Role of basic leucine zipper proteins in transcriptional regulation of the steroidogenic acute regulatory protein gene

Abstract

The regulation of steroidogenic acute regulatory protein (StAR) gene transcription by cAMP-dependent mechanisms occurs in the absence of a consensus cAMP response element (CRE, TGACGTGA). This regulation is coordinated by multiple transcription factors that bind to sequence-specific elements located approximately 150 bp upstream of the transcription start site. Among the proteins that bind within this region, the basic leucine zipper (bZIP) family of transcription factors, i.e. CRE binding protein (CREB)/CRE modulator (CREM)/activating transcription factor (ATF), activator protein 1 (AP-1; Fos/Jun), and CCAAT enhancer binding protein beta (C/EBPbeta), interact with an overlapping region (-81/-72 bp) in the StAR promoter, mediate stimulus-transcription coupling of cAMP signaling and play integral roles in regulating StAR gene expression. These bZIP proteins are structurally similar and bind to DNA sequences as dimers; however, they exhibit discrete transcriptional activities, interact with several transcription factors and other properties that contribute in their regulatory functions. The 5'-flanking -81/-72 bp region of the StAR gene appears to function as a key element within a complex cAMP response unit by binding to different bZIP members, and the StAR promoter displays variable states of cAMP responsivity contingent upon the occupancy of these cis-elements with these transcription factors. The expression and activities of CREB/CREM/ATF, Fos/Jun and C/EBPbeta have been demonstrated to be mediated by a plethora of extracellular signals, and the phosphorylation of these proteins at several Ser and Thr residues allows recruitment of the transcriptional coactivator CREB binding protein (CBP) or its functional homolog p300 to the StAR promoter. This review will focus on the current level of understanding of the roles of selective bZIP family proteins within the complex series of processes involved in regulating StAR gene transcription.

Fig. 1

Schematic model of the bZIP motif bound to double stranded DNA. The DNA is in red, while bZIP α-helices are in blue and the leucines of their heptad repeats is in grey (diagram revised and reprinted with permission, Vinson et al., 2006).

Fig. 2

Schematic representations of the CREB/CREM/ATF-1, Fos and Jun, and C/EBPβ proteins. These families of proteins are characterized by a highly conserved bZIP DNA binding domain consisting of basic region (BR) and leucine zipper (LZ). Two glutamine rich regions Q1 and Q2 for CREB, CREM and ATF-1 are labeled along with the phosphorylated region/kinase inducible domain (P-Box/KID). Phosphorylation of Ser133 and Ser117, in CREB and CREM respectively, turns them into powerful activators through interaction with CBP. Ser142 in CREB attenuates its activity. The Fos and Jun proteins possess several domains including the bZIP domain, transactivation domains and docking sites for several kinases. The regions rich in prolines (P), prolines and glutamines (P–Q), prolines and glycines (P–G), and threonines (S) in Fos and Jun are labeled. Structure of the C/EBPβ protein indicating different regions (AD, activation domain; RD, repression domain) is provided. All of these bZIP proteins can be phosphorylated on several Ser and Thr residues by multiple kinases; however, only selected phosphorylation sites are illustrated in the diagrams. Numbers indicate the approximate positions of amino acids in the proteins; note that protein structures are not drawn to scales.

Fig. 3

Proposed models of CREB, c-Fos/c-Jun, C/EBPβ, or CREB and c-Fos/c-Jun interactions on a single cis-element known to coordinate both the positive and negative regulation of StAR gene transcription. Three different families of proteins have been shown to bind to a motif, labeled as CRE2/AP-1/C/EBPβ (−81/−72), which displays overlapping similarity to multiple consensus binding sites. Crosstalk of CREB and c-Fos/c-Jun at the −81/−72 site results in trans-repression of the StAR gene due to competition of these factors with limiting amounts of intracellular CBP. The functional cooperation, interaction and/or inhibition of CREB-C/EBPβ or Fos/Jun-C/EBPβ at the same site, is not currently known.

Fig. 4

A model illustrating the involvement of CBP/p300 function as a cointegrator of different signaling. The interaction of LH/ACTH with specific receptors results in the activation of G proteins (G), which in turn activate membrane associated adenylyl cyclases (AC) that catalyzes cAMP formation from ATP. cAMP then activates PKA which results in the phosphorylation of transcription factors regulating StAR gene expression. Activation of the PKC pathway results in an increase in the transcription and translation of StAR. Growth factors and peptide hormones bind to specific membrane receptors and can stimulate the steroidogenic response via different signaling pathways. cAMP and/or different factors are capable of activating a cascade of protein kinases including Raf/Ras or related kinases leading to the ERK/MAPK pathway that phosphorylates different transcription factors. Trans-activation of the StAR gene requires simultaneous interaction of CREB/CREM, Fos/Jun and C/EBPβ to the CRE2/AP-1/C/EBPβ overlapping site in the StAR promoter. cAMP-dependent PKA, PKC and other kinases phosphorylate these bZIP proteins, and the subsequent activation of these factors lead to the recruitment of CBP/p300 to the StAR promoter. CBP/p300 act as bridging proteins between the transcription factors and the general transcription machinery, and thus enhance the transcriptional activation of the StAR gene.