Oxalate-inducible AMBP gene and its regulatory mechanism in renal tubular epithelial cells

The AMBP [AIM (alpha(1)microglobulin)/bikunin precursor] gene encodes two plasma glycoproteins: A I M, an immunosuppressive lipocalin, and bikunin, a member of plasma serine protemase inhibitor family with prototypical Kunitz-type domain. Although previously believed to be constitutively expressed exclusively in liver, the present study demonstrates the induction of this gene by oxalate in porcine proximal tubular LLC-PKI cells and rat kidney. In liver, the precursor protein is cleaved in the Golgi network by a furin-like enzyme to release constituent proteins, which undergo glycosylation before their export from the cell. In the renal tubular cells, A I M and bikunin co-precipitate, indicating lack of cleavage of the precursor protein. As the expression of the AMBP gene is regulated by A1M-specific cis elements and transcription factors, A I M protein was studied as a representative of AMBP gene expression in renal cells. Oxalate treatment (500 mu M) resulted in a time- and dose-dependent induction of A I M protein in LLC-PK1 cells. Of the four transcription factors, HNF-4 (hepatocyte nuclear factor-4) has been reported previously to be a major regulator of AMBP gene expression in liver. Electrophoretic mobility-shift assay, supershift assay, immunoreactivity assay and transfection-based studies showed the presence of an HNF-4 or an HNF-4-like protein in the kidney, which can affect the expression of the AMBP gene. In situ hybridization and immunocytochemical studies showed that the expression of this gene in kidney was mainly restricted to cells lining the renal tubular system.