Dexamethasone-stimulated expression of a proviral copy of mouse mammary tumor virus env mRNA

A correlation between dexamethasone inducibility and basal expression levels of retroviral vector proviruses

Identical transcription units inserted at different positions of mammalian chromosomes may vary widely in transcriptional activity. We have used a set of ten cell clones with random unselected single integrations of retroviral vectors to study such position effects. The vector used carries a neo gene driven by the Akv murine leukemia virus long terminal repeat that has only a weak promoter-enhancer activity in the target cell, the lymphoid cell line L691. Under transient expression conditions, the strength of the Akv promoter-enhancer in the L691 cells is increased by dexamethasone. In cell clones with single vector integrations, a correlation is observed between the non-induced expression levels and the degree of dexamethasone induction. The strongest relative induction is found for the integrated vectors with the lowest non-induced expression levels and approaches the inducibility under transient expression. These results indicate that expression levels are composed of distinct contributions from the integrated vector and from the site of integration and are best explained in terms of a model in which the sites of chromosomal integration exert variable positive enhancer effects upon vector transcription.

Inhibitory effect of glucocorticoid hormones on the growth of Inoue-Melnick virus

Glucocorticoid hormones inhibited the early stage of propagation of Inoue-Melnick virus in a dose-dependent manner in human diploid cell cultures. Prednisolone and hydrocortisone achieved complete inhibition of replication of the three Inoue-Melnick virus serotypes at concentrations of 0.2 X 10(-4) and 10(-5) M, respectively; strong inhibition was found even at 10(-5) and 10(-6) M, respectively. The steroid hormones did not show a direct effect on the virus.

Identification and characterization of a mouse mammary tumor virus protein uniquely expressed on the surface of BALB/cV mammary tumor cells

A unique subline of BALB/c mice, designated BALB/cV, exhibits an intermediate mammary tumor incidence (47%) and harbors a distinct milk-transmitted mouse mammary tumor virus (MMTV). The BALB/cV subline was used to study the molecular basis of potential virus-host interactions involving cell surface-expressed MMTV proteins. Cell surface iodination identified virus-specific proteins expressed on BALB/cV primary mammary tumor cells grown in culture. In contrast to (C3H)MMTV-producing cell lines which expressed MMTV gp52, BALB/cV tumor cells lacked gp52 and expressed instead a 68K, env-related protein. The 68Kenv protein was also detected on the surface of metabolically labeled BALB/cV tumor cells by an external immunoprecipitation technique. The expression of 68Kenv was restricted to mammary tissues of BALB/cV mice that also expressed other MMTV proteins. Biochemical analysis established that 68Kenv was not modified by N-linked glycosylation. 125I-labeled 68Kenv was rapidly released into the media of tumor cell cultures and was recovered both in the form of a soluble protein and in a 100,000 g pellet. The biologic function of this cell surface-expressed viral protein remains unknown.