Activator Protein-1 (AP-1) is a family of transcription factors that are involved in the final step of different signal transduction cascades that end in the binding of these AP-1 transcription factors to specific DNA-binding sites called the AP-1 active sites.(Seldeen, 2011) The family includes leucine zipper proteins Jun (, JunB and JunD) and Fos (, FosB, Fra-1 and Fra-2) factors, as well as activation partner proteins and small subfamilies.3 The active sites are promoters on a multitude of genes and the specific DNA sequences are different for the different transcription factors within the family, but are conserved among those specific factors (Seldeen, 2011). This family of transcription factors are mainly involved in regulation of cell proliferation and death as well as various immune responses (Shaulian, 2001)(Li, 2014). The AP-1 site has been studied for its ability to contribute to the oncogenetic ability of a cell, but physiological information on the AP-1 binding site is still lacking.1
Structure of Fos and Jun
What are Fos and Jun? Fos and Jun are DNA binding proteins that form dimeric complexes through a . This leucine zipper is necessary for them to form the heterodimer between the two proteins (Seldeen, 2011). Jun can form stable dimers with itself while Fos is unstable and cannot, but the most stable conformation is when the two dimerize with each other. These proteins are associated with oncogenes, as overexpression of these proteins can lead to cancer while underexpression stunts cellular growth, leading to such diseases as osteoporosis and behavioral abnormalities. Fos and Jun alter cellular phenotypes by regulating expression of the target genes.
DNA Interaction
The C-terminal end of the leucine zipper contains basic amino acids that help stabilize the dimer and the DNA. This is only facilitated by a few nucleotides and residues (Seldeen, 2011). These basic amino acids allow for a bipartite DNA binding domain with each Fos and Jun. These basic regions in the DNA interacting section of the heterodimer also form with the negative phosphate groups in the major groove of the DNA. This dimer binds to Activator Protein-1 (AP-1) and the cAMP responsive element on a DNA strand. This, in turn, regulates the targeted gene expression. The binding of the Fos/Jun dimer is inhibited by inhibitory protein-1 (IP-1). Also, the binding of Fos/Jun is regulated by the phosphorylation of either end of the protein. The phosphorylation of the amino acids near the N-terminal increases DNA binding, but phosphorylation of them at the C-terminal inhibits binding. The Fos/Jun dimer is naturally expressed in low basal levels inside the cell, but can be induced rapidly by extracellular stimuli.
AP-1 Function
AP-1 function is both biologically diverse and not fully understood. The AP-1 site has been shown to be involved in both basal and differential gene expression (Shaulian, 2001). It also has the ability to regulate cell growth, apoptosis, cell survival, and inflammation and innate immune responses (Shaulian, 2001). While most AP-1 site interaction is activating in nature, there have been examples of direct negative regulation with specific genes.
Basal and Differential Gene Expression: AP-1 basally regulates genes involved in RNA polymerase activity as well as differentially regulates genes through multiple avenues. This includes inflammation, UV radiation, tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), and proto/oncoproteins.
Cell Growth: AP-1 transcription factors have various affects on cell growth including cooperation with oncoproteins and oncogenetic ability. C-Jun and c-Fos are the mammalian homologs to v-Fos and v-Jun, which are retroviral oncoproteins associated with cell proliferation and growth in chickens (Shaulian, 2001). Further studies have shown that these proteins are related in their ability to induce cell proliferation. A study showed that alone, c-Fos cannot induce transformation of rat embryo fibroblasts, but the proto-oncogene requires c-Fos in order to complete the transformation (Shaulian, 2001). Antibody studies have also suggested that c-Fos and c-Jun interact to progress cells through the S-phase of the cell cycle, implying that they could be involved in how often a cell can divide. However, further investigation has shown that c-Jun can negatively regulate p53, a tumor suppressor gene, which is what is responsible for the progression through the cell cycle (Shaulian, 2001).
Apoptosis and Cell Survival: There have been multiple studies that have shown AP-1’s involvement in cell apoptosis and survival, depending on the cell type, environment and developmental stage. Nervous system studies on mice have shown that continuous c-Fos expression leads to neural cell death, while conversely, inhibition of c-Jun activity in neural cells can protect cells from apoptosis (Shaulian, 2001). C-Jun also has been shown to be an implicit mediator of apoptosis when there is persistent activity. During embryonic development in mice, AP-1 has been shown to play a protective role. When c-Jun is mutated, there is a high level of liver cell apoptosis, but not when the c-Jun activity is restored (Shaulian, 2001). This differing involvement shows the multiple roles AP-1 plays, through both direct activation of genes and homeostatic function that arises from change in growth, development and environment of cells.
Inflammation and Innate Immune Responses: AP-1 along side with a β enhancer binding protein can positively regulate the protoinflammatory factor S100A12. The AP-1 factors have synergetic effects with the β enhancer binding proteins and when both are mutated, it completely deactivated the transcriptional activity of the S100A12. The AP-1 sites are located on the 1,200 base pair promoter region of the porcine S100A8 and S100A9 genes (Li, 2014). The β enhancer binding proteins are also partially regulated by AP-1 activity.4 AP-1 also has innate responses due to environmental stressors including UV radiation. AP-1 proteins c-Jun and ATF2 are thought to be some of the most sensitive UV response genes. They are phosphorylated by JNK, which is apart of the MAPK cascade (Li, 2014).
