Differential Interactions of Thrombospondin-1, -2, and -4 with CD47 and
Effects on cGMP Signaling and Ischemic Injury
Responses |
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Authors: | Jeff S Isenberg Douglas S Annis Michael L Pendrak Malgorzata Ptaszynska William A Frazier Deane F Mosher and David D Roberts |
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Institution: | ‡Laboratory of Pathology, NCI, National Institutes of Health, Bethesda, Maryland 20892, the §Departments of Biomolecular Chemistry and Medicine, University of Wisconsin, Madison, Wisconsin 53706, and the ¶Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110 |
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Abstract: | Thrombospondin-1 regulates nitric oxide (NO) signaling in vascular cells
via CD47. Because CD47 binding motifs are conserved in the C-terminal
signature domains of all five thrombospondins and indirect evidence has
implied CD47 interactions with other family members, we compared activities of
recombinant signature domains of thrombospondin-1, -2, and -4 to interact with
CD47 and modulate cGMP signaling. Signature domains of thrombospondin-2 and -4
were less active than that of thrombospondin-1 for inhibiting binding of
radiolabeled signature domain of thrombospondin-1 or SIRPα
(signal-regulatory protein) to cells expressing CD47. Consistent with this
binding selectivity, the signature domain of thrombospondin-1 was more potent
than those of thrombospondin-2 or -4 for inhibiting NO-stimulated cGMP
synthesis in vascular smooth muscle cells and downstream effects on cell
adhesion. In contrast to thrombospondin-1- and CD47-null cells, primary
vascular cells from thrombospondin-2-null mice lack enhanced basal and
NO-stimulated cGMP signaling. Effects of endogenous thrombospondin-2 on
NO/cGMP signaling could be detected only in thrombospondin-1-null cells.
Furthermore, tissue survival of ischemic injury and acute recovery of blood
flow in thrombospondin-2-nulls resembles that of wild type mice. Therefore,
thrombospondin-1 is the dominant regulator of NO/cGMP signaling via CD47, and
its limiting role in acute ischemic injury responses is not shared by
thrombospondin-2.Nitric oxide (NO) is a major mediator of intracellular and paracellular
signal transduction. NO preserves vascular health by minimizing the adhesion
of inflammatory cells to the vessel wall, limiting platelet activation, and
increasing blood vessel diameter and blood flow by relaxing vascular smooth
muscle cells (VSMC).3
These actions of NO are mediated by activating soluble isoforms of guanylate
cyclase (sGC) to increase cGMP levels, resulting in downstream activation of
cGMP-dependent protein kinases and ion channels
(1).Physiological NO/cGMP signaling is limited by several phosphodiesterases
that degrade cGMP and by thrombospondin-1 (TSP). TSP1 is a secreted protein
that is produced by vascular and inflammatory cells that regulates cellular
behavior by engaging several cell surface receptors. Recently we reported that
TSP1 potently blocks NO-stimulated prosurvival responses in endothelial and
VSMC (2,
3). TSP1 also plays a role in
promoting platelet thrombus formation and hemostasis by antagonizing the
antithrombotic activity of NO
(4). In all of these vascular
cells, picomolar concentrations of TSP1 are sufficient to block NO-stimulated
fluxes in cGMP by engaging its receptor CD47
(5). Nanomolar concentrations
of TSP1 further inhibit the same signaling pathway by inhibiting CD36-mediated
uptake of myristate into vascular cells
(6). In vivo, mice
lacking TSP1 demonstrate elevated basal tissue cGMP levels and greater
increases in regional blood flow in response to a NO challenge than wild type
controls (4). After an ischemic
insult, the absence of TSP1 or CD47 in transgenic mice is associated with
better maintenance of tissue perfusion and enhanced tissue survival.
Similarly, targeting TSP1 or CD47 using function blocking antibodies enhances
ischemic tissue perfusion and survival in wild type mice and pigs
(7,
8).TSP1 belongs to a family of five secreted glycoproteins that share an
evolutionarily conserved C-terminal signature domain
(9). TSP1 and TSP2 form a
distinct subfamily of trimeric proteins that exhibit similar anti-angiogenic
activities for endothelial cells in vitro and activities in
vivo to block tumor growth. Despite their similarities in structure, TSP1
and TSP2 have markedly different expression patterns after tissue injury, with
TSP1 being immediately expressed and maximal at day 3, whereas TSP2 was not
expressed until day 7 and was maximal 10 days after injury
(10). In addition, large
amounts of TSP1 but not TSP2 are stored in platelet α-granules and
released into the wound environment. Polymorphisms in TSP1 and TSP2 have been
linked to altered risk of premature myocardial infarction
(11,
12). A 3′-untranslated
region polymorphism in TSP2 is also associated with type 2 diabetes in men
(13). The molecular basis for
these associations is unclear.Less is known about the roles of the pentameric TSP3–5 in vascular
cells. TSP3 and TSP5 (also known as cartilage oligomeric matrix protein)
appear to serve their primary functions in bone development
(14,
15). However, a polymorphism
in TSP4 is associated with premature myocardial infarcts in certain
populations (11,
16,
17). A proatherogenic activity
for the A387P variant of TSP4 was proposed based on its differential ability
to modulate proliferation of endothelial and VSMC
(18). Cardiovascular functions
of TSP4 may also be linked to the high expression of TSP4 in heart
(19) and its altered
expression in that tissue during hypertensive heart failure
(20).The C-terminal domain of TSP1 is sufficient to mediate CD47-dependent
inhibition of cGMP signaling
(5). Of the two CD47 binding
VVM motifs identified in this domain of TSP1, the first is conserved among all
five TSPs, suggesting that CD47 binding could be a universal attribute of this
family (21). Based on
structural evidence that the VVM motifs may not be accessible
(22,
23), however, conservation of
VVM motifs may not be sufficient to predict CD47 binding. Uncertainty
regarding the location of the CD47 binding site in the G domain of TSP1
therefore limits interpretation of the known sequence homology to predict CD47
binding to other TSP family members.Although CD47 recognition of other TSPs has not been demonstrated
experimentally, a local deficiency of inflammation-associated T cell apoptosis
shared by TSP1-, CD47-, and TSP2-null mice is consistent with this hypothesis
(24). Furthermore, a
21-residue peptide from the C-terminal domain of TSP4 was found to decrease
human umbilical vein endothelial cell proliferation similar to the CD47
binding peptides from TSP1, although it lacks the VVM motif and no interaction
with CD47 was demonstrated
(25).To directly address whether other TSP family members can inhibit NO
responses and signaling in vascular cells, we now compare binding of
recombinant signature domains of TSP1, TSP2, and TSP4 to cell surface CD47 and
inhibition of NO-stimulated cell responses and cGMP signaling by these
domains. We also compared acute tissue blood flow and perfusion responses to
ischemic challenge in TSP1 and TSP2-null mice and cGMP responses in primary
cultures of vascular cells isolated from these mice. These studies clearly
demonstrate that CD47 selectively interacts with TSP1 and that the signature
domains of TSP2 and TSP4 are less potent inhibitors of NO signaling in
vascular cells in vitro. Furthermore, we show that the role of TSP1
to acutely limit recovery from ischemic injury in vivo is not shared
by TSP2. |
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