Cancer-associated fibroblasts that restrain cancer progression: Hypotheses and perspectives

Abstract

The roles of cancer-associated fibroblasts (CAF) in the progression of various types of cancers are well established. CAF promote cancer progression through pleiotropic mechanisms, including the secretion of soluble factors and extracellular matrix, physical interactions with cancer cells, and the regulation of angiogenesis, immunity and metabolism. Their contribution to therapeutic resistance is also well appreciated. Therefore, CAF have been considered as a therapeutic target in cancer. However, recent studies in autochthonous pancreatic cancer models suggest that specific subset(s) of CAF exhibit cancer-restraining roles, indicating that CAF are functionally and molecularly heterogeneous, which is supported by recent single-cell transcriptome analyses. While cancer-promoting CAF (pCAF) have been extensively studied, the nature and specific marker(s) of cancer-restraining CAF (rCAF) have remained uncharacterized. Interestingly, a recent study provided insight into the nature of rCAF and suggested that they may share molecular properties with pancreatic stellate cells (PSC) and mesenchymal stem/stromal cells (MSC). Complicating this finding is that PSC and MSC have been shown to promote the formation of a tumor-permissive and tumor-promoting environment in xenograft tumor models. However, these cells undergo significant transcriptional and epigenetic changes during ex vivo culture, which confounds the interpretation of experimental results based on the use of cultured cells. In this short review, we describe recent studies and hypotheses on the identity of rCAF and discuss their analogy to fibroblasts that suppress fibrosis in fibrotic diseases. Finally, we discuss how these findings can be exploited to develop novel anticancer therapies in the future.

Keywords: Meflin; cancer-restraining cancer-associated fibroblasts; fibrosis; mesenchymal stem/stromal cells; tumor microenvironment.

Figure 1

Cancer‐associated fibroblast (CAF) distribution and heterogeneity in human pancreatic ductal adenocarcinoma (PDAC). A, Representative histological images of human PDAC. Serial sections from two tumor lesions from a patient with PDAC stained with H&E are shown (upper panel). Residual acini are indicated in a dotted yellow‐colored area. In middle and lower panels, serial sections were stained for the CAF marker α‐smooth muscle actin (α‐SMA) (brown). Note that smooth muscle cells in the vessel walls (V) are also positive for α‐SMA. Tumor cells (T) as well as normal ducts (ND) are negative for α‐SMA. Magnified images of the boxed areas (lower panel) suggest that α‐SMA expression is heterogeneous: some CAF are strongly positive for α‐SMA (white arrowheads), whereas others are moderately or weekly positive for α‐SMA (yellow arrowheads). Note that some CAF that are predominantly positive for α‐SMA preferentially localize immediately adjacent to the tumor glands (red arrows in the middle panels). B, Meflin⁺ cancer‐restraining cancer‐associated fibroblasts (rCAF) and α‐SMA⁺ CAF in human PDAC. CAF in human PDAC were double stained for Meflin (ISLR; red) and α‐SMA (ACTA2; green) mRNA by in‐situ hybridization (ISH), revealing CAF heterogeneity in the tumor stroma. Boxed areas are magnified in lower panels. T, tumor glands

Figure 2

A possible mechanism underlying cancer‐associated fibroblast (CAF) heterogeneity. A, In the pancreas, Meflin marks pancreatic stellate cells (PSC), undifferentiated mesenchymal stem/stromal cells (MSC) and resident fibroblasts that localize around the acini, ducts and the islets of Langerhans. In the early stages of cancer development, including ADM and preinvasive (PanIN) stages, Meflin⁺ CAF or PSC that express low levels of α‐SMA start to proliferate around neoplastic cells, which we termed cancer‐restraining cancer‐associated fibroblasts (rCAF). Our recent studies showed that Meflin expression is downregulated by TGF‐β, stiff substrate and hypoxia, which we speculate are major factors that drive the differentiation of rCAF into α‐SMA⁺ and Meflin^−/low CAF in advanced‐stage cancer. Aging and ex vivo culture of cells also cause downregulation of Meflin expression, although the relevance of these factors in cancer development and progression has not been demonstrated. In pancreatic ductal adenocarcinoma (PDAC), some CAF that are predominantly positive for α‐SMA preferentially localize immediately adjacent to the tumor glands, although there is currently no evidence that these CAF are derived from Meflin⁺ cells. The effect of mechanical stress imposed by the expanded tumor glands on α‐SMA expression in the CAF should also be considered. B, t‐distributed stochastic neighbor embedding (t‐SNE) plot showing the myCAF, iCAF and Meflin⁺ CAF subpopulations identified by scRNA‐seq of all cells isolated from tumors of a PDAC mouse model. Each dot is a CAF, and the intensity of the purple represents the expression level of the indicated genes. Reprinted from ref. 50, Copyright (2019), with permission from the American Association for Cancer Research

Figure 3

Meflin⁺ cancer‐associated fibroblasts (CAF) first emerge around metaplastic or tumor cells, followed by appearance of α‐SMA‐positive CAF in early stages of pancreatic cancer. Serial sections prepared from the pancreas of a 12‐week‐old KPC mouse were stained for Meflin and α‐SMA mRNA (left and middle panels) and with H&E (right panels). Different regions of the stained sections, including a nearly normal region (top panels), ADM lesions (middle panels) and a PanIN lesion (bottom panels) are shown. Yellow arrowheads indicate Meflin⁺ CAF or pancreatic stellate cells (PSC). White arrowheads denote α‐SMA⁺ CAF that preferentially emerge around metaplastic or tumor cells. A, normal acini; IL, islet of Langerhans; ND, normal duct

Figure 4

Analogy of cancer‐restraining cancer‐associated fibroblasts (rCAF) in pancreatic ductal adenocarcinoma (PDAC) to antifibrotic fibroblasts in cardiac fibrosis. In pancreatic ductal adenocarcinoma (PDAC), Meflin marks CAF that first emerge around metaplastic or transformed cells, which we showed behave as rCAF and later give rise to α‐SMA⁺ CAF with low Meflin expression, resulting in CAF heterogeneity in an advanced stage of cancer (upper panel). In cardiac infarction and fibrosis, we speculate that Meflin⁺ fibroblasts first proliferate in the injured area, and later yield α‐SMA⁺ myofibroblasts in the fibrotic phase (lower panel). The primary function of Meflin⁺ fibroblasts is to promote tissue repair and inhibit fibrosis, whereas loss of Meflin expression results in the differentiation of fibroblasts to myofibroblasts that contribute to the stiffening of cardiac tissue