In a model of colitis-associated cancer, inhibition of IKKβ in either enterocytes or myeloid cells dramatically decreases tumor incidence. Indpendently of IKKβ inhibition, tumors in this model contain mutations in exon 3 of β-catenin leading to activation of the β-catenin/TCF-4 pathway. The cover image taken with a Zeiss Axioscope shows nuclear accumulation of c-Myc in epithelial cells of a colonic tumor as a result of this activation. IKKβ deficiency does not affect c-Myc expression in established tumors, but it influences early stages of tumor development to decrease tumor numbers, thus revealing the importance of IKKβ and NK-κB activation during tumor promotion in an inflammation-associated malignancy.
许多年来，炎症一直被怀疑和癌症密切相关，因为反复发生的炎症作用和慢性感染会大大提高癌症的发病率。而且，能够抑制NF-κB以及其他炎症调节因子的非固醇类抗炎药物（non-steroidal anti-inflammatory drugs, DSAIDs）也被采用，来降低胃肠道癌症的发病风险。不过，一直没有任何有力的证据来证实这种猜想。
最新的一期《cell》的封面文章，报道了University of California, San Deigo的科学家的新发现，证明了IKKβ是连接炎症和肿瘤的桥梁。
患有慢性大肠炎的患者，极易诱发大肠炎相关性癌症（colitis associated cancer，CAC）。实验中，研究者利用了小鼠CAC作为研究模型，通过利用人为注射化学药剂诱发炎症反应和随之而来的肿瘤。
由于转录因子NF-κB是炎症过程中的重要作用，而IKKβ又对NF-κB的活化起决定性的作用。实验除了测试正常的小鼠试验组，还有研究了两组经过遗传改造的小鼠：一组小鼠的肠道上皮细胞缺乏IKKβ，另一组小鼠的骨髓细胞（myeloid cells）缺乏IKKβ——骨髓细胞能够产生在炎症反应中起重要作用的巨噬细胞（macrophage）。
观察发现，上皮细胞缺乏IKKβ后，虽然NF-κB无法活化，但炎症反应依然发生，不过肿瘤的发生几率却下降了80% 。进一步对组织进行生化研究表明，NF-κB的活化抑制提高了促凋亡蛋白Bax 和 Bak的表达，同时降低抑制凋亡的BclxL。这样，细胞凋亡过程被加强，所以肿瘤的发生明显减少。
截然不同的是，在骨髓细胞缺乏IKKβ的小鼠中，许多对炎症过程有重要作用的基因的表达都降低。由于NF-κB不能活化，肿瘤发生几率下降了50%，而且，即使出现肿瘤生长，其大小也比正常的小鼠要小75%。那么，骨髓细胞中的IKKβ的缺乏对细胞凋亡有什么影响呢？结果是没有明显影响。
两组实验的结果显示，上皮细胞中的IKKβ通过对NF-κB途径的活化，从而抑制细胞凋亡来诱发肿瘤。值得注意的是，骨髓细胞中的IKKβ对NF-κB途径的活化，促进炎症的发生，而炎症又对肿瘤的发生和生长有重大影响。
IKK Links Inflammation and Tumorigenesis in a Mouse Model of Colitis-Associated Cancer
Abstract
A link between inflammation and cancer has long been suspected, but its molecular nature remained ill defined. A key player in inflammation is transcription factor NF-B whose activity is triggered in response to infectious agents and proinflammatory cytokines via the IB kinase (IKK) complex. Using a colitis-associated cancer model, we show that although deletion of IKK in intestinal epithelial cells does not decrease inflammation, it leads to a dramatic decrease in tumor incidence without affecting tumor size. This is linked to increased epithelial apoptosis during tumor promotion. Deleting IKK in myeloid cells, however, results in a significant decrease in tumor size. This deletion diminishes expression of proinflammatory cytokines that may serve as tumor growth factors, without affecting apoptosis. Thus, specific inactivation of the IKK/NF-B pathway in two different cell types can attenuate formation of inflammation-associated tumors. In addition to suppressing apoptosis in advanced tumors, IKK may link inflammation to cancer.
Figure 1. Enterocyte-Specific Deletion of IKK Decreases Tumor Incidence(A) Schematic overview of the CAC model; each rectangle represents one week. After initial AOM injection (12.5 mg/kg), DSS was given in drinking water (gray areas) followed by regular water.(B) Tumor incidence in IkkF/Δ and villin-Cre/IkkF/Δ mice (n ≥ 9, p < 0.001)(C–J) H&E stainings of tumor morphology.(C and D) Overview of representative sections of the "Swiss-rolls" that were used for tumor counting. Tumors are marked by black lines.(E and F) Representative tumors, lines mark borders between adenomas (T) and normal epithelium, 40× magnification.(G and H) Adenomas show infiltration with inflammatory cells and ulceration on the luminal surface, 100× magnification(I and J) Nuclei of adenomas (T) show pseudostratification and increased mitotic figures relative to normal tissue (N), 400× magnification.(K and L) Tumor apoptotic and proliferation indices were determined by TUNEL and BrdU staining, respectively.(M) Histogram showing size distribution of tumors. Tumor areas were determined by computerized image analysis.
Figure 2. Tumors Harbor -Catenin Mutations and Exhibit Activation of the -Catenin Pathway(A) Mutations within exon 3 of the -catenin gene. DNA was eluted from microdissected tumor cells. Exon 3 that codes for the GSK-3 phosphorylation sites (boxed exons) was amplified and sequenced. Codons containing mutations are in bold.(B and C) Activation of -catenin. Immunohistochemical analysis of -catenin in normal epithelium (B) and in tumors (C).(D and E) Immunhistochemical analysis of c-Myc in normal epithelium (D) and in tumors (E).(F and G) Immunohistochemical analysis of Cyclin D1 in normal epithelium (F) and in tumors (G).(H) Analysis of IKK deletion in tumor DNA. DNA from microdissected normal epithelium of villin-Cre/IkkF/Δ mice (a), normal epithelium and tumors from IkkF/Δ mice (b) and from tumors of villin-Cre/IkkF/Δ mice (c) was analyzed by PCR for deletion of Ikk exon 3. ΔCt values were obtained by subtracting Ct values of two other genes not affected by the deletion from the Ikk Ct values
Figure 3. Increased Colonic Inflammation in villin-Cre/IkkF/Δ Mice Challenged with High Levels of DSS(A) Weight loss during DSS (3.5%) colitis in IkkF/Δ and villin-Cre/IkkF/Δ mice.(B) Histological damage (n ≥ 7, p < 0.05) and (C) ulcer number (n ≥ 7, p < 0.05) in mice treated with 3.5% DSS.(D) Histology of untreated IkkF/Δ and (E) and untreated villin-Cre/IkkF/Δ mice. Representative histologies of (F) IkkF/Δ mice and (G) villin-Cre/IkkF/Δ mice five days after the termination of DSS administration.
Figure 4. Increased Expression of Proinflammatory Factors during DSS-Colitis in villin-Cre/IkkF/Δ Mice(A) Expression of inflammatory genes. Relative mRNA expression levels were examined in whole colon tissues of mice at day 15 of the CAC regimen, 5 days after DSS (2.5%). The levels of the indicated mRNAs were quantitated by real-time PCR and normalized to the level of cyclophilin mRNA (n ≥ 3).(B) NF-B binding activity, COX-2 and MMP-9 expression in whole colonic extracts prepared at days 0 and 15 of the CAC regimen five days after DSS (2.5%) challenge. NF-B binding activity was determined by EMSA. COX-2 and MMP-9 expression were determined by Western blotting (WB).(C and D) BrdU incorporation into intestinal epithelial cells at day 15 of the CAC regimen, five days after termination of DSS challenge.
