Proto-oncogenes encode proteins with three main sites of action: the cell-surface membrane, the cytoplasm and the nucleus. Although the exact biochemical function of most proto-oncogene products is not understood, several of them are known to be involved in signal transduction. A role in gene regulation through DNA binding has been suggested for a recently isolated member of the group of oncogenes acting at the nucleus, v-jun. The C-terminus of the putative v-jun-encoded protein is similar in sequence to the C-terminus of the yeast transcriptional activator GCN4 (refs 8, 9), which forms its minimal DNA-binding domain. GCN4 binds to specific sites whose consensus sequence is highly similar to the recognition sequence of the mammalian transcriptional activator AP-1 (refs 12, 13). Like GCN4, AP-1 binds to promoter elements of specific genes and activates their transcription. Because of the similarity between the recognition sites for GCN4 and AP-1, we examined the possibility that AP-1 could be the product of the c-jun proto-oncogene. The experimental results reported here indicate that the JUN oncoprotein is a sequence-specific transcriptional activator similar to AP-1.

We previously reported that the chromatin high-mobility group protein 1 (HMG-1) enhances the sequencespecific DNA binding activity of progesterone receptor (PR) in vitro, thus providing the first evidence that HMG-1 may have a coregulatory role in steroid receptor-mediated gene transcription. Here we show that HMG-1 and the highly related HMG-2 stimulate DNA binding by other steroid receptors, including estrogen, androgen, and glucocorticoid receptors, but have no effect on DNA binding by several nonsteroid nuclear receptors, including retinoid acid receptor (RAR), retinoic X receptor (RXR), and vitamin D receptor (VDR). As highly purified recombinant full-length proteins, all steroid receptors tested exhibited weak binding affinity for their optimal palindromic hormone response elements (HREs), and the addition of purified HMG-1 or -2 substantially increased their affinity for HREs. Purified RAR, RXR, and VDR also exhibited little to no detectable binding to their cognate direct repeat HREs but, in contrast to results with steroid receptors, the addition of HMG-1 or HMG-2 had no stimulatory effect. Instead, the addition of purified RXR enhanced RAR and VDR DNA binding through a heterodimerization mechanism and HMG-1 or HMG-2 had no further effect on DNA binding by RXR-RAR or RXR-VDR heterodimers. HMG-1 and HMG-2 (HMG-1/-2) themselves do not bind to progesterone response elements, but in the presence of PR they were detected as part of an HMG-PR-DNA ternary complex. HMG-1/-2 can also interact transiently in vitro with PR in the absence of DNA; however, no direct protein interaction was detected with VDR. These results, taken together with the fact that PR can bend its target DNA and that HMG-1/-2 are non-sequence-specific DNA binding proteins that recognize DNA structure, suggest that HMG-1/-2 are recruited to the PR-DNA complex by the combined effect of transient protein interaction and DNA bending. In transient-transfection assays, coexpression of HMG-1 or HMG-2 increased PR-mediated transcription in mammalian cells by as much as 7-to 10-fold without altering the basal promoter activity of target reporter genes. This increase in PR-mediated gene activation by coexpression of HMG-1/-2 was observed in different cell types and with different target promoters, suggesting a generality to the functional interaction between HMG-1/-2 and PR in vivo. Cotransfection of HMG-1 also increased reporter gene activation mediated by other steroid receptors, including glucocorticoid and androgen receptors, but it had a minimal influence on VDR-dependent transcription in vivo. These results support the conclusion that HMG-1/-2 are coregulatory proteins that increase the DNA binding and transcriptional activity of the steroid hormone class of receptors but that do not functionally interact with certain nonsteroid classes of nuclear receptors.Steroid hormone receptors are members of a superfamily of ligand-dependent transcriptional activators which direct the expression of specific gene networks involved in regulating the differen...

ATF is a cellular transcription factor involved in the regulation of multiple adenovirus E1A-and cellular cAMP-inducible promoters. Using DNA affinity chromatography, we have purified ATF and found that a series of polypeptides copurify in a sequence-specific manner. We demonstrate that these polypeptides represent a family of proteins that are related by DNA-binding specificity and by immunological cross-reactivity. This family includes the transcription factor AP-1, whose recognition sequence, GTGAGTCAA, differs from the ATF consensus, GTGACGTCAA, by the absence of a cytosine residue. Our results further indicate that there are multiple forms of both ATF and AP-1. The immunological cross-reactivity and related DNA-binding specificities suggest that ATF and AP-1 contain similar amino acid sequences and may have originated from a common gene.

These novel VDR modulators may have potential as therapeutics for cancer, leukemia and psoriasis with less calcium mobilization side effects than are associated with secosteroidal 1,25(OH)2D3 analogs.

Recent studies have suggested that the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3, can inhibit the growth and/or induce the differentiation of a variety of cell types and that these characteristics might be useful in the treatment of some cancers. Retinoids also promote the differentiation and inhibit the growth of some cells. That the vitamin D receptor acts as a heterodimer with the retinoid X receptor (RXR) suggests that there may be functional interactions between 1,25-dihydroxyvitamin D3 and retinoids. In this study, we show that the combination of 1,25-dihydroxyvitamin D3 and 9-cis retinoic acid synergistically inhibits the growth of LNCaP prostate cancer cells. That this effect is mediated by RXR rather than retinoic acid receptors was shown using RXR- and retinoic acid receptor-specific ligands. The vitamin D3 analog, EB1089, inhibited growth more effectively than 1,25-dihydroxyvitamin D3 and also acted synergistically with 9-cis-retinoic acid. These treatments caused cells to accumulate in the G1 phase of the cell cycle, suggesting that 1,25-dihydroxyvitamin D3 can regulate one or more factors critical for the G1/S transition.