Peter Angel

The jun proto-oncogene is positively autoregulated by its product, Jun/AP-1

Binding of the human transcription factor Jun/AP-1 to a conserved 8 bp nucleotide sequence (TRE) is responsible for increased transcription of different cellular genes in response to tumor promoters, such as TPA, and serum factors. Enhanced Jun/AP-1 activity in TPA-stimulated cells is regulated by two different mechanisms: a posttranslational event acting on pre-existing Jun/AP-1 molecules, and transcriptional activation of jun gene expression leading to an increase in the total amount of Jun/AP-1. Induction of jun transcription in response to TPA is mediated by binding of Jun/AP-1 to a high-affinity AP-1 binding site in the jun promoter region. Site-specific mutagenesis of this binding site prevents TPA induction and trans-activation by Jun/AP-1. These results clearly demonstrate that jun transcription is directly stimulated by its own gene product. This positive regulatory loop is likely to be responsible for prolonging the transient signals generated by activation of protein kinase C.

Activation of protein kinase C decreases phosphorylation of c-Jun at sites that negatively regulate its DNA-binding activity

In resting human epithelial and fibroblastic cells, c-Jun is phosphorylated on serine and threonine at five sites, three of which are phosphorylated in vitro by glycogen synthase kinase 3 (GSK-3). These three sites are nested within a single tryptic peptide located just upstream of the basic region of the c-Jun DNA-binding domain (residues 227-252). Activation of protein kinase C results in rapid, site-specific dephosphorylation of c-Jun at one or more of these three sites and is coincident with increased AP-1-binding activity. Phosphorylation of recombinant human c-Jun proteins in vitro by GSK-3 decreases their DNA-binding activity. Mutation of serine 243 to phenylalanine blocks phosphorylation of all three sites in vivo and increases the inherent trans-activation ability of c-Jun at least 10-fold. We propose that c-Jun is present in resting cells in a latent, phosphorylated form that can be activated by site-specific dephosphorylation in response to protein kinase C activation.

Heterodimer formation of cJun and ATF-2 is responsible for induction of c-jun by the 243 amino acid adenovirus E1A protein

The adenovirus E1A proteins differentially regulate AP‐1‐responsive genes. Collagenase and stromelysin are repressed by E1A, whereas the expression of c‐jun is elevated. Inhibition of collagenase has been found to be exerted through the consensus AP‐1 binding site TGAGTCA. Here we show that the distal AP‐1 binding site in the c‐jun promoter, the jun2TRE (TTACCTCA), is the decisive element of this promoter in mediating the positive response to the 243 amino acid E1A product. In vitro binding studies revealed that, in contrast to the consensus AP‐1 site which is preferentially targeted by dimers composed of the Jun and Fos families, the jun2TRE binds heterodimers composed of cJun and ATF‐2(‐like) proteins. Since stimulation of c‐jun transcription is a function of the transforming domain of E1A encoded by conserved region 1, cJun‐‐ATF‐2 may be one of the effector factors involved in transformation. The data further suggest that E1A can distinguish between cJun‐‐cJun and cJun‐‐ATF‐2 in imposing opposite states of activity.

The transactivating domain of the c-Jun proto-oncoprotein is required for cotransformation of rat embryo cells

The nuclear phosphoprotein c-Jun, encoded by the proto-oncogene c-jun, is a major component of the AP-1 complex. A potent transcriptional regulator, c-jun is also able to transform normal rat embryo cells in cooperation with an activated c-Ha-ras gene. By deletion analysis, we identified the regions of c-Jun encoding transformation and transactivation functions. Our studies indicate that there is a direct correlation between the ability of the c-Jun protein to activate transcription and cotransform rat embryo cells. The regions involved in these functions include the conserved leucine zipper/DNA binding domain and an effector domain near its N terminus. This N-terminal region spans amino acids 61 to 146 of the c-Jun protein and is highly conserved among all Jun family members. These results support the hypothesis that c-Jun transforms cells by stimulating the expression of transformation-mediating genes.

Adenovirus E1A negatively and positively modulates transcription of AP-1 dependent genes by dimer-specific regulation of the DNA binding and transactivation activities of Jun

Adenovirus E1A proteins inhibit expression of the collagenase gene but activate expression of the c‐jun gene. Both effects are mediated by TPA‐responsive elements (TREs), the binding sites for members of the AP‐1 transcription factor family. By a process that is independent of the retinoblastoma gene product, E1A distinguishes between different AP‐1 factors: in vivo binding of Jun/Jun homodimers and Jun/Fos heterodimers to the collagenase TRE is totally blocked by E1A while, in contrast, there is no inhibition of Jun/ATF‐2 binding to the TRE sequences in the c‐jun promoter. Altered phosphorylation of the DNA binding domain of cJun is not involved in the inhibition of cJun/cJun and cJun/cFos binding. E1A does, however, cause hyperphosphorylation of the transactivation domain of cJun, which is likely to be responsible for the enhanced c‐jun transcription by E1A mediated through cJun/ATF‐2 heterodimers.

The fos and jun families of transcription factors

The AP-1 family plays crucial roles in cell growth, proliferation, differentiation and apoptosis. It is the endpoint of several pathways of signal transduction, including one which triggers cancerous growth. The control of its activity is an issue both of basic science and of therapy for cancer and other diseases. Chapters are written by European experts in different areas. AP-1 is looked at as a factor regulating genes involved in metastatic properties of cancer, as a factor addressed by viral gene products and by steroid and retinoic acid receptors during the process of anti-inflammation. It covers the important role of AP-1 in cell transformation, induced by either chemical or physical mutagens or by expression of oncogenes.

The AP-1 Family of Transcription Factors

The history of the exploration of transcriptional regulation can be told as a tale of increasing complexity, from single regulatory proteins selecting genes and obeying linear arrays of signals, to huge multiprotein complexes that are embedded in a network of circuitry—involving so many and even seemingly redundant components that it has become difficult to recognize a pattern of rationale. The factor AP-1 can serve as a prime example for this development.