In this issue of Cancer Discovery, Biffi and colleagues report that IL1 signaling cascades resulted in JAK/STAT activation and promoted an inflammatory cancer-associated fibroblast (iCAF) state, which contributed to the establishment of distinct fibroblast niches in the pancreatic ductal adenocarcinoma (PDAC) microenvironment to support the growth of PDAC cells. Furthermore, the investigators demonstrated that TGFβ signaling inhibited IL1R1 expression, antagonized IL1α responses, and promoted differentiation of CAFs into myofibroblasts; thus, IL1α signaling is an important therapeutic target for both PDAC cells and the iCAFs in the tumor microenvironment.
See related article by Biffi et al. p. 282.
Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer mortality in the United States, with 55,440 new cases estimated in 2018 by the American Cancer Society, and the 5-year survival rate has remained at 6% for the past 50 years. PDAC is projected to surpass breast, prostate, and colorectal cancers as the second leading cause of cancer-related deaths by 2030 (1). Approximately 80% of patients with PDAC present with locally advanced or metastatic therapy-resistant disease at the time of diagnosis, and the median survival after diagnosis is 6 months. Despite major advances, current chemotherapy and radiotherapy regimens remain largely ineffective, as most PDACs are resistant to standard therapy. Hence, a better understanding of the molecular basis of PDAC inception and development is crucial to identifying effective therapies to improve patient survival, which is currently one of the greatest challenges in pancreatic cancer research.
Genetic and molecular profiles of PDAC are emerging and include some of the following major alterations (2). Mutational KRAS activation is an early event in pancreatic carcinogenesis and is detected in 80% to 95% of PDAC cases, whereas inactivation of SMAD4 and TP53 tumor suppressor genes is identified in approximately 50% of PDAC cases. Additionally, constitutive NF-κB activation is found in nearly 70% of PDAC, most PDAC cell lines, and in many other tumor types (3). IL1 includes two major agonistic pleiotropic ligands, IL1α and IL1β. IL1β is a secreted active ligand, whereas IL1α is active in the cell membrane–bound form. IL1α is overexpressed in many types of cancers and plays a key role in tumorigenesis and metastasis (4). Knockdown and knockout of IL1a or IKK2 inhibit NF-κB activation and tumorigenesis in PDAC cells. NF-κB activation in RAS-transformed cells is induced by IκB kinase (IKK) and by autocrine IL1α/p62 feed-forward pathways (5). These results demonstrate that NF-κB activation by IL1α is required for mutant KRAS–induced tumorigenesis. Activated NF-κB integrates proinflammatory signals and orchestrates antiapoptotic responses; thus, NF-κB signaling has been implicated as a hallmark of cancer development and a potential therapeutic target (6). Previously, Ohlund and colleagues (7) demonstrated that pancreatic stellate cells (PSC) in the tumor microenvironment differentiated into two subtypes of cancer-associated fibroblasts (CAF): inflammatory CAFs (iCAF) and myofiboblast CAFs (myCAF). The current finding of Biffi and colleagues shows that inhibition of NF-κB activation impaired the ability of tumor organoid–conditioned media to induce inflammatory CAF marker genes in PSCs, which suggests that NF-κB activation is required for the formation of iCAFs (8). They also demonstrate that IL1α-induced autocrine stimulation of LIF in PSCs activates JAK/STAT signaling and enhances iCAF formation. Therefore, their new findings provide an important mechanism of the IL1α signaling cascade in inducing iCAF in PDAC development and strengthen the rationale for therapeutic targeting of IL1α signaling in both PDAC and the tumor microenvironment (Fig. 1).
The SMAD4 tumor suppressor gene encodes a transcription factor that is a central effector of TGFβ and is frequently deleted or mutated in PDAC, resulting in defective TGFβ signaling (9). The activated KRASG12D allele with concomitant haploinsufficiency of expression of SMAD4 tumor suppressor gene engenders a distinct class of pancreatic tumors, mucinous cystic neoplasms, which culminate in invasive ductal adenocarcinomas and evolve along a progression scheme analogous to, but distinct from, the classic PanIN-to-ductal adenocarcinoma sequence (10). In this report, Biffi and colleagues identified that TGFβ signaling antagonized IL1α-induced JAK/STAT signaling in myCAFs. Furthermore, the results from Biffi and colleagues showed that the inhibition of the TGFβ or JAK/STAT pathway shifted iCAFs to myofibroblastic phenotype in vivo (ref. 8; Fig. 1). The authors demonstrated that activation of the NF-κB pathway was associated with the expression of various cytokines and chemokines in iCAFs, which may play a role in recruiting inflammatory cells into PDAC.
The combined mouse genetic and functional studies using organoid cultures have revealed that IL1 signaling cascades result in JAK/STAT activation and promote an inflammatory CAF state, facilitating the establishment of distinct fibroblast niches in the PDAC microenvironment to support PDAC development (Fig. 1). Moreover, in PDAC cells, loss or mutation of SMAD4, a common event in PDAC development, plays a key role in the progression of KRAS-initiated PDAC in the Pdx1-Cre; KrasG12D; Smad4L/L PDAC mouse model by enhancing activation of the NF-κB signaling pathway, which in turn increases the expression of proinflammatory cytokines and chemokines that modulate the PDAC tumor microenvironment. Thus, inactivation of TGFβ signaling intensified IL1α signaling cascades in PDAC cells. However, significant gaps still exist in our understanding of how such genetic alterations act accordingly to induce activation of NF-κB and JAK/STAT signaling for PDAC development and progression. This question has now been answered in part by the findings of Biffi and colleagues.
These new findings raise several important questions. (i) Can the IL1R signaling pathway be effectively inhibited by recombinant IL1RA, IL1R antibodies, and IL1R chemical inhibitors in patients? (ii) Can the tumorigenesis of PDAC cells be further reduced with a combination treatment with IL1 inhibitors and a chemotherapeutic agent? (iii) Is there an IL1α-independent pathway responsible for the development of PDAC-related inflammatory responses and iCAF? (iv) What is the expression profile of various NF-κB–dependent cytokines and chemokines in the epithelial cells expressing mutant KRAS? (v) How is the cooperation between tumor cells and their inflammatory surroundings orchestrated? (vi) Do the high levels of cytokines and chemokines in the tumor microenvironment further amplify the NF-κB activation through the IL1α/p62 feed-forward mechanism in TGFβ-deficient cells to promote PDAC development? (vii) Is there an IL1α autocrine stimulatory pathway in myCAF cells?
The stroma consists of cellular components, predominantly CAFs, immune cells, and a rich extracellular matrix (ECM), many of which closely interact with cancer cells to promote growth and metastasis. The activation of the autocrine and paracrine oncogenic signaling pathways by stroma-derived growth factors and cytokines has been implicated in promoting tumor cell proliferation and metastasis, indicating that targeting tumor–stroma interactions is a promising strategy in the search for novel treatment modalities in human cancer. Comprehensive analysis of the PDAC genome and evaluation of the profiles of various signaling pathways are needed to identify potential therapeutic targets, such as IL1 and NF-κB pathways, in a large human PDAC sample. The findings by Biffi and colleagues have provided an enlightening understanding of PDAC heterogeneity, and this knowledge is important for developing therapeutic approaches that selectively target tumor-promoting CAFs. The discovery of the role of the IL1α signaling pathway in promoting iCAFs that function in CAFs of the tumor microenvironment may lead to the identification of novel and effective therapeutic targets. Thus, directly inhibiting the key cytokine signaling pathways in PDAC harboring SMAD4 deficiencies presents a promising application for targeted therapy. IL1-induced signaling cascades lead to JAK/STAT activation and promote an inflammatory CAF state, which suggests that there may be multiple possibilities in targeting these cells in vivo, such as the JAK/STAT and NF-κB signaling pathways for generating inflammatory CAFs. Further, TGFβ antagonizes this process by downregulating IL1R1 expression and promoting differentiation into myofibroblasts. These results provide a mechanism by which Smad4 loss and IL1 overexpression are altered in PDAC cells. Such molecular hallmarks cooperate to accelerate pancreatic cancer development by enhancing NF-κB activation and upregulating downstream cytokine genes, thus promoting a protumorigenic and metastatic microenvironment. Importantly, the findings by Biffi and colleagues offer unique therapeutic opportunities through the development of drugs that specifically target and inhibit well-defined inflammatory pathways in both PDAC cells and the iCAFs (Fig. 1).
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
P.J. Chiao is supported by a grant from the NIH/NCI (CA207031). We thank Dr. Huaiqiang Ju for help with graphics.