Biology Reference
In-Depth Information
changes in cAMP-PDEs, PKA, Epac, adenylyl cyclases, or any number of down-
stream signaling components.
2.4 GM-CSF
Monocyte to macrophage differentiation can also occur when monocytes are
exposed to the cytokine GM-CSF (Geissmann et al.
2010
). These macrophages
have a phenotype similar to that observed for alveolar macrophages. The cGMP-PDE
profile in CD14+ monocytes isolated from human blood consists of mostly PDE5
activity, with some PDE1 and PDE2 activity and low PDE3 and modest PDE4.
Upon differentiation for 7 days in GM-CSF, PDE1 emerges as the predominant
cGMP-PDE present in these macrophages, consisting mostly of the PDE1B isoform
and the cAMP-PDE activity was attributed equally to PDE3 and PDE4 (Bender and
Beavo
2004
; Bender et al.
2004
; Hertz et al.
2009a
). Also characterized in these
studies was the cGMP-PDE composition of the monocytic cell line THP-1 which
was found to be similar to GM-CSF-derived macrophages upon differentiation
with PMA. THP-1 is a human acute monocytic leukemia cell line with high levels
of PDE1B, but very low levels of PDE1A and PDE1C and very little PDE2 activity.
These cells can be differentiated to a macrophage-like cell upon stimulation with
PMA. While these THP-1-derived macrophages contained high levels of PDE1B,
the levels of all PDEs were consistently high in the undifferentiated monocyte and
throughout differentiation, in contrast to CD14+ monocyte-derived macrophages
(Bender et al.
2004
).
Upon further investigation, it was discovered that a unique splice variant,
PDE1B2 (Fidock et al.
2002
), is the form upregulated upon monocyte to macro-
phage differentiation in the presence of GM-CSF (Bender et al.
2005
). Interest-
ingly, this isoform of PDE1B has a threefold lower EC
50
for calmodulin than its
counterpart PDE1B1. This calmodulin-sensitive PDE has a separate transcriptional
start site that can be activated by monocytic differentiation to a macrophage, but is
suppressed upon the addition of IL-4 and subsequent differentiation of the mono-
cyte to a dendritic cell. The authors postulate that selective upregulation of a
calcium/calmodulin-sensitive cGMP-preferring PDE could provide a mechanism
by which calcium transients can control the duration and amplitude of the cGMP
signal.
The cAMP-PDE activities in GM-CSF-differentiated monocytes were deter-
mined using a PDE activity assay, with PDE4 showing the largest amount of
activity in monocytes, with decreasing amounts of activity during differentiation.
However, in fully differentiated macrophages PDE3 and PDE4 comprised equiva-
lent amounts of cAMP PDE activity (Gantner et al.
1997a
,
b
,
1999
; Hertz et al.
2009a
). It should also be noted that PDE1B is a dual specificity PDE, able to
hydrolyze both cAMP and cGMP. Although it prefers to hydrolyze cGMP over
cAMP, a moderate amount of cAMP-PDE activity is contributed by PDE1B in
these cells.
