Regulation of Plasma Membrane Localization of the Na⁺-Taurocholate Co-Transporting Polypeptide by...

Hydrophobic bile salts, such as glycochenodeoxycholate (GCDC) can trigger hepatocyte apoptosis, which is prevented by tauroursodesoxycholate (TUDC), but the effects of GCDC and TUDC on sinusoidal bile salt uptake via the Na⁺-taurocholate transporting polypeptide (Ntcp) are unclear.

The effects of GCDC and TUDC on the plasma membrane localization of Ntcp were studied in perfused rat liver by means of immunofluorescence analysis and super-resolution microscopy. The underlying signaling events were investigated by Western blotting and inhibitor studies.

GCDC (20 µmol/l) induced within 60 min a retrieval of Ntcp from the basolateral membrane into the cytosol, which was accompanied by an activating phosphorylation of the Src kinases Fyn and Yes. Both, Fyn activation and the GCDC-induced Ntcp retrieval from the plasma membrane were sensitive to the NADPH oxidase inhibitor apocynin, the antioxidant N-acetylcysteine and the Src family kinase inhibitors SU6656 and PP-2, whereas PP-2 did not inhibit GCDC-induced Yes activation. Internalization of Ntcp by GCDC was also prevented by the protein kinase C (PKC) inhibitor Gö6850. TUDC (20 µmol/l) reversed the GCDC-induced retrieval of Ntcp from the plasma membrane and prevented the activation of Fyn and Yes in GCDC-perfused rat livers. Reinsertion of Ntcp into the basolateral membrane in GCDC-perfused livers by TUDC was sensitive to the protein kinase A (PKA) inhibitor H89 and the integrin-inhibitory peptide GRGDSP, whereas the control peptide GRADSP was ineffective. Ex posure of cultured rat hepatocytes to GCDC (50 µmol/l, 15min) increased the fluorescence intensity of the reactive oxygen fluorescent indicator DCF to about 1.6-fold of untreated controls in a TUDC (50 µmol/l)-sensitive way. GCDC caused a TUDC-sensitive canalicular dilatation without evidence for Bsep retrieval from the canalicular membrane.

The present study suggests that GCDC triggers the retrieval of Ntcp from the basolateral membrane into the cytosol through an oxidative stress-dependent activation of Fyn. TUDC prevents the GCDC-induced Fyn activation and Ntcp retrieval through integrin-dependent activation of PKA.