Molecular biology of animal virus infection

The potential of nucleotide analogs as inhibitors of retroviruses and tumors

The biologically active form of most purine or pyrimidine analogs is the nucleoside 5'-mono, di- or triphosphate. The nucleoside form is most often administered because of the ease with which it penetrates cells by facilitated transport. However, many nucleoside derivatives fail to exhibit significant antiviral or antitumor activity because they are not phosphorylated by cellular enzymes to the active nucleotide form. In this review, the potential use of suitable nucleotide analogs as selective inhibitors of ribonucleotide reductase and viral reverse transcriptase is considered. Masked nucleotides such as phosphoramidates or methyl phosphates could be employed to allow transport across cellular membranes. Furthermore, phosphonocarboxamide, phosphonoformate or sulfamidophosphoramidate may mimic nucleotide di- and triphosphates. Tumor cells and virally infected cells are often more permeable to nucleotides and their analogs than normal cells, which could provide a therapeutic advantage. There could be considerable therapeutic potential for nucleotide analogs that can penetrate the tumor cell membranes and that are resistant to enzymatic hydrolysis and are non-incorporable into DNA or RNA.

Molecular mechanisms of virus-mediated cytopathology

Lytic virus infections of animal cells usually lead to a variety of morphological and biochemical lesions that include inhibition of cellular macromolecular syntheses. These cytopathic effects vary in intensity for different virus-cell combinations and probably involve several overlapping mechanisms. Inhibition may be mediated by components of parental virions or require viral gene expression. In many infected cell systems the initiation of host protein synthesis is selectively blocked. This shut-off phenomenon can result from changes in membrane permeability that alter the intracellular ionic environment in favour of viral expression, successful competition of viral mRNAs for limited translational components, or a decrease in the level of cell mRNAs by inhibition of synthesis or nucleocytoplasmic transport. However, the early onset and rapidity of virus-induced inhibition, sometimes under non-permissive conditions, implies more direct mechanisms of translational inactivation. These include enhanced degradation of cellular mRNAs or specific modification of the translation apparatus in infected cells. A dramatic example of the latter occurs in poliovirus-infected HeLa cells in which intact, functional cellular mRNA persists but host protein synthesis is almost completely inhibited. The virus-induced defect is apparently related to inactivation of a protein factor that binds to the 5' end of m7G-capped mRNAs and is required for translation of host (capped) mRNAs but not for the expression of poliovirus RNA, which is not capped. This process and other possible molecular mechanisms of virus-mediated cytopathology are discussed.

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.

Hygromycin B therapy of a murine coronaviral hepatitis

Hepatitis caused by mouse hepatitis virus (MHV-A59), a murine coronavirus, is accompanied by direct infection and replication of virus within the liver. We demonstrate here that the aminoglycoside hygromycin B is able to eliminate MHV-A59 infection from mouse peritoneal macrophages and cultured liver cells in vitro and is also able to reduce levels of virus replication and necrotic liver foci in vivo.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Interaction of delta-9-tetrahydrocannabinol with rat B103 neuroblastoma cells

The effect of delta-9-tetrahydrocannabinol (delta-9-THC) on the growth kinetics and morphology of rat B103 neuroblastoma cells was assessed. Delta-9-THC in doses ranging from 10(-4) to 10(-7) M inhibited cellular growth in a dose-dependent fashion as evidenced by delay in doubling time, decrease in saturation density, and decrease in efficiency of plating. The inhibition in cellular growth was paralleled by dose-related alterations in cell morphology. Modifications included rounding of cells, retraction of neurites, blebbing of the cell surface, and exfoliation of the plasma membrane. Cytoplasmic alterations included distension of the endoplasmic reticulum, Golgi apparatus, and perinuclear space, and macrovacuolization. Intracytoplasmic laminated inclusions and vesicular clusters were suggestive of membrane repair in drug-treated cells. These morphological changes were accompanied by cytoskeletal rearrangement in the absence of significant alteration in the concentration of total cytoskeletal protein. Autoradiographic examination of the intracellular fate of 3H-delta-9-THC demonstrated that the drug was confined to the cytoplasmic compartment and often associated with macrovacuoles. These results suggest that delta-9-THC interacts with cellular membranes, thereby altering neuroblastoma cell growth and behavior.

Potential targets for antiviral chemotherapy

Serendipity and random screening have been successful in producing effective antiviral agents. The increase in our knowledge of the basic biochemistry of viral replication and of virus-host interrelationships has revealed not only an understanding of the targets upon which existing antiviral agents exert their inhibitory effect, but also has uncovered new potential targets. The hope is that such molecular understanding will afford the synthesis of compounds with selective antiviral activity. A review of various viral targets which are potentially susceptible to attack, and a few approaches for development of antiviral agents are presented.

Increased sugar transport in BHK cells infected with Semliki Forest virus or with herpes simplex virus

Infection of BHK cells by SFV increases the rate of uptake of [3H]MeGlc and of [3H]dGlc at approximately 2 hours p.i. Infection by HSV increases the uptake of [3H]MeGlc and [3H]dGlc at approximately 10 hours p.i.; the increased uptake is prevented by acyclovir. It is concluded that an increased sugar uptake by infected cells reflects an increased rate of transport across the plasma membrane and is the result of cellular changes caused by virus infection.

Biochemical changes induced by Coxsackie B4 virus in short-term culture of mouse pancreatic islets

Isolated mouse pancreatic islets were infected in vitro with two strains of Coxsackie B4 virus--a tissue culture-adapted strain and a mouse pancreas-adapted strain. Within 48 h of infection changes had occurred in the biochemical activities of islets infected with the mouse pancreas-adapted strain of virus. Basal insulin release was increased two-fold in these islets, while glucose-induced insulin secretion remained unchanged. Insulin biosynthesis was greatly reduced at a stimulatory concentration of glucose (20 mM), thus leading to a reduced insulin content in these islets. These effects are of importance because they demonstrate that certain strains of Coxsackie B4 virus, like encephalomyocarditis virus, may selectively alter beta-cell function in vitro.

New antiviral compounds

Several potent and selective antiviral agents against herpes virus infections have been developed. However, the majority of compounds against other viral diseases has not yet reached such high standard. Based on progress in molecular virology it can, however, be anticipated that similar concepts of selective inhibition will also be developed for other virus groups. In addition to virus-induced enzymes, viral proteins other than enzymes with specific activities will be identified. The identification of active sites will lead to the design of new and specific inhibitors. Moreover, studies on the mode of action of the huge number of known antiviral compounds may provide the basis for new and potent approaches to specific virus chemotherapy. New inhibitors of viral replication may also be derived from 2'-5'A and other mediators of the interferon induced antiviral state. However, since 2'-5'A does not enter cells, is rapidly degraded by phosphodiesterases, and affects viral and cellular protein synthesis, only analogs which do not have these disadvantages may qualify as antiviral drugs. In addition to refinements at the molecular level quantitative assays for a better evaluation of antiviral agents for clinical use are required. For clinical trials, rapid diagnosis, early initiation of treatment, and quantitative evaluation of the antiviral effects of a drug need to be developed. Moreover, new methods of drug delivery and/or drug targeting will improve potency and selectivity of antiviral compounds. Drug carriers have already successfully been used in cancer therapy (Poste and Fidler, 1981) they should be also applicable to virus chemotherapy. Finally, a better understanding of the pathogenesis and the natural course of viral diseases will contribute to the development of more effective and safe antiviral agents.

Synthesis of heat-shock proteins in HeLa cells: inhibition by virus infection

Incubation of HeLa cells at supraoptimal temperatures induces the synthesis of a class of proteins known as heat-shock or stress-response proteins. After restoration of cells to physiological temperatures heat-shock protein synthesis continues for several hours. Normal cellular translation eventually recovers even if cells are treated with actinomycin D, indicating that neither normal cellular mRNAs nor components of the translation machinery are irreversibly modified by heat treatment. The synthesis of heat-shock proteins after poliovirus infection is more resistant to inhibition than normal host proteins. Nevertheless, their synthesis is also inhibited in infected cells, even in cells treated with human interferon under which conditions viral RNA replication and viral translation are blocked. Translation of heat-shock proteins is also resistant to hypertonic shock indicating that their mRNAs bind ribosomes with high affinity.

Action of oligomycin on cultured mammalian cells. Permeabilization to translation inhibitors

Oligomycin, an inhibitor of ATP synthesis, has been used as a model to study the effects of ATP depletion on macromolecular synthesis and modification of membrane permeability. Protein synthesis is totally blocked by the antibiotic, whereas RNA and DNA synthesis are less inhibited. Different concentrations of monovalent and divalent cations do not revert the inhibition of protein synthesis. Measurement of cellular ATP and 86Rb+ content indicate that the blockade of translation depends on the ATP content. A significant decrease in cellular ATP does not lead to the reduction of monovalent ions in the cell, although hyperpolarization of the cell membrane does take place. An increased membrane permeability to some inhibitors develops when the cells are hyperpolarized by oligomycin.

Effect of interferon treatment on blockade of protein synthesis induced by poliovirus infection

Treatment of HeLa cells with lymphoblastoid interferon leads to a drastic inhibition of infective poliovirus. Even relatively high concentrations of human lymphoblastoid interferon HuIFN-alpha (Ly) (400 IU/ml) do not prevent destruction of the cell monolayer after most of the cells have been infected with poliovirus. Analysis of macromolecular synthesis in a single step growth cycle of poliovirus in interferon-treated cells detected no viral protein synthesis. In spite of this inhibition of viral translation, the shut-off of host protein synthesis in interferon-treated cells is apparent when they are infected both at low and high multiplicities. Although viral RNA synthesis is inhibited considerably in cells treated with interferon, a certain amount is detected, suggesting that some viral replication takes place. Analysis of membrane permeability after poliovirus infection shows a leakage to 86Rb+ ions and modification of membrane permeability to the translation inhibitor hygromycin B at the moment when the bulk of virus protein synthesis occurs. These changes are delayed and even prevented if cells are pretreated with interferon. A situation is described in which host protein synthesis is shut-down with no major changes in membrane permeability, as studied by the two tests mentioned above. Prevention of viral gene expression by inactivation with ultraviolet light of the input virus or by treatment with cycloheximide blocks the shut-off of protein synthesis. This does not occur in the presence of 3 mM guanidine. These observations are in agreement with the idea that some poliovirus protein synthesis takes place in interferon-treated cells and this early gene expression is necessary to block cellular protein synthesis.

Protein synthesis in cells infected with Semliki Forest virus is not controlled by intracellular cation changes

Treatment of BHK cells with 1 microM nigericin results in a 55% decrease in K+ and a 3.3-fold increase in intracellular Na+; protein synthesis under these conditions is depressed by 35%. In BHK cells infected with Semliki Forest virus (SFV), protein synthesis is depressed by 76% 6.5 h after infection; intracellular K+ is unchanged, and intracellular Na+ is increased 1.8-fold at this time. These results suggest that the increase in intracellular Na+ in SFV-infected BHK cells does not adequately account for the decrease in protein synthesis, and makes it likely that an increased Na+ concentration is a consequence, not a cause, of alterations in protein synthesis in virally-infected cells. No evidence was obtained for the purported [Alonso, M. A. and Carrasco, L. (1980) Eur. J. Biochem. 109, 535-540; (1981) Eur. J. Biochem. 118, 289-294; (1981) FEBS Lett. 127, 112-114] ability of 1 microM nigericin to permeabilize' cells.

Myxoviruses do not induce non-specific alterations in membrane permeability early on in infection

The permeability characteristics of cells infected with myxoviruses have been studied by measuring the concentrative uptake of nutrients, the concentration of intracellular K+, and the maintenance of the Na+ gradient across the plasma membrane. Cells either show no change at all (Sendai virus-infected BHK cells and measles virus-infected Vero cells) or they show a decreased ability to concentrate nutrients, while intracellular K+ and the Na+ gradient remain unchanged (Sendai and influenza virus-infected L-1210 cells, measles virus-infected lymphocytes and mumps virus-infected L-41 cells). In no case, therefore, was a change observed that resembles the non-specific increase in membrane permeability induced by haemolytic paramyxoviruses (35, 42) or the non-specific membrane leakiness postulated to take place in infected cells (8, 9). A preliminary account of some of these findings has been presented (39).

Poliovirus-induced alterations in HeLa cell membrane functions

Protein synthesis, amino acid uptake, membrane potential, cell volume, Na+ and K+ levels, and ATPase (Na+,K+ activated; EC 3.6.1.3) activity were investigated in control and poliovirus-infected HeLa cells. Inhibition of protein synthesis was first observed 60 min postinfection and reached a maximum at 120 min. The onset of protein synthesis inhibition coincided with a decrease in cell volume and with an elevation of ATPase activity in isolated HeLa cell membranes. Some 3 h after virus adsorption, ATPase activity was inhibited, the Na+-K+ gradient of the cell collapsed, both membrane potential-dependent tetraphenylphosphonium ion uptake and amino acid uptake were reduced, and the cell volume increased. These results provide further experimental support for the hypothesis that modification of the cell membrane plays an important role in the strategy of cytopathogenic viruses in the shutoff of host metabolism and cell death.

Membrane changes during viral infection

The effect of viruses on the surface membrane of susceptible cells during the entry and exit process has been studied. Haemolytic paramyxoviruses induce a non-specific leakage to low-molecular-weight compounds during entry; other viruses do not show this effect. During exit, no such changes occur with any virus so far studied: some viruses are released without any obvious change at all in surface membrane function; in other cases, uptake of some nutrients is altered and there is a fall in membrane potential. These results do not support the hypothesis that a generalized membrane leakiness is a prerequisite for the synthesis and release of virus particles.

Acute membrane responses to viral action

The effects of Sendai, a paramyxovirus, on the functional activity of 3 cell types, have been studied in vitro to establish whether a virus alone can cause pathophysiological changes. Neuronal cells are depolarized and suffer a loss of excitability which was attributed to an increase in membrane conductance. Spontaneously beating cardiac cells initially stop beating and then beat more rapidly and asynchronously. Anterior pituitary cells release hormones. In all 3 cases the effects are transient and the cells recover completely.

Calcium ions and the control of proliferation in normal and cancer cells

Several lines of evidence suggest tha Ca2+ ions control cell proliferation: Ca2+ entry into cytoplasm acts as a general mitogen; serum and serum-replacements induce Ca2+ influx; the Ca2+ concentrations in growth media required to support the proliferation of normal cells are much higher than those required for cancer cells; serum and growth factors reduce the Ca2+ requirements of normal cells; tumour promoters alter Ca2+ fluxes via a mechanism used principally by growth factors. Minor supporting evidence includes the effects of various drugs and viruses, and the behaviour of tumour cell mitochondria and intercellular junctions. It is still not possible to decide exactly where and when inside cells the critical effect of Ca2+ on proliferation occurs, but we discuss at length the practical problems of understanding Ca2+ movements in tissue-culture cells. Carried to its logical conclusion, present evidence suggests that an overridden or bypassed Ca2+ control process may be the key, common determinant of unrestrained proliferation in cancer cells.

Reversion by hypotonic medium of the shutoff of protein synthesis induced by encephalomyocarditis virus

Infection of human HeLa cells by picornaviruses produces a drastic inhibition of host protein synthesis. Treatment of encephalomyocarditis virus-infected HeLa cells with hypotonic medium reversed this inhibition; no viral protein synthesis was detected. The blockade of viral translation by hypotonic conditions was observed for a wide range of multiplicities of infection. However, only with low virus-to-cell ratios did cellular protein synthesis resume. The ratio of cellular to viral mRNA translation was strongly influenced by the concentration of monovalent ions present in the culture medium: a high concentration of NaCl or KCl favored the translation of viral mRNA and strongly inhibited cellular protein synthesis, whereas the opposite was true when NaCl was omitted from the culture medium. Once viral protein synthesis had been blocked by hypotonic medium treatment, it resumed when the infected cells were placed in a normal or hypertonic medium, indicating that the viral components synthesized in the infected cells were not destroyed by this treatment. These observations reinforced the idea that ions play a role in discriminating between viral and cellular mRNA translation in virus-infected animal cells.