The AP-1 (activator protein-1) transcription factor is a sensor of changes affecting the extracellular environment. It is regulated by a great variety of stimuli, such as growth factors, cytokines, stress signals, infections and oncoproteins and is implicated in the modulation of many processes including proliferation, apoptosis, differentiation, transformation and migration (Shaulian and Karin, 2001; Shaulian and Karin, 2002).
1.4.1 Structure and composition of the AP-1 Transcription factor
AP-1 is a dimeric transcription factor composed of one member of the Fos family of proteins (c-Fos, FosB, Fra1 or Fra2) and one member of the Jun family (c-Jun, JunB or JunD) (Zenz et al., 2008). In addition, Jun members can also form homo-dimers or associate with the structural similar members of the MAF, ATF (ATF-2, ATF-3 or B-ATF) and JDP (JDP-1 and JDP-2) subfamily (Karin et al., 1997; Shaulian and Karin, 2001). AP-1 belongs to the family of basic leucin zipper (bZIP) proteins in which the dimerisation occurs through a leucine zipper, a process required for DNA binding via a basic domain. The consensus sequence bound by AP-1, 5′-TGAG/CTCA-3′, is called TRE (TPA (12-O-tetradecanoylphorbol-13-acetate) responsive element). In addition to a bZIP domain, Jun proteins and the Fos protein members c-Fos and FosB consist of transactivation domains and binding and phosphorylation sites for different kinases (Hess et al., 2004).
Figure 1.4: Schematic representation of the AP-1 transcription factor and structure of Jun and Fos proteins. Dimerisation of a Jun and a Fos family member and DNA binding to the consensus sequence occurs through a bZIP domain (blue: bZIP domain of Jun, red: bZIP domain of Fos, yellow: DNA). In addition to the bZIP domain, Fos and Jun proteins consist of transactivation domains and binding sites for different kinases.
From (Hess et al., 2004).
1.4.2 Regulation of transcriptional activity
1 activity is regulated on the transcriptional level, leading to a differing availability of AP-1 family members depending on celltype, differentiation status and cell context. Based on the composition of the heterodimer, AP-1 complexes diverge in their transactivation potential. In
particular, c-Fos, c-Jun and FosB have a high potential to activate transcription. Other members, like JunB, JunD, and in particular Fra1 and Fra2, the two Fos members that lack the C-terminal transactivation domain, could be weak transactivators or even exert an inhibitory function by competing for more active AP-1 complexes to be formed or to bind to the consensus sequence. However, transcription can also be regulated positively by recruiting further transcription factors (Chinenov and Kerppola, 2001; Wisdon and Verma, 1993). In addition, Jun-Fos heterodimers exhibit higher stability and stronger DNA binding affinities than Jun homodimers (Hess et al., 2004; Karin et al., 1997). Dimerisation with other structurally unrelated transcription factors and transcriptional regulators was also described including members of the C/EBP family (Wagner, 2002).
The activity of AP-1 is also regulated post-transcriptionally for instance by different kinases mainly belonging to the mitogen activated protein kinase (MAPK) pathway (Young and Colburn, 2006). The Jun N terminal Kinase (JNK) phosphorylates c-Jun, JunD and ATF-2 (Karin et al., 1997), phosphorylation by ERK is essential for Fra1 stability and activity (Doehn et al., 2009; Young and Colburn, 2006) and in combination with RSK2 for c-Fos transforming activity (Chen et al., 1996; David et al., 2005).
1.4.3 Functions of AP-1
c-Fos and c-Jun (cellular Fos and Jun, respectively) have been identified as cellular homologes of the viral oncogenes v-Fos and v-Jun, depicting their role in cell proliferation and oncogenic transformation. JunB has been described to inhibit cell proliferation, while JunD exerts pro- and anti-proliferative effects. However, both, JunB and JunD reduce cell transformation (Eferl and Wagner, 2003).
AP-1 family members also play a role in embryogenesis and organogenesis (Wagner, 2002).
In regard to the Fos family members, Fra1 was described to be essential for embryogenesis as Fra1 deficiency leads to embryonal death, due to placental defects and a lack of vascularisation (Schreiber et al., 2000). In Fra2 knockout mice, an early postnatal lethality is also observed probably as a result of heart and gastrointestinal tract defects (Karreth et al., 2004). c-Fos and Fos B, however, are not necessary during embryogenesis (Wagner, 2002). In contrast to JunD, c-Jun and JunB are required for embryonic development due to their essential role in heart and liver development and angiogenesis in the extra-embryonal tissue, respectively (Eferl et al., 1999; Schorpp-Kistner et al., 1999; Wagner, 2002).
Furthermore, functions in the formation of fibrosis and in tumour progression (Fra1, Fra2) (Eferl et al., 2008; Kireva et al., 2011; Schroder et al., 2010; Young and Colburn, 2006), for keratinocytes (c-Jun) (Zenz and Wagner, 2006), spermatogenesis (JunD ko) (Thepot et al., 2000) and an influence on immune cells (JunB, JunD, ∆FosB) (Eferl and Wagner, 2003; Zenz et al., 2008) was described.
1.4.4 Functions of AP-1 in bone development
An important role for AP-1, especially for all the Fos protein members has been described in the control of bone homeostasis. In particular, overexpression of c-Fos leads to osteosarcoma formation due to a transformation of osteoblasts (Grigoriadis et al., 1993), and a block in osteoclast differentiation in c-Fos knockout mice was reported, resulting in osteopetrosis (Grigoriadis et al., 1994). Osteoclast size and survival however is influenced by Fra2 (Bozec et al., 2008). In addition, Fra2 knockout mice exhibit a reduction in chondrocyte (Karreth et al., 2004) and osteoblast differentiation (Bozec et al., 2010), while an increased differentiation to osteoblasts in the Fra2tg mice results in an osteosclerotic phenotype (Bozec et al., 2010).
Overexpression of Fra1 also leads to the development of osteosclerosis due to an accelerated osteoblast differentiation (Jochum et al., 2000) and a similar phenotype was described for mice overexpressing a naturally occurring splice variant of FosB (∆FosB) that lacks the C-terminal transactivation domain (Kveiborg et al., 2004; Sabatakos et al., 2000). For all three, Fra1, Fra2 and ∆FosB transgenic mice, the phenotype was reported to be due to a cell-autonomous increased osteoblast activity (Bozec et al., 2010; Jochum et al., 2000; Kveiborg et al., 2004; Sabatakos et al., 2000).
Like the Fra2 knockout mice, Fra1 deficient mice are osteopenic due to a decreased osteoblast activity (Eferl et al., 2004). No bone phenotype was described for FosB knockout mice (Zenz et al., 2008).
Deletion of c-Jun resulted in defects of the axial skeleton (Behrens et al., 2003). In addition, a role for c-Jun in promoting osteoclastogenesis was described (David et al., 2002). Mice lacking JunB develop osteopenia due to a decreased osteoblast activity (Kenner et al., 2004) and an increased bone formation occurs in JunD deficient mice (Kawamata et al., 2008).
1.4.5 Functions of AP-1 in adipocyte development
Implications of AP-1 members in adipocyte development and function were also described. In particular, mice overexpressing ∆FosB display a reduced mass of adipose tissue (Kveiborg et al., 2004). Originally, overexpression of ∆FosB was shown to inhibit adipogenic differentiation in vitro, indicating a cell-autonomous defect within the adipocytes (Sabatakos et al., 2000). However, the decreased adipogenesis in vivo was later described to be caused by increased energy expenditure and insulin sensitivity (Rowe et al., 2009).
Recently, a reduced adipose tissue mass was described for JunB knockout mice. In these mice, no change in adipogenesis was observed as differentiation capacities of cells with a reduced level of JunB were not altered and the expression of marker genes for differentiation was not changed. Therefore, it was proposed that the reduced amount of fat is caused by a high lipolytic activity due to an increased expression of lipolytic enzymes (Pinent et al., 2011).
In addition to FosB and JunB, a possible role on adipocyte differentiation was also proposed for c-Jun, c-Fos, and Fra1, as adipogenic stimulation induced their expression in vitro (White and Stephens, 2009). However, no adipose-related phenotype has yet been described in vivo.