We investigated the use of purinergic receptors as a new treatment modality for nonmelanoma skin cancers. Purinergic receptors, which bind adenosine 5′-tri-phosphate, are expressed on human cutaneous keratinocytes. Previous work in rat and human epidermis suggested functional roles for purinergic receptors in the regulation of proliferation, differentiation, and apoptosis. Immunohistochemical analysis of frozen sections in human basal cell carcinomas and squamous cell carcinomas for P2X5, P2X7, P2Y1, P2Y2, and P2Y4 receptors was performed, accompanied by detailed analysis of archive material of tumor subtypes in paraffin sections. Functional studies were performed using a human cutaneous squamous cell carcinoma cell line (A431), where purinergic receptor subtype agonists were applied to cells and changes in cell number were quantified via a colorimetric assay. Immunostaining in paraffin sections was essentially the same as that in frozen sections, although more detail of the subcellular composition was visible. P2X5 and P2Y2 receptors were heavily expressed in basal cell carcinomas and squamous cell carcinomas. P2X7 receptors were expressed in the necrotic center of nodular basal cell carcinomas and in apoptotic cells in superficial multifocal and infiltrative basal cell carcinomas, and squamous cell carcinomas. P2Y1 receptors were only expressed in the stroma surrounding tumors. P2Y4 receptors were found in basal cell carcinomas but not in squamous cell carcinomas. P2X5 receptors appear to be associated with differentiation. The P2X7 receptor agonist benzoylbenzoyl-adenosine 5′-triphosphate and high concentrations of adenosine 5′-triphosphate (1000–5000 μM) caused a significant reduction in A431 cell number (p<0.001), whereas the P2Y2 receptor agonist uridine 5′-triphosphate caused a significant amount of proliferation (p<0.001). We have demonstrated that non-melanoma skin cancers express functional purinergic receptors and that P2X7 receptor agonists significantly reduce cell numbers in vitro.
