Inflammation driving cancer development
There is a strong connection between cancer and inflammation. Cancer occurs due to accumulation of genetic mutations and it is fostered by chronic inflammation that can drive proliferation and promote mutations, for example, trough radical production.
Furthermore, an inflammatory microenvironment helps malignant cells to escape the immune surveillance mechanisms (Hanahan & Weinberg, 2011). Although acute inflammation helps modulating immune system against tumor cells, chronic inflammation supports tumor progression, growth, metastasization and immune evasion (Shalapour & Karin, 2015). Many inflammatory pathways including the inflammasome were associated with cancer development and progression. However, the role of the inflammasome is still controversial as there are many publications showing contrasting findings (Terlizzi et al., 2014).
Some gastrointestinal tumors are associated with chronic inflammation. A dysregulated inflammasome activity, accompanied by high IL-1β secretion, was reported in a model of colorectal cancer (Kolb, Liu, Janowski, Sutterwala, & Zhang, 2014). However, IL-18 was found to be rather protective by inducing repair of colonic tissue after injury (Dupaul-Chicoine et al., 2015). In another work about inducible colorectal cancer, the authors show that NLRP3, ASC, and caspase-1 knockout mice had a more severe phenotype because of recurrent colitis and associated tumorigenesis (Allen et al., 2010). Controversially, in another study NLRP3 and caspase-1 knockouts had instead decreased DSS-induced inflammation and colitis (Bauer et al., 2010). These opposite results were rather confusing and did not explain the actual role of the inflammasome in gastrointestinal cancers. However, one study convincingly explained, by using a DSS induced colitis models, that the composition of the gut microbiota determines the disease severity and the relative contribution of the inflammasome to inflammation and carcinogenesis. The authors report that ASC knockout females can transmit an enhanced colitis phenotype to wildtype pups through the transmission of a colitogenic microflora (Elinav et al., 2011). The differences in the gut microbiome could be the underlying effect that determined the discordant findings obtained for these models.
The inflammasome was also associated with development and progression of skin cancer. In one study, the authors found that human melanoma cells secrete IL-1β because of a constitutively assembled NLRP3 inflammasome (Okamoto et al., 2010).
In a second study, the role of ASC was central in melanoma progression. When ASC is expressed in non-metastatic melanoma cells, it reduces expression of NFκB and reduces tumorigenesis. In metastatic cells, ASC increases NFκB signaling and increases pro-IL-1β transcription which is cleaved and secreted by the inflammasome thus enhancing tumor progression (W. Liu et al., 2013). A third study of a murine model of induced skin-carcinogenesis, showed a differential role for ASC depending on the cell type. By using conditional ASC knockout mice, the authors found that ASC has a tumor-promoting role in myeloid cells by taking part to the inflammasome with consequent IL-1β secretion, driving inflammation. In keratinocytes, it is a suppressor gene, as cancer cell progression is significantly delayed when ASC is knocked out in these cells. The authors finally indicate a physical interaction between ASC and p53 as the basis for the cancer cell-intrinsic phenotype (Drexler et al., 2012).
As our research group works prevalently with immune cells we became interested in exploring the role of inflammasome in hematologic malignancies. There are few papers so far that explored in this direction. However, it is a matter of fact that patients suffering from several types of cancers, leukemia included, have high peripheral blood levels of inflammatory cytokines, two of which are IL-1β and IL-18. It was therefore proposed to test IL-1β blocking agents as an anticancer treatment (Dinarello, 2011).
In one study, the authors suggest that NLRP3 helps ALL cells to escape glucocorticoid receptor blockade (Paugh et al., 2015). In another one, the oncosuppressor protein PML was proposed to interact physically with NLRP3 to license full inflammasome activation. The authors proposed PML as a new target for inflammasome related diseases (Lo et al., 2013). A recent work from our collaborators, is the first to put the inflammasome in close relation to the progression of acute myeloid leukemia (AML).
The necroptosis-related kinase RIPK3 restricts malignant proliferation by activating the inflammasome, which promotes differentiation and cell death through IL-1beta in a mouse model of AML (Höckendorf et al., 2016). The link between inflammasome and cancer is still under debate and more studies are required to clarify it.
ASC as a tumor suppressor?
Although the role of the inflammasome in cancer needs further investigation, there is also another research line that, even before the discovery of the inflammasome, investigated the role of ASC as oncosuppressor gene. The ASC protein was discovered independently by two research groups in the year 2000. The group of Jyunji Sagara at the Shinshu University of Nagano, coined the name “ASC” which stands for: apoptosis-associated speck-like protein containing a CARD (Masumoto et al., 1999). This work also opened the research line that led the discovery of the inflammasome. In parallel, the group of Paula Vertino at Emory University, identified and named it “target of methylation-induced silencing 1” (TMS1) as the gene’s promoter was found hypermethylated in primary human breast cancer cells (Conway et al., 2000). After this publication, other work showed that ASC/TMS1 has a CpG island in proximity of its promoter that is frequently hypermethylated in primary cells and cell lines of different kind of tumors including brain (Grau et al., 2010; Martinez, Schackert, & Esteller, 2007; Stone et al., 2004), lung (Virmani et al., 2003), liver (C.
Zhang et al., 2007), gastrointestinal tract (Ohtsuka et al., 2006), prostate (Collard, Harya, Monzon, Maier, & O'Keefe, 2006; Das et al., 2006), and skin (Guan et al., 2003). Despite these many in vitro reports, only two studies reported in vivo results so far, and were conducted by using ASC deficient mice. One study identified ASC as pro-oncogenic in a murine model of medulloblastoma (Knight et al., 2014). A second work, already mentioned in the previous paragraph, suggests that ASC has an oncosuppressive role in keratinocytes and a pro-oncogenic one in myeloid cells in model of induced skin-carcinogenesis (Drexler et al., 2012).
These results led the scientific community to think that ASC may have a dual function, one potentially independent from the other. In the first one, it serves as the inflammasome adaptor to trigger IL-1β secretion and promote pyroptotic cell death. In the other function, it might contribute to apoptosis by interacting with other pro-apoptotic proteins, like BAX, BID, caspase-8 and p53. This hypothesis has been recently described in a review article that capitulates the story of ASC/TMS-1 in both pyroptosis and apoptosis (Salminen, Kauppinen, Hiltunen, & Kaarniranta, 2014). The first research branch of ASC, brought important findings like the inflammasome and pyroptosis. The second branch, mainly focusing on apoptosis, is still under question and has not generated breakthrough findings so far.
There are therefore three different aspects to consider. First, ASC might contribute in controlling cell death pathways other than pyroptosis, and this can be apoptosis.
Second, a major factor in cancer development is the suppression of cell death pathways which leads to tumor expansion (Hanahan & Weinberg, 2011). Third, ASC was found blocked in its expression in several types of cancers cells. For these reasons, it appears possible that cancer cells may require to block ASC as either an inflammasome or an apoptotic adaptor. Parallelly, ASC could have a role in promoting cancer as those inflammasome-competent cells nearby the tumoral area may boost neoplastic growth by undergoing pyroptosis and secreting IL-1β while fighting the malignant cells. In this thesis work, we tried to contribute to this research issue and investigated the role of ASC in hematologic malignancies.
Figure 2: A diagram of the physical interactions of ASC/TMS1 in cell death pathways. Inspired and adapted from Salminem et al. (2014), Cellular and Molecular Life Sciences, 71 (10).