Evidence for NFAT1 as a Tumor Suppressor in T-ALL

NFAT transcription factors are highly phosphorylated proteins residing in the cytoplasm of resting cells. Upon dephosphorylation by the phosphatase calcineurin, NFAT translocates to the nucleus, where it orchestrates developmental and activation programs in diverse cell types. NFAT is rephosphorylated and inactivated through the concerted action of at least three different kinases: CK1, GSK-3 and DYRK. NFAT signalling is further implicated in lymphocyte homeostasis and its deregulation has been suggested to be involved in the pathogenesis of different malignancies. Previous studies have primarily concentrated on NFAT2, which was found to be overexpressed and constitutively activated in a majority of biopsies from Diffuse Large B-Cell lymphomas and Burkitt’s lymphomas, presumably reflecting activation of the Ca/NFAT signalling pathway as part of their pathogenesis. Other recent studies have demonstrated that NFAT2 activation can lead to increased expression of different cell survival factors (CD154, BLyS) in several NHL subtypes. Here, we analyzed the role of NFAT1 in malignant transformation and in the pathogenesis of T-ALL using a transgenic mouse model for the disease. We show that sustained activity of NFAT1 and NFAT2 induces opposite phenotypes in NIH 3T3 cells. While NIH 3T3 cells infected with empty retrovirus showed normal viability and stopped growing upon reaching confluence, cells infected with a constitutively active version of NFAT2 overgrew the monolayer and continued to proliferate beyond confluence. NIH 3T3 cells expressing a constitutively active form of NFAT1 on the contrary exhibited a remarkable reduction in cell proliferation and never reached confluence, suggesting opposite roles for NFAT1 and NFAT2 in the control of cell growth and proliferation. To directly test whether NFAT1 has the capability to suppress tumor growth and to subvert a transformed cell type, we generated NIH 3T3 cell clones harboring the oncogene H-rasV12, which showed a typical transformed phenotype. Expression of constitutively active NFAT1 in the transformed NIH 3T3-HrasV12 cells leads to a significant reduction in cell proliferation which was accomanied by a large decrease in DNA replication and an accumulation in the G1 phase of the cell cycle. To test if NFAT1 has tumor suppressor characteristics in vivo, we generated transgenic mice conditionally expressing a hyperactivable form of NFAT1 from the ROSA26 locus. These mice were subsequently bred to Tel-Jak2 transgenic mice, which develop T-ALL at 8–20 wks of age. Mice that expressed hyperactivable NFAT1 in their T cells in addition to the disease inducing Tel-Jak2 transgene exhibited a significantly attenuated phenotype of the disease. Infiltration of vital organs such as lung, liver and bone marrow was significantly delayed and overall survival was almost twice as long in the animals that expressed hyperactivable NFAT1 in their T cell compartment (88 days vs. 159 days). To summarize, our data identify NFAT1 as a novel tumor suppressor gene in T-ALL and emphasize the importance of the Ca/NFAT signalling pathway in the pathogenesis of hematologic malignancies.