Dynamic localization of canalicular transporters in health and disease

ROMA M.G.

Santiago, Chile

Conferencia; International Symposium: "Lipids and the Liver: New Insights in Health and Disease"; 2010

Dpto. de Gastroenterología, Facultad de Medicina, Pontificia Universidad Católica de Chile

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} p.MsoBodyText3, li.MsoBodyText3, div.MsoBodyText3 {margin:0cm; margin-bottom:.0001pt; text-align:justify; line-height:200%; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} @page Section1 {size:595.3pt 841.9pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:35.4pt; mso-footer-margin:35.4pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Vesicle-based trafficking of canalicular transporters involves delivery of newly-synthesized carriers from the rough endoplasmic reticulum to either the canalicular membrane or to an endosomal, subapical compartment, followed by its exocytic targeting to the plasma membrane. From there, the transporters can undergo recycling towards and from the subapical, endosomal compartment, which serves as a reservoir of pre-existing transporters available on demand. The balance between exocytic targeting and endocytic internalization is therefore a chief determinant of the overall capability of the hepatocyte to secrete bile and to detoxify endo and xenobiotics. Hence, it is a highly regulated process. A cAMP-sensitive stimulatory pathway for the apical targeting of transporters exists, which is Ca2+-dependent, via Ca2+-CaM complex formation. This process is counter-regulated by activation of Ca2+-dependent PKCs (cPKC). cAMP-stimulatory effects may also involve PI3K activation, the novel PKCd isoform being the downstream effector. Bile salts also stimulate apical trafficking of transporters by signalling mechanisms. Whereas taurocholate promotes this process by stimulating a PI3K-dependent pathway, the anticholestatic, therapeutic bile salt taroursodeoxycholate does so by stimulating MAPKs of both p38MAPK and Erk types. Transporters traffics from their synthesis place towards the surroundings of the canalicular membrane in a microtubule-dependent fashion, followed by the microfilament-dependent fusion of transporter-containing vesicles with the apical membrane. Actin can regulate transporter localization via binding to plasma membrane-actin cross-linking proteins, such as the ezrin-radixin-moesin (ERM) family of proteins, or by binding to interacting-partner proteins, such as PDZK1 and HAX-1. Exacerbated internalization of canalicular transporters, resulting in bile secretory failure, occurs in several experimental models of hepatocellular cholestasis (e.g., in 17ß-estradiol glucuronide- and in tautolithocholate-induced cholestasis). This also occurs in most human cholestatic hepatopathies, including obstructive extrahepatic cholestasis, inflammatory cholestasis associated to autoimmune hepatitis, primary biliary cirrhosis and primary sclerosing cholangitis. Sustained internalization of canalicular tansporters may lead to delivery to the lysosomal compartment and further degradation, thus explaining the diminished post-transcriptional expression of transporters often associated with these hepatopathies. The mechanism by which the endocytic internalization of canalicular transporters occurs is poorly known. Most hepatopathies are associated with oxidative stress, and pro-oxidant conditions induces actin disorganization and further transporter internalization. In addition, a disturbed colocalization of canalicular transporters and radixin without actin disorganization has been reported both in experimental and in human cholestasis. The participation of “cholestatic” signaling pathways in this effect is being actively studied in experimental settings, as in 17ß-estradiol glucuronide-induced cholestasis, where activation of cPKC was shown to play a key role. Based on the information above, a number of experimental anticholestatic strategies have been tested in experimental settings to restore the normal insertion/internalization balance. They include the stimulation of the exocytic insertion of canalicular transporters (e.g., by cAMP or tauroursodeoxycholate), or the administration of signalling modulators able to block cholestatic signalling pathways (e.g., the cPKC inhibitor Gö6976). Additional strategies are expected to emerge in parallel to the discovery of new molecular mechanisms of transporter internalization, in an attempt to prevent their accelerated degradation by better assuring proper localization.