Internalization of ricin in Chinese hamster ovary cells

Chinese hamster cell variants resistant to the A chain of ricin carry altered ribosome function

Ricin, a toxic lectin from Ricinus communis, is composed of two different polypeptide chains, A and B, and the ricin A chain (RA) blocks protein synthesis. We studied cell lines resistant to cytotoxic action of RA. One low-RA-resistant cell line, AR10, isolated from Chinese hamster ovary (CHO) cells, was resistant to a low dose of RA (1 microgram/ml) and showed a 10-fold-higher resistance to RA and ricin than that of CHO. We further mutagenized AR10 to isolate high-RA-resistant cell lines AR100-6, AR100-9, and AR100-13, which were resistant to higher doses of RA and ricin (100- to 1,000-fold) than CHO was. The binding of [125I]ricin to AR10, AR100-6, AR100-9, and AR100-13 cells was decreased to about 30% of that of CHO. The internalization of [125I]ricin in AR10 cells and in the high-RA-resistant clones was the same. Polyuridylate-dependent polyphenylalanine synthesis, using S-30 extracts from either AR100-9 or AR100-13, was about 100-fold more resistant to the inhibitory action of RA than when CHO, AR10, and AR100-6 cells extracts were used. The protein synthesis with ribosomes (80S) from AR100-9 or AR100-13 was 10- to 100-fold more resistant to RA than it was with parental ribosomes when combined with the S-100 fraction of CHO cells. The polyphenylalanine synthesis assay using the ribosomes constituted from the 60S subunit of AR100-9 and the 40S subunit of CHO indicated that the resistant phenotype of AR100-9 cells is due to an alteration of the 60S ribosomal subunit.

Chinese hamster cell variants resistant to the A chain of ricin carry altered ribosome function

Ricin, a toxic lectin from Ricinus communis, is composed of two different polypeptide chains, A and B, and the ricin A chain (RA) blocks protein synthesis. We studied cell lines resistant to cytotoxic action of RA. One low-RA-resistant cell line, AR10, isolated from Chinese hamster ovary (CHO) cells, was resistant to a low dose of RA (1 microgram/ml) and showed a 10-fold-higher resistance to RA and ricin than that of CHO. We further mutagenized AR10 to isolate high-RA-resistant cell lines AR100-6, AR100-9, and AR100-13, which were resistant to higher doses of RA and ricin (100- to 1,000-fold) than CHO was. The binding of [125I]ricin to AR10, AR100-6, AR100-9, and AR100-13 cells was decreased to about 30% of that of CHO. The internalization of [125I]ricin in AR10 cells and in the high-RA-resistant clones was the same. Polyuridylate-dependent polyphenylalanine synthesis, using S-30 extracts from either AR100-9 or AR100-13, was about 100-fold more resistant to the inhibitory action of RA than when CHO, AR10, and AR100-6 cells extracts were used. The protein synthesis with ribosomes (80S) from AR100-9 or AR100-13 was 10- to 100-fold more resistant to RA than it was with parental ribosomes when combined with the S-100 fraction of CHO cells. The polyphenylalanine synthesis assay using the ribosomes constituted from the 60S subunit of AR100-9 and the 40S subunit of CHO indicated that the resistant phenotype of AR100-9 cells is due to an alteration of the 60S ribosomal subunit.

Enhanced internalization of ricin in nigericin-pretreated Chinese hamster ovary cells

Biochemical and electron microscopic autoradiographic studies with [125I] ricin have revealed that nigericin-pretreated Chinese hamster ovary cells are more efficient than untreated cells in the internalization of the toxin into the cells. These results suggest that the enhanced rate of internalization of ricin in nigericin-pretreated cells may account for the enhancement of cytotoxicity of ricin in Chinese hamster ovary cells by nigericin.

Enhancement of ricin cytotoxicity in Chinese hamster ovary cells by depletion of intracellular K+: evidence for an Na+/H+ exchange system in Chinese hamster ovary cells

Depletion of intracellular K+ has been reported to result in an arrest of the formation of coated pits in human fibroblasts (Larkin, J.M., M.S. Brown, J.L. Goldstein, and R.G.W. Anderson, 1983, Cell, 33:273-285). We have studied the effects of K+ depletion on the cytotoxicities of ricin, Pseudomonas exotoxin A, and diphtheria toxin in Chinese hamster ovary (CHO) cells. The cytotoxicities of ricin and Pseudomonas toxin were enhanced in K+-depleted CHO cells whereas the cytotoxicity of diphtheria toxin was reduced by K+ depletion. The effects of NH4Cl on the cytotoxicities of ricin, Pseudomonas toxin, and diphtheria toxin were found to be similar to those of K+ depletion, and there were no additive or synergistic effects on ricin cytotoxicity by NH4Cl in K+-depleted medium. The enhancement of ricin cytotoxicity by K+ depletion could be completely reversed by the addition of K+, Rb+, and partially by the addition of Cs+, before the ricin treatment, whereas Li+ was ineffective. These protective effects of K+ or Rb+ requires a functional Na+/K+ ATPase. CHO cells grown in K+-depleted media were found to contain 6.3-fold increase in intracellular Na+ level, concomitant with a 10-fold reduction in intracellular K+ level. The enhanced cytotoxicity of ricin in K+-free medium and the increased uptake of Na+ could be abolished by amiloride or amiloride analogues, which are known to be potent inhibitors of the Na+/H+ antiport system. Our results suggest that a depletion of intracellular K+ results in an influx of Na+, which is accompanied by the extrusion of H+. Consequently, there is an alkalinization of the cytosol and the ricin-containing endosomes. As a result, ricin is more efficiently released from the endosomes in-K+-depleted cells. Results from the studies of the binding, internalization, and degradation of 125I-ricin, and the kinetics of inhibition of protein synthesis by ricin in K+-depleted cells are consistent with this working hypothesis.

Role of vesicles during adenovirus 2 internalization into HeLa cells

In this investigation, the early period of adenovirus type 2 (Ad2)-HeLa cell interaction was analyzed by electron microscopy and biochemical techniques. Events observed in this period ranged from the disappearance of virions from the cell surface to their subsequent association with the cell nucleus. Destabilization of the virions attached to the intact cell was necessary for virions to escape from intracellular vesicles. Strong temperature dependence and rapid escape from a vesicular compartment were shown in temporal kinetic experiments. These vesicles appeared to be acidic, since lysosomotropic agents partly inhibited the release of virions from vesicles. Studies of Ad2 binding to cells in buffers of different pH values suggested that adenovirus binds to cells by two different mechanisms. At low pH the binding was most probably mediated by the penton base and at neutral pH by the fiber protein. The number of receptor sites per cell was 25,000 and 6,000 at low and neutral pH, respectively. This study suggests that the low-pH affinity between the penton base and a vesicular membrane is important inside acid vesicles when Ad2 quickly enters the cytoplasm. However, a significant fraction of the virions was possibly internalized by a pathway not requiring a passage through such vesicles.

Chinese hamster ovary cell mutants defective in the internalization of ricin

Chinese hamster ovary mutants simultaneously resistant to ricin and Pseudomonas toxin have been isolated. Two mutant cell lines (4-10 and 11-2) were found to retain normal levels of binding of both ricin and Pseudomonas toxin. They were defective in the internalization of [125I]ricin into the mutant cells, as measured by both a biochemical assay for ricin internalization and electron microscopic autoradiographic studies. Although pretreatment of Chinese hamster ovary cells with a Na+/K+ ionophore, nigericin, resulted in an enhancement of the cytotoxicities of ricin and Pseudomonas toxin in the wild-type Chinese hamster ovary cells, preculture of the mutant cells did not alter the susceptibility of the mutant cells to either toxin. These results provide further evidence that there is a common step in the internalization process for ricin and Pseudomonas toxin.

Chinese hamster ovary cell mutants defective in the internalization of ricin

Chinese hamster ovary mutants simultaneously resistant to ricin and Pseudomonas toxin have been isolated. Two mutant cell lines (4-10 and 11-2) were found to retain normal levels of binding of both ricin and Pseudomonas toxin. They were defective in the internalization of [125I]ricin into the mutant cells, as measured by both a biochemical assay for ricin internalization and electron microscopic autoradiographic studies. Although pretreatment of Chinese hamster ovary cells with a Na+/K+ ionophore, nigericin, resulted in an enhancement of the cytotoxicities of ricin and Pseudomonas toxin in the wild-type Chinese hamster ovary cells, preculture of the mutant cells did not alter the susceptibility of the mutant cells to either toxin. These results provide further evidence that there is a common step in the internalization process for ricin and Pseudomonas toxin.

Enhanced internalization of ricin in nigericin-pretreated Chinese hamster ovary cells

Biochemical and electron microscopic autoradiographic studies with [125I] ricin have revealed that nigericin-pretreated Chinese hamster ovary cells are more efficient than untreated cells in the internalization of the toxin into the cells. These results suggest that the enhanced rate of internalization of ricin in nigericin-pretreated cells may account for the enhancement of cytotoxicity of ricin in Chinese hamster ovary cells by nigericin.