New oncogenic mechanisms in lymphoma via extracellular vesicles discovered

Japanese collaborative research teams have discovered new mechanisms of action by extracellular vesicles (EV) ⁽¹⁾ for the development and malignant progression of lymphomas.

Professor Ai Kotani and Kai Kudo, a graduate student, at the Tokai University has been investigating the roles of EV in the development of Epstein-Barr virus (EBV) ⁽²⁾ -positive B-cell malignant lymphomas, but the mechanism of its action has not been fully clarified. Meanwhile, Professor Makoto Murakami and Yoshimi Miki researcher at the University of Tokyo has been clarifying a number of vital phenomena to which “secretory phospholipase A2 (sPLA₂) ⁽³⁾” is involved. sPLA₂ is a phospholipid-degrading enzyme existing in the extracellular environment; however, the source of the phospholipids that sPLA₂ uses as a substrate outside cells has been unknown.

In this study, the research groups demonstrated that sPLA₂ secreted from  tumor-associated macrophage  (TAM) ⁽⁴⁾ in malignant lymphoma tissue degrades phospholipid ⁽⁵⁾ of EV derived from tumor cells. They also found that this degradation dramatically improves EV function, such as “ease of cellular uptake” and “immunosuppressive effect,” and induction of various vital phenomena.  Moreover, they discovered that lysophospholipids, degraded products of EV phospholipids, transmit signals to cells, namely, novel mechanisms of action that have not been known in EV biology. Using model mice that reproduce the development of lymphomas in humans, the research groups demonstrated that EV-degradation by sPLA₂ is essential for tumorigenesis. Analysis of samples from patients with lymphoma also showed that sPLA₂ is involved in tumorigenesis and malignant progression.

On the other hand, the study demonstrated that not only lymphoma-derived EVs but also EVs derived from other cancer cells are degraded by sPLA₂ and revealed that the sPLA₂-EV axis is a common phenomenon in tumorigenesis. Therefore, it is expected that this sPLA₂-EV axis will become a new drug target for cancer treatment as a new “immune checkpoint.” In addition, based on sPLA₂’s EV function enhancement demonstrated in this study, they are currently testing hypotheses that sPLA₂ may enhance inherent abilities of various types of EV, such as “EV with tissue protecting and anti-inflammatory effects” previously reported by the Kotani’s group, and further development for therapeutic applications in the future is expected.

This research was conducted under the JST Strategic Basic Research Programs and the AMED Cancer Research Program in collaboration with following members. Professor Kiyoshi Ando, Professor Naoya Nakamura, and Instructor Joaquim Carreras, Tokai University School of Medicine, Associate Professor Asuka Inoue, Tohoku University, Associate Professor Kei Yamamoto, Tokushima University, Researcher Hiroshi Higuchi, Massachusetts General Hospital and Harvard Medical School, Professor Shinya Morita, Shiga University of Medical Science Hospital, and Professor Junken Aoki, The University of Tokyo.

(1) Extracellular Vesicle (EV)

A particle released from the cell, which has no nuclei and is surrounded by a lipid bilayer membrane.  In cancer research, EVs are drawing attention because of their role in transporting various proteins and nucleic acids for promoting cancer survival and malignancy.

(2) Epstein-Barr Virus (EBV)

A species of herpesvirus that has double-stranded DNA as a genome. Many EBV-related diseases are hematologic neoplasms, including malignant lymphoma. However, they are also known to cause epithelial cell cancers, such as gastric cancer in rare cases. Two viral strains, the Akata strain having strong tumorigenic potential and a weaker B95-8 strain, were used in this research.

(3) Secretory phospholipase A2 (sPLA₂)

Among two fatty acids that bind to phospholipids, the site called sn-2 position is often bound to unsaturated fatty acids, and sPLA₂ is an enzyme that cleaves this portion and liberates fatty acids and lysophospholipids from phospholipids. Since it is known that released fatty acids and lysophospholipids are converted into a potent lipid mediators known as bioactive lipids, sPLA₂ is considered the rate-limiting factor for lipid mediator synthesis. sPLA₂ is known to, in vivo, induce tissue-specific vital responses by showing distinct tissue distributions and substrate selectivities, such as braking acquired immunity through the release of omega (ω)-3 fatty acids in lymphoid tissues (sPLA₂-IID), and aggravation of asthma by promoting activation of natural immune lymphocyte ILC2 in respiratory epithelia (sPLA₂-X).

(4) Tumor-Associated Macrophage (TAM)

Macrophage is one of the essential immune cells involved in eliminating foreign substances, such as viruses and bacteria that have invaded the body. Tumor-associated macrophage is a collective term for macrophages that infiltrate into tumor tissues. M2-type macrophages that show predominantly immunosuppressive phenotype are included. Therefore, it is known that normal antitumor immunity is suppressed in tumor tissue to which this cell has infiltrated, thus promoting tumor growth.

(5) Phospholipid

Amphiphilic lipids with a hydrophilic molecule, such as choline and serine, and two fatty acids showing hydrophobicity. It is a major component of biological membranes such as plasma membranes and membranes of extracellular vesicles. Further, it has been clarified that there is a lot of combination due to any hydrophilic molecule and many kinds of two fatty acids. Therefore, there are a vast number of molecular species of phospholipid.