Sedimentation velocity distributions of cells from adult and fetal rat liver and ascites hepatoma

Keratin 8 modulation of desmoplakin deposition at desmosomes in hepatocytes

Keratins, the intermediate filament proteins of epithelial cells, connect to desmosomes, the cell-cell adhesion structures at the surface membrane. The building elements of desmosomes include desmoglein and desmocollin, which provide the actual cell adhesive properties, and desmoplakins, which anchor the keratin intermediate filaments to desmosomes. In the work reported here, we address the role of keratin 8 in modulating desmoplakin deposition at surface membrane in mouse hepatocytes. The experimental approach is based on the use of keratin 8- and keratin 18-null mouse hepatocytes as cell models. In wild-type mouse hepatocytes, desmoplakin is aligned with desmoglein and keratin 8 at the surface membrane. In keratin 8-null hepatocytes, the intermediate filament loss leads to alterations in desmoplakin distribution at the surface membrane, but not of desmoglein. Intriguingly, a significant proportion of keratin 18-null hepatocytes express keratin 8 at the surface membrane, associated with a proper desmoplakin alignment with desmoglein at desmosomes. A Triton treatment of the monolayer reveals that most of the desmoplakin present in either wild-type, keratin 8- or keratin 18-null hepatocytes is insoluble. Deletion analysis of keratin 8 further suggests that the recovery of desmoplakin alignment requires the keratin 8 rod domain. In addition, similarly to other works revealing a key role of desmoplakin phosphorylation on its interaction with intermediate filaments, we find that the phosphorylation status of the keratin 8 head domain affects desmoplakin distribution at desmosomes. Together, the data indicate that a proper alignment/deposition of desmoplakin with keratins and desmoglein in hepatocytes requires keratin 8, through a reciprocal phosphoserine-dependent process.

Dexamethasone and dimethylsulfoxide as distinct regulators of growth and differentiation of cultured suckling rat hepatocytes

Dexamethasone can promote the differentiation of different tissues in vivo while dimethylsulfoxide is a commonly used inducer of differentiation in various tumor cell types in culture. In the present study, the effects of dexamethasone and dimethylsulfoxide on growth and functional activities of cultured differentiating suckling rat hepatocytes stimulated with various combinations of EGF, insulin, and glucagon were evaluated. Hepatocytes stimulated with EGF and either insulin or glucagon entered S phase and mitosis after a lag period of 24 h. These hormonal factors thus provide simple combinations of hepatocyte-growth regulators. Dexamethasone in the presence of EGF and glucagon inhibited the initiation of DNA synthesis and mitosis, but it had no effect on EGF-insulin stimulated cultures. Such a differential effect of dexamethasone was observed at concentrations ranging from 4 nM to 200 microM. alpha-Fetoprotein, albumin, and tyrosine aminotransferase were used as typical markers of hepatocyte differentiation status. Irrespective of the combinations of growth-promoting factors used, dexamethasone inhibited alpha 1-fetoprotein production and maintained albumin production and tyrosine aminotransferase inducibility. In contrast, dimethylsulfoxide at 2% inhibited hepatocyte growth and supported the maintenance of the production of both alpha 1-fetoprotein and albumin, independent of the hormonal growth regulators used. On this basis, dexamethasone and dimethylsulfoxide act as distinct modulators of growth and maturation of cultured differentiating suckling rat hepatocytes.

Differential responsiveness of cultured suckling and adult rat hepatocytes to growth-promoting factors: entry into S phase and mitosis

The capacity of suckling and adult rat hepatocytes in culture to enter into S phase and mitosis in response to EGF, insulin, and glucagon was measured. Both cell types were isolated in high yield and purity and cultured in the absence of serum under identical conditions. At the time of isolation, suckling rat hepatocytes were all diploid and in the G1 phase of the cell cycle. Adult rat hepatocytes constituted a population of mixed ploidy level, as shown by flow cytometry. Upon stimulation, both suckling and adult rate hepatocytes entered S phase after a minimum lag period of 24 h. For suckling rat hepatocytes EGF was required, but its stimulating action was dependent on insulin and/or glucagon. In contrast, adult rat hepatocytes entered into S phase in response to EGF alone; insulin and glucagon did not significantly potentiate its effect. Under optimal hormonal stimulation for entry into S phase a large proportion of suckling rat hepatocytes underwent mitosis, whereas only a few mitoses were observed in the case of adult rat hepatocytes. Therefore, there is a differential response of suckling and adult rat hepatocytes to growth factors which correlates with ploidy level, and this difference may be associated with the degree of maturation.

Heterogeneity of endothelial cells of adult rat liver as resolved by sedimentation velocity and flow cytometry

Sinusoidal cells isolated from adult rat liver were fractionated by velocity sedimentation at 1 X g ( primarily on the basis of size) and the various cell fractions were further analysed by flow cytometry on the basis of forward and perpendicular light scattering and autofluorescence. Cell volume was also measured electronically using a Coulter counter. At least four enriched cell populations were resolved after velocity sedimentation. They corresponded to cells having a modal diameter of 6.5, 7.5, 9, and 11 microns, respectively. Transmission electron microscopy (TEM) analysis of the various cell populations revealed that the 7.5- and 9-microns cell fractions represented two distinct classes of endothelial cells while the 11-microns cells corresponded to Kupffer cells. The 6.5-microns cells were identified as lymphocytes. Fat-storing cells, identified by their autofluorescence and lipid content, were included in the Kupffer population. Further information about the nature of the two physically distinct endothelial cell populations was obtained by TEM. It demonstrated that the smaller endothelial cells possessed quantitatively and relatively less retracted sieve plates than the larger ones. This ultrastructural feature can be possibly correlated to a differential localization of the two classes of endothelial cells within the liver acinus.

Cell types in rat liver cultures: their identification and isolation

This paper reviews the various types of cells in the liver in vivo and in hepatic cellular suspensions produced by perfusion of the liver with collagenase solutions. Methods to identify and isolate different types of hepatic cells are discussed. In vitro culture of various types of liver cells is reviewed and the identification of cultured cells is considered.

Growth and functional activities of neonatal and adult rat hepatocytes cultured on fibronectin coated substratum in serum-free medium

Hepatocytes isolated from neonatal (NN) and adult (AD) rats were seeded on fibronectin coated substratum and cultured in arginine-free medium supplemented with various combinations of insulin, dexamethasone, triiodothyronine (T3), albumin, and transferrin, in presence or absence of fibronectin depleted serum (FDS). The main finding is that in response to certain hormone mixtures, both NN and AD hepatocytes can be stimulated to proliferate, as revealed by an increase in cell number, a [3H]thymidine incorporation into nuclei, and extractable DNA as well as the appearance of mitotic figures. Moreover, this proliferative activity is associated with changes in hepatocyte ploidy. However, the proliferative response of NN hepatocytes to hormone action is much different from that of AD hepatocytes, and the addition of FDS amplifies this activity in NN but inhibits it in AD hepatocyte cultures. Measurements of tyrosine aminotransferase and lactate dehydrogenase activities indicate a good preservation of NN and AD hepatocyte functional integrity under certain culture conditions. A good maintenance of albumin production in NN and AD hepatocyte cultures requires the presence of dexamethasone, whereas the alpha-fetoprotein production in NN hepatocyte cultures is reduced quite rapidly under most conditions. No alpha-fetoprotein is detectable in AD hepatocyte cultures.

The relationship between cell volume, ploidy. and functional activity in differentiating hepatocytes

Liver cells were isolated by collagenase perfusion from rats of 1 day, and 1, 3, 5, and 12 weeks of age, fractionated by velocity sedimentation at 1 g (STAPUT), and the major cell types were identified in terms of specific functions. Alphafetoprotein and albumin were used as markers of differentiating hepatocytes and these functional activities were evaluated in a quantitative manner using a radioimmunoassay. The capacity of this cell type to store 35S-BSP, an indicator of bile formation, was also evaluated. Sinusoidal cells and hematopoietic cells were identified on the basis of their ability to take up 99mTC-colloid sulfur and to incorporate 59Fe, respectively. The fractionation procedure allowed a good separation of sinusoidal cells from hepatocytes at all postnatal ages and also of erythroid cells still present during the first week after birth. With increasing age, alphafetoprotein-producing hepatocytes exhibited changes in sedimentation velocities that parallelled those of albumin-producers. In turn, the latter hepatocyte subpopulation underwent gradual shifts in modal peak velocities similar to those of bile-forming hepatocytes. The fractionated hepatocytes obtained at different ages were further analyzed in terms of cell volume and nuclear ploidy using a Coulter counter system. This quantitative analysis obtained at the cellular level demonstrated that during the age-related differentiation of hepatocytes, which occurs during the postnatal period and results in the gradual appearance of cells of higher ploidy levels, the extent of albumin production and bile formation can be correlated with the hepatocyte volume.

Surface membrane differentiation of hemopoietic cells as observed by radioactive labeling

Bone marrow cells from normal rats were separated by velocity sedimentation into three distinct populations corresponding to granulocytes, lymphocytes and reticulocytes. Cells from each population were surface labelled via the lactoperoxidase radioiodination or the tritiated borohydride reduction. Distinct labeling patterns were observed on SDS-PAGE for each cell populations. Separation of bone marrow cells from anemic rats injected with TAB vaccine led to four populations corresponding to successive stages of erythroid cell maturation. Labelled protein patterns were not in this case as different from one population to the other, except for one species which increased in intensity with the degree of maturation. The tritiated glycoprotein profiles show a shift from high to low molecular weight species during the process of maturation.