The tumor microenvironment (TME) in ovarian cancer (OC) has much greater complexity than previously understood. In response to aggressive pro-angiogenic stimulus, blood vessels form rapidly and are dysfunctional, resulting in poor perfusion, tissue hypoxia, and leakiness, which leads to increased interstitial fluid pressure (IFP). Decreased perfusion and high IFP significantly inhibit the uptake of therapies into the tumor. Within the TME, there are numerous inhibitor cells, such as myeloid-derived suppressor cells (MDSCs), tumor association macrophages (TAMs), regulatory T cells (Tregs), and cancer-associated fibroblasts (CAFs) that secrete high numbers of immunosuppressive cytokines. This immunosuppressive environment is thought to contribute to the lack of success of immunotherapies such as immune checkpoint inhibitor (ICI) treatment. This review discusses the components of the TME in OC, how these characteristics impede therapeutic efficacy, and some strategies to alleviate this inhibition.

Muscle loss with cancer causes weakness, worsens quality of life, and predicts reduced overall survival rates. Recently, muscle weakness was identified during early-stage cancer before atrophy develops. This discovery indicates that mechanisms independent of muscle loss must contribute to progressive weakness. While mitochondrial stress responses are associated with early-stage 'pre-cachexia' weakness, a causal relationship has not been established. Here, using a mouse model of metastatic ovarian cancer cachexia, we identified that the well-established mitochondrial-targeted plastoquinone SkQ1 partially prevents muscle weakness occurring before the development of atrophy in the diaphragm. Furthermore, SkQ1 improved force production during atrophy without preventing atrophy itself in the tibialis anterior and diaphragm. These findings indicate that atrophy-independent mechanisms of muscle weakness occur in different muscle types throughout ovarian cancer. Ovarian cancer reduced flexor digitorum brevis (FDB) whole muscle force production and myoplasmic free calcium ([Ca²⁺]_i) during contraction in intact single muscle fibers, both of which were prevented by SkQ1. Remarkably, changes in mitochondrial reactive oxygen species and pyruvate metabolism were heterogeneous across time and between muscle types which highlights a considerable complexity in the relationships between mitochondria and muscle remodeling throughout ovarian cancer. These discoveries identify that muscle weakness can occur independent of atrophy throughout ovarian cancer in a manner that is linked to improved calcium handling. The findings also demonstrate that mitochondrial-targeted therapies exert a robust effect in preserving muscle force early during ovarian cancer during the pre-atrophy period and in late stages once cachexia has become severe.

Cancer cachexia, and the related loss of muscle and strength, worsens quality of life and lowers overall survival. Recently, a novel ‘pre-atrophy’ muscle weakness was identified during early-stage cancer. While mitochondrial stress responses are associated with early-stage pre-atrophy weakness, a causal relationship has not been established. Using a robust mouse model of metastatic epithelial ovarian cancer (EOC)-induced cachexia, we found the well-established mitochondrial-targeted plastoquinone SkQ1 partially prevents pre-atrophy weakness in the diaphragm. Furthermore, SkQ1 improved force production during atrophy without preventing atrophy itself in the tibialis anterior and diaphragm. EOC reduced flexor digitorum brevis (FDB) force production and myoplasmic free calcium ([Ca ²⁺ ] _i ) during contraction in single muscle fibers, both of which were prevented by SkQ1. Remarkably, changes in mitochondrial reactive oxygen species and pyruvate metabolism were heterogeneous across time and between muscle types which highlights a considerable complexity in the relationships between mitochondria and muscle remodeling throughout EOC. These discoveries identify that muscle weakness can occur independent of atrophy throughout EOC in a manner that is linked to improved calcium handling. The findings also demonstrate that mitochondrial-targeted therapies exert a robust effect in preserving muscle force during the early pre-atrophy period and in late-stage EOC once cachexia has become severe.

The degree to which mitochondrial-linked cell death pathways contribute to skeletal muscle atrophy during cancer remain unknown. Here, we combined a novel and robust mouse model of metastatic ovarian cancer with chronic administration of the mitochondrial-targeted antioxidant SkQ1 to determine the time-dependent and muscle-specific relationships of mitochondrial-linked apoptosis and necroptosis to the development of muscle atrophy in the type II B-rich gastrocnemius. Early-stage ovarian cancer reduced type II B fibre cross-sectional area in the gastrocnemius but did not alter mitochondrial H ₂ O ₂ emission despite increased activities of mitochondrial-linked caspase-9 and-3 regulators of apoptosis. During late-stage ovarian cancer, sustained atrophy was associated with increased mitochondrial H ₂ O ₂ emission potential in vitro , a greater probability of calcium-triggered mitochondrial permeability transition and increases in downstream caspase-9 and −3 activity. SkQ1 attenuated mitochondrial H ₂ O ₂ emission and caspase-9 and −3 activity in late-stage ovarian cancer but did not prevent atrophy. Necroptosis markers were heterogeneous across time with total RIPK1 increasing during early-stage cancer which reverted to normal levels by late-stages while phosphorylated RIPK3 decreased below control levels. These discoveries indicate that preventing increases in mitochondrial-linked apoptotic caspase-9 and −3 activity during late-stage ovarian cancer with SkQ1 does not prevent atrophy of type II B fibres. Furthermore, necroptotic markers are inconclusive during cancer in this muscle type but are not modified by SkQ1. These results do not support a causal relationship between mitochondrial H ₂ O ₂ -linked apoptosis or necroptosis and atrophy in type II B fibres during ovarian cancer but do not rule out potential relationships in other muscle types. Key Points: Cancer increases mitochondrial reactive oxygen species (ROS) in skeletal muscle during atrophy, but the role of ROS in regulating cell death remains unknown. We show that attenuating gastrocnemius mitochondrial ROS with the mitochondrial-targeted antioxidant SkQ1 prevented mitochondrial-linked pro-apoptotic caspase 9- and 3-activities but did not affect markers of necroptosis in a mouse model of ovarian cancer. Reductions in gastrocnemius muscle fibre cross-sectional areas and the wet weights of several muscles were not prevented by SkQ1. These findings demonstrate that mitochondrial ROS regulate apoptotic caspases but not necroptosis, and neither pathway is linked to gastrocnemius atrophy in mice with ovarian cancer. The degree to which mitochondrial ROS-linked cell death pathways regulate muscle mass in other muscle types and cancer models requires further investigation.