The Fusarium mycotoxin, 2-Amino-14,16-dimethyloctadecan-3-ol (AOD) induces vacuolization in HepG2 cells.

The mycotoxin 2-Amino-14,16-dimethyloctadecan-3-ol (AOD) has been isolated from cultures of the fungus Fusarium avenaceum, one of the most prevalent Fusarium species. AOD is an analogue of sphinganine and 1-deoxysphinganine, important intermediates in the de novo biosynthesis of cellular sphingolipids. Here we studied cellular effects of AOD using the human liver cell line HepG2 as a model system. AOD (10 µM) induced a transient accumulation of vacuoles in the cells. The effect was observed at non-cytotoxic concentrations and was not linked to cell death processes. Proteomic analyses indicated that protein degradation and/or vesicular transport may be a target for AOD. Further studies revealed that AOD had only minor effects on the initiation rate of macropinocytosis and autophagy. However, the AOD-induced vacuoles were lysosomal-associated membrane protein-1 (LAMP-1) positive, suggesting that they most likely originate from lysosomes or late endosomes. Accordingly, both endosomal and autophagic protein degradation were inhibited. Further studies revealed that treatment with concanamycin A or chloroquine completely blocked the AOD-induced vacuolization, suggesting that the vacuolization is dependent of acidic lysosomes. Overall, the results strongly suggest that the increased vacuolization is due to an accumulation of AOD in lysosomes or late endosomes thereby disturbing the later stages of the endolysosomal process.

Autophagy and senescence, stress responses induced by the DNA-damaging mycotoxin alternariol.

The mycotoxin alternariol (AOH), a frequent contaminant in fruit and grain, is known to induce cellular stress responses such as reactive oxygen production, DNA damage and cell cycle arrest. Cellular stress is often connected to autophagy, and we employed the RAW264.7 macrophage model to test the hypothesis that AOH induces autophagy. Indeed, AOH treatment led to a massive increase in acidic vacuoles often observed upon autophagy induction. Moreover, expression of the autophagy marker LC3 was markedly increased and there was a strong accumulation of LC3-positive puncta. Increased autophagic activity was verified biochemically by measuring the degradation rate of long-lived proteins. Furthermore, AOH induced expression of Sestrin2 and phosphorylation of AMPK as well as reduced phosphorylation of mTOR and S6 kinase, common mediators of signaling pathways involved in autophagy. Transmission electron microscopy analyzes of AOH treated cells not only clearly displayed structures associated with autophagy such as autophagosomes and autolysosomes, but also the appearance of lamellar bodies. Prolonged AOH treatment resulted in changed cell morphology from round into more star-shaped as well as increased ß-galactosidase activity. This suggests that the cells eventually entered senescence. In conclusion, our data identify here AOH as an inducer of both autophagy and senescence. These effects are suggested to be to be linked to AOH-induced DSB (via a reported effect on topoisomerase activity), resulting in an activation of p53 and the Sestrin2-AMPK-mTOR-S6K signaling pathway.

Internalization of cholera toxin by different endocytic mechanisms.

The mechanism of cholera toxin (CT) internalization has been investigated using Caco-2 cells transfected with caveolin to induce formation of caveolae, HeLa cells with inducible synthesis of mutant dynamin (K44A) and BHK cells in which antisense mRNA to clathrin heavy chain can be induced. Here we show that endocytosis and the ability of CT to increase the level of cAMP were unaltered in caveolin-transfected cells grown either in a non-polarized or polarized manner. Treatment of Caco-2 cells with filipin reduced CT-uptake by less than 20%, suggesting that caveolae do not play a major role in the uptake. Extraction of cholesterol by methyl-beta-cyclodextrin, which removes caveolae and inhibits uptake from clathrin-coated pits, gave 30-40% reduction of CT-endocytosis. Also, CT-uptake in HeLa K44A cells was reduced by 50-70% after induction of mutant dynamin, which inhibits both caveolae- and clathrin-dependent endocytosis. These cells contain few caveolae, and nystatin and filipin had no effect on CT-uptake, indicating major involvement of clathrin-coated pits in CT-internalization. Similarly, in BHK cells, where clathrin-dependent endocytosis is blocked by induction of antisense clathrin heavy chain, the CT-uptake was reduced by 50% in induced cells. In conclusion, a large fraction of CT can be endocytosed by clathrin-dependent as well as by caveolae- and clathrin-independent endocytosis in different cell types.

Ricin transport into cells: studies of endocytosis and intracellular transport.

The plant toxin ricin binds to both glycoproteins and glycolipids with terminal galactose, and the toxin will therefore be endocytosed by the different mechanisms operating in a given cell. After endocytosis the toxin is transported to the Golgi apparatus by a process that differs from the Rab9-dependent transport of mannose-6-phosphate receptors. Retrograde toxin transport from the Golgi apparatus to the endoplasmic reticulum (ER) seems to be a requirement for subsequent toxin translocation to the cytosol where the toxin inhibits protein synthesis enzymatically. By using ricin we have characterized different types of endocytosis and the transport steps used by this toxin.

Endocytic mechanisms responsible for uptake of GPI-linked diphtheria toxin receptor.

We have here used diphtheria toxin as a tool to investigate the type of endocytosis used by a glycosylphosphatidylinositol-linked molecule, a glycosylphosphatidylinositol-linked version of the diphtheria toxin receptor that is able to mediate intoxication. The receptor is expressed in HeLa cells where clathrin-dependent endocytosis can be blocked by overexpression of mutant dynamin. Diphtheria toxin intoxicates cells by first binding to cell-surface receptors, then the toxin is endocytosed, and upon exposure to low endosomal pH, the toxin enters the cytosol where it inhibits protein synthesis. Inhibition of protein synthesis by the toxin can therefore be used to probe the entry of the glycosylphosphatidylinositol-linked receptor into an acidic compartment. Furthermore, degradation of the toxin can be used as an indicator of entry into the endosomal/lysosomal compartment. The data show that although expression of mutant dynamin inhibits intoxication mediated via the wild-type receptors, mutant dynamin does not affect intoxication or endocytosis and degradation of diphtheria toxin bound to the glycosylphosphatidylinositol-linked receptor. Confocal microscopy demonstrated that diphtheria toxin is transported to vesicles containing EEA1, a marker for early endosomes. Biochemical and ultrastructural studies of the HeLa cells used reveal that they have very low levels of caveolin-1 and that they contain very few if any caveolae at the cell surface. Furthermore, the endocytic uptake of diphtheria toxin bound to the glycosylphosphatidylinositol-linked receptor was not reduced by methyl-beta-cyclodextrin or by nystatin which both disrupt caveolar structure and functions. Thus, uptake of a glycosylphosphatidylinositol-linked protein, in this case the diphtheria toxin receptor, into the endosomal/lysosomal system can occur independently of both caveolae and clathrin-coated vesicles.

Hepatocyte growth factor (HGF) induces interleukin-11 secretion from osteoblasts: a possible role for HGF in myeloma-associated osteolytic bone disease.

Multiple myeloma is associated with unbalanced bone remodeling causing lytic bone lesions. Interleukin-11 (IL-11) promotes osteoclast formation and inhibits osteoblast activity and may, thus, be one factor involved in cancer-induced bone destruction. We have previously shown that myeloma cells produce hepatocyte growth factor (HGF). We now report that HGF induces IL-11 secretion from human osteoblast-like cells and from the osteosarcoma cell lines Saos-2 and HOS. In coculture experiments, both the myeloma cell line JJN-3 and primary myeloma cells from 3 patients induced IL-11 secretion from osteoblasts, whereas no induction was observed with the non-HGF producing myeloma cell line OH-2. Enhanced IL-11 induction was observed with physical contact between osteoblasts and myeloma cells as compared with experiments in which contact was prohibited by tissue inserts. Anti-HGF serum strongly reduced the myeloma cell-induced IL-11 secretion. Furthermore, we show that JJN-3 cells express HGF on the cell-surface. Removal of surface-bound HGF on JJN-3 cells reduced IL-11 production induced in cocultures. Transforming growth factor beta1 and IL-1 potentiated the effect of HGF on IL-11 secretion, whereas an additive effect was observed with tumor necrosis factor. Thus, myeloma-derived HGF can influence the bone marrow environment both as a soluble and a surface-bound factor. Furthermore, HGF emerges as a possible factor involved in myeloma bone disease by its ability to induce IL-11.