Induction of uridine kinase in rat peripheral blood lymphocytes: dependence on mitogen specificity and effect of pyrimidine analogs

The role of de novo purine synthesis in lymphocyte transformation

Genetic defects in purine metabolism are associated with severe immunodeficiency. Adenosine deaminase deficiency impairs the function of both B- and T-lymphocytes whereas in purine nucleoside (inosine) phosphorylase deficiency there is more severe impairment of T-lymphocyte functions than of B-lymphocyte functions. The relative unimportance of the salvage pathway catalysed by hypoxanthine-guanine phosphoribosyltransferase is shown by the normal responses of T-lymphocytes from patients with the Lesch-Nyhan syndrome to antigenic and mitogenic stimulation. A mild deficiency of B-lymphocyte function is found in these patients. Agents inhibiting the de novo pathway of purine synthesis, including azaserine, 6-mercaptopurine and azathioprine in low doses, block the responses of normal human lymphocytes to mitogenic stimulation. These observations emphasize the importance of the de novo pathway of purine synthesis in lymphocyte responses to antigenic and mitogenic stimulation. There is considerable heterogeneity in the amount of labelled uridine incorporated into human and rat lymphocytes. This does not appear to reflect only a difference between T- and B-lymphocytes

Uridine kinase: altered subunit size or enzyme expression as a function of cell type, growth stimulation, or mutagenesis

Using antibody prepared against pure uridine kinase from Ehrlich ascites cells, we have measured the expression of enzyme protein by the Western blot technique. Variations were observed in the Mr of the enzyme subunit for uridine kinase from different species: 32,000 (mouse Ehrlich ascites cells), 30,000 (normal human lymphocytes), 28,000 (mouse tissues), 27,500 (rat tissues). For different normal tissues from the same species, there was no significant variation in the subunit size. Transformed human and mouse cell lines, selected for a deficiency of uridine kinase activity in the presence of inhibitors activated by this enzyme, expressed two cross-reacting proteins, one with a normal (30,000) and one with a smaller (21,000) subunit molecular weight than was found in the parental cell line (human lymphoma), or only a smaller protein of Mr 25,000 (mouse lymphoma). Our results show that selection protocols using metabolite inhibitors do not always repress the expression of the enzyme but instead may lead to selection of those cells that have a mutation in the uridine kinase gene, resulting in the expression of an inactive enzyme. The expression of uridine kinase protein changes when cells are stimulated to divide. For both mouse fibroblasts and human lymphocytes, expression of uridine kinase protein as well as activity clearly increased after cells were stimulated to grow. In fibroblasts, increases are seen by 3 hr after stimulation, and plateau after 9 hr at a sevenfold increase. In lymphocytes, no change is seen until 12 hr after stimulation, and a plateau is not reached until 72 hr, with a total increase of approximately 50-fold. There has been considerable interest in the possibility of uridine kinase isozymes. Except for cells that have been mutagenized, the present results show that, as judged by subunit molecular weight, there appears to be only one enzyme form in normal and neoplastic cells or in cells in which uridine kinase activity is induced.

Uridine kinase activity in human tumors

The activity of uridine kinase, a key enzyme in the salvage pathway for pyrimidine bases and nucleosides and for the activation of the corresponding chemotherapeutic analogs, varied 10-fold in a series of human colon adenosarcomas; similar variations were observed with other tumor types. In contrast to the leukemias where only the adult isozyme appears to be present, both the adult and embryonic forms were present in the solid tumors examined. The qualitative and quantitative differences may account, in part, for differences in the innate (initial) sensitivity of the tumors to pyrimidine base and nucleoside analogs.

Immunological observations on patients with Lesch-Nyhan syndrome, and on the role of de-novo purine synthesis in lymphocyte transformation

Three patients with the Lesch-Nyhan syndrome were found to have normal delayed hypersensitivity, peripheral-blood T-lymphocyte counts, lymphocyte responses to P.H.A., and serum IgM, IgA, and IgE levels. However, the percentages of B-lymphocytes, IgG levels, serum-isohaemagglutinin titres, and lymphocyte responses to pokeweed mitogen (P.W.M.) were subnormal. These observations suggest that activity of the salvage pathway of purine synthesis catalysed by hypoxanthine-guanine phosphoribosyl transferase (H.G.P.R.T.) is not required for the responses of T-lymphocytes to mitogenic or antigenic stimulation, but may contribute to the proliferation and function of B lymphocytes. The major role of the de-novo pathway of purine synthesis in human lymphocyte responses to mitogenic or antigenic stimulation is shown by the effects of inhibitors of this pathway, including immunosuppressive agents, and by the effects of congenital deficiency or inhibition of adenosine deaminase.