Activation of ATF2
ATF2 is activated by stress kinases, including JNK (MAPK8, MAPK9, MAPK10) and p38 (MAPK1, MAPK11, MAPK12, MAPK13, MAPK14) and is implicated in transcriptional regulation of immediate early genes regulating stress and DNA damage responses (Gupta et al., 1995; van Dam et al., 1995) and cell cycle control under normal growth conditions.(up-regulation of the CCNA2 (cyclin A) promoter at the G1/S boundary) (Nakamura et al., 1995). In response to stimuli, ATF2 is phosphorylated on threonine 69 and/or 71 by JNK or by p38. Phosphorylation on Thr69 and Thr71 of ATF2 and its dimerization are required to activate ATF2 transcription factor activity. Phosphorylation on Thr69 occurs through the RALGDS-SRC-P38 pathway, and phosphorylation on Thr71 occurs through the RAS-MEK-ERK pathway (MAPK1, MAPK3 (ERK1), MAPK11, MAPK12 and MAPK14) (Gupta et al., 1995; Ouwens et al., 2002).
The intrinsic histone acetylase activity of ATF2 promotes its DNA binding ability (Abdel-Hafiz et al., 1992; Kawasaki et al., 2000).
Interaction of ATF2 with CREBBP (CREB-binding protein, also called p300/CBP) is dependent upon phosphorylation at Ser121 induced by PRKCA. ATF2 and CREBBP cooperate in the activation of transcription (Kawasaki et al., 1998; Yamasaki et al., 2009).
VRK1 activates and stabilizes ATF2 through direct phosphorylation of Ser62 and Thr73 (Sevilla et al., 2004).

Down regulation of ATF2
Among other down regulation mechanisms, ATF2 is down regulated, by MIR26B in response to ionizing radiation (Arora et al., 2011).
Transcriptionally active dimers of ATF2 protein are regulated by ubiquitylation and proteosomal degradation (Fuchs et al., 1999); phosphorylation of ATF2 on Thr69 and Thr71 promotes its ubiquitylation and degradation (Firestein and Feuerstein, 1998).
A cytoplasmic alternatively spliced isoform of ATF7, ATF7-4, is a cytoplasmic negative regulator of both ATF2 and ATF7. It impairs ATF2 and ATF7 phosphorylation (ATF7-4 indeed sequesters the Thr53-phosphorylating kinase in the cytoplasm, preventing Thr53 phosphorylation of ATF7) and transcriptional activity (Diring et al., 2011).
The activity of ATF2 is repressed by an intramolecular interaction between the N-terminal domain and the b-ZIP domain (Li and Green, 1996). The N-terminal nuclear export signal (NES) of ATF2 negatively regulates ATF2 transcriptional activity (Hsu and Hu, 2012).

Dimerization of ATF2
The basic leucine zipper (basic motif + leucine zipper, "b-ZIP") of ATF2 enables homo- or hetero-dimerization.
The main dimerization partners of ATF2 are the following: ATF2, BRCA1, CREB1, JDP2, JUN, JUNB, JUND, MAFA, NF1, NFYA, PDX1, POU2F1, TCF3 (Lau and Ronai, 2012).
ATF2 homodimers have a low transcriptional activity.
MAPKAP1 (SIN1) binds to the b-ZIP region of ATF2, and also binds MAPK14, and is required for MAPK14-induced phosphorylation of ATF2 in response to osmotic stress, and activates the transcription of apoptosis-related genes (Makino et al., 2006).
In response to stress, ATF2 binds to POU2F1 (OCT1), NFI, and BRCA1 to activate transcription of GADD45. The b-Zip region of ATF2 is critical for binding to BRCA1. ATF2 also binds and activates SERPINB5 (Maspin) (Maekawa et al., 2008).
ATF2 also forms a heterodimer with JDP2, a repressor of AP-1. JDP2 inhibits the transactivation of JUN by ATF2 (Jin et al., 2002).

ATF2 target genes
Under genotoxic stress, a study showed that 269 genes were found to be bound by ATF2/JUN dimers. Immediate-early genes were a notable subset and included EGR family members, FOS family members, and JUN family members, but the largest group of genes belonged to the DNA repair machinery (Hayakawa et al., 2004, see below).
Among the ATF2 target genes are :
- Transcription factors, such as JUN, ATF3, DDIT3 (CHOP), FOS, JUNB,
- DNA damage proteins (see below),
- Cell cycle regulators (CCNA2, CCND1), see below,
- Regulators of apoptosis (see below and Hayakawa et al., 2004),
- Growth factor receptors and cytokines such as PDGFRA (Maekawa et al., 1999), IL8 (Agelopoulos and Thanos, 2006), FASLG (Fas ligand), TNF (TNFalpha), TNFSF10 family (Herr et al., 2000; Faris et al., 1998),
- Proteins related with invasion such as MMP2 (Hamsa and Kuttan, 2012) and PLAU (UPA): ATF2/JUN heterodimer binds to and activates PLAU (De Cesare et al., 1995),
- Cell adhesion molecules, such as SELE, SELP, and VCAM1 (Reimold et al., 2001),
- Proteins engaged in the response to endoplasmic reticulum (ER) stress. ATF2/CREB dimers bind the CRE-like element TGACGTGA of HSPA5 (Grp78) and activates it (Chen et al., 1997),
- Genes encoding extracellular matrix components seem to constitute an important subset of ATF2/JUN-target genes (van Dam and Castellazzi, 2001).
- PTEN, a negative regulator of the PI3K/AKT signaling pathway, is positively regulated by ATF2 (Qian et al., 2012).
- ATF1 and ATF2 regulate TCRA and TCRB gene transcription.

Histones, Chromatin
UV treatment or ATF2 phosphorylation increases its histone acetyltransferase (HAT) activity as well as its transcriptional activities. Lys296, Gly297 and Gly299, are essential both for histone acetyltransferase activity and for transactivation (Kawasaki et al., 2000).
Binding of ATF2 to the histone acetyltransferase RUVBL2 (TIP49b) suppresses ATF2 transcriptional activity. RUVBL2s association with ATF2 is phosphorylation dependent and requires amino acids 150 to 248 of ATF2 (Cho et al., 2001).
ATF2 interacts with the acetyltransferase domains of CREBBP. ATF2 b-ZIP could serve as an acetyltransferase substrate for the acetyltransferase domains of CREBBP. ATF2 is acetylated on Lys357 and Lys374 by CREBBP, which contributes to its transcriptional activity (Karanam et al., 2007).
ATF2 and ATF4 are essential for the transcriptional activation of DDIT3 (CHOP) upon amino acid starvation.
ATF2 is essential in the acetylation of histone H4 and H2B, and thereby may be involved in the modification of the chromatin structure. An ATF2-independent HAT activity is involved in the amino acid regulation of ASNS transcription (Bruhat et al., 2007).
The histone variant macroH2A recruitement into nucleosomes could confer an epigenetic mark for gene repression. The constitutive DNA binding of the ATF2/JUND heterodimer to the IL-8 enhancer recruits macroH2A-containing nucleosomes in B cells, thus inhibiting transcriptional activation (Agelopoulos and Thanos, 2006).
Heat shock or osmotic stress induces phosphorylation of dATF2 (ATF2 in Drosophila), results in its release from heterochromatin, and heterochromatic disruption. dATF2 regulates heterochromatin formation. ATF2 may be involved in the epigenetic silencing of target genes in euchromatin. The stress-induced ATF2-dependent epigenetic change was maintained over generations, suggesting a mechanism by which the effects of stress can be inherited (Seong et al., 2011).

DNA damage response
Phosphorylation on Ser490 and Ser498 by ATM is required for the activation of ATF2 in DNA damage response. Phosphorylation of ATF2 results in the localisation of ATF2 in ionizing radiation induced foci (in cells exposed to ionizing radiation (IR), several proteins phosphorylated by ATM translocate and colocalize to common intranuclear sites. The resulting IR-induced nuclear foci (IRIF) accumulate at the sites of DNA damage). ATF2 expression contributes to the selective recruitment of MRE11A, RAD50, and NBN (NBS1) into IRIF. ATF2 is required for the IR-induced S phase checkpoint, and this function is independant of its transcriptional activity (Bhoumik et al., 2005).
KAT5 (TIP60) is a histone acetyltransferase and chromatin-modifying protein involved in double strand breaks (DSB) repair, interacting with and acetylating ATM. ATF2 associates with KAT5 and RUVBL2. Under non-stressed conditions, ATF2 in cooperation with the ubiquitin ligase CUL3 promotes the degradation of KAT5 (Bhoumik et al., 2008a).
Following genotoxic stress, 269 genes were found to be bound by ATF2/JUN dimers (see above), of which were 23 DNA repair or repair-associated genes (ERCC1, ERCC3, XPA, MSH2, MSH6, RAD50, RAD23B, MLH1, HIST1H2AC, PMS2, FOXN3 (CHES1), LIG1, ERCC8 (CKN1), UNG, XRCC6 (G22P1), TREX1, PNP, GTF2H1, ATM, FOXD1, DDX3X, DMC1, and the DNA repair-associated GADD45G), derived from several recognized DNA repair mechanisms (Hayakawa et al., 2004).

Cell Cycle
RB1 constrains cellular proliferation by activating the expression of the inhibitory growth factor TGFB2 (TGF-beta 2) through ATF2 (Kim et al., 1992).
CREB1 dimerizes with ATF2 to bind to the CCND1 (cyclin D1) promoter, to increase CCD1 expression (Beier et al., 1999).
JUND dimerizes with ATF2 to repress CDK4 transcription, a protein necessary for the G1-to-S phase transition during the cell cycle, by binding to the proximal region of the CDK4-promoter, contributing to the inhibition of cell growth. The physical interactions of ATF2 with JUND implicates the b-ZIP domain of ATF2 (Xiao et al., 2010).
Heterodimerization of JUND with ATF2 activates CCNA2 (cyclin A) promoter. CCNA2 is essential for the control at the G1/S and the G2/M transitions of the cell cycle. In contrast, ATF4 expression suppresses the promoter activation mediated by ATF2 (Shimizu et al., 1998).

Apoptosis
ATF2/CREB1 heterodimer binds to the CRE element of the BCL2 promoter (Ma et al., 2007).
ATF2 induces BAK upregulation (Chen et al., 2010).
MAP3K5 (ASK1) activates ATF2 and FADD-CASP8-BID signalling, resulting in the translocation of BAX and BAK, and subsequently mitochondrial dysregulation (Hassan et al., 2009).
ATF2/JUN heterodimers bind and activate CASP3, a key executor of neuronal apoptosis (Song et al., 2011).
Following death receptor stimulation, there is phosphorylation and binding of ATF2/JUN to death-inducing ligands promoters (FASLG, TNF, TNFSF10), which allows the spread of death signals (Herr et al., 2000). Neuronal apoptosis requires the concomitant activation of ATF2/JUN and downregulation of FOS (Yuan et al., 2009).
Many drugs are currently being tested for their ability to inhibit cell proliferation and induce apoptosis through various pathways, including ATF2 pathway.
In the cytoplasm, ATF2 abrogates formation of complexes containing HK1 and VDAC1, deregulating mitochondrial outer-membrane permeability and initiating apoptosis. This function is negatively regulated phosphorylation of ATF2 by PRKCE, which dictates its nuclear localization (Lau et al., 2012).

Metabolic control and Insulin signalling
ATF2 has been implicated in the regulation of proteins involved in metabolic control, including the control of the expression of UCP1, a protein involved in thermogenic response in brown adipose tissue (Cao et al., 2004) and phosphoenolpyruvate carboxykinase (PEPCK), a protein regulating gluconeogenesis (Cheong et al., 1998).
Insulin activates ATF2 by phosphorylation of Thr69 and Thr71 (Baan et al., 2006).
Co-expression of ATF2, MAFA, PDX1, and TCF3 results in a synergistic activation of the insulin promoter in endocrine cells of pancreatic islets. ATF2, MAFA, PDX1, and TCF3 form a multi-protein complex to facilitate insulin gene transcription (Han et al., 2011).
ATF2 target genes in insulin signalling are ATF3, JUN, EGR1, DUSP1 (MKP1), and SREBF1. Deregulation of these genes is linked to the pathogenesis of insulin resistance, beta-cell dysfunction and vascular complications found in type 2 diabetes. Therefore, aberrant ATF2 activation under conditions of insulin resistance may contribute to the development of type 2 diabetes (Baan et al., manuscript in preparation).

Iron depletion
Iron depletion induced by chelators increases the phosphorylation of JNK and MAPK14, as well as the phosphorylation of their downstream targets p53 and ATF2 (Yu and Richardson, 2011).