Inhibition of asialoglycoprotein endocytosis and degradation in rat hepatocytes by protein phosphatase inhibitors

Enhancement of cell membrane invaginations, vesiculation and uptake of macromolecules by protonation of the cell surface

The different pathways of endocytosis share an initial step involving local inward curvature of the cell’s lipid bilayer. It has been shown that to generate membrane curvature, proteins or lipids enforce transversal asymmetry of the plasma membrane. Thus it emerges as a general phenomenon that transversal membrane asymmetry is the common required element for the formation of membrane curvature. The present study demonstrates that elevating proton concentration at the cell surface stimulates the formation of membrane invaginations and vesiculation accompanied by efficient uptake of macromolecules (Dextran-FITC, 70 kD), relative to the constitutive one. The insensitivity of proton induced uptake to inhibiting treatments and agents of the known endocytic pathways suggests the entry of macromolecules to proceeds via a yet undefined route. This is in line with the fact that neither ATP depletion, nor the lowering of temperature, abolishes the uptake process. In addition, fusion mechanism such as associated with low pH uptake of toxins and viral proteins can be disregarded by employing the polysaccharide dextran as the uptake molecule. The proton induced uptake increases linearly in the extracellular pH range of 6.5 to 4.5, and possesses a steep increase at the range of 4> pH>3, reaching a plateau at pH≤3. The kinetics of the uptake implies that the induced vesicles release their content to the cytosol and undergo rapid recycling to the plasma membrane. We suggest that protonation of the cell’s surface induces local charge asymmetries across the cell membrane bilayer, inducing inward curvature of the cell membrane and consequent vesiculation and uptake.

Hepatocyte-targeting single galactose-appended naphthalimide: a tool for intracellular thiol imaging in vivo

We present the design, synthesis, spectroscopic properties, and biological evaluation of a single galactose-appended naphthalimide (1). Probe 1 is a multifunctional molecule that incorporates a thiol-specific cleavable disulfide bond, a masked phthalamide fluorophore, and a single galactose moiety as a hepatocyte-targeting unit. It constitutes a new type of targetable ligand for hepatic thiol imaging in living cells and animals. Confocal microscopic imaging experiments reveal that 1, but not the galactose-free control system 2, is preferentially taken up by HepG2 cells through galactose-targeted, ASGP-R-mediated endocytosis. Probe 1 displays a fluorescence emission feature at 540 nm that is induced by exposure to free endogenous thiols, most notably GSH. The liver-specificity of 1 was confirmed in vivo via use of a rat model. The potential utility of this probe in indicating pathogenic states and as a possible screening tool for agents that can manipulate oxidative stress was demonstrated in experiments wherein palmitate was used to induce lipotoxicity in HepG2 cells.

Receptor-mediated endocytosis involves tyrosine phosphorylation of cortactin

Efficient internalization of cell surface receptors requires actin polymerization mediated by Arp2/3 complex and cortactin, a prominent substrate of the protein-tyrosine kinase Src. However, the significance of cortactin tyrosine phosphorylation in endocytosis is unknown. We found that overexpression of a cortactin mutant deficient in tyrosine phosphorylation decreased transferrin uptake. Suppression of cortactin expression by RNA interference also reduced transferrin internalization. Such inhibition was effectively rescued by overexpressing wild-type cortactin but not a cortactin mutant deficient in tyrosine phosphorylation or a mutant with deletion of the Src homology 3 domain. Likewise, purified phosphorylation-null cortactin failed to restore the formation of clathrin-coated vesicles in a cortactin-depleted cell extract. In vitro analysis revealed that Src-mediated phosphorylation enhanced the association of cortactin with dynamin-2 in a tyrosine phosphorylation-dependent manner. Quantitative analysis demonstrated that Src enhances the affinity of cortactin for dynamin-2 by more than 3-fold. On the other hand, Src-treated dynamin-2 had no effect on its interaction with cortactin. These data indicate that Src kinase is implicated in clathrin-mediated endocytosis by phosphorylation of cortactin.

Naringin-sensitive phosphorylation of plectin, a cytoskeletal cross-linking protein, in isolated rat hepatocytes

To identify phosphoproteins that might play a role in naringin-sensitive hepatocellular cytoskeletal disruption and apoptosis induced by algal toxins, hepatocyte extracts were separated by gel electrophoresis and immunostained with a phosphothreonine-directed antibody. Use of dilute (5%) polyacrylamide gels containing 6 m urea allowed the resolution of one very large (approximately 500-kDa) okadaic acid- and naringin-sensitive phosphoprotein, identified by tryptic fingerprinting, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and immunostaining as the cytolinker protein, plectin. The naringin-sensitive phosphorylation induced by okadaic acid and microcystin-LR probably reflected inhibition of a type 2A protein phosphatase, whereas the naringin-resistant phosphorylation induced by calyculin A, tautomycin, and cantharidin probably involved a type 1 phosphatase. Okadaic acid caused a collapse of the plectin-immunostaining bile canalicular sheaths and the general cytoskeletal plectin network into numerous medium-sized plectin aggregates. Inhibitors of protein kinase C, cAMP-dependent protein kinase, or Ca(2+)/calmodulin-dependent kinase II had moderate or no protective effects on plectin network disruption, whereas naringin offered 86% protection. Okadaic acid induced a naringin-sensitive phosphorylation of AMP-activated protein kinase (AMPK), the stress-activated protein kinases SEK1 and JNK, and S6 kinase. The AMPK-activating kinase (AMPKK) is likely to be the target of inhibition by naringin, the other kinases serving as downstream components of an AMPKK-initiated signaling pathway.

Effects of inhibitors of the vacuolar proton pump on hepatic heterophagy and autophagy

Bafilomycin A(1) (BAF) and concanamycin A (ConcA) are selective inhibitors of the H(+)-ATPases of the vacuolar system. We have examined the effects of these inhibitors on different steps in endocytic pathways in rat hepatocytes, using [(125)I]tyramine-cellobiose-labeled asialoorosomucoid ([(125)I]TC-AOM) and [(125)I]tyramine-cellobiose-labeled bovine serum albumin ([(125)I]TC-BSA) as probes for respectively receptor-mediated endocytosis and pinocytosis (here defined as fluid phase endocytosis). The effects of BAF and ConcA were in principle identical, although ConcA was more effective than BAF. The main findings were as follows. (1) BAF/ConcA reduced the rate of uptake of both [(125)I]TC-AOM and [(125)I]TC-BSA. The reduced uptake of [(125)I]TC-AOM was partly due to a redistribution of the asialoglycoprotein receptors (ASGPR) such that the number of surface receptors was reduced approximately 40% without a change in the total number of receptors. (2) BAF/ConcA at the same time increased retroendocytosis (recycling) of both probes. The increased recycling of the ligand ([(125)I]TC-AOM) is partly a consequence of the enhanced pH in endosomes, which prevents dissociation of ligand. (3) It was furthermore found that the ligand remained bound to the receptor in presence of BAF/ConcA and that the total amount of ligand molecules internalized in BAF/ConcA-treated cells was only slightly in excess of the total number of receptors. These data indicate that reduced pH in endosomes is the prime cause of receptor inactivation and release of ligand in early endosomes. (4) Subcellular fractionation experiments showed that [(125)I]TC-AOM remained in early endosomes, well separated from lysosomes in sucrose gradients. The fluid phase marker, [(125)I]TC-BSA, on the other hand, seemed to reach a later endosome in the BAF/ConcA-treated cells. This organelle coincided with lysosomes in the gradient, but hypotonic medium was found to selectively release a lysosomal enzyme (beta-acetylglucosaminidase), indicating that even [(125)I]TC-BSA remained in a prelysosomal compartment in the BAF/ConcA-treated cells. (5) Electron microscopy using horseradish peroxidase (HRP) as a fluid phase marker verified that BAF/ConcA inhibited transfer of material from late endosomes ('multivesicular bodies'). (6) BAF/ConcA led to accumulation of lactate dehydrogenase (LDH) in autophagic vacuoles, but although the drugs partly inhibited fusion between autophagosomes and lysosomes a number of autolysosomes was formed in the presence of BAF/ConcA. This observation explains the reduced buoyant density of lysosomes (revealed in sucrose density gradients). In conclusion, BAF/ConcA inhibit transfer of endocytosed material from late endosomes to lysosomes, but do not at the same time prevent fusion between autophagosomes and lysosomes.

Hyperphosphorylation of the asialoglycoprotein receptor in isolated rat hepatocytes following ethanol administration

Ethanol administration leads to altered function and impaired receptor-mediated endocytosis of the hepatocyte asialoglycoprotein receptor (ASGP-R). The purpose of the present study was to examine the effects of ethanol on the phosphorylation of the ASGP-R to determine whether this post-translational modification could contribute mechanistically to the observed ethanol-induced alterations. The methodological approach of this work involved the measurement of the phosphorylation state of the receptor obtained from isolated rat hepatocytes, using a combination of experimental designs from the biosynthetic incorporation of phosphate to the determination of steady-state phosphotyrosine levels. We report here that both short-term (1- to 2-week) and chronic (5- to 7-week) periods of ethanol administration resulted in a significant increase in the steady-state phosphotyrosine protein in the ASGP-R. In addition, in vitro incorporation of [gamma-(32)P]ATP using a permeabilized cell assay system similarly showed an increase in tyrosine-phosphorylated receptors. Furthermore, metabolic radiolabeling of hepatocytes with [(32)P]orthophosphate demonstrated hyperphosphorylation of the ASGP-R in cells obtained from chronically ethanol-fed animals. Finally, our results revealed that dephosphorylation of the ASGP-R was unaffected by ethanol administration, indicating that kinase activity rather than impaired phosphatase action contributes to the increased phosphorylation state of the receptor. Overall, the results presented in this study demonstrated that the extent of tyrosine phosphorylation of the receptor is significantly higher in hepatocytes obtained from ethanol-fed animals. We conclude that hyperphosphorylation of the ASGP-R may be a contributing factor to the impaired function of the receptor elicited by ethanol administration.

Targeting endocytosis and motor proteins to enhance DNA persistence

Gene therapy provides a major new therapeutic strategy for the treatment of disease. Despite its potential for the inhibition of disease progression at the molecular level, gene therapy has faced numerous challenges. Foremost amongst these is the introduction of a sufficient amount of DNA-based drug to the target cell under conditions that encourage persistence of the introduced DNA. Because many DNA-based drugs enter cells via receptor-mediated endocytosis, clearly modulation of this process is a key issue in maximizing DNA persistence. In this review, a particular protein that participates in receptor-mediated endocytosis, the microtubule-based motor protein, cytoplasmic dynein, is introduced. In addition, recent advances in the study of cytoplasmic dynein in receptor-mediated endocytosis are discussed, and there is consideration of the potential of cytoplasmic dynein as a critical target for the regulation of DNA-uptake and persistence.

Human mesangial cells in culture and in kidney sections fail to express Fc alpha receptor (CD89)

The mechanism of deposition of IgA in the renal mesangium in primary IgA-nephropathy is poorly understood. It has been suggested that membrane receptors for IgA on mesangial cells (MC) of the kidney may be involved. To obtain more insight in the occurrence of the myeloid receptor for IgA (CD89) on MC, both in situ and in culture, rabbit and goat polyclonal antibodies and mouse monoclonal antibody against recombinant CD89 were raised. Kidney sections from five control subjects and five patients with primary IgA-nephropathy failed to be positive for CD89 in the mesangium, using our polyclonal and monoclonal antibodies. Also, five primary human MC cultures assessed for CD89 expression showed no protein expression of CD89. Furthermore, reverse transcription-PCR failed to detect mRNA expression of CD89 in the cultured MC. It was demonstrated that all five human primary MC bound human IgA in a dose-dependent manner, which was not inhibitable by blocking monoclonal anti-CD89 antibody (My43). In contrast, binding of IgA to U937 cells was blocked efficiently by My43. Finally, incubation of human MC with either human or rat IgA led to increased interleukin-6 production, whereas only human IgA, but not rat IgA, was able to bind to human CD89. Therefore, it is concluded that human MC do not express CD89 (to a significant extent). These results strongly suggest that binding of IgA to human MC occurs via an IgA receptor distinct from CD89.

Inhibition of protein phosphatase activity and changes in protein phosphorylation following acetaminophen exposure in cultured mouse hepatocytes

Protein phosphorylation was determined in cultured mouse hepatocytes exposed to an hepatotoxic concentration of acetaminophen (APAP) for selected times up to 12 h. Cultures were radiolabled with 32P-orthophosphoric acid and the cell extracts were analyzed by 2D gel electrophoresis and autoradiography. APAP exposure selectively increased the phosphorylation state of proteins of molecular weight 22, 25, 28, and 59 kDa and decreased the phosphorylation of a 26-kDa protein. Evidence is presented that these changes (1) are dependent on cytochrome P-450 activation of APAP; (2) occur well before enzyme leakage in this in vitro model; (3) are not likely attributed to GSH depletion alone; (4) are in part mimicked by okadaic acid, calyculin A, and cantharidic acid, three structurally distinct inhibitors of protein phosphatases 1 and 2A; and (5) are paralleled by a decline in protein phosphatase activity. The physiological consequences of protein phosphatase inactivation could be significant in APAP overdose since these enzymes are involved in the dephosphorylation of regulatory proteins that control many cell functions. This study also provides the first evidence for disruption in signal transduction pathways as a response to or component of APAP-induced hepatic injury.

Purification and characterization of autophagosomes from rat hepatocytes

To investigate the properties and intracellular origin of autophagosomes, a procedure for the purification and isolation of these organelles from rat liver has been developed. Isolated hepatocytes were incubated with vinblastine to induce autophagosome accumulation; the cells were then homogenized and treated with the cathepsin C substrate glycyl-l-phenylalanine 2-naphthylamide to cause osmotic disruption of the lysosomes. Nuclei were removed by differential centrifugation, and the postnuclear supernatant was fractionated on a discontinuous Nycodenz density gradient. The autophagosomes, recognized by their content of autophagocytosed lactate dehydrogenase (LDH), could be recovered in an intermediate-density fraction, free from cytosol and mitochondria. Finally, the autophagosomes were separated from the endoplasmic reticulum and other membranous elements by centrifugation in a Percoll colloidal density gradient, followed by flotation in iodixanol to remove the Percoll particles. The final autophagosome preparation represented a 24-fold purification of autophagocytosed LDH relative to intact cells, with a 12% recovery. The purified autophagosomes contained sequestered cytoplasm with a normal ultrastructure, including mitochondria, peroxisomes and endoplasmic reticulum in the same proportions as in intact cells. However, immunoblotting indicated a relative absence of cytoskeletal elements (tubulin, actin and cytokeratin), which may evade autophagic sequestration. The autophagosomes showed no enrichment in protein markers typical of lysosomes (acid phosphatase, cathepsin B, lysosomal glycoprotein of 120 kDa), endosomes (early-endosome-associated protein 1, cation-independent mannose 6-phosphate receptor, asialoglycoprotein receptor) or endoplasmic reticulum (esterase, glucose-regulated protein of 78 kDa, protein disulphide isomerase), suggesting that the sequestering membranes are not derived directly from any of these organelles, but rather represent unique organelles (phagophores).

Isolation and characterization of rat liver amphisomes. Evidence for fusion of autophagosomes with both early and late endosomes

Amphisomes, the autophagic vacuoles (AVs) formed upon fusion between autophagosomes and endosomes, have so far only been characterized in indirect, functional terms. To enable a physical distinction between autophagosomes and amphisomes, the latter were selectively density-shifted in sucrose gradients following fusion with AOM-gold-loaded endosomes (endosomes made dense by asialoorosomucoid-conjugated gold particles, endocytosed by isolated rat hepatocytes prior to subcellular fractionation). Whereas amphisomes, by this criterion, accounted for only a minor fraction of the AVs in control hepatocytes, treatment of the cells with leupeptin (an inhibitor of lysosomal protein degradation) caused an accumulation of amphisomes to about one-half of the AV population. A quantitative electron microscopic study confirmed that leupeptin induced a severalfold increase in the number of hepatocytic amphisomes (recognized by their gold particle contents; otherwise, their ultrastructure was quite similar to autophagosomes). Leupeptin caused, furthermore, a selective retention of endocytosed AOM-gold in the amphisomes at the expense of the lysosomes, consistent with an inhibition of amphisome-lysosome fusion. The electron micrographs suggested that autophagosomes could undergo multiple independent fusions, with multivesicular (late) endosomes to form amphisomes and with small lysosomes to form large autolysosomes. A biochemical comparison between autophagosomes and amphisomes, purified by a novel procedure, showed that the amphisomes were enriched in early endosome markers (the asialoglycoprotein receptor and the early endosome-associated protein 1) as well as in a late endosome marker (the cation-independent mannose 6-phosphate receptor). Amphisomes would thus seem to be capable of receiving inputs both from early and late endosomes.

Endocytosed ricin and asialoorosomucoid follow different intracellular pathways in hepatocytes

Earlier studies have suggested that fluid phase endocytosis in rat hepatocytes takes place via a clathrin-independent mechanism [1,2]. This observation suggests that a relatively large amount of plasma membrane outside coated pits may be involved in hepatic endocytosis. Ricin, which binds to galactose residues on glycoproteins and glycolipids, has, in this report, been used as a general marker for the plasma membrane of hepatocytes. The endocytosis of ricin was compared with that of asialoorosomucoid (AOM) which is taken up exclusively via clathrin-coated pits. Hypertonic medium has been shown to inhibit uptake via coated pits more effectively than clathrin-independent uptake [3-5]. It was found, in this study, that the addition of 100 mM sucrose to the incubation medium inhibited the uptake of 125I-tyramine-cellobiose-asialoorosomucoid (125I-TC-AOM) more extensively than that of 125I-tyramine-cellobiose-ricin (125I-TC-ricin), compatible with the notion that the two probes are internalised via different mechanisms. Subcellular fractionation experiments indicated that 125I-TC-ricin entered a denser endocytic organelle than that receiving 125I-TC-AOM. To determine whether the separation of the two probes was due to a different transport kinetics (i.e. that 125I-TC-ricin is transported more rapidly to a later, denser compartment than 125I-TC-AOM) the cells were incubated at 18 degreesC to allow a slower internalisation/transport of the labelled probes. The results obtained showed, again, that the early endosomes containing 125I-TC-ricin were significantly denser than those containing 125I-TC-AOM. We also employed the horseradish peroxidase (HRP)-diaminobenzidine (DAB) density shift technique of Courtoy et al. [6] to determine whether 125I-TC-ricin and 125I-TC-AOM were in separate endosomes early after their uptake. The results showed that early endosomes containing 125I-TC-AOM were density shifted whereas those containing 125I-TC-ricin were unaffected by the density shift procedure. The use of probes labelled with 125I-TC allowed us to identify compartments involved in the degradation of 125I-TC-AOM and 125I-TC-ricin, by measuring acid soluble radioactivities in the gradient fractions. It was found that 125I-TC-ricin was degraded mainly in endosomes, whereas 125I-TC-AOM, as expected, was degraded mainly in lysosomes.

Dissection of hepatic receptor-mediated endocytic pathways using self-generated gradients of iodixanol (Optiprep)

Iodixanol is a new, nonionic, iodinated density gradient medium which has an advantage over other similar media in that it rapidly forms self-generated gradients in vertical or near-vertical rotors. Endocytosis of 99mTc-labeled neogalactosyl albumin (99mTc-NGA), a synthetic ligand for the asialoglycoprotein receptor, was studied by administering the ligand as a short pulse to perfused rat livers operating under single-pass conditions. Intracellular processing was arrested at various times after the pulse and the resultant homogenate cleared of nuclei and heavy mitochondria by centrifugation at 3000 g for 10 min. After adjustment to 12.5% (w/v) iodixanol, the 3000 g supernatants were centrifuged at 350,000 g for 60 min to form the gradients in which early, clathrin-containing vesicles, low-density endosomes, and lysosomes were well-resolved. 99mTc-NGA bound to the sinusoidal membrane could be partially resolved from clathrin-containing vesicles by inclusion of 1 mM CaCl2 in the homogenization and gradient buffers. Two populations of early clathrin-containing vesicles could be resolved by rate-zonal centrifugation in preformed iodixanol gradients. Thus, iodixanol is an excellent density gradient medium for the rapid and efficient resolution of endosome compartments.