RGC-32 is a novel regulator of the T-lymphocyte cell cycle

We have previously shown that RGC-32 is involved in cell cycle regulation in vitro. To define the in vivo role of RGC-32, we generated RGC-32 knockout mice. These mice developed normally and did not spontaneously develop overt tumors. To assess the effect of RGC-32 deficiency on cell cycle activation in T cells, we determined the proliferative rates of CD4⁺ and CD8⁺ T cells from the spleens of RGC-32^-/- mice, as compared to wild-type (WT, RGC-32^+/+) control mice. After stimulation with anti-CD3/anti-CD28, CD4⁺ T cells from RGC-32^-/- mice displayed a significant increase in [³H]-thymidine incorporation when compared to WT mice. In addition, both CD4⁺ and CD8⁺ T cells from RGC-32^-/- mice displayed a significant increase in the proportion of proliferating Ki67⁺ cells, indicating that in T cells, RGC-32 has an inhibitory effect on cell cycle activation induced by T-cell receptor/CD28 engagement. Furthermore, Akt and FOXO1 phosphorylation induced in stimulated CD4⁺ T-cells from RGC-32^-/- mice were significantly higher, indicating that RGC-32 inhibits cell cycle activation by suppressing FOXO1 activation. We also found that IL-2 mRNA and protein expression were significantly increased in RGC-32^-/- CD4⁺ T cells when compared to RGC-32^+/+ CD4⁺ T cells. In addition, the effect of RGC-32 on the cell cycle and IL-2 expression was inhibited by pretreatment of the samples with LY294002, indicating a role for phosphatidylinositol 3-kinase (PI3K). Thus, RGC-32 is involved in controlling the cell cycle of T cells in vivo, and this effect is mediated by IL-2 in a PI3K-dependent fashion.