XENOBIOTIC REGULATION OF THE ATP BINDING CASSETTE TRANSPORTER ABCB6 AND ITS SIGNIFICANCE TO HEPATIC HEME HOMEOSTASIS

Heme is indispensable for mammalian life. It is an essential component of numerous heme proteins, with functions including oxygen transport and storage, energy metabolism, drug and steroid metabolism and signal transduction. Under normal physiological conditions intracellular free heme levels are extremely low because increased levels of free heme are cytotoxic and accordingly, heme biosynthesis is tightly regulated. Although, 5-aminolevulinic acid synthase (ALAS) mediated regulation of heme synthesis is considered the key step in heme biosynthesis, recent reports have identified a second regulatory step in heme biosynthesis mediated by the mitochondrial ATP binding cassette transporter b6 (Abcb6). Abcb6 expression is directly related to enhanced de novo porphyrin biosynthesis, and Abcb6 overexpression activates the expression of genes important for heme biosynthesis. Thus, Abcb6 represents a previously unrecognized rate-limiting step in heme biosynthesis. The dissertation outlines the progress made since its initiation in understanding the mechanism(s) that regulate Abcb6 expression and the significance of Abcb6 expression to cellular heme homeostasis. Exposure to therapeutic drugs and environmental contaminants leads to an increase in heme demand to compensate for the increased expression of the heme-dependent cytochrome P450s (P450s) detoxifying enzymes. Cells respond to this increasing heme demand by increasing heme synthesis. Thus, exposure to environmental contaminants serves as an optimal in vivo and in vitro model system to study mechanisms that regulate heme synthesis. In this model, Abcb6 expression was induced in response to exposure to xenobiotics [polyaromatic hydrocarbons (PAHs), 1,4-Bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and pregnenolone 16alpha -carbonitrile (PCN)] suggesting a co-ordinate induction of Abcb6 to support the increased heme synthesis. Increased Abcb6 expression in response to cellular heme demands was mediated by the xenobiotic sensing nuclear receptors aryl-hydrocarbon receptor (AhR), the constitutive androstane receptor (CAR), and the pregnane-X receptor (PXR). Exposure to environmental contaminants also leads to the generation of oxidative stress, a primary mechanism by which these compounds cause cellular damage. Cells respond to this increased oxidative stress by activating anti-oxidant defense mechanisms, whose principal components include hemo-proteins (such as catalase, superoxide dismutase, etc). Arsenic, an environmental contaminant and a major hazard following occupational exposure exerts its chronic toxicity through the generation of reactive oxygen species. Of importance, exposure to arsenic also activates the antioxidant defense mechanism. Thus, exposure to arsenic serves as a good model system to evaluate in vivo and in vitro oxidative stress response. In this model system, sodium arsenite induced Abcb6 expression in a dose-dependent manner both in mice fed sodium arsenite in drinking water and in cells exposed to sodium arsenite in vitro. Arsenite-induced Abcb6 expression was transcriptionally regulated but was not mediated by the redox sensitive transcription factor nuclear factor-erythroid 2-related factor 2 (Nrf2). The significance of Abcb6 expression to cellular heme homeostasis under conditions of heme demand was evaluated in vitro by both gain of function (cells engineered to overexpress Abcb6) and loss of function (cells where endogenous Abcb6 expression was knocked down using Abcb6 specific ShRNA) analysis. Loss of Abcb6 expression in these in vitro model systems significantly compromises the ability of cells to respond to increased heme demand and the ability to protect against oxidative stress following exposure to environmental contaminants. To understand the significance of Abcb6 function to heme homeostasis in vivo, we generated mice carrying homozygous deletion of the Abcb6 allele (Abcb6 null mice). Abcb6 null animals appear phenotypically normal with a trend towards decreased hepatic heme levels, although, the decreased heme levels did not appear to be statistically significant. Interestingly however, Abcb6 null mice demonstrate genotypic changes that suggest a role for Abcb6 in lipid and cholesterol homeostasis. Abcb6 null mice have increased fasting serum cholesterol and increased accumulation of androstone metabolites. Abcb6 null mice also show decreased expression and activity of a specific set of P450s suggesting a role for Abcb6 in drug metabolism and disposition. Mitochondrial ABC transporters are difficult to study because of the two-membrane architecture of mitochondria, problems associated with analyzing transport process, and the high abundance of other ATPases and carriers/transporters. Thus, the development of an in vitro system with pure and active protein is a prerequisite toward understanding the mechanistic relationships between ATP binding and hydrolysis and coupling of these events to translocation of substrates across the lipid membranes. Towards this end, we developed an in vitro liposomal transport system with pure and active Abcb6 protein. Reconstitution of Abcb6 into liposomes allowed biochemical characterization of the ATPase including (i) substrate stimulated ATPase activity (ii) transport kinetics of its proposed endogenous substrate coproporphyrinogen III and (iii) transport kinetics of substrates identified using a HTS assay. In summary, this dissertation provides insight into the mechanisms that regulate Abcb6 expression in response to increasing heme demand and the in vitro significance of Abcb6 to cellular heme homeostasis. Development of the Abcb6-null mice suggests that loss of Abcb6 does not severely affect heme-dependent functions in the liver probably because of the activation of compensatory mechanisms that balance the loss of Abcb6. More interestingly, Abcb6-null mice show a phenotype that is characteristic of the deficiency of a protein that is involved in cholesterol and lipid homeostasis. Development of the Abcb6-null mice and the development of an in vitro system with purified Abcb6 should serve as useful tools to understand the transport function of Abcb6 and its role in normal and patho-physiology

Double deletion of PINK1 and Parkin impairs hepatic mitophagy and exacerbates acetaminophen-induced liver injury in mice

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.Mitochondria damage plays a critical role in acetaminophen (APAP)-induced necrosis and liver injury. Cells can adapt and protect themselves by removing damaged mitochondria via mitophagy. PINK1-Parkin pathway is one of the major pathways that regulate mitophagy but its role in APAP-induced liver injury is still elusive. We investigated the role of PINK1-Parkin pathway in hepatocyte mitophagy in APAP-induced liver injury in mice. Wild-type (WT), PINK1 knockout (KO), Parkin KO, and PINK1 and Parkin double KO (DKO) mice were treated with APAP for different time points. Liver injury was determined by measuring serum alanine aminotransferase (ALT) activity, H&E staining as well as TUNEL staining of liver tissues. Tandem fluorescent-tagged inner mitochondrial membrane protein Cox8 (Cox8-GFP-mCherry) can be used to monitor mitophagy based on different pH stability of GFP and mCherry fluorescent proteins. We overexpressed Cox8-GFP-mCherry in mouse livers via tail vein injection of an adenovirus Cox8-GFP-mCherry. Mitophagy was assessed by confocal microscopy for Cox8-GFP-mCherry puncta, electron microscopy (EM) analysis for mitophagosomes and western blot analysis for mitochondrial proteins. Parkin KO and PINK1 KO mice improved the survival after treatment with APAP although the serum levels of ALT were not significantly different among PINK1 KO, Parkin KO and WT mice. We only found mild defects of mitophagy in PINK1 KO or Parkin KO mice after APAP, and improved survival in PINK1 KO and Parkin KO mice could be due to other functions of PINK1 and Parkin independent of mitophagy. In contrast, APAP-induced mitophagy was significantly impaired in PINK1-Parkin DKO mice. PINK1-Parkin DKO mice had further elevated serum levels of ALT and increased mortality after APAP administration. In conclusion, our results demonstrated that PINK1-Parkin signaling pathway plays a critical role in APAP-induced mitophagy and liver injury.NIH R01 AA 020518NIH R01 DK 102142NIH U01 AA 024733NIH P20 GM 103549NIH P30 GM 118247NIH COBRE grant 9P20GM104936NIH S10RR02756

A Novel Flow Cytometric HTS Assay Reveals Functional Modulators of ATP Binding Cassette Transporter ABCB6

ABCB6 is a member of the adenosine triphosphate (ATP)-binding cassette family of transporter proteins that is increasingly recognized as a relevant physiological and therapeutic target. Evaluation of modulators of ABCB6 activity would pave the way toward a more complete understanding of the significance of this transport process in tumor cell growth, proliferation and therapy-related drug resistance. In addition, this effort would improve our understanding of the function of ABCB6 in normal physiology with respect to heme biosynthesis, and cellular adaptation to metabolic demand and stress responses. To search for modulators of ABCB6, we developed a novel cell-based approach that, in combination with flow cytometric high-throughput screening (HTS), can be used to identify functional modulators of ABCB6. Accumulation of protoporphyrin, a fluorescent molecule, in wild-type ABCB6 expressing K562 cells, forms the basis of the HTS assay. Screening the Prestwick Chemical Library employing the HTS assay identified four compounds, benzethonium chloride, verteporfin, tomatine hydrochloride and piperlongumine, that reduced ABCB6 mediated cellular porphyrin levels. Validation of the identified compounds employing the hemin-agarose affinity chromatography and mitochondrial transport assays demonstrated that three out of the four compounds were capable of inhibiting ABCB6 mediated hemin transport into isolated mitochondria. However, only verteporfin and tomatine hydrochloride inhibited ABCB6’s ability to compete with hemin as an ABCB6 substrate. This assay is therefore sensitive, robust, and suitable for automation in a high-throughput environment as demonstrated by our identification of selective functional modulators of ABCB6. Application of this assay to other libraries of synthetic compounds and natural products is expected to identify novel modulators of ABCB6 activity

Arsenite Effects on Mitochondrial Bioenergetics in Human and Mouse Primary Hepatocytes Follow a Nonlinear Dose Response

Arsenite is a known carcinogen and its exposure has been implicated in a variety of noncarcinogenic health concerns. Increased oxidative stress is thought to be the primary cause of arsenite toxicity and the toxic effect is thought to be linear with detrimental effects reported at all concentrations of arsenite. But the paradigm of linear dose response in arsenite toxicity is shifting. In the present study we demonstrate that arsenite effects on mitochondrial respiration in primary hepatocytes follow a nonlinear dose response. In vitro exposure of primary hepatocytes to an environmentally relevant, moderate level of arsenite results in increased oxidant production that appears to arise from changes in the expression and activity of respiratory Complex I of the mitochondrial proton circuit. In primary hepatocytes the excess oxidant production appears to elicit adaptive responses that promote resistance to oxidative stress and a propensity to increased proliferation. Taken together, these results suggest a nonlinear dose-response characteristic of arsenite with low-dose arsenite promoting adaptive responses in a process known as mitohormesis, with transient increase in ROS levels acting as transducers of arsenite-induced mitohormesis

Targeting the Enterohepatic Bile Acid Signaling Induces Hepatic Autophagy via a CYP7A1âAKTâmTOR Axis in MiceSummary

Background & Aims: Hepatic cholesterol accumulation and autophagy defects contribute to hepatocyte injury in fatty liver disease. Bile acid synthesis is a major pathway for cholesterol catabolism in the liver. This study aims to understand the molecular link between cholesterol and bile acid metabolism and hepatic autophagy activity. Methods: The effects of cholesterol and cholesterol 7α-hydroxylase (CYP7A1) expression on autophagy and lysosome function were studied in cell models. The effects and mechanism of disrupting enterohepatic bile acid circulation on hepatic autophagy were studied in mice. Results: The results first showed differential regulation of hepatic autophagy by free cholesterol and cholesterol ester, whereby a modest increase of cellular free cholesterol, but not cholesterol ester, impaired lysosome function and caused marked autolysosome accumulation. We found that CYP7A1 induction, either by cholestyramine feeding in mice or adenovirus-mediated CYP7A1 expression in hepatocytes, caused strong autophagy induction. Mechanistically, we showed that CYP7A1 expression markedly attenuated growth factor/AKT signaling activation of mechanistic target of rapamycin (mTOR), but not amino acid signaling to mTOR in vitro and in vivo. Metabolomics analysis further found that CYP7A1 induction not only decreased hepatic cholesterol but also altered phospholipid and sphingolipid compositions. Collectively, these results suggest that CYP7A1 induction interferes with growth factor activation of AKT/mTOR signaling possibly by altering membrane lipid composition. Finally, we showed that cholestyramine feeding restored impaired hepatic autophagy and improved metabolic homeostasis in Western dietâfed mice. Conclusions: This study identified a novel CYP7A1âAKTâmTOR signaling axis that selectively induces hepatic autophagy, which helps improve hepatocellular integrity and metabolic homeostasis. Keywords: Cholesterol, Cholestyramine, Fatty Liver, Nuclear Recepto

Impaired TFEB-Mediated Lysosome Biogenesis and Autophagy Promote Chronic Ethanol-Induced Liver Injury and Steatosis in Mice

Defects in lysosome function and autophagy contribute to the pathogenesis of alcoholic liver disease. We investigated the mechanisms by which alcohol consumption affects these processes by evaluating the functions of transcription factor EB (TFEB), which regulates lysosomal biogenesis