Dynamic changes in protein components of the tight junction during liver regeneration

The construction of the hepatocyte tight junction is one of the most import ant events during liver regeneration leading to the reorganization of the b ile canaliculi and the repolarization of hepatocytes after cell division. T o understand this event at the molecular level, we examined the expression of tight junction proteins by Western blot analysis and their cellular loca lization by immunofluorescence microscopy in regenerating rat liver after t wo-thirds hepatectomy. The levels of tight junction components such as clau din-3, ZO-1 and atypical protein kinase C (PKC)-specific interacting protei n (ASIP) increased two- to three-fold over control levels in coordination w ith a peak 2-3 days after partial hepatectomy, whereas occludin levels rema ined unchanged. The bile canaliculi outlined by tight junction components a nd actin filaments reveal significant morphological changes from 2-3 days a fter partial hepatectomy. During this period, claudin-3/ZO-1 and ASIP/ZO-1 were nearly co-localized, whereas occludin was locally reduced or almost ab sent on the bile canaliculi outlined by ZO-1 staining. The uncoupled locali zation of F-actin and tight junction components was often observed. The fun ction of hepatocytes, as revealed by the serum bile acids level, was distor ted temporally at an early stage of regeneration but mostly restored 3 days after partial hepatectomy. These observations suggest that the de novo con struction of tight junctions proceeds mainly 2-3 days after partial hepatec tomy in parallel with the cell polarization required for hepatocyte functio n. However, the complete normalization of the composition of the tight junc tion components, such as occludin and the association with F-actin, require s additional time, which may support the regeneration of fully polarized no rmal hepatocytes.