Date Published: January 31, 2019
Publisher: Impact Journals
Author(s): Fengyan Jin, Xiangyu Zheng, Yanping Yang, Gang Yao, Long Ye, Thorsten R. Doeppner, Dirk M. Hermann, Haifeng Wang, Yun Dai.
Hypoxia inducible factors (HIFs) mediate angiogenesis via up-regulation of various pro-angiogenic factors (particularly VEGF) in response to hypoxia. Here, we report that hypoxia unexpectedly induced robust production of the pro-inflammatory factor TNFα by endothelial cells (ECs), suggesting an autocrine loop that in turn activated HIFs via an NF-κB-dependent process, resulting in production of VEGF and thereby promotion of angiogenesis. In contrast, low-density lipoprotein (LDL) prevented expression of HIFs in ECs exposed to either hypoxia or TNFα, while knockdown of either HIF-1α or HIF-2α strikingly attenuated hypoxia-induced production of VEGF by ECs as well as EC colony formation and tube formation. Significantly, LDL attenuated hypoxia-induced angiogenesis by disrupting the TNFα/NF-κB/HIF/VEGF signaling cascade via down-regulation of the TNF receptor TNF-R1, rather than TNFα itself, and multiple key components of both canonical and non-canonical NF-κB pathways. By doing so, LDL was able to either inhibit or down-regulate a wide spectrum of HIF-dependent pro-angiogenic downstream targets and signals. Together, these findings argue existence of a self-regulatory TNFα/NF-κB/HIF/VEGF signaling network in ECs, which mediates and fine-tones angiogenesis, at least in response to hypoxia. They also suggest that LDL impairs angiogenesis by disrupting this network, which might represent a novel mechanism underlying anti-angiogenic property of LDL.
Hypoxia-driven angiogenesis represents one of the major adaptive responses to low oxygen supply, which is essential for maintenance of cellular metabolism, survival, and function, thereby representing a prominent feature of embryo development, ischemic injury recovery, tissue regeneration, inflammation, tumor growth, etc . This event is primarily governed by the transcription factor hypoxia-inducible factors (HIFs), including HIF-1α and HIF-2α (originally named endothelial PAS domain protein-1, EPAS1), HIF-3α, and HIF-1β (also known as aryl hydrocarbon receptor nuclear translocator, ARNT) . Different HIF isoforms may have their distinct functions in regulation of hypoxia-induced metabolic switch in endothelial cells (ECs) and angiogenesis [2,3]. It has been well established that as a major transcription factor, the HIF complex consisting of an oxygen-sensitive subunit (HIF-1α or HIF-2α) and a constitutively expressed HIF-1β plays an essential role in driving angiogenesis under various physiological and pathological conditions [4,5]. In the latter case, the significance of HIF-mediated angiogenesis (via VEGF) has been well documented in a wide variety of disease types, especially aging-related diseases such as atherosclerotic (e.g., coronary artery disease, ischemic stroke)  and degenerative disorders (e.g., neurodegeneration  and macular degeneration ). In diverse types of cells (especially ECs), downstream genes of HIF are also involved in vascular remodeling, inflammation, cell survival and proliferation, apoptosis, autophagy, cell migration and invasion, DNA damage responses, extracellular matrix metabolism, as well as glucose metabolism (e.g., glucose uptake and glycolysis) [8,9].
The transcription factor HIFs are central regulators of adaptive responses to hypoxia in various physiological and pathological processes, including angiogenesis and inflammation . In the former, HIFs act as a master switch during angiogenesis through inducing production of VEGFs and other growth factors by diverse types of cells (particularly ECs and macrophages) , to recover blood supply and restore oxygenation in hypoxic tissues (e.g., caused by ischemic diseases, such as stroke and myocardial infarction) [43,44]. However, such a protective role is often impaired due to hyperlipidemia that is common in patients with aging-related disorders, such as atherosclerotic diseases [24–27]. In the latter, HIFs (HIF-1α in particular) plays an important role in inflammation through promoting production of TNFα and other cytokines or chemokines by immune cells (especially macrophages) . As TNFα also serves as a potent pro-angiogenic factor, HIF-1α-mediated production of TNFα by macrophages could be considered as an alternative mechanism for increased angiogenesis in certain circumstances , such as within atherosclerotic plaques where macrophages are the most abundant cellular component . However, the role of HIFs and NF-κB (another major transcription factor related to both TNFα and hypoxia) in the potential cross-link between pro-inflammatory TNFα and pro-angiogenic VEGFs remains to be defined in ECs whose activation and dysfunction are the hallmark of age-related vascular diseases (e.g., atherosclerosis  and neovascular macular degeneration ). Here we showed for the first time that hypoxia could induce autocrine of TNFα by ECs, which in turn activated the HIF pathways via a NF-κB-dependent process, thereby promoting VEGF production by ECs and angiogenesis, a process that might not need macrophages. Notably, the anti-angiogenic property of LDL likely involved disruption of this self-regulatory HIF-centered signaling network in ECs. Thus, these findings might provide a new insight into the mechanism underlying hypoxia-induced angiogenesis as well as its defect in patients with hyperlipidemia-related diseases.