The main findings from this study are that pericytes play key roles in maintenance of ovarian cancer vasculature and may provide a protective mechanism against anti-angiogenic therapies. These effects reflect complex paracrine interactions between the tumor endothelium, pericytes, and tumor cells (). However, our findings suggest opportunities for novel anti-vascular therapeutic strategies based on dual-targeting of tumor endothelial cells and pericytes.
Figure 7 Hypothetical model of interactions between tumor, endothelial cells, and pericytes in ovarian cancer. The tumor and endothelial cells produce PDGF-BB, which promotes local VEGF production by pericytes, providing local survival signals for the mature tumor (more ...)
Despite many advances in surgical and medical management, ovarian cancer remains the most deadly gynecologic malignancy.33,34,35
Ongoing studies in the Gynecologic Oncology Group (GOG) suggest that addition of more chemotherapy drugs may not result in substantial improvements in the outcome of ovarian cancer patients. It is likely that combination of biologically-targeted agents will be required for additional gains in patient survival. Targeting the tumor vasculature is attractive because angiogenesis is required for tumor growth and endothelial cells are thought to be more genetically stable than the highly aneuploid cancer cells. Indeed, VEGF-targeted therapies have improved the outcome of patients with colorectal and other cancers.13,14
Remarkably, bevacizumab monotherapy resulted in an 18% response rate in recurrent ovarian cancer.12
While these therapies are improving survival in many cancer patients, the cure rates do not appear substantially higher and new strategies are needed to make additional gains.
There is growing evidence that pericytes may play a role in limiting the efficacy of VEGF-targeted therapies.20
The pathophysiological interaction of tumor cells, tumor-associated endothelial cells, and pericytes in the tumor microenvironment is complex and poorly understood. Nonetheless, understanding this relationship is critical to effective angiogenic targeting. We documented that pericytes are normally tightly associated with endothelial cells and extensively cover normal vasculature, but in ovarian cancer, pericytes are loosely associated with endothelial cells and irregularly oriented relative to the stroma. The interplay between these cells likely reflects the shifts in PDGF-BB gradients in the tumor microenvironment. Nevertheless, pericytes recruited and stimulated by PDGF-BB appear to provide survival signals for the tumor endothelial cells. Using highly specific decoy receptors for ligands (PDGF-Trap and VEGF-Trap), we demonstrated that while blockade of PDGF had little effect on tumor growth, VEGF blockade and dual targeting significantly reduced tumor growth. Mechanistically, this effect appears to be due, in part, to endothelial cell apoptosis following VEGF blockade, which is enhanced in the presence of PDGF blockade. We also observed that pericyte coverage of endothelial cells in ovarian cancer is reduced when PDGF-BB signaling is blocked, which enhances the effects of anti-VEGF therapy in ovarian cancer.
We previously examined the potential of endothelial and pericyte targeting using non-specific inhibitors of PDGFR (imatinib mesylate) and VEGFR (AEE788, Novartis, Basel, Switzerland) signaling in ovarian cancer models.21
In combination with paclitaxel, the triple-drug combination was highly effective in chemotherapy-sensitive and resistant tumors. As was observed in the current study, interruption of the PDGF receptor/ligand axis reduced pericyte coverage and blockade of VEGF receptor phosphorylation induced endothelial cell apoptosis. However, we recognized that a confounding factor in our previous study was the broad end-target spectrum of the agents, particularly the co-blockade of epidermal growth factor receptor (EGFR) phosphorylation with AEE788. In the current study, we extend and confirm these results using highly specific decoy receptors for PDGF and VEGF ligand.
The consistency of these findings combined with those of others20,36,37
support the hypothesis that pericytes modulate endothelial cell function partially by producing survival signaling. Since PDGF-BB promotes VEGF production by pericytes and VEGF is a known survival factor for endothelial cells, pericytes appear to promote endothelial cell survival in a paracrine fashion. It is possible then that pericytes may serve as a local source of VEGF and other factors for the adjacent endothelial cells. Based on our data and published literature, we have developed a model () to describe the interactions between tumor, perivascular, and endothelial cells. In this model, the tumor and endothelial cells produce PDGF-BB, which promotes VEGF production by pericytes. Among other functions, VEGF protects endothelial cells from apoptosis.
We recognize that there are limitations to this work, which should be discussed. We have focused primarily on PDGF-BB due to its known significance for pericyte biology. However, other PDGF ligands may be important for production of survival factors from pericytes. In addition, other pathways such as Ang1/Tie2, TGF-β1/Alk5, and MMPs may be involved in pericyte recruitment during angiogenesis,15
and could offer additional opportunities for therapeutic targeting. While the presence of pericytes is altered in cancer vasculature relative to normal, there are no standard ways to quantify this effect. We used the percent of vessels with 50% coverage of pericytes; however, a more precise measure of functional deficiency may be required for using this feature as a potential biomarker. Finally, we co-administered PDGF- and VEGF-Trap to demonstrate the differential improvement in VEGF sensitivity. However, it is possible that specific temporal sequencing of agents such as these, particularly in combination with cytotoxic chemotherapy, may be required for optimal anti-tumor effects.
In summary, our data show that paracrine signaling from pericytes provides survival cues for tumor vasculature. Blocking PDGF signaling enhances the effect of anti-VEGF therapy in ovarian cancer treatment, in part, by decreasing pericyte coverage and increasing apoptosis of tumor-associated endothelial cells. Therefore, dual targeting of endothelial cells and pericytes is more efficacious than targeting either cell type alone. This study provides the preclinical rationale for the development of more effective anti-vascular therapeutic strategies for human ovarian cancer.