Manganese toxicity and effects on polarized hepatocytes

Abstract

Manganese (Mn) is an essential metal involved in several cellular metabolic pathways including DNA synthesis, sugar metabolism and protein modification. The majority of Mn is obtained through the diet in food products such as nuts, whole grains and leafy greens. Abundant in most diets, Mn deficiency is rare while excess exposure in the occupational environment leads to cytotoxic levels. Labour workers commonly inhale Mn in settings like welding and mining. Metal inhalation bypasses many of the body's homeostatic pathways leading to accumulation in the brain. Physiologically, the presence of Mn in Mnsensitive brain regions, such as the globus pallidus, has been linked to neurodegeneration and induction of a Parkinsonian-like syndrome known as manganism. Mn homeostasis is therefore critical for brain health. The liver controls the redistribution of excess Mn to specific tissues/organs and hepatobiliary clearance.Mutations in Mn transporters, however, compromises homeostasis causing hepatic damage and surplus body Mn. Understanding Mn toxicity in hepatocytes is crucial for developing new medicines that prevent blood Mn build-up. To understand the molecular changes attributed to excess hepatic Mn, we sought to determine changes in hepatocyte viability under Mn hepatotoxicity. For these experiments, polarized hepatocytoma WIF-B cells were grown for 12-14 days to achieve maximal polarity. Immunocytochemistry, Western blot and MTT viability assays helped characterize Mn's effect on the Golgi. We found that WIF-B cell viability was maintained during 4 h exposures of up to 100 μMMn. Under these conditions, we identified no change at the cis-Golgi but levels of the trans-Golgi marker TGN38 fell in a dosedependent manner. Immunofluorescence (IF) images confirmed that Mn-induced TGN38 loss, while the cis-Golgi marker GM130 remained unaffected. Treatment with the lysosomal inhibitor Bafilomycin A1 for 16 h prevented degradation of TGN38 when cells were exposed to Mn for 4 h and increased its co-localization with the late endosomal marker mannose-6-phosphate receptor (M6PR). Our results suggest disrupting Mn homeostasis negatively affects the integrity of the Golgi apparatus, altering normal Mn trafficking in WIF-B cells. Understanding how Mn-induced changes in the Golgi architecture affect toxicity is key to developing therapeutic treatments for Mn toxicity.