Aquaporin 3 in keratinocyte differentiation

Abstract

Aquaporin 3 (AQP3) is a channel that transports both water and glycerol. AQP3- null mutant mice exhibit skin defects, including impairment of water holding capacity, barrier recovery and wound healing and decreased glycerol content. We hypothesized that AQP3 is involved in the regulation ofkeratinocyte proliferation and differentiation and this regulation is mediated, at least in part, by the functional interaction between AQP3 and phospholipase D (PLD). Here we demonstrate that AQP3 expression was down-regulated at the transcriptional level and glycerol uptake was reduced when primary mouse keratinocytes were induced to differentiate. In co-transfection experiments, we found that AQP3 decreased the promoter activity of keratin 5, a keratinocyte proliferation marker, but increased the promoter activity of keratin 10 and involucrin, an early and intermediate keratinocyte differentiation marker respectively. These results suggest that AQP3 is a regulator of early keratinocyte differentiation. In further investigatjons to determine the sigualing function of AQP3 in regulating keratinocyte differentiation, we found using sucrose gradient centrifugation, irnmunoprecipitation analysis, confocal microscopy that AQP3 and PLD2 were colocalized in lipid rafts. In addition, we demonstrated that AQP3 could contribute to the synthesis of phosphatidylglycerol (PG) and that PLD2 was able to utilize glycerol as a substrate to synthesize PG. These data suggest that AQP3 transports glycerol for use as a physiological primary alcohol substrate for-adjacent PLD2 to generate PG. Our results, together with the reports that PG is an activator of protein kinases (PKqm and PKCe) and also contributes to protein-protein interactions in membranes, suggest that glycerol AQP3-PLD2-PG is a potential signaling pathway in regulating keratinocyte differentiation.