Distinct Cargos of Small Extracellular Vesicles Derived from Hypoxic Cells and Their Effect on Cancer Cells

Abstract

Hypoxia is a common hallmark of solid tumors and is associated with aggressiveness, metastasis and poor outcome. Cancer cells under hypoxia undergo changes in metabolism and there is an intense crosstalk between cancer cells and cells from the tumor microenvironment. This crosstalk is facilitated by small extracellular vesicles (sEVs; diameter between 30 and 200 nm), including exosomes and microvesicles, which carry a cargo of proteins, mRNA, ncRNA and other biological molecules. Hypoxia is known to increase secretion of sEVs and has an impact on the composition of the cargo. This sEV-mediated crosstalk ultimately leads to various biological effects in the proximal tumor microenvironment but also at distant, future metastatic sites. In this review, we discuss the changes induced by hypoxia on sEV secretion and their cargo as well as their effects on the behavior and metabolism of cancer cells, the tumor microenvironment and metastatic events.

Keywords: TME; biomarker; cancer; exosomes; extracellular vesicles; hypoxia; immunity.

Figure 1

Overview of small extracellular vesicles (sEVs), their content, mechanisms of hypoxia and their potential biological roles. (A) sEVs are constituted of exosomes and small microvesicles. Exosomes are secreted after fusion of multivesicular bodies (MVB) with the plasma membrane and microvesicles are released by direct budding from the plasma membrane. sEVs can carry DNA fragments, mRNAs, microRNAs, lncRNAs, proteins, lipids and all other biological molecules. (B) Under normal O₂ availability, prolyl hydroxylase domain proteins (PHDs) hydroxylate proline residues on HIF-1α, which triggers the binding of the von Hippel-Lindau (VHL) tumor suppressor protein to HIF-1α leading to ubiquitination and degradation of HIF-1α. Under hypoxia, HIF-1α is stabilized and binds hypoxic response elements (HRE), thereby triggering target gene expression. This leads to changes in the metabolism of the cells and may stimulate invasion and angiogenesis. (C) Hypoxic sEVs play roles in invasion, migration, angiogenesis, epithelial to mesenchymal transition (EMT) and drug resistance of cancer cells. They also regulate immune responses, metabolism and hypoxia tolerance of target cells, details of which are discussed below.

Figure 2

Hypoxic sEVs promote progression and metastasis of cancer cells. Hypoxic sEVs enriched with a disintegrin and metalloproteinase with thrombospondin motifs 1 (ADAMTS1), carbonic anhydrase 9 (CA9), protein-lysine 6-oxidase (LOX), tissue factor (TF), thrombospondin-1 (TSP1), vascular endothelial growth factor (VEGF), Wnt4, miR-135b, miR-155, miR-210, miR-23a, miR-494 and/or lnc-p21 promote angiogenesis. Hypoxic sEVs loaded with C4.4A, CD171, α6β4 integrin, matrix metalloproteinases (MMP2, MMP9, MMP13 and MMP14), protein arginine methyltransferase 5 (PRMT5) and/or miR-1290 drive migration and invasion of target cancer cells. HIF-1α, TGF-β and/or Lnc-UCA1 contained in hypoxic sEVs can enhance epithelial to mesenchymal transition (EMT). miR-21 and/or miR-1273f, secreted in hypoxic sEV, promote metastasis of cancer cells.

Figure 3

Hypoxic sEVs modulate the immune system. miR-23a and TGF-β, loaded in hypoxic sEVs, decrease cytotoxicity of natural killer (NK) cells. The transfer of miR-1246, miR-125b-5p, miR-181d-5p, miR-21-3p, miR-21-5p, miR-301-3p, miR-940, let7a and/or lncRNA BRCT1 by hypoxic sEVs induce M2 macrophage polarization. Hypoxic sEVs loaded with miR-10 and miR-21 decrease cytotoxicity of myeloid derived suppressor cells. Hypoxic sEVs affect the proliferation of T cells and differentiation of Th1, Th17 and regulatory T cells (T-reg) by transferring miR-21, miR-24-3p and TGF-β.