Transcriptional coactivator PC4, a chromatin-associated protein, induces chromatin condensation

Periodic changes of cyclin D1 mRNA stability are regulated by PC4 modifications in the cell cycle

The cell cycle is a highly regulated process in which proteins involved in cell cycle progression exhibit periodic expression patterns, controlled by specific mechanisms such as transcription, translation, and degradation. However, the precise mechanisms underlying the oscillations of mRNA levels in cell cycle regulators are not fully understood. In this study, we observed that the stability of cyclin D1 (CCND1) mRNA fluctuates during the cell cycle, with increased stability during interphase and decreased stability during the M phase. Additionally, we identified a key RNA binding protein, positive coactivator 4 (PC4), which plays a crucial role in stabilizing CCND1 mRNA and regulating its periodic expression. Moreover, the binding affinity of PC4 to CCND1 mRNA is modulated by two cell cycle-specific posttranslational modifications: ubiquitination of K68 enhances binding and stabilizes the CCND1 transcript during interphase, while phosphorylation of S17 inhibits binding during the M phase, leading to degradation of CCND1 mRNA. Remarkably, PC4 promotes the transition from G1 to S phase in the cell cycle, and depletion of PC4 enhances the efficacy of CDK4/6 inhibitors in hepatocellular carcinoma, suggesting that PC4 could serve as a potential therapeutic target. These findings provide valuable insights into the intricate regulation of cell cycle dynamics.

Human inborn errors of immunity associated with IRF4

The transcription factor interferon regulatory factor 4 (IRF4) belongs to the IRF family and has several important functions for the adaptive immune response. Mutations affecting IRF family members IRF1, IRF3, IRF7, IRF8, or IRF9 have been described in patients presenting with inborn errors of immunity (IEI) highlighting the importance of these factors for the cellular host defense against mycobacterial and/or viral infections. IRF4 deficiency and haploinsufficiency have been associated with IEI. More recently, two novel IRF4 disease-causing mechanisms have been described due to the characterization of IEI patients presenting with cellular immunodeficiency associated with agammaglobulinemia. Here, we review the phenotypes and physiopathological mechanisms underlying IEI of IRF family members and, in particular, IRF4.

Targeting positive cofactor 4 induces autophagic cell death in MYC-expressing diffuse large B-cell lymphoma

MYC-expressing diffuse large B-cell lymphoma (DLBCL) is one of the refractory lymphomas. Currently, the pathogenesis of MYC-expressing DLBCL is still unclear, and there is a lack of effective therapy. We characterized positive cofactor 4 (PC4) as an upstream regulator of c-Myc, and PC4 is overexpressed in DLBCL and is closely related to clinical staging, prognosis, and c-Myc expression. Furthermore, our in vivo and in vitro studies revealed that PC4 knockdown can induce autophagic cell death and enhance the therapeutic effect of doxorubicin in MYC-expressing DLBCL. Inhibition of c-Myc-mediated aerobic glycolysis and activation of the AMPK/mTOR signaling pathway are responsible for the autophagic cell death induced by PC4 knockdown in MYC-expressing DLBCL. Using dual-luciferase reporter assay and electrophoretic mobility shift assay assays, we also found that PC4 exerts its oncogenic functions by directly binding to c-Myc promoters. To sum up, our study provides novel insights into the functions and mechanisms of PC4 in MYC-expressing DLBCL and suggests that PC4 may be a promising therapeutic target for MYC-expressing DLBCL.

The Catalytic Subunit of Schizosaccharomyces pombe CK2 (Cka1) Negatively Regulates RNA Polymerase II Transcription through Phosphorylation of Positive Cofactor 4 (PC4)

Positive cofactor 4 (PC4) is a transcriptional coactivator that plays important roles in transcription and DNA replication. In mammals, PC4 is phosphorylated by CK2, and this event downregulates its RNA polymerase II (RNAPII) coactivator function. This work describes the effect of fission yeast PC4 phosphorylation on RNAPII transcription in a cell extract, which closely resembles the cellular context. We found that fission yeast PC4 is strongly phosphorylated by the catalytic subunit of CK2 (Cka1), while the regulatory subunit (Ckb1) downregulates the PC4 phosphorylation. The addition of Cka1 to an in vitro transcription assay can diminish the basal transcription from the Ad-MLP promoter; however, the addition of recombinant fission yeast PC4 or Ckb1 can stimulate the basal transcription in a cell extract. Fission yeast PC4 is phosphorylated in a domain which has consensus phosphorylation sites for CK2, and two serine residues were identified as critical for CK2 phosphorylation. Mutation of one of the serine residues in PC4 does not completely abolish the phosphorylation; however, when the two serine residues are mutated, CK2 is no longer able to phosphorylate PC4. The mutant which is not phosphorylated is able to stimulate transcription even though it is previously phosphorylated by Cka1, while the wild type and the point mutant are inactivated by Cka1 phosphorylation, and they cannot stimulate transcription by RNAPII in cell extracts. Those results demonstrate that CK2 can regulate the coactivator function of fission yeast PC4 and suggests that this event could be important in vivo as well.

Peptide and protein chemistry approaches to study the tumor suppressor protein p53

The tumor suppressor and master gene regulator protein p53 has been the subject of intense investigation for several decades due to its mutation in about half of all human cancers. However, mechanistic studies of p53 in cells are complicated by its many dynamic binding partners and heterogeneous post-translational modifications. The design of therapeutics that rescue p53 functions in cells requires a mechanistic understanding of its protein-protein interactions in specific protein complexes and identifying changes in p53 activity by diverse post-translational modifications. This review highlights the important roles that peptide and protein chemistry have played in biophysical and biochemical studies aimed at elucidating p53 regulation by several key binding partners. The design of various peptide inhibitors that rescue p53 function in cells and new opportunities in targeting p53-protein interactions are discussed. In addition, the review highlights the importance of a protein semisynthesis approach to comprehend the role of site-specific PTMs in p53 regulation.

Phosphorylation-dependent association of human chromatin protein PC4 to linker histone H1 regulates genome organization and transcription

Human Positive Coactivator 4 (PC4) is a multifaceted chromatin protein involved in diverse cellular processes including genome organization, transcription regulation, replication, DNA repair and autophagy. PC4 exists as a phospho-protein in cells which impinges on its acetylation by p300 and thereby affects its transcriptional co-activator functions via double-stranded DNA binding. Despite the inhibitory effects, the abundance of phosphorylated PC4 in cells intrigued us to investigate its role in chromatin functions in a basal state of the cell. We found that casein kinase-II (CKII)-mediated phosphorylation of PC4 is critical for its interaction with linker histone H1. By employing analytical ultracentrifugation and electron microscopy imaging of in vitro reconstituted nucleosomal array, we observed that phospho-mimic (PM) PC4 displays a superior chromatin condensation potential in conjunction with linker histone H1. ATAC-sequencing further unveiled the role of PC4 phosphorylation to be critical in inducing chromatin compaction of a wide array of coding and non-coding genes in vivo. Concordantly, phospho-PC4 mediated changes in chromatin accessibility led to gene repression and affected global histone modifications. We propose that the abundance of PC4 in its phosphorylated state contributes to genome compaction contrary to its co-activator function in driving several cellular processes like gene transcription and autophagy.

Identification, characterization and expression analyses of PC4 genes in Yesso scallop (Patinopecten yessoensis) reveal functional differentiations in response to ocean acidification

Transcriptional coactivator p15 (PC4), considered a multifunctional chromosome associated protein, is actively involved in transcription regulation, DNA replication, damage repair and chromosome formation. Although studies have reported significant effects of PC4 in most vertebrates and some invertebrates, the complete PC4 gene members are less systematically identified and characterized in scallops. In this study, seven PC4 genes (PyPC4s) were identified in the Yesso scallop Patinopecten yessoensis using whole-genome scanning via bioinformatic analyses. Phylogenetic and protein structural analyses were performed to determine the identities and evolutionary relationships of the seven genes. Expression profiles of PyPC4s were further investigated in embryos/larvae at all developmental stages, healthy adult tissues, and mantles that were exposed to low pH stress (pH 6.5 and 7.5) with different time durations (3, 6, 12 and 24 h). Spatiotemporal expression patterns indicated the functional roles of PyPC4s at all development stages and in healthy adult tissues, with PY-3235.33 demonstrating remarkably high constitutive expressions. Expression regulations (up- and down-regulation) of PyPC4s under low pH stress levels demonstrated a time-dependent pattern with functional complementation and/or enhancement, revealing that PyPC4s exhibited differentiated functions in response to ocean acidification (OA). Collectively, our data offer a novel perspective stating that low pH is a potential inducer leading to functional differentiation of PyPC4s in scallops. The results provide preliminary information on the versatile roles of PC4(s) in bivalves in response to OA.

Genome-Wide Association Study Reveals PC4 as the Candidate Gene for Thermal Tolerance in Bay Scallop (Argopecten irradians irradians)

The increasing sea temperature caused by global warming has resulted in severe mortalities in maricultural scallops. Therefore, improving thermal tolerance has become an active research area in the scallop farming industry. Bay scallop (Argopecten irradians irradians) was introduced into China in 1982 and has developed into a vast aquaculture industry in northern China. To date, genetic studies on thermal tolerance in bay scallops are limited, and no systematic screening of thermal tolerance-related loci or genes has been conducted in this species. In the present study, we conducted a genome-wide association study (GWAS) for thermal tolerance using the Arrhenius break temperature (ABT) indicators of 435 bay scallops and 38,011 single nucleotide polymorphism (SNP) markers. The GWAS identified 1,906 significant thermal tolerance-associated SNPs located in 16 chromosomes of bay scallop. Gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses showed that 638 genes were enriched in 42 GO terms, while 549 annotated genes were enriched in aggregation pathways. Additionally, the SNP (15-5091-20379557-1) with the lowest P value was located in the transcriptional coactivator p15 (PC4) gene, which is involved in regulating DNA damage repair and stabilizing genome functions. Further analysis in another population identified two new thermal tolerance-associated SNPs in the first coding sequence of PC4 in bay scallops (AiPC4). Moreover, AiPC4 expression levels were significantly correlated (r = 0.675–0.962; P < 0.05) with the ABT values of the examined bay scallops. Our data suggest that AiPC4 might be a positive regulator of thermal tolerance and a potential candidate gene for molecular breeding in bay scallop aiming at thermal tolerance improvement.

Protocol for in vitro BCR-mediated plasma cell differentiation and purification of chromatin-associated proteins

Molecular-level understanding of plasma cell (PC) differentiation has been modeled using lipopolysaccharide (LPS) stimulation in vitro. However, this system does not involve the B-cell receptor (BCR)-a critical component of B cell biology. Here, we present a protocol for in vitro PC differentiation system dependent on BCR signaling that easily scales up for cell number-demanding applications, including protein complex purification. We describe how to set up this system and detail applications for endogenous complex purification of chromatin-associated proteins. For further details on the use and execution of this protocol, please refer to Sciammas et al. (2011) and Ochiai et al. (2018, 2020).

Acceleration of ageing via disturbing mTOR-regulated proteostasis by a new ageing-associated gene PC4

Research on ageing‐associated genes is important for investigating ageing and anti‐ageing strategies. Here, we firstly reported that the human positive cofactor 4 (PC4), a multifunctional and highly conserved nucleoprotein, is accumulated and activated during ageing and causes global accelerated ageing process by disrupting proteostasis. Mechanistically, PC4 interacts with Sin3‐HDAC complex and inhibits its deacetylated activity, leads to hyper‐acetylation of the histones at the promoters of mTOR‐related genes and causes mTOR signalling activation. Accordingly, mTOR activation causes excessive protein synthesis, resulting in impaired proteostasis and accelerated senescence. These results reveal a new biological function of PC4 in vivo, recognizes PC4 as a new ageing‐associated gene and provides a genetically engineered mouse model to simulate natural ageing. More importantly, our findings also indicate that PC4 is involved in histone acetylation and serves as a potential target to improve proteostasis and delay ageing.

Knockdown of PC4 increases chemosensitivity of Oxaliplatin in triple negative breast cancer by suppressing mTOR pathway

As a multifunctional nuclear protein, the human positive cofactor 4 (PC4) is highly expressed in various tumors including breast cancer and has potential roles in cancer development and progression. However, the functional signatures and molecular mechanisms of PC4 in triple negative breast cancer (TNBC) progression and chemotherapeutic response are still unknown. In this study, we found that PC4 is significantly upregulated in TNBC cells compared with non-TNBC cells, implying its potential role in TNBC. Then, in vivo and in vitro studies revealed that knockdown of PC4 increased chemosensitivity of Oxaliplation (Oxa) in TNBC by suppressing mTOR pathway. Therefore, our findings demonstrated the signatures and molecular mechanisms of PC4 in TNBC chemotherapeutic response, and indicated that PC4 might be a promising therapeutic target for TNBC.

Chromatin Protein PC4 Orchestrates B Cell Differentiation by Collaborating with IKAROS and IRF4

The chromatin protein positive coactivator 4 (PC4) has multiple functions, including chromatin compaction. However, its role in immune cells is largely unknown. We show that PC4 orchestrates chromatin structure and gene expression in mature B cells. B-cell-specific PC4-deficient mice show impaired production of antibody upon antigen stimulation. The PC4 complex purified from B cells contains the transcription factors (TFs) IKAROS and IRF4. IKAROS protein is reduced in PC4-deficient mature B cells, resulting in de-repression of their target genes in part by diminished interactions with gene-silencing components. Upon activation, the amount of IRF4 protein is not increased in PC4-deficient B cells, resulting in reduction of plasma cells. Importantly, IRF4 reciprocally induces PC4 expression via a super-enhancer. PC4 knockdown in human B cell lymphoma and myeloma cells reduces IKAROS protein as an anticancer drug, lenalidomide. Our findings establish PC4 as a chromatin regulator of B cells and a possible therapeutic target adjoining IKAROS in B cell malignancies.

Interaction of ASOs with PC4 Is Highly Influenced by the Cellular Environment and ASO Chemistry

The activity of PS-ASOs is strongly influenced by association with both inter- and intracellular proteins. The sequence, chemical nature, and structure of the ASO can have profound influences on the interaction of PS-ASOs with specific proteins. A more thorough understanding of how these pharmacological agents interact with various proteins and how chemical modifications, sequence, and structure influence interactions with proteins is needed to inform future ASO design efforts. To better understand the chemistry of PS-ASO interactions, we have focused on human positive cofactor 4 (PC4). Although several studies have investigated the in vitro binding properties of PC4 with endogenous nucleic acids, little is known about the chemistry of interaction of PS-ASOs with this protein. Here we examine in detail the impact of ASO backbone chemistry, 2'-modifications, and buffer environment on the binding affinity of PC4. In addition, using site-directed mutagenesis, we identify those amino acids that are specifically required for ASO binding interactions, and by substitution of abasic nucleotides we identify the positions on the ASO that most strongly influence affinity for PC4. Finally, to confirm that the interactions observed in vitro are biologically relevant, we use a recently developed complementation reporter system to evaluate the kinetics and subcellular localization of the interaction of ASO and PC4 in live cells.

PC4 serves as a negative regulator of skin wound healing in mice

Background

Human positive cofactor 4 (PC4) was initially characterized as a multifunctional transcriptional cofactor, but its role in skin wound healing is still unclear. The purpose of this study was to explore the role of PC4 in skin wound healing through PC4 knock-in mouse model.

Methods

A PC4 knock-in mouse model (PC4^+/+) with a dorsal full-thickness wound was used to investigate the biological functions of PC4 in skin wound healing. Quantitative PCR, Western blot analysis and immunohistochemistry were performed to evaluate the expression of PC4; Sirius red staining and immunofluorescence were performed to explore the change of collagen deposition and angiogenesis. Proliferation and apoptosis were detected using Ki67 staining and TUNEL assay. Primary dermal fibroblasts were isolated from mouse skin to perform cell scratch experiments, cck-8 assay and colony formation assay.

Results

The PC4^+/+ mice were fertile and did not display overt abnormalities but showed an obvious delay in cutaneous healing of dorsal skin. Histological staining showed insufficient re-epithelialization, decreased angiogenesis and collagen deposition, increased apoptosis and decreased cell proliferation in PC4^+/+ skin. Our data also showed decreased migration rate and proliferation ability in cultured primary fibroblasts from PC4^+/+ mice in vitro.

Conclusions

This study suggests that PC4 might serve as a negative regulator of skin wound healing in mice.

Regulation of epigenetic state by non-histone chromatin proteins and transcription factors: Implications in disease

Besides the fundamental components of the chromatin, DNA and octameric histone, the non-histone chromatin proteins and non-coding RNA play a critical role in the organization of functional chromatin domains. The non-histone chromatin proteins therefore regulate the transcriptional outcome in both physiological and pathophysiological state as well. They also help to maintain the epigenetic state of the genome indirectly. Several transcription factors and histone interacting factors also contribute in the maintenance of the epigenetic states, especially acetylation by the induction of autoacetylation ability of p300/CBP. Alterations of KAT activity have been found to be causally related to disease manifestation, and thus could be potential therapeutic target.

Chromatin protein PC4 is downregulated in breast cancer to promote disease progression: Implications of miR-29a

The human transcriptional coactivator PC4 has numerous roles to play in the cell. Other than its transcriptional coactivation function, it facilitates chromatin organization, DNA damage repair, viral DNA replication, etc. Although it was found to be an essential protein in vivo, the importance of this multifunctional protein in the regulation of different cellular pathways has not been investigated in details, particularly in oncogenesis. In this study, PC4 downregulation was observed in a significant proportion of mammary tissues obtained from Breast cancer patient samples as well as in a subset of highly invasive and metastatic Breast cancer patient-derived cell lines. We have identified a miRNA, miR-29a which potentially reduce the expression of PC4 both in RNA and protein level. This miR-29a was found to be indeed overexpressed in a substantial number of Breast cancer patient samples and cell lines as well, suggesting one of the key mechanisms of PC4 downregulation. Stable Knockdown of PC4 in MCF7 cells induced its migratory as well as invasive properties. Furthermore, in an orthotopic breast cancer mice model system; we have shown that reduced expression of PC4 enhances the tumorigenic potential substantially. Absence of PC4 led to the upregulation of several genes involved in Epithelial to Mesenchymal Transition (EMT), indicating the possible mechanism of uniform tumour progression in the orthotropic mice. Collectively these data establish the role of PC4 in tumour suppression.

Multifunctional transcriptional coactivator PC4 is a global co-regulator of p53-dependent stress response and gene regulation

Human positive coactivator 4 (PC4), a multifunctional chromatin-associated protein, is known to directly interact with p53 and modulate expressions of a few p53-dependent genes. However, the role of PC4 in p53's myriad of other regulatory functions is not known. The p53-PC4 interaction was selectively perturbed by a small peptide which led to abrogation of genotoxic stress-induced up-regulation of many p53-dependent genes and reduction of apoptosis in A549 cells. Over-expression of a PC4 point mutant, incapable of binding p53, recapitulated many of the effects of the peptide. Global gene expression profiling in A549 cells, upon peptide treatment, revealed PC4's involvement in the regulation of many p53-dependent pathways, including the Hippo pathway. Introduction of the peptide in neuronal cells significantly reduced its amyloid-β-induced death. Thus, PC4 emerges as a global co-regulator of p53 and a therapeutic target against pathogeneses where the p53-dependent cell death process plays a crucial role.

Nonhistone human chromatin protein PC4 is critical for genomic integrity and negatively regulates autophagy

Multifunctional human transcriptional positive co-activator 4 (PC4) is a bona fide nonhistone component of the chromatin and plays a pivotal role in the process of chromatin compaction and functional genome organization. Knockdown of PC4 expression causes a drastic decompaction which leads to open conformation of the chromatin, and thereby altered nuclear architecture, defects in chromosome segregation and changed epigenetic landscape. Interestingly, these defects do not induce cellular death but result in enhanced cellular proliferation, possibly through enhanced autophagic activity. Moreover, PC4 depletion confers significant resistance to gamma irradiation. Exposure to gamma irradiation further induced autophagy in these cells. Inhibition of autophagy by small molecule inhibitors as well as by silencing of a critical autophagy gene drastically reduces the ability of PC4 knockdown cells to survive. On the contrary, complementation with wild-type PC4 could reverse this phenomenon, confirming the process of autophagy as the key mechanism for radiation resistance in the absence of PC4. These data connect the unexplored role of chromatin architecture in regulating autophagy during stress conditions such as radiation.

Transcriptional positive cofactor 4 promotes breast cancer proliferation and metastasis through c-Myc mediated Warburg effect

Background

The human positive cofactor 4 (PC4) is initially identified as a transcriptional cofactor and has an important role in embryonic development and malignant transformation. However, the clinical significance and the molecular mechanisms of PC4 in breast cancer development and progression are still unknown.

Methods

We investigated PC4 expression in 114 cases of primary breast cancer and matched normal breast tissue specimens, and studied the impact of PC4 expression as well as the molecular mechanisms of this altered expression on breast cancer growth and metastasis both in vitro and in vivo.

Results

PC4 was significantly upregulated in breast cancer and high PC4 expression was positively correlated with metastasis and poor prognosis of patients. Gene set enrichment analysis (GSEA) demonstrated that the gene sets of cell proliferation and Epithelial-Mesenchymal Transition (EMT) were positively correlated with elevated PC4 expression. Consistently, loss of PC4 markedly inhibited the growth and metastasis of breast cancer both in vitro and in vivo. Mechanistically, PC4 exerted its oncogenic functions by directly binding to c-Myc promoters and inducing Warburg effect.

Conclusions

Our study reveals for the first time that PC4 promotes breast cancer progression by directly regulating c-Myc transcription to promote Warburg effect, implying a novel therapeutic target for breast cancer.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0348-0) contains supplementary material, which is available to authorized users.

The human positive cofactor 4 promotes androgen-independent prostate cancer development and progression through HIF-1α/β-catenin pathway

Androgen-dependent prostate cancer (ADPC) eventually progresses to androgen-independent prostate cancer (AIPC), that has a poor prognosis owing to its unclear mechanism and lack of effective therapeutic targets. The human positive cofactor 4 (PC4) is a transcriptional cofactor, and plays a potential role in cancer development. However, the significance and mechanism of PC4 in AIPC progression are unclear. By analyzing the clinical data, we find that PC4 is overexpressed in prostate cancer and closely correlated with the progression, metastasis and prognosis of patients. Additionally, PC4 is significantly upregulated in AIPC cells compared with ADPC cells, implying its importance in the development and progression of AIPC. Then, in vivo and in vitro studies reveal that loss of PC4 inhibits cell growth by suppressing c-Myc/P21 pathway and inducing cell cycle arrest at G1/S phase transition in AIPC. PC4 knockdown also attenuates EMT-mediated metastasis in AIPC. Moreover, for the first time, we find that PC4 exerts its oncogenic functions by promoting the expression of HIF-1α and activating β-catenin signaling. Therefore, our findings determine the signatures and molecular mechanisms of PC4 in AIPC, and indicate that PC4 might be a promising therapeutic target for AIPC.

Interaction of positive coactivator 4 with histone 3.3 protein is essential for transcriptional activation of the luteinizing hormone receptor gene

The luteinizing hormone receptor (LHR) is essential for sexual development and reproduction in mammals. We have established that Sp1 has a central role in derepression of LHR gene transcription induced by Trichostatin A (TSA) in MCF7 cells. Moreover, the co-activator PC4 which associates directly with Sp1 at the LHR promoter is essential for TSA-mediated LHR transcription. This study explores interactions of PC4 with histone proteins, which presumably triggers chromatin modifications during LHR transcriptional activation. TSA treatment of MCF7 cells expressing PC4-Flag protein induces acetylation of histone 3 (H3) and immunoprecipitation (IP) studies revealed its interaction with PC4-Flag protein. MS/MS analysis of the protein complex obtained after IP from TSA treated samples detected H3.3 acetylated at K9, K14, K18, K23 and K27 as a PC4 interacting protein. The association of PC4 with H3.3 was corroborated by IP and re-ChIP using H3.3 antibody. Similarly, IP and re-ChIP showed association of PC4 with H3 acetylated protein. Knockdown of PC4 in MCF7 cells reduced H3.3 enrichment, H3 acetylation at the Lys sites and LHR promoter activity in TSA treated cells despite an increase in H3 and H3.3 protein induced by TSA, linking PC4 to H3 acetylation and LHR transcription. Depletion of H3.3 A/B in MCF7 cells impair chromatin accessibility and enrichment of Pol II and TFIIB at the LHR promoter and its activation, resulting in marked reduction of LHR gene expression. Together, these findings point to the critical role of PC4 and its association with acetylated H3.3 in TSA-induced LHR gene transcription.

Life and Death of mRNA Molecules in Entamoeba histolytica

In eukaryotic cells, the life cycle of mRNA molecules is modulated in response to environmental signals and cell-cell communication in order to support cellular homeostasis. Capping, splicing and polyadenylation in the nucleus lead to the formation of transcripts that are suitable for translation in cytoplasm, until mRNA decay occurs in P-bodies. Although pre-mRNA processing and degradation mechanisms have usually been studied separately, they occur simultaneously and in a coordinated manner through protein-protein interactions, maintaining the integrity of gene expression. In the past few years, the availability of the genome sequence of Entamoeba histolytica, the protozoan parasite responsible for human amoebiasis, coupled to the development of the so-called “omics” technologies provided new opportunities for the study of mRNA processing and turnover in this pathogen. Here, we review the current knowledge about the molecular basis for splicing, 3′ end formation and mRNA degradation in amoeba, which suggest the conservation of events related to mRNA life throughout evolution. We also present the functional characterization of some key proteins and describe some interactions that indicate the relevance of cooperative regulatory events for gene expression in this human parasite.

Nuclear fragments of the neural cell adhesion molecule NCAM with or without polysialic acid differentially regulate gene expression

The neural cell adhesion molecule (NCAM) is the major carrier of polysialic acid (PSA) which modulates NCAM functions of neural cells at the cell surface. In previous studies, we have shown that stimulation of cultured neurons with surrogate NCAM ligands leads to the generation and nuclear import of PSA-lacking and -carrying NCAM fragments. Here, we show that the nuclear import of the PSA-carrying NCAM fragment is mediated by positive cofactor 4 and cofilin, which we identified as novel PSA-binding proteins. In the nucleus, the PSA-carrying NCAM fragment interacts via PSA with PC4 and cofilin, which are involved in RNA polymerase II-dependent transcription. Microarray analysis revealed that the nuclear PSA-carrying and -lacking NCAM fragments affect expression of different genes. By qPCR and immunoblot analysis we verified that the nuclear PSA-carrying NCAM fragment increases mRNA and protein expression of nuclear receptor subfamily 2 group F member 6, whereas the PSA-lacking NCAM fragment increases mRNA and protein expression of low density lipoprotein receptor-related protein 2 and α-synuclein. Differential gene expression evoked by nuclear NCAM fragments without and with PSA indicates that PSA-carrying and -lacking NCAM play different functional roles in the nervous system.

Down regulation of human positive coactivator 4 suppress tumorigenesis and lung metastasis of osteosarcoma

Osteosarcoma is a kind of primary malignant bone tumor with the highest incidence and an extraordinarily poor prognosis and early pulmonary metastasis formation as a frequent occurrence. Transcriptional positive coactivator 4 (PC4) has multiple functions in DNA replication, transcription, repair and chromatin organization, even in tumorigenesis. However, the precise function of PC4 in osteosarcoma is still unclear and controversial. In this paper we found PC4 was upregulated in patient-derived osteosarcoma tissues compared to normal. Moreover, higher expression of PC4 was correlated with poorer overall survival and advanced clinicopathological tumor staging. Down regulation of PC4 in the highly metastatic osteosarcoma cells reduced the malignant behaviors in vitro and in vivo. Analyzing the downstream genes affected obviously by shPC4 with RNA sequencing, we found knocking down PC4 will inhibit the propensity for lung metastasis through transcriptional suppression of MMPs pathways. Taken together, PC4 may be an attractive therapeutic strategy for osteosarcoma, especially in preventing lung metastasis formation.

A biochemical and biophysical model of G-quadruplex DNA recognition by positive coactivator of transcription 4

DNA sequences that are guanine-rich have received considerable attention because of their potential to fold into a secondary, four-stranded DNA structure termed G-quadruplex (G4), which has been implicated in genomic instability and some human diseases. We have previously identified positive coactivator of transcription (PC4), a single-stranded DNA (ssDNA)-binding protein, as a novel G4 interactor. Here, to expand on these previous observations, we biochemically and biophysically characterized the interaction between PC4 and G4DNA. PC4 can bind alternative G4DNA topologies with a low nanomolar K_d value of ∼2 nm, similar to that observed for ssDNA. In consideration of the different structural features between G4DNA and ssDNA, these binding data indicated that PC4 can interact with G4DNA in a manner distinct from ssDNA. The stoichiometry of the PC4-G4 complex was 1:1 for PC4 dimer:G4 substrate. PC4 did not enhance the rate of folding of G4DNA, and formation of the PC4-G4DNA complex did not result in unfolding of the G4DNA structure. We assembled a G4DNA structure flanked by duplex DNA. We find that PC4 can interact with this G4DNA, as well as the complementary C-rich strand. Molecular docking simulations and DNA footprinting experiments suggest a model where a PC4 dimer accommodates the DNA with one monomer on the G4 strand and the second monomer bound to the C-rich strand. Collectively, these data provide a novel mode of PC4 binding to a DNA secondary structure that remains within the framework of the model for binding to ssDNA. Additionally, consideration of the PC4-G4DNA interaction could provide insight into the biological functions of PC4, which remain incompletely understood.

How does chromatin package DNA within nucleus and regulate gene expression?

The human body is made up of 60 trillion cells, each cell containing 2 millions of genomic DNA in its nucleus. How is this genomic deoxyribonucleic acid [DNA] organised into nuclei? Around 1880, W. Flemming discovered a nuclear substance that was clearly visible on staining under primitive light microscopes and named it 'chromatin'; this is now thought to be the basic unit of genomic DNA organization. Since long before DNA was known to carry genetic information, chromatin has fascinated biologists. DNA has a negatively charged phosphate backbone that produces electrostatic repulsion between adjacent DNA regions, making it difficult for DNA to fold upon itself. In this article, we will try to shed light on how does chromatin package DNA within nucleus and regulate gene expression?

Transcriptional Coactivator and Chromatin Protein PC4 Is Involved in Hippocampal Neurogenesis and Spatial Memory Extinction

Although the elaborate combination of histone and non-histone protein complexes defines chromatin organization and hence regulates numerous nuclear processes, the role of chromatin organizing proteins remains unexplored at the organismal level. The highly abundant, multifunctional, chromatin-associated protein and transcriptional coactivator positive coactivator 4 (PC4/Sub1) is absolutely critical for life, because its absence leads to embryonic lethality. Here, we report results obtained with conditional PC4 knock-out (PC4(f/f) Nestin-Cre) mice where PC4 is knocked out specifically in the brain. Compared with the control (PC4(+/+) Nestin-Cre) mice, PC4(f/f) Nestin-Cre mice are smaller with decreased nocturnal activity but are fertile and show no motor dysfunction. Neurons in different areas of the brains of these mice show sensitivity to hypoxia/anoxia, and decreased adult neurogenesis was observed in the dentate gyrus. Interestingly, PC4(f/f) Nestin-Cre mice exhibit a severe deficit in spatial memory extinction, whereas acquisition and long term retention were unaffected. Gene expression analysis of the dorsal hippocampus of PC4(f/f) Nestin-Cre mice revealed dysregulated expression of several neural function-associated genes, and PC4 was consistently found to localize on the promoters of these genes, indicating that PC4 regulates their expression. These observations indicate that non-histone chromatin-associated proteins like PC4 play a significant role in neuronal plasticity.

Novel Candidate Biomarkers of Chemoradiosensitivity in Esophageal Squamous Cell Carcinoma: A Systematic Review

There is no doubt that, along with surgery, chemoradiotherapy is an important treatment for esophageal squamous cell carcinoma (ESCC). Patients who respond well to chemoradiotherapy obtain great benefits toward overcoming their cancer, and so a more favorable prognosis. On the other hand, patients who do not respond well have wasted valuable time and experienced severe toxicity and seriously diminished quality of life, only to have their cancer recur with an unfavorable prognosis. For this reason, a reliable biomarker of chemoradiosensitivity in ESCC has long been sought. In this review, we will enumerate recently reported candidate biomarkers of chemoradiosensitivity in ESCC that have the potential for future clinical application.

Multinucleation and Polykaryon Formation is Promoted by the EhPC4 Transcription Factor in Entamoeba histolytica

Entamoeba histolytica is the intestinal parasite responsible for human amoebiasis that is a leading cause of death in developing countries. In this protozoan, heterogeneity in DNA content, polyploidy and genome plasticity have been associated to alterations in mechanisms controlling DNA replication and cell division. Studying the function of the transcription factor EhPC4, we unexpectedly found that it is functionally related to DNA replication, and multinucleation. Site-directed mutagenesis on the FRFPKG motif revealed that the K127 residue is required for efficient EhPC4 DNA-binding activity. Remarkably, overexpression of EhPC4 significantly increased cell proliferation, DNA replication and DNA content of trophozoites. A dramatically increase in cell size resulting in the formation of giant multinucleated trophozoites (polykaryon) was also found. Multinucleation event was associated to cytokinesis failure leading to abortion of ongoing cell division. Consistently, genome-wide profiling of EhPC4 overexpressing trophozoites revealed the up-regulation of genes involved in carbohydrates and nucleic acids metabolism, chromosome segregation and cytokinesis. Forced overexpression of one of these genes, EhNUDC (nuclear movement protein), led to alterations in cytokinesis and partially recapitulated the multinucleation phenotype. These data indicate for the first time that EhPC4 is associated with events related to polyploidy and genome stability in E. histolytica.

Yeast transcription co-activator Sub1 and its human homolog PC4 preferentially bind to G-quadruplex DNA

Using a G-quadruplex bait, we identified the transcription co-activator Sub1 as a G-quadruplex binding protein by quantitative LC-MS/MS and demonstrated in vivo G-quadruplex binding by ChIP. In vitro, Sub1, and its human homolog PC4, bind preferentially to G-quadruplexes. This provides a possible mechanism by which G-quadruplexes can influence gene transcription.

Selective Recognition of H3.1K36 Dimethylation/H4K16 Acetylation Facilitates the Regulation of All-trans-retinoic Acid (ATRA)-responsive Genes by Putative Chromatin Reader ZMYND8

ZMYND8 (zinc finger MYND (Myeloid, Nervy and DEAF-1)-type containing 8), a newly identified component of the transcriptional coregulator network, was found to interact with the Nucleosome Remodeling and Deacetylase (NuRD) complex. Previous reports have shown that ZMYND8 is instrumental in recruiting the NuRD complex to damaged chromatin for repressing transcription and promoting double strand break repair by homologous recombination. However, the mode of transcription regulation by ZMYND8 has remained elusive. Here, we report that through its specific key residues present in its conserved chromatin-binding modules, ZMYND8 interacts with the selective epigenetic marks H3.1K36Me2/H4K16Ac. Furthermore, ZMYND8 shows a clear preference for canonical histone H3.1 over variant H3.3. Interestingly, ZMYND8 was found to be recruited to several developmental genes, including the all-trans-retinoic acid (ATRA)-responsive ones, through its modified histone-binding ability. Being itself inducible by ATRA, this zinc finger transcription factor is involved in modulating other ATRA-inducible genes. We found that ZMYND8 interacts with transcription initiation-competent RNA polymerase II phosphorylated at Ser-5 in a DNA template-dependent manner and can alter the global gene transcription. Overall, our study identifies that ZMYND8 has CHD4-independent functions in regulating gene expression through its modified histone-binding ability.

Cooperation between SMYD3 and PC4 drives a distinct transcriptional program in cancer cells

SET and MYND domain containing protein 3 (SMYD3) is a histone methyltransferase, which has been implicated in cell growth and cancer pathogenesis. Increasing evidence suggests that SMYD3 can influence distinct oncogenic processes by acting as a gene-specific transcriptional regulator. However, the mechanistic aspects of SMYD3 transactivation and whether SMYD3 acts in concert with other transcription modulators remain unclear. Here, we show that SMYD3 interacts with the human positive coactivator 4 (PC4) and that such interaction potentiates a group of genes whose expression is linked to cell proliferation and invasion. SMYD3 cooperates functionally with PC4, because PC4 depletion results in the loss of SMYD3-mediated H3K4me3 and target gene expression. Individual depletion of SMYD3 and PC4 diminishes the recruitment of both SMYD3 and PC4, indicating that SMYD3 and PC4 localize at target genes in a mutually dependent manner. Artificial tethering of a SMYD3 mutant incapable of binding to its cognate elements and interacting with PC4 to target genes is sufficient for achieving an active transcriptional state in SMYD3-deficient cells. These observations suggest that PC4 contributes to SMYD3-mediated transactivation primarily by stabilizing SMYD3 occupancy at target genes. Together, these studies define expanded roles for SMYD3 and PC4 in gene regulation and provide an unprecedented documentation of their cooperative functions in stimulating oncogenic transcription.

PC4 induces lymphangiogenesis dependent VEGF-C/VEGF-D/VEGFR-3 axis activation in lung adenocarcinoma

Human transcriptional positive cofactor 4 (PC4) is a novel marker for diagnosis and treatment of advanced human cancers metastasis. In human lung adenocarcinoma, tumor lymphangiogenesis, an important early event, can promotes lymphatic metastasis, while it has been reported that VEGF-C/VEGF-D/VEGFR-3 axis plays an important role in lymphangiogenesis. The proposed study aims to explore whether PC4 correlates with VEGF-C/VEGF-D/VEGFR-3 axis of lymphangiogenesis in the lymph node metastasis during lung adenocarcinoma. Here, small interfering RNA technique was employed to investigate the relationship of PC4 and the VEGF-C/VEGF-D/VEGFR-3 axis in lung adenocarcinoma cell lines as well as tumor xenografts of mice model. And then mRNA and protein levels of PC4, VEGF-C, VEGF-D and VEGFR-3 were analyzed. Moreover, the correlation between PC4 expression and lymphatic vessel density or the rate of metastatsis in vivo was also revealed. Down-regulating PC4 expression resulted in the lower expression of VEGFC, VEGF-D and VEGFR-3 in mRNA and protein levels, and PC4 expression was significantly related with the factor of VEGF-C/VEGF-D/VEGFR-3 axis expression (P<0.05). Meanwhile, high expression level of PC4 was accompanied by the higher density of tumor lymphatic vessels and the rate of metastatsis in vivo (P<0.05). PC4 expression correlated with the levels of VEGF-C, VEGF-D and VEGFR-3 during the development of lymphangiogenesis and lymphatic metastasis in lung adenocarcinoma in vitro and in vivo, which may be a novel marker in the development of lymphangiogenesis and lymphatic metastasis of tumors.

Identification and characterization of nonhistone chromatin proteins: human positive coactivator 4 as a candidate

The highly dynamic nucleoprotein structure of eukaryotic genome is organized in an ordered fashion, the unit of which is the nucleosome. The nucleosome is composed of core histones and DNA of variable size wrapped around it. Apart from the histone proteins, several nonhistone proteins also interact with the complex consisting of the DNA, the core and linker histones conferring highly regulated fluidity on the chromatin and permitting fine tuning of its functions. The nonhistone proteins are multifunctional and accentuate diverse cellular outcomes. In spite of the technical challenges, the architectural role of the nonhistone proteins altering the topology of the chromatin has been studied extensively. To appreciate the significance of the chromatin for genome function, it is essential to examine the role of the nonhistone proteins in different physiological conditions. Here, taking the example of a highly abundant chromatin protein, PC4 (Positive coactivator 4), we describe strategies for the identification of the chromatin-associated proteins and their structural and functional characterization.

Sub1 and Maf1, two effectors of RNA polymerase III, are involved in the yeast quiescence cycle

Sub1 and Maf1 exert an opposite effect on RNA polymerase III transcription interfering with different steps of the transcription cycle. In this study, we present evidence that Sub1 and Maf1 also exhibit an opposite role on yeast chronological life span. First, cells lacking Sub1 need more time than wild type to exit from resting and this lag in re-proliferation is correlated with a delay in transcriptional reactivation. Second, our data show that the capacity of the cells to properly establish a quiescent state is impaired in the absence of Sub1 resulting in a premature death that is dependent on the Ras/PKA and Tor1/Sch9 signalling pathways. On the other hand, we show that maf1Δ cells are long-lived mutant suggesting a connection between Pol III transcription and yeast longevity.

Inhibition of human positive cofactor 4 radiosensitizes human esophageal squmaous cell carcinoma cells by suppressing XLF-mediated nonhomologous end joining

Radiotherapy has the widest application to esophageal squamous cell carcinoma (ESCC) patients. Factors associated with DNA damage repair have been shown to function in cell radiosensitivity. Human positive cofactor 4 (PC4) has a role in nonhomologous end joining (NHEJ) and is involved in DNA damage repair. However, the clinical significance and biological role of PC4 in cancer progression and cancer cellular responses to chemoradiotherapy (CRT) remain largely unknown. The aim of the present study was to investigate the potential roles of PC4 in the radiosensitivity of ESCC. In this study, we showed that knockdown of PC4 substantially increased ESCC cell sensitivity to ionizing radiation (IR) both in vitro and in vivo and enhanced radiation-induced apoptosis and mitotic catastrophe (MC). Importantly, we demonstrated that silencing of PC4 suppressed NHEJ by downregulating the expression of XLF in ESCC cells, whereas reconstituting the expression of XLF protein in the PC4-knockdown ESCC cells restored NHEJ activity and radioresistance. Moreover, high expression of PC4 positively correlated with ESCC resistance to CRT and was an independent predictor for short disease-specific survival of ESCC patients in both of our cohorts. These findings suggest that PC4 protects ESCC cells from IR-induced death by enhancing the NHEJ-promoting activity of XLF and could be used as a novel radiosensitivity predictor and a promising therapeutic target for ESCCs.

Radiation-induced conformational changes in chromatin structure in resting human peripheral blood mononuclear cells

Abstract Background: Ionizing radiation induces a plethora of DNA damage including double-strand breaks (DSB) that may trigger a series of events such as transcription, DNA repair and alteration in the conformation of chromatin structure in human cells. We have made an attempt to study the conformational changes in chromatin fibers in irradiated human peripheral blood mononuclear cells (PBMC) using Dynamic Light Scattering (DLS) as a new tool.

MATERIALS AND METHODS

Venous blood samples were collected from 10 random, healthy individuals with written informed consent, approved by institutional ethics committee. PBMC were separated from blood, irradiated with different doses of gamma radiation from 0.25-1.0 Gy. Native chromatin was isolated from irradiated PBMC and changes in the hydrodynamic diameter of the chromatin fiber were measured using DLS. Both dose response and time kinetics was studied in order to see the chromatin changes. Radiation-induced DNA double-strand breaks were measured using gamma-H2AX (histone 2A member X) as a biomarker using flow cytometry and foci were visualized in confocal microscopy.

RESULTS

A significant alteration in hydrodynamic diameter of the chromatin fiber was observed at lower doses (0.25 and 0.50 Gy), whereas at higher dose (1.0 Gy), the size of the chromatin fiber was comparable to unirradiated control. Among the 10 individuals studied, five individuals showed significant increase (p ≤ 0.002) in hydrodynamic size at 0.25 Gy whereas four individuals showed significant decrease (p ≤ 0.009) at 0.25 Gy. One individual did not show any significant difference as compared to control. However, dose-dependent increase in gamma-H2AX fluorescence signals as well as foci number was observed. Increased fragmentation of chromatin fiber was also observed using Atomic Force Microscopy at higher doses.

CONCLUSION

Radiation-induced DNA damage response can lead to individual specific conformational changes in chromatin structure at lower doses (0.25 Gy and 0.50 Gy) which can be detected using dynamic light scattering method in resting human PBMC.

The DNA intercalators ethidium bromide and propidium iodide also bind to core histones

Eukaryotic DNA is compacted in the form of chromatin, in a complex with histones and other non-histone proteins. The intimate association of DNA and histones in chromatin raises the possibility that DNA-interactive small molecules may bind to chromatin-associated proteins such as histones. Employing biophysical and biochemical techniques we have characterized the interaction of a classical intercalator, ethidium bromide (EB) and its structural analogue propidium iodide (PI) with hierarchical genomic components: long chromatin, chromatosome, core octamer and chromosomal DNA. Our studies show that EB and PI affect both chromatin structure and function, inducing chromatin compaction and disruption of the integrity of the chromatosome. Calorimetric studies and fluorescence measurements of the ligands demonstrated and characterized the association of these ligands with core histones and the intact octamer in absence of DNA. The ligands affect acetylation of histone H3 at lysine 9 and acetylation of histone H4 at lysine 5 and lysine 8 ex vivo. PI alters the post-translational modifications to a greater extent than EB. This is the first report showing the dual binding (chromosomal DNA and core histones) property of a classical intercalator, EB, and its longer analogue, PI, in the context of chromatin.

Systematic nucleo-cytoplasmic trafficking of proteins following exposure of MCF7 breast cancer cells to estradiol

We have used a proteomics subcellular spatial razor approach to look at changes in total protein abundance and in protein distribution between the nucleus and cytoplasm following exposure of MCF7 breast cancer cells to estradiol. The dominant response of MCF7 cells to estrogen stimulation involves dynamic changes in protein subcellular spatial distribution rather than changes in total protein abundance. Of the 3604 quantitatively monitored proteins, only about 2% show substantial changes in total abundance (>2-fold), whereas about 20% of the proteins show substantial changes in local abundance and/or redistribution of their subcellular location, with up to 16-fold changes in their local concentration in the nucleus or the cytoplasm. We propose that dynamic redistribution of the subcellular location of multiple proteins in response to stimuli is a fundamental characteristic of cells and suggest that perturbation of cellular spatial control may be an important feature of cancer.

Genome wide chromatin occupancy of mrhl RNA and its role in gene regulation in mouse spermatogonial cells

Mrhl RNA is a nuclear lncRNA encoded in the mouse genome and negatively regulates Wnt signaling in spermatogonial cells through p68/Ddx5 RNA helicase. Mrhl RNA is present in the chromatin fraction of mouse spermatogonial Gc1-Spg cells and genome wide chromatin occupancy of mrhl RNA by ChOP (Chromatin oligo affinity precipitation) technique identified 1370 statistically significant genomic loci. Among these, genes at 37 genomic loci also showed altered expression pattern upon mrhl RNA down regulation which are referred to as GRPAM (Genes Regulated by Physical Association of Mrhl RNA). p68 interacted with mrhl RNA in chromatin at these GRPAM loci. p68 silencing drastically reduced mrhl RNA occupancy at 27 GRPAM loci and also perturbed the expression of GRPAM suggesting a role for p68 mediated mrhl RNA occupancy in regulating GRPAM expression. Wnt3a ligand treatment of Gc1-Spg cells down regulated mrhl RNA expression and also perturbed expression of these 27 GRPAM genes that included genes regulating Wnt signaling pathway and spermatogenesis, one of them being Sox8, a developmentally important transcription factor. We also identified interacting proteins of mrhl RNA associated chromatin fraction which included Pc4, a chromatin organizer protein and hnRNP A/B and hnRNP A2/B1 which have been shown to be associated with lincRNA-Cox2 function in gene regulation. Our findings in the Gc1-Spg cell line also correlate with the results from analysis of mouse testicular tissue which further highlights the in vivo physiological significance of mrhl RNA in the context of gene regulation during mammalian spermatogenesis.

Subcellular proteomics reveals a role for nucleo-cytoplasmic trafficking at the DNA replication origin activation checkpoint

Depletion of DNA replication initiation factors such as CDC7 kinase triggers the origin activation checkpoint in healthy cells and leads to a protective cell cycle arrest at the G1 phase of the mitotic cell division cycle. This protective mechanism is thought to be defective in cancer cells. To investigate how this checkpoint is activated and maintained in healthy cells, we conducted a quantitative SILAC analysis of the nuclear- and cytoplasmic-enriched compartments of CDC7-depleted fibroblasts and compared them to a total cell lysate preparation. Substantial changes in total abundance and/or subcellular location were detected for 124 proteins, including many essential proteins associated with DNA replication/cell cycle. Similar changes in protein abundance and subcellular distribution were observed for various metabolic processes, including oxidative stress, iron metabolism, protein translation and the tricarboxylic acid cycle. This is accompanied by reduced abundance of two karyopherin proteins, suggestive of reduced nuclear import. We propose that altered nucleo-cytoplasmic trafficking plays a key role in the regulation of cell cycle arrest. The results increase understanding of the mechanisms underlying maintenance of the DNA replication origin activation checkpoint and are consistent with our proposal that cell cycle arrest is an actively maintained process that appears to be distributed over various subcellular locations.

Human positive coactivator 4 is a potential novel therapeutic target in non-small cell lung cancer

Transcriptional positive coactivator 4 (PC4) is a multifunctional nuclear protein that has important roles in DNA transcription, replication, repair and heterochromatinization. However, the role of PC4 in cancer remains to be clarified. Several studies propose that PC4 may act as a putative tumor suppressor. Here, we demonstrate for the first time that PC4 may represent a potential therapeutic target in non-small cell lung cancer (NSCLC). PC4 protein expression is significantly upregulated in NSCLC carcinoma tissues compared with their adjacent noncancerous counterparts as shown by immunohistochemical staining and western blotting in 104 pairs of formalin-fixed human NSCLC specimens and 6 fresh NSCLC samples. Knockdown of PC4 expression by sequence-specific small interfering RNA (siRNA) in human NSCLC cells (A549, H460 and H358) significantly inhibits the growth of cancer cells by the induction of cell cycle arrest and the increase of cell apoptosis in vitro. Interrupting the PC4 signaling pathway by injection of the PC4 siRNA liposome complex produced an effective regression of pre-established A549 cell xenografts in mice through growth inhibition and increased apoptosis. These results indicated that PC4 could be an attractive new therapeutic target for the treatment of NSCLC.