BOX 1: Targeting pericytes to overcome resistance to VEGF-targeted
therapy
Mural pericytes stabilize nascent vessels by inhibiting endothelial proliferation
and migration, promoting endothelial survival, tightening the endothelial barrier
via deposition of a common basement membrane and formation of junctions, and
controlling, through vasoregulation, vessel function and perfusion.7,S1 Members
of the PDGF family control the recruitment of mural cells around naked
endothelial channels.S2 This family consists of five ligands (PDGF-AA, PDGF-AB,
PDGF-BB, PDGF-CC and PDGF-DD), binding to three receptor dimers
(PDGFR, PDGFRß and PDGFRß).S3 Certain family members stimulate tumor
angiogenesis and growth through effects on endothelial or malignant cells, and
increase interstitial tumor pressure via effects on myofibroblasts; indirect effects
also rely on the upregulation of angiogenic factors.S4
During normal vessel branching, endothelial tip cells release PDGF-BB to
recruit PDGFRß+ mural cells; this is also the case in inflamed tissues, when
cytokines such as TNF upregulate PDGF-BB.S5 Loss of PDGF-BB or PDGFRß
in development does not affect microvessel density, length or number of branch
points, but the absence of pericytes correlates with EC hyperplasia, increased
capillary diameter, abnormal EC shape and ultrastructure, changed distribution of
junctional proteins, and morphological signs of increased transendothelial
permeability;S6 similar findings were observed in the ischemic retina.S7 Tumor-cell
production of PDGF-AA recruits PDGFR+ myofibroblasts, which produce VEGF
to rescue angiogenesis in VEGF-deficient tumors.S8 VEGF(R) inhibitor refractory
tumors
can
overcome
inhibition
of
VEGF-driven
angiogenesis
through
upregulation of PDGF-C, a known angiogenic factor.S9
Unlike mature vessels in healthy organs, vessels in tumors are more
immature and surrounded by fewer pericytes, which contributes to their “vessel
abnormalization”. Genetic studies show that immature, pericyte-poor tumor
vessels are more sensitive to VEGF withdrawal.S10 Hence, it was postulated that
strategies to deprive the tumor vasculature of pericytes would increase its
sensitivity towards VEGF-targeted therapy. Indeed, pharmacological inhibition,
using multi-targeted tyrosine kinase inhibitors, of PDGFRß signaling in tumor
pericytes amplifies VEGFR inhibitors in suppressing tumor growth.S1 Similar
findings were obtained with neutralizing antibodies against VEGFR2 (VEGFR2)
and PDGFRß (PDGFRß).S11 Combination therapy with a PDGRß-trap and
VEFR2-trap yielded a modest additive effect, but only when a submaximal dose
of the VEGFR-trap was used.S12 Contradictory results have been reported when
PDGF-BB is overexpressed in experimental tumors; while this leads to increased
pericyte coverage of tumor vessels in all cases, tumor growth was either
increased (presumably because malignant cells express PDGFRß and proliferate
upon autocrine overproduction of PDGF-BB)S4 or reduced (presumably because,
in this case, excess pericyte recruitment inhibits EC proliferation and vascular
density).S13 Deprivation of tumor vessels from pericytes may cause harm by
tightening the EC barrier, pericytes impede tumor cell intravasation and
metastasis, explaining why inhibition of PDGFs or othter pericyte recruitment
signals favoured metastasis.S14
BOX 2: Targeting leukocytes to overcome resistance to VEGF-targeted
therapy
Preclinical evidence indicates that tumor infiltration of myeloid cells, such as
CD11b+Gr1+ myeloid cells and F4/80+ macrophages, confers refractoriness of
tumors to VEGF(R) inhibitors.74,S15,S16 Indeed, depletion of F4/80+ macrophages
from mice, bearing tumors that are refractory to VEGFR2 restored their
sensitivity to this inhibitor.74 Likewise, depletion of CD11b+Gr1+ myeloid cells in
preclinical models sensitized tumors to VEGF therapy, while, conversely,
infusion of these cells in tumors, sensitive to VEGF therapy, rendered these
tumors refractory to VEGF inhibition.S15 It is somewhat surprising that VEGF(R)
inhibitors fail to inhibit the recruitment of angiocompetent VEGFR1+ myeloid cells,
since VEGF chemoattracts VEGFR1-expressing myeloid cells.68 These infiltrating
inflammatory cells promote tumor angiogenesis by secreting a myriad of growth
factors such as PlGF, VEGF-C, FGFs, PDGFs, angiopoietins, chemokines,
proteinases and many other molecules;S17 they also stimulate tumor growth by
suppressing the immune response against malignant cells.S16
Recent studies revealed that Bv8 promotes the recruitment of resistanceconferring angiogenic myeloid cells, and stimulates angiogenesis directly.S18 Bv8
is a mitogen for a subset of ECs and haematopoietic progenitors, besides having
other non-vascular activities.S18,S19 This protein is upregulated by G-CSF (often
expressed by tumor and stromal cells), but also enhances G-CSF induced
recruitment of resistance-conferring CD11b+Gr1+ myeloid cells from the bone
marrow into tumors.S15 Tumors that are refractory to VEGF spontaneously
recruit CD11b+Gr1+ myeloid cells, while others do this only in response to VEGFtargeted therapy.S16 Bv8 contributes to the angiogenic switch in early but not in
advanced stages of tumor growth in the Rip-Tag model of pancreatic
insulinoma.S20
Anti-Bv8
(Bv8)
antibodies
impede
tumor
infiltration
of
CD11b+Gr1+ cells, and inhibit the growth and angiogenesis of tumors, not only of
those that are sensitive, but also those that are resistant to VEGF-inhibitor
therapy. Combined delivery of Bv8 with VEGF elicits an additive anti-tumor
effect in implanted tumor models, but not in the pancreatic Rip-Tag tumor
model.S20,S21 Consistent with findings that chemotherapy induces mobilization of
hematopoietic cells from the bone marrow, and that cytotoxic tumor necrosis
upregulate chemokines such as G-CSF with subsequent generation of
neutrophils, combined delivery of Bv8 with cytotoxic agents enhanced tumor
growth inhibition.S21
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