Exosomes - Pioneers and transporters in the hypoxic tumor microenvironment

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Non-small cell lung cancer (NSCLC) is one of leading cause of fatal malignant tumors. Radiotherapy is still one of the main treatment methods. However, radioresistance greatly limits the efficacy of radiotherapy 1. Therefore, it is increasingly urgent to decode the molecular mechanism of radioresistance and find new targets of individualized radiotherapy. Recently, exosomes have been widely studied due to their easy availability, stable existence and ability to carry a variety of important signaling factors such as proteins, RNA, DNA and lipids 2. At present, exosomes have become a hot research field of radiotherapy in mediating radiobiological effect between irradiated and unirradiated bystander cells. As an intermediator, exosome participates in a variety of physiological and pathological processes. Tumor-derived exosomes can be considered as a promising vehicle for regulating cellular radiosensitivity 3.

Tumor microenvironment (TME) is a unique environmental network composing of variety of non-malignant cells, tumor cells, extracellular matrix (ECM), surrounding blood vessels, and cell secreted products such as exosomes, cytokines and chemokines 4, which can be maintained and affected by many factors. Hypoxia, a prominent feature of TME, has long been considered as a critical regulator of radioresistance 5. In solid tumors, hypoxic and normoxic regions exist alternatively or coexist locally. Among them, hypoxia can alter the expression of pro-survival genes and then mediate cell-cell interaction through extracellular environmental factors 6. Hypoxia adapts NSCLC cells to the environment through a variety of mechanisms, makes NSCLC more survivable and aggressive, and then more prone to metastasis and radioresistance. Although hypoxic cells themselves have been extensively studied, the mechanism underlying the distal transfer of radioresistance mediated by the crosstalk between hypoxic cells and surround normoxic cells under TME remains elusive 7.

As a secretory glycoprotein, ANGPTL4 (angiopoietin-like 4 protein) is widely regarded as a decisive regulator of angiogenesis and an inflammatory carcinogenic mediator that participates in the occurrence of human cancer in the manner of autocrine and paracrine activity8-10. However, so far, there is no mechanism study on the role of exosomes carrying ANGPTL4 in tumor microenvironment.

Through TMT proteomic analysis, we found that hypoxia significantly induced the high expression of ANGPTL4 in lung cancer cells and corresponding exosomes. In addition, the high expression of ANGPTL4 and HIF-1α (a classical hypoxic marker) have co-regional characteristics and significantly mediate the radioresistance of hypoxic NSCLC cells. Further studies showed that ferroptosis played an important role in the radioresistance of hypoxic tumor cells mediated by ANGPTL4. Hypoxic-induced ANGPTL4 further promoted the expression of key negative regulatory proteins of ferroptosis such as GPX4, FTH1 and FLH, and thus inhibited the occurrence of ferroptosis, which further mediated the radioresistance of hypoxic tumor cells. Interestingly, ANGPTL4 encapsulated in the exosomes released from hypoxic cells could be absorbed by neighboring normoxic cells, resulting in radioresistance of the bystanders in a GPX4 dependent manner, which was diminished when the expression level of exosomal ANGPTL4 was downregulated. These results revealed for the first time that ANGPTL4 not only contributes to the radioresistance of hypoxic tumor cells but also improved the radioresistance of adjacent normoxic cells due to the transports of exosomes within the TME, suggesting that ANGPTL4 might applicable as a promising biomarker and therapeutic target to improve the therapeutic efficacy of NSCLC. 

Our results confirmed the important role of exosomes in the TME and laterally revealed its potential implication as a diagnostic and therapeutic target.

References

1     Vinod S K & Hau E. Radiotherapy treatment for lung cancer: Current status and future directions. Respirology 25 Suppl 2, 61-71 (2020).

2     Zhang N, Nan A, Chen L, Li X, Jia Y, Qiu M et al. Circular RNA circSATB2 promotes progression of non-small cell lung cancer cells. Mol Cancer 19, 101 (2020).

3     Yue X, Lan F & Xia T. Hypoxic Glioma Cell-Secreted Exosomal miR-301a Activates Wnt/β-catenin Signaling and Promotes Radiation Resistance by Targeting TCEAL7. Mol Ther 27, 1939-1949 (2019).

4     Suwa T, Kobayashi M, Nam J M & Harada H. Tumor microenvironment and radioresistance. Exp Mol Med 53, 1029-1035 (2021).

5     Chen F, Xu B, Li J, Yang X, Gu J, Yao X et al. Hypoxic tumour cell-derived exosomal miR-340-5p promotes radioresistance of oesophageal squamous cell carcinoma via KLF10. J Exp Clin Cancer Res 40, 38 (2021).

6     Barker H E, Paget J T, Khan A A & Harrington K J. The tumour microenvironment after radiotherapy: mechanisms of resistance and recurrence. Nat Rev Cancer 15, 409-425 (2015).

7     Wilson W R & Hay M P. Targeting hypoxia in cancer therapy. Nat Rev Cancer 11, 393-410 (2011).

8     Kolb R, Kluz P, Tan Z W, Borcherding N, Bormann N, Vishwakarma A et al. Obesity-associated inflammation promotes angiogenesis and breast cancer via angiopoietin-like 4. Oncogene 38, 2351-2363 (2019).

9     Lichtenstein L, Mattijssen F, de Wit N J, Georgiadi A, Hooiveld G J, van der Meer R et al. Angptl4 protects against severe proinflammatory effects of saturated fat by inhibiting fatty acid uptake into mesenteric lymph node macrophages. Cell Metab 12, 580-592 (2010).

10    Shen C J, Chang K Y, Lin B W, Lin W T, Su C M, Tsai J P et al. Oleic acid-induced NOX4 is dependent on ANGPTL4 expression to promote human colorectal cancer metastasis. Theranostics 10, 7083-7099 (2020).

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