Functional studies on colorectal cancer cells indicated a protective role of the interferon‐inducible dsDNA sensor Absent in Melanoma 2 (AIM2) in cancer progression. Given that a high mutation rate and lack of AIM2 expression was previously detected in a subset of colorectal cancers, we here investigated the association of AIM2 expression in tumor cells and patient prognosis (5‐year follow‐up). A tissue microarray analysis of 476 matched tissue pairs (colorectal tumor and adjacent normal colon epithelium) was performed by two independent observers. Samples from 62 patients were excluded because of missing follow‐up information or due to neo‐adjuvant therapy before tissue sampling. Out of the remaining 414 tissue pairs, 279 (67.4%) displayed reduced AIM2 expression in cancer cells when compared to epithelial cells of their normal counterpart. Thirty‐eight patients (9.18%) had completely lost AIM2 expression in tumor cells. After adjustment for sex, age, cancer stage, tumor site, tumor grade and chemotherapy, complete lack of AIM2 expression was associated with an up to 3‐fold increase in overall mortality (HR = 2.40; 95% CI = 1.44–3.99) and disease specific mortality (HR = 3.14; 95% CI = 1.75–5.65) in comparison to AIM2‐positive tumor samples. Our results demonstrate that lack of AIM2 expression is closely associated with poor outcome in colorectal cancer. The data thus strongly substantiate a protective role of AIM2 against progression of colorectal tumors. Further studies are required to assess whether lack of AIM2 expression may be used as a biomarker for the identification of colorectal cancer patients with poor prognosis.

Absent in melanoma 2 (AIM2) is a member of the interferon‐inducible HIN‐200 protein family. Recent findings point to a role of AIM2 function in both inflammation and cancer. In response to foreign cytoplasmic DNA, AIM2 forms an inflammasome, resulting in caspase activation in inflammatory cells. Moreover, AIM2 reduces breast cancer cell proliferation and mammary tumor growth in a mouse model and shows a high frequency of frameshift mutations in microsatellite unstable (MSI‐H) gastric, endometrial and colorectal cancers. However, the consequences of AIM2 restoration in AIM2‐deficient colon cancer cells have not yet been examined. Using different constructs for expression of AIM2 fusion proteins, we found that AIM2 restoration clearly suppressed cell proliferation and viability in HCT116 cells as well as in cell lines derived from other entities. In contrast to previous reports from breast cancer cells, our cell cycle analyses of colon cancer cells revealed that AIM2‐mediated inhibition of cell proliferation is associated with accumulation of cells at late S‐phase, resulting in G2/M arrest. The latter correlated well with upregulation of cyclin D3 and p21^Waf1/Cip1 as well as with inhibition of cdc2 activity through Tyr‐15 phosphorylation. Furthermore, AIM2 restoration affected the adhesion of colorectal cancer cells to fibronectin and stimulated the invasion through extracellular matrix‐coated membrane in transwell assays. Consistent with this phenotype, AIM2 induced the expression of invasion‐associated genes such as VIM and MCAM, whereas ANXA10 and CDH1 were downregulated. Our data suggest that AIM2 mediates reduction of cell proliferation by cell cycle arrest, thereby conferring an invasive phenotype in colon cancer cells.

The serine/threonine kinase AKT plays a critical role in controlling the balance between cell survival and apoptosis. Several reports implicated AKT in the molecular pathogenesis of different human malignancies and overexpression of AKT was recently demonstrated to be an early event in colorectal carcinogenesis. We report here the identification of nine putative Tcf/Lef-binding elements (TBEs) upstream to the ATG initiation site of the AKT1 gene. Four of these TBEs are located upstream of the transcriptional start, whereas five TBEs are situated in Exon 1 of the AKT1 gene. Accordingly, we hypothesized that AKT1 expression might be regulated by Wnt/β-catenin signaling. To elucidate the regulation of AKT expression in colon cancer cells, we generated reporter constructs containing the luciferase gene under the control of different regions derived from the AKT1 promoter/enhancer. Transient expression of the constructs in colorectal cancer (CRC) cell lines resulted in significant activation of the reporter gene. Luciferase was stimulated 20- to 50-fold in SW480, SW948 and HCT116 CRC cells. In contrast, the AKT1 promoter/enhancer constructs showed only a weak response in 293 embryonic kidney cells. Coexpression of a constitutively active β-catenin mutant in colon cancer cells further enhanced reporter gene activation from the AKT1 promoter/enhancer, whereas it was downregulated by introduction of either wild-type APC or dnTcf-4. In addition, immunohistochemical staining of tumor sections derived from CRC patients showed elevated expression levels of AKT1, correlating with enhanced cytoplasmic/nuclear expression of β-catenin. In summary our data suggest that β-catenin/Tcf contributes to the transcriptional regulation of the AKT1 gene.