Background
The normal tissue microenvironment plays a major role in the maintenance of organ structure, tissue specificity, and homeostasis, and in suppressing malignant progression [
1]. Parenchymal cells, fibroblasts, endothelial and inflammatory cells all communicate with each other through secreted extracellular matrix molecules and growth factors [
2]. Negative feedback loops maintain the correct balance between the different cell types and avoid uncontrolled cell proliferation. In certain circumstances, particularly following infection, inflammation, trauma or other insults, the microenvironment undergoes profound alterations which can reestablish homeostasis or, conversely, promote disease progression [
3,
4].
Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal tumors in the western world. It involves deregulated extracellular matrix deposition and abundant fibrosis, with qualitative and quantitative alterations in the equilibrium between proteases and inhibitors. Extracellular proteases originally considered favorable to malignancy, have a much more complex role than previously assumed, with some enzymes acting in an opposing fashion to block cancer growth and maintain tissue homeostasis [
5].
Trypsinogen is zymogen of one of the major pancreatic enzymes secreted by exocrine acinar cells. In physiological conditions, its conversion to trypsin by enterokinase and the digestive activity of trypsin are closely regulated to prevent tissue damage [
6]. Events causing acinar cell damage lead to premature enzyme activation and secretion [
6]. Depending on the context, trypsin can have pro- or anti- tumor effects by directly triggering specific cell signaling pathways or by generating biologically active fragments of extracellular matrix proteins that cooperate with – or antagonize – the effect of growth factors in tumor progression [
7,
8].
Here, to investigate the role of the pancreatic microenvironment in PDAC progression, we studied the morphological and functional changes in PDAC cells induced by spheroids isolated from human and murine healthy pancreas. We found that the pancreatic microenvironment counteracted the growth of PDAC through the activity of trypsin-generated FN fragments, pointing to a role of proteases and protease-generated extracellular matrix fragments in the control of tumor growth. Finally, the involvement of the FGF2/FGFR1 and integrin β1/FAK pathways in this process supports the use of combinations of FAK and FGFR inhibitors in PDAC treatment.
Methods
PDAC models
Tumor cells
The MIAPaCa2 and BxPC3 human pancreatic cancer cell lines were obtained, respectively, from Istituto Zooprofilattico Sperimentale (Brescia, Italy) and from American Type Culture Collection (ATCC) and authenticated using the AmpFlSTR® Identifiler® PCR Amplification Kit (Applied Biosystems, Merk, Milano, Italy). The FC1199 pancreatic cancer cell line [
9] was kindly provided by D.A. Tuveson (Cold Spring Harbor, NY, USA). MIAPaCa2, FC1199 and BxPC3 were cultured respectively in Dulbecco modified Eagle medium (DMEM) and RPMI 1640 Medium (ATCC modification) (Gibco, ThermoFisher, Rodano, Italy) supplemented with 10% FCS (Euroclone, Milano, Italy) and 1% L-glutamine (Gibco) and were routinely tested for mycoplasma infection. Stocks of cell lines were stored frozen in liquid nitrogen and kept in culture for no more than 3 weeks.
Pancreatic spheroids
Human spheroids were obtained from the exocrine pancreas of human donors after pancreatic islet isolation [
10] in the Pancreatic Islet Processing Unit, San Raffaele Scientific Institute, Milan, Italy. Their collection and use was approved by the local scientific ethic committees. Murine spheroids were obtained as follows: pancreatic fragments from four-months-old female C57BL/6 mice (Charles River Laboratories, Lecco, Italy) were soaked for ten minutes in cold P-collagenase (1 mg/mL, Roche, Monza, Italy) and then incubated at 37 °C for 15 min under agitation. After two washes with cold Hank’s Balanced Salt Solution (HBSS) (Gibco), cells were filtered with a 500 μm strainer and washed again. After removing islets by centrifugation density gradient, isolated cells were left to spontaneously aggregate and form spheroids in non-adherent tissue culture plates. Cell vitality was checked using Trypan Blue (Sigma-Aldrich, Merk, Milano, Italy).
Preparation of spheroid conditioned media
Spheroids were seeded in serum-free DMEM with or without trypsin inhibitors: aprotinin (1.7 µg/mL, Sigma-Aldrich) or Tosyl-L-lysyl-chloromethane hydrochloride (TLCK 50–100 µM, abcam, Cambridge, UK). After 24 h spheroids were washed in serum-free DMEM and reseeded with or without inhibitors. After 72 h, conditioned media were collected, centrifuged at 3000 rpm for 10 min at 4 °C, and stored frozen at -80 °C.
In vivo tumor models
Mice were maintained under specific pathogen-free conditions and handled using aseptic procedures. Procedures involving animals and their care were conducted in conformity with institutional guidelines that comply with national (Lgs 26/2014) and EU directives laws and policies (EEC Council Directive 2010/63, in line with guidelines for the welfare and use of animals in cancer research [
11]. Animal studies were approved by the Mario Negri Institute Animal Care and Use Committee and by the Italian Ministry of Health (DM 85/2013-B and Authorization no.519/2021-PR and no.125/2016-PR).
Patient-derived PDAC xenograft HuPa4 (5 × 10
5 cells), MIAPaCa2-luc and FC1199 (5 × 10
4 cells) were injected with or without a suspension of spheroids (at the estimated ratio of 1:300) in the pancreas of six- to eight-week-old female severe combined immunodeficiency (SCID) mice for HuPA4 [
12] (Envigo, Correzzana, Italy), or female athymic Foxn1 nu/nu mice (Envigo) MIAPaCa2-luc, or female C57BL/6 mice for FC1199.
The experiments were concluded when the first animals showed sign of suffering. After euthanasia the pancreas was weighted and collected for further analysis.
To test the effects of spheroid conditioned media, HuPa4 cells (2 × 106) were injected subcutaneously in the flank of six- to eight-week-old male severe combined immunodeficiency (SCID) mice. Mice were injected subcutaneously with spheroid conditioned or control medium (200 µL) 24 h before tumor injection, and five days a week until euthanasia. Subcutaneous tumor growth was monitored three times a week with a digital caliper, and tumor volume (mm3) was calculated as [length (mm) x width2 (mm2)/2].
De-adhesion and proliferation assay
MIAPaCa2 and FC1199 cells were seeded in 96-well plates (3000/well) in complete medium. After 24 h, different stimuli were added as indicated and incubated for 1 h (de-adhesion) or 72 h (proliferation). De-adhesion was measured with crystal violet (Sigma-Aldrich). Proliferation was measured with MTS (Promega, Madison, Wisconsin, USA) or crystal violet. Each condition was tested in triplicate. Data are expressed as percentages of control (absence of stimuli).
Immunofluorescence
MIAPaCa2 cells were seeded in 8-well ibi-treat micro-slides (Ibidi, Giemme, Milano, Italy) (30,000/well) in DMEM with 10% FCS. After 48 h, different stimuli were added and incubated for the times indicated. Cells were then fixed with 2% PFA (Sigma-Aldrich) in 4% sucrose for 10 min at room temperature (RT), permeabilized with 0.1% Triton X-100 (Biorad, Segrate, Italy) for 3 min and then incubated for 30 min with 3% BSA (Sigma-Aldrich) at RT. The samples were stained with the primary antibody diluted in 3% BSA overnight at 4 °C followed by incubation with the appropriate secondary antibody for 1 h at RT. Actin was stained with rhodamine phalloidin (Invitrogen, ThermoFisher) at the dilution of 1:40 for 1 h at RT. Nuclei were counterstained with DAPI for 5 min at RT. Images were taken using a Leica SP8 confocal microscope (Leica Microsystems, Wetzlar, Germany).
Trypsin digestion of fibronectin
50 µg of human plasma fibronectin (FN, Sigma-Aldrich) was digested at 1:50 (w/w) with trypsin (Trypsin Sequencing Grade, Sigma-Aldrich) for 10, 60 and 240 min at 37 °C with gentle agitation. Proteolysis was stopped by the addition of TLCK 800 µM (abcam) at 4 °C.
Western blot
Cells were lysed with RIPA buffer (ThermoFisher), containing protease inhibitors (Roche) and phosphatase inhibitors (Sigma-Aldrich) and centrifuged at 12000xg at 4 °C.
Proteins were separated by 4–12% SDS-PAGE (Genscript, Twin Helix, Rho, Italy) under reducing conditions and transferred to nitrocellulose membranes (GE Healthcare, Milano, Italy). After blocking with 5% BSA in Tris-buffered saline (TBS) 0.1% Tween-20, membranes were incubated with primary antibodies in 2% BSA overnight at 4 °C, followed by IR- or peroxidase-labeled secondary antibody. Alpha-tubulin 1:2000 or GAPDH 1:2500 (Sigma-Aldrich) antibodies were used to confirm equal loading. Signals were detected using an Odyssey FC Imaging System (LI-COR,Biosciences, Lincoln, Nevada, USA). Bands were quantified using Image Studio Lite 5.0 (LI-COR) software.
Proteomics
Label-free proteomics of inhibitory and stimulatory CM was carried out as previously described [
13] and detailed in the supplementary material.
Statistical analysis
Statistical analysis was done with GraphPad Prism 9.3.1 (GraphPad, La Jolla, CA). Differences in tumor growth were analyzed by two-way ANOVA followed by Bonferroni’s multiple comparison test. For other analysis, a nonparametric Mann-Whitney test was used for comparison of two groups, and one-way ANOVA, followed by Dunnett’s or Tukey’s multiple comparison test, for three or more groups. P-values less than 0.05 were considered statistically significant.
Discussion
This study demonstrates that FN fragments generated by pancreatic trypsin inhibit PDAC cell adhesion, proliferation and in vivo tumor growth by inhibiting the functional connections among β1 integrins, FAK and FGFR, hence contributing to pancreatic homeostasis and limiting PDAC progression. Our findings also support the use of FAK inhibitors together with FGFR TKI for PDAC treatment.
Several studies reported that stromal and epithelial components surrounding neoplastic cells can positively or negatively influence tumor growth [
2,
14,
28,
29]. However, to the best of our knowledge, this is the first study in which 3D spheroids isolated from the pancreas have been used to study the effect of the pancreatic microenvironment on PDAC growth. This system reproduces the complex interactions among cells composing the pancreas, maintains the ratio between cell populations and cell interaction is not affected by exogenous extracellular matrix or synthetic scaffolds. Of note, spheroids also survive in vivo, transplanted orthotopically in mice, where they survive for at least four weeks ensuring a long-lasting effect on the co-transplanted tumors.
The finding that the spheroids release active trypsin, normally absent in healthy pancreas, indicates that, to some extent, the spheroids reproduce a damaged environment, and might be considered a model of damaged pancreas. In agreement, we found that experimental pancreatitis was able to inhibit PDAC growth. Other studies have reported that that trypsin in vitro can exert antitumor activity on different tumor cell lines by suppressing the EMT program and promoting cancer cell differentiation [
30,
31]. A combination of trypsinogen and chymotrypsinogen inhibited tumor growth, invasion and angiogenesis both in vitro and in vivo and demonstrated clinical efficacy on a cohort of 46 patients with advanced tumors of different origin [
32,
33]. Nevertheless, because of its complex activities, trypsin is not suitable for therapeutic purposes, and its proteolytic activity has to be tightly regulated to avoid tissue damage [
34], inflammation, and even chronic pancreatitis and consequently PDAC [
6,
7]. Instead, this study points to specific pathways downstream trypsin, specifically the FGFR and FAK signalling pathways as valid therapeutic targets. Our in vivo preclinical studies do in fact confirm the feasibility and efficacy of this approach.
Due to the large number of trypsin substrates, the role of trypsin in cancer is complex, depending on the molecular context and the signaling pathway directly or indirectly modulated in the target cells [
8,
35‐
37]. This study indicates that pancreatic trypsin, besides its digestive functions, seems necessary to maintain pancreatic tissue homeostasis and counteract tumor growth. By generating FN fragments with antitumor activity trypsin has a protective effect at the site of primary tumor growth affecting FGFR and FAK tumor signaling.
Assembly of FN into a three-dimensional network is essential for maintaining tissue architecture and to regulate cell adhesion [
38]. We speculate that trypsin-generated FN fragments initially restrain tumor progression in the pancreas. Then, when FN secretion and deposition by activated stroma and tumor cells exceed trypsin’s proteolytic capacity for generating FN fragments with antitumor activity, tumor cells spread and proliferate on intact FN scaffolds, escape normal environmental surveillance, and progress towards a more malignant phenotype. It has in fact been demonstrated that an intact FN matrix is vital for cell adhesion [
39], and that proteolysis is an important mechanism to control FN turnover and assembly [
40‐
42]. The functional importance of this is supported by our findings that proteolytic FN fragments were identified only in the CM of spheroids with tumor restraining activity, whereas high MW, intact FN molecules were found in the CM of spheroids with pro-tumorigenic activity. Additional studies are needed to validate the importance of these pathways in the complex tumor-microenvironment interactions in in vivo experimental and clinical settings.
The connection between trypsin activity, FN fragment generation and the effect of spheroid CM on tumor cell adhesion/proliferation is further supported by the fact that spheroids treated with FCS lost their inhibitory activity and did not generate the FN fragments because of the copious trypsin inhibitors in serum.
Plasma and cellular FN have distinct structural (splice isoforms) and functional properties. In our study, trypsin-generated FN fragments from both cellular (spheroids) and plasma (purified) FN had the same inhibitory effect on PDAC, indicating that the FN fragment responsible for these effects belongs to a domain conserved in the two forms [
5,
43]. The FN type III-8 domain identified in medium conditioned by pancreatic spheroids with inhibitory activity is present in most FN isoforms. Indeed in some physiological and pathological processes it has been suggested that plasma and cellular FN perform the same functions, with cellular FN compensating for plasma FN deficiency [
44]. In other processes such as wound healing, plasma and cellular FN play distinct roles in the different phases of tissue repair [
45].
This study establishes a direct cause-effect relationship between trypsin-generated FN fragments and the effect on FGFR and FAK phosphorylation and co-localization with integrin α5β1. Other proteases, such as MMP2 [
46], MMP9 [
47], elastase [
48], TAT-2 [
35], and granzyme [
49] cleave FN and influence several cell functions including cell adhesion and migration.
FN fragments have been previously identified both locally in tissues and body fluids and suggested to regulate cell activities [
50]. In agreement with our findings, a time course study [
51] indicated that FN cleavage occurred rapidly proximal to the C-terminal residues containing the interdimeric disulfides bonds, near the N-terminus (between I5 and I6 in Fig.
6M), and near the C-terminus (between III14 and III15), followed by slower cleavages that yield multiple bands. Here proteomic analysis identified peptides in the ~ 80 KD fragment that encompassed a ~ 150 KDa region. Considering the discrepancy between the sequence coverage of FN (150 KDa) and the fragment molecular weight (80 KDa), we assumed the concomitant presence of two FN fragments of similar molecular weight. Using a panel of monoclonal antibodies, we identified the type III-8 domain as the putative active region within the type III domain of FN, but further analysis are required to define the active site more precisely.
The FN multimodular structure and cell- and tissue-specific isoforms [
52] allow various different and multiple interactions with cell-surface receptors, growth factors and other ECM proteins [
20,
45]. Our findings are in agreement with other studies supporting the existence of a complex interaction between FN-FN fragments, FGFR1 and integrins. FN transactivates FGFR1 through β1 integrin in liver endothelial cells [
21]. FNIII 9–10 collaborates with FGFRs to promote neuronal cell adhesion [
53]. Changes in integrin receptor affinity have been reported to be triggered by soluble rather than immobilized FN [
54]. A direct interaction of FGF1 with integrin αvβ3 and FGFR has been demonstrated [
55].
To our knowledge, our study is the first one to demonstrate that FN proteolytic fragments generated by pancreatic cells can reduce P-FGFR, P-FAK and α5β1 co-localization in PDAC cells.
FGFR and FAK pathways drive PDAC progression [
56‐
58] and have been targeted separately in clinical and preclinical trials in combination with cytotoxic agents and immune-checkpoint inhibitors, but with limited efficacy [
59,
60]. Our in vitro and in vivo results confirm the partial inhibitory activity of the single inhibitors, but also indicate that combining FGFR and FAK inhibitors results in additive tumor growth inhibition and therefore could be a valid therapeutic approach to be considered in the clinical setting.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.