SUMMARY It is not understood why healthy tissues can exhibit varying levels of sensitivity to the same toxic stimuli. Using BH3 Profiling, we find that mitochondria of many adult somatic tissues, including brain, heart and kidneys, are profoundly refractory to pro-apoptotic signaling, leading to cellular resistance to cytotoxic chemotherapies and ionizing radiation. In contrast, mitochondria from these tissues in young mice and humans are primed for apoptosis, predisposing them to undergo cell death in response to genotoxic damage. While expression of the apoptotic protein machinery is nearly absent by adulthood, in young tissues its expression is driven by c-Myc, linking developmental growth to cell death. These differences may explain why pediatric cancer patients have a higher risk of developing treatment-associated toxicities.

Apoptosis is a form of programmed cell death that is critical for basic human development and physiology. One of the more important surprises in cell biology in the last two decades is the extent to which mitochondria represent a physical point of convergence for many apoptosis inducing signals in mammalian cells. Mitochondria not only adjudicate the decision of whether or not to commit to cell death, but also release toxic proteins culminating in widespread proteolysis, nucleolysis, and cell engulfment. Interactions among Bcl-2 family proteins at the mitochondrial outer membrane controls the release of these toxic proteins, and by extension control cellular commitment to apoptosis. This pathway is particularly relevant to cancer treatment, as most cancer chemotherapies trigger mitochondrial mediated apoptosis. In this review, we discuss recent advances in the Bcl-2 family interactions, their control by upstream factors, and how the mitochondria itself alters these interactions. We also highlight recent clinical insights into mitochondrial mediated apoptosis and novel cancer therapies that exploit this pathway. Introduction Mitochondria adjudicate the cell death decision in response to many physiological and therapeutic stimuli. The review we highlight seminal and recent advances on how mitochondria and the Bcl-2 family of proteins regulate cell death. In particular we discuss recent advances in the Bcl-2 family interactions, their control by upstream factors, and how the mitochondria itself alters these interactions. We also highlight recent clinical insights into mitochondrial mediated apoptosis and how cancer therapies that exploit this pathway.

Persistent myofibroblast activation distinguishes pathological fibrosis from physiological wound healing, suggesting that therapies selectively inducing myofibroblast apoptosis could prevent progression and potentially reverse established fibrosis in diseases such as scleroderma, a heterogeneous autoimmune disease characterized by multiorgan fibrosis. We demonstrate that fibroblast-to-myofibroblast differentiation driven by matrix stiffness increases the mitochondrial priming (proximity to the apoptotic threshold) of these activated cells. Mitochondria in activated myofibroblasts, but not quiescent fibroblasts, are primed by death signals such as the proapoptotic BH3-only protein BIM, which creates a requirement for tonic expression of the antiapoptotic protein BCL-X to sequester BIM and ensure myofibroblast survival. Myofibroblasts become particularly susceptible to apoptosis induced by "BH3 mimetic" drugs inhibiting BCL-X such as ABT-263. ABT-263 displaces BCL-X binding to BIM, allowing BIM to activate apoptosis on stiffness-primed myofibroblasts. Therapeutic blockade of BCL-X with ABT-263 (navitoclax) effectively treats established fibrosis in a mouse model of scleroderma dermal fibrosis by inducing myofibroblast apoptosis. Using a BH3 profiling assay to assess mitochondrial priming in dermal fibroblasts derived from patients with scleroderma, we demonstrate that the extent of apoptosis induced by BH3 mimetic drugs correlates with the extent of their mitochondrial priming, indicating that BH3 profiling could predict apoptotic responses of fibroblasts to BH3 mimetic drugs in patients with scleroderma. Together, our findings elucidate the potential efficacy of targeting myofibroblast antiapoptotic proteins with BH3 mimetic drugs in scleroderma and other fibrotic diseases.