Regulation of vascular endothelial growth factor expression in human colon carcinoma cells by activity of src kinase

Abstract

Background. The c-src protooncogene encodes a protein tyrosine kinase, pp60 c-src, that is a mediator in many signal transduction pathways. One pathway in which pp60 c-src protein tyrosine kinase activity is implicated involves regulation of vascular endothelial growth factor (VEGF), an angiogenic factor important to neovascularization of growing tumors. Recently we demonstrated that decreased activity of pp60 c-src in colon tumor cells contributes to decreased expression of VEGF. This study examined the relationship between pp60 c-src activation, cell density, and VEGF production in a colon tumor cell line. Methods. Parental HT-29 colon adenocarcinoma cells and stable subclones created by transfection with c-src antisense and sense (control) expression vectors were plated under sparse (2 × 10 4 cells/cm 2) and confluent (20 × 10 4 cells/cm 2) conditions and grown for 36 hours. Protein and RNA were extracted from cells to determine pp60 c-src levels, c-Src tyrosine kinase activity, and VEGF mRNA expression. Results. The pp60 c-src kinase activity of HT-29 cells and control sense-transfected clones grown under confluent conditions was increased threefold to fivefold compared with cells grown under sparse conditions. In contrast, the ability of confluent culture conditions to increase pp60 c-src activity was blunted in antisense transfectants. By regression analysis, VEGF expression was found to vary directly with pp60 c-src levels (r 2 = 0.886). Conclusions. Cell density contributes to the regulation of c-src kinase activity and VEGF expression in HT-29 cells. When the steady-state level of pp60 c-src is reduced in antisense transfectants, not only is the steady-state level of VEGF reduced, but the ability of confluence to stimulate pp60 c-src activity and VEGF production is too. These data suggest that c-src may be an intermediary of both constitutive and inducible pathways for VEGF production in colon tumor cells.