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

- An RGC-32 knockout mouse was generated.
- After stimulation, RGC-32−/− T cells exhibited a significant increase in cell cycle.
- Akt and FOXO1 phosphorylation were significantly higher in RGC-32−/− CD4+ T cells.
- IL-2 expression was significantly increased in RGC-32−/− CD4+ T cell mice.
- The effect of RGC-32 on the cell cycle is PI3K dependent.

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.