Alteration of homeobox gene expression by N-ras transformation of PA-1 human teratocarcinoma cells

Overexpression of melanoma inhibitory activity (MIA) enhances extravasation and metastasis of A-mel 3 melanoma cells in vivo

The secreted MIA protein is strongly expressed by advanced primary and metastatic melanomas but not in normal melanocytes. Previous studies have shown that MIA serum levels correlate with clinical tumour progression in melanoma patients. To provide direct evidence that MIA plays a role in metastasis of malignant melanomas, A-mel 3 hamster melanoma cells were transfected with sense- and antisense rhMIA cDNA and analysed subsequently for changes in their tumorigenic and metastatic potential. Enforced expression of MIA in A-mel 3 cells significantly increased their metastatic potential without affecting primary tumour growth, cell proliferation or apoptosis rate in hamsters, compared with control or antisense transfected cells. Additionally, MIA overexpressing transfectants showed a higher rate of both tumour cell invasion and extravasation. Cells transfected with MIA antisense generally exerted an opposite response. The above changes in function attributed to the expression of MIA may underlie the contribution of MIA to the malignant phenotype.

Functional interaction between a RARE and an AP-2 binding site in the regulation of the human HOX A4 gene promoter

HOX A genes are induced in a temporal fashion after retinoic acid (RA) treatment in non-N-ras-transformed PA-1 human teratcarcinoma cells. However, In N-ras-transformed PA-1 cells, RA-Induced expression of HOX A genes is delayed. The mRNA for the transcriptional activator AP-2 is overexpressed in these ras-transformed cells, but AP-2 transcriptional activity is inhibited relative to non ras-transformed PA-1 cells. Constitutive expression of AP-2 mimics the effect of ras by transforming cells and inhibiting differentiation in culture. We analyzed 4 kb of the human HOX A4 gene promoter and identified seven putative AP-2-binding sites in the DNA sequence. Transcription assays with variably sized HOX A4 promoter reporter constructs revealed that a 365 bp region of the promoter, -2950 to -3315 relative to the mRNA start, controls RA responsiveness and ras-mediated inhibition of HOX A4 activity. This region contains an AP-2 binding site and a RARE. Elimination of the AP-2 site by site-directed mutagenesis demonstrated that the AP-2 site is involved in RA-mediated transcriptional activation of the human HOX A4 promoter in combination with the RA receptor response element (RARE). In N-ras-transformed cells, low HOX A4 promoter activity results from ras inhibition of AP-2 transactivation.

The genomic structure of the human AP-2 transcription factor

The transcription factor AP-2 is encoded by a gene located on chromosome 6 near the HLA locus. Here we describe the genomic organization of the AP-2 gene including an initial characterization of the promoter. We have mapped two mRNA initiation sites, the entire exon-intron structure and located two polyadenylation sites. The mature AP-2 mRNA is spliced from 7 exons distributed over a region of 18 kb genomic DNA. A recently cloned inhibitory AP-2 protein is generated by alternative usage of a C-terminal exon. The proline-rich transactivation motif is encoded by a single exon within the N-terminal region in contrast to the complex DNA binding and dimerization motif which involves amino acid residues located on four different exons. The sites of mRNA initiation are located 220 and 271 bases upstream from the ATG translation start site. Although the promoter contains no canonical sequence motifs for basal transcription factors, such as TATA-, CCAAT- or SP-1 boxes, it mediates cell-type-specific expression of a CAT reporter gene in PA-1 human teratocarcinoma cells and is inactive in murine F9 teratocarcinoma cells. We demonstrate that the promoter of the AP-2 gene is subject to positive autoregulation by its own gene product. A consensus AP-2 binding site is located at position -622 with respect to the ATG. This site binds specifically to bacterially expressed AP-2 as well as to multiple proteins, including AP-2, present in PA-1 and HeLa cell nuclear extracts. A partial AP-2 promoter fragment including the AP-2 consensus binding site is approximately 5-fold transactivated by cotransfection of an AP-2 expression plasmid.

An alternatively spliced mRNA from the AP-2 gene encodes a negative regulator of transcriptional activation by AP-2

AP-2 is a retinoic acid-inducible and developmentally regulated activator of transcription. We have cloned an alternative AP-2 transcript (AP-2B) from the human teratocarcinoma cell line PA-1, which encodes a protein differing in the C terminus from the previously isolated AP-2 protein (AP-2A). This protein contains the activation domain of AP-2 and part of the DNA binding domain but lacks the dimerization domain which is necessary for DNA binding. Analysis of overlapping genomic clones spanning the entire AP-2 gene proves that AP-2A and AP-2B transcripts are alternatively spliced from the same gene. Both transient and stable transfection experiments show that AP-2B inhibits AP-2 transactivator function, as measured by an AP-2-responsive chloramphenicol acetyltransferase reporter plasmid. Furthermore, constitutive AP-2B expression in PA-1 cells causes a retinoic acid-resistant phenotype, anchorage-independent growth in soft agar, and tumorigenicity in nude mice, in a fashion similar to transformation of these cells by oncogenes. To determine the mechanism by which AP-2B exerts its inhibitory function, we purified bacterially expressed AP-2A and AP-2B proteins. While bacterial AP-2B does not bind an AP-2 consensus site, it strongly inhibits binding of the endogenous AP-2 present in PA-1 cell nuclear extracts. However, DNA sequence-specific binding of bacterially expressed AP-2A cannot be inhibited by bacterially expressed AP-2B. Therefore, inhibition of AP-2 activity by the protein AP-2B may require an additional factor or modification supplied by nuclear extracts.

An alternatively spliced mRNA from the AP-2 gene encodes a negative regulator of transcriptional activation by AP-2

AP-2 is a retinoic acid-inducible and developmentally regulated activator of transcription. We have cloned an alternative AP-2 transcript (AP-2B) from the human teratocarcinoma cell line PA-1, which encodes a protein differing in the C terminus from the previously isolated AP-2 protein (AP-2A). This protein contains the activation domain of AP-2 and part of the DNA binding domain but lacks the dimerization domain which is necessary for DNA binding. Analysis of overlapping genomic clones spanning the entire AP-2 gene proves that AP-2A and AP-2B transcripts are alternatively spliced from the same gene. Both transient and stable transfection experiments show that AP-2B inhibits AP-2 transactivator function, as measured by an AP-2-responsive chloramphenicol acetyltransferase reporter plasmid. Furthermore, constitutive AP-2B expression in PA-1 cells causes a retinoic acid-resistant phenotype, anchorage-independent growth in soft agar, and tumorigenicity in nude mice, in a fashion similar to transformation of these cells by oncogenes. To determine the mechanism by which AP-2B exerts its inhibitory function, we purified bacterially expressed AP-2A and AP-2B proteins. While bacterial AP-2B does not bind an AP-2 consensus site, it strongly inhibits binding of the endogenous AP-2 present in PA-1 cell nuclear extracts. However, DNA sequence-specific binding of bacterially expressed AP-2A cannot be inhibited by bacterially expressed AP-2B. Therefore, inhibition of AP-2 activity by the protein AP-2B may require an additional factor or modification supplied by nuclear extracts.