Single‐cell RNA sequencing to explore cancer‐associated fibroblasts heterogeneity: “Single” vision for “heterogeneous” environment

Cancer‐associated fibroblasts (CAFs), a phenotypically and functionally heterogeneous stromal cell, are one of the most important components of the tumour microenvironment. Previous studies have consolidated it as a promising target against cancer. However, variable therapeutic efficacy—both protumor and antitumor effects have been observed not least owing to the strong heterogeneity of CAFs. Over the past 10 years, advances in single‐cell RNA sequencing (scRNA‐seq) technologies had a dramatic effect on biomedical research, enabling the analysis of single cell transcriptomes with unprecedented resolution and throughput. Specifically, scRNA‐seq facilitates our understanding of the complexity and heterogeneity of diverse CAF subtypes. In this review, we discuss the up‐to‐date knowledge about CAF heterogeneity with a focus on scRNA‐seq perspective to investigate the emerging strategies for integrating multimodal single‐cell platforms. Furthermore, we summarized the clinical application of scRNA‐seq on CAF research. We believe that the comprehensive understanding of the heterogeneity of CAFs form different visions will generate innovative solutions to cancer therapy and achieve clinical applications. The origin heterogeneity of CAFs: Multiple types of cells, under certain conditions, may become tumour‐associated fibroblasts (CAFs). A variety of normal fibroblasts, including resident breast fibroblasts, pancreatic stellate cells (PSCs) and hepatic stellate cells (HSCs), bone marrow‐derived mesenchymal stem cells (BM‐MSCs) transformed into CAFs under the action of cytokines such as TGF‐β. Endothelial cells convert into CAFs through endothelial‐mesenchymal transition (EndMT), and epithelial cells differentiate toward CAFs through epithelial mesenchymal‐transition (EMT). Adipocytes, pericytes and smooth muscle cells can also be the source of CAFs.