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. ADP and 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 [Ca 2+ ] i , PKC‐δ/PKC‐α, 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‐δ, and PKC‐α. Through the inhibition of ADP‐activated transductional kinases it was found that PKC‐δ 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‐α 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.