Fibroblast Heterogeneity in the Pancreatic Tumor Microenvironment

Abstract

The poor prognosis for patients with pancreatic ductal adenocarcinoma (PDAC) impels an improved understanding of disease biology to facilitate the development of better therapies. PDAC typically features a remarkably dense stromal reaction, featuring and established by a prominent population of cancer-associated fibroblasts (CAF). Genetically engineered mouse models and increasingly sophisticated cell culture techniques have demonstrated important roles for fibroblasts in PDAC progression and therapy response, but these roles are complex, with strong evidence for both tumor-supportive and tumor-suppressive or homeostatic functions. Here, we review the recent literature that has improved our understanding of heterogeneity in fibroblast fate and function in this disease including the existence of distinct fibroblast populations, and highlight important avenues for future study. SIGNIFICANCE: Although the abundant stromal reaction associated with pancreatic cancer has long been appreciated, the functions of the CAF cells that establish this stromal reaction remain unclear. An improved understanding of the transcriptional and functional heterogeneity of pancreatic CAFs, as well as their tumor-supportive versus tumor-suppressive capacity, may facilitate the development of effective therapies for this disease.

Figure 1.

Summary of tumor-promoting and potential tumor-suppressive functions of PDAC CAFs. TIME: tumor immune microenvironment. CAFs can promote tumor progression via paracrine regulation of oncogenic signal transduction, including via IGF1/GAS6 and LIF signaling and reciprocal regulation of the PDAC phosphoproteome (17, 18). Through the release of exosomes and metabolites such as deoxycytidine (–16), CAFs also regulate cancer cell metabolism and drug sensitivity. Metabolic regulation through secretion of lipids (12) and direct or indirect provision of amino acids (10, 11) enables proliferation within the nutrient-poor tumor microenvironment. CAFs also orchestrate growth-permissive regulation of the immune microenvironment through secretion of cytokines and other immune-modulatory factors such as CXCL12 (20), IL6 (26, 27), and βig-h3 (31). While tumor-suppressive CAF functions remain poorly understood, results from various CAF depletion models suggest that a Shh-dependent, aSMA-positive subset of CAFs promote a more differentiated and less aggressive tumor phenotype (–35). This may be mediated by release of differentiation cues such as BMPs (38). CAF interactions with the immune system can be tumor-suppressive in part, through suppressing T_reg infiltration (33), though interactions between CAFs and the anti-tumor immune response are complex and an important area for further study.

Figure 2.

Overview of PDAC CAF subtypes identified from transcriptional profiling, including overlap of key markers for each population. In normal pancreas tissue, early or PanIN lesions, and PDAC, fibroblast transcriptional programs generally fall into two categories: inflammatory signatures including cytokines and other immune-modulatory molecules (indicated in shades of pink), and myofibroblastic signatures including classical markers of activated fibroblasts (Acta2, Tagln), ECM components and remodeling factors, and in some myofibroblastic sub-populations, genes encoding the MHC II complex (indicated in shades of blue). Single-cell RNA-seq has identified 2 (ntFib1 and ntFib2) (46) or 3 (FB1, FB2, and FB3) (44) fibroblast populations in normal pancreas, and computational modeling suggests that these tissue-resident fibroblast populations likely give rise to CAFs (46). Based on transcriptional similarities, it seems that these fibroblasts in normal pancreas tissue give rise to inflammatory (eCAF1 in (46), FB1/FB2 in (44)) or myofibroblastic (eCAF2 in (46), FB3 in (44)) fibroblasts in early lesions (per analysis in (46) and indicated by the dashed arrows). Depending on analysis method, model used, and perhaps other factors, 2–4 CAF populations are found in established PDAC, though boundaries seem non-discrete (for example, subtype A in (45)) and sub-populations exist within these CAF designations. An inflammatory population of CAFs (FB1 in (44), IL1 CAFs in (46), iCAF in (26), subtype C in (45)) seems to arise from tissue-resident inflammatory fibroblasts (46) as a result of IL1 signaling (46, 47). Myofibroblastic CAFs (FB3 in (44), TGF-β CAFs in (46), myCAF in (26), subtypes B and D in (45), and sub-populations of myofibroblastic CAFs including antigen-presenting CAFs or apCAFs (25) and cancer-associated mesenchymal stem cells or CA-MSC (55, 56)) seem to arise from tissue-resident myofibroblastic cells (46) as a result of TGF-β signaling (46, 47). Additional populations not fitted to these categories may also be present in some PDAC cases perhaps dependent on genotype or stage, indicated here in orange. Key genes expressed by these defined fibroblast or CAF subtypes are listed.