Cross Talk between Phosphatidylinositol 3-Kinase and Cyclic AMP (cAMP)-Protein Kinase A Signaling Pathways at the Level of a Protein Kinase B/β-Arrestin/cAMP Phosphodiesterase 4 Complex

Abstract

Engagement of the T-cell receptor (TCR) in human primary T cells activates a cyclic AMP (cAMP)-T-cell receptor (TCR) stimulation alone is insufficient for activation of T cells, and sustainable T-cell immune responses require a second signal in addition to the TCR-mediated signal. The second signal is typically elicited by ligands B7-1 or B7-2 on antigen-presenting cells engaging the coreceptor CD28 to prevent anergy and apoptosis and enhancing interleukin-2 (IL-2) production and clonal expansion (4). Although CD28 plays a central role in T-cell activation in vivo (5), relatively little is known about the molecular basis for the increased efficacy of T-cell activation upon TCR and CD28 costimulation. Involvement of Lck, Itk, phosphatidylinositol 3-kinase (PI3K), SLP-76, Vav-1, and phospholipase C-␥ (PLC-␥) has, however, been reported (43). CD28-mediated signals are transmitted via a short intracellular stretch in the receptor containing a conserved YMNM motif (32). Phosphorylation of Tyr173 in this motif by Lck and Fyn following CD28 ligation is key to efficient signal transduction (41), generating a binding site for the SH2 domain of the p85 regulatory subunit of PI3K (37, 40). CD28 may also contribute to TCR-dependent PI3K activity without recruiting PI3K directly (18). Whether engagement of CD28 alone can also induce PI3K activity has been a matter of controversy. However, recent reports confirming phosphorylation of the protein kinase B (PKB) substrate glycogen synthase kinase 3 (GSK3) upon CD28 ligation has demonstrated that this is indeed the case (6, 15). In addition, CD28 can recruit growth factor receptor-bound protein 2 (Grb2), and such association of Grb2 occurs via the phosphorylated YMNM motif as well as via the C-terminal PXXP motif (22,35). The PXXP motif also binds and regulates Src family kinases (SFKs) (21, 47), and knock-in mice mutated in this motif were recently reported to have impaired IL-2 secretion (16).Ligation of the TCR induces cyclic AMP (cAMP) production (27). However, the significance of this observation is still not fully understood, as it is well established that cAMP potently inhibits T-cell function and proliferation (2,45,46,50). The spatiotemporal dynamics of the activation-induced cAMP gradient also are not completely appreciated. We have previously shown that cAMP is rapidly produced in lipid rafts following engagement of the TCR in primary T cells (3). This activates a pool of PKA type I targeted to rafts by association with the anchoring protein Ezrin, forming part of a supramolecular complex where Ezrin, EBP50, and PAG provide a scaffold that is able to coordinate PKA phosphorylation and activation of Csk, thereby inhibiting T-cell activation (44, 50). In addition, we have demonstrated that CD3/CD28 costimulation leads to recruitment of type 4 phosphodiesterase (PDE4) isoforms to rafts, resulting in degradation of the TCR-induced cAMP pool (3). Thus, we envisage that TCR-induced cAMP production constitutes a negative feedback loop capable of abrogating T-cell activation ...

Phosphodiesterase-8A binds to and regulates Raf-1 kinase

“…Peptide array technology has been successfully used by us (28)(29)(30) and others (31,32) to map the interfaces between interacting proteins. This information can be used to design cell-permeable disrupting peptides and to inform mutagenesis strategies aimed at creating null-binding mutants (33,34).…”

Interferon-α Suppresses cAMP to Disarm Human Regulatory T Cells

“…By interfering with the function of cAMP-producing adenylate cyclases or cAMP-degrading PDEs, both the suppressive activity and functional state of Treg can be selectively manipulated (15,27,29). It is currently not clear whether differential cAMP upregulation in human Treg results from differential enzyme endowment or activity; however, it is at least known that human T cells predominantly express the short isoforms PDE4B and PDE4D, which are functionally regulated by ERK2 MAP kinase, and that engagement of the TCR leads to recruitment of a b-arrestin/PDE4 complex to lipid rafts that serves to degrade the local, TCR-induced production of cAMP (30,31,48). Activation of the MEK/ERK pathway again belongs to the prominent actions of IFN-a in human CD4 þ T cells (22).…”

Treatment of Skin Inflammation with Benzoxaborole Phosphodiesterase Inhibitors: Selectivity, Cellular Activity, and Effect on Cytokines Associated with Skin Inflammation and Skin Architecture Changes

“…Optimal T-cell activation requires the CD28 costimulatory signal, which recruits the protein kinase B/b-arrestin/PDE4 complex to the proximity of plasma membrane and allows PDE4 to hydrolyze cAMP to overcome the inhibitory signal elicited by cAMP (Bjørgo et al, 2010). Increase of cAMP and activation of PKA as results of PDE4 inhibition directly or indirectly modulate the activities of NF-kB, AP-1, and nuclear factor of activated T cells, which regulate transcription of cytokines in T cells (Supplemental Fig.…”