PARP-1 Expression and ERK Activation are negatively modulated by PJ-34 in an in vitro model of Glioma-Conditioned Blood Brain Barrier

PARP-1(Poly(ADP-ribose) polymerase-1) inhibitors were pro- posed to play a protective role in many pathological conditions characterized by PARP-1 overactivation. They act by competing with NAD+, the enzyme physiological substrate. It has been shown that PARP-1 also promotes tumor growth and progres- sion through its DNA repair activity. Since angiogenesis is an essential requirement for these activities, we sought to determine whether PARP inhibition might affect rat brain microvascular endothelial cells (GP8.3) migration, stimulated by C6-glioma con- ditioned medium (CM). Through wound-healing experiments and MTT analysis, we demonstrated that PARP-1 inhibitor PJ-34 [N- (6-Oxo-5,6-dihydrophenanthridin-2-yl)-N,N-dimethylacetamide] abolishes the migratory response of GP8.3 cells and reduces their viability. PARP-1 also acts in a DNA independent way within the Extracellular-Regulated-Kinase (ERK) signaling cascade, which regulates cell proliferation and differentiation. By western analysis and confocal laser scanning microscopy (LSM), we anal- ysed the effects of PJ-34 on PARP-1 expression, phospho-ERK and phospho-Elk-1 activation. The effect of MEK (mitogen-acti- vated-protein-kinase-kinase) inhibitor PD98059 (2-(2-Amino-3- methoxyphenyl)-4H-1-benzopyran-4-one) on PARP-1 expression in unstimulated and in CM-stimulated GP8.3 cells was analyzed by RT-PCR. PARP-1 expression and phospho-ERK activation were significantly reduced by treatment of GP8.3 cells with PJ-34 or PD98059. By LSM, we further demonstrated that PARP-1 and phospho-ERK are coexpressed and share the same subcellu- lar localization in GP8.3 cells, in the cytoplasm as in nucleo- plasm. Based on these data, we propose that PARP-1 and phospho-ERK interact in the cytosol and then traslocate to the nucleus, where they trigger a proliferative response. We also pro- pose that PARP-1 inhibition blocks CM-induced endothelial migration by interfering with ERK signal-transduction pathway.