The
aryl hydrocarbon receptor (AhR) mediates many toxic effects of
environmental pollutants. AhR also interacts with multiple
growth factor-driven signaling pathways. In the course of examining effects of
growth factors on proliferation of human
colon cancer cells, we identified cross talk between AhR and the
epidermal growth factor receptor (EGFR). In the present work, we explored underlying signal transduction mechanisms and functional consequences of this interaction. With the use of two human
colon cancer cell lines, H508 and SNU-C4, we examined the effects of AhR
ligands including
2,3,7,8-tetrachlorodibenzo-p-dioxin (
TCDD) on cell proliferation and activation of EGFR, ERK1/2, and
Src kinases. In
colon cancer cells, 5-day incubation with
TCDD stimulated a twofold dose-dependent increase in cell proliferation that was detectable with 1 nM and maximal with 30 nM
TCDD.
TCDD induced dose- and time-dependent phosphorylation of EGFR (Tyr845) and ERK1/2; maximal phosphorylation was observed 5 to 10 min after addition of 30 nM
TCDD. Both
TCDD-induced ERK1/2 phosphorylation and cell proliferation were abolished by AhR
small interfering RNA, AhR-specific inhibitor
CH223191,
Src kinase inhibitor PP2,
neutralizing antibodies against
matrix metalloproteinase 7,
heparin-binding-EGF-like growth factor and EGFR, EGFR inhibitors (
AG1478 and
PD168393), and MEK1 inhibitor
PD98059. Coimmunoprecipitation experiments revealed that AhR forms a
protein complex with Src and regulates Src activity by phosphorylating Src (Tyr416) and dephosphorylating Src (Tyr527). These data support novel observations that, in human
colon cancer cells, Src-mediated cross talk between aryl
hydrocarbon and EGFR results in ERK1/2 activation, thereby stimulating cell proliferation.