Proteins that interact with 14-3-3 isoforms are involved in regulation of the cell cycle, intracellular trafficking/targeting, signal transduction, cytoskeletal structure and transcription. Recent novel roles for 14-3-3 isoforms include nuclear trafficking the direct interaction with cruciform DNA and with a number of receptors, small G-proteins and their regulators. Recent findings also show that the mechanism of interaction is also more complex than the initial finding of the novel phosphoserine/threonine motif. Non-phosphorylated binding motifs that can also be of high affinity may show a more isoform-dependent interaction and binding of a protein through two distinct binding motifs to a dimeric 14-3-3 may also be essential for full interaction. Phosphorylation of specific 14-3-3 isoforms can also regulate interactions. In many cases, they show a distinct preference for a particular isoform(s) of 14-3-3. A specific repertoire of dimer formation may influence which of the 14-3-3-interacting proteins could be brought together. Mammalian and yeast 14-3-3 isoforms show a preference for dimerization with specific partners in vivo.

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TNBC is a highly heterogeneous and aggressive breast cancer subtype associated with high relapse rates, and for which no targeted therapy yet exists. Protein arginine methyltransferase 5 (PRMT5), an enzyme which catalyzes the methylation of arginines on histone and non‐histone proteins, has recently emerged as a putative target for cancer therapy. Potent and specific PRMT5 inhibitors have been developed, but the therapeutic efficacy of PRMT5 targeting in TNBC has not yet been demonstrated. Here, we examine the expression of PRMT5 in a human breast cancer cohort obtained from the Institut Curie, and evaluate the therapeutic potential of pharmacological inhibition of PRMT5 in TNBC. We find that PRMT5 mRNA and protein are expressed at comparable levels in TNBC, luminal breast tumors, and healthy mammary tissues. However, immunohistochemistry analyses reveal that PRMT5 is differentially localized in TNBC compared to other breast cancer subtypes and to normal breast tissues. PRMT5 is heterogeneously expressed in TNBC and high PRMT5 expression correlates with poor prognosis within this breast cancer subtype. Using the small‐molecule inhibitor EPZ015666, we show that PRMT5 inhibition impairs cell proliferation in a subset of TNBC cell lines. PRMT5 inhibition triggers apoptosis, regulates cell cycle progression and decreases mammosphere formation. Furthermore, EPZ015666 administration to a patient‐derived xenograft model of TNBC significantly deters tumor progression. Finally, we reveal potentiation between EGFR and PRMT5 targeting, suggestive of a beneficial combination therapy. Our findings highlight a distinctive subcellular localization of PRMT5 in TNBC, and uphold PRMT5 targeting, alone or in combination, as a relevant treatment strategy for a subset of TNBC.

14-3-3 proteins mediate interactions between proteins involved in signal transduction and cell cycle regulation. Phosphorylation of target proteins as well as 14-3-3 are important for protein-protein interactions. Here, we describe the purification of a protein kinase from porcine brain that phosphorylates 14-3-3 on Thr-233. This protein kinase has been identified as casein kinase I␣ (CKI␣) by peptide mapping analysis and sequencing. Among mammalian 14-3-3, only 14-3-3 possesses a phosphorylatable residue at the same position (Ser-233), and we show that this residue is also phosphorylated by CKI. In addition, we show that 14-3-3 is exclusively phosphorylated on Thr-233 in human embryonic kidney 293 cells. The residue 233 is located within a region shown to be important for the association of 14-3-3 to target proteins. We showed previously that, in 293 cells, only the unphosphorylated form of 14-3-3 associates with the regulatory domain of c-Raf. We have now shown that in vivo phosphorylation of 14-3-3 at the CKI␣ site (Thr-233) negatively regulates its binding to c-Raf, and may be important in Raf-mediated signal transduction.The name 14-3-3 was given to an abundant mammalian brain protein family due to its particular migration pattern on two-dimensional DEAE-cellulose chromatography and starch gel electrophoresis (1). The proteins were subsequently named by Greek letters according to their respective elution positions on HPLC.1 Seven mammalian forms of 14-3-3 (␤, ␥, ⑀, , , , and ) have been found, and two are specifically expressed in T cells () and epithelial cells (). The 14-3-3 family is highly conserved, and individual proteins are either identical or contain a few conservative substitutions over a wide range of mammalian species. All are dimeric proteins with a pI around 4.5 and a subunit mass of 30 -33 kDa. Homologues of 14-3-3 proteins have also been found in a broad range of eukaryotic organisms.Although the exact function of 14-3-3 is not known, various biological activities have been ascribed for 14-3-3: activation of tyrosine and tryptophan hydroxylases (2), regulation of protein kinase C (3), stimulation of calcium-dependent exocytosis (4), cofactor activity for ADP-ribosylation by Pseudomonas aeruginosa exoenzyme S (5), and a role in cell cycle control (6). New findings have suggested many additional roles for the 14-3-3 family, in particular mediating interactions between components involved in intracellular signal transduction (7). The discovery of the interaction of specific 14-3-3 proteins with Raf (3,8,9) generated much interest in the 14-3-3 family. Whether 14-3-3 directly activates Raf is still controversial, and activation of Raf by 14-3-3 may in fact be due to stabilization rather than stimulation of Raf activity (10). However, it has been shown that dimerization may provide a mechanism for Raf activation (11,12), and 14-3-3 may be involved in this process (13). 14-3-3 have also been shown to interact with other important signaling proteins including polyoma middle T antigen (14), Cdc25 ph...

In this study, we assessed the role of annexin V, a Ca2+-dependent phospholipid-binding protein, as a regulator of protein kinase C (PKC) and characterized its mechanism of inhibition. Several mutants obtained by oligonucleotide site-directed mutagenesis were tested in vitro on PKC activity in cytosolic fractions from Jurkat cells and on purified PKCalpha. Annexin V inhibited phosphorylation of annexin II by endogenous PKC and phosphorylation of myelin basic protein by PKCalpha. In both systems, the use of single Ca2+-binding-site mutants of annexin V led to a partial reversal of inhibition, and the Ca2+-binding site located in the first domain of annexin V was found to have the most important role. An increase in the number of mutated Ca2+-binding sites led to a greater loss of inhibition. These results corroborated those showing the progressive loss of binding of these mutants to phospholipid liposomes. In conclusion, we show that PKC inhibition by annexin V is the consequence of a mechanism involving phospholipid sequestration by annexin V, and that the Ca2+-binding site located in domain 1 of annexin V plays a predominant role in this process. In addition, we show that the R122AIK site, which may act analogously to a PKC-inhibitory pseudosubstrate site, is not involved in PKC inhibition, and that a peptide corresponding to the C-terminal tail of annexin V inhibits PKC activity but to a lesser extent than annexin V itself.

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Annexin V belongs to a family of proteins that interact with phospholipids in a Ca 2؉ -dependent manner. This protein has been demonstrated to have anti-phospholipase A 2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA 2 (cPLA 2 ). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA 2 activity after stimulation by calcium, GTP␥S (guanosine 5-O-(3-thiotriphosphate)), formyl-Met-LeuPhe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA 2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca 2؉ ] of 3 and 10 M. The mechanism of inhibition was also analyzed. cPLA 2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA 2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA 2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca 2؉ site. We demonstrate that the Ca 2؉ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA 2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA 2 . Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA 2 activity by depletion of the phospholipid substrate.

MicroRNAs play important roles in many biological processes. Their aberrant expression can have oncogenic or tumor suppressor function directly participating to carcinogenesis, malignant transformation, invasiveness and metastasis. Indeed, miRNA profiles can distinguish not only between normal and cancerous tissue but they can also successfully classify different subtypes of a particular cancer. Here, we focus on a particular class of transcripts encoding polycistronic miRNA genes that yields multiple miRNA components. We describe ‘clustered MiRNA Master Regulator Analysis (ClustMMRA)’, a fully redesigned release of the MMRA computational pipeline (MiRNA Master Regulator Analysis), developed to search for clustered miRNAs potentially driving cancer molecular subtyping. Genomically clustered miRNAs are frequently co-expressed to target different components of pro-tumorigenic signaling pathways. By applying ClustMMRA to breast cancer patient data, we identified key miRNA clusters driving the phenotype of different tumor subgroups. The pipeline was applied to two independent breast cancer datasets, providing statistically concordant results between the two analyses. We validated in cell lines the miR-199/miR-214 as a novel cluster of miRNAs promoting the triple negative breast cancer (TNBC) phenotype through its control of proliferation and EMT.