Concentration dependence of transcriptional transactivation in inducible E1A-containing human cells

The nuclear tyrosine kinase c-Abl negatively regulates cell growth

c-Abl is a tyrosine kinase localized primarily in the nucleus. Previous assays for abl function rely on cellular transformation by abl mutants, which are cytoplasmic. Using a conditional overexpression strategy, we have developed a functional assay for c-abl. Overexpression of c-abl inhibits growth by causing cell cycle arrest. Growth suppression requires tyrosine kinase activity, nuclear localization, and an intact SH2 domain. Overexpression of dominant negative c-abl disrupts cell cycle control and enhances transformation by tyrosine kinases, G proteins, and transcription factor oncogenes. These findings suggest that c-abl acts as a negative regulator of cell growth. This growth suppressive activity is functionally similar to that of tumor suppressor genes such as p53 and Rb.

Nucleotide sequence and regulation of the adenovirus type 3 E2A early promoter

The nucleotide sequence of the adenovirus serotype 3 E2A early promoter has been determined. In contrast to Ad2, the Ad3 E2A early promoter possessed only one TATA-like box and one nuclear transcription factor E2F binding site and lacked the silencer sequences; however, as in Ad2, the ATF binding site was present. Moreover, the Ad3 E2A promoter harbored a protein binding sequence recognized by the SP1 factor. By transient expression analysis in HeLa cells, we demonstrated that the E1A gene products of Ad3 and Ad2 stimulated Ad3 E2A transcription. In competition experiments, the Ad3 E2A promoter was used in preference to the Ad2 E2A promoter.

Suppression of adenovirus type 5 E1A-mediated transformation and expression of the transformed phenotype by caffeic acid phenethyl ester (CAPE)

Viral transformation and DNA-transfection assays were employed to investigate the differential toxic effect of caffeic acid phenethyl ester (CAPE), an extract of the honeybee hive product propolis, on adenovirus type 5 (Ad5)-transformed cloned rat embryo fibroblast (CREF) cells. CAPE inhibited, in a dose-dependent manner, both de novo and carcinogen-enhanced transformation of CREF cells by H5hr1, the cold-sensitive (cs) host-range mutant of Ad5. CAPE had a selective inhibitory effect on Ad5-induced transformation when a wild-type (wt) Ad5 E1A gene or a cs Ad5 E1A gene (at 37 degrees C, but not at 32 degrees C) was cotransfected into CREF cells with a dominant-acting bacterial hygromycin-resistance gene. A requirement for the expression of Ad5 E1A-encoded mRNAs and transforming proteins and sensitivity to CAPE was demonstrated using CREF cells stably transformed by a cs Ad5 E1A gene and an Ad5 E1A gene under the transcriptional control of a mouse mammary tumor virus promoter. To distinguish between the effects of the two Ad5 E1A-encoded proteins of 289 amino acids (aa) and 243 aa, CREF cells were stably transformed with cDNAs encoding either the 13S or the 12S E1A mRNA. CREF cells expressing the 13S E1A-encoded 289-aa protein were more sensitive to the growth-suppressing effect of CAPE than cells producing only the 12S E1A-encoded 243-aa protein. However, the growth-suppressing and toxic effects of CAPE were greatest in cells expressing both E1A-encoded transforming proteins. Analysis of the effect of CAPE on E1A and beta-actin gene expression in wt and cs E1A and H5hr1-transformed CREF cells indicated that low levels of CAPE, which were growth suppressive, did not selectively suppress E1A expression. These results demonstrated that cellular changes induced in CREF cells by the 13S E1A-encoded 289-aa protein of Ad5, when expressed alone or in combination with the 12S E1A-encoded 243-aa protein, rendered transformed cells sensitive to the growth-suppressing and toxic effects of CAPE.

Adenovirus E1A under the control of heterologous promoters: wide variation in E1A expression levels has little effect on virus replication

Adenovirus early region 1A(E1A) encodes a heterogeneous family of proteins some of which function as transactivators and are required for efficient viral replication in HeLa cells. We have constructed adenovirus type 5 (Ad 5) mutants in which the E1A transcription unit is placed under the control of either the human beta-actin promoter or the human cytomegalovirus immediate early promoter. The level of E1A expression in cells infected with these mutants was several times higher than that in wild-type virus infections. When the same heterologous promoters were inserted upstream of, but in the opposite orientation to, the E1A transcription unit, the level of expression was greatly reduced with respect to wild-type levels of E1A. Despite this variation of at least 40-fold in the concentration of E1A proteins in infected cells, there was no significant difference between wild-type Ad 5 and any of the mutants in their ability to replicate in HeLa cells. These results suggest that very low levels of E1A proteins are sufficient for virus production in cultured cells and that wild-type Ad 5 produces an amount of E1A in excess of that required.

Localization of newly synthesized vimentin subunits reveals a novel mechanism of intermediate filament assembly

We have assessed the mechanism of intermediate filament assembly by assaying the sites of incorporation of chicken vimentin subunits expressed under the control of an inducible promoter in transfected mouse fibroblasts. The localization of newly synthesized vimentin was determined by immunofluorescence and immunoelectron microscopy at short time periods of induced synthesis, using antibodies specific for chicken vimentin. Under conditions where neither the soluble subunit pools nor the steady-state distribution of endogenous filaments are affected, newly synthesized vimentin incorporates into the vimentin filament network at numerous and discrete sites throughout the cell. Over time, the pattern of newly assembled vimentin converts to a continuous array coincident with preexisting vimentin filaments. These results are consistent with a novel mechanism of intermediate filament assembly, whereby growth of intermediate filaments occurs by topographically restricted and localized subunit addition, necessitating a transient disruption of filament integrity.

Molecular oncogenetics of metastasis

Metastasis is a complex non-stochastic process that is most likely the result of genetic and epigenetic interactions of a wide variety of genes. The search for a single gene which can encompass such a pleiotropic response as to account for the observed phenotypic characteristics of metastatic tumour populations has been unsuccessful. Particular studies involving gene transfection, subtractive hybridisation and cell fusion are beginning to identify specific genes which contribute to metastasis in some cell types. However, such analyses are complicated by the inherent genetic instability and phenotypic heterogeneity present in tumour populations. A more detailed understanding of the metastatic process may require an abandoning of current generalised approaches to metastasis in favour of concentrating on key components of the metastatic cascade such as adhesion and invasion.

Adeno-associated virus P5 promoter contains an adenovirus E1A-inducible element and a binding site for the major late transcription factor

Activity of the adeno-associated virus P5 transcriptional control region was found to be induced by adenovirus E1A gene products. A pair of adjacent sequence elements was found to mediate both basal and E1A-induced P5 activity. The first element is a binding site for the major late transcription factor (MLTF), a factor first identified on the basis of its binding to a specific sequence within the adenovirus major late promoter. The second element is a tandemly repeated 10-base-pair sequence whose relationship to previously described binding sites is unclear. Each element individually conferred E1A responsiveness on a heterologous promoter, and deletion analysis demonstrated that each contributed to the level of P5 activity in the presence of E1A products both in transfection- and infection-based assays. Although deletion of the MLTF binding site led to reduced P5 transcriptional activity in the presence of E1A proteins, the deletion generated enhanced P5 basal activity in the absence of the transcriptional activator. The negative effect of the MLTF binding site in the absence of activator and its positive effect in the presence of activator combine to enhance the magnitude of the response by the P5 control region to E1A gene products.