Involvement of the Gut Microbiome in the Local and Systemic Immune Response to Pancreatic Ductal Adenocarcinoma.

The systemic and local immunosuppression exhibited by pancreatic ductal adenocarcinoma (PDAC) contributes significantly to its aggressive nature. There is a need for a greater understanding of the mechanisms behind this profound immune evasion, which makes it one of the most challenging malignancies to treat and thus one of the leading causes of cancer death worldwide. The gut microbiome is now thought to be the largest immune organ in the body and has been shown to play an important role in multiple immune-mediated diseases. By summarizing the current literature, this review examines the mechanisms by which the gut microbiome may modulate the immune response to PDAC. Evidence suggests that the gut microbiome can alter immune cell populations both in the peripheral blood and within the tumour itself in PDAC patients. In addition, evidence suggests that the gut microbiome influences the composition of the PDAC tumour microbiome, which exerts a local effect on PDAC tumour immune infiltration. Put together, this promotes the gut microbiome as a promising route for future therapies to improve immune responses in PDAC patients.

Interplay in galectin expression predicts patient outcomes in a spatially restricted manner in PDAC.

BACKGROUND: Galectins (Gal's) are a family of carbohydrate-binding proteins that are known to support the tumour microenvironment through their immunosuppressive activity and ability to promote metastasis. As such they are attractive therapeutic targets, but little is known about the cellular expression pattern of galectins within the tumour and its neighbouring stromal microenvironment. Here we investigated the cellular expression pattern of Gals within pancreatic ductal adenocarcinoma (PDAC). METHODS: Galectin gene and protein expression were analysed by scRNAseq (n=4) and immunofluorescence imaging (n=19) in fibroblasts and epithelial cells of pancreatic biopsies from PDAC patients. Galectin surface expression was also assessed on tumour adjacent normal fibroblasts and cancer associated primary fibroblasts from PDAC biopsies using flow cytometry. RESULTS: scRNAseq revealed higher Gal-1 expression in fibroblasts and higher Gal-3 and -4 expression in epithelial cells. Both podoplanin (PDPN+, stromal/fibroblast) cells and EpCAM+ epithelial cells expressed Gal-1 protein, with highest expression seen in the stromal compartment. By contrast, significantly more Gal-3 and -4 protein was expressed in ductal cells expressing either EpCAM or PDPN, when compared to the stroma. Ductal Gal-4 cellular expression negatively correlated with ductal Gal-1, but not Gal-3 expression. Higher ductal cellular expression of Gal-1 correlated with smaller tumour size and better patient survival. CONCLUSIONS: In summary, the intricate interplay and cell-specific expression patterns of galectins within the PDAC tissue, particularly the inverse correlation between Gal-1 and Gal-4 in ducts and its significant association with patient survival, highlights the complex molecular landscape underlying PDAC and provides valuable insights for future therapeutic interventions.

Spatial determination and prognostic impact of the fibroblast transcriptome in pancreatic ductal adenocarcinoma.

Pancreatic ductal adenocarcinoma has a poor clinical outcome and responses to immunotherapy are suboptimal. Stromal fibroblasts are a dominant but heterogenous population within the tumor microenvironment and therapeutic targeting of stromal subsets may have therapeutic utility. Here, we combine spatial transcriptomics and scRNA-Seq datasets to define the transcriptome of tumor-proximal and tumor-distal cancer-associated fibroblasts (CAFs) and link this to clinical outcome. Tumor-proximal fibroblasts comprise large populations of myofibroblasts, strongly expressed podoplanin, and were enriched for Wnt ligand signaling. In contrast, inflammatory CAFs were dominant within tumor-distal subsets and expressed complement components and the Wnt-inhibitor SFRP2. Poor clinical outcome was correlated with elevated HIF-1α and podoplanin expression whilst expression of inflammatory and complement genes was predictive of extended survival. These findings demonstrate the extreme transcriptional heterogeneity of CAFs and its determination by apposition to tumor. Selective targeting of tumor-proximal subsets, potentially combined with HIF-1α inhibition and immune stimulation, may offer a multi-modal therapeutic approach for this disease.

Pancreatic cancer is one of the deadliest and most difficult cancers to treat. It responds poorly to immunotherapy for instance, despite this approach often succeeding in enlisting immune cells to fight tumours in other organs. This may be due, in part, to a type of cell called fibroblasts. Not only do these wrap pancreatic tumours in a dense, protective layer, they also foster complex relationships with the cancerous cells: some fibroblasts may fuel tumour growth, while other may help to contain its spread. These different roles may be linked to spatial location, with fibroblasts adopting different profiles depending on their proximity with cancer calls. For example, certain fibroblasts close to the tumour resemble the myofibroblasts present in healing wounds, while those at the periphery show signs of being involved in inflammation. Being able to specifically eliminate pro-cancer fibroblasts requires a better understanding of the factors that shape the role of these cells, and how to identify them. To examine this problem, Croft et al. relied on tumour samples obtained from pancreatic cancer patients. They mapped out the location of individual fibroblasts in the vicinity of the tumour and analysed their gene activity. These experiments helped to reveal the characteristics of different populations of fibroblasts. For example, they showed that the myofibroblast-like cells closest to the tumour exhibited signs of oxygen deprivation; they also produced podoplanin, a protein known to promote cancer progression. In contrast, cells further from the cancer produced more immune-related proteins. Combining these data with information obtained from patients' clinical records, Croft et al. found that samples from individuals with worse survival outcomes often featured higher levels of podoplanin and hypoxia. Inflammatory markers, however, were more likely to be present in individuals with good outcomes. Overall, these findings could help to develop ways to selectively target fibroblasts that support the growth of pancreatic cancer. Weakening these cells could in turn make the tumour accessible to immune cells, and more vulnerable to immunotherapies.

Tissue-Resident Memory T Cells in Pancreatic Ductal Adenocarcinoma Coexpress PD-1 and TIGIT and Functional Inhibition Is Reversible by Dual Antibody Blockade.

Pancreatic ductal adenocarcinoma (PDAC) has a poor clinical outlook. Responses to immune checkpoint blockade are suboptimal and a much more detailed understanding of the tumor immune microenvironment is needed if this situation is to be improved. Here, we characterized tumor-infiltrating T-cell populations in patients with PDAC using cytometry by time of flight (CyTOF) and single-cell RNA sequencing. T cells were the predominant immune cell subset observed within tumors. Over 30% of CD4+ T cells expressed a CCR6+CD161+ Th17 phenotype and 17% displayed an activated regulatory T-cell profile. Large populations of CD8+ tissue-resident memory (TRM) T cells were also present and expressed high levels of programmed cell death protein 1 (PD-1) and TIGIT. A population of putative tumor-reactive CD103+CD39+ T cells was also observed within the CD8+ tumor-infiltrating lymphocytes population. The expression of PD-1 ligands was limited largely to hemopoietic cells whilst TIGIT ligands were expressed widely within the tumor microenvironment. Programmed death-ligand 1 and CD155 were expressed within the T-cell area of ectopic lymphoid structures and colocalized with PD-1+TIGIT+ CD8+ T cells. Combinatorial anti-PD-1 and TIGIT blockade enhanced IFNγ secretion and proliferation of T cells in the presence of PD-1 and TIGIT ligands. As such, we showed that the PDAC microenvironment is characterized by the presence of substantial populations of TRM cells with an exhausted PD-1+TIGIT+ phenotype where dual checkpoint receptor blockade represents a promising avenue for future immunotherapy.

mRNA or ChAd0x1 COVID-19 Vaccination of Adolescents Induces Robust Antibody and Cellular Responses With Continued Recognition of Omicron Following mRNA-1273.

Children and adolescents generally experience mild COVID-19. However, those with underlying physical health conditions are at a significantly increased risk of severe disease. Here, we present a comprehensive analysis of antibody and cellular responses in adolescents with severe neuro-disabilities who received COVID-19 vaccination with either ChAdOx1 (n=6) or an mRNA vaccine (mRNA-1273, n=8, BNT162b2, n=1). Strong immune responses were observed after vaccination and antibody levels and neutralisation titres were both higher after two doses. Both measures were also higher after mRNA vaccination and were further enhanced by prior natural infection where one vaccine dose was sufficient to generate peak antibody response. Robust T-cell responses were generated after dual vaccination and were also higher following mRNA vaccination. Early T-cells were characterised by a dominant effector-memory CD4+ T-cell population with a type-1 cytokine signature with additional production of IL-10. Antibody levels were well-maintained for at least 3 months after vaccination and 3 of 4 donors showed measurable neutralisation titres against the Omicron variant. T-cell responses also remained robust, with generation of a central/stem cell memory pool and showed strong reactivity against Omicron spike. These data demonstrate that COVID-19 vaccines display strong immunogenicity in adolescents and that dual vaccination, or single vaccination following prior infection, generate higher immune responses than seen after natural infection and develop activity against Omicron. Initial evidence suggests that mRNA vaccination elicits stronger immune responses than adenoviral delivery, although the latter is also higher than seen in adult populations. COVID-19 vaccines are therefore highly immunogenic in high-risk adolescents and dual vaccination might be able to provide relative protection against the Omicron variant that is currently globally dominant.

Robust SARS-CoV-2-specific and heterologous immune responses in vaccine-naïve residents of long-term care facilities who survive natural infection.

We studied humoral and cellular immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 152 long-term care facility staff and 124 residents over a prospective 4-month period shortly after the first wave of infection in England. We show that residents of long-term care facilities developed high and stable levels of antibodies against spike protein and receptor-binding domain. Nucleocapsid-specific responses were also elevated but waned over time. Antibodies showed stable and equivalent levels of functional inhibition against spike-angiotensin-converting enzyme 2 binding in all age groups with comparable activity against viral variants of concern. SARS-CoV-2 seropositive donors showed high levels of antibodies to other beta-coronaviruses but serostatus did not impact humoral immunity to influenza or other respiratory syncytial viruses. SARS-CoV-2-specific cellular responses were similar across all ages but virus-specific populations showed elevated levels of activation in older donors. Thus, survivors of SARS-CoV-2 infection show a robust and stable immunity against the virus that does not negatively impact responses to other seasonal viruses.

DNA and modified vaccinia Ankara prime-boost vaccination generates strong CD8+ T cell responses against minor histocompatibility antigen HA-1.

Allogeneic immune responses underlie the graft-versus-leukaemia effect of stem cell transplantation, but disease relapse occurs in many patients. Minor histocompatibility antigen (mHAg) peptides mediate alloreactive T cell responses and induce graft-versus-leukaemia responses when expressed on patient haematopoietic tissue. We vaccinated nine HA-1-negative donors against HA-1 with a 'prime-boost' protocol of either two or three DNA 'priming' vaccinations prior to 'boost' with modified vaccinia Ankara (MVA). HA-1-specific CD8+ T cell responses were observed in seven donors with magnitude up to 1·5% of total CD8+ T cell repertoire. HA-1-specific responses peaked two weeks post-MVA challenge and were measurable in most donors after 12 months. HA-1-specific T cells demonstrated strong cytotoxic activity and lysed target cells with endogenous HA-1 protein expression. The pattern of T cell receptor (TCR) usage by HA-1-specific T cells revealed strong conservation of T cell receptor beta variable 7-9 (TRBV7-9) usage between donors. These findings describe one of the strongest primary peptide-specific CD8+ T cell responses yet recorded to a DNA-MVA prime-boost regimen and this may reflect the strong immunogenicity of mHAg peptides. Prime-boost vaccination in donors or patients may prove of substantial benefit in boosting graft-versus-leukaemia responses.

NK cells in pancreatic cancer demonstrate impaired cytotoxicity and a regulatory IL-10 phenotype.

Pancreatic ductal adenocarcinoma (PDAC) is one of the most common tumor subtypes and remains associated with very poor survival. T cell infiltration into tumor tissue is associated with improved clinical outcome but little is known regarding the potential role of NK cells in disease control. Here we analyze the phenotype and function of NK cells in the blood and tumor tissue from patients with PDAC. Peripheral NK cells are present in normal numbers but display a CD16hiCD57hi phenotype with marked downregulation of NKG2D. Importantly, these cells demonstrate reduced cytotoxic activity and low levels of IFN-γ expression but instead produce high levels of intracellular IL-10, an immunoregulatory cytokine found at increased levels in the blood of PDAC patients. In contrast, NK cells are largely excluded from tumor tissue where they display strong downregulation of both CD16 and CD57, a phenotype that was recapitulated in primary NK cells following co-culture with PDAC organoids. Moreover, expression of activatory proteins, including DNAM-1 and NKP30, was markedly suppressed and the DNAM-1 ligand PVR was strongly expressed on tumor cells. As such, in situ and peripheral NK cells display differential features in patients with PDAC and indicate local and systemic mechanisms by which the tumor can evade immune control. These findings offer a number of potential options for NK-based immunotherapy in the management of patients with PDAC.

Azidothymidine Sensitizes Primary Effusion Lymphoma Cells to Kaposi Sarcoma-Associated Herpesvirus-Specific CD4+ T Cell Control and Inhibits vIRF3 Function.

Kaposi sarcoma-associated herpesvirus (KSHV) is linked with the development of Kaposi sarcoma and the B lymphocyte disorders primary effusion lymphoma (PEL) and multi-centric Castleman disease. T cell immunity limits KSHV infection and disease, however the virus employs multiple mechanisms to inhibit efficient control by these effectors. Thus KSHV-specific CD4+ T cells poorly recognize most PEL cells and even where they can, they are unable to kill them. To make KSHV-infected cells more sensitive to T cell control we treated PEL cells with the thymidine analogue azidothymidine (AZT), which sensitizes PEL lines to Fas-ligand and TRAIL challenge; effector mechanisms which T cells use. PELs co-cultured with KSHV-specific CD4+ T cells in the absence of AZT showed no control of PEL outgrowth. However in the presence of AZT PEL outgrowth was controlled in an MHC-restricted manner. To investigate how AZT sensitizes PELs to immune control we first examined BJAB cells transduced with individual KSHV-latent genes for their ability to resist apoptosis mediated by stimuli delivered through Fas and TRAIL receptors. This showed that in addition to the previously described vFLIP protein, expression of vIRF3 also inhibited apoptosis delivered by these stimuli. Importantly vIRF3 mediated protection from these apoptotic stimuli was inhibited in the presence of AZT as was a second vIRF3 associated phenotype, the downregulation of surface MHC class II. Although both vFLIP and vIRF3 are expressed in PELs, we propose that inhibiting vIRF3 function with AZT may be sufficient to restore T cell control of these tumor cells.

Primary B Lymphocytes Infected with Kaposi's Sarcoma-Associated Herpesvirus Can Be Expanded In Vitro and Are Recognized by LANA-Specific CD4+ T Cells.

UNLABELLED: Kaposi's sarcoma-associated herpesvirus (KSHV) has tropism for B lymphocytes, in which it establishes latency, and can also cause lymphoproliferative disorders of these cells manifesting as primary effusion lymphoma (PEL) and multicentric Castleman disease (MCD). T cell immunity is vital for the control of KSHV infection and disease; however, few models of B lymphocyte infection exist to study immune recognition of such cells. Here, we developed a model of B lymphocyte infection with KSHV in which infected tonsillar B lymphocytes were expanded by providing mitogenic stimuli and then challenged with KSHV-specific CD4(+)T cells. The infected cells expressed viral proteins found in PELs, namely, LANA and viral IRF3 (vIRF3), albeit at lower levels, with similar patterns of gene expression for the major latency, viral interleukin 6 (vIL-6), and vIRF3 transcripts. Despite low-level expression of open reading frame 50 (ORF50), transcripts for the immune evasion genes K3 and K5 were detected, with some downregulation of cell surface-expressed CD86 and ICAM. The vast majority of infected lymphocytes expressed IgM heavy chains with Igλ light chains, recapitulating the features seen in infected cells in MCD. We assessed the ability of the infected lymphocytes to be targeted by a panel of major histocompatibility complex (MHC) class II-matched CD4(+)T cells and found that LANA-specific T cells restricted to different epitopes recognized these infected cells. Given that at least some KSHV latent antigens are thought to be poor targets for CD8(+)T cells, we suggest that CD4(+)T cells are potentially important effectors for thein vivocontrol of KSHV-infected B lymphocytes. IMPORTANCE: KSHV establishes a latent reservoir within B lymphocytes, but few models exist to study KSHV-infected B cells other than the transformed PEL cell lines, which have likely accrued mutations during the transformation process. We developed a model of KSHV-infected primary B lymphocytes that recapitulates features seen in PEL and MCD by gene expression and cell phenotype analysis, allowing the study of T cell recognition of these cells. Challenge of KSHV-infected B cells with CD4(+)T cells specific for LANA, a protein expressed in all KSHV-infected cells and malignanciesin vivo, showed that these effectors could efficiently recognize such targets. Given that the virus expresses immune evasion genes or uses proteins with intrinsic properties, such as LANA, that minimize epitope recognition by CD8(+)T cells, CD4(+)T cell immunity to KSHV may be important for maintaining the virus-host balance.

The RNA helicase/transcriptional co-regulator, p68 (DDX5), stimulates expression of oncogenic protein kinase, Polo-like kinase-1 (PLK1), and is associated with elevated PLK1 levels in human breast cancers.

p68 (DDX5) acts both as an ATP-dependent RNA helicase and as a transcriptional co-activator of several cancer-associated transcription factors, including the p53 tumor suppressor. p68 is aberrantly expressed in a high proportion of cancers, but the oncogenic drive for, or the consequences of, these expression changes remain unclear. Here we show that elevated p68 expression in a cohort of human breast cancers is associated significantly with elevated levels of the oncogenic protein kinase, Polo-like kinase-1 (PLK1). Patients expressing detectable levels of both p68 and PLK1 have a poor prognosis, but only if they also have mutation in the TP53 gene (encoding p53), suggesting that p68 can regulate PLK1 levels in a manner that is suppressed by p53. In support of this hypothesis, we show that p68 stimulates expression from the PLK1 promoter, and that silencing of endogenous p68 expression downregulates endogenous PLK1 gene expression. In the absence of functional p53, p68 stimulates the expression of PLK1 both at basal levels and in response to the clinically relevant drug, etoposide. In keeping with a role as a transcriptional activator/co-activator, chromatin immuno-precipitation analysis shows that p68 is associated with the PLK1 promoter, irrespective of the p53 status. However, its recruitment is stimulated by etoposide in cells lacking p53, suggesting that p53 can oppose association of p68 with the PLK1 promoter. These data provide a model in which p68 and p53 interplay regulates PLK1 expression, and which describes the behavior of these molecules, and the outcome of their interaction, in human breast cancer.

DEAD-box RNA helicases as transcription cofactors.

It is established that several DEAD box RNA helicases perform multiple functions in the cell, often through interactions with different partner proteins in a context-dependent manner. Several studies have shown that some DEAD box proteins play important roles as regulators of transcription, particularly as coactivators or cosuppressors of transcription factors that are themselves highly regulated. Two such RNA helicases are DDX5 (p68) and DDX17 (p72). These proteins are known to function in RNA processing/alternative splicing, but they have also been shown to interact with, and act as coregulators of, transcription factors that are themselves highly regulated. In this chapter, we shall describe protocols we have used to investigate the factors that influence the function of p68 and p72 in transcriptional regulation. These include the interactions of p68 and p72 with transcription factors and/or components of the transcription machinery and posttranslational modification by the small ubiquitin-related modifier, SUMO.

An evolutionarily conserved, alternatively spliced, intron in the p68/DDX5 DEAD-box RNA helicase gene encodes a novel miRNA.

The DEAD-box RNA helicase p68 (DDX5) plays important roles in several cellular processes, including transcription, pre-mRNA processing, and microRNA (miRNA) processing. p68 expression is growth and developmentally regulated, and alterations in p68 expression and/or function have been implicated in tumor development. The p68 gene encodes an evolutionarily conserved, alternatively spliced, intron the function of which has to date remained unclear. Although the intron-containing p68 RNA does not appear to yield an alternative p68 protein, it is differentially expressed in cell lines and tissues, indicating regulation of expression. Here we show that the p68 conserved intron encodes a novel putative miRNA, suggesting a previously unknown possible regulatory function for the p68 intron. We show that this miRNA (referred to as p68 miRNA) is processed from the intron via the canonical miRNA-processing pathway and that it associates with the Argonaute protein Ago2. Finally we show that the p68 miRNA suppresses an mRNA bearing complementary target sequences, suggesting that it is functional. These findings suggest a novel mechanism by which alterations in p68 expression may impact on the cell.

Analysis of the RNA helicase p68 (Ddx5) as a transcriptional regulator.

The DEAD box RNA helicase p68 (Ddx5) has been demonstrated to act as a transcriptional co-activator for a number of highly regulated transcription factors (e.g. estrogen receptor alpha and the tumour suppressor p53) and to be recruited to promoters of genes that are responsive to activation of these transcription factors, suggesting that it may play a role in transcription initiation. We have investigated the function of p68 as a co-activator of the tumour suppressor p53, with a particular emphasis on the importance of p68 in the induction of p53 transcriptional activity by DNA damage. These studies have involved RNAi-mediated suppression of p68 in cells expressing wild-type p53 and determining its effect on the expression of cellular p53 target genes in response to DNA damage. Additionally a significant amount of our research has focused on the study of the role of p68 in transcriptional initiation; this has included an investigation of the recruitment of p68 to the promoters of p53-responsive genes and of the importance of p68 in influencing recruitment of p53. Here we present detailed methods for RNAi knock-down of p68 expression, determination of its effect on expression of p53-responsive genes by quantitative RT-PCR and Western blotting, and chromatin immunoprecipitation techniques for determining recruitment of p68 and p53 to p53-responsive promoters.

SUMOylation of nuclear actin.

Actin, a major component of the cytoplasm, is also abundant in the nucleus. Nuclear actin is involved in a variety of nuclear processes including transcription, chromatin remodeling, and intranuclear transport. Nevertheless, the regulation of nuclear actin by posttranslational modifications has not been investigated. We now show that nuclear actin is modified by SUMO2 and SUMO3 and that computational modeling and site-directed mutagenesis identified K68 and K284 as critical sites for SUMOylating actin. We also present a model for the actin-SUMO complex and show that SUMOylation is required for the nuclear localization of actin.

FKBP25, a novel regulator of the p53 pathway, induces the degradation of MDM2 and activation of p53.

The p53 tumour suppressor protein is tightly controlled by the E3 ubiquitin ligase, mouse double minute 2 (MDM2), but maintains MDM2 expression as part of a negative feedback loop. We have identified the immunophilin, 25kDa FK506-binding protein (FKBP25), previously shown to be regulated by p53-mediated repression, as an MDM2-interacting partner. We show that FKBP25 stimulates auto-ubiquitylation and proteasomal degradation of MDM2, leading to the induction of p53. Depletion of FKBP25 by siRNA leads to increased levels of MDM2 and a corresponding reduction in p53 and p21 levels. These data are consistent with the idea that FKBP25 contributes to regulation of the p53-MDM2 negative feedback loop.

The RNA helicase p68 is a novel androgen receptor coactivator involved in splicing and is overexpressed in prostate cancer.

The androgen receptor (AR) is a member of the nuclear steroid hormone receptor family and is thought to play an important role in the development of both androgen-dependent and androgen-independent prostatic malignancy. Elucidating roles by which cofactors regulate AR transcriptional activity may provide therapeutic advancement for prostate cancer (PCa). The DEAD box RNA helicase p68 (Ddx5) was identified as a novel AR-interacting protein by yeast two-hybrid screening, and we sought to examine the involvement of p68 in AR signaling and PCa. The p68-AR interaction was verified by colocalization of overexpressed protein by immunofluorescence and confirmed in vivo by coimmunoprecipitation in the PCa LNCaP cell line. Chromatin immunoprecipitation in the same cell line showed AR and p68 recruitment to the promoter region of the androgen-responsive prostate-specific antigen (PSA) gene. Luciferase reporter, minigene splicing assays, and RNA interference (RNAi) were used to examine a functional role of p68 in AR-regulated gene expression, whereby p68 targeted RNAi reduced AR-regulated PSA expression, and p68 enhanced AR-regulated repression of CD44 splicing (P = 0.008). Tyrosine phosphorylation of p68 was found to enhance coactivation of ligand-dependent transcription of AR-regulated luciferase reporters independent of ATP-binding. Finally, we observe increased frequency and expression of p68 in PCa compared with benign tissue using a comprehensive prostate tissue microarray (P = 0.003; P = 0.008). These findings implicate p68 as a novel AR transcriptional coactivator that is significantly overexpressed in PCa with a possible role in progression to hormone-refractory disease.

p68 (Ddx5) interacts with Runx2 and regulates osteoblast differentiation.

Runx2 is an essential transcription factor for osteoblast development from mesenchymal progenitors. Runx2 regulates gene expression by interacting with numerous transcription factors and co-activators to integrate signaling events within the nucleus. In this study we used affinity purification and proteomic techniques to identify novel Runx2 interacting proteins. One of these proteins is the DEAD box RNA helicase, p68 (Ddx5). p68 regulates many aspects of RNA expression, including transcription and splicing. p68 co-localized with Runx2 in punctate foci within the nucleus. In transcription assays, p68 functioned as a co-activator of Runx2, but its helicase activity was not essential for co-activation. In accordance, Runx2 transcriptional activity was muted in p68-suppressed cells. Surprisingly, osteoblast differentiation of the multipotent progenitor C2C12 cell line was accelerated by p68 suppression and Runx2 suppressed p68 expression in calvarial progenitor cells. Together these data demonstrate that p68 is a novel co-activator for Runx2, but it inhibits osteogenic differentiation of progenitor cells. Moreover Runx2 has an active role in regulating p68 levels in osteoblast precursors. Thus, crosstalk between Runx2 and p68 controls osteoblast specification and maturation at multiple levels.

The DEAD box protein p68: a novel transcriptional coactivator of the p53 tumour suppressor.

The DEAD box RNA helicase, p68, has been implicated in various cellular processes and has been shown to possess transcriptional coactivator function. Here, we show that p68 potently synergises with the p53 tumour suppressor protein to stimulate transcription from p53-dependent promoters and that endogenous p68 and p53 co-immunoprecipitate from nuclear extracts. Strikingly, RNAi suppression of p68 inhibits p53 target gene expression in response to DNA damage, as well as p53-dependent apoptosis, but does not influence p53 stabilisation or expression of non-p53-responsive genes. We also show, by chromatin immunoprecipitation, that p68 is recruited to the p21 promoter in a p53-dependent manner, consistent with a role in promoting transcriptional initiation. Interestingly, p68 knock-down does not significantly affect NF-kappaB activation, suggesting that the stimulation of p53 transcriptional activity is not due to a general transcription effect. This study represents the first report of the involvement of an RNA helicase in the p53 response, and highlights a novel mechanism by which p68 may act as a tumour cosuppressor in governing p53 transcriptional activity.

A novel site of AKT-mediated phosphorylation in the human MDM2 onco-protein.

MDM2 is an E3 ubiquitin ligase which mediates ubiquitylation and proteasome-dependent degradation of the p53 tumor suppressor protein. Phosphorylation of MDM2 by the protein kinase AKT is thought to regulate MDM2 function in response to survival signals, but there has been uncertainty concerning the identity of the sites phosphorylated by AKT. In the present study, we identify Ser-166, a site previously reported as an AKT target, and Ser-188, a novel site which is the major site of phosphorylation of MDM2 by AKT in vitro. Analysis of MDM2 in cultured cells confirms that Ser-166 and Ser-188 are phosphorylated by AKT in a physiological context.