Transcriptional and Post-Transcriptional Regulation of Thrombospondin-1 Expression: A Computational Model

Hypoxia is an important physiological stress signal that drives angiogenesis, the formation of new blood vessels. Besides an increase in the production of pro-angiogenic signals such as vascular endothelial growth factor (VEGF), hypoxia also stimulates the production of anti-angiogenic signals. Thrombospondin-1 (TSP-1) is one of the anti-angiogenic factors whose synthesis is driven by hypoxia. Cellular synthesis of TSP-1 is tightly regulated by different intermediate biomolecules including proteins that interact with hypoxia-inducible factors (HIFs), transcription factors that are activated by receptor and intracellular signaling, and microRNAs which are small non-coding RNA molecules that function in post-transcriptional modification of gene expression. Here we present a computational model that describes the mechanistic interactions between intracellular biomolecules and cooperation between signaling pathways that together make up the complex network of TSP-1 regulation both at the transcriptional and post-transcriptional level. Assisted by the model, we conduct in silico experiments to compare the efficacy of different therapeutic strategies designed to modulate TSP-1 synthesis in conditions that simulate tumor and peripheral arterial disease microenvironment. We conclude that TSP-1 production in endothelial cells depends on not only the availability of certain growth factors but also the fine-tuned signaling cascades that are initiated by hypoxia.Author Summary: Research evidence show that thrombospondin-1 (TSP-1) is an anti-angiogenic protein which potently inhibits the downstream signaling of vascular endothelial growth factor receptor 2 (VEGFR2), an important pathway that promotes endothelial cell proliferation, migration and permeability. As demonstrated by numerous studies, expression of TSP-1 is often upregulated in peripheral arterial disease (PAD) and downregulated in many solid tumors, primarily because of its inhibitory effect on angiogenesis and tumor growth. Given the established anti-angiogenic property of TSP-1 and its dysregulation in diseases, it holds great value to design novel therapeutic strategies that aim to restore TSP-1 expression in tumors and limit its expression in PAD by regulating the biomolecules that control TSP-1 synthesis. The computational, mechanistic, experiment-based model of TSP-1 intracellular regulation presented here is a solid integration of the current knowledge and is substantially validated against published data. Our model simulations reproduce the experimental time-course dynamics of key proteins within the regulatory network and suggest interesting behavior in hypoxia- and cytokine-driven regulation of TSP-1. In addition, we assess different model-based strategies to modulate TSP-1 synthesis in silico; our model is the essential module of an integrated computational platform that can provide research insights for future investigations of TSP-1 and angiogenesis.