Reversion of bovine papillomavirus-induced transformation and immortalization by a xanthate compound

Sphingomyelin synthase as a potential target for D609-induced apoptosis in U937 human monocytic leukemia cells

Tricyclodecan-9-yl-xanthogenate (D609) is a selective tumor cytotoxic agent. However, the mechanisms of action of D609 against tumor cells have not been well established. Using U937 human monocytic leukemia cells, we examined the ability of D609 to inhibit sphingomyelin synthase (SMS), since inhibition of SMS may contribute to D609-induced tumor cell cytotoxicity via modulating the cellular levels of ceramide and diacylglycerol (DAG). The results showed that D609 is capable of inducing U937 cell death by apoptosis in a dose- and time-dependent manner. The induction of U937 cell apoptosis was associated with an inhibition of SMS activity and a significant increase in the intracellular level of ceramide and decrease in that of sphingomyelin (SM) and DAG, which resulted in an elevation of the ratio between ceramide and DAG favoring the induction of apoptosis. In addition, incubation of U937 cells with C(6)-ceramide and/or H7 (a selective PKC inhibitor) reduced U937 cell viability; whereas pretreatment of the cells with a PKC activator, PMA or 1-oleoyl-2-acetylglycerol (OAG), attenuated D609-induced U937 cell apoptosis. These results suggest that SMS is a potential target of D609 and inhibition of SMS may contribute to D609-induced tumor cell death via modulation of the cellular levels of ceramide and DAG.

Prodrug Modification Increases Potassium Tricyclo[5.2.1.02,6]-decan-8-yl Dithiocarbonate (D609) Chemical Stability and Cytotoxicity against U937 Leukemia Cells

Potassium tricyclo[5.2.1.0(2,6)]-decan-8-yl dithiocarbonate (D609) is a selective antitumor agent, potent antioxidant, and cytoprotectant. It has the potential to be developed as a unique chemotherapeutic agent that may provide dual therapeutic benefits against cancer, e.g., enhancing tumor cell death while protecting normal tissues from damage. However, D609 contains a dithiocarbonate (xanthate) group [O-C(=S)S(-)/O-C(=S)SH], which is chemically unstable, being readily oxidized to form a disulfide bond with subsequent loss of all biological activities. Therefore, we developed the synthesis of a series of S-(alkoxyacyl) D609 prodrugs by connecting the xanthate group of D609 to an ester via a self-immolative methyleneoxyl group. These S-(alkoxylacyl)-D609 prodrugs are designed to release D609 in two steps: esterase-catalyzed hydrolysis of the acyl ester bond followed by conversion of the resulting hydroxymethyl D609 to formaldehyde and D609. Three S-(alkoxyacyl) D609 prodrugs were synthesized by varying the steric bulkiness of the acyl group. These prodrugs are stable to ambient conditions, but readily hydrolyzed by esterases to liberate D609 in a controlled manner. More importantly, the lead prodrug methyleneoxybutyryl D609 is biologically more effective than D609 in inhibiting sphingomyelin synthase, thereby increasing the level of ceramide and inducing apoptosis in U937 leukemia cells. The prodrug has a significantly lower LD(50) value than that of D609 (56.6 versus 117 microM) against U937 cells. These findings demonstrate that prodrug modification of the xanthate moiety with an alkoxyacyl group can improve D609 oxidative stability and enhance its antitumor activity.

Priming of alveolar macrophage respiratory burst by H(2)O(2) is prevented by phosphatidylcholine-specific phospholipase C inhibitor Tricyclodecan-9-yl-xanthate (D609)

The respiratory burst in alveolar macrophages is enhanced in vitro by pre-exposure to nontoxic concentrations of hydroperoxides before stimulation by an agonist, which may represent a feed-forward regulatory mechanism. Tricyclodecan-9-yl-xanthate (D609), an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), suppresses this priming of the respiratory burst by pre-exposure to H(2)O(2) in NR8383 alveolar macrophages (up to 100 microM D609, 400 nmol of H(2)O(2) added to 5 x 10(6) cells 15 min before stimulation with ADP). D609 has potential as an antioxidant due to its dithiocarbonate functional group that allows it to slowly react with H(2)O(2) and rapidly reduce cytochrome c, which interferes with a common assay for the respiratory burst. Nonetheless, the antioxidant properties of D609 do not account for its inhibition of priming of the respiratory burst by H(2)O(2). Reduction of nitro blue tetrazolium is the basis for an assay for superoxide production with which D609 does not interfere. With this assay, it was found that D609 does not inhibit the respiratory burst per se, but prevents its enhancement by pre-exposure to H(2)O(2). Consistent with a role of diacylglycerol generation by phospholipase C, this enhancement was mimicked by pre-exposure to phorbol ester. In contrast with priming, receptor-mediated stimulation of the respiratory burst depends on the better characterized phosphatidylinositol-specific phospholipase C. Priming of the respiratory burst by H(2)O(2) joins the list of inflammatory responses that are inhibited by D609. Nevertheless, the results herein indicate that caution should be exercised in the interpretation of the effects of D609 to consider both antioxidant effects and inhibition of PC-PLC.

Stimulation of DNA synthesis in untransformed cells by the antiviral and antitumoral compound tricyclodecan-9-yl-xanthogenate (D609)

The antiviral and antitumor xanthate compound tricyclodecan-9-yl-xanthogenate (D609) is best known for its inhibitory effect on phosphatidylcholine-specific phospholipase C activity. Now we report that in NIH 3T3 cells, but not in several transformed cell types tested, D609 stimulated DNA synthesis when phosphocholine (PCho), insulin, or ATP was also present. Maximal co-mitogenic effects of D609 were observed at 5 microg/mL, a concentration 4-6 times lower than that required to inhibit phospholipase C activity. The synergistic mitogenic effects of D609 and PCho, but not of D609 and insulin, were associated with activation of p42 and, to a lesser extent, p44 mitogen-activated protein (MAP) kinases. The results raise the possibility that the mitogenic activity of D609 in untransformed cells may contribute to its antiviral and antitumor effects.

The antiviral xanthate compound D609 inhibits herpes simplex virus type 1 replication and protein phosphorylation

The mechanism of antiviral action of tricyclodecan-9-yl-xanthogenate (D609) was investigated in vitro. D609 inhibited herpes simplex virus type 1 (HSV-1) replication without apparent cytotoxicity. It reduced phosphorylation of virus-infected cell polypeptides and inhibited the HSV-1 encoded protein kinase (US3 PK) and, to a lesser extent, cellular protein kinase C in vitro. Virus production was reduced by D609 at concentrations greater than 3.8 microM, with complete inhibition at 75.2 microM at an MOI of 1 PFU/cell or less. Addition of D609 could be delayed until 7 h post-infection and still inhibit virus replication. Phosphorylation of infected cell viral polypeptides of 34 (similar molecular weight to the substrate of the viral US3 protein kinase) and 69 kDa was inhibited at 18.4 microM. Treatment of infected or uninfected cells with 37.6 microM D609 reduced protein phosphorylation to background levels. A concentration of 1.9 microM D609 in vitro inhibited the viral US3-encoded PK, which had been purified from infected cell lysates by affinity chromatography and identified by specific antibody. Purified cellular protein kinase C was inhibited at 75.2 microM D609 whereas other cellular kinases including casein kinase 1 and cAMP dependent kinase were not inhibited at concentrations as high as 188 microM D609. Collectively these data indicate that the mechanism of antiviral action of D609 is by inhibition of protein kinases and protein phosphorylation affecting a late step in HSV replication.

Binding of NF-kB to the HIV-1 LTR is not sufficient to induce HIV-1 LTR activity

Human immunodeficiency virus type 1 (HIV-1) spends a significant part of its life cycle as latent provirus in nonactivated cells. It induction requires mitogen stimulation. TPA treatment induces HIV-1 transcription by protein kinase C (PKC)-mediated activation of the cellular transcription factor NF-kB. PKC activation induces the dissociation of NF-kB from its inhibitor protein (IkB). The liberated NF-kB then binds to its proviral recognition sequence in the HIV-1 long terminal repeat (LTR) sequence. This step, however, is not sufficient to augment transcription. We demonstrate that NF-kB-mediated HIV-1 LTR activation is regulated by an additional event that is not dependent on IkB. A further phosphorylation event is proposed, since this step could be blocked by an inhibitor of a phospholipase C (PLC) type reaction. This inhibitor precludes the formation of diacylglycerols, which are required for activation of PKC isoenzymes. As an alternative pathway that is not dependent on PLC reactions, high-level transcription from the HIV-1 LTR is shown to require binding of both NF-kB and TAT.

Viruses and oral cancer

Oral cancer is a disease with a complex etiology. There is evidence for important roles of smoking, drinking, and genetic susceptibility, as well as strong indications that DNA viruses could be involved. The herpes simplex virus type 1 has been associated with oral cancer by serological studies, and animal models and in vitro systems have demonstrated that it is capable of inducing oral cancer. Papillomaviruses are found in many oral cancers and are also capable of transforming cells to a malignant phenotype. However, both virus groups depend on co-factors for their carcinogenic effects. Future research on viruses and oral cancer is expected to clarify the role of these viruses, and this will lead to improvements in diagnosis and treatment of the disease.

DNA methylation and cellular ageing

Methylated cytosine (m5C) in DNA appears to be an important modulator of the expression of some genes. There are several lines of evidence that gradual loss of m5C is relevant to in vitro cellular ageing: m5C loss occurs during cell culture; m5C loss is detectable at an early stage of culture; m5C loss appears to be related to cell division not just duration in culture; the rate of m5C loss appears to be related to in vitro lifespan of the cell strain in question; and the total loss of m5C during an in vitro lifespan is significant by comparison with induced-changes in m5C levels which effect cell growth, or cause cell-death in culture. Progressive loss of m5C in dividing cells may thus produce the multi-step cell division "clock" which underlies the Hayflick phenomenon.

Inhibition of c-fos transcription and phosphorylation of the serum response factor by an inhibitor of phospholipase C-type reactions

Phospholipase C activity is necessary for transcriptional c-fos activation by providing diacylglycerol as an activator of protein kinase C. We found that transcriptional activation of c-fos and the phosphorylation of its major transcription factor were inhibited by tricyclodecan-9-yl xanthogenate, which blocks phospholipase C-type reactions. Transcription of the c-ras and beta-actin genes in the same cells remained unaffected.

Inhibition of c-fos transcription and phosphorylation of the serum response factor by an inhibitor of phospholipase C-type reactions

Phospholipase C activity is necessary for transcriptional c-fos activation by providing diacylglycerol as an activator of protein kinase C. We found that transcriptional activation of c-fos and the phosphorylation of its major transcription factor were inhibited by tricyclodecan-9-yl xanthogenate, which blocks phospholipase C-type reactions. Transcription of the c-ras and beta-actin genes in the same cells remained unaffected.

Restoration of the responsiveness to growth factors in senescent cells by an embryonic cell extract

In senescent fibroblast cell cultures which have approached a postmitotic stage in vitro, responsiveness to growth factors is restored upon exposure to an embryonic sheep cell extract. The extract contains molecules below a molecular weight of 1 x 10(5) Da in aqueous solution. Following a transient exposure to the extract, mitotic activity is resumed, and the cells keep dividing over several passages. The target cells which respond to the treatment were identified in a single-cell assay as those that still had the capacity to undergo at least several mitotic divisions before entering the final stage of senescence.

Tumor necrosis factor induces necrosis of human carcinoma xenografts in the presence of tricyclodecan-9-yl-xanthogenate and lauric acid

Recombinant human tumor necrosis factor (rh TNF) when administered intravenously together with the phospholipase C inhibitor tricyclodecan-9-yl-xanthogenate (D609) and lauric acid (C12), leads to the partial regression of various human tumor transplants in athymic mice. Extensive necrosis occurred after a single intravenous infusion, with no detectable side effects. TNF-mediated cytotoxicity was found to be correlated with the depletion of energy in HeLa cells. The activity of rh TNF was enhanced by the absence of glucose, while it was reduced by addition of extraneous ATP. In the presence of rh TNF, D609, and C12, cellular energy metabolism was almost completely switched to glycolysis. Under these conditions the cytocidal activity of rh TNF on HeLa cells was amplified at least 60-fold.

Interruption of TPA-induced signals by an antiviral and antitumoral xanthate compound: inhibition of a phospholipase C-type reaction

The effect of tricyclodecan-9-yl-xanthogenate on the phorbolester TPA induced changes in phosphatidylcholine metabolism was investigated. In the simultaneous presence of the xanthate TPA failed to stimulate the metabolic [32P] turnover of the major phospholipids. The precursor molecule [3H] choline was incorporated into phosphatidylcholine after pulse labeling in TPA/D609-treated cells. Thus, the reduction of the [32P] phosphatidylcholine turnover did not appear to result from an inhibition of the TPA-stimulated phosphatidylcholine biosynthesis. However, the xanthate exerted an inhibitory effect on the TPA-stimulated liberation of [3H] phosphorylcholine from [3H] phosphatidylcholine in cells prelabeled with [3H] choline. Furthermore, the TPA-induced rise in the diacylglycerol level was reduced in the presence of the compound. Thus, these results provide evidence that the xanthate inhibits a TPA-induced phospholipase C activity in the intact cell.

Tumor prevention by a xanthate compound in experimental mouse-skin tumorigenesis

The antiviral and antitumoral compound tricyclodecan-9-yl-xanthogenate (D609), which is an inhibitor of protein kinase C activation, has been used for tumor prevention in vivo. When applied chronically together with 12-O-tetradecanoylphorbol-13-acetate (TPA) in the classic initiation-promotion mouse-skin model, D609 prevented tumor induction in a dose-dependent manner. At the concentration that inhibited tumor formation by 97%, no toxic effects were detected and the TPA-induced hyperplasia remained unaffected. As D609 failed to prevent the activity of a chronically applied carcinogen, it is concluded that the observed tumor prevention achieved with D609 is tumor-promotion-specific and is not due to killing of tumor cells.

Interruption of growth signal transduction by an antiviral and antitumoral xanthate compound

The binding of growth factors to the cellular receptors elicits the phosphorylation of proteins which transmit growth signals to the nucleus [E. Rozengurt (1986) Science 234, 161-166]. Both the tyrosine-specific kinase (growth factor receptor) and the threonine-serine phosphorylating protein kinase C (pkC) become activated upon binding of the epidermal growth factor (EGF) to its receptor. Here we describe the selective inhibition of the pkC activation by tricyclodecane-9-yl-xanthogenate (D609) in the presence of unsuppressed receptor tyrosine autophosphorylation. As a consequence the affinity of EGF to the receptor was not down-regulated and the complex failed to be internalized.

"Hit and run" transformation of mouse C127 cells by bovine papillomavirus type 4: the viral DNA is required for the initiation but not for maintenance of the transformed phenotype

Morphological transformation of C127 mouse fibroblasts by bovine papillomavirus type 4 (BPV-4) DNA depends on additional factors, including cell density, the presence of TPA, the concentration of fetal calf serum, and the physical state of the input DNA. Low cell density or the presence of TPA allows the achievement of full transformation, suggesting that disturbance of cell-to-cell contact may be necessary for the expression of the malignant phenotype. TPA also induces a burst of viral DNA synthesis and transcription. The frequency of focus formation is fivefold higher in the presence of high serum than in low serum, indicating that growth factors are required during the early stages of transformation. Although the same number of foci is obtained with either circular or linear DNA, the cells transfected with linear DNA are at a more advanced stage of transformation, being capable of anchorage-independent growth and tumorigenic in nude mice; only 5 out of 21 of these cell lines contain viral DNA in integrated and rearranged forms. In contrast cells transfected with circular DNA are anchorage-dependent but tumorigenic in mice, and do not contain any viral DNA. The linear DNA has an interrupted E1 ORF and fully transformed cells are obtained also with a subgenomic viral DNA fragment containing part of the L1 ORF, the complete E6, E7, and E8 ORFs, and the truncated E1 ORF under the control of the BPV-4 regulatory region; viral DNA is not maintained in any of these cell lines. In total, only 15% of the transformed cell lines contains viral DNA. These results are discussed in the light of the "hit and run" model of cell transformation.

Phenotypic transformation of primary mouse fibroblasts by BPV 1 DNA

Cultures of primary fibroblasts of C57BL/6J mice were used as targets for transformation by bovine papillomavirus type 1 (BPV 1) DNA. Although no foci were observed, several lines of transformed cells were established by subculturing. These immortalized cell lines had in vitro growth characteristics in high and low serum media and saturation densities typical of transformed cells. Karyotype analyses revealed extensive aneuploidic changes. In two of the three cell lines analyzed, viral DNA was present in monomeric episomal form, in the third cell line all viral sequences were found in the high molecular weight region of a Southern blot. Despite the transformed phenotype, only one of the cell lines was tumorigenic in nude mice at a low level.

Inhibition of HIV-1 replication by an antiviral xanthate compound in vitro

The antiviral xanthate compound tricyclodecan-9-yl-xanthogenate (code name D609) is capable of inhibiting DNA and RNA viruses in vitro. It can also inhibit the shedding of infectious HIV into the tissue culture medium from chronically infected lymphoma cells (KE37-III) as shown by infectivity assays and Western blots of the supernatant. HIV-specific proteins, however, were accumulated intracellularly. The initiation of a de novo HIV replication after infection of permissive KE37-1 cells was completely inhibited at concentrations of D609 which still permitted mitotic divisions of the cells. Furthermore, the selective antiviral activity of the xanthate compound was evidenced by the absence of HIV replicative intermediate DNA. The expression of cellular genes, such as c-myc, remained unimpaired within these cells.

Selective killing of tumor cells by xanthates

Xanthate derivatives of primary alcohols with antiviral properties exert, in combination with monocarboxylic C11 or C12 acids a pronounced anti-tumor activity in vitro and in vivo. Tricyclodecan-9-yl-xanthogenate (D609) or cyclododecyl xanthogenate (D435) when administered together with either undecanoic or dodecanoic acid to various transformed animal and human tumor cells (displaying low serum requirement) cause cell death. In contrast, normal cells from which transformed derivatives arose, were unaffected.

The in vitro lifespan of MRC-5 cells is shortened by 5-azacytidine-induced demethylation

The minor base 5-methylcytosine (5mC) in DNA may be important for the regulation of gene expression. Random loss of 5mC may occur during pre-replicative DNA synthesis in mortal cell strains, and thus give rise to biochemical aberrations in aging cells. 5-Azacytidine (5azaC) was used to induce loss of 5mC in DNA of human diploid fibroblasts (MRC-5) in an attempt to accelerate in vitro senescence. The 5mC content of DNA was measured by incorporation of [3H]uridine into dividing cells, hydrolysis of DNA and separation of bases by HPLC. In untreated MRC-5 cells, 5mC was 3.6% of the total cytosine (C+5mC) at population doubling (PD) 20 (28% of lifespan) and fell to 1.6% at PD 67 (97% of lifespan). A single pulse treatment with 5azaC (1 microgram/ml) induced demethylation and shortened the lifespan by 10% (6.8 PDs loss). Pulse-treated cells showed temporary growth inhibition, though they subsequently regained normal growth rate and morphology. However, uniform treatment with 0.1 microgram/ml 5azaC between PD 20 and 23 produced no immediate growth inhibition, but a 22% loss of 5mC and 25% decrement in lifespan (16.6 PDs loss). The present results indicate that 5mC levels fall during normal aging of MRC-5 cells and accelerated 5mC loss shortens the in vitro lifespan of these cells. Hypomethylation may thus be responsible for some aspects of in vitro aging.