The filters were dried and dissolved in Clearsol-I for quantitation of radioactivity. Bidirectional trans-cellular transport study in Caco-2 cells Caco-2 cells were obtained from the American Type Culture Collection (Rockville, MD). Furthermore, a Bcrp inhibitor, Oroxin B elacridar, caused a decrease of intestinal secretion of uric acid. In Caco-2 cells, uric acid showed a polarized flux from the basolateral to apical side, and this flux was almost abolished in the presence of elacridar. These results demonstrate that BCRP contributes at least in part to the intestinal excretion of uric acid as extra-renal elimination pathway in humans and rats. Introduction Uric acid is usually a final product of purine nucleoside metabolism in humans, and it is thought that its level is usually well controlled, mainly by the balance between production in liver from purine nucleosides and excretion into urine. Although its physiological role is usually poorly comprehended, uric acid is thought not only to protect neuronal cells due to its antioxidant activity, but also to play a role in maintaining blood pressure [1], [2]. It has been suggested that serum uric acid (SUA) should be kept below 7 mg/dL to prevent hyperuricemia, which is a clinically important risk factor for cardiovascular diseases, chronic kidney disease and gout [3], [4]. In addition, it is known that several drugs in clinical use alter the level of SUA. For example, angiotensin II receptor blockers such as losartan decrease SUA level, while others increase it [5]C[8]. Salicylic acid causes an increase in SUA level at low dose, but a decrease at high dose [9]. Accordingly, it is important to clarify the mechanisms that control SUA level in order to understand the effects of drugs on uric acid disposition and to find better ways of regulating SUA. Uric acid handling in the kidney is very complex, involving glomerular filtration, tubular reabsorption and secretion. The uric acid transporters URAT1 Rabbit polyclonal to ACAD11 and URATv1, located at the apical and basolateral membranes of the proximal tubular Oroxin B cells, respectively, are involved in renal reabsorption of uric acid [8]C[13]. On the other hand, organic anion transporters including OAT1 [14], OAT2 [15], [16], and OAT3 [17] at the basolateral membrane and other transporters such as BCRP [18], NPT1 [19], [20], NPT4 [21] and MRP4 [22] at the apical membranes have been reported to be involved in renal secretory transport of uric acid. Among them, BCRP, NPT1 and NPT4 may make significant contributions to renal handling of uric acid, because genetic polymorphisms of these transporters are associated with gout and/or hyperuricemia [13], [23]. Uric acid is also excreted from the body extra-renally, although urinary excretion is usually predominant. Nevertheless, it has been suggested that one-third to one-fourth of uric acid is recovered in feces, indicating that biliary and/or intestinal secretion is an important alternative pathway(s) Oroxin B of uric acid excretion [24], [25]. However, there has been little mechanistic study on extra-renal excretion of uric acid. Accordingly, the purpose of the present study is to examine the mechanism(s) of uric acid excretion via liver and intestine, focusing on hepatobiliary excretion and intestinal secretion directly from blood into the lumen. Although there are significant species difference in uric acid handling between human and animals, transporters such as URAT1 [10], [26], [27], URATv1 [9], [28], BCRP [29], [30], OATs [14]C[17], [31] and NPTs [19]C[21], [32] are expressed in both humans and rats. However, rats exhibit a significantly lower SUA level than humans owing to the contribution of uricase, which metabolizes uric acid to allantoin as a final metabolic product of purine; this is different from the situation in humans [33]. However, the uricase inhibitor oxonate can decrease uricase activity contribution of extra-renal excretion of uric acid. Furthermore, Caco-2 cells, a well-established model of human intestinal epithelial cells, were used as a model to examine the human intestinal transport mechanism of uric acid. BCRP/ABCG2 (breast cancer resistance protein) is usually abundantly expressed at the apical membrane of small intestinal epithelial cells and in liver, and impaired BCRP function is usually associated with an increase of SUA level [13]..
