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1 Vegetative Physiologie und Pathophysiologie, Zentrum Physiologie und Pathophysiologie, Goettingen, Germany
* To whom correspondence should be addressed. E-mail: hagos{at}veg-physiol.med.uni-goettingen.de.
The sodium dicarboxylate cotransporter located at the basolateral side supplies renal proximal tubule cells with Krebs cycle intermediates, and maintains the driving force for the exchange of organic anions like p-aminohippurate against 
-ketoglutarate through the organic anion transporter 1 (OAT1). Recently, we cloned the sodium dicarboxylate cotransporter 3 from winter flounder kidney (fNaDC-3). To understand the regulation of fNaDC-3, we preincubated fNaDC-3 expressing oocytes with phorbol-12-myristate-13-acetate (PMA), a protein kinase C (PKC) activator. PMA dose- and time-dependently inhibited fNaDC-3-mediated succinate uptake. Simultaneous preincubation of fNaDC-3 expressing oocytes with 50 nM PMA and either staurosporine or RO 31-8220 for 30 min attenuated PKC-mediated inhibition of succinate uptake. Site-directed mutagenesis of the five putative PKC sites [S7, T167, S174, T188 and S396] resulted in no change in PKC-mediated inhibition of the transporter. In electrophysiological studies performed at -60 mV, the K0.5 for succinate was not significantly affected (56 ± 13 µM vs 42 ± 19 µM), but 
Imax was reduced from -139 ± 49 nA to -20 ± 8 nA by PMA (50 nM, 30 min). Immunofluorescence analysis of fNaDC-3 expressing oocytes revealed that PMA leads to an endocytosis of fNaDC-3 protein. In conclusion, fNaDC-3 expressed in oocytes is down-regulated by PMA through endocytosis. PKC consensus sites appear not to be important for this process.
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