Inhibition of ZL55 cell proliferation by ADP via PKC-dependent signalling pathway

Extracellular nucleotides can regulate cell proliferation in both normal and tumorigenic tissues. Here, we studied how extracellular nucleotides regulate the proliferation of ZL55 cells, a mesothelioma-derived cell line obtained from bioptic samples of asbestos-exposed patients. ADPand 2-MeS-ADP inhibited ZL55 cell proliferation, whereas ATP, UTP, and UDP were inactive. The nucleotide potency profile and the blockade of the ADP-mediated inhibitory effect by the phospholipase C inhibitor U-73122 suggest that P2Y1 receptor controls ZL55 cell proliferation. The activation of P2Y1 receptor by ADP leads to activation of intracellular transduction pathways involving [Ca2+](i), PKC-delta/PKC-alpha, and MAPKs, ERK1/2 and JNK1/2. Cell treatment with ADP or 2-MeS-ADP also provokes the activation of p53, causing an accumulation of the G1 cyclin-dependent kinase inhibitors p21(WAF1) and p27(Kip). Inhibition of ZL55 cell proliferation by ADP was completely reversed by inhibiting MEK1/2, or JNK1/2, or PKC-delta, and PKC-alpha. Through the inhibition of ADP-activated transductional kinases it was found that PKC-delta was responsible for JNK1/2 activation. JNK1/2 has a role in transcriptional up-regulation of p53, p21(WAF1/CIP1), and p27(kip1). Conversely, the ADP-activated PKC-a provoked ERK1/2 phosphorylation. ERK1/2 increased p53 stabilization, required to G1 arrest of ZL55 cells. Concluding, the importance of the study is twofold: first, results shed light on the mechanism of cell cycle inhibition by ADP; second, results suggest that extracellular ADP may inhibit mesothelioma progression.