Dual role of the ddx5/ddx17 RNA helicases in the control of the pro-migratory NFAT5 transcription factor.

Ddx5 and ddx17 are two highly related RNA helicases involved in both transcription and splicing. These proteins coactivate transcription factors involved in cancer such as the estrogen receptor alpha, p53 and beta-catenin. Ddx5 and ddx17 are part of the splicing machinery and can modulate alternative splicing, the main mechanism increasing the proteome diversity. Alternative splicing also has a role in gene expression level regulation when it is coupled to the nonsense-mediated mRNA decay (NMD) pathway. In this work, we report that ddx5 and ddx17 have a dual role in the control of the pro-migratory NFAT5 transcription factor. First, ddx5 and ddx17 act as transcriptional coactivators of NFAT5 and are required for activating NFAT5 target genes involved in tumor cell migration. Second, at the splicing level, ddx5 and ddx17 increase the inclusion of NFAT5 exon 5. As exon 5 contains a pre-mature translation termination codon, its inclusion leads to the regulation of NFAT5 mRNAs by the NMD pathway and to a decrease in NFAT5 protein level. Therefore, we demonstrated for the first time that a transcriptional coregulator can simultaneously regulate the transcriptional activity and alternative splicing of a transcription factor. This dual regulation, where ddx5 and ddx17 enhance the transcriptional activity of NFAT5 although reducing its protein expression level, suggests a critical role for ddx5 and ddx17 in tumor cell migration through the fine regulation of NFAT5 pathway.

NFAT3 transcription factor inhibits breast cancer cell motility by targeting the Lipocalin 2 gene.

NFAT1 and NFAT5 act as pro-invasive and pro-migratory transcription factors in breast carcinoma, contributing to the formation of metastases. We report that NFAT3 is specifically expressed in estrogen receptor alpha positive (ERA+) breast cancer cells. We show that NFAT3 inhibits by itself the invasion capacity of ERA+ breast cancer cells and needs to cooperate with ERA to inhibit their migration. Conversely, NFAT3 downregulation results in actin reorganization associated with increased migration and invasion capabilities. NFAT3 signaling reduces migration through inhibition of Lipocalin 2 (LCN2) gene expression. Collectively, our study unravels an earlier unknown NFAT3/LCN2 axis that critically controls motility in breast cancer.

gp160 of HIV or anti-CD4 monoclonal antibody ligation of CD4 induces inhibition of JNK and ERK-2 activities in human peripheral CD4+ T lymphocytes.

Under physiological conditions, activation of CD4+ T cells by major histocompatibility complex (MHC)antigen complexes requires engagement of both the T cell receptor and the CD4 molecule. However, CD4 ligands binding to the CD4 molecule has also been shown to inhibit T cell proliferation and interleukin (IL)-2 production in human CD4+ T cells, in an MHC-independent way. We have previously shown that this inhibition was associated with a diminished binding activity of the IL-2 transcription factors NF-AT, NF-kappaB, and AP-1. AP-1 plays a key role in the regulation of IL-2 transcription, and ERK and JNK activities are necessary for regulating AP-1 at both the transcriptional and the post-transcriptional levels. We therefore studied, in human peripheral CD4+ T cells, the regulation of the activities of extracellular signal-regulated protein kinases (ERK) and c-Jun N-terminal kinases (JNK) by two CD4 ligands, gp160 the envelope glycoprotein of human immunodeficiency virus (HIV) and an anti-CD4 monoclonal antibody (mAb). Pre-incubation of CD4+ T lymphocytes in the presence of anti-CD4 mAb or gp160 inhibits the activation of JNK in response to phorbol 12-myristate 13-acetate and ionomycin. In the same conditions, phosphorylation and activation of ERK-2 were also inhibited. Inhibition of both JNK and ERK-2 activities are specific for binding of CD4 ligands to the CD4 molecule. They were not observed in CD8+ T lymphocytes. These results suggest that a specific inhibition of JNK and ERK-2 activities contributes to defective IL-2 production in T lymphocytes pre-incubated with CD4 ligands.

Sam68 association with p120GAP in CD4+ T cells is dependent on CD4 molecule expression.

p120 GTPase-activating protein (p120GAP) is a major negative regulator of p21ras activity in several cell types including T cells. Catalytic activity of this enzyme is regulated in part by its interaction with several associated tyrosine-phosphorylated proteins. Sam68 was initially described as associated with p120GAP. It has been further established that Sam68 is a substrate of src kinases in mitosis and that it is not associated with p120GAP in transformed fibroblasts. We describe herein that Sam68 associates with p120GAP and PLC gamma 1 in human mature T cells and in a T cell line expressing the CD4 molecule HUT78 CD4+. This association is present in nonactivated cells and increases after anti-CD3 activation. It is dependent on CD4 expression and, in part, on the association of CD4 with p56lck, as shown by the strongly decreased association of Sam68 with p120GAP in the CD4- mutants, HUT78 CD4-, and by the reduced association of Sam68 with both p120GAP and p56lck in the HUT78 T cell line expressing a CD4 mutant unable to interact with p56lck, HUT78 C420/22. We propose that recruitment of Sam68, via CD4/p56lck, to the inner face of the plasma membrane may permit, via its docking properties, the correct association of key signaling molecules including PLC gamma 1 and p120GAP. This formation of transduction modules will enable the activation of different signaling cascades including the p21ras pathway and an array of downstream events, ultimately leading to T cell activation.

Differential CD4-dependent inhibition of JNK but not Erk-2 activities in human naive and memory CD4+ T cell populations.

CD4 ligand binding to the CD4 molecules has been shown to inhibit T cell proliferation and IL-2 transcription and synthesis. We have recently shown that this inhibition correlated with a CD4-mediated inhibition of the kinase Erk-2 and c-Jun-N-terminal kinases (JNK) which play a key role in IL-2 transcription. Moreover, we have previously reported that antigen-independent adhesion of CD45RObright/CD4+ T cells to B cells is negatively regulated by CD4 ligands, whereas that of CD45RAbright/CD4+ naive T cells is not. Other groups have described, in murine models, a differential sensitivity of memory and naive T cells to CD4-mediated inhibitory effects on T cell activation. The aim of the present report was to study the sensitivity of the naive and memory CD4+ T cell populations to the CD4-mediated inhibition of Erk-2 and JNK activation. Our data show that preincubation with anti-CD4 mAb, of the CD45RAbright/CD4+ naive and the CD45RObright/CD4+ memory human T cell populations, induces inhibition of both Erk-2 phosphorylation and Erk-2 activation by phorbol ester or anti-CD3 mAb. In contrast, CD3 mediated JNK activation was inhibited in the memory but not in the naive CD4+ T cell population, whereas JNK activation by phorbol ester or phorbol esters plus Ca2+ ionophore was inhibited by anti-CD4 mAb in both T cell populations. These data further demonstrate a differential sensitivity of naive and memory CD4+ T cell populations to the CD4-mediated negative signaling.

Ligands of CD4 inhibit the association of phospholipase Cgamma1 with phosphoinositide 3 kinase in T cells: regulation of this association by the phosphoinositide 3 kinase activity.

We have previously shown that CD4 ligands inhibit interleukin-2 (IL-2) production and T cell proliferation in human peripheral CD4+ T lymphocytes, in an MHC-independent way. Two major pathways implicated in T cell activation are inhibited by binding of CD4 ligands to the CD4 molecule, i.e. Ca2+ signaling by phospholipase Cgamma1 (PLCgamma1), and ERK-2 activation, suggesting a p21ras inhibition. We have correlated these inhibitions with the disruption of multifunctional complexes containing PLCgamma1, p120GAP and Sam68, induced by T cell activation. We report here that T cell activation through the TCR/CD3 induces an association of the phosphoinositide 3 kinase (PI3 kinase) with PLCgamma1, both in peripheral CD4+ T lymphocytes and the HUT-78 CD4+ T cell line. PI3 kinase is present in the multifunctional complexes that we have described previously. Preincubation of human peripheral CD4+ T cells and HUT-78 CD4+ T cells with gp160 or a peptide analogue of the HLA class II DR molecule precludes the association of PLCgamma1 with PI3 kinase. We also demonstrate, using two specific inhibitors of PI3 kinase activity (LY294002 and wortmannin), that this activity plays a key role in the association of PLCgamma1 with PI3 kinase. Moreover, we demonstrate the implication of the PI3 kinase activity in the negative signal mediated by HIV gp160 binding to CD4 molecules. We propose that the products of the PI3 kinase are important mediators of the negative signaling induced by the binding of CD4 ligands to the CD4 molecule implicated in the regulation of the formation of multifunctional complexes.

The protein tyrosine kinase p60c-Src is not implicated in the pathogenesis of the human autosomal recessive form of osteopetrosis: a study of 13 children.

Osteopetrosis has been described in mice generated by homozygous gene disruption of c-src gene encoding for the p60c-Src protein tyrosine kinase (Src-/- mice). The similarities of bone histologic findings in this murine model to those observed in some patients first seen with autosomal recessive osteopetrosis, "malignant" osteopetrosis, led us to investigate the potential role of p60c-Src in the pathogenesis of malignant osteopetrosis in 13 children. In 4 patients a c-src mutation was ruled out by an intragenic microsatellite segregation study. In the other 9 we analyzed p60c-Src expression and function, as well as c-src sequence. The expression was normal in all of the patients tested. In addition, the tyrosine phosphorylation and kinase activity of p60c-Src were also normal in all of the patients. Moreover, in these patients, sequences of the coding region of c-src were identical to the published sequence of the human c-src complementary DNA. These results exclude a role for c-src in the pathogenesis of human malignant osteopetrosis in the 13 patients analyzed.