Phosphodiesterases as Targets for Modulating T-Cell Responses - Phosphodiesterases as Drug Targets - page 351

anchoring protein Ezrin, forming part of a multimolecular complex where Ezrin,
EBP50, and Cbp/PAG provide a scaffold able to coordinate PKA phosphorylation
and activation of Csk, thereby inhibiting T-cell activation (Ruppelt et al. 2007 ;
Vang et al. 2001 ). It has been shown that G i ,G s , and AC segregate into lipid rafts
(Oh and Schnitzer 2001 ), and our data indicate that raft recruitment of the stimulatory
G-protein G s and dissociation of the inhibitory G-protein G i play an important role in
the cAMP production that occurs after TCR ligation. A local increase in cAMP is,
therefore, generated in T-cell lipid rafts upon TCR stimulation alone. In contrast,
TCR/CD28 costimulated T cells revealed decreased cAMP levels compared to control
cells and an increase in cAMP levels was only observed in the presence of
the nonselective PDE inhibitor IBMX. Furthermore, PKA substrates in lipid rafts
were rapidly phosphorylated in cells only activated through the TCR (Abrahamsen
et al. 2004 ). In conclusion, this suggests that TCR-induced cAMP production
constitutes a negative feedback loop in the absence of a second stimulatory signal
and that this inhibition can only be lifted through cAMP degradation by PDEs, thus
allowing a complete T-cell activation to proceed (see Fig. 2 for an overview).
1.3.1 PDE4 is Recruited to T-Cell Lipid Rafts Upon
TCR/CD28 Costimulation
As already mentioned, isoforms from the PDE4 subfamily account for the majority
of the cAMP hydrolyzing activity in T cells (Erdogan and Houslay 1997 ; Giembycz
et al. 1996 ), and in accordance with this, we have observed PDE4 activity in
lipid rafts upon T-cell activation. In contrast, no PDE3 activity was detected
(Abrahamsen et al. 2004 ). In particular, TCR/CD28 costimulation resulted in a
rapid recruitment of PDE4 activity to lipid rafts, indicating that temporal changes in
PDE4 activity can play a key role in tuning intracellular activation-induced gradi-
ents of cAMP in T-cell lipid rafts and thereby increase signal propagation upon
costimulation (Abrahamsen et al. 2004 ). The mechanisms by which PDE4 isoforms
are recruited to specific locations upon T-cell activation are now being unravelled
(Abrahamsen et al. 2004 ; Arp et al. 2003 ; Bjorgo et al. 2010 ). PDE4B has been
reported to associate with the TCR complex (Baroja et al. 1999 ) and transfected
PDE4B has been shown to relocalize to the synapse area between the Jurkat T cell
and the antigen-presenting cell (APC) upon contact (Arp et al. 2003 ). We have
demonstrated that PDE4A4, PDE4B2, and PDE4D1/2 are recruited to lipid rafts
upon TCR/CD28 costimulation in human primary T cells (Abrahamsen et al. 2004 ).
Spatial organization and recruitment of mediators of specific pathways are
essential to ensure both signaling specificity and amplification. The scaffolding
b -arrestin proteins have been reported to confer crosstalk with a growing list of
molecules important in cellular trafficking and signal transduction, including Src
family protein tyrosine and MAP kinases (reviewed in DeWire et al. 2007 ).
Members of the PDE4 family have previously been described to associate with
b -arrestin, and this scaffolding protein has been shown to be responsible for
bringing PDE4 to the plasma membrane of HEK293 (Bolger et al. 2003 ;