Murine leukemic lymphoblasts: homogenization and fractionation procedures for plasma membrane vesicles isolation

Isolation of cell surface proteins for mass spectrometry-based proteomics

Defining the cell surface proteome has profound importance for understanding cell differentiation and cell-cell interactions, as well as numerous pathogenic abnormalities. Owing to their hydrophobic nature, plasma membrane proteins that reside on the cell surface pose analytical challenges and, despite efforts to overcome difficulties, remain under-represented in proteomic studies. Limitations in the classically employed ultracentrifugation-based approaches have led to the invention of more elaborate techniques for the purification of cell surface proteins. Three of these methods--cell surface coating with cationic colloidal silica beads, biotinylation and chemical capture of surface glycoproteins--allow for marked enrichment of this subcellular proteome, with each approach offering unique advantages and characteristics for different experiments. In this article, we introduce the principles of each purification method and discuss applications from the recent literature.

Aqueous polymer two-phase systems: effective tools for plasma membrane proteomics

Plasma membranes (PMs) are of particular importance for all living cells. They form a selectively permeable barrier to the environment. Many essential tasks of PMs are carried out by their proteinaceous components, including molecular transport, cell-cell interactions, and signal transduction. Due to the key role of these proteins for cellular function, they take center-stage in basic and applied research. A major problem towards in-depth identification and characterization of PM proteins by modern proteomic approaches is their low abundance and immense heterogeneity in different cells. Highly selective and efficient purification protocols are hence essential to any PM proteome analysis. An effective tool for preparative isolation of PMs is partitioning in aqueous polymer two-phase systems. In two-phase systems, membranes are separated according to differences in surface properties rather than size and density. Despite their rare application to the fractionation of animal tissues and cells, they represent an attractive alternative to conventional fractionation protocols. Here, we review the principles of partitioning using aqueous polymer two-phase systems and compare aqueous polymer two-phase systems with other methods currently used for the isolation of PMs.

Role of cell cholesterol in modulating vincristine uptake and resistance

The relationship between cell-membrane permeability to vincristine and cholesterol/phospholipid levels was studied in L5178Y murine leukemic lymphoblasts and in 2 multidrug-resistant cell sublines, VCR/P60 and VCR/P200, which expressed increasing levels of vincristine resistance. The uptake of 3H-vincristine was measured in all cell lines and in cholesterol-depleted and -reloaded L5178Y and VCR/P200 cells. The initial rate of drug entry in resistant cells was lower than that measured in the parental cell line and it decreased as the relative resistance increased. An increment of cholesterol content, characterized in resistant cells, was directly proportional to the relative resistance to vincristine. Cholesterol depletion in both sensitive and resistant cells resulted in an increase in the rate of vincristine uptake, which reverted to the respective basal levels when each cell line was cholesterol-reloaded. The rate of drug uptake was inversely correlated with the molar ratio of cholesterol to phospholipids. Although both VCR/P cell sublines, but not the sensitive parental cells, expressed the P-glycoprotein in their plasma membrane, there were no differences in drug efflux and retention between resistant and parental cells. These results indicate that cholesterol modulates the permeation of vincristine through the plasma membrane and strongly suggest that increased levels of cholesterol/phospholipid account for the lower drug accumulation and greater resistance in these multidrug-resistant cells.

Vincristine cellular pharmacokinetic changes associated with multidrug resistance

The correlation between vincristine cellular pharmacokinetics and its biological action was analyzed in L5178Y cells and in a multidrug resistant subline (VCR/G40) at the nM drug concentration range attained in serum during the clinical use of the drug. The reduced rate of drug influx and the higher drug efflux measured in VCR/G40 cells justify the lower drug accumulation characterized in these cells compared to the parental cells. Nevertheless this does not seem to be the sole reason accounting for the resistance expression since similar cytotoxic effects of vincristine on both cell lines were only attained when the intracellular drug concentration was ten times higher in resistant than in parental cells. Bearing in mind that the drug-binding kinetics have not been modified during the resistance development, our results indicate that some vincristine accumulated by resistant cells may be compartmentalized into these cells before its interaction with microtubules.