The Role of BAX/BAK-Mediated Apoptosis for the Cytotoxic Action of Anti-Myeloma Agents

Multiple myeloma is a malignancy of plasma cells. The abnormal clonal accumulation of plasma cells interferes with the blood cell production, thereby causing cytopenias and aberrant, excessive antibody production. Patients can also experience bone pain, neurological symptoms and renal failure. With the exception of bone marrow transplantation that is associated with significant morbidity and mortality, there is currently no cure for patients with this disease. Thus, novel therapies are desperately needed for treating patients especially those harboring disease with adverse prognostic markers or when they become refractory to current treatment regimens.

Recently, we have screened a panel of well-established myeloma cell lines against multiple anti-myeloma agents, including standard-of-care agents such as bortezomib, lenalidomide, dexamethasone or melphalan, as well as novel targeted agents such as the BH3 mimetic compounds (e.g. venetoclax/ABT-199 to target BCL2) or HDAC inhibitors (e.g. panobinostat). Specifically, we sought to determine whether their cytotoxic action depends on the induction of BAX/BAK-mediated apoptosis. When activated, these two proteins drive permeabilization of the outer mitochondrial membrane. In their absence, apoptosis cannot proceed and we could readily establish if BAX/BAK-mediated apoptosis is critical by comparing the sensitivity of isogenic cell lines lacking these two cell death mediators to their wild-type parental counterparts.

As anticipated, the action of the BH3 mimetic compounds such as venetoclax/ABT-199 to inhibit BCL2 or small molecule inhibitors to selectively target relatives of BCL2, such as BCLxL and MCL1, rely exclusively on BAX and BAK. In the their absence, the treated cells remained viable. While the action of some drugs such as dexamethasone is already known to be mediated in part by BAX/BAK-driven apoptosis, we were very surprised that lenalidomide, an immunomodulatory drug (IMiD), can also kill some myeloma cell lines through BAX/BAK mediated apoptosis. A more potent relative of lenalidomide, pomalidamide, does likewise. While the cytotoxic action of these IMiDs, most notably the induction of cell cycle arrest upon activation of the E3 ligase cereblon (CRBN) is well known, our data is the first to definitively demonstrate that the IMiDs can actively induce BAX/BAK-mediated apoptosis.

Next we explored the mechanisms by which the IMiDs can trigger apoptosis. Most stress signals and many cytotoxic drugs activate the BH3-only proteins such as BIM, BAD, BID or PUMA to initiate apoptosis. We checked whether killing by lenalidomide might also depend on specific BH3-only proteins and found that the genetic deletion of BIM conferred almost complete resistance to treatment with lenalidomide in vitro. We are currently investigating the precise mechanism by which the IMiDs can activate BIM to drive apoptosis. While lenalidomide can trigger BAX/BAK-mediated apoptosis in some myeloma cell lines, many others were spared.

We expect that by deciphering the mechanisms by which anti-myeloma agents can induce BAX/BAK-mediated apoptosis of myeloma cell lines, we can probably improve the utility of drugs such as the IMiDs and understand why they might fail in some patients.