Regulation of mOAT-mediated organic anion transport by okadaic acid and protein kinase C in LLC-PK1 cells

Organic anion transporters in the kidney proximal tubule play an essential role in eliminating a wide range of organic anions including endogenous compounds, xenobiotics, and their metabolites, thereby preventing their potentially toxic effects within the body. We have previously cloned a cDNA encoding an organic anion transporter from mouse kidney (mOAT) (Lopez-Nieto, C, E., You, G., Bush, K. T., Barros, E. J. G., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478; Kuze, K., Graces, P., Leahy, A, Wilson, P., Stuhlmann, H., and You, G. (1999) J. Biol. Chem, 274, 1519-1524). In the present study, we assessed the potential for regulation of this transporter by heterologous expression of mOAT in the pig proximal tubule-like cell Line, LLC-PK1. We report here that both protein phosphatase (PP1/PP2A) inhibitor, okadaic acid, and protein kinase C (PKC) activators down-regulate mOAT-mediated transport of para-aminohippuric acid (PAH), a prototypic organic anion, in a time- and concentrationdependent manner. However their mechanisms of action for this down-regulation are distinct. Okadaic acid modulated PAH transport, at least in part, through phosphorylation/dephosphorylation of mOAT; phosphoamino acid analysis indicated this phosphorylation occurs on serine. In contrast, PKC activation induced a decrease in the maximum transport velocity (V-max) of PAH transport without direct phosphorylation of the transporter protein. Together these results provide the first demonstration that regulation of organic anion transport by mOAT is likely to be tightly controlled directly and indirectly by phatase PP1/PP2A and PKC. Our results also suggest that kinases other than PKC are involved in this process.