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K Butz

Publications and source records attributed to K Butz.

28 records · Page 2Linked to original sources
PubMed1994

Cellular control of human papillomavirus oncogene transcription.

Specific types of human papillomaviruses (HPVs) are closely associated with the development of cervical cancer. The transforming ability of these high-risk HPV types depends on the expression of the viral E6 and E7 oncogenes. It is therefore of particular interest to elucidate the molecular mechanisms that result in the activation of E6/E7 expression during HPV-associated tumorigenesis. Recently, much progress has been made in characterizing the proteins involved in the regulation of HPV oncogene transcription. This review describes the functional significance of cellular factors involved in the transcriptional control of the E6/E7 promoter for the two most common HPV types associated with cervical cancer, HPV16 and HPV18. In addition, we discuss regulatory pathways that may contribute to the epithelial cells specificity of E6/E7 transcription. The definition of the factors that regulate HPV oncogene transcription could provide new insights into the molecular mechanisms activating viral oncogene expression during cervical carcinogenesis and forms an experimental basis for investigating the specific biochemical pathways that contribute to HPV-associated malignant cell transformation.

Base Sequence↗
PubMed1993

A novel cis-stimulatory element maps to the 5' portion of the human papillomavirus type 18 upstream regulatory region and is functionally dependent on a sequence-aberrant Sp1 binding site.

Gene expression of the cancer-associated human papillomavirus (HPV) type 18 is modulated by cis-regulatory elements located within the viral upstream regulatory region (URR). All cellular factors identified so far involved in viral gene control bind to the 3' portion of the HPV-18 URR. In contrast, very little is known about regulatory elements within the 5' portion of the URR. We therefore analysed this region of unknown function to delineate potential cis-regulatory elements contained therein. By utilizing transient expression assays, an 84 bp fragment could be identified in the 5' portion of the URR that exhibits orientation-independent cis-stimulatory activity in HeLa cervical carcinoma cells and primary human fibroblasts. Gel retardation assays and competition experiments indicated specific binding of cellular proteins to the 84 bp fragment. By functional dissection, a regulatory element with intrinsic cis-stimulatory activity could be mapped within the 84 bp fragment. Binding studies indicate that this cis-stimulatory element contains a sequence-aberrant Sp1 recognition site. Transient luciferase assays performed with mutated templates demonstrate that this Sp1 binding site behaves as a functional Sp1 element in vivo and is a main determinant of the cis-stimulatory activity exerted by the 84 bp fragment. These data show that the 5' portion of the HPV-18 URR contains cis-activating elements and indicate an important functional role for the cellular transcription factor Sp1 in their regulation.

Base Sequence↗
PubMed1993

Transcriptional control of human papillomavirus (HPV) oncogene expression: composition of the HPV type 18 upstream regulatory region.

The malignant transformation potential of high-risk human papillomaviruses (HPVs) is closely linked to the expression of the viral E6 and E7 genes. To elucidate the molecular mechanisms resulting in HPV oncogene expression, a systematic analysis of the cis-regulatory elements within the HPV type 18 (HPV18) upstream regulatory region (URR) which regulate the activity of the E6/E7 promoter was performed. As the functional behavior of a given cis-regulatory element can be strongly influenced by the overall composition of a transcriptional control region, individual elements were inactivated by site-directed mutagenesis in the physiological context of the complete HPV18 URR. Subsequently, the effects of these mutations on the activity of the E6/E7 promoter were assessed by transient transfection assays. We found that the transcriptional stimulation of the E6/E7 promoter largely depends on the integrity of cis-regulatory elements bound by AP1, SP1, and in certain epithelial cells, KRF-1. In contrast to previous reports by implying a key role for NF1 and Oct-1 recognition motifs in the stimulation of papillomavirus oncogene expression, the inactivation of these elements in the context of the HPV18 URR did not strongly affect the transcriptional activity of the E6/E7 promoter. Mutation of a promoter-proximal glucocorticoid response element completely abolished dexamethasone inducibility of the HPV18 E6/E7 promoter and resulted in an increase of its basal activity. Functional dissection of the HPV18 constitutive enhancer region indicates that its transcriptional activity is largely generated by functional synergism between a centrally located AP1 module and thus far undetected cis-active elements present in the 5' flank of the enhancer. Furthermore, comparative analyses using homologous and heterologous promoters show that the transcriptional activity of HPV18 enhancer elements is influenced by the nature of the test promoter in a cell-type-specific manner.

Base Sequence↗
PubMed1993

Repression of endogenous p53 transactivation function in HeLa cervical carcinoma cells by human papillomavirus type 16 E6, human mdm-2, and mutant p53.

Somatic mutations in the p53 tumor suppressor gene represent the single most common genetic alteration observed in human cancers. Interestingly, the great majority of malignant tumors of the cervix uteri contain wild-type p53 alleles together with the DNA of specific types of human papillomaviruses (HPVs), while the small portion of HPV-negative cervical carcinomas often carry alterations in the p53 tumor suppressor gene. Transcriptional activation of yet-undefined cellular regulatory genes has been implicated to play a key role for the tumor-suppressive activity of wild-type p53, as mutant p53 in general has lost the activity to stimulate p53-responsive reporter plasmids. The detection of DNA-binding-competent and transcriptionally active p53 protein in HeLa cervical carcinoma cells enabled us to investigate the in vivo effects of putative modulators on endogenous p53 function in cervical cancer cells. We show that the transcriptional stimulatory activity of HeLa cell p53 is strongly repressed by overexpression of E6 protein from oncogenic HPV type 16 (HPV16) but is not influenced by low-risk HPV6 E6. Similar to HPV16 E6, cellular oncoproteins such as mutant p53 or the product of the human mdm-2 gene also negatively interfere with p53-mediated transactivation in HeLa cells. Our findings indicate that, within a cervical cancer cell, the expression of E6 protein from high-risk HPV16, but not from low-risk HPV6, can lead to the same functional consequences as a mutation of the p53 gene. These results could provide a biochemical basis for the inverse correlation between the presence of HPV sequences and somatic mutations of the p53 gene in cervical carcinomas.

Base Sequence↗
PubMed1992

Activation of human papillomavirus type 18 E6-E7 oncogene expression by transcription factor Sp1.

The human papillomavirus 18 (HPV18) E6 and E7 proteins are considered to be primarily responsive for the transforming activity of the virus. In order to analyse the molecular mechanisms resulting in viral oncoprotein expression, it is necessary to identify the factors involved in the transcriptional regulation of the E6/E7 genes. Here we define by gel retardation experiments a sequence aberrant Sp1 binding site present in the promoter proximal part of the viral transcriptional control region (Upstream Regulatory Region, URR). Functional analyses employing transient reporter assays reveal that this Sp1 element is required for an efficient stimulation of the HPV18 E6/E7-promoter. Mutation of the Sp1 element in the natural context of the HPV18 URR leads to a strong decrease in the activity of the E6/E7-promoter in several cell lines. The magnitude of reduction varies between different cell types and is higher in cell lines of epithelial origin when compared with nonepithelial cells. Cotransfection assays using Sp1 expression vector systems further define the promoter proximal HPV18 Sp1 binding motif as a functional Sp1 element in vivo and show that its integrity is essential for the stimulation of the E6/E7-promoter by augmented levels of Sp1. These results indicate, that the cellular transcription factor Sp1 plays an important role for the stimulation of the E6/E7-promoter by the viral URR and represents a major determinant for the expression of HPV18 transforming genes E6 and E7.

3T3 Cells↗
PubMed1991

Repression of the human papillomavirus type 18 enhancer by the cellular transcription factor Oct-1.

The role of cellular factors involved in the transcriptional regulation of the cancer-associated human papillomavirus type 18 (HPV18) is yet poorly understood. The presence of an Oct-1-binding site within the HPV18 upstream regulatory region led us to investigate the influence of Oct-1 on viral transcription. Cotransfection of Oct-1 expression plasmids together with luciferase reporter constructs containing HPV18 regulatory sequences indicated that Oct-1 can transcriptionally repress the HPV18 upstream regulatory region. In contrast, heterologous control regions were not affected by Oct-1. HPV18 cis elements that can be repressed by Oct-1 mapped to a 135-bp subregion of the viral constitutive enhancer. Analysis of an Oct-1 mutant defective in DNA binding suggested that HPV18 down-modulation does not require direct binding of Oct-1 to DNA. These results make Oct-1 a candidate factor involved in the intracellular surveillance of HPV18 transcription and support the notion of a host cell mechanism that can specifically repress HPV E6-E7 transforming gene expression.

DNA-Binding Proteins↗
PubMedDate not supplied

Human tumor viruses.

Tumor viruses play an important role for the development of a substantial fraction of human malignancies, including common cancers, such as carcinomas of the cervix uteri, hepatocellular carcinomas, or lymphomas. In the recent past, much progress has been made in elucidating the molecular mechanisms by which human tumor viruses contribute to cellular growth deregulation and carcinogenesis. The picture emerges that different tumor viruses target similar cellular pathways for growth deregulation but, in addition, also have unique properties contributing to oncogenesis. Malignant transformation typically requires additional genetic alterations of the host cell, to which tumor viruses can contribute by destabilizating the cellular genome.

Cell Transformation, Viral↗