Inhibition of autophagy and multiple steps in asialoglycoprotein endocytosis by inhibitors of tyrosine protein kinases (tyrphostins)

Mitophagy in the Pathogenesis of Liver Diseases

Autophagy is a catabolic process involving vacuolar sequestration of intracellular components and their targeting to lysosomes for degradation, thus supporting nutrient recycling and energy regeneration. Accumulating evidence indicates that in addition to being a bulk, nonselective degradation mechanism, autophagy may selectively eliminate damaged mitochondria to promote mitochondrial turnover, a process termed “mitophagy”. Mitophagy sequesters dysfunctional mitochondria via ubiquitination and cargo receptor recognition and has emerged as an important event in the regulation of liver physiology. Recent studies have shown that mitophagy may participate in the pathogenesis of various liver diseases, such as liver injury, liver steatosis/fatty liver disease, hepatocellular carcinoma, viral hepatitis, and hepatic fibrosis. This review summarizes the current knowledge on the molecular regulations and functions of mitophagy in liver physiology and the roles of mitophagy in the development of liver-related diseases. Furthermore, the therapeutic implications of targeting hepatic mitophagy to design a new strategy to cure liver diseases are discussed.

Diverse Functions of Autophagy in Liver Physiology and Liver Diseases

Autophagy is a catabolic process by which eukaryotic cells eliminate cytosolic materials through vacuole-mediated sequestration and subsequent delivery to lysosomes for degradation, thus maintaining cellular homeostasis and the integrity of organelles. Autophagy has emerged as playing a critical role in the regulation of liver physiology and the balancing of liver metabolism. Conversely, numerous recent studies have indicated that autophagy may disease-dependently participate in the pathogenesis of liver diseases, such as liver hepatitis, steatosis, fibrosis, cirrhosis, and hepatocellular carcinoma. This review summarizes the current knowledge on the functions of autophagy in hepatic metabolism and the contribution of autophagy to the pathophysiology of liver-related diseases. Moreover, the impacts of autophagy modulation on the amelioration of the development and progression of liver diseases are also discussed.

The Multifaceted Roles of Autophagy in Flavivirus-Host Interactions

Autophagy is an evolutionarily conserved cellular process in which intracellular components are eliminated via lysosomal degradation to supply nutrients for organelle biogenesis and metabolic homeostasis. Flavivirus infections underlie multiple human diseases and thus exert an immense burden on public health worldwide. Mounting evidence indicates that host autophagy is subverted to modulate the life cycles of flaviviruses, such as hepatitis C virus, dengue virus, Japanese encephalitis virus, West Nile virus and Zika virus. The diverse interplay between autophagy and flavivirus infection not only regulates viral growth in host cells but also counteracts host stress responses induced by viral infection. In this review, we summarize the current knowledge on the role of autophagy in the flavivirus life cycle. We also discuss the impacts of virus-induced autophagy on the pathogeneses of flavivirus-associated diseases and the potential use of autophagy as a therapeutic target for curing flavivirus infections and related human diseases.

Analysis of Tyrosine Kinase Inhibitor-Mediated Decline in Contractile Force in Rat Engineered Heart Tissue

Introduction

Left ventricular dysfunction is a frequent and potentially severe side effect of many tyrosine kinase inhibitors (TKI). The mode of toxicity is not identified, but may include impairment of mitochondrial or sarcomeric function, autophagy or angiogenesis, either as an on-target or off-target mechanism.

Methods and Results

We studied concentration-response curves and time courses for nine TKIs in three-dimensional, force generating engineered heart tissue (EHT) from neonatal rat heart cells. We detected a concentration- and time-dependent decline in contractile force for gefitinib, lapatinib, sunitinib, imatinib, sorafenib, vandetanib and lestaurtinib and no decline in contractile force for erlotinib and dasatinib after 96 hours of incubation. The decline in contractile force was associated with an impairment of autophagy (LC3 Western blot) and appearance of autophagolysosomes (transmission electron microscopy).

Conclusion

This study demonstrates the feasibility to study TKI-mediated force effects in EHTs and identifies an association between a decline in contractility and inhibition of autophagic flux.

Trafficking of the human transferrin receptor in plant cells: effects of tyrphostin A23 and brefeldin A

Plant cells possess much of the molecular machinery necessary for receptor-mediated endocytosis (RME), but this process still awaits detailed characterization. In order to identify a reliable and well-characterized marker to investigate RME in plant cells, we have expressed the human transferrin receptor (hTfR) in Arabidopsis protoplasts. We have found that hTfR is mainly found in endosomal (Ara7- and FM4-64-positive) compartments, but also at the plasma membrane, where it mediates binding and internalization of its natural ligand transferrin (Tfn). Cell surface expression of hTfR increases upon treatment with tyrphostin A23, which inhibits the interaction between the YTRF endocytosis signal in the hTfR cytosolic tail and the mu2-subunit of the AP2 complex. Indeed, tyrphostin A23 inhibits Tfn internalization and redistributes most of hTfR to the plasma membrane, suggesting that the endocytosis signal of hTfR is functional in Arabidopsis protoplasts. Co-immunoprecipitation experiments show that hTfR is able to interact with a mu-adaptin subunit from Arabidopsis cytosol, a process that is blocked by tyrphostin A23. In contrast, treatment with brefeldin A, which inhibits recycling from endosomes back to the plasma membrane in plant cells, leads to the accumulation of Tfn and hTfR in larger patches inside the cell, reminiscent of BFA compartments. Therefore, hTfR has the same trafficking properties in Arabidopsis protoplasts as in animal cells, and cycles between the plasma membrane and endosomal compartments. The specific inhibition of Tfn/hTfR internalization and recycling by tyrphostin A23 and BFA, respectively, thus provide valuable molecular tools to characterize RME and the recycling pathway in plant cells.

Testing for endocytosis in plants

For many years endocytosis has been regarded with great scepsis by plant physiologists. Although now generally accepted, care must still be taken with experiments designed to demonstrate endocytic uptake at the plasma membrane. We have taken a critical look at the various agents which are in use as markers for plant endocytosis, pointing out pitfalls and precautions which should be taken. We also take this opportunity to introduce the tyrphostins--tyrosine kinase inhibitors--, which also seem to prevent endocytosis in plants.

Stimulation of hepatocytic AMP-activated protein kinase by okadaic acid and other autophagy-suppressive toxins

Autophagic activity in isolated rat hepatocytes is strongly suppressed by OA (okadaic acid) and other PP (protein phosphatase)-inhibitory toxins as well as by AICAR (5-aminoimidazole-4-carboxamide riboside), a direct activator of AMPK (AMP-activated protein kinase). To investigate whether AMPK is a mediator of the effects of the toxin, a phosphospecific antibody directed against the activation of phosphorylation of the AMPK alpha (catalytic)-subunit at Thr172 was used to assess the activation status of this enzyme. AICAR as well as all the toxins tested (OA, microcystin-LR, calyculin A, cantharidin and tautomycin) induced strong, dose-dependent AMPKalpha phosphorylation, correlating with AMPK activity in situ (in intact hepatocytes) as measured by the AMPK-dependent phosphorylation of acetyl-CoA carboxylase at Ser79. All treatments induced the appearance of multiple, phosphatase-sensitive, low-mobility forms of the AMPK alpha-subunit, consistent with phosphorylation at several sites other than Thr172. The flavonoid naringin, an effective antagonist of OA-induced autophagy suppression, inhibited the AMPK phosphorylation and mobility shifting induced by AICAR, OA or microcystin, but not the changes induced by calyculin A or cantharidin. AMPK may thus be activated both by a naringin-sensitive and a naringin-resistant mechanism, probably involving the PPs PP2A and PP1 respectively. Neither the Thr172-phosphorylating protein kinase LKB1 nor the Thr172-dephosphorylating PP, PP2C, were mobility-shifted after treatment with toxins or AICAR, whereas a slight mobility shifting of the regulatory AMPK beta-subunit was indicated. Immunoblotting with a phosphospecific antibody against pSer108 at the beta-subunit revealed a naringin-sensitive phosphorylation induced by OA, microcystin and AICAR and a naringin-resistant phosphorylation induced by calyculin A and cantharidin, suggesting that beta-subunit phosphorylation could play a role in AMPK activation. Naringin antagonized the autophagy-suppressive effects of AICAR and OA, but not the autophagy suppression caused by cantharidin, consistent with AMPK-mediated inhibition of autophagy by toxins as well as by AICAR.

Characterization of chronic low-level proteasome inhibition on neural homeostasis

Increasing evidence suggests that proteasome inhibition plays a causal role in promoting the neurodegeneration and neuron death observed in multiple disorders, including Alzheimer's disease (AD) and Parkinson's disease (PD). The ability of severe and acute inhibition of proteasome function to induce neuron death and neuropathology similar to that observed in AD and PD is well documented. However, at present the effects of chronic low-level proteasome inhibition on neural homeostasis has not been elucidated. In order to determine the effects of chronic low-level proteasome inhibition on neural homeostasis, we conducted studies in individual colonies of neural SH-SY5Y cells that were isolated following continual exposure to low concentrations (100 nm) of the proteasome inhibitor MG115. Clonal cell lines appeared morphologically similar to control cultures but exhibited significantly different rates of both proliferation and differentiation. Elevated levels of protein oxidation and protein insolubility were observed in clonal cell lines, with all clonal cell lines being more resistant to neural death induced by serum withdrawal and oxidative stress. Interestingly, clonal cell lines demonstrated evidence for increased macroautophagy, suggesting that chronic low-level proteasome inhibition may cause an excessive activation of the lysosomal system. Taken together, these data indicate that chronic low-level proteasome inhibition has multiple effects on neural homeostasis, and suggests that studying the effects of chronic low-level proteasome inhibition may be useful in understanding the relationship between protein oxidation, protein insolubility, proteasome function, macroautophagy and neural viability in AD and PD.

Phosphoinositide 3-kinase regulates maturation of lysosomes in rat hepatocytes

To obtain information about the role of phosphoinositide 3-kinase (PI3K) in the endocytic pathway in hepatocytes, the uptake and intracellular transport of asialo-orosomucoid (ASOR) was followed in cells treated with wortmannin or LY294002. The two inhibitors, at concentrations known to inhibit the enzyme, did not affect internalization or the number of surface asialoglycoprotein receptors, but they caused a paradoxical increase (approx. 50% above control values) in the degradation of ASOR labelled with [(125)I]tyramine cellobiose ([(125)I]TC). Wortmannin or LY204002 inhibited the autophagic sequestration of lactate dehydrogenase very effectively, and the enhanced degradation of [(125)I]TC-ASOR could be an indirect effect of reduced autophagy, as an amino acid mixture known to inhibit autophagy also caused increased degradation of [(125)I]TC-ASOR, and its effect was not additive to that of wortmannin or LY294002. Wortmannin or LY294002 had pronounced effects on the late parts of the endocytic pathway in the hepatocytes: first, dense lysosomes disappeared and were replaced by swollen vesicles; secondly, degradation of [(125)I]TC-ASOR took place in an organelle of lower buoyant density (in a sucrose gradient) than the bulk of lysosomes (identified in the gradient by lysosomal marker enzymes). With increasing length of incubation with wortmannin or LY294002, the density distributions of the lysosomal markers also shifted to lower density and gradually approached that of the labelled degradation products. The labelled degradation products formed from [(125)I]TC-labelled proteins were trapped at the site of formation, because they did not penetrate the vesicle membranes. The results obtained indicate that internalization and intracellular transport of ASOR to lysomes may take place in the absence of PI3K activity in rat hepatocytes. On the other hand, fusion of late endosomes with lysosomes seems to produce 'hybrid organelles' (active lysosomes) that are unable to mature into dense lysosomes.

Tyrphostin A23 inhibits internalization of the transferrin receptor by perturbing the interaction between tyrosine motifs and the medium chain subunit of the AP-2 adaptor complex

Several intracellular membrane trafficking events are mediated by tyrosine-containing motifs within the cytosolic domains of integral membrane proteins. Many such motifs conform to the consensus YXXPhi, where Phi represents a bulky hydrophobic residue. This motif interacts with the medium chain (mu) subunits of adaptor complexes that link the cytosolic domains of integral membrane proteins to the clathrin coat involved in vesicle formation. The YXXPhi motif is similar to motifs in which the tyrosine residue is phosphorylated by tyrosine kinases. Tyrphostins (structural analogs of tyrosine) are inhibitors of tyrosine kinases and function by binding to the active sites of the enzymes. We previously showed that, in vitro and in yeast two-hybrid interaction assays, some tyrphostins can inhibit the interaction between YXXPhi motifs and the mu2 subunit of the AP-2 adaptor complex (Crump, C., Williams, J. L., Stephens, D. J., and Banting, G. (1998) J. Biol. Chem. 273, 28073-28077). A23 is such a tyrphostin. We now show that molecular modeling of tyrphostin A23 into the tyrosine-binding pocket in mu2 provides a structural explanation for A23 being able to inhibit the interaction between YXXPhi motifs and mu2. Furthermore, we show that A23 inhibited the internalization of (125)I-transferrin in Heb7a cells without having any discernible effect on the morphology of compartments of the endocytic pathway. Control tyrphostins, active as inhibitors of tyrosine kinase activity, but incapable of inhibiting the YXXPhi motif/mu2 interaction, did not inhibit endocytosis. These data are consistent with A23 inhibition of the YXXPhi motif/mu2 interaction in intact cells and with the possibility that different tyrphostins may be used to inhibit specific membrane trafficking events in eukaryotic cells.

Src-dependent tyrosine phosphorylation regulates dynamin self-assembly and ligand-induced endocytosis of the epidermal growth factor receptor

Endocytosis of ligand-activated receptors requires dynamin-mediated GTP hydrolysis, which is regulated by dynamin self-assembly. Here, we demonstrate that phosphorylation of dynamin I by c-Src induces its self-assembly and increases its GTPase activity. Electron microscopic analyses reveal that tyrosine-phosphorylated dynamin I spontaneously self-assembles into large stacks of rings. Tyrosine 597 was identified as being phosphorylated both in vitro and in cultured cells following epidermal growth factor receptor stimulation. The replacement of tyrosine 597 with phenylalanine impairs Src kinase-induced dynamin I self-assembly and GTPase activity in vitro. Expression of Y597F dynamin I in cells attenuates agonist-driven epidermal growth factor receptor internalization. Thus, c-Src-mediated tyrosine phosphorylation is required for the function of dynamin in ligand-induced signaling receptor internalization.

Protein kinase C-dependent inhibition of the lysosomal degradation of endocytosed proteins in rat hepatocytes

We studied the role of protein kinase C (PKC) in the lysosomal processing of endocytosed proteins in isolated rat hepatocytes. We used [14C]sucrose-labeled horseradish peroxidase ([14C]S-HRP) to simultaneously evaluate endocytosis and lysosomal proteolysis. The PKC activator phorbol 12-myristate 13-acetate (PMA) inhibited the lysosomal degradation of [14C]S-HRP (1 microM PMA: 40% inhibition, P<.05), without affecting either the endocytic uptake or the delivery to lysosomes. However, PMA was not able to affect the lysosomal processing of the beta-galactosidase substrate dextran galactosyl umbelliferone. The PKC inhibitors, chelerytrine (Che), staurosporine (St) and Gö 6976, prevented PMA inhibitory effect on lysosomal proteolysis. Nevertheless, purified PKC failed to alter proteolysis in [14C]S-HRP-loaded isolated lysosomes, suggesting that intracellular intermediates are required. PMA induced phosphorylation and hepatocyte membrane-to-lysosome redistribution of the myristoylated alanine-rich C kinase substrate (MARCKS) protein, raising the possibility that MARCKS mediates the PKC-induced inhibition of lysosomal proteolysis.

A novel assay to study autophagy: regulation of autophagosome vacuole size by amino acid deprivation

Autophagy is a normal degradative pathway that involves the sequestration of cytoplasmic portions and intracellular organelles in a membrane vacuole called the autophagosome. These vesicles fuse with lysosomes and the sequestered material is degraded. Owing to the complexity of the autophagic pathway and to its inaccessibility to external probes, little is known about the molecular mechanisms that regulate autophagy in higher eukaryotic cells. We used the autofluorescent drug monodansylcadaverine (MDC), a specific autophagolysosome marker to analyze at the molecular level the machinery involved in the autophagic process. We have developed a morphological and biochemical assay to study authophagy in living cells based on the incorporation of MDC. With this assay we observed that the accumulation of MDC was specifically induced by amino acid deprivation and was inhibited by 3-methlyadenine, a classical inhibitor of the autophagic pathway. Additionally, wortmannin, an inhibitor of PI3-kinases that blocks autophagy at an early stage, inhibited the accumulation of MDC in autophagic vacuoles. We also found that treatment of the cells with N-ethylmaleimide (NEM), an agent known to inhibit several vesicular transport events, completely blocked the incorporation of MDC, suggesting that an NEM-sensitive protein is required for the formation of autophagic vacuoles. Conversely, vinblastine, a microtubule depolymerizing agent that induces the accumulation of autophagic vacuoles by preventing their degradation, increased the accumulation of MDC and altered the distribution and size of the autophagic vacuoles. Our results indicate that in the presence of vinblastine very large MDC-vacuoles accumulated mainly under starvation conditions, indicating that the expansion of autophagosomes is upregulated by amino acid deprivation. Furthermore, these MDC-vacuoles were labeled with LC3, one of the mammalian homologues of the yeast protein Apg8/Aut7 that plays an important role in autophagosome formation.

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.

Ultrastructural characterization of the delimiting membranes of isolated autophagosomes and amphisomes by freeze-fracture electron microscopy

The delimiting membranes of isolated autophagosomes from rat liver had extremely few transmembrane proteins, as indicated by the paucity of intramembrane particles in freeze-fracture images (about 20 particles/microm2, whereas isolated lysosomes had about 2000 particles/microm2). The autophagosomes also appeared to lack peripheral surface membrane proteins, since attempts to surface-biotinylate intact autophagosomes only yielded biotinylation of proteins from contaminating damaged mitochondria. All the membrane layers of multilamellar autophagosomes were equally particle-poor; the same was true of the autophagosome-forming, sequestering membrane complexes (phagophores). Isolated amphisomes (vacuoles formed by fusion between autophagosomes and endosomes) had more intramembrane particles than the autophagosomes (about 90 particles/microm2), and freeze-fracture images of these organelles frequently showed particle-rich endosomes fusing with particle-poor or particle-free autophagosomes. The appearence of multiple particle clusters suggested that a single autophagic vacuole could undergo multiple fusions with endosomes. Only the outermost membrane of bi- or multilamellar autophagic vacuoles appeared to engage in such fusions.

Fluid phase endocytosis and galactosyl receptor-mediated endocytosis employ different early endosomes

Endocytosis may originate both in coated pits and in uncoated regions of the plasma membrane. In hepatocytes it has been shown that fluid phase endocytosis (here defined as 'pinocytosis') is unaffected by treatments that arrest coated pit-mediated endocytosis, indicating that pinocytosis is primarily a clathrin-independent process. In this study we have tried to determine possible connections between pinocytosis and clathrin-dependent endocytosis in rat hepatocytes by means of subcellular fractionation, electron microscopy, and by assessing the influence of inhibitors of clathrin-dependent endocytosis on pinocytosis. As marker for clathrin-dependent endocytosis was used asialoorosomucoid (AOM) labelled with [(125)I]tyramine cellobiose ([(125)I]TC). [(125)I]TC-labelled bovine serum albumin ([(125)I]TC-BSA) was found to be a useful marker for pinocytosis. Its uptake in the cells is not saturable, and any remnants of [(125)I]TC-BSA associated with the cell surface could be removed by incubating the cells with 0.3% pronase at 0 degrees C for 60 min. The data obtained by electron microscopy and by subcellular fractionation suggested that early after initiation of uptake (<15 min) [(125)I]TC-BSA and [(125)I]TC-AOM were present in different endocytic vesicles. The two probes probably join prior to their entrance in the lysosomal compartment. The relation between endocytosis via coated pits and pinocytosis was also studied with techniques that induced a selective density shift either in the clathrin-dependent pathway (by AOM-HRP) or in the pinocytic pathway (by allowing uptake of AuBSA). Both treatments indicated that the two probes ([(125)I]TC-AOM and [(125)I]TC-BSA) were early after uptake, at least partly, in separate endocytic compartments. The different distribution of the fluid phase marker and the ligand (internalised via coated pits) was not due to a difference in the rate at which they enter a later compartment, since a lowering of the incubation temperature to 18 degrees C, which should keep the probes in the early endosomes, did not affect their early density distribution. Incubation of cells in a hypertonic medium reduced uptake both of [(125)I]TC-AOM and [(125)I]TC-BSA; the uptake of [(125)I]TC-AOM was, however, reduced much more than that of the fluid phase marker. This finding supports the notion that the two probes enter the cells via different routes.

EGF receptor signaling stimulates SRC kinase phosphorylation of clathrin, influencing clathrin redistribution and EGF uptake

Epidermal growth factor (EGF) binding to its receptor causes rapid phosphorylation of the clathrin heavy chain at tyrosine 1477, which lies in a domain controlling clathrin assembly. EGF-mediated clathrin phosphorylation is followed by clathrin redistribution to the cell periphery and is the product of downstream activation of SRC kinase by EGF receptor (EGFR) signaling. In cells lacking SRC kinase, or cells treated with a specific SRC family kinase inhibitor, EGF stimulation of clathrin phosphorylation and redistribution does not occur, and EGF endocytosis is delayed. These observations demonstrate a role for SRC kinase in modification and recruitment of clathrin during ligand-induced EGFR endocytosis and thereby define a novel effector mechanism for regulation of endocytosis by receptor signaling.

Endocytosis at the apical plasma membrane of pancreatic acinar cells is regulated by tyrosine kinases

We have shown that endocytosis at the apical plasma membrane of pancreatic acinar cells is regulated by the pH of the acinar lumen and is associated with cleavage of GP2, a glycosyl phosphatidylinositol-anchored protein. The aim of this study was to determine the transduction pathway by which endocytosis is activated. Apical endocytosis was studied in rat pancreatic acini by prestimulation with cholecystokinin followed by measurement of horseradish peroxidase (HRP) uptake. Lanthanum, staurosporine, and forskolin had no effect on HRP uptake. Cytochalasin D significantly inhibited endocytosis, indicating a dependence on actin filament integrity. Genistein and the specific tyrphostin inhibitor B42 also inhibited HRP uptake, implicating tyrosine kinases in the regulation of HRP uptake. With the use of an Src kinase-specific substrate, Src kinase activity was temporally related to activation of endocytosis. The tyrosine-dependent phosphorylation of an 85-kDa substrate in both rat and mouse pancreatic acini correlated with Src kinase activation and pH-dependent regulation of HRP uptake. These results indicate that apical endocytosis in acinar cells is associated with tyrosine kinase activation and is dependent on the actin cytoskeleton.

Src-mediated tyrosine phosphorylation of dynamin is required for beta2-adrenergic receptor internalization and mitogen-activated protein kinase signaling

Some forms of G protein-coupled receptor signaling, such as activation of mitogen-activated protein kinase cascade as well as resensitization of receptors after hormone-induced desensitization, require receptor internalization via dynamin-dependent clathrin-coated pit mechanisms. Here we demonstrate that activation of beta2-adrenergic receptors (beta2-ARs) leads to c-Src-mediated tyrosine phosphorylation of dynamin, which is required for receptor internalization. Two tyrosine residues, Tyr231 and Tyr597, are identified as the major phosphorylation sites. Mutation of these residues to phenylalanine dramatically decreases the c-Src-mediated phosphorylation of dynamin following beta2-AR stimulation. Moreover, expression of Y231F/Y597F dynamin inhibits beta2-AR internalization and the isoproterenol-stimulated mitogen-activated protein kinase activation. Thus, agonist-induced, c-Src-mediated tyrosine phosphorylation of dynamin is essential for its function in clathrin mediated G protein-coupled receptor endocytosis.

Inhibition of the interaction between tyrosine-based motifs and the medium chain subunit of the AP-2 adaptor complex by specific tyrphostins

Several intracellular membrane trafficking events are mediated by tyrosine-containing motifs found within the cytosolic domains of certain integral membrane proteins. Many of these tyrosine motifs conform to the consensus YXXPhi (where Phi represents a bulky hydrophobic residue). This YXXPhi motif has been shown to interact with the medium chain subunits of adaptor complexes that generally link relevant integral membrane protein cytosolic domains to the clathrin coat involved in vesicle formation. The motif YXXPhi is also very similar to motifs that are targets for phosphorylation by tyrosine kinases. Tyrosine kinase inhibitors known as tyrphostins are structural analogues of tyrosine, and so it is possible that tyrphostins could also inhibit interactions between medium chains and YXXPhi motifs. TGN38 is a type I integral membrane protein containing a tyrosine motif, YQRL, within the cytosolic domain. We have previously shown that this motif interacts directly with the medium chain subunit of the plasma membrane localized AP-2 adaptor complex (mu2). We have investigated a range of tyrphostins and demonstrated a specific inhibition of the interaction between mu2 and the TGN38 cytosolic domain by tyrphostin A23 through in vitro analysis and the yeast two-hybrid system. These data raise the exciting possibility that different membrane traffic events could be inhibited by specific tyrphostins.

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).

Microtubule-dependent vesicle transport: modulation of channel and transporter activity in liver and kidney

Microtubule-based vesicle transport driven by kinesin and cytoplasmic dynein motor proteins facilitates several membrane-trafficking steps including elements of endocytosis and exocytosis in many different cell types. Most early studies on the role of microtubule-dependent vesicle transport in membrane trafficking focused either on neurons or on simple cell lines. More recently, other work has considered the role of microtubule-based vesicle transport in other physiological systems, including kidney and liver. Investigation of the role of microtubule-based vesicle transport in membrane trafficking in cells of the kidney and liver suggests a major role for microtubule-based vesicle transport in the rapid and directed movement of ion channels and transporters to and from the apical plasma membranes, events essential for kidney and liver function and homeostasis. This review discusses the evidence supporting a role for microtubule-based vesicle transport and the motor proteins, kinesin and cytoplasmic dynein, in different aspects of membrane trafficking in cells of the kidney and liver, with emphasis on those functions such as maintenance of ion channel and transporter composition in apical membranes that are specialized functions of these organs. Evidence that defects in microtubule-based transport contribute to diseases of the kidney and liver is also discussed.

Inhibition of hepatocytic autophagy by adenosine, aminoimidazole-4-carboxamide riboside, and N6-mercaptopurine riboside. Evidence for involvement of amp-activated protein kinase

To examine the role of AMP-activated protein kinase (AMPK; EC 2.7.1. 109) in the regulation of autophagy, rat hepatocytes were incubated with the AMPK proactivators, adenosine, 5-amino-4-imidazole carboxamide riboside (AICAR), or N6-mercaptopurine riboside. Autophagic activity was inhibited by all three nucleosides, AICAR and N6-mercaptopurine riboside being more potent (IC50 = 0.3 mM) than adenosine (IC50 = 1 mM). 2'-Deoxycoformycin, an adenosine deaminase (EC 3.5.4.4) inhibitor, increased the potency of adenosine 5-fold, suggesting that the effectiveness of adenosine as an autophagy inhibitor was curtailed by its intracellular deamination. 5-Iodotubercidin, an adenosine kinase (EC 2.7.1.20) inhibitor, abolished the effects of all three nucleosides, indicating that they needed to be phosphorylated to inhibit autophagy. A 5-iodotubercidin-suppressible phosphorylation of AICAR to 5-aminoimidazole-4-carboxamide riboside monophosphate was confirmed by chromatographic analysis. AICAR, up to 0.4 mM, had no significant effect on intracellular ATP concentrations. Because activated AMPK phosphorylates and inactivates 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.88), the rate-limiting enzyme in cholesterol synthesis, the strong inhibition of hepatocytic cholesterol synthesis by all three nucleosides confirmed their ability to activate AMPK under the conditions used. Lovastatin and simvastatin, inhibitors of HMG-CoA reductase, strongly suppressed cholesterol synthesis while having no effect on autophagic activity, suggesting that AMPK inhibits autophagy independently of its effects on HMG-CoA reductase and cholesterol metabolism.

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.

Two T Cell Epitopes from the M5 Protein of Viable Streptococcus pyogenes Engage Different Pathways of Bacterial Antigen Processing in Mouse Macrophages

We studied the mechanisms of MHC class II-restricted bacterial Ag processing of the surface fibrillar M5 protein from viable Streptococcus pyogenes in murine macrophages. Two previously defined T cell epitopes were studied using T cell hybridomas specific for 308–319/Ad, associated with the cell wall on the surface of streptococci, and 17–31/Ed, located at the protruding amino terminus of M5. Studies with metabolic inhibitors showed that slow (1 h) processing of M5 308–319 occurred in late endosomes and was dependent on newly synthesized MHC class II molecules and microtubules and on communications between early and late endosomes, consistent with engagement of the classical MHC class II processing pathway. In contrast, fast (15 min) bacterial Ag processing of 17–31 occurred in early endosomes independently of newly synthesized MHC class II molecules and microtubules and of trafficking between early and late endosomes, consistent with the recycling MHC class II processing pathway. Finally, bacterial Ag processing of the epitopes exhibited differential sensitivity to blocking with anti-MHC class II Abs. Thus, two T cell epitopes of a single protective Ag from the surface of whole bacteria are routed to distinct MHC class II processing pathways.

Molecular motors and their role in membrane traffic

Recent investigations support a role for the vesicle motor proteins (kinesin, cytoplasmic dynein, and myosin) in numerous membrane trafficking events including endocytosis and transcytosis. Kinesin and cytoplasmic dynein are responsible for movement of membrane vesicles along cellular microtubules to and from cellular membrane compartments, while certain members of the myosin family also appear to drive membrane vesicles along actin filaments to and from membrane compartments. In this review, our current understanding of the role of these vesicle motors in membrane trafficking is highlighted. Future areas of interest which may be able to make use of these vesicle motors as potential targets for drug delivery are also discussed.

Block of the cell cycle of the yeast Saccharomyces cerevisiae by tyrphostin, an inhibitor of protein tyrosine kinase

Tyrphostins are inhibitors of the epidermal growth factor receptor tyrosine kinase. To elucidate the biological function of protein tyrosine kinases in yeast cells, a mutant hypersensitive to tyrphostin was isolated and investigated for its response to the drug. The mutation was recessive and was designated tpt1 for tyrphostin hypersensitive. A tpt1 strain cannot grow in the presence of tyrphostin, implying that a biological process sensitive to tyrphostin is essential for cell growth. Microscopic observation indicated that large-budded cells were accumulated in the presence of the inhibitor. The results suggest the involvement of protein tyrosine phosphorylation in the cell cycle progression of Saccharomyces cerevisiae.

Differential effect of vanadate on receptor-mediated endocytosis of the asialoglycoprotein receptor in hepatocytes from normal and diabetic rats

Insulin-dependent diabetes has been shown to affect several aspects of receptor-mediated endocytosis. Since vanadate, a phosphate analogue, is known to exert insulin-like actions in target tissues, we studied the effects of vanadate on the endocytosis of the asialoglycoprotein receptor (ASGP-R) after its administration either in vivo (oral therapy) and/or in vitro by direct incubation of isolated hepatocytes with vanadate. The surface binding, internalization, and degradation of 3H-asialoorosomucoid (3H-ASOR), a prototype ligand of the ASGP-R, were decreased in diabetic rats by approximately 36.5%, 22.3%, and 12.9%, respectively. These values were normalized in diabetic rats treated by vanadate. Similarly, vanadate treatment normalized the biphasic dissociation of 3H-ASOR/ASGP-R complexes by restoring the rapid dissociation process. In contrast, vanadate treatment did not affect any of these endocytic parameters in normal rats. In vitro experiments were monitored by direct incubation of isolated hepatocytes with 10 mM vanadate. This incubation created an inhibitory effect on the endocytic parameters. In this work, we have demonstrated that vanadate treatment can reverse the alterations induced by diabetes on receptor-mediated endocytosis of the ASGP-R.

Analyses of APG13 gene involved in autophagy in yeast, Saccharomyces cerevisiae

We have isolated 14 apg mutants defective in autophagy in yeast Saccharomyces cerevisiae (Tsukada and Ohsumi, 1993). Among them, APG1 encodes a novel Ser/Thr protein kinase whose kinase activity is essential for autophagy. In the course of searching for genes that genetically interact with APG1, we found that overexpression of APG1 under control of the GAL1 promoter suppressed the autophagy-defective phenotype of apg13-1 mutant. Cloning and sequencing analysis showed that the APG13 gene encodes a novel hydrophilic protein of 738 amino acid residues. APG13 gene is constitutively expressed bot not starvation-inducible. Though dispensable for cell proliferation, APG13 is important for maintenance of cell viability under starvation conditions. apg13 disruptants were defective in autophagy like apg13-1 mutants. Morphological and biochemical investigation showed that a defect in autophagy of delta apg13 was also suppressed by APG1 overexpression. These results imply genetic interaction between APG1 and APG13.

Apg1p, a novel protein kinase required for the autophagic process in Saccharomyces cerevisiae

Autophagic protein degradation includes bulk protein turnover with dynamic membrane reorganization, in which formation of novel organelles autophagosomes play key roles. We have shown that Saccharomyces cerevisiae performs the autophagy in the vacuole, a lytic compartment of yeast, in response to various kinds of nutrient starvation. Here we show that the APG1 gene, involved in the autophagic process in yeast, encodes a novel type of Ser/Thr protein kinase. Our results provide direct evidence for involvement of protein phosphorylation in regulation of the autophagic process. We found overall homology of Apglp with C. elegans Unc-51 protein, suggesting that homologous molecular mechanisms, conserved from unicellular to multicellular organisms, are involved in dynamic membrane flow.

Autophagic proteolysis: control and specificity

The rate of proteolysis is an important determinant of the intracellular protein content. Part of the degradation of intracellular proteins occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include amino acids, cell swelling and insulin. In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role. Activation of a signal transduction pathway, ultimately leading to phosphorylation of ribosomal protein S6, accompanies inhibition of macroautophagy. Components of this pathway may include a heterotrimeric Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence indicates that lysosomal protein degradation can be selective and occurs via ubiquitin-dependent and -independent pathways.

Signal transduction pathways in macroautophagy

Macroautophagy is a major cellular catabolic pathway involved in the regulation of cell homeostasis. It is initiated by the sequestration of intracellular material by a wrapping membrane and terminates with the fusion of autophagic vacuoles with the lysosomal compartment. Macroautophagy has been extensively studied at the morphological level and in terms of environmental responses (nutrient deprivation, hormones). Recently a burst of data has emerged concerning the intracellular molecular events involved in the control of macroautophagic sequestration. It is becoming clear that the initial sequestration step of macroautophagy is under the control of different signalling pathways.

A novel method for the study of autophagy: destruction of hepatocytic lysosomes, but not autophagosomes, by the photosensitizing porphyrin tetra(4-sulphonatophenyl)porphine

A photoactivatable porphyrin, tetra(4-sulphonatophenyl)porphine (TPPS4), was shown to accumulate in rat hepatocytes as a linear function of dose after intravenous injection, and to localize predominantly in hepatocytic lysosomes. A major fraction of the lysosomal enzymes acid phosphatase and N-acetyl-beta-D-glucosaminidase was inactivated by TPPS4 after 20 h of contact with the drug in vivo in the absence of photoactivation. On exposure of isolated hepatocytes to light, photoactivated TPPS4 caused additional inactivation of the lysosomal enzymes as well as inactivation of intralysosomal lactate dehydrogenase (LDH), a cytosolic enzyme that accumulated in lysosomes as a result of autophagy during a 2 h incubation of hepatocytes at 37 degrees C in the dark (in the presence of the proteinase inhibitor leupeptin to prevent degradation of intralysosomal LDH). Photoactivation of TPPS4 also induced lysosomal rupture, with a loss of lysosomal enzymes, autophagocytosed LDH, endocytosed 125I-tyramine-cellobiose-asialo-orosomucoid and TPPS4 from the lysosomes. However, LDH-containing autophagosomes, accumulated in the presence of vinblastine (a microtubule inhibitor used to prevent the fusion of lysosomes with autophagosomes or endosomes), were not affected by TPPS4. TPPS4 may thus be useful as a selective lysosomal (or endosomal) perturbant in the study of autophagic-endocytic-lysosomal interactions.

Guanine nucleotide exchange on heterotrimeric Gi3 protein controls autophagic sequestration in HT-29 cells

Recent results have shown that autophagic sequestration in the human colon cancer cell line HT-29 is controlled by the pertussis toxin-sensitive heterotrimeric Gi3 protein. Here we show that transfection of an antisense oligodeoxynucleotide to the alphai3-subunit markedly inhibits autophagic sequestration, whereas transfection of an antisense oligodeoxynucleotide to the alphai2-subunit does not change the rate of autophagy in HT-29 cells. Autophagic sequestration was arrested in cells transfected with a mutant of the alphai3-subunit (Q204L) that is restricted to the GTP-bound form. In Q204L-expressing cells, 3-methyladenine-sensitive degradation of long lived [14C]valine-labeled proteins was severely impaired and could not be stimulated by nutrient deprivation. Autophagy was also reduced when dissociation of the betagamma dimer from the GTP-bound alphai3-subunit was impaired in cells transfected with the G203A mutant. In contrast, a high rate of pertussis toxin-sensitive autophagy was observed in cells transfected with an alphai3-subunit mutant (S47N) which has an increased guanine nucleotide exchange rate and increased preference for GDP over GTP. Cells that express pertussis toxin-insensitive mutants of either wild-type alphai3-subunit (C351S) or S47N alphai3-subunit (S47N/C351S) exhibit a high rate of autophagy.