**2.3. ACs, PDEs and AKAPs are essential to cAMP signaling compartimentalization**

The idea of compartimentalized pools of cAMP originated in 1979 when Brunton et al. showed that while both the β-adrenergic receptor agonist isoprotrenol and prostaglandin E1 increased cAMP concentration in perfused rat hearts, only isoproterenol increased glycogen metabolism and phosphorylation of troponin [49]. These results illustrated the fact that different hormones may act through the same messenger to generate different pools of cAMP and mediate distinct physiological responses. An increasing number of results support now the existence of distinct cAMP microdomains that control cAMP signalling. ACs, PDEs and the scaffolding proteins AKAPs are at the foundation of this cAMP signalling compartmentalization [50,51]. As mentioned, -ACs can orchestrate their own microenvironment by recruiting a variety of signalling and scaffolding molecules, - PDEs mediate local cAMP degradation and literally sculpt gradients of cAMP surrounding specific signalling complexes and therefore regulate

the availability of cAMP/cGMP to their effectors –AKAPs dynamically assemble the three different cAMP effectors to control the cellular actions of cAMP [37]. As their name implies, AKAPs were originally described to target PKA to distinct subcellular locations and confine activation to only a subset of potential targets. In reality, these proteins have the ability to form complexes with other signalling molecules including Epac proteins, protein kinases, phos‐ phatases, phosphodiesterases, AC, as well as GPCR and ion channels. AKAPs are localized to numerous cellular sites, including the plasma membrane, Golgi, centrosome, nucleus, mitchondria and cytosol. The first AKAP to be characterized was microtubule associated protein-2 (MAP2), initially identified because of it co-purified with RII from brain extract [52]. The AKAP family has grown and includes more than 50 structurally diverse, but functionally similar members. Despite their diversity, AKAP orthologues have been identified in a range of species, including yeast, nematodes, mice and humans. All AKAPs share common proper‐ ties: 1) they contain a PKA-anchoring domain 2) compartmentalization of individual AKAP-PKA units occur through specialized targeting domains that are present on each anchoring protein 3) they have the ability to form complexes with other signalling molecules including protein kinases, phosphatases, phosphodiesterases, AC, as well as GPCR and ion channels 4) AKAPs are recruited into much larger multiprotein complexes through the interactions with other adaptator molecules such as PDZ and SH3 domain containing proteins. These four properties of AKAPs allow these proteins to integrate multiple signalling pathways, allowing the convergence of signals to a common target [36,37].

type 6 adenylyl cyclase (AC6). Indeed, overexpression of only AC6 (and not AC5, AC2, or AC1) in primary aortic VSMCs enhances smooth muscle relaxation [59]. Furthemore, a recent study using selective short interfering RNA sequences reveals that AC6 is the predominant isoenzyme involved in vasodilator-mediated cAMP accumulation in aortic VSMCs, account‐

The Role of Cyclic 3'-5' Adenosine Monophosphate (cAMP) in Differentiated and Trans-Differentiated...

http://dx.doi.org/10.5772/54726

129

A cause to effect relationship between the decreased expression of some specific components of the cAMP signalling and proliferative capacity of VSMC has been demonstrated. Inversely,

The cAMP Response Element Binding Protein (CREB) is a transcription factor, well known to be phosphorylated and activated by PKA. CREB expression has been shown to be dramatically decreased in cultured trans-differentiated VSMCs and in the media of numerous rodent and porcine models of vascular diseases. Depletion of this transcription factor *in vivo* elicits changes consistent with those observed in SMCs from pathologically remodelled arteries whereas forced depletion of CREB with small interfering RNA in aortic SMCs is sufficient to induce their proliferation, hypertrophy, migration, de-differentiation, and ECM production. Furthe‐ more, CREB is inactivated in VSMCs by several proliferative stimuli and overexpression of wild type or constitutively active CREB, in primary cultures of SMC arrests cell cycle progres‐ sion induced by these stimuli [61-66]. Additionally, Transforming growth factor beta and thiazolidinediones activate CREB to oppose to aortic SMC proliferation induced by growth factors [62,67]. Nevertheless, some apparent contradictory studies show that CREB is involved

AKAP12β, a member of the AKAP family, is markedly decreased in human and rodent vascular lesions. Overexpression of AKAP12 β attenuates serum-induced SMC growth in *vitro* and a causal relationship exists between the induction of the expression of this protein and the inhibition of serum-induced VSMC proliferation by all trans retinoic acid [70]. An other AKAP shown to repress VSMC growth is AKAP5 (AKAP79/AKAP75/AKAP150 in human, bovine, rat respectively) since over-expression of this protein inhibits serum-induced VSMC prolifer‐ ation and local delivery of AKAP5 to balloon-injured vessels wall reduced the extent of

PDE1C, a PDE isoform hydrolyzing both cAMP and cGMP, is expressed in proliferating human VSMCs but is absent in quiescent cells. In *vivo*, PDE1C is expressed in human foetal aortas containing proliferating SMCs, but not in newborn aortas in which SMC proliferation has ceased. Moreover, a causal relationship has been established between the emergence of

ing for 60% of the total response to β-adrenoceptor (β-AR) stimulation [60].

**3.2. cAMP maintains a low rate of proliferation in differentiated VSMC**

emergence of PDEs in trans-differentiated VSMC allows them to proliferate.

in VSMC proliferation induced by ATP and thrombin [68,69].

*3.2.2. Role of CREB AKAP12β and AKAP5*

neointimal burden [71].

*3.2.3. Role of PDE1-C*

*3.2.1. Role of CREB*
