Characterization and regulation of human kim-1 gene promoter: the role of activator protein 1 (AP1)transcription factor

Kidney Injury Molecule-1 (KIM-1) is an immunoglobulin superfamily cell sur-face protein highly upregulated on the surface of dedifferentiated renal proximal tubule epithelial cells regenerating after toxic or ischemic injury. KIM-1 is known to be the most sensitive biomarker of kidney injury approved by FDA for preclinical safety stud-ies in 2008. Therefore, characterization of the human Kim-1 gene promoter and the associated transcription factors regulating its transcription under toxic injury is very crucial. For this purpose, a human-derived proximal tubule epithelial cell line HK2 was utilized. Ochratoxin A (OTA), Gentamicin (GM) and Cisplatin (CP), which are known to induce nephrotoxicity, were used in in vitro HK2 cell culture system as chemotoxic stress inducers. Significant changes were observed in KIM-1 protein and mRNA amounts under chemotoxic stress as shown by Western Blot Analysis and Quantitative Real Time PCR, respectively. The minimal promoter region of Kim-1 gene was characterized by deletional mutation analysis of Kim-1 upstream region. AP1 protein, a well-known stress response transcription factor, was demonstrated to bind to the promoter region of Kim-1 gene by employing Electrophoretic Mobility Shift Assay (EMSA) and Chromatin Immunoprecipitation (ChIP) assay. However, luciferase reporter assays did not reveal any significant change between the wild type and the mutant AP1 binding site at -1010 position bearing plasmids in transient transfections. In addition, KIM-1 open reading frame (ORF) was cloned into bacterial and eukaryotic recombinant protein expression vector systems in order to produce KIM-1 antibody and reveal the functional role(s) of KIM-1 in human cells, respectively. Our results suggest that, the 700 bp upstream region of Kim-1 gene contains the minimal promoter region and Kim-1 gene might be regulated by the cooperative activity of AP1 with other trans-acting or cis-regulatory elements.