Abstract
BackgroundThe study tested the hypothesis that apoptosis can prevent and control growth of neoplastic cells. Previous studies in-vitro have shown that the pro-apoptotic P2X7 receptor regulates growth of epithelial cells. The specific objective of the present study was to understand to what degree the P2X7 system controls development and growth of skin cancer in vivo, and what cellular and molecular mechanisms are involved in the P2X7 action.MethodsSkin neoplasias in mice (papillomas, followed by squamous spindle-cell carcinomas) were induced by local application of DMBA/TPA. Experiments in-vitro utilized cultured epidermal keratinocytes generated from wild-type or from P2X7-null mice. Assays involved protein immunostaining and Western blots; mRNA real-time qPCR; and apoptosis (evaluated in situ by TUNEL and quantified in cultured keratinocytes as solubilized DNA or by ELISA). Changes in cytosolic calcium or in ethidium bromide influx (P2X7 pore formation) were determined by confocal laser microscopy.Results(a) Co-application on the skin of the P2X7 specific agonist BzATP inhibited formation of DMBA/TPA-induced skin papillomas and carcinomas. At the completion of study (week 28) the proportion of living animals with cancers in the DMBA/TPA group was 100% compared to 43% in the DMBA/TPA+BzATP group. (b) In the normal skin BzATP affected mainly P2X7-receptor – expressing proliferating keratinocytes, where it augmented apoptosis without evoking inflammatory changes. (c) In BzATP-treated mice the degree of apoptosis was lesser in cancer than in normal or papilloma keratinocytes. (d) Levels of P2X7 receptor, protein and mRNA were 4–5 fold lower in cancer tissues than in normal mouse tissues. (e) In cultured mouse keratinocytes BzATP induced apoptosis, formation of pores in the plasma membrane, and facilitated prolonged calcium influx. (f) The BzATP-induced apoptosis, pore-formation and augmented calcium influx had similar dose-dependence for BzATP. (g) Pore formation and the augmented calcium influx were depended on the expression of the P2X7 receptor, while the BzATP-induced apoptosis depended on calcium influx. (h) The BzATP-induced apoptosis could be blocked by co-treatment with inhibitors of caspase-9 and caspase-3, but not of caspase-8.Conclusion(a) P2X7-dependent apoptosis is an important mechanism that controls the development and progression of epidermal neoplasia in the mouse. (b) The P2X7-dependent apoptosis is mediated by calcium influx via P2X7 pores, and involves the caspase-9 (mitochondrial) pathway. (c) The diminished pro-apoptotic effect of BzATP in mouse cancer keratinocytes is possibly the result of low expression of the P2X7 receptor. (d) Activation of P2X7-dependent apoptosis, e.g. with BzATP could be a novel chemotherapeutic growth-preventive modality for papillomas and epithelial cancers in vivo.
Highlights
The study tested the hypothesis that apoptosis can prevent and control growth of neoplastic cells
The main finding of the study was that pharmacological activation of P2X7-mediated apoptosis, by local skin application of the P2X7-receptor agonist BzATP, inhibited DMBA/TPA-induced formation of skin papillomas and squamous spindle-cell carcinomas
The present findings support the hypothesis that P2X7 is an important physiological pro-apoptotic system in epithelia, those derived from the ectoderm, the uro-genital sinus, and the distal paramesonephric duct [9,38,41], and Li, Qi, Zhou, Fu, AbdulKarim, MacLennan, and Gorodeski GI: P2X7 receptor expression is decreased in epithelial cancer cells of ectodermal, uro-genital sinus, and distal paramesonephricduct origin
Summary
The study tested the hypothesis that apoptosis can prevent and control growth of neoplastic cells. Previous studies in-vitro have shown that the pro-apoptotic P2X7 receptor regulates growth of epithelial cells. The natural ligand of the receptor is ATP [11,12] which is present in the extracellular fluid of epithelial cells at high nanomolar, low micromolar levels [14,15,16,17,18]. Studies in epithelial cells of the female reproductive tract showed a threshold effect and activation of P2X7-mediated apoptosis already by nanomolar concentrations of ATP [8,18], suggesting that ATP levels which are present in the extracellular fluid suffice to activate the receptor
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