Understanding Sex Differences in the Regulation of Cancer-Induced Muscle Wasting

Abstract

Purpose:

We highlight evidence for sexual dimorphism in preclinical and clinical studies investigating the etiology and treatment of cancer cachexia.

Recent Findings:

Cancer cachexia is unintended bodyweight loss occurring with cancer, and skeletal muscle wasting is a critical predictor of negative outcomes in the cancer patient. Skeletal muscle exhibits sexual dimorphism in fiber type, function, and regeneration capacity. Sex differences have been implicated in skeletal muscle metabolism, mitochondrial function, immune response to injury, and myogenic stem cell regulation. All of these processes have the potential to be involved in cancer-induced muscle wasting. Unfortunately, the vast majority of published studies examining cancer cachexia in preclinical models or cancer patients either have not accounted for sex in their design or have exclusively studied males. Preclinical studies have established that ovarian function and estradiol can affect skeletal muscle function, metabolism and mass; ovarian function has also been implicated in the sensitivity of circulating inflammatory cytokines and the progression of cachexia.

Summary:

Females and males have unique characteristics that effect skeletal muscle’s microenvironment and intrinsic signaling. These differences provide a strong rationale for distinct etiologies for cancer cachexia development and treatment in males and females.

1. INTRODUCTION

Cancer-induced cachexia is a complex and progressive wasting condition that is clinically identified by >5% bodyweight loss over a 6-month period (1, 2). Cancer-induced cachexia commonly occurs in 40–60% of male and 40–50% of female patients > 60 years of age (3, 4). While bodyweight loss is an indicator of cachexia, skeletal muscle wasting is a critical predictor of negative outcomes (5–7). Muscle mass loss is also related to reduced physical function and fatigue (1, 2, 8, 9), which can further aggravate cachexia through sedentary behavior. Systemic inflammation and hypogonadism are cancer-induced disruptions to the systemic environment that are directly linked to muscle wasting (1, 2, 8, 9).

Outcomes following cancer diagnosis and treatment are influenced by many factors, including age, body composition, and activity level (8, 10, 11). However, sex-based differences in cancer outcomes and treatment success only recently have garnered significant interest (3, 12–14). Although males and females both experience cachexia-induced functional and metabolic decrements (15–17), the vast majority of published studies examining cancer cachexia in preclinical models and cancer patients either have not accounted for sex in their design or have exclusively studied males. Despite the high prevalence of females presenting as cachectic, little is known about the biological differences between sexes during wasting, despite estrogens and androgens being acknowledged as important regulators of muscle mass and function (18–20). The role of sex in the etiology and treatment of cancer cachexia certainly warrants further study. The National Institutes of Health (NIH) is requiring experimental designs that account for sex as a biological variable (12, 14, 21). Through this effort the NIH intends to increase the reproducibility and translational value of biomedical research (13). Therefore, we highlight the rationale for investigating sex differences in the development and treatment of cancer-induced muscle wasting. The review emphasizes several well-described sex differences in skeletal muscle physiology and function that have clear implications for advancing the understanding of mechanistic drivers of cancer-induced muscle wasting.

2. CANCER-INDUCED DISRUPTION to SKELETAL MUSCLE & SEXUAL DIMORPHISM

Skeletal Muscle Fiber Type and Function

Skeletal muscle has been classically characterized by contractile, fatigue, and metabolic properties (22–24). This characterization has led to broad fiber type classifications: glycolytic (II(B,X)), oxidative-glycolytic (IIA), and oxidative (I) (23). Type II glycolytic myofibers have been reported to be more sensitive to cancer-induced muscle wasting than type I myofibers in cancer patients and mouse models (25). Cancer-induced muscle wasting is also associated with reduced myofiber oxidative metabolism (26, 27). Age-related wasting, sarcopenia, also causes pronounced type II fiber atrophy (28–30). However, both elderly people and cancer patients tend to engage in high levels of sedentary behavior, which can exacerbate atrophy (8, 31). Interestingly, muscle disuse regulates muscle mass and metabolism (31), and preferentially atrophies oxidative myofibers (25). Shifts in muscle fiber distribution with cancer cachexia have not been observed clinically (32), but altered contractile function of type II fibers has been found (32). Only recently have studies acknowledged a role for muscle disuse and sedentary behavior as drivers of cancer-induced muscle wasting (31). While several studies have demonstrated reduced physical function with cancer cachexia (33–36), the role of skeletal muscle fiber type in these changes warrants further investigation.

While skeletal muscle sexual dimorphisms have been well-documented and reviewed (24, 37, 38), the mechanisms responsible for these differences remain to be elucidated. For example, skeletal muscle gene expression profiling has revealed thousands of genes that exhibit sexually dimorphic expression (37), including evidence that muscle fiber type distribution is impacted by sex (24, 38). However, differences are not easy to quantify and interpret. Muscle fiber type is very heterogenous, varying by muscle type and species (23). Males have been reported to have greater muscle mass and ratio of type II:I fibers than females, demonstrating a more glycolytic phenotype (23, 24, 37). Type I fibers have been reported to account for a greater percentage of muscle area in females (23, 24, 38). Interestingly, estrogen status has been reported to not affect muscle fiber type distribution in humans (39) and mice (24). Since glycolytic and oxidative fibers can differentially respond to cancer and disuse, further research is warranted to determine if inherent fiber type differences contribute to differential responses to cancer-induced muscle wasting between men and women.

Sex-based differences have been widely reported for muscle fatigue susceptibility (24, 40), including evidence that male muscles are more fatigable than female muscles (16, 41). While the source of female fatigue resistance has not yet been established, it does coincide with increased oxidative fiber incidence. Interestingly, estrogen signaling can regulate fatigue and muscle contractile response in males and females; estrogen receptor (ER) beta loss decreases female endurance (42). Further work is needed to establish regulators of fatigue resistance in females (43). While estrogen status does not play a role in fiber type distribution, it can impact myofiber morphology, contraction, and function (24). Strength losses occur earlier in the aging process for postmenopausal females than males of the same age, but can be restored with hormone replacement therapy (44). Furthermore, cachectic males have greater deficits in handgrip strength when compared to cachectic female (15), and male cancer patients have increased muscle fatigue when compared to female patients (16). While whole body (45) and muscle (46) fatigue have been reported in preclinical cachexia models, sex regulation of cachexia-induced fatigue requires further study. Nevertheless, tumor-bearing female mice with decrements in estrogenic signaling have demonstrated increased tumor growth (47–49) and exacerbated cachectic pathology (33) that coincided with diminished muscle function (48, 49). While the cancer can increase muscle fatigue and decrease contractile function in preclinical cancer cachexia models (46), the sex-specific mechanisms that regulate these critical outcomes have not been established. A better understanding of sexual dimorphism’s role in cachexia-induced decrements to muscle function is needed and should benefit the development of therapeutics to improve cancer patient quality of life and survival.

Skeletal Muscle Protein Turnover Regulation

Muscle mass regulation comprises the constant flux of protein turnover related to protein degradation and synthesis (22). Endocrine, immune, physical activity, and nutritional signaling are integrated to provide this vital and dynamic regulation (22). When protein synthesis consistently exceeds degradation, net protein accretion results in hypertrophy (22). Insulin-like growth factor-1 (IGF-1) induces muscle protein synthesis and hypertrophy (22), and conversely the suppression of this pathway occurs with atrophic conditions (50, 51). The protein kinase B (Akt)/ mammalian target of rapamycin complex 1 (mTORC1) axis is a critical downstream effector of IGF-1 that induces muscle protein synthesis (22, 52, 53). mTORC1 has also emerged as a critical integration point for resistance exercise and amino acid induction of protein synthesis (54–58), which is independent of Akt signaling (22). Furthermore, protein synthesis regulation by nutrition and exercise is a critical indicator of skeletal muscle metabolic health (22, 59, 60). Anabolic resistance, the inability for anabolic stimuli to induce muscle protein synthesis, has been investigated as a driver of age-related sarcopenia (61, 62) and disuse atrophy (31). While anabolic resistance has been reported in cancer patients (56, 63), further investigation is needed to determine the mechanistic underpinnings of this resistance, and to identify the upstream mediators responsible for suppressed anabolic signaling in cachectic muscle (64). Protein degradation is also a tightly regulated process mediated by ubiquitin-proteasomal activation and proteasomal-autophagy-lysosomal pathways (65). Akt/mTORC1 signaling also regulates these degradation pathways (65, 66). Muscle ubiquitin-proteasomal degradation and autophagy pathways are induced during cancer cachexia (65, 67), and can be regulated by systemic inflammation and inflammatory cytokines (8, 52). Furthermore, 5’-adenosine monophosphate-activated protein kinase (AMPK) can promote autophagy through UNC-51 like kinase (ULK1) (68, 69). Additional research is warranted to determine if muscle AMPK and mTORC1 signaling can serve as therapeutic targets for cancer-induced autophagy (67, 70, 71).

The male and female response to anabolic stimuli has been well documented (24, 43, 72). Protein turnover regulation in young adult muscles does not exhibit overt sexual dimorphism (44, 72). However, differential protein turnover regulation in post-menopausal females has been reported (20, 72). Mediators of muscle anabolism decrease with advancing age and exhibit estrogen sensitivity (73, 74). Estrogen regulation has been implicated in AMPK signaling (75, 76) and upregulation of the IGF-1/mTORC1/Akt axis (73). Estradiol also promotes anabolism in preclinical models (18, 77), and disrupts muscle protein turnover through autophagy regulation by the Akt/forkhead-BoxO (FoxO) signaling pathway (78). Estrogen can also regulate inflammatory signaling (33, 79) and apoptosis (80). Therefore, decreased circulating estrogen has the potential to alter cachectic pathology through dysregulated protein turnover driven by increased muscle inflammation, attenuated anabolic metabolism, and increased autophagy (3, 77) (Figure 1). While the importance of sex on the ubiquitin-proteasome and autophagy-lysosomal pathways has only begun to be investigated (81), it remains an exploitable target for treating cachexia. Since muscle inflammatory signaling mediated through STAT3 and NF-κB can induce ubiquitin proteasome degradation during cachexia (27, 82), altered inflammatory signaling has the potential to suppress or activate these pathways. In general, a greater understanding of the sex-specific regulation of protein turnover should provide valuable insight for the successful development of therapies to prevent cancer-induced muscle wasting in both males and females.

Figure 1:

Potential Targets of Estrogen in Muscle in the Systemic Cachectic Environment

Skeletal Muscle Mitochondrial Quality Control and Oxidative Metabolism

The impact of mitochondrial function on skeletal muscle homeostasis and function has been widely acknowledged and recently and thoroughly reviewed (83–85). Mitochondrial quality control is a highly coordinated process that facilitates mitochondrial homeostasis (84, 85). Mitochondrial quality control encompasses the regulation of mitochondrial biogenesis, dynamics, and mitophagy processes (84). These pathways are integrated with muscle anabolic and catabolic signaling (83) that impacts muscle metabolic health (9, 63, 85). Cancer cachexia is associated with mitochondrial dysfunction and loss of muscle oxidative capacity and recent research has identified these changes as critical drivers of cancer cachexia progression (26, 27, 46, 86–88). Indeed, mitochondrial dysfunction and degeneration can precede muscle wasting in tumor-bearing (86) and sarcopenic (28) mice. Disrupted mitophagy regulation has also been investigated as a central mechanism for muscle wasting (67, 70, 88–91). Additionally, reactive oxygen species (ROS) and oxidative stress can accompany mitochondrial dysfunction and have also been widely investigated in muscle wasting (92, 93). While cachexia research has largely focused on defining the role of mitochondrial dysfunction as a driver of wasting, there is strong rationale for investigating the function of sex hormones in the cancer-induced disruption to muscle oxidative metabolism.

Mitochondrial homeostasis can be regulated by sex and has been comprehensively reviewed (77, 80, 94, 95). Basal mitochondria respiratory function and content is higher in females, recapitulating the oxidative phenotype (94). Sexually dimorphic properties of mitochondria quality control and oxidative metabolism can also be observed. Integration of cellular signaling pathways involving the mitochondria regulation, such as AMPK activity (18, 75, 76), are estrogen sensitive (96, 97). These are well-defined sex-mediated regulators of oxidative metabolism (77, 98). Mitochondrial gene expression is influenced by estrogen (80, 96), and has demonstrated sexual dimorphisms in genomic (37) and proteomic (38) analyses. Recently it has been shown that mitochondrial membrane viscosity can be regulated by estrogenic signaling and that it promotes electron transport chain activity (98). Research examining drivers of cancer cachexia in males and females provides a strong premise to investigate the role of sex in mitochondrial processes that regulate cancer cachexia (94).

3. POTENTIAL THERAPEUTIC INTERVENTIONS

Resistance Exercise

Resistance exercise is a non-pharmacological intervention that can modulate muscle fiber composition, protein turnover, oxidative phosphorylation, and hormones, and it has been extensively reviewed (64, 99, 100). Resistance exercise can benefit the cancer patient’s quality of life and physical function (11, 99–102). Regular physical activity during chemotherapy treatment (103) and prior to cancer can have positive effects on skeletal muscle metabolism and attenuate comorbidities (104–106). A multimodal approach to cachexia treatment could impact patient survival, and a recent clinical trial validated the feasibility of an exercise intervention in cachectic patients (107). A clinical trial is being conducted to investigate the effect of resistance exercise on skeletal muscle during chemotherapy treatment (NIH#:NCT02330926). While resistance exercise has the ability to attenuate wasting (56, 108, 109), mimicked resistance exercise in preclinical cancer models has demonstrated a blunted anabolic response in cachectic muscle (110). Exercise benefits have been as observed in male cancer patients (105, 111) and female cancer patients (104, 108), cancer patients of both sexes (99, 100, 107, 112, 113) and those with sarcopenia (56, 62, 114). However, there is a strong rationale for resistance exercise to impact cancer-induced muscle wasting (64) in a sexually dimorphic manner (99). Resistance exercise regulates many sexually dimorphic cellular signaling pathways in skeletal muscle (18, 24, 38, 72, 115, 116). Furthermore, resistance exercise can modulate sex hormone levels in men and women (99, 104, 117, 118). Given that exercise is a modifiable risk factor for chronic disease, resistance exercise adaptation and responses in both male and female cancer patients suffering from cachexia warrants further investigation.

Hormone Therapy

Hypogonadism is associated with decreased circulating sex hormones that can result from aging, gonad dysfunction, and chronic disease (9, 20). Testosterone impacts skeletal muscle hypertrophy and anabolic signaling (119, 120). Estrogen has genomic and nongenomic properties (18, 19, 116), and skeletal muscle fibers are highly sensitive to changes in circulating estrogen (18). While the prevalence of hypogonadism increases with age (20), it is more common among cancer patients (3, 121) and is related to cachexia development (122, 123). While sex hormones play established roles in many aspects of skeletal muscle function, their role in cancer-induced muscle wasting is not fully understood (19). Testosterone treatment in male and female cancer patients has positive effects on skeletal muscle function and health (124) (NIH#:NCT00878995), but may exacerbate comorbidities (119, 125). Similarly, the positive effects of estrogen-related therapy (18, 126) have the potential to be offset by adverse consequences (127, 128) and risk of cancer recurrence (129, 130). Therefore, hormone therapy alternatives have generated interest and ongoing study. Selective androgen receptor modulators (SARMs) are a class of tissue-selective androgen receptor ligands that target selected androgen effects and avoid comorbidities observed with traditional hormone therapy (3, 131–133). SARMs have been shown to benefit muscle mass, inflammation, and catabolism in hypogonadal states of both sexes (134, 135). The complex interaction of sex hormones with cancer-induced muscle metabolic and contractile dysfunction warrants further investigation.

4. CONCLUSION

Males and females have demonstrated sex-specific regulation related to aging and several disease states (12, 20, 136). This understanding extends to well-described sexual dimorphism in aspects of skeletal muscle function and metabolism. Sex differences also extend to essential muscle properties involving muscle regeneration after injury (3). Despite a strong premise for sexual dimorphism in mechanisms that drive muscle wasting, we have a very limited understanding of how sex can influence cancer cachexia progression. We highlight several mechanistic regulators of skeletal muscle wasting that have potential for sexually dimorphic responses that could impact the prevention and treatment of cancer cachexia in both males and females (Figure 2).

Figure 2:

Cancer Environment’s Effect on Dysregulated Protein Turnover in Skeletal Muscle

Key Points:

1. Skeletal muscle has sexual dimorphic properties that have strong potential to influence the mechanistic underpinnings of cancer-induced muscle wasting.

2. Cancer-induced systemic inflammation can disrupt the IGF-1/Akt/mTORC1 axis and mitochondria function, which have been widely investigated as regulators of cachexia progression and also have strong potential to be differentially regulated in males and females.

3. The role of hypogonadism in cancer-induced muscle wasting has significant implications for the identification of sexually dimorphic mechanisms of cachexia regulation.

4. Resistance exercise has strong therapeutic potential for the maintenance and recovery of muscle mass in both male and female cancer patients.

5. Sex is an important biological variable that needs to be accounted for in the design of human and preclinical cancer cachexia research.