Relevance of canalicular transporter endocytosis in hepatocellular cholestasis

CROCENZI, FERNANDO A.

Rosario

Congreso; Reunión Anual de la Sociedad Argentina de Fisiología SAFIS 2019; 2019

Sociedad Argentina de Fisiología (SAFIS)

Bile formation is an osmotic process driven by the gradient generated by osmotically active compounds between sinusoidal blood and the bile canaliculi. Creation and maintenance of this gradient depend on the activity of canalicular hepatocellular transporters belonging to the ABC superfamily, such as BSEP and MRP2, which actively transport bile salts and organic anions such as glutathione, respectively, into bile. Activity of these transporters depends on their proper localization at the apical domain of hepatocytes, and acute changes at this level take place in both physiological and pathological conditions, by modulating their recycling between the canalicular membrane and intracellular subapical endosomal compartments. Thus, an increase in the number of transporters inserted in the canalicular membrane domain, and consequently in transporter secretory function, can occur in physiological conditions under demand (e.g., by intracellular increase of cAMP or bile salts). On the other hand, these transporters can suffer endocytic internalization and intracellular retention in cholestatic conditions; this leads to a decreased secretion of osmotically active biliary solutes and impairment of canalicular bile flow. Eventually, long lasting intracellular retention leads to increased degradation of the endocytosed transporters. Endocytic retrieval of canalicular transporters has been described in several experimental models of hepatocellular cholestasis, for example, that induced by monohydroxylated bile salts, hyperosmolarity, estrogens and endotoxin. More importantly, this phenomenon has been reported in several human cholestatic pathologies, such as primary sclerosing cholangitis, obstructive cholestasis, and primary biliary cholangitis. The mechanisms underlying the endocytic retrieval of canalicular transporters in hepatocellular cholestasis are not fully understood at present. Our group has focused on the study of the molecular mechanisms governing this process in the acute cholestasis induced by the endogenous metabolite of estradiol, estradiol 17beta-D-glucuronide (E17G), a main responsible of intrahepatic cholestasis of pregnancy. According to the evidence gathered so far, E17G activates signaling proteins in three complementary pathways; two pathways initiate with the estrogen receptor GPR30, and the remaining one initiates with the activation of PKCalfa. These signaling pathways lead to i) a shift of the transporters from "rafts" to "non-rafts" membrane microdomains followed by an increased microtubule-independent and clathrin-dependent endocytic retrieval of canalicular transporters, and ii) a delayed microtubule-dependent exocytic reinsertion of the canalicular transporter- containing vesicles occurring during the recovery phase of cholestasis. The understanding of this crucial pathomechanism at a molecular level opens new possibilities for treatment of cholestatic hepatopathies.