Jun, Fos and the AP-1 complex

AP-1 is the collective name for a variety of dimers
composed of members of the Jun and Fos protein
families (Angel & Karin, 1991). Among these proteins,
c-Jun is the major component of the dimeric
complex and c-Fos is its best-known partner. AP-1
is a transcriptional activator that binds to promoter
elements in genes that are required for cell proliferation.
Typical genes are those that encode collagenase,
stromelysin and osteocalcin. The response element
that is recognized by AP-1 (the AP-1 binding site)
functions as an inducible enhancer (Angel et al.,
1987). Its consensus DNA sequence is a palindromic
5´-TGA(G/C)TCA-3´. Jun and Fos proteins are described
as bZIP proteins because of their conserved
basic regions and leucine zipper motifs that are
required for DNA binding and dimerization, respectively.
Their activation domains, which contain high
densities of specifi c amino acids, are envisioned to
interact directly or indirectly with other regulatory
proteins and with the basal transcription machinery.
AP-1 is an example of a transcription factor that
responds to extracellular signals. The occupancy of
specifi c cell-surface receptors by growth factors and
cytokines triggers tightly coordinated multistep signal
transduction cascades to the nucleus. In this cascade
AP-1 is the terminal acceptor responsible for converting
the transient stimulation at the cell surface into a
long-term transcriptional response.
The functional forms of AP-1 are Jun/Fos heterodimers
and Jun homodimers; Fos proteins do not
form homodimers. Dimerization is a prerequisite for
DNA binding. The various heterodimers differ not
only in their direct effects on transcription, but also
in their interactions with other transcription factors,
thereby increasing the combinatorial possibilities for
gene regulation. There is also evidence that Jun family
members can modulate each other’s action.
c-Jun and c-Fos are the respective products of the
genes c-jun and c-fos, whose own transcription is
induced by growth factors, cytokines, tumour promoters
and various other stimuli which trigger signal
transduction pathways to the nucleus via protein
kinase C. c-jun and c-fos are primary response genes,
whose transcription is activated independently of
concomitant protein synthesis within a few minutes
of cell stimulation. Induction of c-jun is long lasting,
varying from a few hours to several days in a manner
that is dependent on cell type and stimulus. It is
noteworthy that c-jun transcription is stimulated by
its own protein product. This positive autoregulation
serves an important role in amplifying the initial transient
stimulus to produce the long-term response. In
contrast to c-jun, c-fos induction is rapid and highly
transient, resulting in appearance of the c-Fos protein
within 1 hour of the initial stimulus. Induction of cfos
transcription depends on another transcription
factor, serum response factor, and is inhibited by its
own product. Thus c-fos acts as a genetic switch, being
rapidly induced, then quickly shut off again.
AP-1 activity is regulated by post-translational
modifi cation of both pre-existing and newly synthesized
AP-1 proteins, as well as through induction
DNA
looping
enhancer
promoter
activator
(VP16)
HAT/coactivator
(CBP/p300)
silencer
repressor
(Ume6)
co-repressor
(Sin3)
HDAC
ACTIVATION REPRESSION
Fig. 6.15 Arrangement of regulatory proteins
involved in transcriptional activation and
repression. Coactivators harbour intrinsic HAT
activity while co-repressors interact with a
complex containing HDAC.
126 Vitamins: their role in the human body
of jun and fos genes. Protein phosphorylation is the
post-translational modifi cation of choice when rapid
modulation of protein activity in response to changes
in environmental conditions is required. c-Jun is
phosphorylated at fi ve regulatory sites bearing threonine
or serine residues. Three sites (Thr231, Ser243
and Ser249) are clustered within the carboxy-terminal
DNA-binding domain and the other two sites
(Ser63 and Ser73) are located in the amino-terminal
activation domain. Phosphorylation of c-Jun at the
carboxy-terminal sites inhibits DNA binding by c-Jun
homodimers, but has no measurable effect on c-Jun/
c-Fos heterodimers (Karin, 1994). Dephosphorylation
of these sites through activation of protein kinase
C removes this inhibition, allowing DNA binding and
contributing to increased AP-1 activity (Boyle et al.,
1991). Phosphorylation of the amino-terminal sites
stimulates the transcriptional activity of c-Jun, without
affecting its DNA-binding activity; this stimulation
also takes place when c-Jun is heterodimerized
with c-Fos (Karin, 1994). In this case, the phosphorylation,
which is mediated by c-Jun N-terminal kinase
(JNK) (Hibi et al., 1993), is required to recruit the
transcriptional coactivator, CBP (Arias et al., 1994).
A third mechanism to regulate AP-1 activity is
physical interaction with nuclear receptors. In contrast
to the interaction between AP-1 and steroid
hormone receptors, which is nonmutual and can be
either negative or positive (Shemshedini et al., 1991),
the interaction between AP-1 and the retinoid receptors
is mutual and solely inhibitory.
In addition to repression by protein–protein interactions,
AP-1 activity is also negatively regulated at the
level of c-jun and c-fos transcription. This negative
regulation is important for normal cellular function.
Because of its ability to positively autoregulate its own
transcription, c-jun is at the risk of being permanently
transcribed. Over-expression of c-jun is dangerous
because it can lead to uncontrolled cell proliferation
and tumour formation.
Another property of Jun and Fos is their ability to
induce DNA bending in opposite orientations (Kerppola
& Curran, 1991). The bending is caused at least in
part by charge interactions. Contact between proteins
bound to separated sites on DNA requires looping of
the intervening DNA. Although long DNA fragments
of several hundred to thousands of base pairs are
fl exible, short DNA fragments of ten to a few hundred
base pairs have the characteristics of stiff rods.
Therefore, interaction between proteins separated by
short distances is constrained by the unfavourable
free energy of DNA looping. The bending of DNA by
Jun and Fos reduces this thermodynamic barrier and
facilitates protein–protein interactions.