How the Liver Rescues the Body from Metal Overload

Manganese (Mn) toxicity is caused by the excessive accumulation of Mn in the human body which can impair the normal functioning of neurotransmitter systems, damages the basal ganglia and contributes to hyperactivity, learning disabilities, and attention deficit disorder in children. The prevalence of Mn toxicity has been attributed to the contamination of drinking water, environmental emissions, occupational hazards, hepatic encephalopathy, the use of soy infant formulas and parenteral nutrition support.

Mn levels in the blood are tightly regulated by hepatobiliary clearance. Previous studies have shown that the disruption of human genes involved in Mn transport and homeostasis (i.e. SLC39A14 (ZIP14), SLC39A8 (ZIP8), and SLC30A10 (ZnT10)) leads to the acquisition and accumulation of Mn. Although the evidence obtained from mouse genetic models and human patients have revealed the possible transport pathways involved in the hepatobiliary clearance of Mn, in vitro models that can recapitulate this process have not been established.

Recently, Harvard University scientists: Dr. Khristy Thompson, Jennifer Hein, Andrew Baez, Jose Carlo Sosa, and Professor Marianne Wessling-Resnick from the Department of Genetics and Complex Diseases investigated the expression and cellular distribution of Mn transporters in WIF-B cells. WIF-B cells have been used to study hepatic basolateral-to-apical transport in humans. The authors also developed an in vitro assay for hepatocyte Mn efflux and identified the roles of Mn transporters in Mn influx and efflux in WIF-B cells. The work is published in the American Journal of Physiology- Gastrointestinal Liver Physiology.

To optimize their in vitro model, the research team detected maximal expression of Fpn, ZIP14, ZIP8, and ZnT10 transcripts in polarized WIF-B cells after 12 days in culture. They observed that ZnT10-positive vesicles were adjacent to apical bile compartments while ZIP8 positive vesicles were uniformly distributed in the cytoplasm of hepatocytes.

WIF-B cells were remarkably resistant to brief 4-h exposures to 500 µM MnCl₂ while the cells showed decreased viability after 16h exposure to more than 250 µM MnCl_2. The morphological and functional features of the polarized WIF-B cells were similar to untreated control cells and the levels of Fpn, ZIP14, ZIP8, and ZnT10 remained the same after 4 h exposures to 500 µM MnCl₂.

The authors looked carefully at the traffic of Mn in WIF-B cells using some pharmacological inhibitors. For example, they found the efflux of Mn taken up by polarized WIF-B cells is time-dependent. Levels of Fpn are reduced in WIF-B cells by hepcidin, while treatment with hepcidin did not affect Mn efflux in WIF-B cells, and the secretory inhibitor, brefeldin A, blocked the release of Mn from WIF-B cells.

The study by Khristy Thompson and her colleagues provide compelling and comprehensive picture of hepatobiliary efflux of Mn and that several metal transporters function at specific cellular locations to ensure the delivery of the metal from the sinusoidal basolateral surface to the bile canalicular apical membrane of polarized WIF-B cells. The new findings documented in their study suggest that ZnT10 may play a major role in the import of Mn into vesicles that fuse with the apical membrane to release their contents into bile. The in vitro model is also expected advance further studies on the secretory mechanisms that mediate the efflux of Mn in hepatocytes.