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 PDGFRß).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.S8VEGF(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).S11Combination 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 resistance-conferring angiogenic myeloid cells, and stimulates angiogenesis directly.S18Bv8 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 VEGF-targeted 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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