Localization of alkaline phosphatase and proteins related to intercellular junctions in rat hepatoma cell line McA-RH 7777

Altered membrane glycoprotein targeting in cholestatic hepatocytes

BACKGROUND

Hepatocytes are polarized epithelial cells with three morphologically and functionally distinct membrane surfaces: the sinusoidal, lateral and canalicular surface domains. These domains differ from each other in the expression of integral proteins, which concur to their polarized functions. We hypothesize that the cholestasis-induced alterations led to partial loss of hepatocyte polarity. An altered expression of membrane proteins may be indicative of functional disorders. Alkaline liver phosphatase (ALP), one of the most representative plasma membrane glycoproteins in hepatocytes, is expressed at the apical (canalicular) pole of the cell. Because the release of ALP protein in the bloodstream is significantly increased in cholestasis, the enzymatic levels of plasma ALP have major relevance in the diagnosis of cholestatic diseases. Here we assess the cholestasis-induced redistribution of membrane glycoproteins to investigate the ALP release.

MATERIALS AND METHODS

We performed enzymatic histochemistry, immunohistochemistry, lectin histochemistry, immunogold and lectin-and immunoblotting studies. Experimental cholestasis was induced in rats by ligation of common bile duct (BDL).

RESULTS

The BDL led to altered membrane sialoglycoprotein targeting as well as to ultrastructural and functional disorders. Disarrangement of the microtubular system, thickening of the microfilamentous pericanalicular ectoplasm and disturbance of the vectorial trafficking of membrane glycoprotein containing vesicles were found.

CONCLUSIONS

Altogether, results indicate that the cholestasis-induced partial loss of hepatocyte cell polarity leads to mistranslocation of ALP to the sinusoidal plasma membrane from where the enzyme is then massively released into the bloodstream.

Effects of colchicine on localization of alkaline phosphatase in McA-RH 7777 rat hepatoma cells

We investigated the changes caused by microtubule disruption in cell contact-induced translocation of alkaline phosphatase (ALP) from the Golgi area to the plasma membrane in McA-RH 7777 cells. When the cells were treated with colchicine, the tubular structure of microtubules in the cytoplasm was lost. Colchicine treatment also resulted in the appearance of numerous dots containing mannosidase II (man II) throughout the cytoplasm. Moreover, ALP was distributed in small dots throughout the cytoplasm, as well as in all regions of the plasma membrane, although it was most concentrated at sites of intercellular contact. On the other hand, when the cells were incubated in basal medium after colchicine treatment, large spots containing ALP reappeared in the perinuclear cytoplasm more quickly than the accumulation of small dots containing man II. These findings suggest that colchicine causes disassembly of the Golgi complex into fragments, which scatter throughout the cytoplasm, but that it does not interfere with translocation of ALP to the plasma membrane. Furthermore, cytoplasmic ALP may be localized at sites other than the Golgi complex.

Change in localization of alkaline phosphatase and mannosidase II by colchicine treatment of primary cultures of fetal rat hepatocytes

We examined the changes in localization of alkaline phosphatase (ALP) and mannosidase II (man II), a Golgi marker, after colchicine treatment of primary cultures of fetal rat hepatocytes, using double immunofluorescence staining and confocal laser microscopy. In hepatocytes cultured in basal medium, ALP was localized in the perinuclear cytoplasm, and man II was observed in the Golgi region of the cytoplasm. When hepatocytes were cultured in dexamethasone-supplemented medium, ALP was also localized in the plasma membrane surrounding the bile canaliculus-like structure that was formed between adjacent cells. In hepatocytes cultured in the same medium containing colchicine, the structure of microtubules in the cytoplasm was lost, man II exhibited granular distribution scattering throughout the cytoplasm, and ALP was localized in coarse granular sites of the cytoplasm. However, ALP was not colocalized at the same sites as man II. The present study indicated that colchicine inhibits the dexamethasone-promoted translocation of ALP to the plasma membrane surrounding the bile canaliculus-like structure in primary cultures of fetal rat hepatocytes by disassembling microtubules and discomposing the Golgi complex.

Localization of alkaline phosphatase and proteins related to intercellular junctions in primary cultures of fetal rat hepatocytes

The localization of alkaline phosphatase (ALP) and four proteins related to intercellular junctions in primary cultures of fetal rat hepatocytes was immunocytochemically investigated using fluorescence-labeled antibodies and confocal laser microscopy in order to determine whether the formation of intercellular junctions at the borders between adjacent rat hepatocytes becomes the trigger of translocation of ALP from the cytoplasm to the plasma membrane. Dexamethasone (DEX) which was supplemented in the base medium promoted the translocation of ALP from the cytoplasm to the plasma membrane surrounding bile canaliculus-like intercellular spaces and the appearance of connexin-32 at cell borders between adjacent fetal hepatocytes. E-cadherin, occludin and ZO-1 were localized at the cell borders between adjacent fetal hepatocytes irrespective of the presence of DEX. Occludin and ZO-1 were further localized along the plasma membrane surrounding bile canaliculus-like intercellular spaces formed by DEX. The present study indicates that the formation of adherens and tight junctions between adjacent rat hepatocytes does not become the trigger of ALP translocation from the cytoplasm to the plasma membrane, although we cannot be certain of whether the formation of gap junctions between adjacent rat hepatocytes triggers ALP translocation.