Characterization of subcellular components in synchronized hepatoma cells as a function of the cell cycle

CE analysis of the acidic organelles of a single cell

The properties of organelles within a cell have been shown to be highly heterogeneous. Until now, it has been unclear just how much of this heterogeneity is endemic to the organelle subpopulations themselves and how much is actually due to stochastic cellular noise. An attractive approach for investigating the origins of heterogeneity among the organelles of a single cell is CE with LIF detection (CE-LIF). As a proof of principle, in this report we optimize and use a single cell CE-LIF method to investigate the properties of endocytic (acidic) organelles. Our results show that the properties of individual acidic organelles containing Alexa Fluor 488 Dextran suggest that there are two groups of CCRF-CEM cells: a group with a high dextran content per cell, and a group with a low dextran content per cell. Furthermore, the individual organelle measurements of the single cells allow us to compare in each group the distributions of doxorubicin content per acidic organelle and electrophoretic mobilities of these organelles.

Cell cycle analysis and synchronization of the Xenopus laevis XL2 cell line: study of the kinesin related protein XlEg5

Cell free extracts prepared from Xenopus eggs are one of the most powerful in vitro systems to analyze cell cycle-regulated mechanisms such as DNA replication, nuclear assembly, chromosome condensation, or spindle formation. Xenopus embryos can complete several synchronous cell cycles in the absence of transcription, consequently Xenopus extracts are very helpful to study the molecular level of cellular mechanisms. Many key cell cycle regulators like p34cdc2 and cdk2 have been discovered and characterized using those extracts, but their regulation during somatic cell cycles have only been studied in mammalian cultured cells. In this paper, we describe optimized conditions to obtain cell cycle arrested Xenopus XL2 cultured cells. Synchronization of XL2 cells at different stages of the cell cycle was achieved by serum starvation and drug treatments such as aphidicolin, nocodazole, and ALLN. The degree of synchronization was assessed by indirect fluorescence microscopy and FACS analysis. This method was used to study the cell cycle expression of the Xenopus kinesin-related protein, XlEg5, a microtubule-based motor protein involved in movement and cell division in early development. We found that the expression of the protein was maximum in mitosis and minimum in G1, which correlated with the expression of its messenger RNA. XL2 cultured cells were also used to analyze the ultrastructural sub-cellular localization of XlEg5. During mitosis, the protein was found around the centrosome in prophase, on the spindle microtubules in metaphase, and, interestingly, around the minus end of the midbody microtubules in telophase.

Cell cycle analysis and synchronization of the Xenopus cell line XL2

We have determined the length of the cell cycle and its different phases in a permanent Xenopus tadpole cell line, XL2. Following BrdU labeling, the total length of the cell cycle was estimated as 28 h. The different phases of the cell cycle, G1, S, G2, and M were, respectively, 14 h, 10 h 45 min, 2 h 30 min, and 54 min. Knowing these parameters, we were able to develop methods that selectively enrich cells in different phases of the cycle. Treatment with aphidicolin resulted in a S phase block in which more than 85% of the cells showed S phase chromosomes. Almost 60% of the cells were arrested in mitosis after a double block with aphidicolin/nocodazole or aphidicolin/ALLN (acetyl-leucyl-leucyl-norleucinal) treatment. This synchronization protocol will greatly facilitate studies of biochemical events associated with specific gene regulation through the cell cycle. Our synchronization protocol does not disturb cell metabolism as the expression of cyclin B2 during the cell cycle is in agreement with the results obtained with mammalian cells.

Post-translational modifications in mitotic yeast cells

We have recently shown that secretion of invertase is not inhibited in the yeast Saccharomyces cerevisiae during mitosis, but continues, as during interphase. This is in contrast with the mammalian cell, where membrane traffic stops at the onset of prometaphase. Here we extend our findings by showing that the bulk of the cell surface glycoproteins and mannans, as well as the yeast pheromone alpha-factor, traverse the secretory pathway during mitosis. We show that the mitotic cells are able to carry out several types of post-translational modification of secretory proteins. (a) The secretory protein invertase was oligomerized and extensively glycosylated, (b) the N-glycan cores of bulk-cell surface mannans were extended with outer chains, (c) some N-glycans were phosphorylated, (d) the protein-bound O-glycans were extended up to tetramannosides, (e) prepro-ka-factor was proteolytically processed to alpha-factor molecules. We conclude that the secretory pathway in yeast remains fully functional throughout the cell cycle.

Lack of protein-mediated alpha-tocopherol transfer between membranes in the cytoplasm of ascites hepatomas

Transfer-stimulating activity for alpha-tocopherol and the concentration of alpha-tocopherol and peroxidized lipids in rat ascites hepatoma cells were compared with those from normal and regenerating liver. The ability of supernatants from ascites hepatomas (AH-13, AH-60C, AH-109A) to enhance the transfer of alpha-tocopherol was much lower than that from normal livers. The alpha-tocopherol per mg protein of supernatant from ascites hepatomas was lower than that from normal liver. Regenerating liver showed almost the same values as normal liver in activity to stimulate the transfer of alpha-tocopherol and alpha-tocopherol content of the supernatant. By gel filtration, about 60% of alpha-tocopherol in the supernatant of normal liver was detected in the fractions containing the 30 K protein, which stimulates transfer of alpha-tocopherol between membranes, whereas no significant amount of alpha-tocopherol was detected in 30 K protein fractions of AH-60C supernatant. Little stimulating activity for alpha-tocopherol transfer was detected in AH-60C, AH-109A and AH-13. All ascites hepatomas tested contained less arachidonic acid and docosahexaenoic acid than normal and regenerating liver. An absorption peak with maximum intensity at 233 nm, which is due to conjugated dienes, was observed in UV-absorption spectra of ascites hepatoma total lipids, indicating that peroxidized lipids accumulate in these cells. With normal and regenerating liver, no significant peak due to conjugated dienes was detected.

Newly synthesized G protein of vesicular stomatitis virus is not transported to the Golgi complex in mitotic cells

Newly synthesized G protein of vesicular stomatitis virus is not transported to the surface of cultured mammalian cells during mitosis (Warren et al., 1983, J. Cell Biol. 97:1623-1628). To determine where intracellular transport is inhibited, we have examined the post-translational modifications of G protein, which are indicators of specific compartments on the transport pathway. G protein in mitotic cells had only endo H-sensitive oligosaccharides containing seven or eight mannose residues, but no terminal glucose, and was not fatty acylated. These modifications were indicative of processing only by enzymes of the endoplasmic reticulum (ER). Quantitative immunocytochemistry was used as an independent method to confirm that transport of G protein out of the ER was inhibited. The density of G protein in the ER cisternae was 2.5 times greater than in infected G1 cells treated similarly. Incubation of infected mitotic cells with cycloheximide, which inhibits protein synthesis without affecting transport, did not result in a decrease in the density of G protein in the ER cisternae, demonstrating that G protein cannot be chased out of the ER. These results suggest that intracellular transport stops at or before the first vesicle-mediated step on the pathway.

DNA in cytosol fractions obtained by differential centrifugation of homogenates of rapidly growing liver tumors

Preliminary work revealed that the cytosol fractions obtained by differential centrifugation of homogenates of the rapidly growing Morris hepatomas 7777 and 7288CTC contained substantially more DNA than could be detected with the more slowly growing hepatoma 5123C or the liver of tumor-bearing rats. The amount of DNA in the cytosol fractions of the rapidly growing hepatomas was decreased by concentrations of cations which are known to promote precipitation of solubilized chromatin. Data on the incorporation of [3H]thymidine into DNA of nuclear and cytosol fractions and the time-dependent effects of hydroxyurea were compatible with initial nuclear synthesis and later release of the DNA seen in the cytosol fractions. The amount of this DNA was increased in larger tumors and was less in isolated cells than in solid tumors. Postmortem changes in the liver and hepatoma 5123C, but not in hepatomas 7777 and 7288CTC, resulted in a large increase in DNA appearing in the cytosol fraction. The results indicated that the measurement of DNA in cytosol fractions may serve as a guide to the existence of necrosis in tumors, but is not always proportional to the degree of necrosis.

Laser flow cytofluorometric analysis of HTC cells synchronized with hydroxyurea, nocodazole and aphidicolin

The technique of laser flow cytofluorometry has been used to monitor the arrival in G1 and the subsequent progression through the cell cycle of HTC cells accumulated in metaphase with colcemid alone or after treatment with hydroxyurea and Nocodazole. Under the experimental conditions used in this study, the latter procedure gives much better results, avoiding in particular the extensive formation of micronucleated cells. Aphidicolin, an inhibitor of DNA polymerase, in combination with Nocodazole, provides a useful method to tightly synchronize these cells at the G1/S border.

Analytical characterization and purification of plasma membrane from cultured hepatoma cells (HTC cells)

The plasma membrane of the hepatoma cell line, HTC cells, has been characterized and purified by cell fractionation techniques. In the absence of true 5'-nucleotidase in HTC cells, alkaline phosphodiesterase I has been used as a marker enzyme, following conclusions gained from differential and isopycnic centrifugation studies (Lopez-Saura, P., Trouet, A. and Tulkens, P. (1978) Biochim. Biophys. Acta 543, 430-449). To confirm this localization, HTC cells were exposed to anti-plasma membrane IgG at 4 degrees C and fractionated. Alkaline phosphodiesterase I and IgG showed superimposable distribution patterns in linear sucrose gradients. Alkaline phosphodiesterase I is, however, only poorly resolved from enzyme markers of other organelles, especially NADPH-cytochrome c reductase (endoplasmic reticulum) and galactosyltransferase (Golgi complex). Maximal purification from the homogenate is only 13-fold, on a protein basis, even when using a microsomal fraction (67 and 13% of alkaline phosphodiesterase I and protein, respectively) as the starting material. Improved resolution can be obtained after the addition of small quantities of digitonin (equimolar with respect to the cholesterol content). Digitonin increases the buoyant density of alkaline phosphodiesterase I by approx. 0.05 g/cm3, whereas the buoyant densities of galactosyltransferase and NADPH-cytochrome c reductase are increased only by 0.03 and 0.015 g/cm3, respectively. Accordingly, a procedure has been designed which yields a fraction containing 22.8% of alkaline phosphodiesterase I with a purification of 21-fold on a protein basis. The content of NADPH-cytochrome c reductase and galactosyltransferase is 1.2 and 2.1%, respectively. Electron microscopy shows smooth surface membrane elements and vesicles, with only occasional other recognizable elements.