Nuclear oncogene products have the potential to induce alterations in gene regulation leading to the genesis of cancer. The biochemical mechanisms by which nuclear oncoproteins act remain unknown. Recently, an oncogene, v-jun, was found to share homology with the DNA binding domain of a yeast transcription factor, GCN4. Furthermore, GCN4 and the phorbol ester-inducible enhancer binding protein, AP-1, recognize very similar DNA sequences. The human proto-oncogene c-jun has now been isolated, and the deduced amino acid sequence indicates more than 80 percent identity with v-jun. Expression of cloned c-jun in bacteria produced a protein with sequence-specific DNA binding properties identical to AP-1. Antibodies raised against two distinct peptides derived from v-jun reacted specifically with human AP-1. In addition, partial amino acid sequence of purified AP-1 revealed tryptic peptides in common with the c-jun protein. The structural and functional similarities between the c-jun product and the enhancer binding protein suggest that AP-1 may be encoded by c-jun. These findings demonstrate that the proto-oncogene product of c-jun interacts directly with specific target DNA sequences to regulate gene expression, and therefore it may now be possible to identify genes under the control of c-jun that affect cell growth and neoplasia.

The Fos protein complex and several Fos-related antigens (FRA) bind specifically to a sequence element referred to as the HeLa cell activator protein 1 (AP-1) binding site. A combination of structural and immunological comparisons has identified the Fos-associated protein (p39) as the protein product of the jun proto-oncogene (c-Jun). The p39/Jun protein is one of the major polypeptides identified in AP-1 oligonucleotide affinity chromatography extracts of cellular proteins. These preparations of AP-1 also contain Fos and several FRA's. Some of these proteins bind to the AP-1 site directly whereas others, like Fos, appear to bind indirectly via protein-protein interactions. Cell-surface stimulation results in an increase in c-fos and c-jun products. Thus, the products of two protooncogenes (and several related proteins), induced by extracellular stimuli, form a complex that associates with transcriptional control elements containing AP-1 sites, thereby potentially mediating the long-term responses to signals that regulate growth control and development.

Biologically active molecular clones of avian sarcoma virus 17 (ASV 17) contain a replication-defective proviral genome of 3.5 kilobases (kb). The genome retains partial gag and env sequences, which flank a cell-derived putative oncogene of 0.93 kb, termed jun. The jun gene lacks preserved coding domains of tyrosine-specific protein kinases. It also shows no significant nucleic acid homology with other known oncogenes. The probable transformation-specific protein in ASV 17-transformed cells is a 55-kDa gag-jun fusion product.Avian sarcoma virus 17 (ASV 17) was isolated from a spontaneous sarcoma in an adult chicken (1). It screened by plaque hybridization (6) using 32P-labeled long terminal repeat (LTR) or pol-specific probes of avian retroviruses. The LTR-and pol-specific DNA fragments were prepared from pEcoRlD and pHindIII Bam plasmids, respectively. These plasmids were gifts from J. M. Bishop and Nancy Quintrell (University of California, San Francisco). All enzymes employed were purchased from Boehringer Mannheim and were used under the conditions recommended by the supplier.Transfection. Transfection experiments were performed on CEF with recombinant X phage DNAs without the addition of helper virus DNA, according to the method described by Kawai and Nishizawa (7).DNA Sequencing. The DNA fragment encompassing the ASV 17 oncogene was inserted into single-stranded DNA phages M13mpl9 and mpl8. Deletion clones were constructed by using BAL-31 nuclease (8). Both strands were sequenced by the dideoxy chain termination method, using the Klenow fragment of DNA polymerase I (9). Ambiguous regions containing a preponderance of G and C residues were sequenced with reverse transcriptase (10). 2848The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

We have previously demonstrated decreased Jun/AP-1 activity in the breast cancer cell line MCF-7 when compared to normal or immortalized mammary epithelial cells. In this paper, we overexpress Jun in MCF-7 cells (MCF7Jun) and demonstrate that it results in diverse biologic and biochemical changes, which mimic those seen clinically in breast cancer. Overexpression of Jun causes signi®cant alterations in the composition of AP-1, decreased junB and increased fra-1 expression and results in an increased biologic aggressiveness. MCF7Jun cells exhibit increased cellular motility, increased expression of a matrix degrading enzyme MMP-9, increased in vitro chemoinvasion and tumor formation in nude mice in the absence of exogenous estrogens. Furthermore, MCF7Jun cells are unresponsive to the growth stimulating e ects of estrogen and growth inhibitory e ects of tamoxifen. Analysis of the estrogen receptor (ER) expression and activity showed that the MCF7Jun cells have no detectable ER. MCF-7 cells overexpressing mutant forms of cJun were responsive to the growth stimulatory e ects of estrogen indicating that full-length cJun is required to acquire the estrogenindependent phenotype in breast cancer cells.

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