Reorganization of the cytoskeleton by interferon in MSV-transformed cells

Amyloid precursor protein induces reactive astrogliosis

AIM

Astrocytes respond to stressors by acquiring a reactive state characterized by changes in their morphology and function. Molecules underlying reactive astrogliosis, however, remain largely unknown. Given that several studies observed increase in the Amyloid Precursor Protein (APP) in reactive astrocytes, we here test whether APP plays a role in reactive astrogliosis.

METHODS

We investigated whether APP instigates reactive astroglios by examining in vitro and in vivo the morphology and function of naive and APP-deficient astrocytes in response to APP and well-established stressors.

RESULTS

Overexpression of APP in cultured astrocytes led to remodeling of the intermediate filament network, enhancement of cytokine production, and activation of cellular programs centered around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion abrogated remodeling of the intermediate filament network and blunted expression of IFN-stimulated gene products in response to lipopolysaccharide. Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein observed canonically in astrocytes in response to TBI.

CONCLUSIONS

The APP thus represents a candidate molecular inducer and regulator of reactive astrogliosis. This finding has implications for understanding pathophysiology of neurodegenerative and other diseases of the nervous system characterized by reactive astrogliosis and opens potential new therapeutic avenues targeting APP and its pathways to modulate reactive astrogliosis.

Amyloid precursor protein induces reactive astrogliosis

We present in vitro and in vivo evidence demonstrating that Amyloid Precursor Protein (APP) acts as an essential instigator of reactive astrogliosis. Cell-specific overexpression of APP in cultured astrocytes led to remodelling of the intermediate filament network, enhancement of cytokine production and activation of cellular programs centred around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion in cultured astrocytes abrogated remodelling of the intermediate filament network and blunted expression of IFN stimulated gene (ISG) products in response to lipopolysaccharide (LPS). Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein (GFAP) observed canonically in astrocytes in response to TBI. Thus, APP represents a molecular inducer and regulator of reactive astrogliosis.

Impaired angiogenic balance and suppression of tumorigenicity in HeLa cells chronically exposed to interferon-alpha

We have previously reported that IFNalpha-chronic treatment for 41 days induced a partial phenotype reversion on HeLa cells along with a down-regulation of HPV18 mRNA levels. However, tumorigenicity of these cells in nude mice was unchanged. Interestingly, after 1 year of IFNalpha-chronic exposition, HeLa cells failed to induce s.c. tumors when injected into nude mice. In such experimental conditions both HPV18 DNA integration pattern and viral DNA copy number present in HeLa cells remained intact in the nontumorigenic phenotype cells. As result of the treatment with IFNalpha, HeLa cells rendered more resistant to lysis mediated by activated natural killer cells in vitro. Furthermore, IFNalpha-chronic treatment was able to induce VEGF and decrease bFGF mRNA expression, suggesting a potential effect on the angiogenic behavior of these tumoral cells. Thus, long-term treatment of HeLa cells with IFNalpha can accomplish a reversion of the malignant phenotype by a sequential multistep mechanism, in which the antiangiogenic effect of IFNalpha could be one of the contributing events.

A Stat1alpha factor regulates the expression of the human vimentin gene by IFN-gamma

Vimentin is an intermediate filament protein normally expressed in cells of mesenchymal origin. Here, we report an increase in vimentin gene transcription induced by the cytokine interferon-y (IFN-gamma). Northern blot analysis and reporter gene assays reveal that IFN-gamma induces vimentin gene transcription in HeLa cells. However, no increase in vimentin mRNA synthesis was observed de novo in MCF-7 cells, which do not already express vimentin. Band shift analysis shows that the Stat1alpha protein mediates vimentin induction by IFN-gamma. A human mutant fibroblast cell line (U3A), which lacks Stat1alpha but expresses vimentin mRNA, yields no increase in vimentin mRNA levels on the addition of IFN-gamma. These results suggest that the induction of vimentin gene expression might be an important part of a complex cellular response to IFN-gamma.

Localization and structure of v-mos in transformed mouse fibroblasts reverted by long-term interferon treatment to nonmalignancy

We have reported previously that Moloney virus-transformed cells, when treated for over 200 passages in the presence of low concentrations of mouse interferon-alpha/beta, can be reverted to a stable nonmalignant status. The cells recover full contact inhibition and are unable to raise tumors when grafted in nude mice. In the present report, we show that whether reverted or malignant, these cells contain deleted v-mos oncogenes, which have lost 392 nucleotides. The truncated oncogenes contain a reduced and modified open reading frame but are able, however, to induce tumors when transfected in mouse 3T3 cells. As there is no difference either in the location or in the structure of this modified v-mos, whether yielded by reverted or malignant cells, we postulate that both cell lines derive from the same population and this modification does not play any role in the reversion process obtained through prolonged IFN-dependent selection. We suggest that reversion could be an epigenetic phenomenon, involving the constitutive synthesis of IFN-beta only in the reverted and not in the malignant cells. The continued persistence of such noncancerous cells could result at least partly from a balance involving the expression of v-mos, IFN-beta, and an IFN antagonist, sarcolectin. These reverted cells can undergo an unlimited number of passages, but they must be trypsinized before day 5 in confluent cultures. Thereafter, the cells stop dividing, cannot proliferate anymore, progressively show signs of apoptosis, and die.

Partial phenotypic reversion of HeLa cells by long-term interferon-alpha treatment

Human papillomaviruses (HPV) are associated with malignant cervical neoplasia. Several HPV-related diseases have been shown to be sensitive to interferon (IFN) treatment. HeLa cells contain and express the HPV type 18 genome and were used as a model for the evaluation of the viral expression regulation and the effect on the malignant phenotype during IFN treatment. Cells were treated continuously with 200 IU/ml IFN-alpha 2b or natural leukocyte INF-alpha for six passages (42 days). Some IFN-induced changes were observed: decrease of HPV-18 mRNA expression, changes of cell morphology, and reduction of clonogenicity in soft agar. Tumorigenicity in nude mice was not modified. Other targets of the IFN system were analyzed, and an increase of the 2',5'-oligoadenylate synthetase mRNA level and a down-regulation of type I IFN receptor were found. These results demonstrate that long-term IFN-alpha treatment induces a partial phenotypic reversion of HeLa cells to a more differentiated stage were down-regulation of HPV-18 expression could play a central role. It therefore confirms that the IFN-alpha treatment may be therapeutically useful in cervix cancer produced by HPV-18.

Enhancement by stable butyrate derivatives of antitumor and antiviral actions of interferon

The use of n-butyric acid has been associated with induction of cell differentiation and bypassing of genetic defects in the suppression of malignancy. This biological response modifier satisfies the requirements for specificity and low toxicity, and its use can be considered as an alternative approach to conventional cancer chemotherapy. However, a lack of clinical efficacy has been observed with butyrate and attributed mainly to the rapid metabolism of the compound. Butyric acid pro-drugs derived from monosaccharides such as 3-O-butanoyl-1,2-O-isopropylidene-alpha-D-glucofuranose (MAG = 3but) have consequently been devised. Pharmacokinetic and biological advantages of MAG = 3but have been previously described. In the present report, we have studied the effect of MAG = 3but on murine interferon-alpha, beta (IFN) anticellular, antitumor and antiviral activities. In vitro, it appears that MAG = 3but predisposes malignant MSV cells to a later, complete establishment of the antiproliferative and the cell-differentiating effects of IFN, and the antiviral action of the latter in the same line of cells infected with encephalomyocarditis (EMC) virus. In vivo, combined treatment with MAG = 3but and IFN protects mice effectively against the fatal development of ascitic sarcoma 180 TG and the lethal effect of EMC virus infection.

Pharmacokinetic study of butyric acid administered in vivo as sodium and arginine butyrate salts

Considering that butyrate-treated malignant cells can recover in a transitory fashion a non-cancerous phenotype, the authors carried out a pharmacokinetics study of butyric acid injected as sodium or arginine salts for possible antitumor therapies. In the case of 1-14C-labelled butyrate, the appearance of radioactivity in the blood of injected mice is rapid and some of it is maintained for relatively long periods in different organs, mainly the liver. However, no precision can be given about the structure of radioactive compounds in blood and tissues. Using gas-liquid chromatography, the authors studied the metabolism of butyrate in both animals and man. In mice and rabbits, the half-life is less than 5 min. In man, the butyric acid elimination curve can be divided into two parts corresponding to two half-lives: for the first (0.5 min), the slope suggests an accelerated excretion, while for the following (13.7 min), a slow plateau is observed. The rapid elimination of butyrate is a limiting factor for practical applications. However, the lack of toxicity supports its use in human therapy.

Expression of the 210 kDa neurofilament subunit in cultured central nervous system from normal and trisomy 16 mice: regulation by interferon

When applied to central nervous system (CNS) cultures taken from normal fetal mice, interferon increases the immunohistochemical expression of the highly phosphorylated 210 kDa neurofilament subunit. This effect can be blocked by the application of an agent which inhibits interferon-mediated metabolic pathways. Murine trisomy 16 is an excellent model for human Down's Syndrome. CNS cultures taken from trisomy 16 fetal mice express greater intensity of 210 kDa neurofilament subunit immunohistochemical staining than do normals. Application of an interferon inhibitor normalizes trisomy 16 CNS neurofilament expression.

Future developments in interferon therapy

The antitumour effects of interferons in animals and humans are well known. Despite the fact, however, that the 3 types of human interferon, leukocyte alpha-interferon, fibroblast beta-interferon and immune gamma-interferon are available in large amounts through recombinant DNA technology, the practical applicability of interferon therapy in cancer is still not clear. An initial approach to this problem is to determine the mechanism of action of interferons and to find out why, in certain circumstances, they are inactive. There are various ways in which interferon may control tumours--i.e. antiviral action, inhibition of cell growth, stimulation of cell differentiation, changes in cells modulating the susceptibility to immune rejection, or effects on the host immune systems (natural killer system and cytotoxic proteins). The implications of these data in the use of interferon in cancer therapy need to be evaluated. Both alpha- and beta-interferons may have beneficial effects on growth inhibition and differentiation, but gamma-interferon is probably more effective in boosting the immune recognition and rejection of tumour cells. A combination of alpha- and gamma-interferon may give the best results in vivo, since they often act synergistically in vitro. The sensitivity of individual tumour cells to the various types of interferon needs to be evaluated by measurement of oncogenes mRNA inhibition, G0/G1 arrest and increase in various H-La antigens. Finally, the aim of any treatment (antiviral action, tumour regression, prevention of metastasis, decreased tumour growth and increased cell differentiation) should be an important consideration in whether interferon therapy is chosen. A major problem remains in understanding why only a small proportion of patients usually show an objective response to interferon.(ABSTRACT TRUNCATED AT 250 WORDS)

Concomitant enhancement of a cytoskeleton-associated 76,000-dalton protein and inhibition of fluid-phase pinocytosis by interferon-alpha in Fujinami sarcoma virus-transformed rat 3Y1 cells

Addition of rat interferon-alpha (IFN-alpha) to Fujinami sarcoma virus-transformed rat 3Y1 cells progressively inhibited fluid-phase pinocytosis [10% inhibition at 3 h; maximal (60%) inhibition by 12 h]. Electrophoretic analysis of the cytoskeletal fraction from cultures exposed to IFN for 24 h revealed a novel 76,000-dalton protein (CKp76). The kinetics of its appearance paralleled the inhibition of fluid-phase pinocytosis. CKp76 was not detected in cultures pretreated with actinomycin D, or prelabeled with [35S]methionine, prior to IFN addition. However, the presence of cycloheximide during incubation with IFN had no effect on the synthesis of CKp76 after removal of both agents. These results suggest that the appearance of CKp76 was due to enhanced transcription of its gene in response to IFN. Subcellular fractionation revealed the presence of induced CKp76 in the nuclear pellet. From these results it is possible that CKp76 may be responsible at leat in part for the effects of IFN on fluid-phase pinocytosis.

Persistent expression of v-mos oncogene in transformed cells that revert to nonmalignancy after prolonged treatment with interferon

BALB/c embryonic fibroblasts productively transformed by Moloney sarcoma virus and cultivated for over 600 generations in the presence of mouse alpha/beta interferon reverted to an apparently normal phenotype and were unable to produce tumors in nude mice. Nevertheless, the presence of an integrated Moloney sarcoma virus genome in the nonmalignant Moloney sarcoma virus-transformed interferon-treated cell DNA could be shown by focus formation upon transfection and by hybridization with a v-mos probe. After digestion with various restriction endonucleases, similar hybridization patterns of v-mos sequences were obtained with DNAs from both reverted and transformed cells. However, additional integration sites and at least twice as many copies of the oncogene were found in the nonmalignant Moloney sarcoma virus-transformed interferon-treated cell DNA. Polyadenylylated RNA extracted from reverted and control cells contained two mos-specific transcripts. Interestingly, the nonmalignant Moloney sarcoma virus-transformed interferon-treated cells produced helper virus, but no detectable mos-containing virions, suggesting that a posttranscriptional block in the v-mos gene expression had occurred in these cells. It should be stressed that, after up to 100 additional passages, cells cultured in the absence of interferon maintained their nontumorigenic character in spite of the persistent transcription of the mos oncogene.

Lightmicroscopical features of interferon-alpha-treated laryngeal papillomatosis in adults

Papillomatous tissue samples from 12 adult patients with laryngeal papillomatosis were examined by light microscopy before and after interferon-alpha (IFN) treatment. Increased atypia, a change in intra-epithelial differentiation, sclerosis of the underlying stroma and infiltration of especially plasma cells were found in tumour tissue removed after 3 weeks of IFN treatment. These morphological findings do not explain the effects exerted by IFN on the growth pattern of adult laryngeal papillomatosis, but provide a basis for further studies.

The interferon receptors

Early studies on the mode of action of interferons have indicated that a receptor system on the cell surface is involved in its action. The first direct evidence to a high-affinity binding site was found only after pure interferon was available. Two different receptors, one specific for interferons-alpha and beta, and the other for interferon-gamma were recognized. A correlation between affinity to the receptor and specific activity was established. Cross-linked complexes of labeled interferons with their receptors were visualized on gel electrophoresis and even partially purified. Internalization of interferons after binding to the receptor was reported. The role of gangliosides as helpers of interferon binding was recently investigated. Fragments of interferons which still retained binding capacity were described and helped in elucidating the binding site on the interferon molecule.

Ultrastructural features of the interferon-treated adult laryngeal papilloma

The ultrastructural features of the interferon-treated adult laryngeal papillomas are described. Compared with untreated papillomas, no differences in ultrastructure were found by transmission electron microscopy. By scanning electron microscopy, significant alterations comprising a smooth and oedematous surface with a uniform microvilli pattern of the superficial cells were observed after interferon therapy. Our observations suggest an increased cell adhesion and maturation of epithelial cells caused by interferon.

Unusual apparently constitutive interferons and antagonists in human placental blood

We have detected seemingly uninduced interferons (IFNs) in 29/37 human placental samples obtained during caesarian sections at different periods of pregnancy, mostly around the 37th week. The amounts were usually low and did not enable us to correlate our findings with any physiological or pathological conditions. Occasionally the presence of IFN was masked by a lectin-like antagonist. Therefore, in a number of cases, substantially higher amounts of IFN were found after purification by affinity chromatography using concanavalin A, Cibacron blue, or antiserum to IFN-alpha, each coupled to Sepharose. Analysis by sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed the presence of IFN-alpha and IFN-beta with molecular masses between 15 and 80 kilodaltons. Some of the high molecular weight components were neutralized either only by monospecific antiserum to IFN-alpha or, to the same extent, by antiserum to IFN-alpha or to IFN-beta, reminiscent of those previously reported after viral induction in the human amniotic membrane. We postulate that both IFNs and antagonist play a physiological role during fetal development.

Interferon affects the formation of adhesion plaques in human monocyte cultures

When monocytes isolated from human blood adhere to glass substratum, actin- and vinculin-containing punctate plaques rapidly appear at the ventral surface of the cells. We show here that highly purified human leukocyte interferon (IFN) can inhibit formation of these adhesion plaques in a dose-dependent manner. Complete inhibition was obtained when 300 IU/ml IFN were added into the cell-seeding medium. Plaques already formed in the absence of IFN were only partially affected by subsequent addition of IFN into the culture medium. Prevention by IFN of the formation of the adhesion plaques was associated with loosened attachment of the cells to the substratum. Effect of IFN on cellular morphology was complex. At higher doses, IFN added to the cultures within 24 h of seeding almost completely inhibited the differentiation of monocytes to macrophages and most of the cells remained rounded. At lower doses, however, an enhancement of the bipolar spreading was seen and the end result was a culture with predominantly elongated fibroblastoid cells. The latter cells, unlike the fibroblastoid cells in untreated monocyte-macrophage cultures, were completely devoid of the actin plaques, while the reorganization of vimentin-type intermediate filaments took place in a normal manner. These results further support the view that the actin- and vinculin-containing plaques have a role in mediating firm adherence of human monocytes to growth substratum.

Detection of an interferon antagonist, sarcolectin, in human sarcomas and muscles

In a variety of human sarcomas we detected the presence of a sarcolectin which reversed an established antiviral protection induced by interferon (IFN). For the same protein concentration, this biological activity was significantly increased when compared to that of normal muscles. All the biological characteristics were comparable to those of a sarcolectin found in hamster tissues; namely the capacity to agglutinate cells and its inhibition by specific sugars, migration in sodium dodecyl sulfate gel, and pepsin, heat and sodium dodecyl sulfate stability. Except for its anti-IFN function and cell agglutinating activity, the biological significances of this sarcolectin is presently poorly understood.

Effect of mouse lymphokines and cloned mouse interferon-gamma on the interaction of Rickettsia prowazekii with mouse macrophage-like RAW264.7 cells

We studied the effects of crude mouse lymphokines and cloned mouse interferon-gamma on the interaction of Rickettsia prowazekii with mouse macrophage-like RAW264.7 cells. Treatment of RAW264.7 cells with lymphokines before infection, after infection, or both before and after infection with R. prowazekii led to killing of a substantial proportion of the RAW264.7 cells. Such cytotoxicity required both lymphokines and viable R. prowazekii and did not occur in mouse fibroblastic L929 cells. Untreated cultures of RAW264.7 cells supported good growth of the Breinl strain of R. prowazekii, but in lymphokine-treated cultures, little or no rickettsial growth occurred in the cells that survived the cytotoxic reaction. In addition, treatment of RAW264.7 cells with lymphokines before rickettsial infection was associated with suppression of the initial infection. The effects of cloned mouse interferon-gamma were similar to the effects of crude mouse lymphokines. Assessment of cytotoxicity, inhibition of the initial infection, and inhibition of rickettsial growth in RAW264.7 cells pretreated with various concentrations of interferon-gamma indicated that the effects of the lymphokines could be explained by the interferon-gamma that was present in these preparations. Treatment of RAW264.7 cells with interferon-gamma makes them unsuitable host cells for R. prowazekii.

Differential effects of calcium deprivation on the cytoskeleton of non-tumorigenic and tumorigenic rat liver cells in culture

Normal rat liver T51B epithelial cells and Morris no. 7795 hepatoma cells growing exponentially were exposed for 24 h to standard medium containing low (0.02 mM) calcium, a concentration which drastically reduces the proliferation of normal but not tumour cells. Cell surface morphology was examined by scanning electron microscopy (SEM); and the distribution and organization of microtubules, cytokeratin and vimentin filaments, and microfilaments were analysed by indirect immunofluorescence microscopy using specific antibodies. Calcium deprivation caused the loss of intercellular cohesion in both cell types and the appearance of some microvilli and blebs, particularly on tumour cells. However, marked differential (normal vs tumour cells) effects on the organizational integrity of the cytoskeleton fibrillar network were observed. Extracellular calcium deprivation led to a particular rearrangement of microtubules, and a perinuclear accumulation of cytokeratin and vimentin filaments in normal, but not in tumour cells. A massive concentration of actin-containing microfilaments was observed in the cell periphery and blebs of hepatoma cells. In the light of the possible involvement of calcium in controlling cytoskeleton assembly, the differing cytoskeletal changes of the two cell types may be linked to their different proliferative capabilities in low-calcium medium.

Effects of beta interferon on human fibroblasts at different population doubling levels. Proliferation, cell volume, thymidine uptake, and DNA synthesis

Cellular aging had no effect on the ability of beta interferon to increase cell volume and population doubling time in 76-109 cells, a line of human skin fibroblasts. However, DNA synthesis in cells at high population doubling levels (PDL 55-70) was inhibited after 72 h of beta interferon treatment (1,000 U/ml) while no inhibition of DNA synthesis was observed in cells at middle population doubling levels (PDL 30-40).

Effect of coordinated therapeutic assays using C. parvum, interferon and arginine butyrate on spontaneous disease and survival of AKR mice

AKR mice, known to develop spontaneous leukemia in almost 100% of cases, were studied throughout their life-span. Different treatments combining a potent immune stimulator, Corynebacterium parvum (CP), with interferon (IFN) and arginine butyrate were initiated at the 15th week of life. In a preliminary series of experiments, CP (200 micrograms), IFN (20,000 units) and butyrate 50 mM) were employed in a well-defined chronological order. In controls, the mean survival time (MST) was 35.17 +/- 1.67 weeks and the final survival rates was 0/50 mice for all experiments. Only CP associated with arginine butyrate significantly augmented the MST (42.5 +/- 3.66 weeks) and final survival rate (9/35 mice). In an adjusted set of experiments, reducing the IFN concentration to 10,000 and 5,000 units and that of butyrate to 6 mM greatly improved the results. The MST was substantially increased with the combinations of CP + IFN + butyrate (41.4 +/- 1.86 weeks), CP + IFN (42.73 +/- 3.29 weeks) and butyrate + IFN (41 +/- 2.34 weeks), as well as the final survival rates (8/15, 10/15 and 6/15 mice respectively). An important finding was that when CP and IFN were used separately, they were ineffective.

Unusual human interferons produced by virus-infected amniotic membranes

Interferon (IFN) induced in the human amniotic membrane contains at least five different molecular species, as shown by analysis in NaDodSO4/polyacrylamide gels after heating and under reducing conditions. Three of the IFN components reported here--migrating at 26, 43, and 80 kilodaltons--are of unusual antigenic structure because they are neutralized to about the same extent by anti-IFN-alpha and anti-IFN-beta antibodies. The 15- to 17-kilodalton species belongs to the IFN-alpha group, while the 21- to 22-kilodalton species, the most frequently detected major peak, is IFN-beta. In addition to their unusual size and antigenic structure, these IFNs could play a role during embryonic development and in the immune tolerance of the mother with regard to the fetus.

Reversibility of the transformed and neoplastic phenotype. III. Long-term treatment with electrophoretically pure mouse interferon leads to the progressive reversion of the phenotype of X-ray transformed C3H/10T1/2 cells

Electrophoretically pure mouse interferon induced the progressive reversion of the transformed phenotype of a clone of X-ray transformed C3H/10T1/2 cells. Cells of this clone did not harbor C-type particles and reverse transcriptase activity was not detected. Interferon-treated transformed cells were aligned without cellular overlapping and attained low cell densities. Morphologic changes were associated with the appearance of a thick layer of submembranous microfilaments. The tumorigenicity of interferon-treated cells was markedly reduced. Back reversion to the transformed phenotype occurred progressively when these cells were passaged in the absence of interferon. These results suggest that interferon may induce the reversion of the transformed phenotype by a mechanism other than by its antiviral activity.

Antitumor effect of arginine butyrate in conjunction with Corynebacterium parvum and interferon

The antitumor effect of arginine butyrate, used alone or in conjunction with interferon (IFN), was studied using randomly selected Swiss mice inoculated i.p. with 10(6) 180 TG Crocker tumor cells. The results of these different therapeutic regimens were estimated by tumor incidence at 10 days, by the time necessary to protect 50% of the animals, and by the final survival rate. Combined treatment by IFN and arginine butyrate was effective; but in order to compensate for IFN's immune repressive effects, a single injection of Corynebacterium parvum was administered in some experiments prior to IFN and/or arginine butyrate. After this immune potentiation, both IFN and arginine butyrate used separately significantly protected the animals. However, optimal results were obtained when a single C. parvum injection was followed by nine daily alternating shots of arginine butyrate and IFN.

Effects of sodium butyrate, a new pharmacological agent, on cells in culture

Sodium butyrate, at millimolar concentrations, when added to cell cultures produces many morphological and biochemical modifications in a reversible manner. Some of them occur in all cell lines. They concern regulatory mechanisms of gene expression and cell growth: an hyperacetylation of histone resulting from an inhibition of histone deacetylase and an arrest of cell proliferation are almost constantly observed. Some other modifications vary from one cell type to another: induction of proteins, including enzymes, hormones, hemoglobin, inhibition of cell differentiation, reversion of transformed characteristics of cells to normal morphological and biochemical pattern, increase in interferon antiviral efficiency and induction of integrated viruses. Most if not all these effects of butyrate could result from histone hyperacetylation, from changes in chromatin structures as measured by accessibility to DNases and from modifications in cytoskeleton assembly. We do not know at the present time whether butyrate acts on a very specific target site in cell or if it acts on several cell components.

Interferon inhibits Sendai virus-induced cell fusion: an effect on cell membrane fluidity

Interferon can affect several cellular functions, in addition to its antiviral activity. We report here that pretreatment of human cells with homologous interferon significantly inhibits cell fusion induced by Sendai virus and that this refractory state is accompanied by a decrease in cell plasma membrane fluidity. Multinucleate cell formation induced by beta-propiolactone-inactivated Sendai virus in human fibroblast cells (a system in which fusion results from an interaction of the viral glycoprotein with the cell membrane) was inhibited by more than 90% after addition of human interferon for 18-24 hr. This inhibition could be neutralized by antiserum to interferon. Furthermore, inhibitor studies with cycloheximide and actinomycin D clearly indicated that synthesis of protein and RNA is necessary to establish the resistant state. To determine whether the inhibition of Sendai virus-induced cell fusion resulted from interferon-induced changes at the cell plasma membrane, experiments were carried out using the fluorescence probe 1,6-diphenyl-1,3,5-hexatriene, which is capable of sensing molecular motions in the hydrocarbon core of the bilayer structure. A significant decrease in the membrane fluidity of interferon-treated cells was observed. It is likely, therefore, that the inhibitory effect on Sendai virus-induced cell fusion observed in interferon-treated cells results from an increased rigidity of the target cell membrane.

Antiretroviral effect of interferon: proposed mechanism

Interferon (IFN) treatment of NIH Swiss mouse embryo cells chronically infected with Rauscher murine leukemia virus (R-MuLV) drastically reduced the release of virus particles from the cells. The characterization of intracellular and extracellular viral specific proteins and polyproteins immunologically with various antisera, and structurally by tryptic digest mapping experiments, indicated that the antiretroviral action of IFN was not due to an IFN-induced alteration in the synthesis of any viral protein. Steady state labeling experiments, however, showed that the processing of three viral specific precursor polyproteins, namely gPr90env, Pr40gag, and Pr25gag, were perceptively slowed in IFN-treated cells. This effect was apparently not related to the ability of these proteins to be modified by phosphorylation or glycosylation after translation since these processes occurred normally in the IFN-treated cells. The treatment of cells with IFN also caused the accumulation of a small amount of a fucosylated viral glycoprotein precursor, termed gP93env, in virus. With the exception of this minor protein, virus released from IFN-treated cells were normal in their content of viral proteins. These virus particles were only slightly less infectious, particle for particle, than virus released from control cultures. Based on these results, we suggest that IFN causes an as yet unelucidated alteration in cell membrane structure of function, or both, which prevents either the insertion of viral core precursor molecules into membrane or the recruitment or clustering of such viral polyproteins into virus assembly centers in the membrane. This suggested mechanism of IFN action is discussed in detail.

Reversibility of the transformed and neoplastic phenotype. I. Progressive reversion of the phenotype of X-ray-transformed C3H/10T1/2 cells under prolonged treatment with interferon

X-ray transformed mouse C3H/10T1/2 cells were cultivated and passaged in the continuous presence of partially purified mouse interferon. This prolonged interferon treatment resulted in a stepwise progressive reversion of the transformed phenotype to the non-transformed phenotype. Thus interferon-treated cells displayed an epithelioid morphology, grew to a lower cell density, and were no longer tumorigenic. Reversion to the non-transformed phenotype was, however, stable only as long as interferon was continuously present in the culture medium. When interferon was removed, the cells "back reverted" to the transformed phenotype. Our results suggest that interferon induced a reversion of the transformed phenotype in the entire cell population rather than a selection of an interferon resistant cell population. C-type viral particles and significant levels of reverse transcriptase were present in transformed cells, but neither present in the parental 10T1/2 cells nor in interferon-treated cells. When interferon was removed form the culture medium, viral particles and reverse transcriptase activity were again detected. It is possible, therefore, that interferon induces reversion through its antiviral activity, or that it induces reversion by its effects on cell function and structure, independently of any antiviral effect. Inhibition of cell multiplication per se does no appear to be sufficient to induce reversion, since cycloheximide inhibited cell multiplication; however, even after ten passages, it did not affect tumorigenicity. Our results suggest the possibility that interferon may act in vivo not only by inhibiting tumor cell multiplication but also by inducing a reversion. Patients with some tumors may therefore benefit from long-term interferon treatment.