Pharmacological inhibition of poly (ADP-ribose) polymerase (PARP) activity down-regulates the expression of syndecan-4 and Id-1 in endothelial cells and increases temozolomide sensitivity of PARP-1 silenced melanoma cells.

We recently demonstrated that poly(ADP-ribose) polymerase (PARP)-1 is involved in angiogenesis and melanoma aggressiveness. In fact, a reduction of blood vessel neo-formation in response to angiogenic stimuli is observed in PARP-1 KO mice and down-regulation of cellular PARP activity, using pharmacological inhibitors, impairs a number of endothelial functions. Notably, abrogation of PARP-1 expression by stable gene silencing reduces the in vivo aggressiveness of melanoma and this effect is associated with a decreased vasculature formation within the tumour. In this study we have investigated the potential mechanisms underlying the anti-angiogenic effect exerted by the PARP inhibitor GPI 15427, analyzing gene expression in human endothelial cells shortly after treatment with this compound. The results indicate that 2 h exposure of endothelial cells to GPI 15427 provokes a rapid decrease of syndecan-4 (SDC-4), a transmembrane protein involved in modulation of cell signalling during angiogenesis, and a reduction of the inhibitor of DNA binding-1 (Id-1), a helix-loop-helix transcription factor also implicated in the control of endothelial functions. Moreover, we evaluated chemosensitivity and cell cycle modifications induced by treatment with the methylating agent temozolomide, as single agent or in combination with GPI 15427, in melanoma cells stably silenced for PARP-1 expression. Silencing of PARP-1 in melanoma results in enhanced sensitivity to the methylating agent temozolomide and in an early cell accumulation at the G2/M phase after drug treatment. The addition of GPI 15427, which inhibits both PARP-1 and PARP-2, increases sensitivity to temozolomide both in PARP-1-proficient and -deficient melanoma cells, but it induces a different cell cycle modulation depending on PARP-1 expression. In conclusion, these findings provide a novel implication for PARP-1 in cancer development and underscore the importance of targeting both PARP-1 and PARP-2 for cancer therapy. Supported by: Italian Ministry of Education and Research (PRIN project to GG) and Italian Ministry of Health (RC07-3.16 project to PML).