Protein kinase Cδ differentially regulates cAMP-dependent translocation of NTCP and MRP2 to the plasma membrane

Park SW, Schonhoff CM, Webster CR, Anwer MS. Protein kinase C delta differentially regulates cAMP-dependent translocation of NTCP and MRP2 to the plasma membrane. Am J Physiol Gastrointest Liver Physiol 303: G657-G665, 2012. First published June 28, 2012; doi:10.1152/ajpgi.00529.2011.-Cyclic AMP stimulates translocation of Na+/taurocholate cotransporting polypeptide (NTCP) from the cytosol to the sinusoidal membrane and multidrug resistance-associated protein 2 (MRP2) to the canalicular membrane. A recent study suggested that protein kinase C delta (PKC delta) may mediate cAMP-induced translocation of Ntcp and Mrp2. In addition, cAMP has been shown to stimulate NTCP translocation in part via Rab4. The aim of this study was to determine whether cAMP-induced translocation of NTCP and MRP2 require kinase activity of PKC delta and to test the hypothesis that cAMP-induced activation of Rab4 is mediated via PKC delta. Studies were conducted in HuH-NTCP cells (HuH-7 cells stably transfected with NTCP). Transfection of cells with wild-type PKC delta increased plasma membrane PKC delta and NTCP and increased Rab4 activity. Paradoxically, overexpression of kinase-dead dominant-negative PKC delta also increased plasma membrane PKC delta and NTCP as well as Rab4 activity. Similar results were obtained in PKC delta knockdown experiments, despite a decrease in total PKC delta. These results raised the possibility that plasma membrane localization rather than kinase activity of PKC delta is necessary for NTCP translocation and Rab4 activity. This hypothesis was supported by results showing that rottlerin, which has previously been shown to inhibit cAMP-induced membrane translocation of PKC delta and NTCP, inhibited cAMP-induced Rab4 activity. In addition, LY294002 (a phosphoinositide-3kinase inhibitor), which has been shown to inhibit cAMP-induced NTCP translocation, also inhibited cAMP-induced PKC delta translocation. In contrast to the results with NTCP, cAMP-induced MRP2 translocation was inhibited in cells transfected with DN-PKC delta and small interfering RNA PKC delta. Taken together, these results suggest that the plasma membrane localization rather than kinase activity of PKC delta plays an important role in cAMP-induced NTCP translocation and Rab4 activity, whereas the kinase activity of PKC delta is necessary for cAMP-induced MRP2 translocation.