Differential Regulation of Toll-Like Receptor 4 Gene Expression in Renal Cells by Angiotensin II: Dependency on AP1 and PU.1 Transcriptional Sites

Abstract

Background: Toll-like receptor 4 (TLR4) is involved in the sensing of lipopolysaccharide and, therefore, plays a central role in innate immune responses to Gram-negative bacteria. Interestingly, TLR4 expression occurs within the kidney. We have previously demonstrated that angiotensin II (ANG II) upregulates TLR4 expression on mesangial cells. However, the factors controlling transcriptional activation of the Tlr4 gene in mesangial cells are not known, and the specificity of this response for other renal cells is unclear. Methods: Cultured murine proximal tubular cells (mouse cortical tubule cell line; MCT cells), murine mesangial cells (MMCs), and murine podocytes were treated with ANG II. The expression of ANG II receptor mRNA and TLR4 mRNA and protein was determined by polymerase chain reaction and Western blotting. The transcriptional activity of wild-type and mutant mouse TLR4 promoter reporter constructs was determined upon transient transfection of the three cell types. Results: Although MMCs, podocytes, and syngeneic proximal MCT cells similarly expressed ANG II receptors, ANG II stimulated TLR4 mRNA and protein expression in MMCs and podocytes only. A mouse TLR4 promoter construct (–518/+129), previously shown to contain all important transcriptional regulatory elements in various cell types, was activated by ANG II in MMCs and podocytes, but not in MCT cells. Mutation of a proximal PU.1-binding consensus site or an AP1 site abolished ANG-II-mediated transcriptional activation of the TLR4 promoter. Finally, basal transcription of the Tlr4 gene depended in all three cell lines on an intact AP1 site and additionally on the proximal PU.1 site in MMCs. Conclusions: ANG II stimulates TLR4 transcription through AP1 and PU.1 sites in a cell-specific manner. Since the intrarenal ANG II concentrations are enhanced in many pathophysiological situations, ANG-II-stimulated transcription of TLR4 on MMCs and podocytes may contribute to renal inflammation.