Biology Reference
In-Depth Information
upon TCR stimulation (Abrahamsen et al. 2004 ; Bjorgo et al. 2010 ). Furthermore,
CD28 ligation was shown to act synergistically with CD3 stimulation in the
recruitment of b -arrestin and PDE4, and we concluded that the CD28 signal plays
a distinct role enhanced by a concomitant TCR signal (Bjorgo et al. 2010 ).
The CD28 coreceptor plays a central role in T-cell activation in vivo . Although
engagement of the TCR provides the necessary scaffolding from which TCR
signaling is propagated, CD28 functions as an amplifier necessary to promote
optimal IL-2 production and clonal expansion and to prevent anergy and cell
death. The CD28-mediated signals are transmitted via a short intracellular stretch
in the receptor containing conserved motifs important for docking of several
signaling proteins (Michel et al. 2001 ). Phosphorylation of a tyrosine residue
(Y173) in the conserved YMNM motif by Lck and Fyn is a key to efficient signal
transduction (Raab et al. 1995 ). This generates a docking site for the SH2 domain of
p85, the regulatory subunit of phosphatidylinositol 3-kinase (PI3K) (Pages et al.
1994 ; Prasad et al. 1994 ). The C-terminal PXXP motif is another important binding
site recognized by both Src family kinases and Grb2 (Holdorf et al. 1999 ; Kim et al.
1998 ; Okkenhaug and Rottapel 1998 ). It has been somewhat controversial whether
CD28 alone can induce PI3K activity. However, phosphorylation of the PKB
substrate glycogen synthase kinase 3 (GSK3) upon CD28 stimulation has recently
been observed (Appleman et al. 2002 ; Diehn et al. 2002 ), demonstrating that
PI3K is indeed activated upon CD28 stimulation alone. Thus, we examined how
b -arrestin and PDE4 levels in T-cell lipid rafts were affected by the inhibition of
either SFKs or PI3K using PP2 and LY294002, respectively. Strikingly, CD28-
induced recruitment of b -arrestin in primary T cells was abolished upon both Lck/
Fyn and PI3K inhibition. Furthermore, PDE4 activity in raft fractions from T cells
pretreated with the PI3K inhibitor prior to CD28 stimulation was reduced below
basal levels and no increase in PDE4 activity was observed upon CD28 ligation
(Bjorgo et al. 2010 ). Taken together, these data suggest a novel mechanism, whereby
PI3K can regulate cAMP degradation through recruitment of a PDE4/ b -arrestin
complex to lipid rafts.
1.3.3 The PDE4/
b
-Arrestin Complex Interacts with PKB
Activation of PI3K results in rapid phosphatidylinositol 3,4,5-trisphosphate (PIP3)
generation in the inner leaflet of the plasma membrane and recruitment of proteins
containing a specialized lipid-binding moiety called the pleckstrin homology (PH)
domain. Thus, PIP3 controls both activity and subcellular localization of a broad
range of signal transduction molecules (Kane and Weiss 2003 ). Vav-1, Itk, and
PKB are three PH-containing proteins that have been implicated both downstream
of PI3K signaling and in TCR/CD28 costimulation (Acuto et al. 2003 ). We recently
demonstrated that b -arrestin interacts with PIP3 in an activation-dependent fashion,
although b -arrestin does not possess a PH domain (Bjorgo et al. 2010 ). We,
therefore, hypothesized the presence of a PH domain-containing interaction partner
inside the complex that confers binding to PIP3 and can mediate the translocation of
Search WWH ::




Custom Search