Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase. ADMA is generated by protein methyltransferase (PRMT) and is metabolized mainly by dimethylarginine dimethylaminohydrolase (DDAH). ADMA levels are reported to increase in patients with chronic kidney disease (CKD), thereby playing a role in the pathogenesis of accelerated atherosclerosis in this population. However, the precise mechanism underlying ADMA accumulation in these patients is not fully understood. This study investigated the molecular mechanism for the elevation of ADMA levels in CKD, using a rat remnant kidney model that represents progressive CKD. After male Sprague-Dawley rats underwent baseline measurement of BP and renal function, 5/6 subtotal nephrectomy (5/6Nx) and 4/6 nephrectomy were performed. Plasma and urinary levels of ADMA and symmetric dimethylarginine, an inert isomer of ADMA, were measured by HPLC. Expression levels of PRMT genes and DDAH proteins were analyzed by semiquantitative reverse transcription-PCR and Western blotting, respectively. Plasma ADMA levels were elevated in the Nx groups in proportion to the degree of nephrectomy despite marked increases in renal clearance of ADMA. In contrast, renal clearance of symmetric dimethylarginine was decreased and its plasma levels were increased in the Nx groups. Furthermore, both liver and kidney gene expression of PRMT was increased, whereas DDAH protein expression was decreased in the 5/6Nx group. Plasma ADMA levels were correlated with systolic BP levels. Moreover, adenovirus-mediated DDAH gene transfer into the 5/6Nx rats prevented the elevation of BP levels, which was associated with the reduction of plasma and urinary ADMA levels. The results presented here suggest that decreased DDAH levels as well as increased PRMT gene expression could cause the elevation of plasma ADMA levels, thereby eliciting hypertension in CKD. Substitution of DDAH protein or enhancement of its activity may become a novel therapeutic strategy for the treatment of hypertension-related vascular injury in CKD.

Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor, is mainly degraded by dimethylarginine dimethylaminohydrolase (DDAH). It was recently reported that reduced DDAH expression could contribute to ADMA accumulation and subsequent elevation of BP in an experimental model of chronic kidney disease (CKD). ADMA is a strong predictor of the progression of CKD as well. However, a role for the ADMA-DDAH in the pathogenesis of CKD remains to be elucidated. This study investigated the effects of DDAH-elicited ADMA lowering on renal function and pathology in a rat remnant kidney model. Four weeks after five-sixths subtotal nephrectomy (Nx), the rats were given tail-vein injections of recombinant adenovirus vector encoding DDAH-I (Adv-DDAH) or control vector expressing bacterial ␤-galactosidase The synthesis of NO can be blocked by inhibition of the NOS active site with guanidino-substituted analogues of l-arginine, such as asymmetric dimethylarginine (ADMA) (1,2). We, along with others, have demonstrated that elevated plasma ADMA is associated with cardiovascular risk factors such as hypertension (3,4), diabetes (4,5), and chronic kidney disease (CKD) (6,7), thereby being one of the useful biomarkers for atherosclerosis and future cardiovascular events (5,7-9). ADMA is mainly metabolized by an enzyme dimethylarginine dimethylaminohydrolase (DDAH) (1,2). We recently found that reduced DDAH expression could contribute to ADMA accumulation and subsequent elevation of BP in an experimental model of CKD (10). Furthermore, plasma level of ADMA is known to be a strong predictor of the progression of renal dysfunction in patients with CKD (11,12). According to the recent comprehensive review on the potential role of ADMA in renal disease progression (13), there may be two major possible mechanisms by which ADMA could contribute to the progression of CKD; one is a BP-dependent effect of ADMA described previously (4,10), and the other is a BP-independent, direct effect of ADMA on renal microvasculature. As to the latter, Kang et al. (14) demonstrated that administration of an inhibitor of NOS accelerated renal injury and impaired angiogenic response and peritubular capillary formation in the remnant kidney model, whose harmful effects were greater than expected from the increase in BP levels, thus suggesting that an important role of NO in maintaining renal microvasculature (15,16). Therefore, it is plausible that DDAH could protect against renal damage by suppressing the inhibitory effect of ADMA on NO generation. In this study, we investigated the effects of DDAH-elicited ADMA lowering on renal function and tubulointerstitial changes in a rat remnant kidney model. To address the issue of whether DDAH could protect against renal injury in a BPindependent manner, we compared the renoprotective effects of DDAH with those of hydralazine (Hyz), an antihypertensive drug with equihypotensive property.

Advanced glycation end products (AGEs) are senescent macroprotein derivatives that are formed at an accelerated rate in patients with chronic renal failure (CRF). AGE formation and accumulation in plasma and vascular tissues contribute to accelerated atherosclerosis in this devastating disorder. AST-120 is an oral adsorbent that attenuates the progression of CRF by removing uremic toxins. Recently, AST-120 has been reported to reduce the progression of atherosclerosis as well. However, whether AST-120 decreases serum levels of AGEs and subsequently exerts atheroprotective properties remains to be elucidated. Ten nondiabetic CRF patients were enrolled in this study. All patients were kept on regular therapeutic diet and medications throughout the study. Serum AGE levels before and after AST-120 treatments were measured using enzyme-linked immunosorbent assay. Effects of patient-derived serum on atherosclerosis-related gene expression in cultured human umbilical vein endothelial cells (HUVECs) were analyzed by semiquantitative RT-PCR. Administration of AST-120 (6 g/day) for 3 months significantly decreased serum levels of AGEs in nondiabetic CRF patients, whereas AGE levels remained unchanged in age-and renal function-matched CRF patients without AST-120 treatment (n = 6). Patient serum after AST-120 treatment significantly reduced mRNA levels of receptor for AGEs, monocyte chemoattractant protein-1, and vascular adhesion molecule-1 in HUVECs compared with serum before treatment. Moreover, in vitro, AST-120 was found to adsorb carboxymethyllysine (CML), one of the well-characterized, digested food-derived AGEs. This study suggests that atheroprotective properties of AST-120 can be ascribed, at least in part, to its AGE-lowering ability via absorption of CML.

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