Smith S, de Lange T
The Rockefeller University, New York, New York 10021, USA.
Curr Biol. 2000 Oct 19;10(20):1299-302. doi: 10.1016/s0960-9822(00)00752-1.
Human telomeres are maintained by telomerase, a reverse transcriptase that adds telomeric repeats to chromosome ends [1,2]. In human tumors and immortalized cells, telomeres are often maintained at a constant length setting [3,4], indicating that telomerase-mediated telomere elongation is tightly regulated. Tankyrase, a telomeric poly(ADP-ribose) polymerase (PARP) [5], was identified through its interaction with TRF1 [6], a negative regulator of telomere extension by telomerase [7]. Tankyrase-mediated ADP-ribosylation inhibits binding of TRF1 to telomeric repeats in vitro [5], suggesting that tankyrase might regulate TRF1 and therefore control telomere dynamics in vivo. Here, we present evidence that tankyrase acts as a positive regulator of telomere elongation in vivo, apparently by inhibiting TRF1. Overexpression of tankyrase in the nucleus diminished the level of unmodified TRF1 in immunoblots and led to reduced immunofluorescence of TRF1 at interphase telomeres. Long-term overexpression of tankyrase in telomerase-positive human cells resulted in a gradual and progressive elongation of telomeres. A PARP-deficient form of tankyrase failed to affect TRF1 and did not alter telomere length dynamics, consistent with ADP-ribosylation of TRF1 as the main cause of altered telomere homeostasis. Our results indicate that tankyrase can induce telomere elongation in human cells. We propose that tankyrase-mediated ADP-ribosylation of TRF1 opens the telomeric complex, allowing access to telomerase.
人类端粒由端粒酶维持，端粒酶是一种逆转录酶，可将端粒重复序列添加到染色体末端[1,2]。在人类肿瘤和永生化细胞中，端粒通常维持在恒定长度[3,4]，这表明端粒酶介导的端粒延长受到严格调控。端锚聚合酶是一种端粒多聚（ADP-核糖）聚合酶（PARP）[5]，通过与TRF1相互作用被鉴定出来，TRF1是端粒酶介导的端粒延长的负调节因子[7]。端锚聚合酶介导的ADP-核糖基化在体外抑制TRF1与端粒重复序列的结合[5]，这表明端锚聚合酶可能调节TRF1，从而在体内控制端粒动态。在这里，我们提供证据表明，端锚聚合酶在体内作为端粒延长的正调节因子发挥作用，显然是通过抑制TRF1。在细胞核中过表达端锚聚合酶会降低免疫印迹中未修饰的TRF1水平，并导致间期端粒处TRF1的免疫荧光减弱。在端粒酶阳性的人类细胞中长期过表达端锚聚合酶会导致端粒逐渐延长。PARP缺陷型的端锚聚合酶未能影响TRF1，也未改变端粒长度动态，这与TRF1的ADP-核糖基化是端粒稳态改变的主要原因一致。我们的结果表明，端锚聚合酶可以诱导人类细胞中端粒延长。我们提出，端锚聚合酶介导的TRF1的ADP-核糖基化打开了端粒复合物，使端粒酶能够接近。
