concanavalin-a and pyrimidine

Brequinar sodium (BQR) has been shown recently to be a potent immunosuppressive agent. This property has been attributed to the capacity of BQR to inhibit de novo pyrimidine nucleoside biosynthesis and consequently, to blockade the synthesis both of DNA and RNA. The influence of this new immunosuppressant on lymphocyte function has not been fully characterized. To determine the potential efficacy of BQR for the control of antibody-mediated graft rejection, which is of particular significance in the context of xenotransplantation, we have examined the influence of the drug on interleukin-6-dependent IgM production by the human B-cell line, SKW 6.4. At concentrations up to 10 micrograms/ml, BQR did not affect concanavalin A (Con A)-induced human peripheral blood lymphocyte proliferation or IL-6 production by blood mononuclear leucocytes. In contrast, the drug was very effective in inhibiting IL-6-stimulated IgM production by SKW 6.4 cells, with an optimal inhibitory concentration of 0.3 microgram/ml. As expected, addition of exogenous uridine (0.1 mM), the precursor of uridine triphosphate (UTP), reversed the inhibitory effect of BQR on antibody production, while cytidine (0.1 mM) potentiated the inhibitory activity of the drug. It was further demonstrated that the inhibition of IgM production was unrelated to DNA synthesis, indicating that BQR may affect IL-6 signal transduction and IgM production in SKW 6.4 cells independent of any effect on cell proliferation.

Topics: B-Lymphocytes; Biphenyl Compounds; Cell Differentiation; Cell Line; Concanavalin A; Cytidine; DNA; Humans; Immunoglobulin M; Immunosuppressive Agents; Interleukin-6; Pyrimidines; Uridine

Pyrazofurin, a pyrimidine nucleoside analogue with antineoplastic activity, inhibits cell proliferation and DNA synthesis in cells by inhibiting uridine 5'-phosphate (UMP) synthase. It has been previously shown in concanavalin A (con A)-stimulated guinea pig lymphocytes (23) that pyrazofurin-inhibited DNA synthesis could be selectively reversed by exogenous uridine (Urd). In this report, we have examined possible mechanisms for the Urd reversal with experiments that determine the ability of exogenous Urd to (a) interfere with either the intracellular transport of pyrazofurin, or the conversion of pyrazofurin to its intracellularly active form, pyrazofurin-5'-phosphate; (b) reverse the pyrazofurin block of [14C]orotic acid incorporation into DNA; and (c) alter the pattern of exogenous [3H]Urd incorporation into DNA-thymine (DNA-Thy) and DNA-cytosine (DNA-Cyt) during pyrazofurin inhibition of pyrimidine de novo biosynthesis. The results of these experiments showed that Urd reversal does not occur through altered pyrazofurin transport or intracellular conversion to pyrazofurin-5'-phosphate, nor does it alter the distribution of [3H]Urd in DNA-Thy and DNA-Cyt. Instead, these findings indicate that the primary mechanism for exogenous Urd reversal of pyrazofurin inhibition of DNA synthesis involves the reversal of pyrazofurin inhibition of UMP synthase, thus restoring orotic acid incorporation into lymphocyte DNA through the pyrimidine de novo pathway.

Topics: Amides; Animals; Antibiotics, Antineoplastic; Cells, Cultured; Concanavalin A; DNA; Guinea Pigs; Intracellular Fluid; Lymphocyte Activation; Lymphocytes; Male; Nucleotides; Orotidine-5'-Phosphate Decarboxylase; Pyrazoles; Pyrimidines; Ribonucleosides; Ribose; Uridine