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J. Biol. Chem., Vol. 280, Issue 20, 19895-19901, May 20, 2005

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Up-regulation of Human myo-Inositol Oxygenase by Hyperosmotic Stress in Renal Proximal Tubular Epithelial Cells^*

K. Sandeep Prabhu, Ryan J. Arner, Hema Vunta, and C. Channa Reddy

From the Department of Veterinary Science and the Center for Molecular Toxicology and Carcinogenesis, the Pennsylvania State University, University Park, Pennsylvania 16802

myo-Inositol oxygenase (MIOX) catalyzes the oxidative cleavage of myo-inositol (MI) to give D-glucuronic acid, a committed step in MI catabolism. D-Glucuronic acid is further metabolized to xylitol via the glucuronate-xylulose pathway. Although accumulation of polyols such as xylitol and sorbitol is associated with MI depletion in diabetic complications, no causal relationship has been established. Therefore we are examining the role of MIOX in diabetic nephropathy. Here we present evidence that the basis for the depletion of MI in diabetes is likely to be mediated by the increased expression of MIOX, which is induced by sorbitol, mannitol, and xylitol in a porcine renal proximal tubular epithelial cell line, LLC-PK1. To understand the molecular mechanism of regulation of MIOX expression by polyols, we have cloned the human MIOX gene locus of 10 kb containing 5.6 kb of the 5' upstream sequence. Analysis of the 5' upstream sequence led to the identification of an osmotic response element (ORE) in the promoter region, which is present 2 kb upstream of the translation start site. Based on luciferase reporter and electrophoretic mobility shift assays, polyols increased the ORE-dependent expression of MIOX. In addition, we demonstrate that the activity of the promoter is dependent on the binding of the transcription factor, tonicity element-binding protein, or osmotic response element-binding protein, to the ORE site. These results suggest that the expression of MIOX is up-regulated by a positive feedback mechanism where xylitol, one of the products of MI catabolism via the glucuronate-xylulose pathway, induces an overexpression of MIOX.

Received for publication, March 9, 2005

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY943307.

To whom correspondence should be addressed: Dept. of Veterinary Science, 115 William L. Henning Bldg., the Pennsylvania State University, University Park, PA 16802. Tel.: 814-865-7696; Fax: 814-863-6140; E-mail: ccr1{at}psu.edu.

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