Protein phosphatase inhibition in normal and keratin 8/18 assembly-incompetent mouse strains supports a functional role of keratin intermediate filaments in preserving hepatocyte integrity.
The function and regulation of keratin 8 (K8) and 18 (K18), intermediate filament (IF) proteins of the liver, are not fully understood. We employed the liver damage induced by microcystin-LR (MC-LR), a liver-specific inhibitor of type-1 and type-2A protein phosphatases, in normal and in keratin assembly-incompetent mouse strains as a model to elucidate the roles of IF phosphorylation in situ. The mouse strains used were wild-type (wt) mice and mice with abnormal filament assembly, caused by a targeted null mutation of the K8 gene or caused by expression of a point-mutated dominant negative human K18. In vivo 32P-labeled wt mice, subsequently injected with a lethal dose of MC-LR, showed hyperphosphorylation, disassembly, and reorganization of K8/K18, in particular K18, indicating high phosphate turnover on liver keratins in situ. At lethal doses, the keratin assembly-incompetent mice displayed liver lesions faster than wt mice, as indicated histopathologically and by liver-specific plasma enzyme elevations. The histological changes included centrilobular hemorrhage in all mouse strains. The assembly-incompetent mice showed a marked vacuolization of periportal hepatocytes. Indistinguishable MC-LR-induced reorganization of microfilaments was observed in all mice, indicating that this effect on microfilaments is not dependent on the presence of functional K8/K18 networks. At sublethal doses of MC-LR, all animals had the same potential to recover from the liver damage. Our study shows that K8/K18 filament assembly is regulated in vivo by serine phosphorylation. The absence or occurrence of defective K8/K18 filaments render animals more prone to liver damage, which supports the previously suggested roles of keratin IFs in maintenance of structural integrity.
Toivola
D M
DM
Department of Biology, Abo Akademi University, BioCity, Turku, Finland.
Omary
M B
MB
Ku
N O
NO
Peltola
O
O
Baribault
H
H
Eriksson
J E
JE
D000818:Animals; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D007381:Intermediate Filament Proteins / Q000502:physiology*; D007633:Keratins / Q000235:genetics* / Q000502:physiology*; D008099:Liver / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008387:Marine Toxins; D051379:Mice; D008807:Mice, Inbred BALB C; D008822:Mice, Transgenic / Q000235:genetics; D052998:Microcystins; D009154:Mutation; D010456:Peptides, Cyclic / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D010766:Phosphorylation / Q000187:drug effects; D012016:Reference Values
D004791:Enzyme Inhibitors; D007381:Intermediate Filament Proteins; D008387:Marine Toxins; D052998:Microcystins; D010456:Peptides, Cyclic; D007633:Keratins; D010749:Phosphoprotein Phosphatases; C057862:cyanoginosin LR