Involvement of a cyclic adenosine monophosphate–dependent signal in the diet‐induced canalicular trafficking of adenosine triphosphate–binding cassette transporter g5/g8

Abstract

The adenosine triphosphate-binding cassette (ABC) half-transporters Abcg5 and Abcg8 promote the secretion of neutral sterol into bile. Studies have demonstrated the dietinduced gene expression of these transporters, but the regulation of their trafficking when the nutritional status changes in the liver remains to be elucidated. Here, we generated a novel in vivo kinetic analysis that can monitor the intracellular trafficking of Abcg5/ Abcg8 in living mouse liver by in vivo transfection of the genes of fluorescent proteintagged transporters and investigated how hypernutrition affects the canalicular trafficking of these transporters. The kinetic analysis showed that lithogenic diet consumption accelerated the translocation of newly synthesized fluorescent-tagged transporters to intracellular pools in an endosomal compartment and enhanced the recruitment of these pooled gene products into the bile canalicular membrane in mouse liver. Because some ABC transporters are reported to be recruited from intracellular pools to the bile canaliculi by cyclic adenosine monophosphate (cAMP) signaling, we next evaluated the involvement of this machinery in a diet-induced event. Administration of a protein kinase A inhibitor, N-(2-{[3-(4-bromophenyl)22-propenyl]amino}ethyl)25-isoquinolinesulfonamide, decreased the canalicular expression of native Abcg5/Abcg8 in lithogenic diet-fed mice, and injection of a cAMP analog, dibutyryl cAMP, transiently increased their levels in standard diet-fed mice, indicating the involvement of cAMP signaling. Indeed, canalicular trafficking of the fluorescent-tagged Abcg5/Abcg8 was enhanced by dibutyryl cAMP administration. Conclusion: These observations suggest that diet-induced lipid loading into liver accelerates the trafficking of Abcg5/Abcg8 to the bile canalicular membrane through cAMP signaling machinery. (HEPATOLOGY 2015;62:1215-1226 T he bile canalicular membrane of mammalian hepatocytes contains several adenosine triphosphate-binding cassette (ABC) transporters that couple adenosine triphosphate hydrolysis to the transport of specific substrates into the bile canaliculus.

Proteasome inhibition protects against diet-induced gallstone formation through modulation of cholesterol and bile acid homeostasis

“…although lXr activation has been shown to increase gallstone formation by regulating aBcG5 and aBcG8 levels (25,33), Bt or cf injection did not affect lXrα and lXrβ levels. PKa has also been shown to regulate aBcG5 and aBcG8 expression (26), and the present study showed that PKa phosphorylation was reduced by proteasome inhibition. in addition, it was found that PKa activation by 8-bromo-camP or Sp-camPS reversed the reduction of aBcG5 and aBcG8 upon proteasome inhibition, confirming the role of PKA in the proteasome inhibition-induced regulation of aBcG5 and aBcG8.…”

“…7a). thus, the PKa signaling pathway, which has also been shown to affect aBcG5 and aBcG8 expression, was analyzed (26). reduced PKa phosphorylation was found in Hep3B cells that were treated with proteasome inhibitors (fig.…”

ABCG5/G8: a structural view to pathophysiology of the hepatobiliary cholesterol secretion

“…Studies of recombinant proteins in heterologous systems reveal that both ABCG5 and ABCG8 are glycosylated, depend on calnexin and calreticulin chaperones proteins for folding, and require dimerization to exit the endoplasmic reticulum [65,66]. Regulation of trafficking and activity of the mature, post-Golgi complex is poorly understood but may be regulated by bile acids, sterols, and cAMP signaling [44,62,67]. Not all ABCG5/G8 appears to reside at the cell surface, suggesting that intercellular pool may be mobilized to promote cholesterol secretion [67,68].…”

“…Regulation of trafficking and activity of the mature, post-Golgi complex is poorly understood but may be regulated by bile acids, sterols, and cAMP signaling [44,62,67]. Not all ABCG5/G8 appears to reside at the cell surface, suggesting that intercellular pool may be mobilized to promote cholesterol secretion [67,68]. However, the underlying molecular mechanisms that regulate the distribution and activity of the ABCG5/G8 transporter have yet to be elucidated.…”

Thematic Review Series: Lipid Transfer Proteins ABCG5 and ABCG8: more than a defense against xenosterols

“…However, biochemical fractionation and immunolocalization approaches suggest a broader intracellular distribution of ABCG5 and ABCG8 (94). More recently, a recombinant fluorescently tagged ABCG5/G8 complex was shown to reside in an intracellular pool that is recruited to the canalicular surface in response to a lithogenic diet (95). In isolated rat liver canalicular membranes, ABCG5 was shown to reside in Triton-soluble Lubrol-resistant microdomains (96).…”