Role for the Related Poly(ADP-Ribose) Polymerases Tankyrase 1 and 2 at Human Telomeres

Abstract

Telomere maintenance is essential for the continuous growth of tumor cells. In most human tumors telomeres are maintained by telomerase, a specialized reverse transcriptase. Tankyrase 1, a human telomeric poly(ADP-ribose) polymerase (PARP), positively regulates telomere length through its interaction with TRF1, a telomeric DNA-binding protein. Tankyrase 1 ADP-ribosylates TRF1, inhibiting its binding to telomeric DNA. Overexpression of tankyrase 1 in the nucleus promotes telomere elongation, suggesting that tankyrase 1 regulates access of telomerase to the telomeric complex. The recent identification of a closely related homolog of tankyrase 1, tankyrase 2, opens the possibility for a second PARP at telomeres. We therefore sought to establish the role of tankyrase 1 at telomeres and to determine if tankyrase 2 might have a telomeric function. We show that endogenous tankyrase 1 is a component of the human telomeric complex. We demonstrate that telomere elongation by tankyrase 1 requires the catalytic activity of the PARP domain and does not occur in telomerase-negative primary human cells. To investigate a potential role for tankyrase 2 at telomeres, recombinant tankyrase 2 was subjected to an in vitro PARP assay. Tankyrase 2 poly(ADP-ribosyl)ated itself and TRF1. Overexpression of tankyrase 2 in the nucleus released endogenous TRF1 from telomeres. These findings establish tankyrase 2 as a bona fide PARP, with itself and TRF1 as acceptors of ADP-ribosylation, and suggest the possibility of a role for tankyrase 2 at telomeres.Telomere integrity is essential for chromosome stability, and the maintenance of telomeric DNA is required for long-term proliferation of eukaryotic cells. Telomeres are maintained by telomerase, a reverse transcriptase that adds telomeric repeats to chromosome ends (14; reviewed in reference 31). In most normal human somatic tissue telomerase is repressed, and as a result, telomeres shorten (17, 18). Critically short telomeres lose their ability to protect chromosome ends, resulting in chromosomal degradation and fusion. In contrast to normal somatic human cells, immortalized cells (including cancer cells) and germ cells express telomerase (21, 34) and maintain their telomeres. In these cells telomere maintenance is regulated by a homeostatic mechanism (reviewed in reference 29). Thus, in the mammalian germ line telomeres show a speciesspecific telomere length setting which is constant over the generations (23). Regulation is also apparent in many human tumor cell lines, where despite the presence of high levels of telomerase telomeres do not grow, but rather, they are stably maintained within a given size range (9, 10).Mammalian telomeres consist of long tandem arrays of TTAGGG repeats bound by the DNA-binding proteins, TRF1 and TRF2 (4, 5, 7; reviewed in reference 8). The TRFs are related in their primary structure; both contain carboxy-terminal Myb-type DNA-binding motifs and internal, conserved domains required for homodimerization (2, 5). The proteins do not form heterodimers (5). A dis...

Functional Interaction between Poly(ADP-Ribose) Polymerase 2 (PARP-2) and TRF2: PARP Activity Negatively Regulates TRF2

“…In conclusion, the present study has identified PARP-2 as an additional contributor to telomere functions by regulating TRF2, whereas tankyrase 1 and 2 have been shown to target TRF1 (15). The identification of several PARP homologues regulating telomere protection and/or synthesis adds to the complexity but undoubtedly reinforces the increasing number of molecular processes needed for functional telomeres (7).…”

Cancer and aging: the importance of telomeres in genome maintenance

“…TANK-mediated ADP-ribosylation causes TRF1 to dissociate from telomeric DNA in vitro [68]. Moreover, when nuclear-targeted TANK1 is overexpressed in telomerase-positive human cells, telomeres elongate, consistent with TANKs inactivating TRF1 in vivo and TRF1 being a negative regulator of telomere length [70,74].…”

“…3). Although the TANKs can be found at telomeres, and even ADP-ribosylate TRF1 in vitro [68,70], these proteins reside predominantly in Golgi-type vesicles, and have also been identified at centrosomes [69,[71][72][73]. The precise functions of TANKs are not known.…”

PARP1 is activated at telomeres upon G4 stabilization: possible target for telomere-based therapy

“…Several of the PARP family proteins may be associated with telomeres. Tankyrases 1 and 2, as well as PARP1 and 2, can directly bind to and poly-ADP ribosylate the telomeric repeat binding factors 1 (TRF1) and 2 (TRF2) and affect their association to telomeric DNA, acting as positive regulators for telomere elongation (Smith and de Lange, 1999;Kaminker et al, 2001;Cook et al, 2002;Rippmann et al, 2002). In addition, upon DNA damage, PARP1 may be recruited to the damaged telomeres, helping to protect telomeres against chromosome end-to-end fusions and genomic instability (Gomez et al, 2006).…”