Stromal deactivation by CSF1: a new feature of the aggressive pancreatic cancer microenvironment

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer with one of the poorest prognosis among all malignancies. The initiation and evolution of this kind of tumor depends on a complex interaction between cancer cells and the tumor microenvironment (TME). In an elegant study, Steins et al [1] used a combination of relevant disease models to show that mesenchymal subtype of PDAC shows a distinct deactivation of stellate cells in a CSF1‐dependent fashion. This study shedding light on a new role for the stroma driving an aggressive PDAC subtypes shifts the current paradigm for the requirement of an activated TME to regulate PDAC growth and maintenance.

This study shedding light on a new role for the stroma driving an aggressive PDAC subtypes shifts the current paradigm for the requirement of an activated TME to regulate PDAC growth and maintenance.

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive cancers, affecting the exocrine pancreas and accounting for around 95% of pancreatic cancers. The 5‐year survival rate for PDAC patients has slightly changed in the past three decades, and it is at only 9.3% 2. Multiple reports have shown that this aggressiveness nature is due in part to the ability of PDAC cells to interplay the surrounding tumor microenvironment (TME) to promote a disease progression and resistance to therapy. However, other studies have shown that in certain instances the activated TME could restrain the ability of PDAC tumors to grow 3, 4. Understanding the evolution and level of activation of the TME and its interaction with cancer cells is essential for the development to new treatment options for PDAC patients.

The typical PDAC TME includes endothelial and immune cells, stellate cells, and carcinoma‐associated fibroblast (CAF) embedded in a dense collagen matrix. In this issue, Steins et al defined a new feature of pancreatic stellate cells (PSCs), a stromal cell population known to promote PDAC development 5. In the activated form, PSCs show an increase in production of cytokines and collagen contributing to the TME remodeling and PDAC cells growth and survival. The authors demonstrated that high‐grade PDACs (mesenchymal subtype) show lower levels of alpha‐smooth muscle actin (a‐SMA)‐positive cells, a well‐established marker of activated PSCs, thus suggesting a decreased presence of activated PSCs in this subtype compared to the well‐differentiated and less aggressive epithelial‐like PDAC subtype.

Using PDAC cells lines having epithelial‐like or mesenchymal‐like phenotypes in 3D co‐cultures and orthotropic tumors, the authors were able to recreate the phenomenon seen in primary tissues. Organotypic cultures and tumors from epithelial‐like PDAC cells had higher collagen content and a‐SMA compared with the ones generated from mesenchymal‐like PDAC cells. Furthermore, to define a potential paracrine effect underlying this phenomenon PSC was cultured with conditioned media (CM) from epithelial‐ and mesenchymal‐like PDAC cells. Similar to aforementioned results, CM from mesenchymal‐like cells reduce PSC activation demonstrated by the decreased levels of a‐SMA, alpha‐1 type I collagen (COL1A1), and fibroblast activation protein (FAP) among others. These findings were expanded in a large panel of PDAC cell lines demonstrating decreased a‐SMA and COL1A1 expression in PSC cultures treated to mesenchymal‐like PDAC cells’ CM. Altogether, these findings point at the presence of soluble factor acting as mediator of the deactivation of PSCs by mesenchymal‐like PDAC cells.

To identify potential mediators of this phenomenon, Steins et al performed a sequence of biochemical and biological experiments in epithelial‐ and mesenchymal‐like PDAC cells as well as PSCs. First, using a molecular size exclusion assay using conditioned medium from Panc1 cells, a mesenchymal PDAC line, determine that the soluble factor(s) regulating PSCs activation were larger than 30 kDa. Next, forward‐phase protein array (FPPA) for cancer‐associated cyto‐ and chemokines in CM revealed that CXCL16, CCL5, and the colony‐stimulating factor 1 (CSF‐1) increase in mesenchymal‐like PDAC cells. Further analysis defined CSF‐1 as the mediator of the deactivation effect. Blockade of CSF‐1 either shRNA targeting in PDAC cells or recombinant antibody in CM increases PSC activation. Finally, immunohistochemical studies in primary PDAC tissue samples showed a negative correlation between CSF‐1 expression and the presence of activated PSCs. In addition, PDAC tissue samples with high levels of CSF‐1 showed low levels of collagen (I and III) and a‐SMA.

Most of the research in PDAC TME has been focused on the mechanisms leading to PSC activation and the biological impact of this activation on tumor growth, metastasis, angiogenesis, and treatment resistance 6. The work by Steins et al represents a change in the current paradigm and expands our conception of the role of PSC in PDAC pathobiology. The authors showed how PSC deactivation induced by PDAC cells can play an important role in development of an aggressive subtypes of pancreatic tumors with a more dismal prognosis (Fig 1). This study opens new avenues for future research aimed at defining the effect of activating stromal in aggressive variants of PDAC. A deeper understanding of the mechanism controlling the transition between activated and deactivated stroma not only at the level of PSC but also other similar cell types like CAFs will serve as foundation for the development of new treatment approaches for PDAC patients.

Figure 1. Schematic of the proposed model of stroma deactivation in mesenchymal PDAC .

Epithelial‐like PDAC is well‐differentiated tumors. The stroma in this type of malignancies contains high number of activated pancreatic stellate cells (PSCs) and a dense collagen matrix. In a new model proposed by Steins et al, in the mesenchymal‐like PDAC, cancer cells secrete the colony‐stimulating factor 1 (CSF‐1) that led to the deactivation of PSCs. This phenotype is characterized by downregulation of markers such us fibroblast activation protein (FAP), fibronectin (FN1), alpha‐1 type I collagen (COL1A1), and alpha‐smooth muscle actin (a‐SMA). In this more aggressive mesenchymal‐like PDAC, the stroma is characterized by low collagen content and deactivated PSC.

EMBO Reports (2020) 21: e50468