Human cytomegalovirus. Stimulation of [3H] release from [3H]-arachidonic acid prelabelled cells

Modulation of Radiation-Induced Genetic Damage by HCMV in Peripheral Blood Lymphocytes from a Brain Tumor Case-Control Study

Human cytomegalovirus (HCMV) infection occurs early in life and viral persistence remains through life. An association between HCMV infection and malignant gliomas has been reported, suggesting that HCMV may play a role in glioma pathogenesis and could facilitate an accrual of genotoxic damage in the presence of g-radiation; an established risk factor for gliomas. We tested the hypothesis that HCMV infection modifies the sensitivity of cells to γ-radiation-induced genetic damage. We used peripheral blood lymphocytes (PBLs) from 110 glioma patients and 100 controls to measure the level of chromosome damage and cell death. We evaluated baseline, HCMV-, γ-radiation and HCMV + γ-radiation induced genetic instability with the comprehensive Cytokinesis-Blocked Micronucleus Cytome (CBMN-CYT). HCMV, similar to radiation, induced a significant increase in aberration frequency among cases and controls. PBLs infected with HCMV prior to challenge with γ-radiation led to a significant increase in aberrations as compared to baseline, γ-radiation and HCMV alone. With regards to apoptosis, glioma cases showed a lower percentage of induction following in vitro exposure to γ-radiation and HCMV infection as compared to controls. This strongly suggests that, HCMV infection enhances the sensitivity of PBLs to γ-radiation-induced genetic damage possibly through an increase in chromosome damage and decrease in apoptosis.

Inhibition of cyclooxygenase activity blocks cell-to-cell spread of human cytomegalovirus

Human cytomegalovirus has previously been shown to induce the accumulation of cyclooxygenase-2 RNA, protein, and enzyme activity. High doses of cyclooxygenase inhibitors substantially block viral replication in cultured fibroblasts. However, doses corresponding to the level of drug achieved in the plasma of patients have little effect on the replication of human cytomegalovirus in cultured cells. Here, we demonstrate that two nonsteroidal anti-inflammatory drugs, tolfenamic acid and indomethacin, markedly reduce direct cell-to-cell spread of human cytomegalovirus in cultured fibroblasts. The block is reversed by addition of prostaglandin E2, proving that it results from the action of the drugs on cyclooxygenase activity. Because direct cell-to-cell spread likely contributes importantly to pathogenesis of the virus, we suggest that nonsteroidal anti-inflammatory drugs might help to control human cytomegalovirus infections in conjunction with other anti-viral treatments.

The chemokine receptor homologue encoded by US27 of human cytomegalovirus is heavily glycosylated and is present in infected human foreskin fibroblasts and enveloped virus particles

Human cytomegalovirus (HCMV), a member of the beta-herpesvirus family, encodes four homologues of cellular G protein-coupled receptors (GPCRs). One of these, the protein product of HCMV open reading frame (ORF) UL33, has been identified in HCMV-infected cells and virus particles and shown to be heat-aggregatable and N-glycosylated. Another, the product of ORF US28, has been functionally characterized as a beta-chemokine receptor. Here we report the use of RT-PCR, coupled in vitro transcription-translation, immunoprecipitation, and Western immunoassays to (i) show that RNA from the open reading frame US27 appears predominantly during the late phase of replication; (ii) identify the protein encoded by HCMV US27 in infected cells and enveloped virus particles; (iii) demonstrate that the US27-encoded protein is heterogeneously N-glycosylated and resolves as two species following treatment with peptide N-glycosidase F; and (iv) show that both the recombinant and deglycoylated infected cell US27 protein aggregate when heated in the presence of SDS prior to electrophoresis in polyacrylamide gels, a property which is abrogated with the addition of urea to sample buffer.

Human cytomegalovirus carries a cell-derived phospholipase A2 required for infectivity

Human cytomegalovirus (HCMV) is known to carry host cell-derived proteins and mRNAs whose role in cell infection is not understood. We have identified a phospholipase A2 (PLA2) activity borne by HCMV by using an assay based on the hydrolysis of fluorescent phosphatidylcholine. This activity was found in all virus strains analyzed and in purified strains. It was calcium dependent and was sensitive to inhibitors of cytosolic PLA2 (cPLA2) but not to inhibitors of soluble PLA2 or calcium-independent PLA2. No other phospholipase activity was detected in the virus. Purified virus was found to contain human cellular cPLA2alpha, as detected by monoclonal antibody. No homology with PLA2 was found in the genome of HCMV, indicating that HCMV does not code for a PLA2. Decreased de novo expression of immediate-early proteins 1 and 2 (IE1 and IE2), tegument phosphoprotein pp65, and virus production was observed when HCMV was treated with inhibitors of cPLA2. Cell entry of HCMV was not altered by those inhibitors, suggesting the action of cPLA2 was postentry. Together, our results indicate a selective sorting of a cell-derived cPLA2 during HCMV maturation, which is further required for infectivity.

Human cytomegalovirus elicits a coordinated cellular antiviral response via envelope glycoprotein B

Previous studies have shown that human cytomegalovirus (CMV) is a potent elicitor of interferon-stimulated gene (ISG) expression. Induction of the interferon pathway does not require replication-competent virus, and envelope glycoprotein B (gB) from CMV is a viral structural component that can directly induce transcription of ISGs. Here we extend these earlier findings by defining the consequences of inducing the interferon pathway. We found that cells respond to CMV or soluble gB by establishing a functional antiviral state within cell types critical in CMV biology, such as fibroblasts and endothelial cells. We have also discovered new insights into the mechanism by which the pathway is initiated. Interferon regulatory factor 3 (IRF3), a key transcriptional regulator of cellular interferon responses, is activated by CMV virions and soluble gB. Thus, IRF3 becomes activated via "outside-in" signal transduction events. This is a novel mechanism of activation of this key transcription factor by viruses. In comparison to soluble gB (gB(1-750)), which comprises the entire ectodomain of gB, a truncation mutant encompassing only the amino-terminal region of gB (gB(1-460)) was markedly less effective at inducing antiviral responses. This indicates that the region of gB from residues 461 to 750 is important for initiation of the antiviral response. In addition, CMV and gB establish an antiviral state in alpha/beta interferon null cells, illustrating that primary induction of ISGs by CMV and gB is sufficient to establish the antiviral response and that interferon secretion is not necessary for the antiviral effect. Taken together, our findings reveal that CMV initiates a coordinated antiviral response through contact between gB and an as-yet-unidentified cell surface receptor(s).

Degradation of p21cip1 in cells productively infected with human cytomegalovirus

Human cytomegalovirus (HCMV) stimulates arrested cells to enter the cell cycle by activating cyclin-dependent kinases (Cdks), notably Cdk2. Several mechanisms are involved in the activation of Cdk2. HCMV causes a substantial increase in the abundance of cyclin E and stimulates translocation of Cdk2 from the cytoplasm to the nucleus. Further, the abundance of the Cdk inhibitors (CKIs) p21cip1/waf1 (p21cip1) and p27kip1 is substantially reduced. The activity of cyclin E/Cdk2 increases as levels of CKIs, particularly p21cip1, fall. We have previously shown that these phenomena contribute to priming the cell for efficient replication of HCMV. In this study, the mechanisms responsible for the decrease in p21cip1 levels after HCMV infection were investigated by measuring p21cip1 RNA and protein levels in permissive human lung (LU) fibroblasts after HCMV infection. Northern blot analysis revealed that p21cip1 RNA levels increased briefly at 3 h after HCMV infection and then decreased to their nadir at 24 h; thereafter, RNA levels increased to about 60% of the preinfection level. Western blot analysis demonstrated that the relative abundance of p21cip1 protein roughly paralleled the observed changes in initial RNA levels; however, the final levels of protein were much lower than preinfection levels. After a transient increase at 3 h postinfection, p21cip1 abundance declined sharply over the next 24 h and remained at a very low level through 96 h postinfection. The disparity between p21cip1 RNA and protein levels suggested that the degradation of p21cip1 might be affected in HCMV-infected cells. Treatment of HCMV-infected cells with MG132, an inhibitor of proteasome-mediated proteolysis, provided substantial protection of p21cip1 in mock-infected cells, but MG132 was much less effective in protecting p21cip1 in HCMV-infected cells. The addition of E64d or Z-Leu-Leu-H, each an inhibitor of calpain activity, to HCMV-infected cells substantially increased the abundance of p21cip1 in a concentration-dependent manner. To verify that p21cip1 was a substrate for calpain, purified recombinant p21cip1 was incubated with either m-calpain or mu-calpain, which resulted in rapid proteolysis of p21cip1. E64d inhibited the proteolysis of p21cip1 catalyzed by either m-calpain or mu-calpain. Direct measurement of calpain activity in HCMV-infected LU cells indicated that HCMV infection induced a substantial and sustained increase in calpain activity, although there was no change in the abundance of either m- or mu-calpain or the endogenous calpain inhibitor calpastatin. The observed increase of calpain activity was consistent with the increases in intracellular free Ca2+ and phospholipid degradation in HCMV-infected LU cells reported previously from our laboratory. Considered together, these results suggest that the increase in calpain activity observed following HCMV infection contributes significantly to the reduction of p21cip1 levels and the resultant cell cycle progression.

Human cytomegalovirus-induced immunosuppression. Relationship to tumor necrosis factor-dependent release of arachidonic acid and prostaglandin E2 in human monocytes

Cytomegalovirus (CMV) has been associated with immunosuppression. Previously CMV was reported to interfere with signal transduction pathways in T cells. In this report the mechanisms underlying CMV-mediated immunosuppression were examined. Supernatants of CMV (Strains C-87, AD-169)-infected primary human monocyte (MO) cultures inhibited mitogenic T cell proliferative responses by > 95%. The inhibitory activity was observed 24 h through day 7 postinfection. The infection of MO was associated with a sustained elevation of intracellular levels of cAMP and the release of arachidonic acid (AA) and its metabolite PGE2 (activator of adenylate cyclase) in culture supernatants. The AA release was incidentally associated with TNF-alpha production. Monoclonal antibodies to TNF-alpha and pentoxyphylline (inhibitor of TNF synthesis) inhibited both AA and PGE2 release. The release of AA required protein synthesis and occurred under conditions consistent with the expression of CMV immediate early genes. Treatment of MO cultures at time of infection with 100 microM indomethacin or 1 microg of TNF-alpha mAb abolished the CMV-induced T cell inhibitory activity of the supernatants by 100%. These data suggest that TNF dependent release of AA and PGE2 contributes to CMV-induced immunosuppression.

Poxvirus-induced alteration of arachidonate metabolism

Recent evidence suggests that orthopoxviruses have an obligate requirement for arachidonic acid metabolites during replication in vivo and in vitro. Our report indicates that a virus family (Poxviridae) possesses multiple genes that function to regulate arachidonate metabolism. Analyses of BS-C-1 cells infected with cowpox virus or vaccinia virus detected enhanced arachidonate product formation from both the cyclooxygenase (specifically prostaglandins E2 and F2 alpha) and lipoxygenase (specifically 15-hydroxyeicosatetraenoic acid and 12-hydroxyeicosatetraenoic acid) pathways. In contrast, human parainfluenza type 3 or herpes simplex virus type 1 infections did not increase arachidonate metabolism. Results were consistent with a virus early-gene product either directly mediating or inducing a host factor that mediated the up-regulation of arachidonate metabolism, although vaccinia growth factor was not responsible. In addition, the cowpox virus 38-kDa protein-encoding gene, which is associated with inhibition of an inflammatory response, correlated with inhibition of formation of a product biochemically characteristic of (14R,15S)-dihydroxyeicosatetraenoic acid. We propose that orthopoxvirus-induced up-regulation of arachidonic acid metabolism during infection renders the infected cells susceptible to generation of inflammatory mediators from both the cyclooxygenase and the lipoxygenase pathways, and poxviruses, therefore, possess at least one gene (38K) that can alter the lipoxygenase-metabolite spectrum.

Modulation of the frequency of human cytomegalovirus-induced chromosome aberrations by camptothecin

The effects of selected DNA repair inhibitors on the frequency of human cytomegalovirus (HCMV)-induced chromosome aberrations were evaluated in human peripheral blood lymphocytes (PBLs). Treatment of HCMV-infected PBLs with camptothecin (0.05 to 0.3 micrograms/ml), an inhibitor of topoisomerase I, for 30 hr resulted in a significant (P less than 0.01) synergistic enhancement of the frequency of HCMV-induced chromosome damage. On the other hand, a significant increase in the frequency of chromosome damage was not noted for infected PBLs treated with either 3-aminobenzamide (3-AB; 3 to 30 micrograms/ml), an inhibitor of poly(ADP-ribose) polymerase, or novobiocin (3 to 30 micrograms/ml), an inhibitor of topoisomerase II or excision repair processes, for 30 hr. Chromatid-type breaks and exchanges were the predominant type of chromosome aberrations observed in the HCMV-infected cells treated with camptothecin, suggesting that HCMV infection is associated with the induction of single-strand DNA breaks. Furthermore, these findings suggest that HCMV infection does not inflict direct DNA damage which is repaired through 3-AB- or novobiocin-sensitive pathways.

Cellular oncogene activation by human cytomegalovirus. Lack of correlation with virus infectivity and immediate early gene expression

The contribution of expression of human cytomegalovirus (HCMV) immediate early (IE) genes to the rapid and transient increase in cellular (c)-oncogene (fos, jun, myc) transcription following HCMV infection was investigated. A partial temporal overlap was observed between the increases in c-oncogene RNA levels and the increase in either transcripts from HCMV IE genes or the number of cells in which HCMV IE proteins were detected. The increases in c-oncogene RNA levels, however, slightly preceded the increase in the detection of HCMV IE transcripts or proteins. To distinguish between the temporal coincidence and a direct relationship between expression of HCMV IE genes and the increased transcription of c-oncogenes, the number of cells synthesizing HCMV IE proteins was reduced by infecting with virus stock enriched in defective particles. Alternatively, the synthesis of HCMV IE proteins was essentially eliminated by ultra-violet (UV) irradiation of virus stock or by inhibitors of protein synthesis. Virus stocks enriched in defective particles demonstrated a substantially reduced capacity to direct the synthesis of HCMV IE proteins, but were more efficient in activating c-oncogene expression than infectious virus stocks. Elimination of expression of HCMV IE genes by UV-irradiation of virus stock or by inhibiting de novo viral and/or cellular protein synthesis with cycloheximide (100 micrograms/ml) or anisomycin (100 micrograms/ml) did not eliminate the HCMV-induced increase in RNA levels of c-oncogenes. These data indicate that activation of these early response cellular genes is independent from de novo expression of HCMV IE proteins, and possibly involves biologically active virion proteins that are related to the induction of a cascade of cellular events associated with the binding of HCMV to its cellular receptor.

Activation of cellular oncogenes by clinical isolates and laboratory strains of human cytomegalovirus

The effect on cellular (c) oncogene RNA levels was investigated after infection of permissive cells with cell culture adapted strains (AD-169, C-87, Davis) and unadapted clinical isolates (82-1, 84-2, 85-1) of human cytomegalovirus (HCMV). The results indicate that both adapted and unadapted strains of HCMV induce substantial increases in c-oncogene RNA levels for fos, jun, and myc measured by Northern blot hybridization. Elimination of immediate early (IE) protein synthesis between 0 and 3 hrs or reduction of virus infectivity (99.99%) by UV-irradiation did not reduce the increase in c-oncogene RNA levels. Inhibition of viral and cellular protein synthesis by cycloheximide resulted in a high abundance (superinduction) of specific RNAs which hybridized to c-oncogene probes after infection with either adapted or unadapted strains of HCMV. These data suggest that IE viral gene expression is not essential for activation of c-oncogenes. Inhibition of DNA-dependent RNA synthesis by blocking RNA elongation with actinomycin-D or by inhibiting the activity of RNA polymerase II with alpha-amanitin significantly reduced the increase in c-oncogene RNA levels, suggesting that activation of cellular genes by HCMV is controlled at the level of transcription. Activation of c-oncogenes by HCMV may be particularly important because their protein products appear to be involved in initiation and regulation of viral and cellular gene expression.

Transcriptional activation of cellular oncogenes fos, jun, and myc by human cytomegalovirus

The mechanisms responsible for the human cytomegalovirus (HCMV)-induced increase in cellular oncogene RNAs for c-jun, c-fos, and c-myc in human embryo lung cells (I. Boldogh, S. AbuBakar, and T. Albrecht, Science 247:561-564, 1990) were investigated. Results of transcription assays indicated that the rapid increase in RNA levels for the above-noted oncogenes was controlled at the transcriptional level and was related to enhanced transcription. The maximum rates of transcription for c-jun and c-fos genes occurred at 40 min postinfection, while for the c-myc gene the maximum rate occurred at about 60 min. The magnitude of HCMV-induced activation of these cellular genes was similar to the activation induced by serum. The half-lives of the cellular oncogenes showed similar decay rates after either serum or HCMV activation when measured by dactinomycin chase. The half-life for c-fos or c-jun was about 20 min, and that for c-myc was about 40 min. Furthermore, inhibition of the RNA increase by dactinomycin or by alpha-amanitin suggested that the increase in RNA levels was due to an increase in the transcriptional activity of oncogenes triggered by HCMV.