Effect of dibutyryl cyclic AMP on the induction of Epstein-Barr virus in hybrid cells

Replication of lymphocytic choriomeningitis virus is restricted in terminally differentiated neurons

We have investigated the replication of lymphocytic choriomeningitis virus (LCMV) before and after the nerve growth factor (NGF)-induced transdifferentiation of PC12 cells from the chromaffin to the neuron-like phenotype. Untreated and NGF-treated cells were equally susceptible to LCMV infection; however, the viral yield was found to be 1,000-fold lower in NGF-differentiated PC12 cells. The reduced viral yield correlated with restricted LCMV replication and transcription within the infected cell, which was not caused by the lack of cell proliferation in the NGF-treated cells but rather was related to the induction or changes in expression levels of specific gene product(s) associated with the cell commitment to a neuronal phenotype. The return to the chromaffin phenotype after withdrawal of NGF restored normal LCMV yields as well as levels of viral replication and transcription. The finding of reduced viral replication in terminally differentiated neuronal cells has important implications for understanding the mechanism by which neurotropic viruses, such as LCMV, are able to establish a long-term persistent infection in the central nervous system in the absence of severe pathological changes.

Reversible repression and activation of measles virus infection in neural cells

Conversion of acute measles virus infection to an indolent state has been achieved by treatment of infected cells of neural origin with agents that affect cyclic nucleotide metabolism. Striking results were obtained with papaverine, an inhibitor of cAMP phosphodiesterase that is capable of enhancing neural differentiation. In papaverine-treated cultures, decreased production of infectious virus was accompanied by selective disappearance of intracellular matrix proton, as detected by immunofluorescence. Viral nucleocapsid protein was enhanced in the cytoplasm while three other structural proteins--polymerase, hemagglutinin, and fusion protein--showed little change in distribution or intensity of staining. These results were specific for cells of neural origin and not observed in CV-1 or Vero cultures. cAMP, dibutyryl cAMP, 8-bromo-cAMP, and isobutylmethylxanthine all inhibited replication of virus but less so than did papaverine. Inhibition of virus replication by any of these agents was rapidly reversible, either by removal of the agent or by addition of cGMP to the culture medium and was accompanied by reappearance of the matrix protein. These results suggest that measles virus replication in neural cells depends on host factors, particularly those affecting endogenous cAMP and cGMP. Viral persistence may thus be related to the state of neural differentiation. This model system may yield information on mechanisms of recrudescence observed in some chronic diseases of the nervous system.

[Intracellular cyclic AMP in vesicular stomatitis or Sendaï virus infected cells (author's transl)]

In KB cells, MRC5 and adult skin fibroblasts infected by low doses of Sendaï virus, intracellular cyclic AMP levels rose and fell in the first hours following infection, then remained lower than basal level during at least 2 days in KB cells and adult skin fibroblasts. When compared to other viruses or cAMP inducers previously described, this effect appeared specific of Sendaï virus. Mechanisms and roles of cAMP variations are discussed. VSV-infected KB cells showed slightly decreased cAMP levels during the first hours following infection.

Interaction between cyclic nucleotides and herpes simplex viruses: productive infection

Infection of human fibroblasts and HEp-2 cells with herpes simplex virus type 1 (HSV-1) produced a decrease in the intracellular levels of cyclic adenosine 5'- monophosphate (cAMP) and a concomitant increase in the cyclic guanosine 5'- monophosphate (cGMP) levels. In both cell cultures, changes in cyclic nucleotide levels were first observed at 6 h after viral inoculation and were maximal at 12 h. In human fibroblasts, the addition of theophylline, dibutyryl cAMP, or papaverine (cAMP-enhancing compounds) decreased significantly the yield of HSV-1, whereas the addition of insulin or dibutyryl cGMP (cGMP-enhancing compounds) increased the viral yield. In HEp-2 cells, only theophylline decreased the yield of HSV-1, and the cGMP-enhancing compounds had no apparent effect. Cyclic nucleotide enhancing compounds exhibited their effect only if added to either cell culture within the first 3 h after inoculation with HSV-1.

Differential induction of Epstein-Barr virus-related antigens in heterokaryon cultures

Different paterns of induction of Epstein-Barr virus (EBV)-related antigens were observed in heterokaryons produced by Sendai virus-mediated fusion of producer and non-producer human lymphoblastoid cells with various other cell types. EBV-related early antigens (EA) and viral capsid antigen (VCA) could obviously be induced in heterokaryons between producer cells (P3HR-1 and QIMR-WIL), normally expressing these natigens at very low frequency, and human FL or HeLa cells. Positive cells were detected as early as 3 h after fusion and there often followed a rapid increase in positive cells. In contrast, in heterokaryons between non-producer cells (Raji and NC-37) and FL or HeLa cells, only EA but not VCA was induced. EA induction was also evident in fusion of human lymphoblastoid cells with monkey cells (Vero) but with mouse cells (L-M(TK-) C11D and MCB-2) no EBV induction occurred. The EBV induction in heterokaryons was significantly enhanced by 5-iododeoxyuridine treatment.

Cyclic AMP regulation of mammary tumor virus production

Addition of dibutyryl cyclic AMP (cAMP) and agents (isoproterenol and epinephrine) that stimulate the activities of adenylate cyclase enhance the stimulation of mammary tumor virus (MTV) production two- to threefold by glucocorticoid in short-term primary cultures of mammary tumors. This cAMP potentiation seems to depend on the stimulated level of MTV production by glucocorticoid alone, which increases MTV production 5- to 10-fold over basal level but varies greatly in absolute terms. When the stimulated level by glucocorticoid alone is suboptimal, cAMP seems to restore the sensitivity of the cells to the stimulatory effect of glucocorticoid to its maximum.

Differential induction of Epstein-Barr virus-related antigens in human lymphoblastoid cells by virus-mediated cell-to-cell contact

Differential induction of Epstein-Barr virus (EBV)-related antigens was observed after Sendai virus-medicated fusion of producer and non-producer cells with various human and mouse cells. The EBV-determined early nuclear antigen (ENA), early antigen (EA) and viral capsid antigen (VCA) could be induced at a high rate in producer cells (P3HR-1 and B95-8), normally expressing these antigens at very low frequency, as early as 12-24 h after fusion with each other or with human amnion FL cells. In contrast, only one antigen, ENA, was induced in producer cells following fusion with non-producer cells. This limited induction occurred also in non-producer cells fused with FL cells, but not after fusion with each other. EBV induction did not result from fusion of either producer or non-producer cells with mouse cells (L-M (TK-) Cl1D and MCB-L). The differential induction was not the result of heterokaryon formation as determined by autoradiography. The implications of these findings are discussed.

Neoplasms, differentiations and mutations

Evidence has been presented to support the concept that malignant tumors are postembryonic differentiations superimposed upon the process of tissue maintenance and renewal. Malignant stem cells are derived from normal stem cells. They have a capacity for proliferation and differentiation that operates at a different level of control than the normal. Even so, malignant stem cells are responsive to enviornmental control, suggesting that it may be possible to direct their differentiation or at least to control their ability to replicate. A tumor is a caricature of normal tissue and appears undifferentiated because of the preponderance of undifferentiated proliferating stem cells in relationship to the number of cells that have differentiated and become benign.

Are viruses important in carcinogenesis?

The role of viruses in the etiology of animal cancers is fairly certain. Information derived under both natural and experimental conditions supports the concept that either DNA- or RNA-containing viruses can fulfill this function. The DNA-containing herpesviruses, especially the Epstein-Barr virus, are currently the primary objects of intense investigation concerning their role in human cancer. This article will focus on the properties of counterpart herpesviruses in lower animals as well as the human virus candidates with an assessment of the observations concerning their oncogenic potential.