Regulation of phospholipase C activation by the number of H 2 receptors during Ca 2+-induced differentiation of mouse keratinocytes

Abstract

We have reported previously that the histamine H 2 receptor (H 2R) can stimulate the phospholipase C (PLC) signaling pathway in mouse keratinocytes. In the present work, we examined the physiological mechanisms involved in this activation by studying histamine metabolism and H 2R expression and coupling during mouse keratinocyte differentiation. Ca 2+-induced differentiation decreased histidine decarboxylase (HDC) mRNA, the enzyme responsible for histamine synthesis, by 68.9±5.0%. Concomitantly, intracellular histamine content and its release into the extracellular medium were reduced significantly by 68.2±2.0 and 74.1±1.7%, respectively. Binding of [ 3 H ]tiotidine to H 2Rs present on the surface of whole cells was also decreased by cellular differentiation [(18.17±2.1)×10 4 vs. (6.27±0.87)×10 4 sites/cell, undifferentiated and differentiated cells, respectively], without affecting H 2R affinity. Northern blot and reverse transcriptase–polymerase chain reaction (RT–PCR) analysis of the H 2R mRNA showed that the expression was also down-regulated at the transcriptional level. Moreover, the inhibition of H 2R expression strongly affected the ability of the receptor to induce PLC activation. Our findings suggest that H 2R signaling through the PLC second messenger system is inhibited during keratinocyte differentiation by an autocrine loop involving down-regulation of H 2R expression and inhibition of histamine metabolism.