Eguchi Hidetoshi, Herschenhous Nicole, Kuzushita Noriyoshi, Moss Steven F
Division of Gastroenterology, Department of Medicine, Rhode Island Hospital/Brown University, 593 Eddy Street, Providence, RI 02903, USA.
Cancer Res. 2003 Aug 1;63(15):4739-46.
Helicobacter pylori infection is associated with increased gastric epithelial cell turnover and is a risk factor for noncardia gastric cancer. H. pylori reduces the expression of p27 protein, a cyclin-dependent kinase inhibitor of the G(1) to S-phase cell cycle transition and gastric tumor suppressor gene. Although cell cycle dysregulation associated with decreased p27 may contribute to gastric carcinogenesis, how H. pylori reduces p27 in gastric epithelial cells remains unknown. In the present study, we investigated the mechanisms of the p27 decrease, using AGS and MKN28 gastric epithelial cells cocultured with H. pylori strains under conditions of defined cell cycle distribution. The expression of p27 protein was reduced by H. pylori in a dose- and time-dependent manner. Northern blot and pulse-chase analyses revealed that this reduction was not regulated at a transcriptional level but by accelerated p27 degradation via a proteasome-dependent pathway. Despite up-regulation of the proteasome-dependent degradation of p27 protein, neither threonine 187-phosphorylated p27 nor skp2 (the ubiquitin ligase for p27) were increased. Furthermore, H. pylori impaired p27 ubiquitination and did not increase global proteasomal function. These results indicate that H. pylori increases the degradation of p27 through a proteasomal pathway distinct from the physiological pathway that degrades p27 during cell cycle progression. Putative virulence genes of H. pylori (cagA, cagE, or vacA) played no role in reducing p27 expression. Increased degradation of p27 by H. pylori through a proteasome-dependent, ubiquitin-independent pathway may contribute to the increased risk of gastric cancer associated with chronic H. pylori infection.
幽门螺杆菌感染与胃上皮细胞更新增加相关，是胃非贲门癌的一个危险因素。幽门螺杆菌可降低p27蛋白的表达，p27蛋白是一种细胞周期蛋白依赖性激酶抑制剂，参与G(1)期至S期的细胞周期转换，也是一种胃肿瘤抑制基因。虽然与p27减少相关的细胞周期失调可能有助于胃癌的发生，但幽门螺杆菌如何降低胃上皮细胞中的p27仍不清楚。在本研究中，我们使用AGS和MKN28胃上皮细胞，在确定的细胞周期分布条件下与幽门螺杆菌菌株共培养，研究了p27减少的机制。幽门螺杆菌以剂量和时间依赖性方式降低p27蛋白的表达。Northern印迹和脉冲追踪分析表明，这种减少不是在转录水平上调节的，而是通过蛋白酶体依赖性途径加速p27降解来实现的。尽管p27蛋白的蛋白酶体依赖性降解上调，但苏氨酸187磷酸化的p27和skp2（p27的泛素连接酶）均未增加。此外，幽门螺杆菌损害p27泛素化，且未增加整体蛋白酶体功能。这些结果表明，幽门螺杆菌通过一种不同于细胞周期进程中降解p27的生理途径的蛋白酶体途径增加p27的降解。幽门螺杆菌的假定毒力基因（cagA、cagE或vacA）在降低p27表达中不起作用。幽门螺杆菌通过蛋白酶体依赖性、非泛素依赖性途径增加p27的降解，可能导致与慢性幽门螺杆菌感染相关的胃癌风险增加。
