Hypoxia-inducible factors and innate immunity in liver cancer

Abstract

The liver has strong innate immunity to counteract pathogens from the gastrointestinal tract. During the development of liver cancer, which is typically driven by chronic inflammation, the composition and biological roles of the innate immune cells are extensively altered. Hypoxia is a common finding in all stages of liver cancer development. Hypoxia drives the stabilization of hypoxia-inducible factors (HIFs), which act as central regulators to dampen the innate immunity of liver cancer. HIF signaling in innate immune cells and liver cancer cells together favors the recruitment and maintenance of pro-tumorigenic immune cells and the inhibition of anti-tumorigenic immune cells, promoting immune evasion. HIFs represent attractive therapeutic targets to inhibit the formation of an immunosuppressive microenvironment and growth of liver cancer.

Figure 1. Hypoxia, innate immunity, and the development of HCC.

Hepatocarcinogenesis is a stepwise process. Chronic hepatitis virus infection and fat-induced steatosis (NASH), the major etiological factors of HCC, drive liver inflammation and tissue damage. Excessive alcohol consumption also induces liver damage. Damage in the liver disrupts the hepatic vasculature and perturbs proper blood flow and O₂ supply, creating a hypoxic microenvironment. Hypoxic Kupffer cells, newly recruited macrophages, and hepatocytes activate hepatic stellate cells (HSCs) in the liver, which robustly deposit collagen, leading to fibrosis and then cirrhosis, which further intensifies hypoxia. When HCC is developed, hypoxia is even more severe. The growth of HCC outpaces the growth of blood vessels. Moreover, HCC cells consume all available O₂. Different current HCC treatments, such as TAE/TACE and TKIs, further induce hypoxia. Hypoxia affects the activities of different innate immune cells, such as NK cells. Hypoxia also induces the infiltration and accumulation of many different types of immunosuppressive innate immune cells, including TAMs, MDSCs, and neutrophils, in the microenvironment of HCC. The O₂ level decreases along with HCC development, driving the formation of an immunosuppressive microenvironment.

Figure 2. Roles of the hypoxia/HIF signaling pathway in innate immune cells in HCC.

(A) In the presence of O₂, the HIF-1/2α subunit is hydroxylated by the PHD enzymes at two specific proline residues, enabling the binding of VHL. VHL targets the hydroxylated HIF-1/2α subunit for ubiquitin-mediated proteasomal degradation. (B) In the absence of O₂, the HIF-1/2α subunit is stabilized and dimerizes with HIF-1β, together with cotranscriptional factors p300 and CBP, to drive the transcription of genes encompassing hypoxia-responsive elements (HREs). (C) The hypoxia/HIF signaling pathway in the innate immune cells directly affects their properties in HCC.

Figure 3. Roles of the hypoxia/HIF signaling pathway in HCC cells that affect innate immune cells in the tumor microenvironment.

HIFs activate the transcription and secretion of the chemokines CCL20, VEGF, and CCL26 in HCC cells. These chemoattractants recruit immunosuppressive TAMs and MDSCs to HCC. HIFs activate the transcription of the don’t-eat-me signal surface markers (CD47, CD24) in cancer cells including HCC cells. CD47 and CD24 are well-characterized liver cancer stem cell markers. CD47 and CD24 prevent cancer cells from being phagocytosed by the macrophages. HIFs activate the transcription of members of the purinergic signaling pathway (CD39, CD39L1, CD73) in HCC cells to create an adenosine- and AMP-rich microenvironment that favors the accumulation of MDSCs. HIFs activate the transcription of members of the LOX family in HCC cells. The LOX family cross-links collagen in the primary cancer and metastatic niches. At the primary liver cancer niche, the LOX family increases tissue stiffening and promotes local invasion of cancer cells. At the metastatic niche in the lung, cross-linking of collagen mediated by the LOX family helps to recruit MDSCs, creating a favorable niche for HCC cell colonization.