Published Online
on August 6, 2007

A more recent version of this article appeared on August 21, 2007

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Submitted on December 18, 2006
Accepted on June 15, 2007

Proteasome-Dependent Degradation of Guanosine 5'-Triphosphate Cyclohydrolase I Causes Tetrahydrobiopterin Deficiency in Diabetes Mellitus

Jian Xu PhD, Yong Wu MD, PhD, Ping Song PhD, Miao Zhang MD, PhD, Shuangxi Wang MD, PhD, and Ming-Hui Zou MD, PhD^*

From the Division of Endocrinology and Diabetes, Department of Medicine, University of Oklahoma Health Sciences Center, Oklahoma City.

^* To whom correspondence should be addressed. E-mail: ming-hui-zou{at}ouhsc.edu.

Background—Tetrahydrobiopterin (BH4) deficiency is reported to uncouple the enzymatic activity of endothelial nitric oxide synthase in diabetes mellitus. The mechanism by which diabetes actually leads to BH4 deficiency remains elusive. Here, we demonstrate that diabetes reduced BH4 by increasing 26S proteasome-dependent degradation of guanosine 5'-triphosphate cyclohydrolase I (GTPCH), a rate-limiting enzyme in the synthesis of BH4, in parallel with increased formation of both superoxide and peroxynitrite (ONOO^-).

Methods and Results—Exposure of human umbilical vein endothelial cells to high glucose concentrations (30 mmol/L D-glucose) but not to high osmotic conditions (25 mmol/L L-glucose plus 5 mmol/L D-glucose) significantly lowered the levels of both GTPCH protein and BH4. In addition, high glucose increased both the 26S proteasome activity and the ubiquitination of GTPCH. Inhibition of the 26S proteasome with either MG132 or PR-11 prevented the high glucose–triggered reduction of GTPCH and BH4. Exposure of human umbilical vein endothelial cells to exogenous ONOO^- increased proteasome activity and 3-nitrotyrosine in 26S proteasome. Furthermore, adenoviral overexpression of superoxide dismutase and inhibition of endothelial nitric oxide synthase with N^G-nitro-L-arginine methyl ester significantly attenuated the high glucose–induced activation of 26S proteasome and the reduction of GTPCH. Finally, administration of MG132 or a superoxide dismutase mimetic, tempol, reversed the diabetes mellitus–induced reduction of GTPCH and BH4 and endothelial dysfunction in streptozotocin-induced diabetes mellitus.

Conclusions—We conclude that diabetes mellitus triggers BH4 deficiency by increasing proteasome-dependent degradation of GTPCH.

Key words: atherosclerosis • endothelium • hyperglycemia • nitric oxide • nitric oxide synthase

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