Cellular senescence, an irreversible growth arrest, is considered to play as safeguard against malignant progression, though such a mechanism is speculative in human
carcinogenesis. In gallbladder
carcinoma,
cholecystolithiasis and
pancreaticobiliary maljunction (
PBM) are major risk factors. Here, by using 113 surgically resected gallbladders and cultures of human gallbladder epithelial cells (HGECs) and gallbladder
carcinoma cell line (TGBC2TKB), we examined
carcinogenesis with respect to cellular senescence. Among 15 cases of
PBM in which
carcinoma was found in 4 cases, nonneoplastic gallbladder mucosa showed diffuse papillary
hyperplasia (PHP). PHP was not found in gallbladders with
cholecystolithiasis. Interestingly, PHP exhibited senescent features such as expression of
p16(INK4A) and low cell proliferative activity. In contrast, EZH2, a
polycomb group protein, was overexpressed in
intraepithelial neoplasm and
carcinoma in gallbladders with
cholecystolithiasis. In
PBM, EZH2 was expressed only in
carcinoma foci but not in PHP. Cultured HGECs treated with
lysolecithin, the level of which is elevated in gallbladder bile of
PBM, showed increased expression of
p16(INK4A) and senescence-associated
beta-galactosidase. Conversely, enforced overexpression of EZH2 in senescent HGECs reduced
p16(INK4A) expression. A knockdown of EZH2 in cultured TGBC2TKB cells increased
p16(INK4a) expression. In conclusion, PHP in
PBM may act as a barrier to malignant transformation for decades. EZH2 may be responsible for the escape from cellular senescence followed by malignant transformation in the gallbladder of
PBM.