The Na+‐Taurocholate Cotransporting Polypeptide Traffics with the Epidermal Growth Factor Receptor

Na+‐taurocholate cotransporting polypeptide (ntcp) mediates bile acid transport, also serving as the hepatitis B virus receptor. It traffics in vesicles along microtubules, requiring activity of protein kinase C (PKC)ζ for motility. We have now found that the epidermal growth factor receptor (EGFR) is the target of PKCζ activity and that EGFR and ntcp colocalize in vesicles. ntcp‐containing vesicles that are not associated with EGFR have reduced microtubule‐based motility, consistent with intracellular accumulation and reduced surface expression of ntcp in cells following EGFR knockdown. Na+‐taurocholate cotransporting polypeptide (ntcp) mediates uptake of bile acids as well as serving as the receptor for hepatitis B virus in human liver. Previous studies showed that ntcp traffics on microtubules between the cell surface and endocytic vesicles. Specific inhibition of protein kinase C (PKC)ζ resulted in loss of microtubule‐based motility of these vesicles in vitro and in living cells. The aim of this study was to characterize the PKCζ target. Incubation of ntcp‐containing endocytic vesicles with γ‐32P‐ATP revealed a 180 kDa phosphoglycoprotein that was identified as the epidermal growth factor (EGF) receptor (EGFR). Surface biotinylation of HuH7 cells expressing green fluorescent protein (GFP)‐ntcp revealed substantially reduced trafficking of ntcp to the cell surface with EGFR knockdown. Microtubule‐based motility of ntcp‐containing endocytic vesicles was also significantly reduced when they were not associated with EGFR. ntcp was also found to undergo cellular redistribution upon stimulation of cells with EGF, consistent with a model in which ntcp and EGF–EGFR internalize into common endocytic vesicles from which they segregate, trafficking EGF–EGFR to lysosomes and recycling ntcp to the plasma membrane. EGF regulation of ntcp trafficking may play a heretofore unanticipated role in subcellular targeting of ntcp ligands such as hepatitis B.