A role of arginase-1-expressing myeloid cells in cachexia

Despite decades of efforts to find successful treatment approaches, cachexia remains a major unmet medical need. This condition, that affects patients with diverse underlying conditions, is characterized by severe muscle loss and is associated with reduced quality of life and limited survival. Search for underlying mechanisms that may guide cachexia treatment has mainly evolved around potential atrophy-inducing roles of inflammatory mediators, and in cancer patients, tumor-derived factors. Recently, a new paradigm emerged as it is becoming evident that specific immune cells inhabit atrophic muscle tissue. Arginase 1 (Arg1) expression is characteristic of these immune cells. Studies of potential contributions of these immune cells to loss of muscle mass and function is in its infancy, and the contribution of ARG1 to these processes remains elusive.

Analyses of RNA sequencing data from murine cachexia models and comprehensive, unbiased open approach proteomics analyses of skeletal myotubes was performed. In vitro techniques were employed to evaluate mitochondrial function and capacity in skeletal muscle cells and cardiomyocytes. Functional bioassays were used to measure autophagy activity. ARG1 level in patients' plasma was evaluated using ELISA, and the association between ARG1 level and patient survival, across multiple types of cancer, was examined using the online database Kaplan-Meier plotter.

In line with arginine-degrading activity of ARG1, we found signs of arginine restriction in atrophic muscles. In response to arginine restriction, mitochondrial functions and ATP generation was severely compromised in both skeletal muscle cells and in cardiomyocytes. In skeletal muscle cells, arginine restriction enhanced the expression of autophagic proteins, suggesting autophagic degradation of cellular content. Reduction in mitochondria marker TIMM23 supports selective autophagic degradation of mitochondria (mitophagy). In arginine starved cardiomyocytes, mitochondrial dysfunction is accompanied by both increased bulk autophagy and mitophagy. In cancer patients, we found an association between ARG1 expression and accelerated weight loss and reduced survival, further supporting a role of ARG1-producing cells in cachexia pathogenesis.

Together, our findings point to a mechanism for cachexia which depends on expansion of ARG1-expressing myeloid cells, local restriction of arginine, loss of mitochondrial capacity and induced catabolism in skeletal muscle cells and in the heart.