondrial integrity by sequestering either "BH3-only" members of the same family, or by neutralizing active Bax or Bak at the outer mitochondrial membrane. Inhibiting anti-apoptotic Bcl-2 homologs constitutes a new avenue for the treatment of cancer and autoimmunity. The mechanisms how anti-apoptotic A1/Bfl-1, a protein reported to enable survival and maturation of early thymic precursors and myeloid cells blocks cell death remains unclear and physiological consequences of loss of A1- function await detailed investigation. Based on biochemical studies it was proposed that it acts similar to Mcl-1, the most critical pro-survival member of the Bcl-2 family. Given their proposed role in the pathogenesis of hematopoietic malignancies and their drug-resistance to novel therapeutics (e.g. BH3-mimetics), inhibiting A1 function offers new therapeutic options. Understanding the consequences of A1 loss-of-function, possible redundancy with Mcl-1 and its molecular mode of action thereby constitute a prerequisite to validate the drug- target potential of A1.

Within this research consortium with partners form Germany and Switzerland, we investigated the role of a particular cell death-inhibitory protein, called A1 or, in humans, BFL1. This protein acts as a suppressor of cell death that is referred to as apoptosis and represents a regulated type of cell death that can be initiated during the development of a multi-cellular organism, including humans. In the adult, apoptosis is essential to maintain a balance of healthy and functional cells in all types of tissues. Apoptosis can also be activated in cells in response to stress, but also, for example, anti-cancer therapy. High levels of anti-apoptotic proteins are often fund in cancer patients and are considered to be causative for treatment failure. A1/BFL1 is one such protein that has been found deregulated in multiple types of cancer, including those of the blood and in melanoma skin cancer, defining it as a putative drug-target. Of note, inhibition of a related protein, BCL2, has demonstrated to be successful and BCL2 inhibitors are now part of different anti-cancer treatment strategies. Limited information, however, was available on the role of A1/BFL1, as genetic models and other suitable reagents needed to conduct meaningful research were missing. Within this project we exploited a new set of reagents, including genetic models and newly generated immune-tools to explore the role of A1/BFL1 in the development of blood cells and blood cancer. To this end we were exploring the consequences of too much vs. too little A1/BFL1 protein in cells of the blood, referred to as leukocytes that are also an essential part of the immune system. Moreover, we also investigated the role of this protein in blood cancer induced by a famous oncogene, MYC, found deregulated in a large number of human cancers, including those of the blood. Finally, we tested the role of this protein for drug-resistance to chemotherapy used to treat blood but also solid cancers.We found that (i) A1/BFL1 is very low in resting leukocytes but accumulates rapidly in cells that were activated by immune-stimulatory molecules; (ii) we noted that A1/BFL1 appears particularly important for the survival of mast cells and B-lymphocytes, cells that patrol the skin or other surface barriers, such as the gut mucosa, or produce antibodies to fight infection. Yet, A1 was not controlling the effector function of mast cells nor the quality of antibodies produced; (iii) in line with our observations in B cells, we noted that during oncogenic transformation of B cells by the oncogene MYC, A1 protein levels appear to be critical for optimal tumor growth and that (iv) high levels of A1/BFL1 dramatically speed up tumor growth. Together these findings highlight a role of A1 in leukocyte homeostasis that is important to maintain immunity against invading pathogens and the drug-target potential of BFL1 in blood cancer and potentially beyond.