Cancer stem cells (CSC) are thought to play a major role in the development and metastatic progression of pancreatic ductal adenocarcinoma (PDAC), one of the deadliest solid tumors. Likewise, the tumor microenvironment contributes critical support in this setting, including from tumor stromal cells and tumor-associated macrophages (TAM) that contribute structural and paracrine-mediated supports, respectively. Here, we show that TAMs secrete the IFN-stimulated factor ISG15, which enhances CSC phenotypes in PDAC in vitro and in vivo. ISG15 was preferentially and highly expressed by TAM present in primary PDAC tumors resected from patients. ISG15 was secreted by macrophages in response to secretion of IFNβ by CSC, thereby reinforcing CSC self-renewal, invasive capacity, and tumorigenic potential. Overall, our work demonstrates that ISG15 is a previously unrecognized support factor for CSC in the PDAC microenvironment with a key role in pathogenesis and progression. Cancer Res; 74(24); 7309–20. ©2014 AACR.
The importance of the stroma in promoting cancer initiation and solid tumor growth has been increasingly recognized during recent years (1–3). Specifically, we have come to understand that apart from providing structural support for tumor development, the tumor-associated microenvironment of many solid tumors provides cues to a subpopulation of tumor-initiating cells, also known as cancer stem cells (CSC), which regulates their self-renewal and tumorigenic and metastatic potential (4). This is certainly the case for pancreatic adenocarcinoma (PDAC), which consists of a heterogeneous population of tumor cells including (i) CSCs (5, 6), (ii) more differentiated cancer cells, and (iii) an extremely high proportion of desmoplastic stromal tissue and immune cells, which accounts for up to 90% of the tumor mass (7). Within the stroma-rich PDAC tumor microenvironment, pancreatic stellate cells (PSC) have been extensively studied and recent reports from our laboratory and other have shown that tumor-associated PSCs can create a protumor paracrine niche for PDAC CSCs via Nodal/Activin A secretion (2, 4). Recent but less conclusive evidence also suggests that inflammatory cells, such as tumor-associated macrophages (TAM; refs. 1, 8), may also play critical roles in the development and progression of numerous tumors, such as PDAC, and the immunomodulatory factors they secrete may also be paracrine-mediated.
IFN-stimulated gene (ISG) 15 is a 165-amino acid (17-kDa) protein that is induced by type I IFN treatment (9). Since its discovery in 1979 (10), ISG15 has been extensively studied as an anti-viral protein (11–13), but we now appreciate that ISG15 has many other functions, including ISGylation, a ubiquitin-like modification process whereby ISG15 can be covalently linked to cytoplasmic and nuclear proteins (14). Like ubiquitin, ISG15 coupling to target proteins involves the ISG15-specific E1-like activating enzyme (UbE1L), the conjugating E2 enzyme, and the ligating E3 enzymes (15, 16). While the consequences of ISGylation of host proteins have been elucidated for only a small set of cellular proteins (e.g., cyclin D1, Filamin B, PML-RARα), it is believed that the biologic effects of ISGylation are dynamic and cell-type/tissue-specific (17). For example, while some reports suggest that like ubiquitylation, ISGylation may function in protein turnover (18), it may also play a previously unrecognized role in protein stability (17). The latter has been explored in systems of bladder, oral, prostate, and breast cancers, where high levels of ISG15 and its conjugates have been detected, suggesting a link between ISG15 and tumorigenesis (19–24). For example, Kiessling and colleagues have shown that in prostate cancer, overexpression of UbE1L increased androgen receptor levels in an ISG15-dependent manner, implying that ISGylation promotes androgen receptor overexpression in cancer cells. In breast cancer, Burks and colleagues have shown that ISG15 stabilizes oncogenic K-ras protein by inhibiting its targeted degradation via lysosomes. Therefore, intracellular ISGylation may very well play an important and previously underappreciated role in cancer.
Apart from its intracellular protein-conjugating functions, ISG15 can also be secreted from cells as free ISG15 where it can act as a cytokine or chemokine stimulating the production of type II IFN, enhancing natural killer cell activity and proliferation or functioning as a strong neutrophil chemoattractant (25, 26). Thus, free ISG15 has strong immunomodulatory properties; however, the biologic role of free ISG15 has been understudied and the mechanisms promoting its liberation are poorly understood. Surprisingly, here we show that ISG15 is present in PDAC tumors, it is expressed and secreted by TAMs in response to IFNβ produced by PDAC cells and, in turn, acts on PDAC CSCs enhancing their inherent “stem-like” properties, including self-renewal and tumorigenicity. Thus, our data suggest a previously unrecognized role for ISG15 (i.e., free ISG15) in the context of pancreatic cancer and highlights a potentially new target for therapeutic intervention.
Materials and Methods
Primary pancreatic cancer cells and macrophages
The use of human material was approved by the local ethics committee of each respective hospital or university, and written informed consent was obtained from all patients. Primary tumors were processed and cultured in vitro as previously detailed (5) and are referred to herein as “Panc-xxx.” Murine PDAC cells were established from tumors extracted from K-ras+/LSL-G12D;Trp53LSL-R172H;PDX1-Cre mice (27) at 20 to 24 weeks of age.
Human blood was obtained from healthy donors with informed consent and in accordance with national regulations for the use of human samples in research. Macrophages were established as previously described (28) and polarized using 60 ng/mL granulocyte macrophage colony-stimulating factor (GM-CSF; M1) or M-CSF (M2; ref. 29). Murine monocytes were isolated from mechanically disrupted spleens and polarized using 10 ng/mL of IFNγ (PeproTech) and lipopolysaccharide (LPS; Sigma; M1) or 10 ng/mL IL4 (M2; PeproTech). Human pancreatic ductal epithelial (HPDE) cells have been previously described (30).
Human PDAC tissue microarrays and RNA samples
Human tissue microarrays (TMA) containing a total of 42 tumors were constructed. RNA from 30 flash-frozen primary human PDAC tumors was isolated by the guanidine thiocyanate method using standard protocols (31).
NU-Foxn1nu nude mice (Charles Rivers), ISG15+/+ and ISG15−/− mice (Klaus-Peter Knobeloch; Universitäts Klinikum, Freiburg, Germany) were housed according to institutional guidelines and all experiments were approved by the Animal Experimental Ethics Committee of the Instituto de Salud Carlos III (Madrid, Spain).
Primary human macrophage cultures were resuspended in Sorting Buffer before analysis with a FACS Canto II instrument (BD). Primary and secondary antibodies and dilutions used are listed in Supplementary Table S1.
In vivo tumorigenicity assay
Primary first-generation sphere-derived pancreatic cells were resuspended in 50 μL of Matrigel (BD) and subcutaneously injected into indicated mice alone or with equal numbers of nonpolarized, M1- or M2-polarized, or CSC conditioned media (CM)-primed primary macrophages. Tumor size was monitored weekly over the course of 6 to 10 weeks.
IFNβ in the supernatant of PDAC cultures was quantified using a commercially available ELISA (PBL Assay Science) as per the manufacturer's instructions. Free ISG15 was quantified using an in-house sandwich ELISA as detailed in Supplementary Materials and Methods.
Immunohistochemistry and immunofluorescence
Formalin-fixed, paraffin-embedded (FFPE) blocks were serially sectioned and immunohistochemical (IHC) or immunfluorescent (IF) analyses performed using standard protocols. Primary antibodies, secondary antibodies, and dilutions used are detailed in Supplementary Table S1.
RNA preparation and RT-qPCR
Total RNA was isolated by the guanidine thiocyanate method using standard protocols (31). cDNA synthesis was performed using the QuantiTect Reverse Transcription Kit (Qiagen), followed by SYBR green RTqPCR (Applied Biosystems). Primers used are listed in Supplementary Table S2.
Sphere formation assay
Pancreatic cancer spheres were generated as previously described (4).
Confluent cultures of primary cancer cells were scratched using a 200 μL pipette tip after overnight starvation. Cells were then incubated at 37°C with indicated treatments.
Results for continuous variables are presented as means ± SEM unless stated otherwise. Treatment groups were compared with the independent samples t test. Pairwise multiple comparisons were performed with the one-way ANOVA (2-sided) with Bonferroni adjustment. P < 0.05 was considered statistically significant. All analyses were performed using SPSS 17.0 (SPSS Inc.).
Additional experimental procedures and details can be found in the Supplementary Materials and Methods.
Macrophages promote PDAC CSC self-renewal, migration, and tumorigenesis
Apart from neutrophils, infiltrating macrophages represent one of the major immune cell types present in the high stroma-rich PDAC tumor microenvironment (Supplementary Fig. S1A; ref. 32). Thus, as it has been shown in other solid tumors that CSC properties can be promoted by microenvironmental factors (33, 34), we aimed to test whether macrophage-secreted factors could also enhance PDAC CSC phenotypes. Because macrophages are not static, but rather are highly plastic and can be differentially polarized into classically “activated”/“M1” macrophages or “alternatively activated,” “M2” or “protumorigenic” macrophages (reviewed in ref. 35), we therefore tested the effects of both M1 and M2 macrophages on PDAC CSCs. First, the self-renewal capacity of 2 different primary PDAC cultures was assessed by culturing Panc354 and Panc185 cells in anchorage-independent conditions and in the presence of control media or CM from M1-polarized or M2-polarized monocyte-derived macrophages. First- and second-generation sphere formation increased when cells were cultured in the presence of macrophage CM by about 1.5- to 2-fold, with the greatest increase observed when PDAC cells were cultured with CM from M2-polarized macrophages (Fig. 1A). In addition, we also observed an increase in the expression of the pluripotency-associated genes Klf4, Sox2, and Nanog, modulation of EMT-associated genes E-cadherin, Zeb-1, and vimentin (Fig. 1B), enhancement of the migratory capacity of PDAC cells when cocultured with CM from M2-polarized macrophages (Fig. 1C), and activation of pErk1/2, a mediator of prosurvival and proproliferation pathways, in treated sphere cultures (Fig. 1D).
Macrophages can also respond to cues from cancer cells and differentiate toward a pro-tumorigenic “M2” phenotype in response to tumor microenvironmental stimuli such as CSF1, IL4, IL13, TGFβ1, or IL10 (36). Therefore, we additionally treated macrophages with CM harvested from PDAC spheres, which are enriched in CSCs (Supplementary Fig. S1B) and factors such as TGFβ1, Nodal, and ActivinA (4). Using CD163 as a macrophage M2 marker (35), we observed that macrophages treated with CSC CM adopted a CD163 expression pattern similar to that of macrophages polarized with M-CSF to an M2 phenotype (Fig. 1E). Likewise, media removed from these CSC-primed macrophages were also able to enhance PDAC sphere formation, promote the expression of pluripotency-associated genes, and increase PDAC cell migration similar to CM from non–CSC-primed MCSF-treated M2-polarized macrophages (Fig. 1A and B and data not shown).
Finally, we injected 5 × 105 primary PDAC cells alone or with equal numbers of M1-polarized, M2-polarized, or CSC CM-primed primary human macrophages and assessed tumor growth over 8 weeks. Consistent with the aformentioned in vitro data, tumor growth was significantly accelerated when PDAC cells were coinjected with M2 macrophages or with macrophages prestimulated (i.e., “primed”) with media from CSC spheres (Fig. 1F and Supplementary Fig. S2). Thus, the sum of these data would suggest that an intricate and intimate crosstalk exists between CSCs and macrophages, where CSCs promote the polarization of macrophages toward an M2-like phenotype, which can then, in turn, promote the “stemness” and tumorgenicity of CSCs.
Macrophages increase the expression and secretion of ISG15 when cocultured with PDAC CSCs
We next cocultured monocyte-derived macrophages with and without primary PDAC cells in Transwell. Seventy-two hours after coculture, RNA was extracted from macrophages and microarray analyses were performed. Genes (n = 3,084) were significantly upregulated and 3,431 genes downregulated [false discovery rate (FDR) < 0.05] compared with control cultures. Of the top 25 upregulated genes (FDR < 10−4, |logFC| > 2), the majority of genes belonged to the family of ISG (Fig. 2A). Of the 19 ISGs detected, we focused on ISG15 as a gene of potential interest as it encodes for a protein that can function intracellularly to modify cytoplasmic and nuclear proteins, it can also be secreted from activated cells as free ISG15, and ISG15 has been shown to play a putative role in other solid tumors such as bladder, oral, prostate, and breast cancers (19–24). We confirmed the microarray results by RT-qPCR and Western blot analysis. Specifically, in monocyte-derived macrophage cultures treated with PDAC CSC sphere CM, we observed a strong increase in ISG15 mRNA and protein levels (conjugated and monomeric) compared with nontreated control macrophage cultures (Fig. 2B and C).
We also observed an increase in the ISG15 deconjugating enzyme USP18 (Fig. 2A and B), which functions to remove ISG15 from its conjugates, thus increasing the overall amount of monomeric ISG15 (37). We hypothesized that the increased amount of monomeric ISG15 present in macrophages treated with PDAC CSC-conditioned sphere media (Fig. 2C) would result in increased secretion of free ISG15. In accordance with this hypothesis, we observed an increase in free ISG15 in the supernatant of treated macrophages compared with nontreated controls (Fig. 2D). M2-polarized macrophages alone secreted more ISG15 than M1-polarized macrophages, and free ISG15 levels could be further and significantly enhanced by first priming macrophages with conditioned sphere media from two different primary cultures of PDAC CSC spheres (Fig. 2D).
These results strongly suggested that PDAC CSC conditioned sphere medium must contain type I IFNs, a potent stimulus of ISG15 expression (9). Because it has been shown that K-ras–transformed breast cancer tumors overexpress IFNβ (38), we next determined whether PDAC CSCs also produce/secrete IFNβ and whether it is biologically active using an ELISA for IFNβ and a vesicular stomatitis virus (VSV)–based antiviral assay, respectively. Both assays confirmed that PDAC CSCs (e.g., spheres) secrete IFNβ (Fig. 2E), it is biologically active (Fig. 2F) and it is the likely causative factor for the ISG15 activation observed in monocyte-derived macrophages.
To study the effect of macrophage-derived free ISG15 release on PDAC cells, we analyzed intracellular ISG15 levels (conjugated and nonconjugated) in adherent PDAC cultures and in CSC sphere cultures left untreated or treated with CM from 48-hour CSC-primed control, M1- or M2-polarized macrophages. Independent of treatment, intracellular monomeric and extracellular free ISG15 levels were significantly higher in CSC-enriched sphere-derived cultures compared with adherent cultures, which contain more differentiated cancer cells (Fig. 2G and H). Moreover, following treatment with M2-polarized macrophage CM, we observed a specific increase in intracellular ISGylation levels and a corresponding decrease in the nonconjugated form of ISG15, indicating that the high levels of free ISG15 released by M2 macrophages (Fig. 2D) act on PDAC CSCs, further enhancing the conjugation of monomeric ISG15 to target proteins (Fig. 2G). However, unlike macrophages, we did not observe differences in the levels of free ISG15 in the supernatant of PDAC cultures following treatment (Fig. 2H). Interestingly, the levels of the deconjugating enzyme USP18 were low to undetectable in PDAC cultures as determined by RT-qPCR analysis (data not shown), providing a possible explanation as to why these cells secrete little free ISG15, even after stimulation.
Primary PDAC tumors express ISG15
To assess whether ISG15 could represent an important mediator of cancer development, we first studied the relationship between ISG15 expression and cancer survival at the genomic level. Using various publically available microarray datasets from PrognoScan (http://www.abren.net/PrognoScan/), a database for meta-analysis of the prognostic value of genes (39), we found that across several tumor entities, higher expression of ISG15 was predictive of significantly lower overall survival (Fig. 3A and B). Because no PDAC datasets are publically available, we evaluated the expression of ISG15 by RT-qPCR in bulk PDAC tumor samples obtained from 30 surgical resections and by IHC analysis using PDAC tissue microarrays. Compared with 4 normal pancreas controls, overexpression of ISG15 mRNA was observed in the majority of PDAC samples evaluated (Fig. 3C). Regarding its expression at the protein level, we observed that ISG15 was expressed in about 95% of all the tumors analyzed; however, distinct differences were observed with respect to the level and type of cells expressing ISG15 (Fig. 3D). For example, while ISG15 expression was detected in neoplastic cells of about 17% of tumors, ISG15 was predominantly expressed by immune cells (e.g., TAMs) in the stroma of PDAC tumors (Fig. 3D and E, Supplementary Figs. S3, S4, and S5A). This was not the case for other tumors such as breast cancer and prostate cancer, in which tumor cells express the majority of ISG15 (Supplementary Fig. S5B and S5C).
ISG15 promotes CSC phenotypes
Because macrophages can secrete free ISG15 and as macrophage CM enhances the “stemness” of PDAC CSCs (Fig. 1), we reasoned that ISG15 might be a pro-CSC factor secreted by macrophages in response to cues from PDAC CSCs (e.g., IFNβ). To test this hypothesis, PDAC cultures were treated with recombinant ISG15 (rISG15). The first-generation sphere-forming capacity of CSCs from 3 primary PDAC cultures increased with rISG15 treatment compared with control-treated cultures (Fig. 4A), and the effect was more pronounced during serial passaging, which further enriches for CSCs (4). The increase in sphere formation also correlated with an overall increase in the expression of pluripotency-associated genes in Panc354 spheres treated with rISG15 during serial passaging compared with untreated cultures (Fig. 4B). In addition, rISG15 treatment also increased the migratory capacity of sphere-derived cells in a standard wound-healing assay as shown in Fig. 4C. Finally, similar to what we observed with M2 macrophage CM (Figs. 1D and 2G), rISG15 treatment of CSCs also increased the level of intracellular ISGylation (Fig. 4D) and the phosphorylation of the prosurvival protein Erk1/2 (Fig. 4E). Taken together, these data strongly suggest that ISG15 alone can potentiate the “stemness” of CSCs and thus, macrophages likely potentiate CSCs, in part, via an ISG15-mediated mechanism.
To further validate our findings, we took advantage of mice with a genetic inactivation of the ISG15 gene. Using murine monocyte-derived macrophages, isolated from ISG15-knockout mice, we show that compared with ISG15+/+ M2-polarized macrophages, the conditioned medium from ISG15−/− M2-polarized macrophage cultures did not similarly enhance murine PDAC sphere formation (Fig. 5A), the expression of stemness genes (Fig. 5B), their migratory capacity in a wound-healing assay (Supplementary Fig. S6), or the phosphorylation of Erk1/2 (Fig. 5D). Likewise, when ISG15−/− macrophages were coinjected with murine PDAC cells in vivo, we observed an intermediate tumor growth phenotype compared with murine PDAC cells injected with wild-type macrophages. Importantly, when primary murine PDAC cells were pretreated with rISG15 before coinjection with ISG15−/− macrophages, a phenotype similar to that seen with ISG15+/+ macrophages was achieved (Fig. 5E), indicating that the lack of secreted ISG15 is responsible for the impaired ability of ISG15−/− cells to promote PDAC tumor growth in vivo.
While these data strongly suggested that a driving factor responsible for PDAC tumorigenesis is ISG15, we next performed a limiting dilution cell transplantation assay to more rigorously determine the effect of ISG15 on the frequency of CSC-initiated tumor formation in vivo. Specifically, primary syngeneic murine PDAC cells were transplanted into recipient wild-type and ISG15−/− mice at increasing doses and tumor formation was determined 8 weeks postinjection. While tumors efficiently formed in wild-type mice at dilutions of 105 (8 of 8), 104 (6 of 8), and 103 (5 of 6) cells, tumor formation and growth in ISG15−/− mice were significantly impaired (Fig. 5E and F), and the frequency of CSC-initiated tumorigenesis in ISG15−/− mice was significantly lower than in wild-type mice (CSC frequency: 1/51,360 vs. 1/3,769, respectively; P = 0.0001; Fig. 5E, Table).
TAMs, also known as M2, “alternatively activated” or “protumorigenic” macrophages (reviewed in ref. 35), are the major cell type of the inflammatory infiltrates present in PDAC tumors (32, 40) and are believed to promote tumorigenesis, matrix remodeling, and metastasis (36, 41, 42). In accordance with the latter, we observed that compared with M1 macrophages, M2 macrophages were able to promote the self-renewal capacity, modulate the expression of pluripotency- and EMT-associated genes, increase the migratory potential, and enhance the tumorigenic capacity of PDAC CSC in a contact-independent manner. It is important to note, however, that we cannot discard the fact that human PDAC cells injected into nude mice may have activated murine macrophages to secrete protumor factors. Thus, the effects observed with human PDAC cells in nude mice may have cross-species contributions.
At the cellular level, primary human macrophages that were treated with CM from PDAC CSC cultures acquired M2-like properties at the level of cell morphology (data not shown) and expression of M2 cell surface markers, such as CD163, and media from these cultures enhanced the “stem-like” properties of PDAC CSCs similar to that of M2 macrophages. Thus, our data confirm that macrophages, specifically “M2/alternatively activated” macrophages, can significantly modulate the properties of PDAC CSCs via secreted factors, and PDAC CSCs can promote the polarization of macrophages toward an M2 phenotype. It is worth noting that the PDAC tumor microenvironment is composed of many other cell types, including T cells, neutrophils, and PSCs, which can also positively influence the “stemness” of PDAC CSCs as well as TAMs. Thus, the effects we observe in vitro are likely an underrepresentation of the cross-talk that exists within the more complex multicellular tumor niche in vivo.
While macrophages can secrete many prostimulatory factors, we indentified by microarray analysis that ISG15 and its deconjugating enzyme USP18 were highly upregulated in macrophages cocultured with PDAC CCSs. This apparent overexpression was not all that surprising as Kras-transformed tumors have been shown to overexpress IFNβ (38), a strong inducer of ISG15, and our IFNβ ELISA and VSV anti-viral assays both confirmed that PDAC cells produce soluble and biologically active IFNβ. Interestingly, a growing body of evidence over the past few years has shown a link between ISG15 and tumorigenesis for several solid tumors (19–24). The general conclusion from these studies is that ISG15 is overexpressed in many tumor cell lines and ISGylation is important for malignant transformation; however, the mechanism(s) by which ISG15 exerts its protumor effects and whether ISG15 functions the same in all solid cancer entities is still unknown. In line with these studies, our retrospective analysis of existing cancer microarray datasets with clinical outcome data confirms that for many solid tumors there exists a significant correlation between high ISG15 expression and poor overall survival, further strengthening the notion that ISG15 is indeed protumorigenic. While we do observe ISG15 expression in primary PDAC cultures (data not shown) and in neoplastic cells of patient-derived tumors, TAMs within the tumor stroma express significantly more ISG15 than other tumor-resident cells. Even more important, we observed that ISG15 is liberated from macrophages and its secretion increases when macrophages are polarized to a protumor “M2” state or primed with CM from PDAC CSCs CM. Thus, for PDAC, the source of intratumoral ISG15 is the tumor microenvironment (i.e., TAMs).
Supporting a protumor role for ISG15, we were able to increase the self-renewal capacity, expression of pluripotency-associated genes, and the activation of the p44/42 MAPK (ERK1/2) signaling in PDAC CSCs from 3 different primary patient-derived cultures using rISG15. We also observed a significant increase in the migration of PDAC cultures when treated with rISG15. ISG15 overexpression in breast cancer cell lines has also been linked to an EMT phenotype. Burks and colleagues, have shown that silencing ISG15 expression in the breast cancer cell line MDA-MB-231 reduced the migratory capacity of these cells compared with cells infected with an shRNA control lentivirus (24). Because TAMs are believed to contribute to EMT in solid tumors (43), promoting CSCs dissemination and metastasis, it is tempting to speculate that free ISG15 secreted from TAMs may play an important role in this process in PDAC.
Our model depicted in Fig. 6 illustrates how ISG15 is regulated within the PDAC tumor. Macrophages with the tumor microenvironment are polarized toward an M2 protumor phenotype via tumor cell secreted factors (e.g., TGFβ1). TAMs can then respond to other tumor microenvironmental stimuli, such as IFNβ, promoting the upregulation of ISG15 and USP18 mRNA and the subsequent secretion of free ISG15. Free ISG15 can then act on PDAC CSCs, enhancing their stem-like properties, including self-renewal and tumorigenicity. While our data support this putative model, 3 questions remain unanswered: (i) what receptor does free ISG15 bind to, (ii) how does free ISG15 exert its effects on PDAC CSC, and (iii) are the protumor effects of free ISG15 independent of intracellular ISGylation or are they interconnected? Unfortunately, the receptor for ISG15 is currently unknown. It is believed that free ISG15 acts via binding to a cell surface receptor rather than passive diffusion into the cell; however, until a receptor is discovered, this question remains a black box. Regarding how ISG15 exerts its effect(s), in this study we show for the first time that rISG15 can activate p44/42 MAPK (ERK1/2) signaling, a pathway that has been shown to be important for cancer cells. Appreciating that other pathways may also be modulated by ISG15, we are currently investigating whether the AKT–PI3K and mTOR/S6K pathways, both of which have been shown to be important in PDAC (44, 45), are also affected by free ISG15. Along these lines, previous work from our laboratory has shown a direct link between AKT signaling and ISG15 in macrophages (11). We have reported that ISG15 plays an important role in the regulation of macrophage functions as ISG15−/− macrophages display reduced activation, phagocytic capacity, and programmed cell death activation in response to vaccinia virus infection. This phenotype is independent of cytokine production and secretion but correlates with impaired activation of the protein kinase AKT in ISG15/− macrophages (11). Thus, as AKT signaling is very important for ISG15-mediated downstream effects in macrophages, it may very well also play an important role in ISG15-mediated enhancement of PDAC CSCs. In the end, understanding the pathways activated by free ISG15 should yield new insights into PDAC cell biology and possibly identify new targets that could be therapeutically inhibited.
Finally, we cannot currently separate the fact that intracellular ISGylation is likely also an active and important process in PDAC cells. The purpose of our study was to investigate the role of TAM-secreted factors on PDAC CSC features, and thus we primarily focused on the effects of free ISG15 on PDAC CSCs, rather than the role of intracellular ISG15 and ISGylated products. Our data do, however, suggest that macrophages increase intracellular ISGylation in PDAC CSCs via secreted free ISG15. Thus, apart from activation of p44/42 MAPK (ERK1/2) signaling in PDAC CSCs, it may very well be that another important mechanism of action of TAM-derived free ISG15 is to potentiate the conjugation of intracellular ISG15 to its target host proteins in PDAC CSCs, a process that has been shown to be beneficial for cancer cells of other tumor entities (22–24), but that has not been shown or studied to date in PDAC.
Finally, using ISG15-knockout mice, we show that the tumorigenic potential of murine PDAC cells coinjected with ISG15−/− macrophages was reduced compared with murine PDAC cells coinjected with wild-type ISG15+/+ macrophages. Assuming the later to be related to the fact that macrophages from ISG15−/− mice neither express nor secrete ISG15, we attempted to rescue the tumorigenic potential of murine PDAC cells coinjected with ISG15−/− macrophages by first pretreating these cells with recombinant ISG15. The result was a near-complete rescue, providing a plausible explanation for the reduced tumor growth observed when murine PDAC cells were injected with ISG15−/− macrophages compared with ISG15+/+ macrophages. It is important to point out that ISG15−/− macrophages were still able to potentiate PDAC CSCs, but to a significantly lesser degree compared with wild-type ISG15+/+ macrophages. This is not all that surprising as ISG15−/− macrophages may still secrete other protumor factors that could still enhance the stem-like and tumorigenic potential of PDAC cells even in the absence of ISG15. Even in our syngeneic experiments, we observed that murine PDAC CSCs were still able to form tumors in ISG15−/− mice, although smaller and with significantly less efficiency. Thus, while ISG15 is certainly important, it is likely not the only CSC-promoting factor secreted by macrophages and the tumor stroma; however, we did not test all CSC-specific phenotypes, such as metastasis, which may rely more on ISG15 than self-renewal or primary tumor initiation. Nonetheless, to the best of our knowledge, this is the first report showing that macrophages can secrete high levels of free ISG15 when polarized toward an M2 phenotype and PDAC CSCs can further potentiate its secretion, which in turn activates PDAC CSC stem-like properties. In summary, our study underscores the importance of the tumor microenvironment in CSC-mediated tumor biology and provides proof of principle that the stem-like nature of PDAC CSCs is strongly influenced by TAMs and their secreted factors, such as free ISG15. Thus, these findings not only advance our understanding of the role TAMs play in PDAC tumorigenesis, but they should also prove useful for future applications in cancer therapy, particularly those focused on targeting the tumor stroma.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Conception and design: B. Sainz Jr, C. Heeschen, S. Guerra
Development of methodology: B. Sainz Jr, B. Martín, M. Tatari, S. Guerra
Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): B. Sainz Jr, S. Guerra
Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): B. Sainz Jr, C. Heeschen, S. Guerra
Writing, review, and/or revision of the manuscript: B. Sainz Jr, C. Heeschen, S. Guerra
Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): M. Tatari
Study supervision: B. Sainz Jr, C. Heeschen, S. Guerra
The authors thank the UAM Animal facility for help with all of the in vivo experiments, Raquel Pajares for IHC assistance, and Daniela Cerezo for helpful discussions. They are grateful to those collaborators listed in Materials and Methods for kindly providing important reagents.
This work was supported by grants from the Spanish Ministry of Health FIS2011-00127, Bayer Group Grants4Grants (ID 2013-08-0982) and UAM-Banco de Santander to S. Guerra and ERC Advanced Investigator Grant (Pa-CSC 233460), the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 256974 (EPC-TM-NET) and no. 602783 (CAM-PaC), the Subdirección General de Evaluación y Fomento de la Investigación, Fondo de Investigación Sanitaria (PS09/02129 and PI12/02643) and the Programa Nacional de Internacionalización de la I+D, Subprogramma: FCCI 2009 (PLE2009-0105; both Ministerio de Economía y Competitividad, Spain) to C. Heeschen.