Reduction of 14-3-3 proteins correlates with increased sensitivity to killing of human lung cancer cells by ionizing radiation

Overexpression of 14-3-3gamma causes polyploidization in H322 lung cancer cells

The 14-3-3 proteins are a family of highly conserved proteins that participate in a wide variety of cellular processes. Mounting evidence suggests that 14-3-3 proteins have a role in human cancers, however their role in tumorigenesis is unclear. Here we report that over-expression of 14-3-3 gamma protein in human lung cancer cell line H322 results in abnormal DNA replication and polyploidization. Cells that overexpress 14-3-3 gamma are resistant to microtubule inhibitors and can reenter the cell cycle in the absence of mitosis suggesting that elevated levels of 14-3-3 gamma may enable cells to bypass the mitotic checkpoint. Taken together, our data indicate that 14-3-3gamma may contribute to tumorigenesis by promoting genomic instability.

Up-regulation of 14-3-3zeta in lung cancer and its implication as prognostic and therapeutic target

A functional genomic approach integrating microarray and proteomic analyses done in our laboratory has identified 14-3-3zeta as a putative oncogene whose activation was common and driven by its genomic amplification in lung adenocarcinomas. 14-3-3zeta is believed to function in cell signaling, cycle control, and apoptotic death. Following our initial finding, here, we analyzed its expression in lung tumor tissues obtained from 205 patients with various histologic and stage non-small cell lung cancers (NSCLC) using immunohistochemistry and then explored the effects of specific suppression of the gene in vitro and in a xenograft model using small interfering RNA. The increased 14-3-3zeta expression was positively correlated with a more advanced pathologic stage and grade of NSCLCs (P = 0.001 and P = 0.006, respectively) and was associated with overall and cancer-specific survival rates of the patients (P = 0.022 and P = 0.018, respectively). Down-regulation of 14-3-3zeta in lung cancer cells led to a dose-dependent increased sensitivity to cisplatin-induced cell death, which was associated with the inhibition of cell proliferation and increased G2-M arrest and apoptosis. The result was further confirmed in the animal model, which showed that the A549 lung cancer cells with reduced 14-3-3zeta grew significantly slower than the wild-type A549 cells after cisplatin treatment (P = 0.008). Our results suggest that 14-3-3zeta is a potential target for developing a prognostic biomarker and therapeutics that can enhance the antitumor activity of cisplatin for NSCLC.

Over-expression of 14-3-3zeta is an early event in oral cancer

Background

The functional and clinical significance of 14-3-3 proteins in human cancers remain largely undetermined. Earlier, we have reported differential expression of 14-3-3ζ mRNA in oral squamous cell carcinoma (OSCC) by differential display.

Methods

The clinical relevance of 14-3-3ζ protein in oral tumorigenesis was determined by immunohistochemistry in paraffin embedded sections of oral pre-malignant lesions (OPLs), OSCCs and histologically normal oral tissues and corroborated by Western Blotting. Co-immunoprecipitation assays were carried out to determine its association with NFκB, β-catenin and Bcl-2.

Results

Intense immunostaining of 14-3-3ζ protein was observed in 61/89 (69%) OPLs and 95/120 (79%) OSCCs. Immunohistochemistry showed significant increase in expression of 14-3-3ζ protein from normal mucosa to OPLs to OSCCs (p_trend < 0.001). Significant increase in expression of 14-3-3ζ protein was observed as early as in hyperplasia (p = 0.009), with further elevation in moderate and severe dysplasia, that was sustained in OSCCs. These findings were validated by Western blotting. Using Co-immunoprecipitation, we demonstrated that 14-3-3ζ protein binds to NFκB, β-catenin and Bcl-2, suggesting its involvement in cellular signaling, leading to proliferation of oral cancer cells.

Conclusion

Our findings suggest that over-expression of 14-3-3ζ is an early event in oral tumorigenesis and may have an important role in its development and progression. Thus, 14-3-3ζ may serve as an important molecular target for designing novel therapy for oral cancer.

Growth factor signalling in endocrine and anti-growth factor resistant breast cancer

Growth factors provide powerful mitogenic and survival signals to breast cancer cells and it is therefore not surprising that they are able to subvert inhibitory responses to anti-hormonal drugs. In this review we discuss several mechanisms by which this may be achieved and expand our observations to encompass recently emerging anti-growth factor treatments. The information presented is underpinned by inhibitor studies that show the targeting of such mechanisms in advance of anti-hormone or anti-growth factor resistance development is able to substantially delay this event, thus pointing the way forward to intelligent combination therapies relevant to the future management of breast cancer.

Toxoplasma gondii: effect of infection on expression of 14-3-3 proteins in human epithelial cells

14-3-3 Proteins are expressed in most eukaryotes organisms and play varied and crucial roles in a wide range of regulatory processes. In mammalian cells, seven 14-3-3 isoforms have been identified. However, it is not known what effect infection has on 14-3-3 isoform expression. In this study human colonic carcinoma cell lines were infected with Toxoplasma gondii for 24h and expression of 14-3-3 proteins was determined by RT-PCR. HT-29 cells only expressed 3 out of the 7 isoforms while 5 and all 7 isoforms were found in HCT-116 and Caco-2 cells, respectively. Infection had little or no effect in the expression of 14-3-3gamma, epsilon, sigma, and xi; but in HCT-116 cells induced expression of 14-3-3eta and sigma, while 14-3-3beta, eta, and xi were induced in HT-29 cells. If 14-3-3 proteins are involved in cell survival and/or prevention of parasite replication, longer incubation times may be required as no differences in percentage of infection were found among the cell lines at 24h post-infection.

Tuning cell cycle regulation with an iron key

14-3-3 proteins are ubiquitously expressed dimeric adaptor proteins that have emerged as key mediators of many cell signaling pathways in multiple cell types. Its effects are mainly mediated by binding to selective phosphoserine/threonine proteins. The importance of 14-3-3 proteins in cancer have only started to become apparent and its exact role in cancer progression as well as the mechanisms by which 14-3-3 proteins mediate cancer cell function remain unknown. While protein 14-3-3σ is widely accepted as a tumor suppressor, 14-3-3ζ, β and γ isoforms have been shown to have tumor promoting effects. Despite the importance of 14-3-3 family in mediating various cell processes, the exact role and mechanism of 14-3-3ζ remain unexplored. In the current study, we investigated the role of protein 14-3-3ζ in prostate cancer cell motility and transendothelial migration using biochemical, molecular biology and electric cell-substrate impedance sensing approaches as well as cell based functional assays. Our study indicated that expression with wild-type protein 14-3-3ζ significantly enhanced Rac activity in PC3 cells. In contrast, expression of dimer-resistant mutant of protein 14-3-3ζ (DM-14-3-3) inhibited Rac activity and associated phosphorylation of p21 activated kinase-1 and 2. Expression with wild-type 14-3-3ζ or constitutively active Rac1 enhanced extracellular matrix recognition, lamellipodia formation, cell migration and trans-endothelial migration by PC3 cells. In contrast, expression with DM 14-3-3ζ or DN-Rac1 in PC3 cells significantly inhibited these cell functions. Our results demonstrate for the first time that 14-3-3ζ enhances prostate cancer cell-matrix interactions, motility and transendothelial migration in vitro via activation of Rac1-GTPase and is an important target for therapeutic interventions for prostate cancer.

Structural basis for protein-protein interactions in the 14-3-3 protein family

The seven members of the human 14-3-3 protein family regulate a diverse range of cell signaling pathways by formation of protein-protein complexes with signaling proteins that contain phosphorylated Ser/Thr residues within specific sequence motifs. Previously, crystal structures of three 14-3-3 isoforms (zeta, sigma, and tau) have been reported, with structural data for two isoforms deposited in the Protein Data Bank (zeta and sigma). In this study, we provide structural detail for five 14-3-3 isoforms bound to ligands, providing structural coverage for all isoforms of a human protein family. A comparative structural analysis of the seven 14-3-3 proteins revealed specificity determinants for binding of phosphopeptides in a specific orientation, target domain interaction surfaces and flexible adaptation of 14-3-3 proteins through domain movements. Specifically, the structures of the beta isoform in its apo and peptide bound forms showed that its binding site can exhibit structural flexibility to facilitate binding of its protein and peptide partners. In addition, the complex of 14-3-3 beta with the exoenzyme S peptide displayed a secondary structural element in the 14-3-3 peptide binding groove. These results show that the 14-3-3 proteins are adaptable structures in which internal flexibility is likely to facilitate recognition and binding of their interaction partners.

Dynamic 14-3-3/client protein interactions integrate survival and apoptotic pathways

The serine/threonine binding protein, 14-3-3, possesses a diverse array of client proteins. It is involved in the regulation of apoptosis through multiple interactions with proteins of the core mitochondrial machinery, pro-apoptotic transcription factors, and their upstream signaling pathways. 14-3-3 coordinates with survival kinases to inhibit multiple pro-apoptotic molecules. One prominent mechanism for the suppression of apoptosis is through 14-3-3-mediated sequestration of pro-apoptotic client proteins. On the other hand, cellular stresses appear to signal through the inhibition of 14-3-3 function to exert their pro-apoptotic effect. Global inhibition of 14-3-3/client protein interaction induces apoptosis, and stands as an attractive intervention in diseases involving overactive survival signaling pathways. Because dysregulation of 14-3-3 has been associated with poor survival of cancer patients, targeting 14-3-3 may provide a novel therapeutic approach for the treatment of cancer.

14-3-3 proteins as potential oncogenes

14-3-3 proteins are a family of highly conserved cellular proteins that play key roles in the regulation of central physiological pathways. More than 200 14-3-3 target proteins have been identified, including proteins involved in mitogenic and cell survival signaling, cell cycle control and apoptotic cell death. Importantly, the involvement of 14-3-3 proteins in the regulation of various oncogenes and tumor suppressor genes points to a potential role in human cancer. The present review summarizes current findings implicating a 14-3-3 role in cancer while discussing potential mechanisms and points of action of 14-3-3 during cancer development and progression.

Epigenetic silencing of 14-3-3sigma in cancer

The 14-3-3sigma gene is a direct target of the p53 tumor suppressor and its product inhibits cell cycle progression. Recently, a proteomic analysis revealed that 14-3-3sigma regulates additional cellular processes relevant to carcinogenesis, as migration and MAP-kinase signalling. The expression of 14-3-3sigma is down-regulated by CpG methylation in several types of human cancer, among them prostate, lung, breast and several types of skin cancer. The epigenetic inactivation of 14-3-3sigma occurs at an early stage of tumor development and may allow evasion from senescence and promote genomic instability. In the future the detection of CpG methylation of 14-3-3sigma may be used for diagnostic and prognostic purposes.

UPREGULATION OF 14-3-3 ISOFORMS IN ACUTE RAT MYOCARDIAL INJURIES INDUCED BY BURN AND LIPOPOLYSACCHARIDE

1. Burn-induced myocardial injuries can be acute due to loss of body fluid and blood redistribution, and subacute due to pathogenic toxins of infecting bacteria. The goal of this study was to examine expression of 14-3-3 in the injured myocardium. 2. Myocardial injury models were created in vivo by subjecting rats to severe burn and administration of lipopolysaccharide. RT-PCR and Western blotting were employed to assess the expression of 14-3-3 proteins and messenger ribonucleic acid (mRNA) for 14-3-3eta and gamma in the myocardium, respectively. 3. In the two models, we found that 14-3-3 proteins were induced in a time-dependent fashion. Such a change is at least in part attributed to increases in mRNAs for 14-3-3gamma and eta. In contrast to 14-3-3xi, whose mRNA was not detectable in the heart, mRNA for 14-3-3gamma was found significantly elevated between 24-48 h after burn. 14-3-3eta mRNA exhibited a marked increase at 3 h continuing to 12 h and then decreased nearly to a normal level after 48 h. In lipopolysaccharide-treated intact rats, 14-3-3gamma mRNA in myocardium showed a significant increase, reaching a peak at 4 h, followed by a decrease at 6 h. In contrast, 14-3-3eta mRNA had a slight increase without significance. 4. Our results suggest that 14-3-3 may play a role in both acute and subacute (postburn infectious) phases of severe burn.

14-3-3 proteins in cell cycle regulation

Co-ordinated progression through the cell cycle is essential for the maintenance of genomic integrity. Several checkpoint mechanisms guarantee that the next step in cell cycle progression is only entered after error-free completion of the previous phase. Cell cycle deregulation caused by changes in 14-3-3 expression has been implicated in cancer formation. 14-3-3 proteins function at several key points in G(1)/S- and G(2)/M-transition by binding to regulatory proteins and modulating their function. In most cases, the association with 14-3-3 proteins requires a specific phosphorylation of the protein ligand and mediates cell cycle arrest. 14-3-3 binding may lead to cytoplasmic sequestration of the protein ligand but may also have other functional consequences. The 14-3-3sigma gene is induced by p53 and its product inhibits G(2)/M progression by cytoplasmatic sequestration of CDC2-cyclin B complexes. In addition, 14-3-3 proteins have been implicated in the transcriptional regulation of CDK-inhibitors as they modulate the transcription factors p53, FOXO and MIZ1. Effects of 14-3-3 proteins on cell cycle progression and the regulation of 14-3-3 activity during the cell cycle are reviewed in this chapter.

Sensitization of DNA damage-induced apoptosis by the proteasome inhibitor PS-341 is p53 dependent and involves target proteins 14-3-3sigma and survivin

Proteasome inhibition following DNA damage results in the synergistic induction of apoptosis via a nuclear factor-kappaB-independent mechanism. In this study, we identify the role of p53 in mediating apoptosis by the sequence-specific treatment involving the DNA-damaging, topoisomerase I-targeting drug SN-38 followed by the proteasome inhibitor PS-341 (SN-38-->PS-341). The p53-dependent sensitization of DNA damage-induced apoptosis by PS-341 is accompanied by persistent inhibition of proteasome activity and increased cytosolic accumulation of p53, including higher molecular weight forms likely representing ubiquitinated species. In contrast, pretreatment with PS-341 followed by treatment with SN-38 (PS-341-->SN-38), which leads to an antagonistic interaction, results in transient inhibition of proteasome activity and accumulation of significantly lower levels of p53 localized primarily to the nucleus. Whereas cells treated with PS-341-->SN-38 undergo G2 + M cell cycle arrest, cells treated with SN-38-->PS-341 exhibit a decreased G2 + M block with a concomitant increase in the sub-G1 population. Decreased accumulation of cells in the G2 + M phase of the cell cycle in SN-38-->PS-341-treated cells compared with PS-341-->SN-38-treated cells correlates with enhanced apoptosis and reduced expression of two p53-modulated proteins, 14-3-3sigma and survivin, both of which play critical roles in regulating G2 + M progression and apoptosis. The functional role of 14-3-3sigma or survivin in regulating the divergent function of p53 in response to SN-38-->PS-341 and PS-341-->SN-38 treatment in inducing apoptosis versus G2 + M arrest/DNA repair, respectively, was confirmed by targeted down-regulation of these proteins. These results provide insights into the mechanisms by which inhibition of proteasome activity modulates DNA damage-induced apoptosis via a p53-dependent pathway.

Human Lymphoblastoid Proteome Analysis Reveals a Role for the Inhibitor of Acetyltransferases Complex in DNA Double-Strand Break Response

A DNA double-strand break (DSB) is highly cytotoxic; it emerges as the type of DNA damage that most severely affects the genomic integrity of the cell. It is essential that DNA DSBs are recognized and repaired efficiently, in particular, prior to mitosis, to prevent genomic instability and eventually, the development of cancer. To assess the pathways that are induced on DNA DSBs, 14 human lymphoblastoid cell lines were challenged with bleomycin for 30 and 240 minutes to establish the fast and more prolonged response, respectively. The proteomes of 14 lymphoblastoid cell lines were investigated to account for the variation among individuals. The primary DNA DSB response was expected to occur within the nucleus; therefore, the nuclear extracts were considered. Differential analysis was done using two-dimensional difference in gel electrophoresis; paired ANOVA statistics were used to recognize significant changes in time. Many proteins whose nuclear levels changed statistically significantly showed a fast response, i.e., within 30 minutes after bleomycin challenge. A significant number of these proteins could be assigned to known DNA DSB response processes, such as sensing DSBs (Ku70), DNA repair through effectors (high-mobility group protein 1), or cell cycle arrest at the G(2)-M phase checkpoint (14-3-3 zeta). Interestingly, the nuclear levels of all three proteins in the INHAT complex were reduced after 30 minutes of bleomycin challenge, suggesting that this complex may have a role in changing the chromatin structure, allowing the DNA repair enzymes to gain access to the DNA lesions.

14-3-3 proteins—an update

14-3-3 is a highly conserved acidic protein family, composed of seven isoforms in mammals. 14-3-3 protein can interact with over 200 target proteins by phosphoserine-dependent and phosphoserine-independent manners. Little is known about the consequences of these interactions, and thus are the subjects of ongoing studies. 14-3-3 controls cell cycle, cell growth, differentiation, survival, apoptosis, migration and spreading. Recent studies have revealed new mechanisms and new functions of 14-3-3, giving us more insights on this fascinating and complex family of proteins. Of all the seven isoforms, 14-3-3sigma seems to be directly involved in human cancer. 14-3-3sigma itself is subject to regulation by p53 upon DNA damage and by epigenetic deregulation. Gene silencing of 14-3-3sigma by CpG methylation has been found in many human cancer types. This suggests that therapy-targeting 14-3-3sigma may be beneficial for future cancer treatment.

Isoform-specific expression of 14-3-3 proteins in human lung cancer tissues

14-3-3 Proteins play important roles in a wide range of vital regulatory processes, including signal transduction, apoptosis, cell cycle progression and DNA replication. In mammalian cells, 7 14-3-3 isoforms (beta, gamma, epsilon, eta, sigma, theta and zeta) have been identified and each of these seems to have distinct tissue localizations and isoform-specific functions. Previous studies have shown that 14-3-3 protein levels are higher in human lung cancers as compared to normal tissues. It is unclear, however, which of the 14-3-3 isoform(s) are overexpressed in these cancers. In our study, the levels of all seven 14-3-3 isoforms were examined by RT-PCR and Western blotting. We show that the message for only two isoforms, 14-3-3epsilon and zeta, could be detected in normal tissues. In lung cancer biopsies, however, four isoforms, 14-3-3beta, gamma, sigma, and theta;, in addition to 14-3-3epsilon and zeta, were present in abundance. The expression frequency of 14-3-3beta, gamma, sigma and theta; isoforms was 11, 10, 13 and 8 of the 14 biopsies examined, respectively. The data from immunohistochemical staining and Western blotting were consistent with the RT-PCR results. Given the prevalence of elevated 14-3-3 expression in human lung cancers we propose that these proteins may be involved in lung cancer tumorigenesis and that specific 14-3-3 proteins may be useful as markers for lung cancer diagnosis and targets for therapy.

G2 checkpoint abrogators as anticancer drugs

Many conventional anticancer treatments kill cells irrespective of whether they are normal or cancerous, so patients suffer from adverse side effects due to the loss of healthy cells. Anticancer insights derived from cell cycle research has given birth to the idea of cell cycle G2 checkpoint abrogation as a cancer cell specific therapy, based on the discovery that many cancer cells have a defective G1 checkpoint resulting in a dependence on the G2 checkpoint during cell replication.

Damaged DNA in humans is detected by sensor proteins (such as hHUS1, hRAD1, hRAD9, hRAD17, and hRAD26) that transmit a signal via ATR to CHK1, or by another sensor complex (that may include γH2AX, 53BP1, BRCA1, NBS1, hMRE11, and hRAD50), the signal of which is relayed by ATM to CHK2. Most of the damage signals originated by the sensor complexes for the G2 checkpoint are conducted to CDC25C, the activity of which is modulated by 14-3-3. There are also less extensively explored pathways involving p53, p38, PCNA, HDAC, PP2A, PLK1, WEE1, CDC25B, and CDC25A.

This review will examine the available inhibitors of CHK1 (Staurosporin, UCN-01, Go6976, SB-218078, ICP-1, and CEP-3891), both CHK1 and CHK2 (TAT-S216A and debromohymenialdisine), CHK2 (CEP-6367), WEE1 (PD0166285), and PP2A (okadaic acid and fostriecin), as well as the unknown checkpoint inhibitors 13-hydroxy-15-ozoapathin and the isogranulatimides. Among these targets, CHK1 seems to be the most suitable target for therapeutic G2 abrogation to date, although an unexplored target such as 14-3-3 or the strategy of targeting multiple proteins at once may be of interest in the future.