Interaction of RIPK1 and A20 modulates MAPK signaling in murine acetaminophen toxicity.

Acetaminophen (APAP)-induced liver necrosis is a form of regulated cell death (RCD) in which APAP activates the mitogen-activated protein kinases (MAPKs) and specifically the c-Jun-N-terminal kinase (JNK) pathway, leading to necrotic cell death. Previously, we have shown that receptor interacting protein kinase-1 (RIPK1) knockdown is also protective against APAP RCD upstream of JNK. However, whether the kinase or platform function of RIPK1 is involved in APAP RCD is not known. To answer this question, we used genetic mouse models of targeted hepatocyte RIPK1 knockout (RIPK1HepCKO) or kinase dead knock-in (RIPK1D138N) and adult hepatocyte specific knockout of the cytoprotective protein A20 (A20HepCKO), known to interact with RIPK1, to study its potential involvement in MAPK signaling. We observed no difference in injury between WT and RIPK1D138N mice post APAP. However, RIPK1HepCKO was protective. We found that RIPK1HepCKO mice had attenuated pJNK activation, while A20 was simultaneously upregulated. Conversely, A20HepCKO markedly worsened liver injury from APAP. Mechanistically, we observed a significant upregulation of apoptosis signal-regulating kinase 1 (ASK1) and increased JNK activation in A20HepCKO mice compared with littermate controls. We also demonstrated that A20 coimmunoprecipitated (co-IP) with both RIPK1 and ASK1, and that in the presence of RIPK1, there was less A20-ASK1 association than in its absence. We conclude that the kinase-independent platform function of RIPK1 is involved in APAP toxicity. Adult RIPK1HepCKO mice are protected against APAP by upregulating A20 and attenuating JNK signaling through ASK1, conversely, A20HepCKO worsens injury from APAP.

Akt2-Dependent Phosphorylation of Radixin in Regulation of Mrp-2 Trafficking in WIF-B Cells.

BACKGROUND: The dominant ezrin/radixin/moesin protein in hepatocytes is radixin, which plays an important role in mediating the binding of F-actin to the plasma membrane after a conformational activation by phosphorylation at Thr564. AIM: Here we have investigated the importance of Akt-mediated radixin Thr564 phosphorylation on Mrp-2 distribution and function in WIF-B cells. Mrp-2 is an adenosine triphosphate (ATP)-binding cassette transporter that plays an important role in detoxification and chemoprotection by transporting a wide range of compounds, especially conjugates of lipophilic substances with glutathione, organic anions, and drug metabolites such as glucuronides. METHODS: Akt1 and Akt2 expression were manipulated using dominant active and negative constructs as well as Akt1 and Akt2 siRNA. Cellular distribution of radixin and Mrp-2 was visualized by fluorescence microscopy. A 5-chloromethylfluorescein diacetate, which is a substrate of the Mrp-2 and is actively transported in canalicular lumina, was used to measure Mrp-2 function. RESULTS: Radixin phosphorylation was significantly increased in wild-type and dominant active Akt2 transfected cells. Furthermore, radixin and Mrp-2 were localized at the canalicular membrane, similar to control cells. In contrast, overexpression of dominant negative Akt2, siRNA knockdown of Akt2 and a specific Akt inhibitor prevented radixin phosphorylation and led to alteration of normal radixin and Mrp-2 localization; inhibition of Akt2, but not Akt1 function led to radixin localization to the cytoplasmic space. In addition, dominant negative and Akt2 knockdown led to a dramatically impaired hepatocyte secretory response, while wild-type and dominant active Akt2 transfected cells exhibited increased 5-chloromethylfluorescein diacetate excretion. In contrast to Akt2, Akt1 was not associated with radixin phosphorylation. CONCLUSIONS: These studies, therefore, identify Akt2 as a critical kinase that regulates radixin phosphorylation and leads to Mrp-2 translocation and function.

Lysosomal Cholesterol Accumulation Sensitizes To Acetaminophen Hepatotoxicity by Impairing Mitophagy.

The role of lysosomes in acetaminophen (APAP) hepatotoxicity is poorly understood. Here, we investigated the impact of genetic and drug-induced lysosomal cholesterol (LC) accumulation in APAP hepatotoxicity. Acid sphingomyelinase (ASMase)(-/-) mice exhibit LC accumulation and higher mortality after APAP overdose compared to ASMase(+/+) littermates. ASMase(-/-) hepatocytes display lower threshold for APAP-induced cell death and defective fusion of mitochondria-containing autophagosomes with lysosomes, which decreased mitochondrial quality control. LC accumulation in ASMase(+/+) hepatocytes caused by U18666A reproduces the susceptibility of ASMase(-/-) hepatocytes to APAP and the impairment in the formation of mitochondria-containing autolysosomes. LC extraction by 25-hydroxycholesterol increased APAP-mediated mitophagy and protected ASMase(-/-) mice and hepatocytes against APAP hepatotoxicity, effects that were reversed by chloroquine to disrupt autophagy. The regulation of LC by U18666A or 25-hydroxycholesterol did not affect total cellular sphingomyelin content or its lysosomal distribution. Of relevance, amitriptyline-induced ASMase inhibition in human hepatocytes caused LC accumulation, impaired mitophagy and increased susceptibility to APAP. Similar results were observed upon glucocerebrosidase inhibition by conduritol ß-epoxide, a cellular model of Gaucher disease. These findings indicate that LC accumulation determines susceptibility to APAP hepatotoxicity by modulating mitophagy, and imply that genetic or drug-mediated ASMase disruption sensitizes to APAP-induced liver injury.

Phosphorylation dynamics of radixin in hypoxia-induced hepatocyte injury.

The most prominent ezrin-radixin-moesin protein in hepatocytes is radixin, which is localized primarily at the canalicular microvilli and appears to be important in regulation of cell polarity and in localizing the multidrug resistance-associated protein 2 (Mrp-2) function. Our aim was to investigate how hypoxia affects radixin distribution and Mrp-2 function. We created wild-type and mutant constructs (in adenoviral vectors), which were expressed in WIF-B cells. The cellular distribution of Mrp-2 and radixin was visualized by fluorescence microscopy, and a 5-chloromethylfluorescein diacetate (CMFDA) assay was used to measure Mrp-2 function. Under usual conditions, cells infected with wild-type radixin, nonphosphorylatable radixin-T564A, and radixin-T564D (active phospho-mimicking mutant) were found to be heavily expressed in canalicular membrane compartment vacuoles, typically colocalizing with Mrp-2. In contrast, after hypoxia for 24 h, both endogenous and overexpressed wild-type radixin and the radixin-T564A mutant were found to be translocated to the cytoplasmic space. However, distribution of the radixin-T564D mutant, which mimics constant phosphorylation, was remarkably different, being associated with canalicular membranes even in hypoxic conditions. This dominant-active construct also prevented dissociation of radixin from the plasma membrane. Hypoxia also led to Mrp-2 mislocalization and caused Mrp-2 to be dissociated from radixin; the radixin phospho-mimicking mutant (T564D) abrogated this effect of hypoxia. Finally, hypoxia diminished the secretory response (measured using the CMFDA assay) in WIF-B cells, and the dominant-active construct (radixin-T567D) rescued this phenotype. Taken collectively, these findings suggest that radixin regulates Mrp-2 localization and function in hepatocytes and is important in hypoxic liver injury.

Distribution dynamics and functional importance of NHERF1 in regulation of Mrp-2 trafficking in hepatocytes.

Na(+)/H(+) exchanger regulatory factor 1 (NHERF1) is a multifunctional scaffolding protein that interacts with receptors and ion transporters in its PDZ domains and with the ezrin-radixin-moesin (ERM) family of proteins in its COOH terminus. The role of NHERF1 in hepatocyte function remains largely unknown. We examine the distribution and physiological significance of NHERF1 and multidrug resistance-associated protein 2 (Mrp-2) in hepatocytes. A WT radixin binding site mutant (F355R) and NHERF1 PDZ1 and PDZ2 domain adenoviral mutant constructs were tagged with yellow fluorescent protein and expressed in polarized hepatocytes to study localization and function of NHERF1. Cellular distribution of NHERF1 and radixin was visualized by fluorescence microscopy. A 5-chloromethylfluorescein diacetate (CMFDA) assay was used to characterize Mrp-2 function. Similar to Mrp-2, WT NHERF1 and the NHERF1 PDZ2 deletion mutant were localized to the canalicular membrane. In contrast, the radixin binding site mutant (F355R) and the NHERF1 PDZ1 deletion mutant, which interacts poorly with Mrp-2, were rarely associated with the canalicular membrane. Knockdown of NHERF1 led to dramatically impaired CMFDA secretory response. Use of CMFDA showed that the NHERF1 PDZ1 and F355R mutants were devoid of a secretory response, while WT NHERF1-infected cells exhibited increased secretion of glutathione-methylfluorescein. The data indicate that NHERF1 interacts with Mrp-2 via the PDZ1 domain of NHERF1 and, furthermore, that NHERF1 is essential for maintaining the localization and function of Mrp-2.

Rab27b localizes to the tubulovesicle membranes of gastric parietal cells and regulates acid secretion.

BACKGROUND & AIMS: Rabs are monomeric guanosine triphosphatases that regulate membrane trafficking and acid secretion in gastric parietal cells. Using a proteomics approach, we identified a new Rab, Rab27b, in tubulovesicle membranes and determined its role in parietal cell activation. METHODS: We used mass spectrometry (MS) to identify Rab27b in purified tubulovesicular membrane fractions and used immunoblot and immunofluorescence analyses to study its expression. Wild-type, constitutively active (Rab27bQ78L), and dominant negative (Rab27bN133I) forms of Rab27b were tagged with yellow fluorescent protein (YFP) and expressed in parietal cells using adenoviral constructs to study localization and function. Localization was visualized by fluorescence microscopy in resting and stimulated cells. Acid secretion in primary cell cultures was measured by aminopyrine accumulation. RESULTS: A tandem MS peptide mass fingerprint was matched to 7 peptides of Rab27b. Rab27b localized to tubulovesicle membranes, based on immunoblot and immunocytochemical analyses. Endogenous Rab27b, YFP/wild-type Rab27b, Rab27bQ78L, and Rab27bN133I all distributed throughout the cytoplasm of resting parietal cells. After stimulation, wild-type Rab27b and YFP-Rab27bQ78L translocated to the apical membrane, but YFPR-ab27bN133I did not. Expression of wild-type YFP-Rab27b or YFP-Rab27bQ78L did not affect acid secretion, whereas expression of Rab27bN133I almost completely inhibited acid secretion. CONCLUSIONS: Rab27b is associated with tubulovesicle membranes in the parietal cell and Rab27b may play a role in stimulation-associated membrane recruitment and gastric acid secretion.

Phosphorylation of radixin regulates cell polarity and Mrp-2 distribution in hepatocytes.

Radixin, the dominant ezrin-radixin-moesin (ERM) protein in hepatocytes, has two important binding domains: an NH(2)-terminal region that binds to plasma membrane and a COOH-terminal region that binds to F-actin after a conformational activation by phosphorylation at Thr564. The present studies were undertaken to investigate the cellular changes in expression of radixin in WIF-B cells and to assess radixin distribution and its influence on cell polarity. We used a recombinant adenoviral expression system encoding radixin wild-type and Thr564 mutants fused to cyan fluorescent protein (CFP), as well as conventional immunostaining procedures. Functional analyses were characterized quantitatively. Similar to endogenous radixin, adenovirus-infected radixin-CFP-wild type and nonphosphorylatable radixin-CFP-T564A were found to be expressed heavily in the compartment of canalicular membrane vacuoles, typically colocalizing with multidrug resistance-associated protein 2 (Mrp-2). Expression of radixin-CFP-T564D, which mimics constant phosphorylation, was quite different, being rarely associated with canalicular membranes. The WIF-B cells were devoid of a secretory response, T567D radixin became predominantly redistributed to the basolateral membrane, usually in the form of dense, long spikes and fingerlike projections, and the altered cell polarity involved changes in apical membrane markers. Differences in polar distribution of radixin suggest a role for the linker protein in promoting formation and plasticity of membrane surface projections and also suggest that radixin might be an organizer and regulator of Mrp-2 and cell polarity in hepatocytes.