Tributylhexadecylphosphonium Bromide, a Novel Nuclear Factor of Activated T Cells Signaling Inhibitor, Blocks Interleukin-2 Induction Associated with Inhibition of p70 Ribosomal Protein S6 Kinase Phosphorylation

Abstract

Transcriptional factors of the nuclear factor of activated T cells (NFAT) family are involved in T cell signaling. Many NFAT signaling inhibitors, such as cyclosporin A (CsA) and tacrolimus, abrogate dephosphorylation of NFAT proteins by inhibiting calcineurin activity. In pursuit of a novel type of NFAT signaling inhibitor, we screened our chemical library using the NFAT-dependent reporter assay and identified tributylhexadecylphosphonium bromide (THPB) as a selective NFAT signaling inhibitor. THPB inhibited NFATdependent reporter activity, and the induction of interleukin-2 (IL-2) at both mRNA and protein levels by calcium stimulation. Moreover, THPB had an additive effect on the inhibition of IL-2 induction with CsA. Unlike CsA, THPB did not affect dephosphorylation of NFAT1, but suppressed phosphorylation of p70 ribosomal protein S6 kinase (p70S6K). These results suggest that THPB may be a novel type of NFAT signaling inhibitor that acts in association with inhibition of p70S6K phosphorylation.Key words nuclear factor of activated T cell; interleukin-2; cyclosporin A Nuclear factor of activated T cells (NFAT) was originally identified as a transcriptional factor that binds the interleukin-2 (IL-2) promoter in activated T cells.1) The NFAT family consists of five members: NFAT1 (NFATc2), NFAT2 (NFATc1), NFAT3 (NFATc4), NFAT4 (NFATc3) and NFAT5. All of them, except NFAT5, share a similar sequence motif (PxIxIT), a calcineurin docking consensus. NFAT1, NFAT2 and NFAT4 mainly function in the immune system. 1,2) In resting T cells, NFAT proteins are hyperphosphorylated and retained in the cytosol. Engagement of T cell receptor activates calcineurin, a Ca 2+ /calmodulin dependent serine/ threonine phosphatase. NFAT proteins are dephosphorylated by calcineurin and translocate into the nucleus.2-4) Calcineurin inhibitors such as cyclosporin A (CsA) and tacrolimus have been used clinically to prevent transplant rejection and treat atopic dermatitis by inhibiting NFAT signaling. 5,6) However, these drugs have serious side-effect such as nephrotoxicity, neurotoxicity and increased risk of cancer.5,7-9) Several small molecules such as NCI3 and BTP-2/YM-58483 have been reported to inhibit IL-2 secretion by suppressing dephosphorylation and nuclear import of NFAT.10-12) Venkatesh et al. identified 14 compounds as nuclear import inhibitors of GFP-NFATc3. Most of them inhibited calcium mobilization involving store-operated calcium (SOC) channels and thereby blocked dephosphorylation of NFATc3. 4,13) Most NFAT signaling inhibitors block dephosphorylation of NFAT proteins, but recently it has been reported that acetate inhibited the nuclear import of NFAT proteins without suppressing dephosphorylation of these proteins. 14) We screened our chemical library using the NFAT-dependent reporter assay to identify novel types of NFAT signaling inhibitors. In this study, we identified tributylhexadecylphosphonium bromide (THPB) as a novel NFAT signaling inhibitor. Moreover, THPB and CsA together had an additive effect on the suppress...

Combination Therapy for KIT-Mutant Mast Cells: Targeting Constitutive NFAT and KIT Activity

“…49], alone or in combination with dasatinib. Combination therapy with rocaglamide plus dasatinib or THPB plus dasatinib decreased cell viability more significantly than monotherapy (Fig.…”

Roles of the tacrolimus-dependent transcription factor IRF4 in acute rejection after liver transplantation

“…TAC, like cyclosporine A (CsA), is a widely accepted immunosuppressant that belongs to the family of calcineurin inhibitors, which inhibit IL-2 expression through NFAT (nuclear factor of activated T cells) in T lymphocytes [6,7]. The NFAT family of transcription factors includes key players in the regulation of gene transcription in response to a variety of immune signals [8,9].…”