Flavonoids modulate cell signaling and inhibit oxidative enzymes. After oral consumption, they circulate in human plasma as amphiphilic glucuronide or sulfate conjugates, but it is unknown how these physiological metabolites permeate into cells. We examined the mechanisms of uptake of these conjugates into hepatocellular carcinoma (HepG2) cells, and found that uptake of quercetin-3′-O-sulfate was saturable and temperature-dependent, indicating the involvement of carrier-mediated transport. Quercetin-3-O-glucuronide was taken up predominantly via passive diffusion in these cells. Quantitative real-time PCR analysis showed high expression of OATP4C1, followed by OAT2, OAT4 and low expression of OATP1B1 in HepG2 cells, and addition of inhibitors of OATs and OATPs resulted in a significant reduction in quercetin-3′-O- sulfate uptake. The accumulation of quercetin-3′-O-sulfate was further evaluated in HEK293 cells expressing OAT2, OAT4 and OATP4C1. Uptake of quercetin-3′-O-sulfate was 2.3- and 1.4-fold higher in cells expressing OAT4 and OATP4C1 at pH 6.0, respectively, than in control HEK293 cells. siRNA knockdown of OATP4C1 expression in HepG2 cells reduced uptake of quercetin-3′-O-sulfate by ∼40%. This study highlights a role for OATs and OATPs in the cellular uptake of biologically active flavonoid conjugates.
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