Freedman Deborah A, Folkman Judah
Vascular Biology Program, Department of Surgery, Children's Hospital, 1 Blackfan Circle, Harvard University Medical School, Karp Family Research Laboratories, Floor 12, Boston, MA 02115, USA.
Exp Cell Res. 2005 Jul 1;307(1):118-30. doi: 10.1016/j.yexcr.2005.03.025. Epub 2005 Apr 21.
Here we report for the first time that loss of CDK2 activity, by translational inhibition and through CDK2 inhibition by p21(Cip1/Waf1), may be responsible for endothelial senescence. We show that expression of dominant-negative p53 extends human umbilical vein endothelial cell (HUVEC) lifespan past senescence. HUVEC expressing telomerase can completely bypass senescence and become immortal (i-HUVEC). Surprisingly, early passage i-HUVEC, like senescent HUVEC, express high levels of the CDK inhibitors p16(INK4a) and p21(Cip1/Waf1). Expression of p16(INK4a) can persist for over 280 population doublings, while p21(Cip1/Waf1) expression was eventually lost in five of six i-HUVEC lines. Senescent HUVEC contain undetectable CDK2 activity, which results from a dramatic reduction of CDK2 protein levels and inhibition of remaining CDK2 by p21(Cip1/Waf1). The decreased CDK2 levels in senescent HUVEC are not due to decreased transcription or protein stability; rather, CDK2 translation declines during senescence. Bypass of endothelial senescence by telomerase entails the restoration of CDK2 translation and activity. These results suggest that p16(INK4a) does not play a role in endothelial senescence. Rather, CDK2 translational down-regulation may be a key regulatory event in replicative senescence of endothelial cells. Understanding the mechanisms regulating endothelial senescence will be critical in determining the role of endothelial senescence in tumor growth.
在此，我们首次报告，通过翻译抑制以及p21（Cip1/Waf1）对CDK2的抑制作用导致的CDK2活性丧失，可能是内皮细胞衰老的原因。我们发现，显性负性p53的表达可使人类脐静脉内皮细胞（HUVEC）的寿命延长至衰老之后。表达端粒酶的HUVEC能够完全绕过衰老并获得永生（i - HUVEC）。令人惊讶的是，早期传代的i - HUVEC，与衰老的HUVEC一样，表达高水平的CDK抑制剂p16（INK4a）和p21（Cip1/Waf1）。p16（INK4a）的表达可持续超过280次群体倍增，而在六个i - HUVEC细胞系中的五个中，p21（Cip1/Waf1）的表达最终消失。衰老的HUVEC中检测不到CDK2活性，这是由于CDK2蛋白水平急剧降低以及p21（Cip1/Waf1）对剩余CDK2的抑制所致。衰老的HUVEC中CDK2水平的降低并非由于转录减少或蛋白质稳定性下降；相反，在衰老过程中CDK2的翻译下降。端粒酶绕过内皮细胞衰老需要恢复CDK2的翻译和活性。这些结果表明，p16（INK4a）在内皮细胞衰老中不起作用。相反，CDK2翻译下调可能是内皮细胞复制性衰老中的关键调控事件。了解调节内皮细胞衰老的机制对于确定内皮细胞衰老在肿瘤生长中的作用至关重要。
