Effect of nitrous oxide on haematopoiesis in vitro: biochemical and functional features

Is there a role of cyclosporine A on total homocysteine export from human renal proximal tubular epithelial cells?

BACKGROUND

Immunosuppressive therapy may influence homocysteine metabolism in allograft recipients. We examined whether cyclosporine A influences the in vitro formation of homocysteine as determined by the measurement of total homocysteine (tHcy) concentrations in supernatants of human renal proximal tubule epithelial cells (hRPTEC), an important site of homocysteine metabolism.

METHODS

Cells were incubated with and without vitamins in the presence of low or high methionine concentrations at different cyclosporine A concentrations for 24, 48 and 72 hours (N = 7 for each experiment). The concentration of tHcy in culture supernatants was measured by a fluorescence polarization immunoassay. Data were analyzed by four-way ANOVA, three-way ANOVA and t tests.

RESULTS

The Hcy export from hRPTEC (tHcy in the culture supernatant) was 2.69 micromol/L during standard cell culture conditions at time point 24 hours and increased by 28.3% at 48 hours and by 44.6% at 72 hours. Comparisons of tHcy levels in culture supernatants over time by four way ANOVA showed that cyclosporine A at 200 or 800 ng/mL had no influence on tHcy export from hRPTEC (P = 0.67991). In contrast, the presence of vitamins in the medium (P = 0.000001), in vitro methionine loading (P < 0.000001), and prolonged incubation time (P < 0.000001) were associated with an increase of tHcy export from hRPTEC. Significant interactions in this analysis were "vitamins x methionine" (P = 0.001804), "vitamins x time" (P = 0.001478), "methionine x time" (P < 0.000001), and "vitamins x methionine x time" (P = 0.018128), pointing to a combined effect of vitamins in the presence of high methionine concentrations at the later time points.

CONCLUSION

Our study shows that hRPTEC export Hcy into the cell culture medium, an effect that is enhanced by in vitro methionine loading and modulated by the presence of vitamins. Cyclosporine A had no major influence on Hcy export from tubule cells. Therefore, our findings do not support the assumption that cyclosporine A elevates total homocysteine plasma levels in organ transplant patients.

Transcobalamin II and in vitro proliferation of leukemic cells

We have recently shown that antibodies to transcobalamin II (TCII) inhibit the in vitro growth of human and murine leukemic cells. This antiproliferative strategy targets the uptake of cobalamin (Cbl), an essential cofactor for two biochemical reactions in humans. To date there has been no appropriate cell culture model available to study antagonism of Cbl as a potential antiproliferative strategy. We have established cell culture conditions which allow reproducible measurements of cell proliferation that is dependent on Cbl and its carrier protein, TCII. This bioassay has allowed us to demonstrate that several monoclonal antibodies, raised against TCII, are potent inhibitors of cell proliferation and that excess Cbl abrogates this inhibitory effect. Thus, supporting our hypothesis that interference with Cbl uptake or metabolism will result in inhibition of cell proliferation. Furthermore, Cbl metabolism appears to provide a useful target for antiproliferative strategies which now involve the use of inactive Cbl analogs. In this review, we update our work on the role of targeting TCII and Cbl as an antiproliferative strategy for leukemic cells. We suggest that this strategy may provide a novel direction for anti cancer reagents.