8. The JAK-STAT pathway

26.7 Cyclic AMP and activation of CREB

Figure 26.33 When cyclic AMP binds to the R subunit of PKA, the C subunit is released; some C subunits diffuse to the nucleus, where they phosphorylate CREB.
Multiple figure

Cyclic AMP is the classic second messenger, and its connection to transcription is by the activation of CREB (cAMP response element binding protein). Figure 26.33 shows how the pathway proceeds through the Ser/Thr kinase, PKA.

Figure 26.10 Classes of G proteins are distinguished by their effectors and are activated by a variety of transmembrane receptors.

The initial step in the pathway is activation of adenylate cyclase at the plasma membrane by an activated G protein (see Figure 26.10). cAMP binds to the regulatory R subunit of PKA, which is anchored to membranes in the perinuclear region. This causes the R subunit to release the catalytic (C) subunit of PKA, which is free to translocate to the nucleus. Translocation occurs by passive diffusion, and involves only a proportion of the released C subunits, which have targets for phosphorylation in both the cytosol and nucleus.

The circuitry also has some feedback loops. The end-targets for PKA are also substrates for the phosphatase PPase I, which in effect reverses the action of PKA. However, PKA also has as a target a protein whose phosphorylation converts it into an inhibitor of PPase I, thus preventing the reversal of phosphorylation.

CREB is one of the major nuclear substrates for PKA. Phosphorylation at a single Ser residue greatly increases the activity of CREB bound to the response element CRE, which is found in genes whose transcription is induced by cAMP (see 21 Regulation of transcription). The rate of transcription of these genes is directly proportional to the concentration of phosphorylated CREB in the nucleus. The kinetics of the response are limited by the relatively slow rate at which the free C subunit diffuses into the nucleus. Typically the phosphorylated C subunit reaches a maximum level in the nucleus after ~30 min, and then is slowly dephosphorylated (over several hours). Several circuits may be involved in the dephosphorylation, including direct control of phosphatases and indirect control by the entry into the nucleus of the protein PKI, which binds to the C subunit and causes it to be re-exported to the cytoplasm. The kinetics of activating PKC in the nucleus may be important in several situations, including learning, in which a weak stimulus of cAMP has only short-term effects, whereas a strong stimulus is required for long-term effects, including changes in transcription. This parallels the different consequences of short-term and long-term stimulation of the MAPK pathway (see above) (Hagiwara et al., 1992; Hagiwara et al., 1993).

Research

Hagiwara, M. et al. (1992). Transcriptional attenuation following cAMP induction requires PPA-mediated dephosphorylation of CREB. Cell 70, 105-113.

Hagiwara, M. et al. (1993). Coupling of hormonal stimulation and transcription via the cAMP-responsive factor CREB is rate limited by nuclear entry of PKA Mol. Mol. Cell Biol. 13, 4852-4859.