Differential Control of Cell Cycle, Proliferation, and Survival of Primary T Lymphocytes by Purine and Pyrimidine Nucleotides

Abstract

Purine and pyrimidine nucleotides play critical roles in DNA and RNA synthesis as well as in membrane lipid biosynthesis and protein glycosylation. They are necessary for the development and survival of mature T lymphocytes. Activation of T lymphocytes is associated with an increase of purine and pyrimidine pools. However, the question of how purine vs pyrimidine nucleotides regulate proliferation, cell cycle, and survival of primary T lymphocytes following activation has not yet been specifically addressed. This was investigated in the present study by using well-known purine (mycophenolic acid, 6-mercaptopurine) and pyrimidine (methotrexate, 5-fluorouracil) inhibitors, which are used in neoplastic diseases or as immunosuppressive agents. The effect of these inhibitors was analyzed according to their time of addition with respect to the initiation of mitogenic activation. We showed that synthesis of both purine and pyrimidine nucleotides is required for T cell proliferation. However, purine and pyrimidine nucleotides differentially regulate the cell cycle since purines control both G1 to S phase transition and progression through the S phase, whereas pyrimidines only control progression from early to intermediate S phase. Furthermore, inhibition of pyrimidine synthesis induces apoptosis whatever the time of inhibitor addition whereas inhibition of purine nucleotides induces apoptosis only when applied to already cycling T cells, suggesting that both purine and pyrimidine nucleotides are required for survival of cells committed into S phase. These findings reveal a hitherto unknown role of purine and pyrimidine de novo synthesis in regulating cell cycle progression and maintaining survival of activated T lymphocytes.

Restriction of De Novo Nucleotide Biosynthesis Interferes with Clonal Expansion and Differentiation into Effector and Memory CD8 T Cells

“…5 and 6). Recently, we have demonstrated that purine and pyrimidine nucleotides play a critical role in the cell cycle progression, proliferation, and survival of human primary T lymphocytes in vitro (7,8). As T cell differentiation is closely linked to proliferation, we investigated to what extent inhibition of nucleotide synthesis could interfere with the acquisition of effector functions.…”

“…This enzyme is strongly expressed in activated T and B cells (11). The antiproliferative effect of mycophenolate acid on T cells was clearly demonstrated in vitro (7,8,12,13). Methotrexate (MTX) is a folate antagonist that blocks dihydrofolate reductase and TS (14), but it can also block the first step of purine biosynthesis (15).…”

“…Activation of T lymphocytes is associated with an increase of purine and pyrimidine pools (4). This expansion of the nucleotide pools is a consequence of increased expression, or activity of key enzymes involved in de novo synthesis, such as inosine monophosphate dehydrogenase and thymidylate synthase (TS 8 ; Refs. 5 and 6).…”

Inhibition of GTPase Rac1 in Endothelium by 6-Mercaptopurine Results in Immunosuppression in Nonimmune Cells: New Target for an Old Drug

“…This effect is well described by the ability of 6-MP to prevent T cell proliferation, relevant in a severe inflammatory condition with rapid T cell expansion (8). This effect is observed at a relatively high dose of 6-MP (8), beyond clinically relevant dosages used chronically for IBD patients. In ∼50% of all patients with IBD, a low dose of azathioprine or 6-MP is given and well tolerated without major complications (9).…”

“…Thereby, normal purine synthesis is hampered and incorporation of these alternative purines into newly synthesized DNA has long been the proposed therapeutic mechanism (7). This effect is well described by the ability of 6-MP to prevent T cell proliferation, relevant in a severe inflammatory condition with rapid T cell expansion (8). This effect is observed at a relatively high dose of 6-MP (8), beyond clinically relevant dosages used chronically for IBD patients.…”