Porcine liver sinusoidal endothelial cells contribute significantly to intrahepatic ammonia metabolism

Authors


  • Potential conflicts of interest: none.

Abstract

Ammonia metabolism in the liver has been largely credited to hepatocytes (HCs). We have shown that liver nonparenchymal cells that include liver sinusoidal endothelial cells (LSECs) produce ammonia. To address the limited knowledge regarding a role for LSECs in ammonia metabolism, we investigated the ammonia metabolism of isolated LSECs and HCs under three different conditions: (1) bioreactors containing LSECs (LSEC-bioreactors), (2) bioreactors containing HCs (HC-bioreactors), and (3) separate bioreactors containing LSECs and HCs connected in sequence (Seq-bioreactors). Our results showed that LSEC-bioreactors released six-fold more ammonia (22.2 nM/hour/106 cells) into the growth media than HC-bioreactors (3.3 nM/hour/106 cells) and Seq-bioreactors (3.8 nM/hour/106 cells). The glutamate released by LSEC-bioreactors (32.0 nM/hour/106 cells) was over four-fold larger than that released by HC-bioreactors and Seq-bioreactors (<7 nM/hour/106 cells). LSEC-bioreactors and HC-bioreactors consumed large amounts of glutamine (>25 nM/hour/106 cells). Glutaminase is known for catalyzing glutamine into glutamate and ammonia. To determine if this mechanism may be responsible for the large levels of glutamate and ammonia found in LSEC-bioreactors, immunolabeling of glutaminase and messenger RNA expression were tested. Our results demonstrated that glutaminase was present with colocalization of an LSEC-specific functional probe in lysosomes of LSECs. Furthermore, using a nucleotide sequence specific for kidney-type glutaminase, reverse-transcription polymerase chain reaction revealed that this isoform of glutaminase was expressed in porcine LSECs. Conclusion: LSECs released large amounts of ammonia, perhaps due to the presence of glutaminase in lysosomes. The ammonia and glutamate released by LSECs in Seq-bioreactors were used by hepatocytes, suggesting an intrahepatic collaboration between these two cell types. (HEPATOLOGY 2009.)

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