Cancer-associated cachexia (CAC) is a complex syndrome characterized by significant loss of skeletal muscle and adipose tissue, accompanied by reduced food intake, increased energy expenditure, excess catabolism, and systemic inflammation. While CAC can occur with or without fat loss, evidence suggests that fat loss may precede muscle loss. Studies using CAC animal models have shown that inhibiting lipolysis in white adipose tissue (WAT) ameliorates muscle loss. Additionally, parathyroid hormone-related protein (PTHrP) secretion from tumors contributes to increased energy expenditure and muscle loss. WAT comprises adipocytes, adipose stem and progenitor cells (ASPCs), endothelial cells, smooth muscle cells, and various immune cells. These cells modulate the adipose microenvironment and contribute to metabolic disorders. Depot-specific differences exist, such as between subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT). Single-cell transcriptomics offers powerful tools to analyze the cellular complexity of adipose tissue under various physiological conditions. Previous studies have largely focused on obesity-associated metabolic syndromes; therefore, this study uses a single-cell strategy to investigate human adipose tissue from CAC patients to understand the cellular and molecular mechanisms underlying adipose wasting in CAC.

Previous research has highlighted the complex etiology of CAC, emphasizing the interplay between systemic inflammation, altered metabolism, and the loss of both muscle and adipose tissue. Studies using animal models have demonstrated the critical role of lipolysis inhibition in preventing muscle wasting in CAC, implicating adipose tissue dysfunction as a potential driver of the syndrome. Furthermore, the involvement of factors like PTHrP in promoting energy expenditure and contributing to muscle loss has been established. However, a comprehensive understanding of the cellular and molecular mechanisms at play within adipose tissue during CAC has been lacking. While single-cell studies have shed light on the cellular heterogeneity of adipose tissue in other contexts, such as obesity, there has been a limited application of this technology to human adipose tissue from CAC patients, leaving a gap in our knowledge regarding the contribution of specific immune cell populations to adipose wasting in this context.

The researchers performed single-cell RNA sequencing (scRNA-seq) on stromal vascular fractions (SVFs) from SAT and VAT samples of eight gastric cancer patients (four with and four without CAC). CAC was defined by a greater than 5% weight loss and systemic inflammation. A total of 42,119 cells were sequenced, and 33,856 high-quality cells were analyzed. Unsupervised clustering and marker gene annotation identified nine major cell populations: adipose progenitors, various immune cells (T cells, NK cells, plasmacytoid dendritic cells), and blood vessel cells. The researchers then performed detailed analysis of progenitor populations and immune cell populations in both VAT and SAT from patients with and without CAC. They investigated the expression of genes related to various cellular processes and functions, including inflammation, metabolism, and immune responses. In vitro and in vivo studies were conducted to assess the causal role of specific immune cells in driving adipose catabolism.

The study identified nine major cell populations in the adipose tissue SVFs. While the overall distribution of cell types between CAC and non-CAC patients did not significantly differ, VAT showed significantly higher immune cell infiltration compared to SAT. Analysis of adipose progenitors revealed cachexia-specific alterations in both SAT and VAT, with decreased expression of genes involved in respiratory chain reactions. Macrophages exhibited a pro-inflammatory signature in CAC patients. Notably, CD8+ T cells in VAT showed a pro-inflammatory state with activated cytolytic effector pathways in CAC patients. In vitro experiments demonstrated a pro-catabolic effect of activated CD8+ T cells on adipocytes, and in vivo studies showed that macrophage depletion significantly reduced adipose catabolism and alleviated cachexia. These findings highlight the critical roles of macrophages and CD8+ T cells in driving adipose wasting in CAC.

The study's findings provide strong evidence for a central role of chronic inflammation and immune cell dysregulation in the pathogenesis of CAC-associated adipose wasting. The identification of specific immune cell populations (macrophages and CD8+ T cells) with pro-catabolic and pro-inflammatory activities opens up new avenues for therapeutic intervention. The depot-specific differences observed, particularly the higher immune infiltration in VAT, suggest that therapies may need to target specific adipose depots. Future research could investigate the precise mechanisms by which these immune cells contribute to adipose catabolism and explore the possibility of targeting these pathways to develop effective CAC treatments.

This study provides novel insights into the cellular mechanisms underlying adipose wasting in CAC using single-cell transcriptomics. The identification of specific immune cells, particularly activated CD8+ T cells and macrophages, as key drivers of adipose catabolism highlights potential therapeutic targets for treating CAC. Further research could focus on characterizing the precise molecular mechanisms involved and exploring the translational potential of these findings for developing novel treatment strategies.

The study is limited by its relatively small sample size (eight patients). Further research with larger cohorts is needed to validate these findings. The study primarily focused on gastric cancer patients; therefore, the generalizability of these findings to other cancer types needs to be established. The functional roles of specific immune cells were investigated using in vitro and in vivo models; however, further mechanistic studies are warranted.