Calcineurin/NFAT Pathway Is a Novel Target of Therapeutic Interest in Lymphoid Malignancies.

Calcineurin (PP2B) is an ubiquitously expressed serine/threonine protein phosphatase dependent on calcium calmodulin. Calcineurin plays a pivotal role in many important biological processes, including the development and function of the immune system. Sustained signalling through calcium/calcineurin results in the activation of transcription factors of the NFAT family (Nuclear Factor of Activated T cells). Mouse genetic studies have demonstrated a strong epistatic relationship between calcineurin and NFAT activation and function in many developmental processes. In lymphoid cells, the calcineurin/NFAT signalling pathway is essential to specific aspects of T cell development and plays a central role in the activation of the immune response and in its homeostatic control.

Despite their central role in T cell development, immune function and homeostasis, deregulation of calcineurin/NFAT pathway have not been shown so far to have a pro-oncogenic role in lymphomagenesis. Using our TEL-JAK2 transgenic mouse model of T-ALL, we investigated the activation status and importance of the calcineurin/NFAT signalling pathway in leukemogenesis. We found that the calcineurin/NFAT pathway is activated in leukemic cells of TEL-JAK2 transgenic mice. Western blot and EMSA experiments showed that all NFAT members expressed in the lymphoid lineage (NFAT1, 2 and 4) are found in an active hypophosphorylated form, they are located in the nucleus, and exhibit an increase in their DNA binding activity. In order to investigate whether constitutive calcineurin/NFAT activity was specific to TEL-JAK2 leukemia or whether it is a more general property of leukemic cells, we analyzed NFAT activation in leukemic cells of transgenic mice aberrantly expressing oncoproteins involved in lymphoma in human. This showed that calcineurin/NFATpathway is not only aberantly active in leukemic cells of TEL-JAK2 transgenic animals, but also in other mouse models of human leukemia. We also found that the activation of the calcineurin/NFAT pathway is only observed when cells are rapidly analyzed following their explantation and dissociation from diseased animals. Indeed, if cells are incubated in tissue culture for a short period of time (60–120min.), essentially stochiometric rephosphorylation, meaning inactivation, of NFAT is observed. This indicates that activation of the calcineurin/NFAT pathway depends upon signals generated in the in vivo tumor cell environment and is not -or not solely- under the control of the initiating oncogene. Using these mouse models we conducted pre-clinical studies that indicated that calcineurin/NFAT pathway is an attractive novel target of therapeutic interest in lymphoid malignancies.