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 ϳ2 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 elementbinding 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.
myo-Inositol (MI),1 the dominant form of the physiological inositol isomers, is utilized in many tissues and cell types as a precursor for the synthesis of second messengers and also as an organic osmolyte (1). The first committed step in MI metabolism is catalyzed by the monooxygenase, myo-inositol oxygenase (MIOX; EC 1.13.99.1), which occurs predominantly in the proximal tubular epithelial cells of the kidney cortex (2). The enzymatic reaction involves the oxidative cleavage of the ring in MI between C-6 and C-1 to give D-glucuronic acid. The D-glucuronate formed in animals by this mechanism is successively converted in subsequent steps to L-gluonate, 3-keto-L-gulonate, L-xylulose, xylitol, D-xylulose, and D-xylulose 5-phosphate, which then enters the pentose phosphate cycle (Fig. 1). Studies in human pentosuric patients confirmed that this is the only pathway of MI catabolism (3). We have recently reported the cloning and expression of MIOX, where we demonstrated that D-chiro-inositol, a MI isomer that exhibits insulin-like signaling properties (4), is also a substrate for MIOX (2).myo-Inositol has been suggested to play an important role in the etiology of diabetes mellitus, particularly with respect to the progression of diabetic nephropathy, neuropathy, retinopathy, and diabetic cataract. In diabetic complications, increased glucose levels are associated with high sorbitol accumu...