Fabp4-tastic Pancreatic Stellate Cells Coddle Kitl to Uphold the Inherent Microenvironmental Tumor Suppression

2 sentence Summary:

Oñate et al. demonstrate that KITL expression in pancreatic stellate cells is crucial for maintaining the inherent tumor-suppressive function of the pancreatic microenvironment, and its loss enables pancreatic cancer development. This pivotal discovery not only reinforces the centuries-old hypothesis of natural microenvironmental tumor suppression but also highlights a promising therapeutic avenue whereby restoring KITL expression could reestablish the tumor-suppressive functions of pancreatic resident fibroblastic cells.

Observations from the late 19^th and mid-20^th centuries have firmly established that tumors can only develop within tumor-permissive microenvironments (1,2). Carpenter et al. recently demonstrated that, despite the high prevalence of precancerous lesions in the human pancreas, “normal” fibroblastic and immune cells—including activated dendritic cells—located near benign lesions predominantly exhibit traits associated with natural tumor-suppressive features (3). Together, these findings underscore the necessity for the local microenvironment to transition from tumor-suppressive to tumor-supportive during pancreatic cancer transformation (Figure 1).

Figure 1: PSC KITL Loss Induces a Transition from Tumor-Suppressive to Tumor-Promoting Microenvironment.

This schematic illustrates the pivotal findings by Oñate et al. (10). The left panel depicts a healthy pancreas, characterized by multiple epithelial compartments (orange circles), potentially representing acini and/or pancreatic ductal epithelium. A single pre-cancerous lesion (orange mixed with purple) contains a few pre-cancerous cells (cyan) and a solitary genetically altered cell (purple), indicative of future cancer. The organ maintains its health due to an intact tumor-suppressive microenvironment, featuring a single Fabp4 lineage-derived and KITL-expressing pancreatic stellate cell (PSC; green, representing GFP, with orange counter, representing KITL expression), a perivascular fibroblast (resident fibroblast; gray), and their associated interstitial extracellular matrix (ECM) fibers. The central gradient arrow signifies the loss of KITL in PSCs, leading to their transition into tumor-promoting cancer-associated fibroblasts. The right panel illustrates a pancreatic tumor-bearing organ, where the microenvironment has shifted to a tumor-promoting state. This transition is marked by a substantial expansion of various CAF lineages (other CAFs; gray), including PSC-derived tumor-promoting CAFs (TP-CAFs; green) resulting from KITL loss, and the presence of some immune-cell infiltrates (light gray small circles). The light gray arrow in the middle suggests the potential for reversing this process, proposing a novel future therapeutic strategy to regain the inherent tumor-suppressive function of the PSCs that had transitioned into TP-CAFs.

While it is well known that pancreatic cancer-associated fibroblasts (CAFs) originate from various organ resident mesenchymal/fibroblastic lineages and can function as both tumorsupporting and tumor-suppressive cells (3–6), the literature has yet to identify the functional molecules responsible for enabling these pancreatic-resident fibroblastic cells to maintain the inherent tumor-suppressive functions, including anti-tumor immune surveillance. This suggests that the stroma might undergo a ‘second hit’ event (7), which is non-cancer cell-autonomous in nature, reciprocally allowing the already genetically altered epithelial cells involved in pancreatic cancer onset (8) to transform.

Notably, lymph node perivascular reticular cells, also known as fibroblastic reticular cells (FRCs), are mesenchymal cells specialized in activating both innate and adaptive immunity. FRCs play a crucial role in orchestrating the human body’s immune responses by providing structural support within lymph nodes and producing chemokines that attract and activate immune cells. In the pancreas, resident pancreatic perivascular cells—expressing MHC class II (CD45-negative) pancreatic fibroblastic cells—are responsible for igniting autoimmunity and even driving Type 1 diabetes (9). These FRC-like pancreatic tissue-resident fibroblastic cells serve as a native defense mechanism by eliminating perturbed islets of Langerhans without affecting unperturbed islets, thus preserving the organ’s endocrine function. Essentially, these fibroblastic cells selectively target malfunctioning islets to maintain pancreatic homeostasis. However, when the assault is chronic, the development of diabetes becomes imminent (9).

It is hence possible that these and/or other pancreas-resident fibroblastic cells also constitute the first line of defense against developing cancer, providing anti-tumor immunity and thus preventing pre-cancerous lesions from fully transforming. Supporting this premise, a recent study published in Cancer Discovery by Oñate and her women-in-STEM co-authors (10) demonstrates that pancreatic stellate cells (PSCs) sustain inherent microenvironmental tumor suppression in the pancreas. PSCs develop from the adipo-FABP4 lineage and have previously been shown by the same team, led by a well-known leader in the pancreatic tumor microenvironment field, Dr. Mara Sherman, to co-evolve with pancreatic cancer, producing a small but significant proportion of pro-tumoral pancreatic CAFs (5).

Importantly, Oñate et al (10) not only supports the premise that the microenvironment serves as a natural tumor-suppressive milieu but also uncovered a crucial role for the KIT ligand gene (Kitl/KITLG: HGNC:6343, MIM: 184745, corresponding to the murine and human genes respectively, and also known as stem cell factor, SCF or KITL) as a fibroblastic/mesenchymal tumor-suppressive agent in pancreatic resident cells. Of note, the activated form of the known KITL receptor, phospho-c-KIT, was detected in both murine and human benign pancreas, correlating with the expression of the ligand in PSCs.

The authors identified PSCs’ KITL as a key microenvironmental tumor-suppressive homeostatic factor responsible for maintaining pancreatic tissue integrity using various orthogonal methods. First, they traced GFP-positive mesenchymal PSCs cells known to originate from the adipo-Fabp4 lineage into a subset of pro-tumoral CAFs (5) and confirmed its adjacent and distant distribution relative to epithelial cells. The same pattern of localization was noted using orthotopic models via murine pancreatic cancer cell implantation into the pancreata of Fabp4-Cre mice.

The authors compared the gene expression patterns of PSCs from control pancreatic mouse samples to the cancer-educated CAFs and observed KITL downregulation and loss of factors crucial for standard tissue architecture, alongside acquiring traits that promote a tumor-supportive microenvironment. KITL depletion was verified by extracting PSCs—naturally expressing KITL and the leptin receptor (Lepr)—from healthy pancreatic tissue and inducing their transition to a CAF-like phenotype in vitro. The same pattern was observed in murine samples comparing PSCs and PDGFRa+ pancreatic fibroblasts, as well as in human samples comparing PSCs and cells negatively selected by epithelial markers. By examining transcriptional signatures obtained from single-cell RNA sequencing, they illustrated how the loss of KITL expression in murine and human PSCs is necessary for pancreatic tumorigenesis. The loss of KITL was also confirmed in the mouse model and human samples using an RNA hybridization assay. Rescue experiments using a shRNA approach for knockdown and subsequent KITL overexpression established a correlation between its levels and important extracellular matrix and related proteins, including key integrins, laminins, and cadherins. Moreover, knocking down KITL expression increased the level of inflammation-related genes, supporting the hypothesis that KITL is essential for preserving healthy tissue homeostasis.

To better understand KITL’s role, the authors focused on its known receptor, c-KIT. They investigated how KITL might exert a paracrine effect on tissue organization and found c-KIT-positive cells likely include mesenchymal and epithelial subsets. Although no direct impact on cancer cell survival was observed when cell cultures, in vitro, were treated with KITL, the authors showed that KITL modulates the organizational and mechanical features of the microenvironment, consistent with their previous findings (5).

To demonstrate that KITL supports inherent tumor-suppressive pancreatic tissue homeostasis, mice lacking Kitl were treated with caerulein to induce pancreatitis, and in a following experiment the authors proceeded to conduct an orthotopic inoculation of otherwise syngeneic pancreatic cancer cells. As a result of the genetic ablation of Kitl, driven by the PSC promoter Fabp4, the authors reported an increase in pancreatic transformation and a decreased overall survival.

The authors convincingly demonstrated that the naturally tumor-suppressive pancreatic parenchyma must downregulate KITL to transition into a pro-tumoral microenvironment, facilitating pancreatic cancer onset. Furthermore, they showed that re-expressing KITL in CAFs restores the tumor-suppressive function of these mesenchymal cells.

Understanding the precise stages of tumor onset and progression in which KITL exerts its tumor-suppressive influence is of paramount importance. Future elucidation of the downstream mechanisms in both KITL-positive ‘sender cells’ and c-KIT-positive potential ‘receiver cells’ is crucial. These mechanisms may reveal potential preventative or therapeutic targets, especially since KITL itself, functioning as a microenvironmental tumor-suppressor, is unlikely to be used as an efficacious treatment/target. Moving forward, it will be essential to dissect the intricate stages of tumorigenesis influenced by PSCs expressing KITL and the tumor-suppressive functions of this ligand, as well as to unravel the complex signaling pathways within KITL-expressing PSCs and c-KIT-positive ‘receiver cells’ in pancreatic tissue. Such insights could pave the way for innovative therapeutic strategies targeting these and related pathways, ultimately enhancing our arsenal for treating and/or intercepting pancreatic cancer, and potentially those for treating other malignancies.

In summary, this discovery suggests that upholding the inherent stroma could serve as a pancreatic cancer interceptive/controlling milieu (Figure 1). It opens up new research avenues by potentially facilitating the identification of related, yet druggable, molecules that can restore KITL expression in CAFs, harnessing these cells’ natural tumor-suppressive functions while effectively inhibiting their tumor-supportive roles, thus avoiding the detrimental effects of eliminating the stromal tumor microenvironment.

Conclusion

Oñate et al. (10) demonstrate that KITL expression in PSCs is crucial for maintaining an inherent pancreatic tumor-suppressive microenvironment, while its loss is essential for pancreatic cancer development (Figure 1). This pivotal discovery not only supports the centuries-long hypothesis of natural microenvironmental tumor suppression but also highlights a novel and promising therapeutic avenue. Restoring or upholding KITL expression could harness the natural tumor-restrictive functions of fibroblastic cells, inhibit pro-tumoral immune tolerance, and unleash an efficacious cascade of anti-tumor immunity.