Pancreatic Fibroblast Heterogeneity: From Development to Cancer

Digestive cancers: mechanisms, therapeutics and management

Cancers of the digestive system are major contributors to global cancer-associated morbidity and mortality, accounting for 35% of annual cases of cancer deaths. The etiologies, molecular features, and therapeutic management of these cancer entities are highly heterogeneous and complex. Over the last decade, genomic and functional studies have provided unprecedented insights into the biology of digestive cancers, identifying genetic drivers of tumor progression and key interaction points of tumor cells with the immune system. This knowledge is continuously translated into novel treatment concepts and targets, which are dynamically reshaping the therapeutic landscape of these tumors. In this review, we provide a concise overview of the etiology and molecular pathology of the six most common cancers of the digestive system, including esophageal, gastric, biliary tract, pancreatic, hepatocellular, and colorectal cancers. We comprehensively describe the current stage-dependent pharmacological management of these malignancies, including chemo-, targeted, and immunotherapy. For each cancer entity, we provide an overview of recent therapeutic advancements and research progress. Finally, we describe how novel insights into tumor heterogeneity and immune evasion deepen our understanding of therapy resistance and provide an outlook on innovative therapeutic strategies that will shape the future management of digestive cancers, including CAR-T cell therapy, novel antibody-drug conjugates and targeted therapies.

Characterization of Pancreatic Collagen-Expressing Fibroblasts in Mouse Acute Pancreatitis

Background and Aims

Pancreatic stellate cells (PSCs) are critical mediators in chronic pancreatitis with an undefined role in acute pancreatitis (AP). PSCs consist of a heterogenous group of cells and are considered interchangeable with pancreatic fibroblasts. This study explored the heterogeneous nature of PSCs by characterizing pancreatic collagen-expressing fibroblasts (PCFs) via lineage tracing in mouse normal and AP pancreas and determining the effect of PCF depletion in AP.

Methods

Tandem dimer Tomato (tdTom+) PCFs in collagen type 1 (Col1)a2CreERtdTomato (Tom) mice receiving tamoxifen were characterized via fluorescence, Oil Red staining, and flow cytometry. AP was induced by cerulein, AP injury was assessed, and tdTom+ PCFs were monitored. The effect of PCF depletion on AP injury was evaluated in Col1a2CreERdiphtheria toxin A mice.

Results

Approximately 13% of pancreatic cells in Col1a2CreERTom mice were labeled by tdTom (tdTom+ PCFs), which surrounded acini, ducts, and blood vessels, and stained with Oil Red, collagen type I, vimentin, and desmin. tdTom+ PCFs increased 2-fold during AP, correlating with AP score, amylase, and alpha-smooth muscle actin+ and Ki67+ staining. PCF depletion in Col1a2CreERdiphtheria toxin A mice receiving tamoxifen resulted in enhanced inflammation compared to control.

Conclusion

PCFs may constitute a subset of PSCs and can be activated during AP. PCF depletion aggravates AP, suggesting a protective role for PCFs.

Tumor Stroma as a Therapeutic Target for Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) has a poor prognosis owing to its desmoplastic stroma. Therefore, therapeutic strategies targeting this tumor stroma should be developed. In this study, we describe the heterogeneity of cancer-associated fibroblasts (CAFs) and their diverse roles in the progression, immune evasion, and resistance to treatment of PDAC. We subclassified the spatial distribution and functional activity of CAFs to highlight their effects on prognosis and drug delivery. Extracellular matrix components such as collagen and hyaluronan are described for their roles in tumor behavior and treatment outcomes, implying their potential as therapeutic targets. We also discussed the roles of extracellular matrix (ECM) including matrix metalloproteinases and tissue inhibitors in PDAC progression. Finally, we explored the role of the adaptive and innate immune systems in shaping the PDAC microenvironment and potential therapeutic strategies, with a focus on immune cell subsets, cytokines, and immunosuppressive mechanisms. These insights provide a comprehensive understanding of PDAC and pave the way for the development of prognostic markers and therapeutic interventions.

Fibroblasts in the Aged Pancreas Drive Pancreatic Cancer Progression

Pancreatic cancer is more prevalent in older individuals and often carries a poorer prognosis for them. The relationship between the microenvironment and pancreatic cancer is multifactorial, and age-related changes in nonmalignant cells in the tumor microenvironment may play a key role in promoting cancer aggressiveness. Because fibroblasts have profound impacts on pancreatic cancer progression, we investigated whether age-related changes in pancreatic fibroblasts influence cancer growth and metastasis. Proteomics analysis revealed that aged fibroblasts secrete different factors than young fibroblasts, including increased growth/differentiation factor 15 (GDF-15). Treating young mice with GDF-15 enhanced tumor growth, whereas aged GDF-15 knockout mice showed reduced tumor growth. GDF-15 activated AKT, rendering tumors sensitive to AKT inhibition in an aged but not young microenvironment. These data provide evidence for how aging alters pancreatic fibroblasts and promotes tumor progression, providing potential therapeutic targets and avenues for studying pancreatic cancer while accounting for the effects of aging.

SIGNIFICANCE

Aged pancreatic fibroblasts secrete GDF-15 and activate AKT signaling to promote pancreatic cancer growth, highlighting the critical role of aging-mediated changes in the pancreatic cancer microenvironment in driving tumor progression. See related commentary by Isaacson et al., p. 1185.

Meflin is a marker of pancreatic stellate cells involved in fibrosis and epithelial regeneration in the pancreas

Pancreatic stellate cells (PSCs) are stromal cells in the pancreas that play an important role in pancreatic pathology. In chronic pancreatitis (CP) and pancreatic ductal adenocarcinoma (PDAC), PSCs are known to get activated to form myofibroblasts or cancer-associated fibroblasts (CAFs) that promote stromal fibroinflammatory reactions. However, previous studies on PSCs were mainly based on the findings obtained using ex vivo expanded PSCs, with few studies that addressed the significance of in situ tissue-resident PSCs using animal models. Their contributions to fibrotic reactions in CP and PDAC are also lesser-known. These limitations in our understanding of PSC biology have been attributed to the lack of specific molecular markers of PSCs. Herein, we established Meflin (Islr), a glycosylphosphatidylinositol-anchored membrane protein, as a PSC-specific marker in both mouse and human by using human pancreatic tissue samples and Meflin reporter mice. Meflin-positive (Meflin+ ) cells contain lipid droplets and express the conventional PSC marker Desmin in normal mouse pancreas, with some cells also positive for Gli1, the marker of pancreatic tissue-resident fibroblasts. Three-dimensional analysis of the cleared pancreas of Meflin reporter mice showed that Meflin+ PSCs have long and thin cytoplasmic protrusions, and are localised on the abluminal side of vessels in the normal pancreas. Lineage tracing experiments revealed that Meflin+ PSCs constitute one of the origins of fibroblasts and CAFs in CP and PDAC, respectively. In these diseases, Meflin+ PSC-derived fibroblasts showed a distinctive morphology and distribution from Meflin+ PSCs in the normal pancreas. Furthermore, we showed that the genetic depletion of Meflin+ PSCs accelerated fibrosis and attenuated epithelial regeneration and stromal R-spondin 3 expression, thereby implying that Meflin+ PSCs and their lineage cells may support tissue recovery and Wnt/R-spondin signalling after pancreatic injury and PDAC development. Together, these data indicate that Meflin may be a marker specific to tissue-resident PSCs and useful for studying their biology in both health and disease. © 2023 The Pathological Society of Great Britain and Ireland.

Pathological analysis of lesions in the exocrine pancreas of rats induced by Zinc Maltol

The pancreas plays an important role in the homeostasis of zinc (Zn), a nutritionally essential metal. In several previous studies, Zn ions induced inflammatory changes in the exocrine pancreas; however, little is known about Zn complexes. In this study, we microscopically, immunohistochemically, and ultrastructurally examined pancreatic lesions in Sprague-Dawley (SD) rats induced by a 4-week repeated oral dose toxicity study of Zinc Maltol (ZM), a zinc (II) complex. ZM induces acinar atrophy and increases the number of duct-like structures. Immunohistochemistry revealed a decrease in the number of trypsin-positive cells, and an increase in the number of SOX9-positive cells. Interstitial fibrosis and macrophage infiltration also correlated with the degree of acinar atrophy. Electron microscopic evaluation revealed that the acinar cells that lost granules were surrounded by fibroblasts and collagen fibers. In conclusion, we provided a detailed description of ZM-induced pancreatic lesions in SD rats.

GTM-decon: guided-topic modeling of single-cell transcriptomes enables sub-cell-type and disease-subtype deconvolution of bulk transcriptomes

Cell-type composition is an important indicator of health. We present Guided Topic Model for deconvolution (GTM-decon) to automatically infer cell-type-specific gene topic distributions from single-cell RNA-seq data for deconvolving bulk transcriptomes. GTM-decon performs competitively on deconvolving simulated and real bulk data compared with the state-of-the-art methods. Moreover, as demonstrated in deconvolving disease transcriptomes, GTM-decon can infer multiple cell-type-specific gene topic distributions per cell type, which captures sub-cell-type variations. GTM-decon can also use phenotype labels from single-cell or bulk data to infer phenotype-specific gene distributions. In a nested-guided design, GTM-decon identified cell-type-specific differentially expressed genes from bulk breast cancer transcriptomes.

PDAC, the Influencer Cancer: Cross-Talk with Tumor Microenvironment and Connected Potential Therapy Strategies

Pancreatic ductal adenocarcinoma (PDAC) is among the leading causes of death by cancer in the world. What makes this pathological condition particularly lethal is a combination of clinical and molecular heterogeneity, lack of early diagnostic indexes, and underwhelming results from current therapeutic protocols. A major cause of PDAC chemoresistance seems to lie in the ability of cancer cells to spread out and fill the pancreatic parenchyma, exchanging nutrients, substrates, and even genetic material with cells from the surrounding tumor microenvironment (TME). Several components can be found in the TME ultrastructure, including collagen fibers, cancer-associated fibroblasts, macrophages, neutrophils, mast cells, and lymphocytes. Cross-talk between PDAC and TME cells results in the latter being converted into cancer-favoring phenotypes; this behavior could be compared to an influencer guiding followers into supporting his activity. Moreover, TME could be a potential target for some of the newest therapeutic strategies; these include the use of pegvorhyaluronidase-α and CAR-T lymphocytes against HER2, FAP, CEA, MLSN, PSCA, and CD133. Other experimental therapy options are being currently studied, aiming to interfere with the KRAS pathway, DNA-repairing proteins, and apoptosis resistance in PDAC cells. Hopefully these new approaches will grant better clinical outcomes in future patients.

Activated fibroblasts in cancer: Perspectives and challenges

Activated fibroblasts in tumors, or cancer-associated fibroblasts (CAFs), have become a popular research area over the past decade. As important players in many aspects of tumor biology, with functions ranging from collagen deposition to immunosuppression, CAFs have been the target of clinical and pre-clinical studies that have revealed their potential pro- and anti-tumorigenic dichotomy. In this review, we describe the important role of CAFs in the tumor microenvironment and the technological advances that made these discoveries possible, and we detail the models that are currently available for CAF investigation. Additionally, we present evidence to support the value of encompassing CAF investigation as a future therapeutic avenue alongside immune and cancer cells while highlighting the challenges that must be addressed for successful clinical translation of new findings.

GLI1 interaction with p300 modulates SDF1 expression in cancer-associated fibroblasts to promote pancreatic cancer cells migration

Carcinoma-associated fibroblasts (CAFs) play an important role in the progression of multiple malignancies. Secretion of cytokines and growth factors underlies the pro-tumoral effect of CAFs. Although this paracrine function has been extensively documented, the molecular mechanisms controlling the expression of these factors remain elusive. In this study, we provide evidence of a novel CAF transcriptional axis regulating the expression of SDF1, a major driver of cancer cell migration, involving the transcription factor GLI1 and histone acetyltransferase p300. We demonstrate that conditioned media from CAFs overexpressing GLI1 induce the migration of pancreatic cancer cells, and this effect is impaired by an SDF1-neutralizing antibody. Using a combination of co-immunoprecipitation, proximity ligation assay and chromatin immunoprecipitation assay, we further demonstrate that GLI1 and p300 physically interact in CAFs to co-occupy and drive SDF1 promoter activity. Mapping experiments highlight the requirement of GLI1 N-terminal for the interaction with p300. Importantly, knockdowns of both GLI1 and p300 reduce SDF1 expression. Further analysis shows that knockdown of GLI1 decreases SDF1 promoter activity, p300 recruitment, and levels of its associated histone marks (H4ac, H3K27ac, and H3K14ac). Finally, we show that the integrity of two GLI binding sites in the SDF1 promoter is required for p300 recruitment. Our findings define a new role for the p300-GLI1 complex in the regulation of SDF1, providing new mechanistic insight into the molecular events controlling pancreatic cancer cells migration.

Barriers and opportunities for gemcitabine in pancreatic cancer therapy

Pancreatic ductal adenocarcinoma (PDA) has become one of the leading causes of cancer-related deaths across the world. A lack of durable responses to standard-of-care chemotherapies renders its treatment particularly challenging and largely contributes to the devastating outcome. Gemcitabine, a pyrimidine antimetabolite, is a cornerstone in PDA treatment. Given the importance of gemcitabine in PDA therapy, extensive efforts are focusing on exploring mechanisms by which cancer cells evade gemcitabine cytotoxicity, but strategies to overcome them have not been translated into patient care. Here, we will introduce the standard treatment paradigm for patients with PDA, highlight mechanisms of gemcitabine action, elucidate gemcitabine resistance mechanisms, and discuss promising strategies to circumvent them.

The splanchnic mesenchyme is the tissue of origin for pancreatic fibroblasts during homeostasis and tumorigenesis

Pancreatic cancer is characterized by abundant desmoplasia, a dense stroma composed of extra-cellular and cellular components, with cancer associated fibroblasts (CAFs) being the major cellular component. However, the tissue(s) of origin for CAFs remains controversial. Here we determine the tissue origin of pancreatic CAFs through comprehensive lineage tracing studies in mice. We find that the splanchnic mesenchyme, the fetal cell layer surrounding the endoderm from which the pancreatic epithelium originates, gives rise to the majority of resident fibroblasts in the normal pancreas. In a genetic mouse model of pancreatic cancer, resident fibroblasts expand and constitute the bulk of CAFs. Single cell RNA profiling identifies gene expression signatures that are shared among the fetal splanchnic mesenchyme, adult fibroblasts and CAFs, suggesting a persistent transcriptional program underlies splanchnic lineage differentiation. Together, this study defines the phylogeny of the mesenchymal component of the pancreas and provides insights into pancreatic morphogenesis and tumorigenesis.

CD73-generated extracellular adenosine promotes resolution of neutrophil-mediated tissue injury and restrains metaplasia in pancreatitis

Pancreatitis is currently the leading cause of gastrointestinal hospitalizations in the US. This condition occurs in response to abdominal injury, gallstones, chronic alcohol consumption or, less frequently, the cause remains idiopathic. CD73 is a cell surface ecto-5'-nucleotidase that generates extracellular adenosine, which can contribute to resolution of inflammation by binding adenosine receptors on infiltrating immune cells. We hypothesized genetic deletion of CD73 would result in more severe pancreatitis due to decreased generation of extracellular adenosine. CD73 knockout (CD73-/- ) and C57BL/6 (wild type, WT) mice were used to evaluate the progression and response of caerulein-induced acute and chronic pancreatitis. In response to caerulein-mediated chronic or acute pancreatitis, WT mice display resolution of pancreatitis at earlier timepoints than CD73-/- mice. Using immunohistochemistry and analysis of single-cell RNA-seq (scRNA-seq) data, we determined CD73 localization in chronic pancreatitis is primarily observed in mucin/ductal cell populations and immune cells. In murine pancreata challenged with caerulein to induce acute pancreatitis, we compared CD73-/- to WT mice and observed a significant infiltration of Ly6G+, MPO+, and Granzyme B+ cells in CD73-/- compared to WT pancreata and we quantified a significant increase in acinar-to-ductal metaplasia demonstrating sustained metaplasia and inflammation in CD73-/- mice. Using neutrophil depletion in CD73-/- mice, we show neutrophil depletion significantly reduces metaplasia defined by CK19+ cells per field and significantly reduces acute pancreatitis. These data identify CD73 enhancers as a potential therapeutic strategy for patients with acute and chronic pancreatitis as adenosine generation and activation of adenosine receptors is critical to resolve persistent inflammation in the pancreas.

Fibroblast heterogeneity in pancreatic ductal adenocarcinoma: Perspectives in immunotherapy

Cancer-associated fibroblasts (CAFs), the key component in pancreatic tumor microenvironment (TME), originate from many sources and are naturally heterogeneous in phenotype and function. Numerous studies have identified their crucial role in promoting tumorigenesis through many routes including fostering cancer proliferation, angiogenesis, invasion, and metastasis. Conversely, research also indicates that subsets of CAFs express anti-tumor activity. These dual effects reflect the complexity of CAF heterogeneity and their interactions with other cells and factors in pancreatic TME. A critical component in this environment is infiltrated immune cells and immune mediators, which can communicate with CAFs. The crosstalk occurs via the production of various cytokines, chemokines, and other mediators and shapes the immunological state in TME. Comprehensive studies of the crosstalk between CAFs and tumor immune environment, particularly internal mechanisms interlinking CAFs and immune effectors, may provide new approaches for pancreatic ductal adenocarcinoma (PDAC) treatments. In this review, we explore the characteristics of CAFs, describe the interplay among CAFs, infiltrated immune cells, other mediators, and provide an overview of recent CAF-target therapies, their limitations, and potential research directions in CAF in the context of PDAC.

Mechanisms of PDAC subtype heterogeneity and therapy response

Pancreatic ductal adenocarcinoma (PDAC) is clinically challenging due to late diagnosis and resistance to therapy. Two major PDAC subtypes have been defined based on malignant epithelial cell gene expression profiles; the basal-like/squamous subtype is associated with a worse prognosis and therapeutic resistance as opposed to the classical subtype. Subtype specification is not binary, consistent with plasticity of malignant cell phenotype. PDAC heterogeneity and plasticity reflect partly malignant cell-intrinsic transcriptional and epigenetic regulation. However, the stromal and immune compartments of the tumor microenvironment (TME) also determine disease progression and therapy response. It is evident that integration of intrinsic and extrinsic factors can dictate subtype heterogeneity, and thus, delineating the pathways involved can help to reprogram PDAC towards a classical/druggable subtype.

Multiomic analysis reveals conservation of cancer-associated fibroblast phenotypes across species and tissue of origin

Cancer-associated fibroblasts (CAFs) are integral to the solid tumor microenvironment. CAFs were once thought to be a relatively uniform population of matrix-producing cells, but single-cell RNA sequencing has revealed diverse CAF phenotypes. Here, we further probed CAF heterogeneity with a comprehensive multiomics approach. Using paired, same-cell chromatin accessibility and transcriptome analysis, we provided an integrated analysis of CAF subpopulations over a complex spatial transcriptomic and proteomic landscape to identify three superclusters: steady state-like (SSL), mechanoresponsive (MR), and immunomodulatory (IM) CAFs. These superclusters are recapitulated across multiple tissue types and species. Selective disruption of underlying mechanical force or immune checkpoint inhibition therapy results in shifts in CAF subpopulation distributions and affected tumor growth. As such, the balance among CAF superclusters may have considerable translational implications. Collectively, this research expands our understanding of CAF biology, identifying regulatory pathways in CAF differentiation and elucidating therapeutic targets in a species- and tumor-agnostic manner.

Exploring the Biology of Cancer-Associated Fibroblasts in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is a lethal malignancy characterised by a stubbornly low 5-year survival which is essentially unchanged in the past 5 decades. Despite recent advances in chemotherapy and surgical outcomes, progress continues to lag behind that of other cancers. The PDAC microenvironment is characterised by a dense, fibrotic stroma of which cancer-associated fibroblasts (CAFs) are key players. CAFs and fibrosis were initially thought to be uniformly tumour-promoting, however this doctrine is now being challenged by a wealth of evidence demonstrating CAF phenotypic and functional heterogeneity. Recent technological advances have allowed for the molecular profiling of the PDAC tumour microenvironment at exceptional detail, and these technologies are being leveraged at pace to improve our understanding of this previously elusive cell population. In this review we discuss CAF heterogeneity and recent developments in CAF biology. We explore the complex relationship between CAFs and other cell types within the PDAC microenvironment. We discuss the potential for therapeutic targeting of CAFs, and we finally provide an overview of future directions for the field and the possibility of improving outcomes for patients with this devastating disease.

Cancer-associated fibroblasts in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a lethal cancer with a prominent extracellular matrix (ECM) deposition and poor prognosis. High levels of ECM proteins derived from tumour cells reduce the efficacy of conventional cancer treatment paradigms and contribute to tumour progression and metastasis. As abundant tumour-promoting cells in the ECM, cancer-associated fibroblasts (CAFs) are promising targets for novel anti-tumour interventions. Nonetheless, related clinical trials are hampered by the lack of specific markers and elusive differences between CAF subtypes. Here, we review the origins and functional diversity of CAFs and show how they create a tumour-promoting milieu, focusing on the crosstalk between CAFs, tumour cells, and immune cells in the tumour microenvironment. Furthermore, relevant clinical advances and potential therapeutic strategies relating to CAFs are discussed.

NetrinG1+ cancer-associated fibroblasts generate unique extracellular vesicles that support the survival of pancreatic cancer cells under nutritional stress

It is projected that in 5 years, pancreatic cancer will become the second deadliest cancer in the United States. A unique aspect of pancreatic ductal adenocarcinoma (PDAC) is its stroma; rich in cancer-associated fibroblasts (CAFs) and a dense CAF-generated extracellular matrix (ECM). These pathogenic stroma CAF/ECM units cause the collapse of local blood vessels rendering the tumor microenvironment nutrient-poor. PDAC cells are able to survive this state of nutrient stress via support from CAF-secreted material, which includes small extracellular vesicles (sEVs). The tumor-supportive CAFs possess a distinct phenotypic profile, compared to normal-like fibroblasts, expressing NetrinG1 (NetG1) at the plasma membrane, and active Integrin α₅β₁ localized to the multivesicular bodies; traits indicative of poor patient survival. We herein report that NetG1⁺ CAFs secrete sEVs that stimulate Akt-mediated survival in nutrient-deprived PDAC cells, protecting them from undergoing apoptosis. Further, we show that NetG1 expression in CAFs is required for the pro-survival properties of sEVs. Additionally, we report that the above-mentioned CAF markers are secreted in distinct subpopulations of EVs; with NetG1 being enriched in exomeres, and Integrin α₅β₁ being enriched in exosomes. Finally, we found that NetG1 and Integrin α₅β₁ were detected in sEVs collected from plasma of PDAC patients, while their levels were significantly lower in plasma-derived sEVs of sex/age-matched healthy donors. The discovery of these tumor-supporting CAF-EVs elucidates novel avenues in tumor-stroma interactions and pathogenic stroma detection.

Targeting hedgehog signaling in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) remains a leading cause of cancer related death. The urgent need for effective therapies is highlighted by the lack of adequate targeting. In PDAC, hedgehog (Hh) signaling is known to be aberrantly activated, which prompted the pathway as a possible target for effective treatment for PDAC patients. Unfortunately, specific targeting of upstream molecules within the Hh signaling pathway failed to bring clinical benefit. This led to the ongoing debate on Hh targeting as a therapeutic treatment for PDAC patients. Additionally, concurrent non-canonical activation routes also result in translocation of Gli transcription factors into the nucleus. Therefore, different downstream targets of the Hh signaling pathway were identified and evaluated in preclinical and clinical research. In this review we summarize the variety of Hh signaling antagonists in different preclinical models of PDAC. Furthermore, we discuss published and ongoing clinical trials that evaluated Hh antagonists and point out the current hurdles and future perspectives in the light of redesigning Hh-targeting therapies for the treatment of PDAC patients.

Combinatorial Gli activity directs immune infiltration and tumor growth in pancreatic cancer

Proper Hedgehog (HH) signaling is essential for embryonic development, while aberrant HH signaling drives pediatric and adult cancers. HH signaling is frequently dysregulated in pancreatic cancer, yet its role remains controversial, with both tumor-promoting and tumor-restraining functions reported. Notably, the GLI family of HH transcription factors (GLI1, GLI2, GLI3), remain largely unexplored in pancreatic cancer. We therefore investigated the individual and combined contributions of GLI1-3 to pancreatic cancer progression. At pre-cancerous stages, fibroblast-specific Gli2/Gli3 deletion decreases immunosuppressive macrophage infiltration and promotes T cell infiltration. Strikingly, combined loss of Gli1/Gli2/Gli3 promotes macrophage infiltration, indicating that subtle changes in Gli expression differentially regulate immune infiltration. In invasive tumors, Gli2/Gli3 KO fibroblasts exclude immunosuppressive myeloid cells and suppress tumor growth by recruiting natural killer cells. Finally, we demonstrate that fibroblasts directly regulate macrophage and T cell migration through the expression of Gli-dependent cytokines. Thus, the coordinated activity of GLI1-3 directs the fibroinflammatory response throughout pancreatic cancer progression.

Tumor-Stroma Interaction in PDAC as a New Approach for Liquid Biopsy and its Potential Clinical Implications

The extremely poor prognosis for patients with pancreatic ductal adenocarcinoma (PDAC) has remained unchanged for decades. As a hallmark of PDAC histology, the distinct desmoplastic response in the tumor microenvironment is considered a key factor exerting pro- and antitumor effects. Increasing emphasis has been placed on cancer-associated fibroblasts (CAFs), whose heterogeneity and functional diversity is reflected in the numerous subtypes. The myofibroblastic CAFs (myCAFs), inflammatory CAFs (iCAFs) and antigen presenting CAFs (apCAFs) are functionally divergent CAF subtypes with tumor promoting as well as repressing effects. Precise knowledge of the underlying interactions is the basis for a variety of treatment approaches, which are subsumed under the term antistromal therapy. Clinical implementation is still pending due to the lack of benefit-as well as paradoxical preclinical findings. While the prominent significance of CAFs in the immediate environment of the tumor is becoming clear, less is known about the circulating (c)CAFs. cCAFs are of particular interest as they seem not only to be potential new liquid biopsy biomarkers but also to support the survival of circulating tumor cells (CTC) in the bloodstream. In PDAC, CTCs correlate with an unfavorable outcome and can also be employed to monitor treatment response, but the current clinical relevance is limited. In this review, we discuss CTCs, cCAFs, secretomes that include EVs or fragments of collagen turnover as liquid biopsy biomarkers, and clinical approaches to target tumor stroma in PDAC.

Metastatic Transition of Pancreatic Ductal Cell Adenocarcinoma Is Accompanied by the Emergence of Pro-Invasive Cancer-Associated Fibroblasts

Cancer-associated fibroblasts (CAFs) are now appreciated as key regulators of cancer metastasis, particularly in cancers with high stromal content, e.g., pancreatic ductal cell carcinoma (PDAC). However, it is not yet well understood if fibroblasts are always primed to be cooperative in PDAC transition to metastasis, if they undergo transformation which ensures their cooperativity, and if such transformations are cancer-driven or intrinsic to fibroblasts. We performed a fibroblast-centric analysis of PDAC cancer, as it transitioned from the primary site to trespass stromal compartment reaching the lymph node using published single-cell RNA sequencing data by Peng et al. We have characterized the change in fibroblast response to cancer from a normal wound healing response in the initial stages to the emergence of subclasses with myofibroblast and inflammatory fibroblasts such as signatures. We have previously posited "Evolved Levels of Invasibility (ELI)", a framework describing the evolution of stromal invasability as a selected phenotype, which explains the large and correlated reduction in stromal invasion by placental trophoblasts and cancer cells in certain mammals. Within PDAC samples, we found large changes in fibroblast subclasses at succeeding stages of PDAC progression, with the emergence of specific subclasses when cancer trespasses stroma to metastasize to proximal lymph nodes (stage IIA to IIB). Surprisingly, we found that the initial metastatic transition is accompanied by downregulation of ELI-predicted pro-resistive genes, and the emergence of a subclass of fibroblasts with ELI-predicted increased invasibility. Interestingly, this trend was also observed in stellate cells. Using a larger cohort of bulk RNAseq data from The Cancer Genome Atlas for PDAC cancers, we confirmed that genes describing this emergent fibroblast subclass are also correlated with lymph node metastasis of cancer cells. Experimental testing of selected genes characterizing pro-resistive and pro-invasive fibroblast clusters confirmed their contribution in regulating stromal invasability as a phenotype. Our data confirm that the complexity of stromal response to cancer is really a function of stage-wise emergence of distinct fibroblast clusters, characterized by distinct gene sets which confer initially a predominantly pro-resistive and then a pro-invasive property to the stroma. Stromal response therefore transitions from being tumor-limiting to a pro-metastatic state, facilitating stromal trespass and the onset of metastasis.

Modulation of the tumor microenvironment by armed vesicular stomatitis virus in a syngeneic pancreatic cancer model

Background

The immunosuppressive microenvironment in pancreatic ductal adenocarcinoma is a major factor that limits the benefits of immunotherapy, especially immune checkpoint blockade. One viable strategy for reverting the immunosuppressive conditions is the use of an oncolytic virus (OV) in combination with other immunotherapy approaches. Infection of PDAC cells with a robust OV can change the tumor microenvironment and increase tumor antigen release by its lytic activities. These changes in the tumor may improve responses to immunotherapy, including immune checkpoint blockade. However, a more potent OV may be required for efficiently infecting pancreatic tumors that may be resistant to OV.

Methods

Vesicular stomatitis virus, a rapid replicating OV, was armed to express the Smac protein during virus infection (VSV-S). Adaptation by limited dilution largely increased the selective infection of pancreatic cancer cells by VSV-S. The engineered OV was propagated to a large quantity and evaluated for their antitumor activities in an animal model.

Results

In a syngeneic KPC model, intratumoral injection of VSV-S inhibited tumor growth, and induced increasing tumor infiltration of neutrophils and elimination of myeloid derived suppressor cells and macrophages in the tumor. More importantly, M2-like macrophages were eliminated preferentially over those with an M1 phenotype. Reduced levels of arginase 1, TGF-β and IL-10 in the tumor also provided evidence for reversion of the immunosuppressive conditions by VSV-S infection. In several cases, tumors were completely cleared by VSV-S treatment, especially when combined with anti-PD-1 therapy. A long-term survival of 44% was achieved.

Conclusions

The improved OV, VSV-S, was shown to drastically alter the immune suppressive tumor microenvironment when intratumorally injected. Our results suggest that the combination of potent OV treatment with immune checkpoint blockade may be a promising strategy to treat pancreatic cancer more effectively.

Multiomic characterization of pancreatic cancer-associated macrophage polarization reveals deregulated metabolic programs driven by the GM-CSF-PI3K pathway

The pancreatic ductal adenocarcinoma microenvironment is composed of a variety of cell types and marked by extensive fibrosis and inflammation. Tumor-associated macrophages (TAMs) are abundant, and they are important mediators of disease progression and invasion. TAMs are polarized in situ to a tumor promoting and immunosuppressive phenotype via cytokine signaling and metabolic crosstalk from malignant epithelial cells and other components of the tumor microenvironment. However, the specific distinguishing features and functions of TAMs remain poorly defined. Here, we generated tumor-educated macrophages (TEMs) in vitro and performed detailed, multiomic characterization (i.e., transcriptomics, proteomics, metabolomics). Our results reveal unique genetic and metabolic signatures of TEMs, the veracity of which were queried against our in-house single-cell RNA sequencing dataset of human pancreatic tumors. This analysis identified expression of novel, metabolic TEM markers in human pancreatic TAMs, including ARG1, ACLY, and TXNIP. We then utilized our TEM model system to study the role of mutant Kras signaling in cancer cells on TEM polarization. This revealed an important role for granulocyte-macrophage colony-stimulating factor (GM-CSF) and lactate on TEM polarization, molecules released from cancer cells in a mutant Kras-dependent manner. Lastly, we demonstrate that GM-CSF dysregulates TEM gene expression and metabolism through PI3K-AKT pathway signaling. Collectively, our results define new markers and programs to classify pancreatic TAMs, how these are engaged by cancer cells, and the precise signaling pathways mediating polarization.

Pancreatic Cancer Microenvironment and Cellular Composition: Current Understandings and Therapeutic Approaches

Pancreatic ductal adenocarcinoma (PDAC) remains one of the most lethal human solid tumors, despite great efforts in improving therapeutics over the past few decades. In PDAC, the distinct characteristic of the tumor microenvironment (TME) is the main barrier for developing effective treatments. PDAC TME is characterized by a dense stroma, cancer-associated fibroblasts, and immune cells populations that crosstalk to the subpopulations of neoplastic cells that include cancer stem cells (CSCs). The heterogeneity in TME is also exhibited in the diversity and dynamics of acellular components, including the Extracellular matrix (ECM), cytokines, growth factors, and secreted ligands to signaling pathways. These contribute to drug resistance, metastasis, and relapse in PDAC. However, clinical trials targeting TME components have often reported unexpected results and still have not benefited patients. The failures in those trials and various efforts to understand the PDAC biology demonstrate the highly heterogeneous and multi-faceted TME compositions and the complexity of their interplay within TME. Hence, further functional and mechanistic insight is needed. In this review, we will present a current understanding of PDAC biology with a focus on the heterogeneity in TME and crosstalk among its components. We also discuss clinical challenges and the arising therapeutic opportunities in PDAC research.

The tumor microenvironment in pancreatic ductal adenocarcinoma: current perspectives and future directions

Pancreatic ductal adenocarcinoma (PDAC) is among the most lethal malignancies and is characterized by a unique tumor microenvironment (TME) consisting of an abundant stromal component. Many features contained with the PDAC stroma contribute to resistance to cytotoxic and immunotherapeutic regimens, as well as the propensity for this tumor to metastasize. At the cellular level, PDAC cells crosstalk with a complex mixture of non-neoplastic cell types including fibroblasts, endothelial cells, and immune cells. These intricate interactions fuel the progression and therapeutic resistance of this aggressive cancer. Moreover, data suggest the polarization of these cell types, in particular immune and fibroblast populations, dictate how PDAC tumors grow, metastasize, and respond to therapy. As a result, current research is focused on how to best target these populations to render tumors responsive to treatment. Herein, we summarize the cell populations implicated in providing a supporting role for the development and progression of PDAC. We focus on stromal fibroblasts and immune subsets that have been widely researched. We discuss factors which govern the phenotype of these populations and provide insight on how they have been targeted therapeutically. This review provides an overview of the tumor microenvironment and postulates that cellular and soluble factors within the microenvironment can be specifically targeted to improve patient outcomes.

The Heterogeneity of the Tumor Microenvironment as Essential Determinant of Development, Progression and Therapy Response of Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is commonly diagnosed at advanced stages and most anti-cancer therapies have failed to substantially improve prognosis of PDAC patients. As a result, PDAC is still one of the deadliest tumors. Tumor heterogeneity, manifesting at multiple levels, provides a conclusive explanation for divergent survival times and therapy responses of PDAC patients. Besides tumor cell heterogeneity, PDAC is characterized by a pronounced inflammatory stroma comprising various non-neoplastic cells such as myofibroblasts, endothelial cells and different leukocyte populations which enrich in the tumor microenvironment (TME) during pancreatic tumorigenesis. Thus, the stromal compartment also displays a high temporal and spatial heterogeneity accounting for diverse effects on the development, progression and therapy responses of PDAC. Adding to this heterogeneity and the impact of the TME, the microbiome of PDAC patients is considerably altered. Understanding this multi-level heterogeneity and considering it for the development of novel therapeutic concepts might finally improve the dismal situation of PDAC patients. Here, we outline the current knowledge on PDAC cell heterogeneity focusing on different stromal cell populations and outline their impact on PDAC progression and therapy resistance. Based on this information, we propose some novel concepts for treatment of PDAC patients.

Ghee Butter from Bovine Colostrum Reduces Inflammation in the Mouse Model of Acute Pancreatitis with Potential Involvement of Free Fatty Acid Receptors

Acute pancreatitis (AP) is an inflammatory disease that causes severe tissue damage. Ghee butter from bovine colostrum (GBBC) is a clarified butter produced by heating milk fat to 40 °C and separating the precipitating protein. As colostrum mainly contains fatty acids (FAs), immunoglobulins, maternal immune cells, and cytokines, we hypothesized that it may exert anti-inflammatory effects. We investigated the effects of GBBC on experimental AP in mice. Two intraperitoneal (ip) injections of L-arginine (8%) were given 1 h apart to generate the AP murine model. After 12 h from the first L-arginine injection, mice were divided into the following experimental groups: AP mice treated with GBBC (oral gavage (po) every 12 h) and non-treated AP mice (po vehicle every 12 h). Control animals received vehicle only. At 72 h, mice were euthanized. Histopathological examination along with myeloperoxidase (MPO) and amylase/lipase activity assays were performed. In a separate set of experiments, FFAR1 and FFAR4 antagonists were used to verify the involvement of respective receptors. Administration of GBBC decreased MPO activity in the pancreas and lungs along with the microscopical severity of AP in mice. Moreover, treatment with GBBC normalized pancreatic enzyme activity. FFAR1 and FFAR4 antagonists tended to reverse the anti-inflammatory effect of GBBC in mouse AP. Our results suggest that GBBC displays anti-inflammatory effects in the mouse model of AP, with the putative involvement of FFARs. This is the first study to show the anti-inflammatory potential of a nutritional supplement derived from GBBC.

Apoptosis in the Pancreatic Cancer Tumor Microenvironment-The Double-Edged Sword of Cancer-Associated Fibroblasts

Pancreatic ductal adenocarcinoma (PDAC) is associated with poor prognosis. This is attributed to the disease already being advanced at presentation and having a particularly aggressive tumor biology. The PDAC tumor microenvironment (TME) is characterized by a dense desmoplastic stroma, dominated by cancer-associated fibroblasts (CAF), extracellular matrix (ECM) and immune cells displaying immunosuppressive phenotypes. Due to the advanced stage at diagnosis, the depletion of immune effector cells and lack of actionable genomic targets, the standard treatment is still apoptosis-inducing regimens such as chemotherapy. Paradoxically, it has emerged that the direct induction of apoptosis of cancer cells may fuel oncogenic processes in the TME, including education of CAF and immune cells towards pro-tumorigenic phenotypes. The direct effect of cytotoxic therapies on CAF may also enhance tumorigenesis. With the awareness that CAF are the predominant cell type in PDAC driving tumorigenesis with various tumor supportive functions, efforts have been made to try to target them. However, efforts to target CAF have, to date, shown disappointing results in clinical trials. With the help of sophisticated single cell analyses it is now appreciated that CAF in PDAC are a heterogenous population with both tumor supportive and tumor suppressive functions. Hence, there remains a debate whether targeting CAF in PDAC is a valid therapeutic strategy. In this review we discuss how cytotoxic therapies and the induction of apoptosis in PDAC fuels oncogenesis by the education of surrounding stromal cells, with a particular focus on the potential pro-tumorigenic outcomes arising from targeting CAF. In addition, we explore therapeutic avenues to potentially avoid the oncogenic effects of apoptosis in PDAC CAF.

New Roles for Vitamin D Superagonists: From COVID to Cancer

Vitamin D is a potent steroid hormone that induces widespread changes in gene expression and controls key biological pathways. Here we review pathophysiology of vitamin D with particular reference to COVID-19 and pancreatic cancer. Utility as a therapeutic agent is limited by hypercalcemic effects and attempts to circumvent this problem have used vitamin D superagonists, with increased efficacy and reduced calcemic effect. A further caveat is that vitamin D mediates multiple diverse effects. Some of these (anti-fibrosis) are likely beneficial in patients with COVID-19 and pancreatic cancer, whereas others (reduced immunity), may be beneficial through attenuation of the cytokine storm in patients with advanced COVID-19, but detrimental in pancreatic cancer. Vitamin D superagonists represent an untapped resource for development of effective therapeutic agents. However, to be successful this approach will require agonists with high cell-tissue specificity.

Palladin isoforms 3 and 4 regulate cancer-associated fibroblast pro-tumor functions in pancreatic ductal adenocarcinoma

Pancreatic Ductal Adenocarcinoma (PDAC) has a five-year survival under 10%. Treatment is compromised due to a fibrotic-like stromal remodeling process, known as desmoplasia, which limits therapeutic perfusion, supports tumor progression, and establishes an immunosuppressive microenvironment. These processes are driven by cancer-associated fibroblasts (CAFs), functionally activated through transforming growth factor beta1 (TGFβ1). CAFs produce a topographically aligned extracellular matrix (ECM) that correlates with reduced overall survival. Paradoxically, ablation of CAF populations results in a more aggressive disease, suggesting CAFs can also restrain PDAC progression. Thus, unraveling the mechanism(s) underlying CAF functions could lead to therapies that reinstate the tumor-suppressive features of the pancreatic stroma. CAF activation involves the f-actin organizing protein palladin. CAFs express two palladin isoforms (iso3 and iso4) which are up-regulated in response to TGFβ1. However, the roles of iso3 and iso4 in CAF functions remain elusive. Using a CAF-derived ECM model, we uncovered that iso3/iso4 are required to sustain TGFβ1-dependent CAF activation, secrete immunosuppressive cytokines, and produce a pro-tumoral ECM. Findings demonstrate a novel role for CAF palladin and suggest that iso3/iso4 regulate both redundant and specific tumor-supportive desmoplastic functions. This study highlights the therapeutic potential of targeting CAFs to restore fibroblastic anti-tumor activity in the pancreatic microenvironment.