Enhanced ligand-induced activation of EGF-receptor and overall tyrosine kinase and phospholipase C in colonocytes isolated from azoxymethane-treated rats

BACKGROUND/AIMS: In order to evaluate the role of epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha) in colorectal cancer, the present investigation examines changes in EGF and TGF-alpha-mediated activation of overall and EGF receptor (EGF-R) associated tyrosine kinase activity in isolated rat colonocytes after administration of the colonic carcinogen azoxymethane. METHODOLOGY: Five days after a single injection of azoxymethane (20 mg/kg) or saline soloution to 3-4 month old Fischer-344 rats, colonocytes were isolated, exposed for 2 minutes to 1 x 10(8) M EGF and TGF-alpha, and assessed for overall and EGF-R associated tyrosine kinase and phospholipase C activity. RESULTS: In colonocytes isolated from control animals, incubation with EGF and TGF-alpha resulted in a small (21-35%) increase in overall tyr-k. However, a marked (113-127%) rise of this enzyme occurred in colonocytes from AOM-treated rats, when compared with the corresponding basal levels. These differences were even more pronounced in colonocytes isolated from the distal part of the colon, as regards to the proximal part. In addition, EGF and TGF-alpha activated EGF-R tyr-k by 40-60% in controls and by 84-85% in AOM-treated animals. Incubation of colonocytes with these growth factors also stimulated PLC activity (in controls by 120-150% and in AOM injected rats by 204-271%) when compared with corresponding basal values. CONCLUSIONS: We conclude that AOM enhances the responsiveness of colonocytes to EGF and TGF-alpha, which may de one of the mechanisms involved in colorectal carcinogenesis.