Novel Mechanism For Induction Research Articles

Cell-free chromatin: A newly described mediator of systemic inflammation

Recent research has shown that cell-free chromatin (cfCh) particles that are released from the billions of cells that die in the body everyday can enter into healthy cells, integrate into their genomes and induce dsDNA breaks and apoptotic responses. Genomic integration of cfCh activates NF κ B suggesting a novel mechanism of induction of systemic inflammation. Since DNA damage and inflammation are underlying pathologies in multiple devastating acute and chronic disease conditions, the discovery of agents that can inactivate cfCh may provide therapeutic possibilities.

Exosomes Secreted by Apoptosis-Resistant Acute Myeloid Leukemia (AML) Blasts Harbor Regulatory Network Proteins Potentially Involved in Antagonism of Apoptosis

Expression of apoptosis-regulating proteins (B-cell CLL/lymphoma 2 - BCL-2, Myeloid Cell Leukemia 1 - MCL-1, BCL-2 like 1 - BCL-X and BCL-2-associated X protein - BAX) in acute myeloid leukemia (AML) blasts at diagnosis is associated with disease-free survival. We previously found that the initially high apoptosis-resistance of AML cells decreased after therapy, while regaining high levels at relapse. Herein, we further explored this aspect of dynamic apoptosis regulation in AML. First, we showed that the intraindividualex vivoapoptosis-related profiles of normal lymphocytes and AML blasts within the bone marrow of AML patients were highly correlated. The expression values of apoptosis-regulating proteins were far beyond healthy control lymphocytes, which implicates the influence of microenvironmental factors. Second, we demonstrated that apoptosis-resistant primary AML blasts, as opposed to apoptosis-sensitive cells, were able to up-regulate BCL-2 expression in sensitive AML blasts in contact cultures (p= 0.0067 andp= 1.0, respectively). Using secretome proteomics, we identified novel proteins possibly engaged in apoptosis regulation. Intriguingly, this analysis revealed that major functional protein clusters engaged in global gene regulation, including mRNA splicing, protein translation, and chromatin remodeling, were more abundant (p= 4.01E-06) in secretomes of apoptosis-resistant AML. These findings were confirmed by subsequent extracellular vesicle proteomics. Finally, confocal-microscopy-based colocalization studies show that splicing factors-containing vesicles secreted by high AAI cells are taken up by low AAI cells. The current results constitute the first comprehensive analysis of proteins released by apoptosis-resistant and sensitive primary AML cells. Together, the data point to vesicle-mediated release of global gene regulatory protein clusters as a plausible novel mechanism of induction of apoptosis resistance. Deciphering the modes of communication between apoptosis-resistant blasts may in perspective lead to the discovery of prognostic tools and development of novel therapeutic interventions, aimed at limiting or overcoming therapy resistance.

Abstract 2945: Nilotinib abrogates DNMT1-depenent DNA methylation: A novel mechanism for induction of AML leukemia regression

Abstract Nilotinib has been approved to treat newly diagnosed, imatinib-resistant or -intolerant leukemia, but the mechanisms underlying its antileukemia actions remain elusive. Here, we report that nilotinib blocks AML growth through its DNA hypomethylating activity. We showed that, in addition to inducing a reduction of KIT and FLT3 protein expression and phosphorylation, nilotinib downregulated DNMT1, 3a and 3b in Kasumi-1, MV4-11 and C1498 cells. Since DNMT1 is maintenance and de novo DNMT, we focused on DNMT1 and proved that nilotinib impaired DNMT1 expression via Sp1 dysfunction, since the luciferase activity driven by DNMT1 promoter containing Sp1 binding sites was decreased by 2.6 fold by nilotinib, while increased by 10.6 fold after Sp1 overexpression. Electrophoretic mobility shift assay disclosed that nilotinib abolished Sp1 binding on DNMT1 promoter. Dotblot using 5mC antibody revealed global DNA hypomethylation and qPCR showed p15INK4B upregulation in Kasumi-1 (3.4 fold) or MV4-11 (5.2 fold). Bisulfite sequencing found a decrease of DNA methylation on p15INK4B promoter from 100% to 89% in Kasumi-1 or 13% to 0% in MV4-11. The functional assays showed a decrease of colony formation in Kasumi-1 (60%) or MV4-11 (58%), and an increase of cell apoptosis in Kasumi-1 (6.9 fold) or MV4-11 (3.2 fold). We verified these findings in AML patient blasts (n = 3) by showing that nilotinib treatment led to the dephosphorylation of KIT, the downregulation of DNMT1, 3a, 3b and Sp1, the reduction of global DNA methylation and the re-expression of p15INK4B (4.4 fold) via promoter demethylation (from 14% to 6%), which was followed by suppressed colony formation (77%), promoted cell apoptosis (1.84 fold) and increased caspase-3 and caspase-8. To test if nilotinib induces AML regression via DNA hypomethylation in vivo, 0.1×106 C1498 cells were injected into C57BL/6 mice (n = 3 mice/group) via the tail vein. The leukemic mice were treated with 5, 10 or 15 mg/kg of nilotinib in PEG400 plus saline via intraperitoneal injection twice a week for a total of 6 doses. No toxicity was observed for the tested drug dosage and schedule, because of no evident change of mouse body weight. Compared to the control group, nilotinib-treated mice had a decrease in spleen size. H&E staining showed that the control mice exhibited increased infiltration of AML cells into the spleen, lung and liver of recipients. The bone marrow (BM) histopathology identified a higher degree of mature post-mitotic cells containing metamyelocytes, bands and segmented neutrophils in nilotinib-treated mice than control. DNMT1 was downregulated (39%), p15INK4B was upregulated and global DNA methylation was reduced (60%) in BM cells from nilotinib-treated mice. Thus, our findings unveil DNA methylation machinery as an innovative target behind nilotinib's anticancer activities, and provide a new therapeutic avenue for nilotinib in overcoming aberrant epigenetics in AML. Note: This abstract was not presented at the meeting. Citation Format: Na Shen, Fei Yan, Jiuxia Pang, Naseema Gangat, Mark R. Litzow, Aref Al-Kali, Shujun Liu. Nilotinib abrogates DNMT1-depenent DNA methylation: A novel mechanism for induction of AML leukemia regression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2945. doi:10.1158/1538-7445.AM2015-2945

HBxAPα/Rsf-1-mediated HBx–hBubR1 interactions regulate the mitotic spindle checkpoint and chromosome instability

Hepatitis B virus (HBV) X protein (HBx), encoded by the HBV genome, is involved in the development of HBV-mediated liver cancer, whose frequency is highly correlated with chromosomal instability (CIN). We reported previously that HBx induces mitotic checkpoint dysfunction by targeting the human serine/threonine kinase BubR1 (hBubR1). However, the underlying mechanism remained unresolved. Here, we show that HBx protein-associated protein α (HBxAPα)/Rsf-1 associates with hBubR1 and HBx in the chromatin fraction during mitosis. Depletion of HBxAPα/Rsf-1 abolished the interaction between HBx and hBubR1, indicating that HBxAPα/Rsf-1 mediates these interactions. Knockdown of HBxAPα/Rsf-1 with small interfering RNA did not affect the recruitment of hBubR1 to kinetochores; however, it did significantly impair HBx targeting to kinetochores. A deletion mutant analysis revealed that two Kunitz domains of HBx, the Cdc20-binding domain of hBubR1 and full-length of HBxAPα/Rsf-1 were essential for these interactions. Thus, binding of HBx to hBubR1, stabilized by HBxAPα/Rsf-1, significantly attenuated hBubR1 binding to Cdc20 and consequently increased the rate of mitotic aberrations. Collectively, our data show that the HBx impairs hBubR1 function and induces CIN through HBxAPα/Rsf-1, providing a novel mechanism for induction of genomic instability by a viral pathogen in hepatocarcinogenesis.

Nilotinib shows prolonged intracellular accumulation upon pulse-exposure: a novel mechanism for induction of apoptosis in CML cells

Nilotinib shows prolonged intracellular accumulation upon pulse-exposure: a novel mechanism for induction of apoptosis in CML cells

Abstract 1498: The combination of immunosuppression and carrier cells significantly enhances poxvirus delivery and the efficacy of its oncolytic virotherapy in the pre-immunized host

Abstract The pre-existing anti-poxvirus immunity in cancer patients presents a hurdle to successful poxvirus-mediated oncolytic virotherapy. To overcome this hurdle, we have explored strategies of immunosuppression (IS) or/and immune evasion (IE) for successful delivery of an oncolytic vaccinia virus to tumor models in mice pre-vaccinated with a poxvirus. These include transient IS using a cocktail of three immunosuppressive drugs (IS drugs) (tacrolimus, mycophenolate mofetil and methylprednisolone sodium succinate) that have been used in organ transplant, carrier cells as a means of IE, or the combined strategy. The three drug cocktail, which targeted multiple components of innate and adaptive immunity, displayed no enhancing effects on viral infection in tumor tissue in the pre-immunized host. Using B cell knockout mice, we confirmed that B cells and the neutralizing antibodies, the later of which were not targeted by the drug cocktail, played a significant role in preventing poxvirus infection. Cancer cells as carrier cells alone were not effective to deliver the virus to tumor. Instead, the application of both IS drugs and carrier cells worked synergistically to promote the delivery to, viral infection, replication and spread inside the tumor mass and thus enhanced the efficacy of oncolytic virotherapy in an ovarian carcinomatosis model. The administraton of these IS drugs led to facilitated development of tumor-associated macrophages (TAMs) into immunosuppressive M2 phenotype (IL-10hi/IL-12low) in the tumor microenvironment. These results revealed a novel mechanism for induction of the immunosuppressive tumor microenvironment by the IS drugs, and demonstrated in principle that it is feasible to treat pre-vaccinated cancer patients with an oncolytic poxvirus when combined with both IS and IE strategies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1498.

Bcl-xL affects the development of functional CD4 Tregs

We read with great interest the article by Haque and colleagues [1] in a recent issue of Arthritis Research & Therapy. They hypothesized that co-transduction of CD4+ T cells with both forkhead box P3 transcription factor (FoxP3) and Bcl-xL will generate highly reactive regulatory T cells (Tregs) that can be used to prevent auto immune disease. The authors showed that the accumulation, persistence, and efficient function of Tregs were attributable to the expression of Bcl-xL in CD4 Tregs. Indications for a potential role of Bcl-xL in the development of functional Tregs were first described by our group, and the results of studies supporting this notion were published in numerous journals (for example, [2-5]). Because this information was not mentioned in the article by Haque and colleagues [1] and because the results presented in their article confirm our previous studies [2-5], we think that it is important, scientifically and ethically, to acknowledge these data. Our group has been studying systemic lupus erythematosus (SLE) and developed a tolerogenic peptide, namely hCDR1, shown to ameliorate manifestations of the disease through several mechanisms of action, including the induction of CD4 Tregs [2]. We showed that Bcl-xL was upregulated in CD4 Tregs of SLE-affected (NZBxNZW)F1 mice following treatment with the tolerogenic peptide [3]. Bcl-xL played a suppressive role in the tolerized mice, as it inhibited the activation of T and B cells, and mediated the downregulating effects of hCDR1 on the production of the pathogenic cytokines interferon-gamma and interleukin-10 and the upregulating effects on the immunosuppressive cytokine transforming growth factor-beta (TGF-β). Furthermore, CD4 Tregs of the tolerized mice elicited the expression of BclxL in the effector CD4 cells, thus contributing to the amelioration of SLE manifestations [3]. Although CD8 Tregs could not trigger the expression of Bcl-xL in effector CD4 cells, the former cells were essential for the optimal inhibitory function of CD4 Tregs [4]. Finally, we demonstrated that Bcl-xL played a role in inducing the regulatory/inhibitory molecules FoxP3, cytotoxic T lymphocyte antigen 4 (CTLA-4), and TGF-β and in repressing PD-1 (programmed death 1) [5]. We showed that Bcl-xL also mediated the induction of CTLA-4 and TGF-β in effector CD4 cells by CD4 Tregs of the tolerized mice, thus explaining the inhibition of proliferation and the decreased activation of effector CD4 cells [5]. These newly described roles of Bcl-xL may provide a novel mechanism of induction of CD4 Tregs. All together, our data [2-5], supported by those presented by Haque and colleagues [1], suggest that immunomodulation of Bcl-xL expression in T cells might be valuable for controlling and treating diseases that are affected by CD4 Tregs.

Cancer vaccine with mimotopes of tumor-associated carbohydrate antigens

The GD2 ganglioside, displayed by five carbohydrate Neu5Acalpha2-8Neu5Acalpha2-3(GalNAcbeta1-4)Galbeta1-4Glcbeta residues attached to a ceramide chain that anchors the ganglioside in the cell membrane, is expressed on neuroectodermally derived tumors. GD2 has been used as a target for passive and active immunotherapy in patients with malignant melanoma and neuroblastoma. We have generated 47-LDA mimotope of GD2 by screening a phage display peptide library with anti-GD2 mAb 14G2a and reported that vaccination with the 47-LDA mimotope elicited GD2 cross-reactive IgG antibody responses as well as MHC class I-restricted CD8(+) T cells to syngeneic neuroblastoma tumor cells. The cytotoxic activity of the vaccine-induced CTLs was independent of GD2 expression, suggesting recognition of a novel tumor-associated antigen cross-reacting with 47-LDA. Immunoblotting studies using 14G2a mAb demonstrated that this antibody cross-reacts with a 105 kDa glycoprotein expressed by GD2(+) and GD2(-) neuroblastoma and melanoma cells. Functional studies of tumor cells grown in three-dimensional (3D) collagen cultures with 14G2a mAb showed decreases in matrix metalloproteinase-2 activation, a process regulated by 105 kDa activated leukocyte cell adhesion molecules (ALCAM/CD166). The CD166 glycoprotein was shown to be recognized by 14G2a antibody, and inhibition of CD166 expression by RNA interference ablated the cell sensitivity to lysis by 47-LDA-induced CD8(+) T cells in vitro and in vivo. These results suggest that the vaccine-induced CTLs recognize a 47-LDA cross-reactive epitope expressed by CD166 and reveal a novel mechanism of induction of potent tumor-specific cellular responses by mimotopes of tumor-associated carbohydrate antigens.

HDAC inhibitor, scriptaid, induces glioma cell apoptosis through JNK activation and inhibits telomerase activity

The present study identified a novel mechanism of induction of apoptosis in glioblastoma cells by scriptaid – a histone deacetylase (HDAC) inhibitor. Scriptaid reduced glioma cell viability by increasing Jun N-terminal kinase (JNK) activation. Although scriptaid induced activation of both p38MAPK and JNK, it was the inhibition of JNK that attenuated scriptaid-induced apoptosis significantly. Scriptaid also increased the expression of (i) p21 and p27 involved in cell-cycle regulation and (ii) γH2AX associated with DNA damage response in a JNK-dependent manner. Treatment with scriptaid increased Ras activity in glioma cells, and transfection of cells with constitutively active RasV12 further sensitized glioma cells to scriptaid-induced apoptosis. Scriptaid also inhibited telomerase activity independent of JNK. Taken together, our findings indicate that scriptaid (i) induces apoptosis and reduces glioma cell proliferation by elevating JNK activation and (ii) also decreases telomerase activity in a JNK-independent manner.

Immunization with a Mimotope of GD2 Ganglioside Induces CD8+ T Cells That Recognize Cell Adhesion Molecules on Tumor Cells

The GD2 ganglioside expressed on neuroectodermal tumor cells has been used as a target for passive and active immunotherapy in patients with malignant melanoma and neuroblastoma. We have reported that immunization of mice with a 47-LDA mimotope of GD2, isolated from a phage display peptide library with anti-GD2 mAb 14G2a, induces MHC class I-restricted CD8(+) T cell responses to syngeneic neuroblastoma tumor cells. The cytotoxic activity of the vaccine-induced CTLs was independent of GD2 expression, suggesting recognition of a novel tumor-associated Ag cross-reacting with 47-LDA. Glycan microarray and immunoblotting studies using 14G2a mAb demonstrated that this Ab is highly specific for the entire carbohydrate motif of GD2 but also cross-reacts with a 105 kDa glycoprotein expressed by GD2(+) and GD2(-) neuroblastoma and melanoma cells. Functional studies of tumor cells grown in three-dimensional collagen cultures with 14G2a mAb showed decreases in matrix metalloproteinase-2 activation, a process regulated by the 105 kDa-activated leukocyte cell adhesion molecule (ALCAM/CD166). A recombinant CD166 glycoprotein was shown to be recognized by 14G2a Ab and inhibition of CD166 expression by RNA interference ablated the cell sensitivity to lysis by 47-LDA-induced CD8(+) T cells in vitro and in vivo. The binding of 14G2a to CD166 was not disruptable by a variety of exo- and endo-glycosidases, implying recognition of a non-glycan epitope on CD166. These results suggest that the vaccine-induced CTLs recognize a 47-LDA cross-reactive epitope expressed by CD166, and reveal a novel mechanism of induction of potent tumor-specific cellular responses by mimotopes of tumor-associated carbohydrate Ags.

Development of Collagen II (CII)‐induced Arthritis Was Associated with High AID and IL‐17 Expression in BXD2 Mice

BXD2 mice develop spontaneous arthritis associated with increased activation‐induced cytidine deaminase (AID), development of pathogenic autoantibodies, and increased TH‐17 T cells. CII arthritis (CIA) was induced to determine the relative contribution of autoantibodies and IL‐17 to the development of arthritis. BXD2 mice with a dominant negative AID transgene (BXD2 AID‐DN Tg) were produced. Autoantibodies for CII and specific triple helical CII epitopes (U1, J1, C1) were determined by isotype‐specific ELISA. Expression of Il17 and Il17r mRNA was determined by RT‐PCR and immunohistochemical staining. Despite the heritage of a CIA‐resistant H2b MHC haplotype, BXD2 mice developed accelerated and more severe CIA compared to DBA/1 mice. Both BXD2 and DBA/1 mice developed significantly higher levels of antibodies to C1, U1, and CII compared to B6 mice (p<0.01). BXD2 mice had an increased T‐ and B‐cell activation after CII injection, and T cells from BXD2 mice exhibited a significantly elevated IL‐17 production, compared to DBA/1 and B6 mice. Anti‐CII antibodies were associated with increased TH‐17 in the spleens and increased IL‐17R on GC B cells in BXD2 mice. The presence of AID‐DN Tg significantly suppressed the development of these autoantibodies. Our results suggest that IL‐17 acting on IL‐17R+ B cells in the spleens may be a novel mechanism for induction of AID leading to the development of CII‐specific antibodies and acceleration of CII arthritis. This work is supported by an NIH T32 Award, VAMC Merit Review Award, American College of Rheumatology, Arthritis Foundation, and Daiichi‐Sankyo Co., Ltd.

Regulation of the acuF gene, encoding phosphoenolpyruvate carboxykinase in the filamentous fungus Aspergillus nidulans.

Phosphoenolpyruvate carboxykinase (PEPCK) is a key enzyme required for gluconeogenesis when microorganisms grow on carbon sources metabolized via the tricarboxylic acid (TCA) cycle. Aspergillus nidulans acuF mutants isolated by their inability to use acetate as a carbon source specifically lack PEPCK. The acuF gene has been cloned and shown to encode a protein with high similarity to PEPCK from bacteria, plants, and fungi. The regulation of acuF expression has been studied by Northern blotting and by the construction of lacZ fusion reporters. Induction by acetate is abolished in mutants unable to metabolize acetate via the TCA cycle, and induction by amino acids metabolized via 2-oxoglutarate is lost in mutants unable to form 2-oxoglutarate. Induction by acetate and proline is not additive, consistent with a single mechanism of induction. Malate and succinate result in induction, and it is proposed that PEPCK is controlled by a novel mechanism of induction by a TCA cycle intermediate or derivative, thereby allowing gluconeogenesis to occur during growth on any carbon source metabolized via the TCA cycle. It has been shown that the facB gene, which mediates acetate induction of enzymes specifically required for acetate utilization, is not directly involved in PEPCK induction. This is in contrast to Saccharomyces cerevisiae, where Cat8p and Sip4p, homologs of FacB, regulate PEPCK as well as the expression of other genes necessary for growth on nonfermentable carbon sources in response to the carbon source present. This difference in the control of gluconeogenesis reflects the ability of A. nidulans and other filamentous fungi to use a wide variety of carbon sources in comparison with S. cerevisiae. The acuF gene was also found to be subject to activation by the CCAAT binding protein AnCF, a protein homologous to the S. cerevisiae Hap complex and the mammalian NFY complex.

A novel mechanism for induction of increased cortical porosity in cases of intracapsular hip fracture

It has been suggested that, in hip fracture, the cortex on the inferoanterior (IA) to superoposterior (SP) axis is thinned and shows increased porosity. This is dependent on the presence of giant canals (i.e., diameter >385 μm), which are related to clusters of remodeling osteons. To investigate further the relationship between remodeling and bone loss, osteonal diameter (On.Dm), wall thickness (W.Th), osteoid width (O.Wi), and extent (OS) were measured in femoral neck biopsies from 12 female intracapsular hip fracture cases and 11 age- and gender-matched controls. Over 83% of giant canals were “composite” osteonal systems in which a single canal was surrounded by multiple packets of osteonal bone. Among smaller canals, over 80% of systems had a canal encircled by a single cement line containing one packet of bone (“simple”). Composites were nearly twice as prevalent in fractures (fracture cases 9.8 ± 0.7/25 mm 2, controls 5.3 ± 0.4/25 mm 2, p < 0.0001), and were dependent ( R 2 = 0.52) on femoral neck region ( p = 0.0008) and the regional distribution of clusters of remodeling osteons ( p = 0.0045). Both the inferior (I) and anterior (A) regions had an elevated number of composites (I: 263% of control values, p = 0.0054; A: 202% of control values, p = 0.0092). On.Dm was similar in fracture cases and controls (simple: fracture cases 183 ± 3 μm, controls 191 ± 4 μm; composites: fracture cases 446 ± 13 μm, controls 460 ± 13 μm). W.Th in simples was similar in fracture cases and controls (fracture cases 51 ± 0.8 μm, controls 49 ± 0.7 μm), but composites had significantly ( p < 0.0001) thinner walls, with the reduction in fracture cases (31%) being twice that of controls (12%, p < 0.0001). There were no differences in O.Wi. It was unusual for osteoid to fully surround the composite canal surface; OS was 38% lower in composite than simple canals ( p < 0.0001). This study indicates that, in the femoral neck cortex, the principal remodeling deficit in hip fracture is specific to composite osteons. Hip fracture cases had zonal increases in composite osteon density with reduced bone formation. The data suggest that generation of composite osteons is a plausible mechanism leading to increasing porosity and trabecularization of the cortex, thus weakening the cortex in regions maximally loaded on fall impact.

Inhibition of PKB/Akt1 by C2-Ceramide Involves Activation of Ceramide-Activated Protein Phosphatase in PC12 Cells

Accumulation of ceramide has been reported in stress- and receptor-induced apoptosis in the nervous system. However, its role in apoptosis signaling remains elusive. We describe here the inhibition of the NGF-activated phosphoinositide 3-kinase (PI3K)-PKB/Akt1 survival pathway by the cell permeable analog C2-ceramide. C2-ceramide did not inhibit ERK, PI3K, or PDK1 activities and did not alter the translocation of PDK1 and Akt1 to the plasma membrane, but blocked nuclear translocation of Akt1. Down-regulation of the Akt pathway was due to enhanced dephosphorylation of Akt1 at residues T308 and S473. Moreover, Akt1 was dephosphorylated in vitro by a cation-independent phosphatase involving ceramide-activated protein phosphatase (CAPP). Membrane-anchored Akt1 was more resistant to dephosphorylation/inactivation by C2-ceramide than wild-type Akt1. Consistently, N-myristylated-Akt1 conferred resistance to the apoptosis induced by C2-ceramide in PC12 cells. These results provide a novel mechanism for induction of apoptosis by ceramide in nerve-derived cells.

Arsenic[III] and heavy metal ions induce intrachromosomal homologous recombination in thehprt gene of V79 Chinese hamster cells

In the present study the carcinogenic metal ions Cd[II], Co[II], Cr[VI], Ni[II], and Pb[II], as well as As[III], were examined for their ability to induce intrachromosomal homologous and nonhomologous recombination in the hprt gene of two V79 Chinese hamster cell lines, SPD8 and Sp5, respectively. With the exception of Pb[II], all of these ions enhanced homologous recombination, the order of potency being Cr>Cd>As>Co>Ni. In contrast, Cr[VI] was the only ion to enhance recombination of the nonhomologous type. In order to obtain additional information on the mechanism of recombination in the SPD8 cell line, individual clones exhibiting metal-induced recombination were isolated, and the sequence of their hprt gene determined. These findings confirmed that all recombinogenic events in this cell line were of the homologous type, involving predominantly a chromatid exchange mechanism. The mechanisms underlying the recombination induced by these ions are discussed in relationship to their genotoxicity, as well as to DNA repair and replication. Induced recombination may constitute a novel mechanism for induction of neoplastic disease.

Monocyte Induction of IL-10 and Down-Regulation of IL-12 by iC3b Deposited in Ultraviolet-Exposed Human Skin

CD11b+ monocytic/macrophagic cells (Mo/Mph), which infiltrate into skin after UV irradiation, play an important role in UV-induced immunosuppression. Because in mice, blockade of CD11b (iC3b receptor) on monocytes and depletion of its ligand, iC3b, reverses UV-induced immunosuppression, we asked whether iC3b is deposited in human skin after UV, and whether iC3b can modulate the cytokine profile of Mo/Mph. Immunofluorescence studies revealed that iC3b was newly deposited in UV-exposed skin and was localized in apposition to infiltrating CD11b+ Mo/Mph. In addition, in situ hybridization studies showed that TNF-alpha mRNA was also induced in a similar microanatomic localization. To model the effects of these complex signals on infiltrating Mo/Mph following UV exposure, we then tested the effects of immobilized iC3b and TNF-alpha on resting blood monocytes. Both IL-10 mRNA synthesis and protein secretion were significantly induced by binding of iC3b in vitro and were synergistically increased by the presence of TNF-alpha. The effect was abrogated by a blocking Ab to CD11b, indicating CD11b-iC3b interaction. In contrast, iC3b binding resulted in suppression of IL-12 p40 mRNA and significantly inhibited the production of IL-12 p70 protein. Our studies thus define a novel mechanism for induction of tissue Mo/Mph into an IL-10high/IL-12low state via iC3b in combination with TNF-alpha.

Unmethylated CpG Motifs Protect Murine B Lymphocytes against Fas-Mediated Apoptosis

CD40 ligand (CD40L) has been shown to increase surface Fas expression and induce B cell sensitivity to Fas-dependent CD4+Th1 cell-mediated cytotoxicity (Th1-CMC). We investigated the role of unmethylated mitogenic CpG motifs in regulating Fas-mediated apoptosis in primary murine B cells. Unmethylated CpG motifs protected CD40L-stimulated B cells from Th1-CMC and apoptosis mediated by Fas-specific antibody. Mitogenic CpG motifs downregulated Fas expression on CD40L-stimulated B cells in a time-dependent fashion. These observations suggest that Fas-mediated apoptosis requires minimum upregulation of surface Fas expression and that CpG motifs protect B cells from Fas-mediated apoptosis by decreasing surface Fas expression. Thus, these results suggest a novel mechanism for induction of Fas resistance in B cells.

Induction of γδ transposition in response to elevated glucose-6-phosphate levels

The nonenzymatic glycosylation of nucleic acids in vitro by the reducing sugars, glucose or glucose-6-phosphate, alters both physical and biological properties. Recent investigations have demonstrated that elevated intracellular levels of glucose-6-phosphate in glycolytic mutants of E. coli resulted in a concentration-associated increase in mutations of a target plasmid. The majority of the plasmid mutations were due to large (> 1 kb) insertions or deletions. We describe here the further analysis of mutant plasmids isolated from bacteria grown under conditions which were conducive to the intracellular accumulation of glucose-6-phosphate. We have found that a number of the insertional plasmid mutations were the result of the movement of the transposable element γδ from the host genome into the plasmid. The frequency of γδ transposition was also associated with the amount of glucose-6-phosphate accumulated in the bacterial cells. Furthermore, the presence of another transposable element, either Tn 5 or Tn 10 in the host genome increased the rate of γδ transposition without affecting its own movement. The observed increase in γδ transposition suggests a novel mechanism of induction by reducing sugar which may be the result of DNA modifications by reducing sugars.