Purinergic receptors are part of a functional signaling system for proliferation and differentiation of human epidermal keratinocytes

We investigated the expression of P2X(5), P2X(7), P2Y(1) and P2Y(2) receptor subtypes in normal human epidermis and in relation to markers of proliferation (PCNA and Ki-67), keratinocyte differentiation (cytokeratin K10 and involucrin) and markers of apoptosis (TUNEL and anticaspase-3). Using immunohistochemistry, we showed that each of the four receptors was expressed in a spatially distinct zone of the epidermis, suggesting different functional roles for these receptors. Functional studies were performed on primary cultures of human keratinocytes and on explanted rat skin, where different P2 receptor subtype agonists and antagonists were applied to cultured keratinocytes or injected subcutaneously into the skin, respectively. An increase in cell number was caused by low doses of the nonspecific P2 receptor agonist ATP, the P2Y(2) receptor agonist UTP (p<0.001), and the P2Y(1) receptor agonist 2MeSADP (p<0.05). There was a significant decrease in cell number as a result of treatment with the P2X(5) receptor agonist ATPgammaS (p<0.001) and the P2X(7) receptor agonist BzATP (p<0.001). Suramin caused a significant block in the effect of 100 mum ATP (p<0.01) and 1000 mum ATP (p<0.001) on cell number. These results imply that different purinergic receptors have different functional roles in the human epidermis with P2Y(1) and P2Y(2) receptors controlling proliferation, while P2X(5) and P2X(7) receptors control early differentiation, terminal differentiation and death of keratinocytes, respectively.