NMR spectroscopy of 14-3-3ζ reveals a flexible C-terminal extension: differentiation of the chaperone and phosphoserine-binding activities of 14-3-3ζ

Intracellular 14-3-3 proteins bind to many proteins, via a specific phosphoserine motif, regulating diverse cellular tasks including cell signalling and disease progression. The 14-3-3ζ isoform is a molecular chaperone, preventing the stress-induced aggregation of target proteins in a manner comparable with that of the unrelated sHsps (small heat-shock proteins). 1H-NMR spectroscopy revealed the presence of a flexible and unstructured C-terminal extension, 12 amino acids in length, which protrudes from the domain core of 14-3-3ζ and is similar in structure and length to the C-terminal extension of mammalian sHsps. The extension stabilizes 14-3-3ζ, but has no direct role in chaperone action. Lys(49) is an important functional residue within the ligand-binding groove of 14-3-3ζ with K49E 14-3-3ζ exhibiting markedly reduced binding to phosphorylated and non-phosphorylated ligands. The R18 peptide binds to the binding groove of 14-3-3ζ with high affinity and also reduces the interaction of 14-3-3ζ ligands. However, neither the K49E mutation nor the presence of the R18 peptide affected the chaperone activity of 14-3-3ζ, implying that the C-terminal extension and binding groove of 14-3-3ζ do not mediate interaction with target proteins during chaperone action. Other region(s) in 14-3-3ζ are most likely to be involved, i.e. the protein's chaperone and phosphoserine-binding activities are functionally and structurally separated.     

The role of epigenetic inactivation of 14-3-3σ in human cancer

Cancer cells show characteristic alterations in DNA methylation patterns. Aberrant CpG methylation of specific promoters results in inactivation of tumor suppressor genes and therefore plays an important role in carcinogenesis. The p53-regulated gene 14-3-3σ undergoes frequent epigenetic silencing in several types of cancer, including carcinoma of the breast, prostate, and skin, suggesting that the loss of 14-3-3σ expression may be causally involved in tumor progression. Functional studies demonstrated that 14-3-3σ is involved in cell-cycle control and prevents the accumulation of chromosomal damage. The recent identification of novel 14-3-3if-associated proteins by a targeted proteomics approach implies that 14-3-3σ regulates diverse cellular processes, which may become deregulated after silencing of 14-3-3σ expression in cancer cells.     

Small-molecule modulators of 14-3-3 protein–protein interactions

14-3-3 Proteins are eukaryotic adapter proteins that regulate a plethora of physiological processes by binding to several hundred partner proteins. They play a role in biological activities as diverse as signal transduction, cell cycle regulation, apoptosis, host-pathogen interactions and metabolic control. As such, 14-3-3s are implicated in disease areas like cancer, neurodegeneration, diabetes, pulmonary disease, and obesity. Targeted modulation of 14-3-3 protein–protein interactions (PPIs) by small molecules is therefore an attractive concept for disease intervention. In recent years a number of examples of inhibitors and stabilizers of 14-3-3 PPIs have been reported promising a vivid future in chemical biology and drug development for this remarkable class of proteins.     

Arachidonic Acid Binds 14-3-3ζ, Releases 14-3-3ζ from Phosphorylated BAD and Induces Aggregation of 14-3-3ζ

Polyunsaturated fatty acids, like arachidonic acid, can bind proteins and affect their function. The 14-3-3 proteins bind phosphorylated sites on a diverse array of client proteins and, in this way, are involved in many intracellular signaling pathways. In this study, we used a novel approach to discover that 14-3-3ζ is able to directly bind arachidonic acid. Furthermore, arachidonic acid, at physiological concentrations, reduced the binding of 14-3-3ζ to phosphorylated BAD, an interaction that is important in regulating apoptosis. In addition, high concentrations of arachidonic acid caused the polymerization of 14-3-3ζ, an event observed in neurodegenerative disorders. Taken together, these results indicate that arachidonic acid directly interacts with 14-3-3ζ and that this interaction may be important in both normal and pathological cellular events. If so, then factors that mediate the release, metabolism and reacylation of arachidonic acid into membranes represent key points of regulation.     

The down-regulated expression of 14-3-3σ may activate the other six 14-3-3 isoforms and they may take over the role of 14-3-3σ in the tumor genesis

Seven isoforms of 14-3-3 protein family have different functions in the cancer genesis and progress. It is found that six isoforms were up-regulated expression and inclined to sustain the cancer survival. Conversely, 14-3-3σ strongly promotes cancer apoptosis. Its down-regulated expression was found in many cancer tissues and thought to be an early event in the tumor genesis. Interestingly, no suggestions are made about the possible effect that the down-regulated expression of 14-3-3σ activated the other six 14-3-3 isoforms and they take over the role of 14-3-3σ in the tumor genesis. The inactivation of 14-3-3σ in the early stage of tumor genesis is a clue to trigger the other six 14-3-3 isoforms activation.     

Targeted proteomic analysis of 14-3-3σ in nasopharyngeal carcinoma

14-3-3σ is a potential tumor suppressor, and loss of 14-3-3σ expression plays an important role in carcinogenesis and metastasis. To explore the possible mechanism of 14-3-3σ in nasopharyngeal carcinoma (NPC) invasion and metastasis, targeted proteomic analysis was performed on 14-3-3σ-associated proteins from NPC cells. As the results, 112 proteins associated with 14-3-3σ were identified, and four 14-3-3σ-interacted proteins: keratin 8, epidermal growth factor receptor (EGFR), small GTP-binding protein RAB7, and p53 were confirmed by coimmunoprecipitation and Western blot analysis. The 14-3-3σ-associated proteins could be grouped into eight clusters based on their molecule functions. Protein–protein interaction (PPI) analysis indicated that 14-3-3σ/EGFR/keratin 8 interactions may be involved in the invasion and metastasis of NPC. 14-3-3σ/EGFR/keratin 8 could form complexes in NPC cells. 14-3-3σ downregulation in NPC may lead to the overexpression of EGFR and keratin 8, which increases the invasion ability of NPC cells possibly by activating the downstream signal molecules and reorganizing cytoskeleton. The data suggest that the biological functions of 14-3-3σ in NPC are diversified, and 14-3-3σ could inhibit the in vitro invasive ability of NPC cells possibly through 14-3-3σ/EGFR/keratin 8 interaction.     

14-3-3 η inhibits chondrogenic differentiation of ATDC5 cell

It was previously shown that 14-3-3η is overexpressed in the synovial fluid of patients with joint inflammation, which is often associated with growth failure. In this study, we investigated the role of 14-3-3η in chondrogenesis using ATDC5 cells. Upon treatment with TNF-α, cells overexpressed 14-3-3η with inhibition of chondrogenesis. Chondrogenesis was also inhibited by overexpression of 14-3-3η without TNF-α treatment, whereas silencing of 14-3-3η promoted chondrogenic differentiation. Further, G1 phase arrest was inhibited by overexpression of 14-3-3η. In summary, we suggest that 14-3-3η plays a regulatory role in chondrogenic differentiation.     

14-3-3ε Overexpression Contributes to Epithelial-Mesenchymal Transition of Hepatocellular Carcinoma

Background

14-3-3ε is implicated in regulating tumor progression, including hepatocellular carcinoma (HCC). Our earlier study indicated that elevated 14-3-3ε expression is significantly associated with higher risk of metastasis and lower survival rates of HCC patients. However, the molecular mechanisms of how 14-3-3ε regulates HCC tumor metastasis are still unclear.

Methodology and Principal Findings

In this study, we show that increased 14-3-3ε expression induces HCC cell migration and promotes epithelial-mesenchymal transition (EMT), which is determined by the reduction of E-cadherin expression and induction of N-cadherin and vimentin expression. Knockdown with specific siRNA abolished 14-3-3ε-induced cell migration and EMT. Furthermore, 14-3-3ε selectively induced Zeb-1 and Snail expression, and 14-3-3ε-induced cell migration was abrogated by Zeb-1 or Snail siRNA. In addition, the effect of 14-3-3ε-reduced E-cadherin was specifically restored by Zeb-1 siRNA. Positive 14-3-3ε expression was significantly correlated with negative E-cadherin expression, as determined by immunohistochemistry analysis in HCC tumors. Analysis of 14-3-3ε/E-cadherin expression associated with clinicopathological characteristics revealed that the combination of positive 14-3-3ε and negative E-cadherin expression is significantly correlated with higher incidence of HCC metastasis and poor 5-year overall survival. In contrast, patients with positive 14-3-3ε and positive E-cadherin expression had better prognostic outcomes than did those with negative E-cadherin expression.

Significance

Our findings show for the first time that E-cadherin is one of the downstream targets of 14-3-3ε in modulating HCC tumor progression. Thus, 14-3-3ε may act as an important regulator in modulating tumor metastasis by promoting EMT as well as cell migration, and it may serve as a novel prognostic biomarker or therapeutic target for HCC.     

Histone Deacetylase 6 (HDAC6) Promotes the Pro-survival Activity of 14-3-3ζ via Deacetylation of Lysines within the 14-3-3ζ Binding Pocket

The phospho-binding protein 14-3-3ζ acts as a signaling hub controlling a network of interacting partners and oncogenic pathways. We show here that lysines within the 14-3-3ζ binding pocket and protein-protein interface can be modified by acetylation. The positive charge on two of these lysines, Lys⁴⁹ and Lys¹²⁰, is critical for coordinating 14-3-3ζ-phosphoprotein interactions. Through screening, we identified HDAC6 as the Lys⁴⁹/Lys¹²⁰ deacetylase. Inhibition of HDAC6 blocks 14-3-3ζ interactions with two well described interacting partners, Bad and AS160, which triggers their dephosphorylation at Ser¹¹² and Thr⁶⁴², respectively. Expression of an acetylation-refractory K49R/K120R mutant of 14-3-3ζ rescues both the HDAC6 inhibitor-induced loss of interaction and Ser¹¹²/Thr⁶⁴² phosphorylation. Furthermore, expression of the K49R/K120R mutant of 14-3-3ζ inhibits the cytotoxicity of HDAC6 inhibition. These data demonstrate a novel role for HDAC6 in controlling 14-3-3ζ binding activity.     

An acid-stable analogue of the 3-β-D-ribofuranoside of Y-base

A cyclonucleoside analogue of Y_TU riboside has been prepared and shown to be relatively stable in M-hydrochloric acid solution at room temperature.     

Synthesis of Derivatives of 3-Methylxantheve and 1-Methyl-3-Isobutylxanthine 7-β-D-Ribofuranosides

Abstract

Methods of synthesis of 7–(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)-8-chloro-3-methylxanthine (5a) and l-methyl-3-isobutylxanthine (5b) were reported. Further nucleophilic displacement of chlorine has provided the corresponding 8-alkylamino and 8-benzylamino derivatives (6a,b-9a,b). Several 5′-acyl analogues of 3-methylxanthine-7–β-D-ribofuranoside (15–18) were synthesized using 7–(2′,3′-di-O-isopropylidene-β-D-ribofuranosyl)-3-methylxanthine (10) as intermediate.     

Superhelical DNA as a preferential binding target of 14-3-3γ protein

The 14-3-3 protein family is a highly conserved and widely distributed group of proteins consisting of multiple isoforms in eukaryotes. Ubiquitously expressed, 14-3-3 proteins play key roles in DNA replication, cell cycle regulation, and apoptosis. The function of 14-3-3 proteins is mediated by interaction with a large number of other proteins and with DNA. It has been demonstrated that 14-3-3γ protein binds strongly to cruciform structures and is crucial for initiating replication. In this study, we analyzed DNA binding properties of the 14-3-3γ isoform to linear and supercoiled DNA. We demonstrate that 14-3-3γ protein binds strongly to long DNA targets, as evidenced by electrophoretic mobility shift assay on agarose gels. Binding of 14-3-3γ to DNA target results in the appearance of blurry, retarded DNA bands. Competition experiments with linear and supercoiled DNA on magnetic beads show very strong preference for supercoiled DNA. We also show by confocal microscopy that 14-3-3 protein in the HCT-116 cell line is co-localized with DNA cruciforms. This implies a role for the 14-3-3γ protein in its binding to local DNA structures which are stabilized by DNA supercoiling.     

Superhelical DNA as a preferential binding target of 14-3-3γ protein

The 14-3-3 protein family is a highly conserved and widely distributed group of proteins consisting of multiple isoforms in eukaryotes. Ubiquitously expressed, 14-3-3 proteins play key roles in DNA replication, cell cycle regulation, and apoptosis. The function of 14-3-3 proteins is mediated by interaction with a large number of other proteins and with DNA. It has been demonstrated that 14-3-3γ protein binds strongly to cruciform structures and is crucial for initiating replication. In this study, we analyzed DNA binding properties of the 14-3-3γ isoform to linear and supercoiled DNA. We demonstrate that 14-3-3γ protein binds strongly to long DNA targets, as evidenced by electrophoretic mobility shift assay on agarose gels. Binding of 14-3-3γ to DNA target results in the appearance of blurry, retarded DNA bands. Competition experiments with linear and supercoiled DNA on magnetic beads show very strong preference for supercoiled DNA. We also show by confocal microscopy that 14-3-3 protein in the HCT-116 cell line is co-localized with DNA cruciforms. This implies a role for the 14-3-3γ protein in its binding to local DNA structures which are stabilized by DNA supercoiling.     

Detection of 14-3-3ζ in cerebrospinal fluid following experimentally evoked seizures

Surrogate and peripheral (bio)markers of neuronal injury may be of value in assessing effects of seizures on the brain or epilepsy development following trauma. The presence of 14-3-3 isoforms in cerebrospinal fluid (CSF) is a diagnostic indicator of Creutzfeldt–Jakob disease but these proteins may also be present following acute neurological insults. Here, we examined neuronal and 14-3-3 proteins in CSF from rats after seizures. Seizures induced by intra-amygdala microinjection of 0.1 µg kainic acid (KA) caused damage which was mainly restricted to the ipsilateral CA3 subfield of the hippocampus. 14-3-3ζ was detected at significant levels in CSF sampled 4 h after seizures compared with near absence in control CSF. Neuron-specific nuclear protein (NeuN) was also elevated in CSF in seizure rats. CSF 14-3-3ζ levels were significantly lower in rats treated with 0.01 µg KA. These data suggest the presence of 14-3-3ζ within CSF may be a biomarker of acute seizure damage.     

Free energy calculations on the stability of the 14-3-3ζ protein

Mutations of cysteine are often introduced to e.g. avoid formation of non-physiological inter-molecular disulfide bridges in in-vitro experiments, or to maintain specificity in labeling experiments. Alanine or serine is typically preferred, which usually do not alter the overall protein stability, when the original cysteine was surface exposed. However, selecting the optimal mutation for cysteines in the hydrophobic core of the protein is more challenging. In this work, the stability of selected Cys mutants of 14-3-3ζ was predicted by free-energy calculations and the obtained data were compared with experimentally determined stabilities. Both the computational predictions as well as the experimental validation point at a significant destabilization of mutants C94A and C94S. This destabilization could be attributed to the formation of hydrophobic cavities and a polar solvation of a hydrophilic side chain. A L12E, M78K double mutant was further studied in terms of its reduced dimerization propensity. In contrast to naïve expectations, this double mutant did not lead to the formation of strong salt bridges, which was rationalized in terms of a preferred solvation of the ionic species. Again, experiments agreed with the calculations by confirming the monomerization of the double mutants. Overall, the simulation data is in good agreement with experiments and offers additional insight into the stability and dimerization of this important family of regulatory proteins.     

Interacting domains of P14-3-3 and actin involved in protein–protein interactions of living cells

14-3-3 proteins are conserved regulatory proteins present in all eukaryotic cells that control numerous cellular activities via targeted protein interactions. To elucidate the interaction between P14-3-3 from Physarum polycephalum and actin in living cells, PCR and DNA recombination were used to generate various P14-3-3 and actin constructs. Yeast two-hybrid assay and FRET were employed to characterize the interaction between P14-3-3 and actin. The two-hybrid assay indicated that P14-3-3 N-terminal 76–108 amino acids and the C-terminal 207–216 amino acids played an important role in mediating interactions with actin, and the actin N-terminal 1–54 amino acids and the C-terminal 326–376 amino acids are also crucial in the interactions with the mPa, a P14-3-3 with mutations at Ser62 (Ser62 → Gly62). Mutations to potential phosphorylation sites did not affect interactions between P14-3-3 and actin. FRET results demonstrated that P14-3-3 co-localized with actin with a FRET efficiency of 22.2% and a distance of 7.4 nm and that P14-3-3 N-terminal 76–108 and C-terminal 207–216 amino acids were important in mediating this interaction, the truncated actin peptides without either the N-terminal 1–54 or C-terminal 326–376 amino acids interacted with P14-3-3, consistent with the results obtained from the yeast two-hybrid assay. Based on data obtained, we identified critical actin and P14-3-3 contact regions.     

Altered Expression of 14-3-3ζ Protein in Spinal Cords of Rat Fetuses with Spina Bifida Aperta

Background

A large number of studies have confirmed that excessive apoptosis is one of the reasons for deficient neuronal function in neural tube defects (NTDs). A previous study from our laboratory used 2-D gel electrophoresis to demonstrate that 14-3-3ζ expression was low in the spinal cords of rat fetuses with spina bifida aperta at embryonic day (E) 17. As a member of the 14-3-3 protein family, 14-3-3ζ plays a crucial role in the determination of cell fate and anti-apoptotic activity. However, neither the expression of 14-3-3ζ in defective spinal cords, nor the correlation between 14-3-3ζ and excessive apoptosis in NTDs has been fully confirmed.

Methodology/Principal Findings

We used immunoblotting and quantitative real-time PCR (qRT-PCR) to quantify the expression of 14-3-3ζ and double immunofluorescence to visualize 14-3-3ζ and apoptosis. We found that, compared with controls, 14-3-3ζ was down-regulated in spina bifida between E12 and E15. Excessive apoptotic cells and low expression of 14-3-3ζ were observed in the dorsal region of spinal cords with spina bifida during the same time period. To initially explore the molecular mechanisms of apoptosis in NTDs, we investigated the expression of microRNA-7 (miR-7), microRNA-375 (miR-375) and microRNA-451 (miR-451), which are known to down-regulate 14-3-3ζ in several different cell types. We also investigated the expression of p53, a molecule that is downstream of 14-3-3ζ and can be down-regulated by it. We discovered that, in contrast to the reduction of 14-3-3ζ expression, the expression of miR-451, miR-375 and p53 increased in spina bifida rat fetuses.

Conclusions/Significance

These data suggest that the reduced expression of 14-3-3ζ plays a role in the excessive apoptosis that occurs in spina bifida and may be partly regulated by the over-expression of miR-451 and miR-375, and the consequent up-regulation of p53 might further promote apoptosis in spina bifida.     

Downregulation of 14-3-3σ Correlates with Multistage Carcinogenesis and Poor Prognosis of Esophageal Squamous Cell Carcinoma

Aims

The asymptomatic nature of early-stage esophageal squamous cell carcinoma (ESCC) results in late presentation and consequent dismal prognosis This study characterized 14-3-3σ protein expression in the multi-stage development of ESCC and determined its correlation with clinical features and prognosis.

Materials and Methods

Western blot was used to examine 14-3-3σ protein expression in normal esophageal epithelium (NEE), low grade intraepithelial neoplasia (LGIN), high grade intraepithelial neoplasia (HGIN), ESCC of TNM I to IV stage and various esophageal epithelial cell lines with different biological behavior. Immunohistochemistry was used to estimate 14-3-3σ protein in 110 biopsy samples of NEE, LGIN or HGIN and in 168 ESCC samples all of whom had follow-up data. Support vector machine (SVM) was used to develop a classifier for prognosis.

Results

14-3-3σ decreased progressively from NEE to LGIN, to HGIN, and to ESCC. Chemoresistant sub-lines of EC9706/PTX and EC9706/CDDP showed high expression of 14-3-3σ protein compared with non-chemoresistant ESCC cell lines and immortalized NEC. Furthermore, the downregulation of 14-3-3σ correlated significantly with histological grade (P = 0.000) and worse prognosis (P = 0.004). Multivariate Cox regression analysis indicated that 14-3-3σ protein (P = 0.016) and T stage (P = 0.000) were independent prognostic factors for ESCC. The SVM ESCC classifier comprising sex, age, T stage, histological grade, lymph node metastasis, clinical stage and 14-3-3σ, distinguished significantly lower- and higher-risk ESCC patients (91.67% vs. 3.62%, P = 0.000).

Conclusions

Downregulation of 14-3-3σ arises early in the development of ESCC and predicts poor survival, suggesting that 14-3-3σ may be a biomarker for early detection of high-risk subjects and diagnosis of ESCC. Our seven-feature SVM classifier for ESCC prognosis may help to inform clinical decisions and tailor individual therapy.