Physiological and Molecular Characterization of Aristolochic Acid Transport by the Kidney

Abstract

Consumption of herbal medicines derived fromAristolochia plants is associated with a progressive tubulointerstitial disease known as aristolochic acid (AA) nephropathy. The nephrotoxin produced naturally by these plants is AA-I, a nitrophenanthrene carboxylic acid that selectively targets the proximal tubule. This nephron segment is prone to toxic injury because of its role in secretory elimination of drugs and other xenobiotics. Here, we characterize the handling of AA-I by membrane transporters involved in renal organic anion transport. Uptake assays in heterologous expression systems identified murine organic anion transporters (mOat1, mOat2, and mOat3) as capable of mediating transport of AA-I. Kinetic analyses showed that all three transporters have an affinity for AA-I in the submicromolar range and thus are likely to operate at toxicologically relevant concentrations in vivo. Structure-activity relationships revealed that the carboxyl group is critical for high-affinity interaction of AA-I with mOat1, mOat2, and mOat3, whereas the nitro group is required only by mOat1. Furthermore, the 8-methoxy group, although essential for toxicity, was not requisite for transport. Mouse renal cortical slices avidly accumulated AA-I, achieving slice-to-medium concentration ratios Ͼ10. Uptake by slices was sensitive to known mOat1 and mOat3 substrates and the organic anion transport inhibitor probenecid, which also blocked the production of DNA adducts formed with reactive intracellular metabolites of AA-I. Taken together, these findings indicate that OAT family members mediate high-affinity transport of AA-I and may be involved in the site-selective toxicity and renal elimination of this nephrotoxin.

Aristolochic Acid I Metabolism in the Isolated Perfused Rat Kidney

“…It was recently shown that organic ion transporters (OATs) 1 and 3 mediate concentrative uptake of AA-I ( 1 ) in the mouse proximal tubule and may be involved in the site-selective toxicity of this nephrotoxin. 31 Thus, in the IPK, OATs 1/3 likely mediate basolateral uptake of AA-I ( 1 ) from the perfusate. The nephrotoxin then undergoes demethylation or nitroreduction in the proximal tubule, followed by secretory efflux of metabolites to the urine and/or reabsortive efflux of metabolites to the perfusate.…”

Probenecid prevents acute tubular necrosis in a mouse model of aristolochic acid nephropathy

“…9,10 In vitro studies confirmed that AAI is much more cytotoxic compared with AAII because of the presence of a methoxy group in position 8. 31,32 On the other hand, the carboxyl group rather than the nitro group is important to facilitate AA entry into tubular cells via OATs. Finally, nitroreduction results in N-hydroxy-aristolactam formation, and these metabolites bind covalently to the exocyclic amino groups of adenine or guanine forming AAspecific DNA adducts.…”

New approaches for biomonitoring exposure to the human carcinogen aristolochic acid

“…51 The elevated levels of dA-AL-I adducts in renal cortex likely are due to the active transport of AA-I into the cortex by organic ion transporters, which also may be involved in the site-selective toxicity and renal elimination of AA-I. 69 Of the 148 Taiwanese subjects assayed by MS, 132 subjects harbored dA-AL-I at a level above 0.3 adducts per 10 8 DNA bases (the LOQ value) in the renal cortex (unpublished data). 25,45,51 The mean level of dA-AL-I was 16.0 adducts per 10 8 DNA bases with a 95% confidence interval range from 10.1 to 21.0 adducts per 10 8 DNA bases (a value of 0.15 adducts per 10 8 DNA bases, one-half the LOQ value for non-positive samples, was used for the calculation of the 95% CI).…”