Abstract
Recently, the organic anion transporting polypeptide 4c1 (4c1) was identified as a novel uptake transporter predominantly expressed on the basolateral membrane of rat kidney proximal tubules. However, in‐vitro studies with MDCKII cells showed that upon transfection 4c1 polarizes to the apical membrane. We used multiple antibodies (N=4) to validate the subcellular localization of 4c1 in rat tissues and in MDCKII and LLCPK1 cells expressing rat 4c1. Antibodies amenable to immunohistochemistry were generated using N‐ and C‐terminal peptide sequences. Apical and basolateral protein markers were used for colocalization studies. Immunofluorescence studies showed that 4c1 polarizes to the apical surface in the in‐vitro cell models. In rat kidney, colocalization of 4c1 with aquaporin 1 but not calbindin in the subcortical region suggest that apical 4c1 expression is localized in proximal straight tubules. 4c1 also colocalizes with P‐glycoprotein and breast cancer resistance protein, but not with multidrug resistance protein 4 in rat kidney. These results were verified by western blot analysis of brush‐border membrane vesicles isolated from the kidney cortex and conclusively demonstrate the apical expression of 4c1 in the rat kidney. To investigate the physiological role of 4c1, future studies will focus on identifying substrates and the driving force for 4c1 mediated transport.
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