The purification of plasma membranes from WI-38 fibroblasts: effects of ageing on their composition

Transport competence of plasma membrane vesicles from cultured human fibroblasts

We obtained plasma membranes from cultured human skin fibroblasts. The preparation was enriched 10-fold with about 40 percent yield. There was minimal contamination with other cell membranes. Various observations indicated vesicular conformation of a portion of the plasma membranes, notably by electron microscopy and from the effect of osmotic pressure on the distribution of solutes between mass and medium at equilibrium. Other studies indicated that these fibroblast plasma membrane vesicles retained mediated transport processes for a variety of substrates. The evidence included: stereospecific and temperature-dependent uptake of glucose; dependence of L-alanine uptake on sodium ion and an inward-directed transmembrane Na+ gradient; stimulation of L-alanine uptake, with overshoot, by enhancement of the interior-negative transmembrane potential; concentration dependent uptake of methotrexate with apparent competitive inhibition by folinic acid; stimulation of L-lysine uptake by trans-L-arginine. These findings indicate that human fibroblast plasma membrane vesicles could be used to study membrane transport processes and, perhaps, expression of mutant genes that cause inborn errors of transport.

Subcellular localization and modification with ageing of glutathione, glutathione peroxidase and glutathione reductase activities in human fibroblasts

Differential centrifugation and isopycnic equilibration in density gradients were used to localize glutathione (GSH), glutathione peroxidase and glutathione reductase in the subcellular organelles of WI-38 fibroblasts. GSH was present in all the subcellular fractions, whereas the glutathione peroxidase and reductase activities were restrained to the cytoplasm and the mitochondrial fractions. After equilibration in density gradients, the results showed the presence of GSH, glutathione peroxidase and glutathione reductase in both the cytoplasm and mitochondria. GSH was also located in plasma membranes and probably in peroxisomes, endoplasmic reticulum and lysosomal membranes. Evolution of GSH in ageing fibroblasts showed a sudden increase of its concentration just before cell death. The glutathione peroxidase activity already decreases in the early passages, while the decrease of the glutathione reductase activity was constant and reached a drastic low level at the end of the culture. In conclusion, GSH is probably involved in the cell degeneration associated with ageing but because of its multiple functions and its ubiquitous localization, it is difficult to assert to which extent this metabolite is implicated in the ageing process.

Vesicles with transport capability isolated from cultured fibroblasts

Plasmalemmas from cultured human skin fibroblasts, isolated by a simple and reproducible method, can be converted to vesicles which are capable of active transport of aminoacid when glutathione is included within the vesicles. In the isolation, the plasmalemmas are stabilized with a Ricinus lectin, with preservation of the classic plasmalemma enzymes. The procedure has been applied successfully to a number of normal and abnormal human skin fibroblasts including those of myotonia dystrophica and progeria victims, and to the lung fibroblast WI-38. As part of a study of the characteristics of transport enzymes related to aging and to the muscular dystrophies in cultured fibroblasts, it was desirable to simplify the system by the use of vesicles prepared from the fibroblast plasmalemmas. The procedure described below is similar in some respects to that applied to another membranous system in the use of a lectin to stabilize the plasmalemma structure. The use of other stabilizing agents such as heavy metals and surfactant polymers which react with the membranes, but could compromise the reliability of the enzyme assays, was avoided. Since the focus of this study was on the enzymic systems of transport, the examination of facilitated diffusion or exchange was excluded. The well defined glutathione-dependent mechanism of aminoacid transport was examined to verify the competence of the vesicles for active transport and to confirm their sidedness. Other enzymes of transport, the ATPases, and membrane marker enzymes were also determined.

Characterization of plasma membranes and rough endoplasmic reticulum of synovial cells cultured from rheumatoid arthritis patients

[35S]methionine-labelled plasma membrane (PM) and rough endoplasmic reticulum (RER) proteins were analysed from synovial cell cultures derived from rheumatoid arthritis (RA) and reference patients. Pure RER was prepared by a modification of CsCl-containing sucrose-cushion method. The PM proteins were resolved into 30-35 bands using sodium dodecyl sulphate-electrophoresis in polyacrylamide gradient gels. The major polypeptides had apparent molecular masses of 170, 140, 110, 60, 40, 33, 23 and 14 kilodaltons. The major RER polypeptides had the apparent molecular masses of 170, 74, 58 and 35 kD. No reproducible differences were demonstrated in the polypeptide patterns or in the specific activities of the PM and ER (endoplasmic reticulum) marker enzymes between cells from RA and reference patients. The results reflect similarities of membrane structures in RA and reference cells, although differences have been reported in their metabolism.

Distribution of DNA between sister cells during serial subcultivation of human fibroblasts

The segregation of DNA at the time of cell division was analysed by measuring the DNA contents of cells in mitosis and the incorporation of tritiated thymidine (3H-TdR) on each half of anaphases and telophases. Results suggest that the spreads in the 2C and 4C DNA contents are due to quantitative differences which could originate during semi-conservative DNA synthesis, chromosome assembly and chromosome segregation. This continuous rearrangement of the genome could lead either to a degenerative process or to a differentiation program.