PARP Family

The role of PARP-1 as a modulator of chromatin stiucture, as well as its chromatin-dependent effects on transcription, has been well established in the literature (D Amours et al, 1999 Kraus and Lis, 2003 Rouleau et al, 2004 Kim et al, 2005). In this section, we will highlight some of the historical studies, as well as more recent studies, that have characterized the role of PARP-1 and PAR in these processes. Note that, when describing this work, the term PARP in the absence of any additional identifiers refers to PAR-generating proteins of ambiguous identity (e.g., as in early studies, which were conducted without the knowledge that multiple PARP family members exist, or when multiple splice variants of a PARP-1-like protein exist in a single organism, as in Drosophila). 

Chemistry. Nicotinamide (NAm), a product of PARP-1 enzymatic action on NAD and a weak inhibitor of PARP-1 activity (Curtin, 2006), was used as a model for the first PARP-1 inhibitors, including benzamide and 3-aminobenzamide (3AB) (Shall, 1975) (Fig. 7a). Benzamide and 3AB inhibit PARP-1 activity in the mM range and may also inhibit other PARP family members e.g., PARP-2 and tankyrase Smith et al, 1998 Ame et al, 1999), thus lacking the potency and specificity required for therapeutic purposes. Banasik et al developed 1,5-dihydroisoquinoline (Fig. 7c) and a variety of other inhibitors (Banasik et al, 1992), which were used in turn to develop even more inhibitors with greater specificity. 

Confinement of biomolectiles within compartments is essential both Ibr the formation and function of the cell. One of the major characteristics of the newly identified members of the PARP family is their wiriotis suhcellular localizations. Figure 5 displays the subcellular localization of PARP-2 compared to that of PARP-1 in HeLa cells in various phases of the cell cycle. During interphase the accumulation of both PARP-1 and PARP-2 is clearly visible in the nucleoli of HeLa cells (panels 5A, 5F). PARP-1 has been shown to translocate from the nucleolus to the nucleoplasm when RNA synthesis is inhibited su esting that its nucleolar location is dependent on the transcriptional state of the nucleolar chromatin. It remains to see whether the same occurs with PARP-2. 

In eukaryotic cells, DNA damage may induce a several thousand fold stimulation of poly(ADP-ribose) metabolism. A few restrictions and rules apply yeast does not express such a response, and in all other eukaryotes tested so far, the most effective types of DNA damages are those that are substrates for the DNA base excision repair pathway. By far the largest amount of ADP-ribose is processed throi the catalytic domains of two nuclear enzymes poly(ADP-ribose)polymerase-l, (PARP-1), and its catabolic counterpart, poly(ADP-ribose)glycohydrolase (PARC). Other members of the growing PARP family may contribute to this metabolism, albeit to a much lesser extent and with mechanisms that await further elucidation (for reviews see refs. 1,2). 

Finally, the recruitment mechanism relying on ADP-ribose polymers as the fishing rods may have a more general application in cellular signaling. All members of the PARP family studied so far, can modify themselves and thus recruit other proteins analogous to the example shown for XRCCl. Moreover, poly(ADP-ribose) becomes attached to other proteins in the course of the heteromodification reaction, catalyzed by several members of the PARP family. The best studied example is PARP-1, which can heteromodify several nuclear proteins, primarily histones, but also enzymes (DNA polymerases, RNA polymerases, topoisomerases, nucleases) and transcription factors (p53, Fos, TFIIF, YY-1) (ref 1,62). 

In addition, it is important to note that the functions of other PARP family members in disorders as well as NF-kB dependent processes are yet completely unknown and have to be addressed. Indeed, there is at least one report, which indicates a functional role of other PARP family members in inflammatory disorders. PARP-1(-/-) mice treated with 5-iodo-6-aminobenzopyrone (INH2BP), a novel potent PARP inhibitor, showed an increase in sensitivity to streptozotocin in the multiple-low-dose-streptozotocin (MDLS) model of type I diabetes induction, characterized by increased blood glucose and incidence of diabetes compared to untreated PARP- (-/-) mice. These results surest that other PARP family members may have opposite functions in inflammatory disorders. 

It is noteworthy that there are also aspects other than involvement in DNA repair that relate the PARP family to the aging process. 

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