Cytotoxic chemotherapy targets elements common to all nucleated human cells, such as DNA and microtubules, yet it selectively kills tumor cells. Here we show that clinical response to these drugs correlates with, and may be partially governed by, the pre-treatment proximity of tumor cell mitochondria to the apoptotic threshold, a property called mitochondrial priming. We used BH3 profiling to measure priming in tumor cells from patients with multiple myeloma, acute myelogenous and lymphoblastic leukemia, and ovarian cancer. This assay measures mitochondrial response to peptides derived from pro-apoptotic BH3 domains of proteins critical for death signaling to mitochondria. Patients with highly primed cancers exhibited superior clinical response to chemotherapy. In contrast, chemoresistant cancers and normal tissues were poorly primed. Manipulation of mitochondrial priming might enhance the efficacy of cytotoxic agents.

Cancer cells demonstrate deviant behavior that induces apoptotic signaling. In order to survive, cancer cells typically acquire changes enabling evasion of death signals. One way they do this is by increasing the expression of anti-apoptotic BCL-2 proteins. Anti-apoptotic BCL-2 family proteins antagonize death signaling by forming heterodimers with pro-death proteins. Heterodimer formation occurs via binding of the pro-apoptotic protein’s BH3 domain into the hydrophobic cleft of anti-apoptotic proteins. The BH3 mimetics are small molecule antagonists of the anti-apoptotic BCL-2 members that function as competitive inhibitors by binding to the hydrophobic cleft. Under certain conditions, antagonism of anti-apoptotic BCL-2 family proteins can unleash pro-death molecules in cancer cells. Thus, the BH3 mimetics are a new class of cancer drugs that specifically target a mechanism of cancer cell survival, to selectively kill cancer cells.

The endoplasmic reticulum (ER) is the main site for protein folding, lipid biosynthesis, and calcium storage in the cell. Disturbances of these critical cellular functions lead to ER stress. The ER responds to disturbances in its homeostasis by launching an adaptive signal transduction pathway, known as the unfolded protein response (UPR). The UPR strives to maintain ER function during stress; however, if the stress is not resolved, apoptotic responses are activated that involve cross talk between the ER and mitochondria. In addition, ER stress is also known to induce autophagy to counteract XBP-1-mediated ER expansion and assist in the degradation of unfolded proteins. One family of proteins involved in the regulation of apoptosis is that of B-cell lymphoma protein 2 (Bcl-2). Complex interactions among the three subgroups within the Bcl-2 family [the antiapoptotic, the multidomain proapoptotic, and the Bcl-2 homology domain 3 (BH3)-only members] control the signaling events of apoptosis upstream of mitochondrial outer membrane permeabilization. These proteins were found to have diverse subcellular locations to aid in the response to varied intrinsic and extrinsic stimuli. Of recent interest is the presence of the Bcl-2 family at the ER. Here, we review the involvement of proteins from each of the three Bcl-2 family subgroups in the maintenance of ER homeostasis and their participation in ER stress signal transduction pathways.

Acute Lymphoblastic Leukemia (ALL) is a hematopoietic malignancy derived from immature B-and T-lymphoid cells (T-ALL). In T-ALL there is an early T-cell progenitor (ETP) subgroup that has a very high risk for relapse. In this study, we utilized mitochondrial BH3 profiling to determine anti-apoptotic dependencies in T-ALL. We found that T-ALL cell lines and primary patient samples are dependent upon BCL-XL, except when the cancer bears ETP phenotype, in which case it is BCL-2 dependent. These distinctions directly relate to differential sensitivity to the BH3 mimetics ABT-263 and ABT-199 both in vitro and in vivo. We thus describe for the first time a change of anti-apoptotic dependence that is related to the differentiation stage of the leukemic clone. Our findings demonstrate that BCL-2 is a clinically relevant target for therapeutic intervention with ABT-199 in ETP-ALL.

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SummaryTargeted therapies designed to exploit specific molecular pathways in aggressive cancers are an exciting area of current research. Mixed Lineage Leukemia (MLL) mutations such as the t(4;11) translocation cause aggressive leukemias that are refractory to conventional treatment. The t(4;11) translocation produces an MLL/AF4 fusion protein that activates key target genes through both epigenetic and transcriptional elongation mechanisms. In this study, we show that t(4;11) patient cells express high levels of BCL-2 and are highly sensitive to treatment with the BCL-2-specific BH3 mimetic ABT-199. We demonstrate that MLL/AF4 specifically upregulates the BCL-2 gene but not other BCL-2 family members via DOT1L-mediated H3K79me2/3. We use this information to show that a t(4;11) cell line is sensitive to a combination of ABT-199 and DOT1L inhibitors. In addition, ABT-199 synergizes with standard induction-type therapy in a xenotransplant model, advocating for the introduction of ABT-199 into therapeutic regimens for MLL-rearranged leukemias.

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Mitochondria are cellular organelles that regulate commitment and execution of apoptosis. The intrinsic apoptotic pathway culminates in the permeabilization of the mitochondrial outer membrane and dismantling of the cell. Apoptosis of cancer cells is a favorable outcome when administering chemotherapeutic treatment yet the basis for why some cancers are sensitive to chemotherapy while others are not has historically been poorly understood. In this review, we present recent work that has demonstrated the importance of mitochondrial apoptotic priming, or how close a cell is to the threshold of apoptosis, in determining whether a cell will undergo apoptosis after chemotherapy treatment. Differential levels of apoptotic priming in tumors create bona fide opportunities and challenges for effective use of targeted and cytotoxic chemotherapies.