Mechanistic insight into activation of MAPK signaling by pro-angiogenic factors

Song and Finley BMC Systems Biology (2018) 12:145 https://doi.org/10.1186/s12918-018-0668-5 RESEARCH ARTICLE Open Access Mechanistic insight into activation of MAPK signaling by pro-angiogenic factors Min Song1 and Stacey D. Finley1,2,3* Abstract Background: Angiogenesis is important in physiological and pathological conditions, as blood vessels provide nutrients and oxygen needed for tissue growth and survival. Therefore, targeting angiogenesis is a prominent strategy in both tissue engineering and cancer treatment. However, not all of the approaches to promote or inhibit angiogenesis lead to successful outcomes. Angiogenesis-based therapies primarily target pro-angiogenic factors such as vascular endothelial growth factor-A (VEGF) or fibroblast growth factor (FGF) in isolation. However, preclinical and clinical evidence shows these therapies often have limited effects. To improve therapeutic strategies, including targeting FGF and VEGF in combination, we need a quantitative understanding of the how the promoters combine to stimulate angiogenesis. Results: In this study, we trained and validated a detailed mathematical model to quantitatively characterize the crosstalk of FGF and VEGF intracellular signaling. This signaling is initiated by FGF binding to the FGF receptor 1 (FGFR1) and heparan sulfate glycosaminoglycans (HSGAGs) or VEGF binding to VEGF receptor 2 (VEGFR2) to promote downstream signaling. The model focuses on FGF- and VEGF-induced mitogen-activated protein kinase (MAPK) signaling and phosphorylation of extracellular regulated kinase (ERK), which promotes cell proliferation. We apply the model to predict the dynamics of phosphorylated ERK (pERK) in response to the stimulation by FGF and VEGF individually and in combination. The model predicts that FGF and VEGF have differential effects on pERK. Additionally, since VEGFR2 upregulation has been observed in pathological conditions, we apply the model to investigate the effects of VEGFR2 density and trafficking parameters. The model predictions show that these parameters significantly influence the response to VEGF stimulation. Conclusions: The model agrees with experimental data and is a framework to synthesize and quantitatively explain experimental studies. Ultimately, the model provides mechanistic insight into FGF and VEGF interactions needed to identify potential targets for pro- or anti-angiogenic therapies. Background Angiogenesis is the formation of new blood capillaries from pre-existing blood vessels. The essential role of blood vessels in delivering nutrients makes angiogenesis important in the survival of tissues, including tumor growth. Angiogenesis also provides a route for tumor metastasis. Thus, targeting angiogenesis is a prominent strategy in many contexts, for example, in both tissue engineering and cancer treatment. * Correspondence: 1 Department of Biomedical Engineering, University of Southern California, Los Angeles, California, USA 2 Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California, USA Full list of author information is available at the end of the article In the context of tissue engineering, there is a large demand for organs needed for transplant surgery, but a great shortage of donors. The long-term viability of engineered tissue constructs depends on growth of new vessels from host tissue, and stimulating new blood vessel formation is an important pro-angiogenic strategy for tissue engineering [1]. Alternatively, the formation of new blood vessels is important for cancer growth and metastasis. Thus, inhibiting angiogenesis is an anti-angiogenic strategy for cancer treatment. Unfortunately, not all approaches to promote or inhibit angiogenesis lead to successful outcomes. For example, clinical trials have shown no effective improvement in blood flow or perfusion by fibroblast growth factor (FGF)-induced [2] or vascular endothelial growth © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Song and Finley BMC Systems Biology (2018) 12:145 factor-A (VEGF)-induced [3] angiogenesis. Specifically, a double-blinded randomized controlled trial studied recombinant FGF-induced angiogenesis and showed no symptomatic improvement (exercise tolerance or myocardial perfusion) following 90 or 180 days of treatment [2]. Similarly, in a double-blinded placebo-controlled trial to study the effects of recombinant human VEGF-induced angiogenesis in animal models, there was no improvement in angina, in comparison with placebo by day 60. Only a high dose of VEGF (50 ng/ kg/min) showed any effect [3]. Also, bevacizumab, an anti-VEGF agent for cancer treatment, has limited effects in certain cancer types, and it is no longer approved for the treatment of metastatic breast cancer due to its disappointing results [4]. Thus, there is a need to better understand the molecular interactions and signaling required for new blood vessel formation, in order to establish more effective therapeutic strategies. The established angiogenesis-based therapies primarily target pro-angiogenic factors such as FGF and VEGF in isolation. However, both FGF and VEGF bind to their receptors to initiate mitogen-activated protein kinase (MAPK) signaling and phosphorylate ERK, the final output of the MAPK pathway [5, 6]. This signaling pathway promotes cell proliferation in the early stages of angiogenesis. Additionally, the combined effects of FGF and VEGF have been shown to be greater than their individual effects [7, 8]. A quantitative understanding of how these promoters combine together to stimulate angiogenesis could greatly benefit the current pro- and anti-angiogenic therapies. Mathematical modeling is a useful tool to predict the molecular response mediated by angiogenic factors. For example, Mac Gabhann and Popel studied interactions between VEGF isoforms, VEGF receptors (VEGFR1, VEGFR2, NRP1), and the extracellular matrix using a molecular-detailed model. The model predicted that blocking Neuropilin-VEGFR coupling is more effective in reducing VEGF-VEGFR2 signaling than blocking Neuropilin-1 expression or binding of VEGF to Neuropilin-1 [9]. Stefanini et al. constructed a pharmacokinetic model that studied VEGF distribution after intravenous administration of bevacizumab, and they found that plasma VEGF was increased after treatment [10]. Filion and Popel explored myocardial deposition and ret (...truncated)