Src Is Dephosphorylated at Tyrosine 530 in Human Colon Carcinomas

Objective: Src is a protein tyrosine kinase that plays important roles in cancer development, and Src kinase activity has been found to be elevated in several types of cancers. However, the cause of the elevation of Src kinase activity in the majority of human colon carcinomas is still largely unknown. We aim at finding the cause of elevated Src kinase activity in human colon carcinomas. Methods: We employed normal colon epithelial FHC cells and examined Src activation in human colon carcinoma specimens from 8 patients. Protein expression levels were determined by Western blotting, and the activity of Src kinase by kinase assay. Results: Actin levels were different between tumor and normal tissues, demonstrating the complexities and inhomogeneities of the tissue samples. Src kinase activities were increased in the majority of the colon carcinomas as compared with normal colon epithelial cells (range 13-29). Src protein levels were reduced in the colon carcinomas. Src Y530 phosphorylation levels were reduced to a higher extent than protein levels in the carcinomas. Conclusion: The results suggest that Src specific activities were highly increased in human colon carcinomas; phosphorylation at Src Y530 was reduced, contributing to the highly elevated Src specific activity and Src kinase activity.