Why do BCL-2 inhibitors work and where should we use them in the clinic?

Abstract

Intrinsic apoptosis is controlled by the BCL-2 family of proteins but the complexity of intra-family interactions makes it challenging to predict cell fate via standard molecular biology techniques. We discuss BCL-2 family regulation and how to determine cells' readiness for apoptosis and anti-apoptotic dependence. Cancer cells often adopt anti-apoptotic defense mechanisms in response to oncogenic stress or anti-cancer therapy. However, by determining their anti-apoptotic addiction, we can use novel BH3 mimetics to overwhelm this apoptotic blockade. We outline the development and uses of these unique anti-apoptotic inhibitors and how to possibly combine them with other anti-cancer agents using dynamic BH3 profiling (DBP) to improve personalized cancer treatment.

Figure 1

The BCL-2 family of proteins. BCL-2 family control over mitochondrial apoptosis representation. In response to therapy or oncogene activation, activators engage effectors, causing mitochondrial permeabilization and apoptotic cell death. PUMA and NOXA, are weaker activators compared with BIM or BID. Anti-apoptotic proteins sequester activators or effector proteins to prevent apoptosis. Sensitizers act as selective antagonists of anti-apoptotic proteins. Modified from Deng et al., 2007

Figure 2

The threshold of apoptosis. We define priming as the proximity of a malignant cell to the threshold of apoptosis, and dynamic BH3 profiling (DBP) measures increases in priming upon treatment. (a) When a cancer cell is exposed to the wrong treatment there is no increase in priming, giving a negative result for DBP, and the cell will survive. (b) In contrast, when a cancer cell is exposed to the right treatment there is an increase in priming that will be detected by DBP in early timepoints preceding apoptotic cell death

Figure 3

Use of BH3 mimetics to overcome tumors’ resistance to therapy. When cancer cells are exposed to therapy, they can adapt their anti-apoptotic strategy to block the pro-apoptotic proteins’ accumulation (activators and active effectors) induced by treatment. By blocking the anti-apoptotic defense using a specific BH3 mimetic, activators will be displaced from anti-apoptotic proteins and restore apoptotic cell death

Figure 4

Using dynamic BH3 profiling to identify BH3 mimetics’ use. To perform dynamic BH3 profiling, we obtain a single cell suspension from a cell line or a primary sample, and we expose the cells for a short incubation with the different treatments of interest. After this incubation, we permeabilize cells with digitonin, and expose the cells to selective BH3 peptides. We then analyze cytochrome c retention in the cells by flow cytometry or microscopy. By comparing non-treated with treated cells, we will determine the Δ%priming for each agent and identify which are most effective to induce apoptosis or detect anti-apoptotic dependency changes. BIM BH3 peptide will indicate overall response to treatment (treatment 1). A BAD BH3-positive signal and negative HRK signal will indicate BCL-2 dependence and a possible use for agents like ABT-199/venetoclax (treatment 2). HRK-positive signal will indicate BCL-XL dependence and possible use for inhibitors against this antiapoptotic protein (treatment 3). NOXA-positive signal will indicate the use of specific MCL-1 inhibitors (treatment 4). Adapted from Montero et al., 2015 and Ryan et al., 2016