The c-Ha-ras oncogene and a tumor promoter activate the polyoma virus enhancer

Purified transcription factor AP-1 interacts with TPA-inducible enhancer elements

The enhancer-binding protein AP-1 has been purified to greater than 95% homogeneity from HeLa cells by sequence-specific DNA affinity chromatography and identified as a 47 kd polypeptide. Purified AP-1 activates transcription in vitro of the wild-type human metallothionein IIA (hMT IIA) gene but not mutant hMT IIA promoters lacking AP-1 recognition sites. DNAase I protection analysis indicates that genetically defined enhancer elements in hMT IIA, SV40, and the human collagenase gene contain high-affinity AP-1-binding sites, each with a conserved recognition motif, TGACTCA. These three genes are transcriptionally induced by treatment of cells with the tumor promoter TPA. Here we demonstrate that multiple synthetic copies of the consensus AP-1-binding site can act as TPA-inducible enhancers in various plasmid constructs after transfection into HeLa cells. These findings suggest that AP-1 is a transcription factor that functions by interacting with a specific enhancer element, and that its activities may be modulated by treatment of cells with TPA, known to stimulate protein kinase C.

Phorbol ester-inducible genes contain a common cis element recognized by a TPA-modulated trans-acting factor

The promoter regions of several phorbol diester-(TPA-) inducible genes (collagenase, stromelysin, hMT IIA, and SV40) share a conserved 9 bp motif. Synthetic copies of these closely related sequences conferred TPA inducibility upon heterologous promoters. Footprinting analysis indicated that these TPA-responsive elements (TREs) are recognized by a common cellular protein: the previously described transcription factor AP-1. A point mutation that eliminated the basal and induced activity of the TRE also interfered with its ability to bind AP-1. Treatment of cultured cells with TPA led to a rapid 3- to 4-fold increase in TRE binding activity, by a posttranslational mechanism. These results strongly suggest that AP-1 is at the receiving end of a complex pathway responsible for transmitting the effects of phorbol ester tumor promoters from the plasma membrane to the transcriptional machinery.

Regulation of inducible and tissue-specific gene expression

Molecular genetics approaches have been used to identify and characterize cis-acting DNA sequences required for eukaryotic gene regulation. These sequences are modular in nature, consisting of arrays of short (10- to 12-base pair) recognition elements that interact with specific transcription factors. Some transcription factors have been extensively purified and the corresponding genes have been cloned, but the mechanisms by which they promote transcription are not yet understood. Positive and negative regulatory elements that function only in specific cell types or in response to extracellular inducers have been identified. A number of cases of inducible and tissue-specific gene expression involve the activation of preexisting transcription factors, rather than the synthesis of new proteins. This activation may involve covalent modification of the protein or an allosteric change in its structure. The modification of regulatory proteins may play a central role in the mechanisms of eukaryotic gene regulation.