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Cytoskeletal changes in hepatocytes induced by Microcystis toxins and their relation to hyperphosphorylation of cell
1Department of Biochemistry, Microbiology and Nutrition, University of New England, Armidale, N.S.W., Australia.
Abstract:
The heptapeptide toxins produced by the blue-green alga (cyanobacterium) Microcystis aeruginosa are selectively hepatotoxic in mammals. The characteristic post-mortem pathology of the liver is extensive lobular disruption due to sinusoidal breakdown, leakage of blood into the tissue and hepatocyte disintegration. Isolated hepatocytes incubated with toxin show severe structural deformity and surface blebbing. This paper demonstrates the effects of Microcystis toxins on the contraction and aggregation of actin microfilaments, and on the relocation and breakdown of cytokeratin intermediate filaments, in cultured hepatocytes. Earlier work did not show changes in the assembly/disassembly of actin; however, this paper demonstrates the change in cytokeratin from intermediate filaments to distributed granules in the cytoplasm of toxin-affected cells. Acrylamide gel electrophoresis of cytoskeletal fractions from hepatocytes did not show changes in total cytokeratins; however, marked changes in the immunogenicity of cytokeratins at 52 and 58 kDa were seen on toxin exposure of cells. Measurement of 32P-phosphorylation of proteins in toxin-affected cells incubated with [32P]orthophosphate showed a dramatic increase compared to control incubations. This is in agreement with research elsewhere describing phosphatase inhibition in vitro by Microcystis toxins. The data indicate that phosphorylated cytokeratin is a major component of cytoplasmic fraction phosphorylated protein after toxin exposure to hepatocytes. It is concluded that the mechanism of Microcystis toxicity to the hepatocyte is through cytoskeletal damage leading to loss of cell morphology, cell to cell adhesion and finally cellular necrosis. The underlying biochemical lesion is likely to be phosphatase inhibition causing hyperphosphorylation of a number of hepatocyte proteins, including those cytokeratins responsible for microfilament orientation and intermediate filament integrity.
Insights
Microcystis toxins damage liver cells by disrupting their internal structure. These cyanotoxins cause hyperphosphorylation of cytokeratins, leading to cell death and liver damage.
Area of Science:
- Hepatology
- Toxicology
- Cell Biology
Background:
- Microcystis aeruginosa produces heptapeptide toxins that are hepatotoxic to mammals.
- Liver pathology includes sinusoidal breakdown, blood leakage, and hepatocyte disintegration.
Purpose of the Study:
- To investigate the effects of Microcystis toxins on cytoskeletal elements in cultured hepatocytes.
- To elucidate the biochemical mechanisms underlying Microcystis-induced hepatotoxicity.
Main Methods:
- Cultured hepatocytes were exposed to Microcystis toxins.
- Changes in actin microfilaments and cytokeratin intermediate filaments were analyzed.
- Cytoskeletal fractions were analyzed by acrylamide gel electrophoresis and immunogenicity assays.
- Protein phosphorylation was measured using [32P]orthophosphate.
Main Results:
- Microcystis toxins induced structural deformity and surface blebbing in hepatocytes.
- Toxin exposure altered cytokeratin from intermediate filaments to cytoplasmic granules.
- Immunogenicity of 52 and 58 kDa cytokeratins changed, though total levels remained constant.
- Hepatocytes exposed to toxins showed significantly increased protein phosphorylation, particularly of cytokeratins.
Conclusions:
- Microcystis toxicity results from cytoskeletal damage, leading to loss of cell morphology, adhesion, and necrosis.
- The primary biochemical lesion is likely phosphatase inhibition, causing hyperphosphorylation of hepatocyte proteins, including cytokeratins.
- This hyperphosphorylation disrupts the cytoskeleton, mediating Microcystis hepatotoxicity.