Mechanisms of uptake and the fate of serum proteins and horseradish peroxidase in cultured human glioma cells

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Abstract
Previous studies have shown that serum proteins are taken up from extracellular oedema fluid by reactive astrocytes and by tumour astrocytes. The present investigation was designed to define the mechanism of this protein uptake. Two or 3-week-old explant cultures from 26 astrocytic gliomas, one anaplastic ependymoma, and five non-glial intracranial tumours were treated with either human IgG (12 mg/ml), human serum albumin, (44 mg/ml) or horseradish peroxidase (0.1–4.0 mg/ml) for 4–24 h. Human IgG and albumin were subsequently detected in cultured cells by the indirect peroxidase-antiperoxidase (PAP) method for light microscopy or by direct peroxidase conjugate technique for electron microscopy. Horseradish peroxidase activity was localised by treatment with diaminobenzidine and hydrogen peroxide. Results of the study show that human serum proteins and horseradish peroxidase are taken up by tumour astrocytes and ependymal cells, and by macrophages, but not by non-glial tumour cells nor by mesenchymal elements in the glioma cultures. Electron immunocytochemistry suggests that the serum proteins are taken up by smooth walled micropinocytic vesicles (approximately 80 nm in diameter) which fuse to form larger endocytic vesicles (200–300 nm); these vacuoles in turn fuse with secondary lysosomes to form cytoplasmic bodies 1.2–3 μm in diameter.