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Dysregulation of integrin-linked kinase (ILK) signaling during colorectal carcinogenesis : modulation of signaling pathways by non-steroidal anti-inflammatory drugs (NSAID) Marotta, Anthony

Abstract

One of the most common events involved in the development of human colon cancer is the mutation of the adenomatous polyposis coli (APC) gene. This protein through its interactions with Axin and GSK3β, serves to regulate the cytosolic levels of β-catenin. Mutation of the APC gene, which impairs complex formation, results in the stabilization of β-catenin. Stabilization is believed to coincide with the translocation of β-catenin to the nucleus, where it up-regulates a number of genes implicated in oncogenesis. Interestingly, stable over-expression of the integrin-linked kinase (ILK) in rat intestinal epithelial cells has been demonstrated to modulate p-catenin sub-cellular localization and function. However, the significance of this finding in human colorectal carcinogenesis is unclear. To determine if ILK signaling was disrupted in colorectal carcinogenesis, this signaling pathway was characterized during various stages of development beginning with the earliest lesion, the adenomatous polyp. The results from these studies demonstrated that ILK was significantly overexpressed and exhibited an increased phosphotransferase activity in polyps resected from patients diagnosed with familial adenomatous polyposis. Changes in ILK activity reflected changes on downstream targets, predominantly GSK3β. In addition to this, dramatic increases in ILK immunoreactivity were observed in all abnormal crypts from sporadic polyps, when compared with the normal appearing crypts, within the same resected specimens. To delineate whether these changes in ILK signaling could be generalized for colon cancer, this signaling nexus was also investigated in both primary lesions as well as secondary deposits within regional lymph nodes. The results from these studies demonstrate that ILK was significantly hyperexpressed in malignant acini from either the primary or secondary site in relation to the normal crypts within the same lesion. Furthermore, over-expression of the ILK protein coincided with an increase in the MBP phosphotransferase activity of the immunoprecipitated ILK in colon cancer in approximately 63% of the primary lesions examined. In addition to this, the data indicated that there was a direct correlation between the protein expression of ILK and the protein levels of Lef-1 in the cases of colon cancer that were analyzed. As aspirin and sulindac have been demonstrated to elicit chemopreventative effects in colon cancer, I tested whether non-steroidal antiinflammatory agents targeted the ILK signaling nexus in vivo. Both of these drugs inhibited the serum-induced activation of ILK and PKB, modulated serine-9 phosphorylation on GSK3β, and down-regulated Tcf-4 transcriptional activity. In addition to this, sulilndac was shown to also inhibit another protein kinase that is known to influence p-catenin, protein kinase CK2. Furthermore, the data demonstrated that over-expression of ILK, PKB or CK2 in a cell culture system, inhibited NSAID mediated apoptosis. In conclusion, dysregulation of the ILK signaling nexus appears to be an early event during the development of colon cancer and it is possible that selective inhibition of this kinase might be an important chemopreventative/chemotherapeutic strategy in the colon.

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