Predictive model of thrombospondin-1 and vascular endothelial growth factor in breast tumor tissue

www.nature.com/npjsba ARTICLE OPEN Predictive model of thrombospondin-1 and vascular endothelial growth factor in breast tumor tissue Jennifer A Rohrs1, Christopher D Sulistio1 and Stacey D Finley1,2 Angiogenesis, the formation of new blood capillaries from pre-existing vessels, is a hallmark of cancer. Thus far, strategies for reducing tumor angiogenesis have focused on inhibiting pro-angiogenic factors, and less is known about the therapeutic effects of mimicking the actions of angiogenesis inhibitors. Thrombospondin-1 (TSP1) is an important endogenous inhibitor of angiogenesis that has been investigated as an anti-angiogenic agent. TSP1 impedes the growth of new blood vessels in many ways, including crosstalk with pro-angiogenic factors. Owing to the complexity of TSP1 signaling, a predictive systems biology model would provide quantitative understanding of the angiogenic balance in tumor tissue. Therefore, we have developed a molecular-detailed, mechanistic model of TSP1 and vascular endothelial growth factor (VEGF), a promoter of angiogenesis, in breast tumor tissue. The model predicts the distribution of the angiogenic factors in tumor tissue, revealing that TSP1 is primarily in an inactive, cleaved form owing to the action of proteases, rather than bound to its cellular receptors or to VEGF. The model also predicts the effects of enhancing TSP1’s interactions with its receptors and with VEGF. To provide additional predictions that can guide the development of new anti-angiogenic drugs, we simulate administration of exogenous TSP1 mimetics that bind specific targets. The model predicts that the CD47-binding TSP1 mimetic markedly decreases the ratio of receptor-bound VEGF to receptor-bound TSP1, in favor of anti-angiogenesis. Thus, we have established a model that provides a quantitative framework to study the response to TSP1 mimetics. npj Systems Biology and Applications (2016) 2, 16030; doi:10.1038/npjsba.2016.30; published online 20 October 2016 INTRODUCTION A hallmark of cancer is angiogenesis, the formation of new blood capillaries from pre-existing vessels. This process enables oxygen and nutrients from the surrounding microenvironment to reach the tumor. In fact, angiogenesis promotes cancer development, invasion, and metastasis. For these reasons, angiogenesis has become a prominent target for cancer drugs.1 Therapies aimed at inhibiting angiogenesis (‘anti-angiogenic therapies’) target many aspects in the process of new blood vessel growth, with a focus on inhibiting pro-angiogenic factors.2 Anti-angiogenic therapeutics that target signaling mediated by the vascular endothelial growth factor-A (VEGF), a potent promoter of angiogenesis, are approved for treatment of various cancer types.3 These agents include drugs that bind to VEGF and prevent it from binding to and activating its receptors, as well as tyrosine kinase inhibitors that impede activation of VEGF receptors intracellularly. These treatment strategies, however, have not been successful in all cancer types. In fact, antibody therapy targeting VEGF is no longer approved for breast cancer treatment. In addition, many tumors, including breast tumors, become resistant to anti-VEGF or other anti-angiogenic treatments.4 Numerous preclinical studies show that targeting a single factor within the angiogenesis signaling network is insufficient to arrest tumor growth and vascularization since tumors may ‘escape’ treatment by utilizing alternative pathways.5 Thus, there is a critical need to better understand the effects of these pro- and anti-angiogenic pathways in order to develop effective treatment strategies, including multi-modal therapies that can address the issue of drug resistance.6,7 Both pro- and anti-angiogenic factors determine the extent of vascularization8 and the response to anti-angiogenic therapy.9 Therefore, another means of increasing the efficacy of antiangiogenic treatment may be to mimic the action of inhibitors of angiogenesis, while simultaneously inhibiting the promoters. For example, in a preclinical model of pancreatic cancer, altering the balance between pro- and anti-angiogenic factors was shown to modulate tumor growth.10 To this end, several anti-angiogenic factors have been identified as potential cancer therapeutics. Thrombospondins (TSPs) are a family of multi-domain, calcium-binding glycoproteins that are highly expressed during development.11 Of the five TSPs, thrombospondin-1 (TSP1) is the most studied, was the first endogenous anti-angiogenic factor identified,12 and has been investigated for anti-angiogenic therapy. TSP1 acts to impede the growth of new blood vessels in multiple ways. First, TSP1 influences growth factor availability. It can bind to VEGF and other pro-angiogenic factors to reduce intracellular signaling through their receptors and to clear the pro-angiogenic growth factors from the cell via the low-density lipoprotein receptorrelated protein 1 (LRP1). TSP1 also inhibits the activation of matrix metalloprotease-9 (MMP9),13 which among its many functions, is able to cleave VEGF.14 In addition to altering growth factor availability, TSP1 inhibits angiogenesis by binding to and activating its own receptors. TSP1 signaling through the CD36 receptor results in reduced cell survival and activation of apoptosis 1 Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA and 2Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, USA. Correspondence: SD Finley (sfi[email protected]) Received 12 March 2016; revised 9 August 2016; accepted 12 August 2016 Published in partnership with the Systems Biology Institute Computational model of TSP1 and VEGF in tumor tissue JA Rohrs et al 2 pathways via caspase-3.13 TSP1-mediated activation of the CD47 receptors antagonizes nitric oxide signaling via endothelial cell-derived nitric oxide synthase,15 which is important in cell migration and proliferation.16 CD47 also couples to the VEGF receptor R2 to inhibit VEGF-mediated activation.17 In addition, TSP1 binds to β1 integrins, which further antagonizes VEGF signaling.13 Interestingly, TSP1 has been shown to elicit both pro- and anti-angiogenic effects, depending on the microenvironment.18 This effect is not fully understood; thus, the complex TSP1 interactome19 and its context-dependent role suggest that a predictive systems biology model would greatly aid in the optimization of TSP1-based therapeutics. Quantitative models of angiogenesis provide insight into the fundamental mechanisms of neovascularization. For example, systems biology models are useful in optimizing anti-angiogenic treatment strategies and identifying prognostic biomarkers,20,21 thereby complementing preclinical and clinical studies. We have previously developed and applied mechanistic, systems biology models to examine the effects of drug and tumor properties on the response to anti-VEGF agents22 and (...truncated)