The effect of chloroquine on lysosomal function and cell-coat glycoprotein transport in the absorptive cells of cultured human small-intestinal tissue

Regulation of late endosomal/lysosomal maturation and trafficking by cortactin affects Golgi morphology

Cortactin is a branched actin regulator and tumor-overexpressed protein that promotes vesicular trafficking at a variety of cellular sites, including endosomes and the trans-Golgi network. To better understand its role in secretory trafficking, we investigated its function in Golgi homeostasis. Here, we report that knockdown (KD) of cortactin leads to a dramatic change in Golgi morphology by light microscopy, dependent on binding the Arp2/3 actin-nucleating complex. Surprisingly, there was little effect of cortactin-KD on anterograde trafficking of the constitutive cargo vesicular stomatitis virus glycoprotein (VSVG), Golgi assembly from endoplasmic reticulum membranes upon Brefeldin A washout, or Golgi ultrastructure. Instead, electron microscopy studies revealed that cortactin-KD cells contained a large number of immature-appearing late endosomal/lysosomal (LE/Lys) hybrid organelles, similar to those found in lysosomal storage diseases. Consistent with a defect in LE/Lys trafficking, cortactin-KD cells also exhibited accumulation of free cholesterol and retention of the retrograde Golgi cargo mannose-6-phosphate receptor in LE. Inhibition of LE maturation by treatment of control cells with Rab7 siRNA or chloroquine led to a compact Golgi morphology similar to that observed in cortactin-KD cells. Furthermore, the Golgi morphology defects of cortactin-KD cells could be rescued by removal of cholesterol-containing lipids from the media, suggesting that buildup of cholesterol-rich membranes in immature LE/Lys induced disturbances in retrograde trafficking. Taken together, these data reveal that LE/Lys maturation and trafficking are highly sensitive to cortactin-regulated branched actin assembly and suggests that cytoskeletal-induced Golgi morphology changes can be a consequence of altered trafficking at late endosomes.

Dominant-negative effect of truncated mannose 6-phosphate/insulin-like growth factor II receptor species in cancer

Oligomerization of the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) is important for optimal ligand binding and internalization. M6P/IGF2R is a tumor suppressor gene that exhibits loss of heterozygosity and is mutated in several cancers. We tested the potential dominant-negative effects of two cancer-associated mutations that truncate M6P/IGF2R in ectodomain repeats 9 and 14. Our hypothesis was that co-expression of the truncated receptors with the wild-type/endogenous full-length M6P/IGF2R would interfere with M6P/IGF2R function by heterodimer interference. Immunoprecipitation confirmed formation of heterodimeric complexes between full-length M6P/IGF2Rs and the truncated receptors, termed Rep9F and Rep14F. Remarkably, increasing expression of either Rep9F or Rep14F provoked decreased levels of full-length M6P/IGF2Rs in both cell lysates and plasma membranes, indicating a dominant-negative effect on receptor availability. Loss of full-length M6P/IGF2R was not due to increased proteasomal or lysosomal degradation, but instead arose from increased proteolytic cleavage of cell-surface M6P/IGF2Rs, resulting in ectodomain release, by a mechanism that was inhibited by metal ion chelators. These data suggest that M6P/IGF2R truncation mutants may contribute to the cancer phenotype by decreasing the availability of full-length M6P/IGF2Rs to perform tumor-suppressive functions such as binding/internalization of receptor ligands such as insulin-like growth factor II.

Does chloroquine, an antimalarial drug, affect autophagy in Tetrahymena pyriformis?

The effect of chloroquine (CQ) on autophagy was studied in starved Tetrahymena pyriformis. When a proliferating Tetrahymena culture is transferred to a starvation medium, autophagy commences although cells most advanced in the cell cycle will divide. The drug was added to 1-h starved cells at different pH values because CQ affects pH dependently. The CQ concentration blocking all cell divisions was determined as the lowest toxic, but sublethal, concentration. Hence, the highest tolerated concentrations at pH 6.8, 7.1, and 7.7 were 1.0, 0.3, and 0.03 mM CQ, respectively. Lower CQ concentrations had a dose-dependent effect on cell increment and higher concentrations induced cell mortality. Rates of cell motility and decreases in cell volume were affected by the drug, while the capacity for endocytosis was unaffected in low concentrations but affected dose dependently in high concentrations. Light microscopically, all drug-treated cells contained small refractive bodies, but in toxic concentrations they also contained conspicuously large vacuoles. After 1 h and 4 h in CQ, fine structure analysis showed autophagosomes with electron-dense material in cells in tolerated concentrations and of enlarged size, but decreased number, in toxic concentrations. The contents of autophagosomes revealed cell organelles in different stages of disintegration. The conclusion is that the drug enhances autophagy in Tetrahymena in a pH-, dose-, and time-dependent manner.

The posttranslational processing of sucrase-isomaltase in HT-29 cells is a function of their state of enterocytic differentiation

The biosynthesis of sucrase-isomaltase was compared in enterocyte-like differentiated (i.e., grown in the absence of glucose) and undifferentiated (i.e., grown in the presence of glucose) HT-29 cells. Unlike differentiated cells, in which the enzyme is easily detectable and active, undifferentiated cells display almost no enzyme activity and the protein cannot be detected by means of cell surface immunofluorescence or immunodetection in membrane-enriched fractions or cell homogenates. Pulse experiments with L-[35S]-methionine show that the enzyme is, however, synthesized in these undifferentiated cells. As compared with the corresponding molecular forms in differentiated cells, the high-mannose form of the enzyme in undifferentiated cells is similarly synthesized and has the same apparent Mr. However, its complex form is less labeled and has a lower apparent Mr. Pulse-chase experiments with L-[35S]methionine show that, although the enzyme is synthesized to the same extent in both situations, the high-mannose and complex forms are rapidly degraded in undifferentiated cells, with an apparent half-life of 6 h, in contrast to differentiated cells in which the enzyme is stable for at least 48 h. A comparison of the processing of the enzyme in both situations shows that the conversion of the high-mannose to the complex form is markedly decreased in undifferentiated cells. These results indicate that the absence of sucrase-isomaltase expression in undifferentiated cells is not the consequence of an absence of biosynthesis but rather the result of both an impaired glycosylation and a rapid degradation of the enzyme.

Biosynthesis of intestinal microvillar proteins. Surface expression of aminopeptidase N is not affected by chloroquine

The effect of chloroquine on the biosynthesis of pig intestinal aminopeptidase N (EC 3.4.11.2) was studied by labelling with [35S]methionine in organ cultured mucosal explants. The lysosomotropic agent did not alter the molecular size of either the transient or the mature form of the enzyme and did not markedly influence the relative intracellular distribution of the two forms. The microvillar expression of aminopeptidase N during labelling periods of 80-120 min was found to be unaffected by chloroquine. Together these data indicate that pH neutralization of the acidic compartments of the cell bears no consequence on the intracellular transport of the newly synthesized microvillar enzyme. This suggests that the acidic compartments are not involved in the post-Golgi transport and that this, in turn, probably occurs via a constitutive rather than a regulated pathway.

Biosynthesis of microvillar proteins

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Light and electron microscopic cytochemistry of glycoconjugates in the rectosigmoid colonic epithelium of the mouse and rat

The several cell types in mouse and rat rectosigmoid colon have been examined with light and electron microscopic methods for localizing and characterizing complex carbohydrates. Mucous cells, also termed vacuolated cells, and goblet cells comprised most of the deep crypt epithelium in both species, and absorptive columnar cells and goblet cells mainly populated the more superficial epithelium of the upper crypts and main lumen. Occasional tuft cells and enteroendocrine cells were also encountered. Transitional cells structurally intermediate between mucous cells and absorptive cells contained granules characteristic of mucous cells and vesicles like those of columnar absorptive cells. These intermediate cells supported the concept of replacement of mucous by absorptive cells through transformation of mucous into absorptive cells. The intermediate cells also contained numerous lysosomes often in apparent fusion with mucous granules, indicating crinophagic disposal of mucous granules as a mechanism in the cell transformation. Glycoconjugate in absorptive cell vesicles resembled that coating the apical plasmalemma and appeared to represent the source of the glycocalyx of the brush border. Complex carbohydrate in these vesicles differed cytochemically from that of the mucous cell granules, which release their content into the crypt lumen. The absorptive cell vesicles, therefore, constitute an organelle distinct from the mucous cell granules rather than an atrophic form of the latter in a more mature cell. Goblet cells differed in failing to transform morphologically with age but changed in the cytochemical characteristic of their secretion during migration up the crypts. Terminal N-acetylglucosamine residues diminished, while terminal sialic acid-galactose dimers increased during the upward migration, indicating activation of glycosyl transferase synthesis in relation to goblet cell maturation. Glycoconjugate in secretion of mucous cell granules differed markedly from that in goblet cell granules, and content of both organelles differed from that of absorptive cell vesicles. However, secretion in mucous cell granules appeared generally similar for mice and rats with minor exceptions, and secretion in goblets of mice generally resembled that in goblets of rats. Cells interpreted tentatively as Kulchitsky cells stained for high content of fucose with the Ulex europeus I lectin. Globoid leukocytes infiltrating the epithelium of the rat but not the mouse rectosigmoid colon resembled globoid leukocytes in rat tracheal epithelium and, like the latter, appeared to derive from mast cells.

Endocytosis in absorptive cells of cultured human small-intestinal tissue: effect of cytochalasin B and D

The effect of cytochalasin B (CB) and cytochalasin D (CD) on the endocytotic uptake of horseradish peroxidase (HRP) by intestinal absorptive cells was investigated by morphometric methods. The results showed that CD inhibited endocytosis considerably, and without any detrimental side-effects. CB had hardly any effect on the endocytosis of HRP, but caused a significant decrease in the number of apical vesicles and tubules involved in the transport of cell-coat glycoproteins from the Golgi apparatus to the brush border. Electron-microscopic autoradiographic analysis of the effect of CD showed that although endocytosis is inhibited significantly by the drug, the amount of radiolabelled cell-coat material entering the lysosome-like bodies was unaltered compared with control cultures. These observations support our hypothesis that the cell-coat glycoproteins of the absorptive cells enter the lysosome-like bodies by a crinophagic rather than by an exocytotic-endocytotic mechanism.

The effect of chloroquine on the intralysosomal degradation of cell-coat glycoproteins in the absorptive cells of cultured human small-intestinal tissue as shown by silver proteinate staining

The effect of chloroquine on the intralysosomal degradation of cell-coat glycoproteins in cultured intestinal absorptive cell was investigated by silver proteinate staining. The results of this staining method, which is specific for carbohydrate containing macromolecules such as glycoproteins and mucopolysaccharides, showed that in the presence of the drug considerable amounts of silver proteinate-positive material accumulated in one type of lysosome-like body: the dense bodies. The staining pattern of other cell organelles was not affected by chloroquine. The presence of the drug in the culture medium also resulted in the occurrence of numerous small vesicular structures in the matrix of the dense bodies. These showed a similar size and structure to those present in the other type of lysosome-like body: the multivesicular bodies. This observation, together with earlier autoradiographical data, suggests that cell-coat material is transferred from multivesicular to dense bodies by fusion between these organelles. This study thus provides further evidence for a regulatory mechanism of cell-coat glycoprotein transport by the lysosome-like bodies in human intestinal absorptive cells.

Endocytosis in absorptive cells of cultured human small-intestinal tissue: horseradish peroxidase, lactoperoxidase, and ferritin as markers

The occurrence of endocytotic mechanisms in human small-intestinal absorptive cells was investigated by culturing biopsy specimens in the presence of horseradish peroxidase (HRP), lactoperoxidase (LPO), and ferritin. The results indicate that both HRP and LPO entered the cells by apical endocytosis, after which they were transported via apical vesicles and tubules to the lysosome-like bodies. Ferritin, which showed a distinct affinity for the cell-coat glycoproteins, was not interiorized by the absorptive cells. These findings suggest that although human absorptive cells have an endocytotic mechanism, possibly fluid-phase endocytosis, cell-coat glycoproteins are not taken up by the cells, as indicated by the absence of ferritin in the apical vesicles and tubules, as well as the lysosome-like bodies. These findings provide indirect support for our hypothesis that the lysosome-like bodies have a function in the regulation of cell-coat glycoprotein transport via a crinophagic mechanism (fusion of apical vesicles and tubules with lysosome-like bodies) rather than via an exocytotic-endocytotic mechanism.

Binding of cationized ferritin to the cell-coat glycoproteins of human and rat small-intestinal absorptive cells

The binding of cationized ferritin (CF) to the cell-coat (glycocalyx) glycoproteins of human and rat intestinal absorptive cells was investigated in relation to the amount of sialic acid in these macromolecules. The cell coat of human absorptive cells exhibited poor binding of CF and contained a small amount of sialic acid. The cell coat of rat absorptive cells had about ten times more sialic acid than that of human cells and showed a strong affinity for the marker. The removal of sialic acid from the cell-coat glycoproteins of rat intestinal cells by neuraminidase treatment abolished CF binding. These results suggest that sialic acid is necessary for CF binding and that human and rat intestinal absorptive cells show a species-specific difference in the sugar composition of the cell coat.