In vitro studies of aquaporins 1 and 3 expression in cultured human proximal tubular cells: Upregulation by transferrin but not albumin

Dysregulated renal water handling is a cardinal feature of nephrotic syndro me that has been shown in animal models of experimental nephrosis to mediat e renal aquaporin (AQP) expression. However, data on the effect of proteinu ria on the proximal tubule, which is heavily vested with AQP1 and therefore may participate in water homeostasis, are limited. To investigate this, we exposed primary human proximal tubular epithelial cells (PTECs) to two key proteinuric components shown to perturb tubule function: human serum album in and transferrin. Using reverse-transcriptase polymerase chain reaction a nd immunocytochemical techniques, PTECs in the quiescent state were found t o express AQP3 in addition to AQP1 gene and protein, which was also validat ed in a human proximal tubule cell line, HK-2. Immunohistochemical staining localized AQP1 synthesis to the apical and basolateral membranes and AQP3 synthesis to the basolateral membrane of proximal tubule epithelium. Transf errin in doses reaching nephrotic range upregulated PTEC transcription and translation of both AQP1 and AQP3 in a time- and dose-dependent manner. Aft er 24 hours of stimulation, transferrin led to a 2.4- and 2.2-fold increase in AQP1 and APQ3 messenger RNA expression, whereas protein synthesis surge d by 40.7% +/- 2.48% and 24.2% +/- 0.9% compared with control, respectively . These effects were not observed with albumin challenge and were not cause d by osmolality fluctuation with transferrin treatment. In summary, our nov el finding of AQP3 in PTECs indicates a role for AQP3 in proximal tubule wa ter reabsorption. The pathophysiological significance of heightened AQP1 an d AQP3 expression in PTECs on protein challenge as occurs in the nephrotic state requires further investigation. (C) 2001 by the National Kidney Found ation, Inc.