Expression of the major red cell sialoglycoprotein, glycophorin A, in the human leukemic cell line K562

We have found that the human leukemic cell line K562 (Lozzio, C.B., and Lozzio, B.B. (1975) Blood 45, 321-334) synthesizes a surface membrane glycoprotein which is identical or closely similar to the major red cell sialoglycoprotein, glycophorin A. The protein can be precipitated by specific anti-glycophorin A antiserum both from surface-labeled and metabolically labeled K562 cells. Cyanogen bromide cleavage of glycophorin A from red cells and the K562 cell protein gives apparently identical fragments, and the glycopeptides and oligosaccharides obtained after Pronase and mild alkaline treatment are closely similar. An antiserum made against intact K562 cells and absorbed with normal human white blood cells precipitated surface-labeled glycophorin A from erythrocytes. The amount of glycophorin A per cell in erythrocytes and K562 cells was very similar when determined by radioimmunoassay. The K562 cells contained blood group MN activity when tested with rabbit anti-M and anti-N sera. When incubated at 37 degrees C with rabbit anti-glycophorin A F(AB)2 fragments and fluorescent sheep anti-rabbit IgG, partial redistribution of glycophorin A (patching and capping) was seen in K562 cells but not in erythrocytes.

Biosynthesis of the major human red cell sialoglycoprotein, glycophorin A, in a continuous cell line

During biosynthesis of glycophorin A in K562 cells a precursor is rapidly transferred through the endoplasmic reticulum membrane with the COOH-terminal remaining in the cytoplasm. This is glycosylated within the cell and appears at the cell surface after about 30 min. The biosynthetic pathway resembles that described for viral membrane glycoproteins.

Expression of the major sialoglycoprotein (glycophorin) on erythroid cells in human bone marrow

The major sialoglycoprotein of human erythrocyte membranes (glycophorin) is one of the most-studied membrane proteins. Although the structure is relatively well known, almost nothing is known about its expression in erythroid cells. To study this we raised an antiserum that reacted specifically with this protein. This was accomplished by immunization of rabbits with a preparation of glycophorin followed by absorption with En(a-) erythrocyte membranes, which lack glycophorin. By use of this antiserum and a staphylococcus protein A technique we could establish that only bone marrow cells of erythrocyte lineage express glycophorin at the cell surface. This occurs in basophilic normoblasts and later stages of erythrocyte differentiation, whereas pronormoblasts do not seem to contain glycophorin.

In vivo and in vitro studies on nonspecific blocking factors of host origin in cancer patients. Role of plasma exchange as an immunotherapeutic modality

Experimental and clinical data are reviewed on elevated glycoprotein levels in tumor-bearing animals and patients at various stages of disease advancement. The authors report their findings in 232 patients with various solid tumors; these confirm and extend the reports in the literature. It is shown that some of these glycoproteins, rich in sialic acid, exhibit immunosuppressive properties in vitro, and it is suggested that tumors may protect themselves by triggering hepatic synthesis of sialoglycoproteins which "coat" the binding sites of both immunocompetent cells and tumor cells and thereby abrogate recognition and killing of the latter by the immune system. This concept of nonspecific blocking factors of host origin has already been substantiated to some extent by observations on the consequences of plasma exchange in 24 patients with metastatic tumors; eight of these patients exhibited an objective tumor regression. It is suggested that such studies should be extended to postoperative patients and that circulating sialoglycoprotein assays could be one of the ways of monitoring tumor growth, including growth during the nonvisible phase.

Chemical and biological properties of B16 murine melanoma cells grown in defined medium containing bovine serum albumin

The addition of 1 percent (w/v) bovine serum albumin (BSA) to the F12 medium utilized for the growth of the B16 melanoma cells significantly stimulated the growth of this cell line. The synthesis of mucopolysaccharides and sialoglycopeptides in this medium is identical with that in Eagle's minimal essential medium with Earle's balanced salt solution supplemented with 2 mM L-glutamine, twice the recommended concentration of vitamins, nonessential amino acids, sodium pyruvate, and 10 percent (v/v) fetal calf serum. Cell volume and morphology did not change significantly, under the different growth conditions and tumorigenicity, as assayed by injection of cultured cells into syngeneic animals, was not decreased. Analysis of the BSA used indicated the presence of a sialoglycoprotein contaminant. This sialoglycoprotein contaminant was present in all lots examined and contains N-acetyl-and N-glycolylneuraminic acid, mannose, galactose, and glucosamine. The sialoglycoprotein can be removed by chromatography on acetate form anion-exchange resin at pH 4.3. F12 media containing the purified BSA plus selenite and the sodium salts of palmitic, oleic, and linoleic acids supported growth of the melanoma cells to the same extent as did the media containing unpurified BSA, indicating that the sialoglycoprotein has no role in sustaining the growth of the cells.

Nucleotide-induced inhibition of surface sialyl transferase activity on cultured Burkitt's lymphoma cells

Sialyl transferase activity was demonstrated on the surfaces of intact, cultured lymphoblastoid cells (RAJI) derived from a Burkitt's lymphoma. Pretreatment of the cells with neuraminidase increased the labeled sialoprotein by severalfold. A nunber of nucleotides were effective in decreasing the amount of sialoprotein assembly. CMP was the most effective inhibitor. UMP, AMP, and GMP were also inhibitory, but to a lesser degree. The diphosphate derivatives were similarly inhibitory, but generally less active than their monophosphate counterparts. The cyclic nucleotides were the least effective of all nucleotides tested; cCMP and cAMP showed a small degree of activity, whereas cUMP and cGMP were without effect. These studies indicated that a number of noncyclic and cyclic nucleotides can influence the activity of the sialyl transferase system.

Prolonged inhibition of protein and glycoprotein synthesis in tumor cells treated with muconomycin A

Several agents were compared for their ability to inhibit protein synthesis for long periods in tumor cells growing in culture. Mouse B16 melanoma cells, treated with high concentrations of cycloheximide or pactamycin for 1 hour and then washed repeatedly, recovered their ability to incorporate [3H]leucine into protein in about 4 hours, while cells treated with emetine recovered in 12 hours. After similar treatment with muconomycin A, however, incorporation of [3H]leucine remained inhibited for at least 30 hours. During this time the cells remained attached to the culture dishes, were able to exclude trypan blue dye, and retained nearly normal levels of rubidium-86 content. When another, untreated, population of cells was added to the muconomycin-treated cells, protein synthesis was not inhibited in the untreated population; action of the drug was thus shown to be confined to the treated cells. In melanoma cells treated with neuraminidase and muconomycin, measurement of glycoprotein synthesis (as determined by sialic acid analysis) showed that muconomycin also inhibited restoration of sialic acid content. Brief treatment with muconomycin, therefore, appeared to be sufficient for prolonged inhibition of protein and glycoprotein synthesis.

Sialic acid uptake by BHK cells and subsequent incorporation into glycoproteins and glycolipids

BHK cells can be grown in the presence of growth medium to which radiolabeled sialic acid has been added. After 24 h, 85 percent of the radioactivity in the cells is covalently bound to glycoproteins and glycolipids. No metabolism of the radiolabeled sialic acid could be detected.