Tumor Progression Locus 2 (Tpl2) Kinase as a Novel Therapeutic Target for Cancer: Double-Sided Effects of Tpl2 on Cancer
The Tpl-2 kinase activates the nuclear factor of activated T cells (NFAT) and induces IL-2 expression in T-cell lines. Here we show that the activation of the IL-2 promoter by Tpl-2 is inhibited by mutant signaling molecules that inhibit the mitogen-activated protein kinase (MAPK) or the calcineurin/NFAT pathways and is promoted by combinations of signaling molecules that activate these pathways. We, therefore, conclude that signals generated by the convergence of the MAPK and the calcineurin/NFAT pathway are necessary and sufficient for the activation of the IL-2 promoter by Tpl-2. The activation of both the IL-2 promoter and an NFAT-driven minimal promoter were shown to depend on signals transduced by Raf1. However, it was only the IL-2 promoter whose activation by Tpl-2 was fully blocked by the dominant negative mutant MEK1S218/222A and the MEK1/MEK2 inhibitor PD098059. Since the activation of NFAT is MAPK-dependent these findings suggested that the activation of MAPK by Tpl-2 is either independent or only partially dependent on MEK1 and MEK2. In addition, they suggested that the activation of the IL-2 promoter is under the control of not only NFAT but also a second factor whose activation is MEK-dependent. Experiments in COS-1 and EL-4 cells confirmed both hypotheses and revealed that the second factor activated by Tpl-2 is NF-κB. While the activation of the IL-2 promoter and an NFAT-driven minimal promoter by Tpl-2 was fully blocked by the dominant negative mutant NFATΔ418, it was only partially blocked by the calcineurin inhibitor cyclosporin A suggesting that the Tpl-2-mediated NFAT activation is under the control of a combination of calcineurin-dependent and independent pathways. Both pathways were fully blocked by Bcl-2 or Bcl-XL.