Evaluation of the role of c-Src and ERK in TCDD-dependent release from contact-inhibition in WB-F344 cells

TCDD ( 2,3,7,8- tetrachlorodibenzo-p-dioxin) is the most potent tumor promoter ever tested in rodents. Although it is known that most of the effects of TCDD are mediated by binding to the aryl hydrocarbon receptor (AhR), the mechanisms leading to tumor promotion remain to be elucidated. Loss of contact-inhibition is one characteristic hallmark in tumorigenesis. In WB-F344 cells, TCDD induces a release from contact-inhibition which is manifested by a twofold increase in DNA-synthesis and cell number when TCDD ( 1 nmol L-1) is given to confluent cells. Because TCDD leads to phosphorylation of the epidermal growth factor receptor and an increase in c-Src-activation in WB-F344 cells, we investigated the functional relevance of this observation. Pharmacological inhibition of c-Src using PP1 ( 10 mu mol L-1) or genistein ( 10 mu mol L-1) did not prevent TCDD-dependent release from contact-inhibition. In accordance, elevation of cyclin A-a previously identified target of TCDD and marker of S-phase entry-was not reduced in the presence of PP1 or genistein. Western blot analysis revealed that phosphorylation of the EGF- receptor downstream target ERK was not induced in response to TCDD. Furthermore, TCDD-dependent increase in DNA-synthesis was not inhibited by the MEK1/2 inhibitor U0126 ( 10 mu mol L-1). Our data show that neither c-Src-activation, nor ERK-activation are required for TCDD-dependent release from contact-inhibition arguing against a functional role of EGF- receptor activation in response to TCDD in WB-F344 cells.