Oxygen, HIF, and Innate Immunity
Take a deep breath. As you feel the air rushing into your lungs, know that you are providing invaluable oxygen to your cells.
At a fundamental level, we all know that our cells need oxygen to survive. They use oxygen to produce the energy they need to perform a countless number of functions. This process can be called aerobic metabolism. However, there are times when our cells can’t get enough oxygen. This lack of enough oxygen is called hypoxia. Sometimes this hypoxia is normal, as certain tissues within our body have low levels of oxygen in their healthy state, such as some of the epithelial cells in our intestines. In these contexts, this hypoxia is called physiologic hypoxia.
Another form of hypoxia is called inflammatory hypoxia, which, as its name suggests, occurs at sites of inflammation. This hypoxia arises from a number of factors, including damage to blood vessels that prevents oxygen from reaching the tissue, elevated oxygen consumption by the inflamed tissue as it rushes to produce a variety of new proteins in response to the inflammatory stress, and the added presence of oxygen-consuming immune cells recruited to the tissue during the inflammatory response.
Because of this link between hypoxia and inflammation, researchers have studied the impact of low oxygen levels on our innate immune system. The innate immune system is one of two major components of our immune system (the other being the adaptive immune system) and is responsible for many of the initial hallmarks of inflammation. Researchers have found that hypoxia and the cellular signaling that it triggers have vital impacts on how our innate immune cells function.
Central to this process is a family of proteins called hypoxia-inducible factors, or HIFs. These proteins are transcription factors, meaning that they are involved in activating the expression of genes in response to the cellular stress caused by hypoxia. In environments with normal levels of oxygen, the HIF proteins are typically constantly synthesized within the cell, but then are quickly degraded in a process involving proteins called von Hippel-Lindau protein (VHL) and prolyl hydroxylase domain proteins (PHDs), which mark HIF for degradation by the proteasome, the cell’s woodchipper. The end result of this process is that while HIF is constitutively made, its constant destruction keeps its levels relatively low in healthy environments.
While at first glance, the continuous production and destruction of HIF may seem counterintuitive, this process allows the cell to very rapidly react to sudden decreases in oxygen. When oxygen levels are lowered and the tissue environment becomes hypoxic, HIF’s destruction is turned off, allowing the existing HIF proteins that would normally be tagged for destruction to instead quickly respond to the new hypoxic stress. As a transcription factor, HIF turns on genes, which are, in turn, involved in a host of biological processes, including the generation of new blood vessels, cell proliferation, cell survival, and cell differentiation.
In inflammation, you see rising HIF levels, or stabilization, which can be due to multiple factors, including increased HIF gene expression, decreased HIF destruction, or a combination of the two. Oftentimes HIF levels rise following hypoxia, but HIF can be stabilized in response to nonhypoxic signals, too. This can be especially true in the inflammatory response, where inflammatory cytokines and signals have been found to regulate HIF levels, including IL-1, IL-6, and TNF. Additionally, certain components of bacteria and their byproducts can trigger HIF levels to increase.
During inflammation, HIF has key roles in innate immune cells. In granulocytes, a category of immune cells that are often the first wave of recruited cells during the inflammatory response, HIF has been shown to be vital for maintaining their cellular metabolism and regulating their migration. Major functions of granulocytes include engulfing (phagocytosis) and killing of invading pathogens, as well as the production of antimicrobial compounds. In both cases, HIF has been shown to be a key mediator. Researchers have demonstrated that the absence of HIF prevents these cells from efficiently killing their targets. Conversely, when researchers blocked HIF degradation to allow HIF levels to rise, the granulocytes exhibited elevated inflammatory responses. A more specific example of HIF’s role in granulocytes comes from a subset of cells known as neutrophils. These cells can produce and release an amalgamation of molecular fibers and antimicrobial compounds called a neutrophil extracellular trap, or NET. As the name implies, these NETs can entrap and kill bacteria and other pathogens. The creation of NETs is dependent on HIF proteins, specifically HIF-1.
Macrophages are an additional member of the innate immune cell family and also rely on HIF for their roles in the inflammatory response. Macrophages can adopt different forms with opposite functions, specifically the inflammatory M1 subtype and the anti-inflammatory M2 subtype. Macrophages convert to the M1 or M2 subtype through a process called polarization. HIF proteins have been implicated in macrophage polarization and impacts the overall function of these cells. During polarization, the cellular metabolism shifts, depending on the subtype. In M1 macrophages, there are higher levels of glycolysis and pentose phosphate metabolism, and this metabolic preference depends on decreased activity of PHDs – the proteins involved in HIF destruction – and higher levels of HIF. In contrast, M2 macrophages predominantly use oxidative phosphorylation instead of glycolysis and seem to favor a different member of the HIF family, HIF-2, instead of HIF-1. By regulating the polarization of macrophages, the HIF proteins can impact how these cells respond during an inflammatory situation.
Dendritic cells (DCs) are another class of innate immune cells, and while the relationship between HIF and DCs has not been as extensively studied as in their brethren, researchers have identified hints that HIF is important in DC function. For example, hypoxia results in lower levels of chemokine receptor expression in DCs. Chemokines are important migratory signals during the inflammatory response, so the lower chemokine receptor levels in DCs during hypoxia should reduce the cells’ ability to detect and respond to migratory signals. This is precisely what the researchers found, though the impact of this, as well as HIF’s other potential roles in DCs, still needs additional study.
A more recently identified innate cell type, innate lymphoid cells (ILCs), also exhibit early indications of a dependency on HIF for regulating their functions. ILCs are innate immune cells but originate from a different precursor cell than DCs, macrophages, and granulocytes. ILCs arise from the lymphoid lineage, making them closely related to T cells and B cells, the components of the adaptive immune system. ILCs reside in tissues and are especially abundant in the barrier surfaces, like the lining of the gut, and they produce copious amounts of inflammatory cytokines and other signals in response to pathogens. Their metabolism appears to be regulated, at least in part, by HIF: high levels of HIF lead to a shift to glycolysis, as well as functional defects in certain subtypes of ILCs.
Complementing its roles in the various innate immune cells, HIF appears to be instrumental in additional aspects in our fight against pathogenic invasion. Our barrier surfaces, like the cells lining our lungs and gut, depend on HIF stabilization to form tight connections between cellular neighbors. These tight junctions help create a consistent barrier layer of cells that prevents pathogens from sneaking between cells into our bodies. These barrier cells also produce a mucous layer, another layer of protection that helps keep pathogens from getting into our bodies. The mucous layer is comprised of a variety of factors, including mucin proteins and antimicrobial compounds, and their production is dependent on HIF signaling.
Due to its role in the innate immune response, the HIF pathway has been associated with inflammatory diseases, including colitis. For this reason, researchers are developing ways to modulate the HIF pathway in the hopes of alleviating these inflammatory conditions. Depending on the target disease, compounds have been developed to either increase HIF levels or decrease HIF levels. To increase HIF levels, there are several compounds being developed called prolyl hydroxylase inhibitors (PHIs) that target the PHDs responsible for HIF degradation. Some of these inhibitors have entered clinical trials, while others have been studied in a number of preclinical models. Data from these studies suggest that targeting the HIF pathway may be a valuable approach for restoring a healthy immune response.
The HIF proteins are vital cellular oxygen sensors that play fundamental roles in a wide range of cell types. While this blog post focused on HIF and the innate immune response, the HIF proteins have been implicated in a number of diseases, including cancer and cardiovascular disease. Since the immune system is also closely associated with these conditions, it will be interesting to see how future studies tie these various threads together, perhaps identifying new therapeutic approaches along the way.
This post was inspired and informed by: Colgan SP, Furuta GT, and Taylor CT. Hypoxia and Innate Immunity: Keeping Up with the HIFsters. Annu. Rev. Immunol. 2020. 38:341-363.
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