RASAL3, a novel hematopoietic RasGAP protein, regulates the number and functions of NKT cells.

Ras GTPase-activating proteins negatively regulate the Ras/Erk signaling pathway, thereby playing crucial roles in the proliferation, function, and development of various types of cells. In this study, we identified a novel Ras GTPase-activating proteins protein, RASAL3, which is predominantly expressed in cells of hematopoietic lineages, including NKT, B, and T cells. We established systemic RASAL3-deficient mice, and the mice exhibited a severe decrease in NKT cells in the liver at 8 weeks of age. The treatment of RASAL3-deficient mice with α-GalCer, a specific agonist for NKT cells, induced liver damage, but the level was less severe than that in RASAL3-competent mice, and the attenuated liver damage was accompanied by a reduced production of interleukin-4 and interferon-γ from NKT cells. RASAL3-deficient NKT cells treated with α-GalCer in vitro presented augmented Erk phosphorylation, suggesting that there is dysregulated Ras signaling in the NKT cells of RASAL3-deficient mice. Taken together, these results suggest that RASAL3 plays an important role in the expansion and functions of NKT cells in the liver by negatively regulating Ras/Erk signaling, and might be a therapeutic target for NKT-associated diseases.

Acute type adult T-cell leukemia cells proliferate in the lymph nodes rather than in peripheral blood.

A massive increase in the number of mature CD4+ T-cells in peripheral blood (PB) is a defining characteristic of acute type of adult T-cell leukemia (ATL). To date, the site of proliferation of ATL cells in the body has been unclear. In an attempt to address this question, we examined the expression of the proliferation marker, Ki-67, in freshly isolated ATL cells from PB and lymph nodes (LNs) of patients with various types of ATL. Our findings reveal that LN-ATL cells display higher expression of the Ki-67 antigen than PB-ATL cells in acute type patients. The gene expression of T-cell quiescence regulators such as Krüppel-like factor 2/6 and forkhead box protein 1 was substantially high in acute type PB-ATL cells. The expression of human telomerase reverse transcriptase, which is involved in T-cell expansion, was significantly low in PB-ATL cells from acute type patients, similar to that in normal resting T-cells. These findings suggest that ATL cells proliferate in the LNs rather than in PB.

Promoter CpG methylation inhibits Krüppel-like factor 2 (KLF2)-Mediated repression of hTERT gene expression in human T-cells.

Constitutive expression of human telomerase reverse transcriptase (hTERT) with DNA methylation of its promoter is a common phenomenon in tumor cells. We recently found that the transcriptional factor Krüppel-like factor 2 (KLF2) binds to the CpG sequences in the hTERT promoter and inhibits hTERT gene expression in normal resting T-cells. The human T-cell line Kit 225 in the resting phase induced by the deprivation of interleukin (IL)-2 showed no decrease in the expression of hTERT, despite the high expression of KLF2. To elucidate the mechanisms of deregulation of hTERT expression in T-cells, we examined the relationship between DNA methylation and KLF2 binding to the hTERT promoter. The hTERT promoter was methylated in Kit 225 cells, resulting in the inhibition of the binding of KLF2 to the promoter. DNA demethylation by the reagent Zebularine recovered KLF2 binding to the hTERT promoter, followed by the downregulation of its gene expression. These findings indicate that the repressive effect of KLF2 on hTERT gene expression is abolished by DNA methylation in T-cell lines.