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Lipoproteins and angiogenesis: A new look at an old problem

Final Report Summary - LIPINTOECTION (Lipoproteins and angiogenesis: A new look at an old problem)

All cells require sufficient amounts of lipids for essential production of energy and synthesis of the plasma membrane. Multicellular organisms have developed a specialized transport system for distributing lipids throughout the body based on lipoprotein (LIP) particles, which travel through the vasculature, reaching all organs. LIP particles are composed of triacylglycerol lipids, phospholipids, cholesterol lipids, and apoproteins, which enable them to be solubilized in the plasma. In spite of the fact that ECs are exposed to LIPs to a higher degree than any other cell type, they were long thought to function as an inert barrier, through which lipids are exchanged between plasma and surrounding tissues. In contrast to this classic view, work from my laboratory challenged this assumption, by uncovering a previously unexplored direct crosstalk between plasma LIPs and ECs (Nat Med, 2012; Exp Cell Res, 2013).
The ERC Stg Grant LIPintoECtion focused on the underpinnings of the interaction between lipoproteins and the vascular system.
1. In order to identify downstream components underlying the effects of lipoproteins within ECs we took advantage of our well-established zebrafish models of hyper-, and hypo-lipidemia, to carry out the first mRNA expression profiling of ECs facing differential LIP levels (ATVB, 2016). These results are especially important in light of the fact that most of the research addressing the interaction between lipoproteins and the endothelium has been carried out in cultured ECs. The results have remained controversial, with reports of both inhibition and stimulation of EC proliferation and migration by LDL, all depending on doses and context. We believe that the series of events activated in ECs upon exposure to LIPs in vivo might be fundamentally different from the program observed in cultured ECs. Therefore, a study of the biology of LIP-EC interactions in vivo is mandatory, despite being much more challenging. Our results showed that different levels of ApoB-LIPs result in differential expression of the secreted enzyme Autotaxin (Atx), which in turn affects EC sprouting and angiogenesis. We further demonstrated that the effects of Atx in vivo are mediated by lysophosphatidic acid (LPA) - a well-known Atx activity product, and that LPA and LPA receptors participate as well in the induction of ectopic sprouting in response to LIP deficiency.
2. The role of ApoB-LIPs in tumor-angiogenesis and metastasis (Asaf, in revision). How the tumor vasculature responds to increased lipid load has been a matter of debate and controversy over the past decades. Several reports describe the enhancement of tumor angiogenesis under hypercholesterolemia, whereas others have presented contradictory results suggesting that cholesterol levels are inversely correlated to tumor vessel growth. Furthermore, the molecular mechanisms governing the regulation of tumor angiogenesis in hyperlipidemia have not been described in these studies, hindering the resolution of this discrepancy. In this project we addressed the contribution of LIPs to tumor-related angiogenesis. To this end we analyzed tumor vascularization and metastasis formation in two different mouse models- Ldlr-/- and ApoE-/-, which share increased cholesterol levels in circulation, but markedly differ in their apoprotein contents. We show that tumor-angiogenesis and metastasis formation inversely correlate to the plasma levels of Apoprotein-E and Apoprotein-B100 but not to cholesterol and triglycerides. These results are strongly supported by in vitro experiments demonstrating the direct effects of these apoproteins on EC behavior. Cholesterol and free fatty acids on the other hand proved beneficial for EC proliferation as shown for other cell types.
We also investigated the molecular mechanisms underlying the effects of apoproteins on tumor vascularization and vessel normalization. Expression analyses on FACS isolated ECs from within
primary tumors pointed to Vegfr1 and Vegfr2 as direct downstream targets of ApoB100 and ApoE within ECs, and demonstrated that endothelial-specific genetic ablation of these receptors in hyperlipidemic mice is sufficient to rescue the tumor-vascularization phenotypes.
As a whole our work demonstrates that the formation and functionality of the tumor-vasculature is controlled by the protein-component of lipoproteins and not by their fatty moieties. Moreover, our results provide important clues as to the potential non-desired contribution of statins to tumor angiogenesis and malignancy, and suggest a molecular mechanism explaining the controversy surrounding this subject. On a broader sense our findings contribute important new insights into the role of apoproteins in tumor progression, their mode of action and their potential benefits for cancer treatment.