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 fac tor (EGF) and transforming growth factor alpha (TGF-alpha) in colorect al cancer, the present investigation examines changes in EGF and TGF-a lpha-mediated activation of overall and EGF receptor (EGF-R) associate d tyrosine kinase activity in isolated rat colonocytes after administr ation of the colonic carcinogen azoxymethane. METHODOLOGY: Five days a fter 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 over all and EGF-R associated tyrosine kinase and phospholipase C activity. RESULTS: In colonocytes isolated from control animals, incubation wit h EGF and TGF-alpha resulted in a small (21-35%) increase in overall t yr-k. However, a marked (113-127%) rise of this enzyme occurred in col onocytes from AOM-treated rats, when compared with the corresponding b asal levels. These differences were even more pronounced in colonocyte s isolated from the distal part of the colon, as regards to the proxim al 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 colo nocytes with these growth factors also stimulated PLC activity (in con trols by 120-150% and in AOM injected rats by 204-271%) when compared with corresponding basal values. CONCLUSIONS: We conclude that AOM enh ances the responsiveness of colonocytes to EGF and TGF-alpha, which ma y de one of the mechanisms involved in colorectal carcinogenesis.