Regulation of the human organic cation transporter hOCT1

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Abstract
The human organic cation transporter type 1 (hOCT1) is an important transport system for small organic cations in the liver. Organic cation transporters are regulated by different signaling pathways, but the regulation of hOCT1 has not yet been studied. In this work, we have for the first time investigated the regulation of hOCT1. hOCT1 was expressed in Chinese hamster ovary cells (CHO-hOCT1) and in human embryonic kidney cells (HEK293-hOCT1). Its activity was monitored using microfluorimetry with the fluorescent organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP+) as substrate. hOCT1 expressed in CHO-cells was inhibited by protein kinase A (PKA) activation (1 µM forskolin, −58 ± 6%, n = 12), calmodulin inhibition (0.1 µM calmidazolium, −68 ± 3%, n = 6; 10 µM ophiobolin A, −48 ± 10%, n = 7), calmodulin-dependent kinase II inhibition (1 µM KN62, −78 ± 4%, n = 12), and inhibition of p56lck tyrosine kinase (10 µM aminogenistein, −35 ± 7%, n = 12). The apparent affinities for TEA+ were lower in CHO-hOCT1 than in HEK293-hOCT1, while those for TPA+ and quinine were almost identical; the rank order of EC50 values (TPA+ > quinine > TEA+) was independent of the expression system. EC50 values for TEA+ in CHO-hOCT1 or HEK293-hOCT1 were increased under calmidazolium incubation (6.3 and 1.4 mM, respectively). hOCT1 was inhibited by PKA and endogenously activated by calmodulin, calmodulin-dependent kinase II, and p56lck tyrosine kinase. Regulation pathways were the same in the two expression systems. Since apparent substrate affinities depend on activity of regulatory pathways, the expression system plays a role in determining the substrate affinities.

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