Colonic
carcinogenesis involves the progressive dysregulation of homeostatic mechanisms that control growth. The
epidermal growth factor (
EGF) receptor (EGFR) regulates colonocyte growth and differentiation and is overexpressed in many human
colon cancers. A requirement for EGFR in colonic premalignancy, however, has not been shown. In the current study, we used a specific EGFR antagonist,
gefitinib, to investigate this role of the receptor in
azoxymethane colonic premalignancy. The
azoxymethane model shares many clinical, histologic, and molecular features of human
colon cancer. Mice received
azoxymethane i.p. (5 mg/kg/wk) or saline for 6 weeks. Animals were also gavaged with
gefitinib (10 mg/kg
body weight) or vehicle (
DMSO) thrice weekly for 18 weeks, a dose schedule that inhibited normal receptor activation by exogenous
EGF. Compared with control colonocytes [
bromodeoxyuridine (BrdUrd), 2.2+/-1.2%],
azoxymethane significantly increased proliferation (BrdUrd, 12.6+/-2.8%), whereas
gefitinib inhibited this hyperproliferation (BrdUrd, 6.2+/-4.0%; <0.005).
Azoxymethane significantly induced pro-
transforming growth factor-alpha (6.4+/-1.3-fold) and increased phospho-(active) EGFR (5.9+/-1.1-fold), phospho-(active) ErbB2 (2.3+/-0.2-fold), and phospho-(active)
extracellular signal-regulated kinase (3.3+/-0.4-fold) in premalignant colonocytes.
Gefitinib inhibited activations of these
kinases by >75% (P<0.05).
Gefitinib also significantly reduced the number of large
aberrant crypt foci and decreased the incidence of colonic microadenomas from 75% to 33% (P<0.05).
Gefitinib concomitantly decreased cell cycle-regulating
cyclin D1 and
prostanoid biosynthetic
enzyme cyclooxygenase-2 in microadenomas, suggesting that these regulators are key targets of EGFR in colonic
carcinogenesis. These results show for the first time that EGFR signaling is required for early stages of colonic
carcinogenesis. Our findings suggest, moreover, that inhibitors of EGFR might be useful in chemopreventive strategies in individuals at increased risk for colonic
malignancies.