HDAC Inhibition with Valproate Improves Direct Cytotoxicity of Monocytes against Mesothelioma Tumor Cells

Our plain-English summary of HDAC Inhibition with Valproate Improves Direct Cytotoxicity of Monocytes against Mesothelioma Tumor Cells, by C. Hoyos, A. Fontaine, J.R. Jacques, et al (Cancers, 2022).

The study

• Authors: C. Hoyos, A. Fontaine, J.R. Jacques, V. Heinen, R. Louis, B. Duysinx, A. Scherpereel, E. Wasielewski, M. Jamakhani, M. Hamaidia, L. Willems
• Journal: Cancers
• Published: 2022;14(9)
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The composition of the tumor microenvironment (TME) mediates the outcome of chemo- and immunotherapies in malignant pleural mesothelioma (MPM). Tumor-associated macrophages (TAMs) and monocyte myeloid-derived immunosuppressive cells (M-MDSCs) constitute a major fraction of the TME. As central cells of the innate immune system, monocytes exert well-characterized functions of phagocytosis, cytokine production, and antibody-dependent cell-mediated cytotoxicity (ADCC). The objective of this study was to evaluate the ability of monocytes to exert a direct cytotoxicity by cell-to-cell contact with MPM cells. The experimental model is based on cocultures between human blood-derived monocytes sorted by negative selection and mesothelioma cell lines. Data show (i) that blood-derived human monocytes induce tumor cell death by direct cell-to-cell contact, (ii) that VPA is a pharmacological enhancer of this cytotoxic activity, (iii) that VPA increases monocyte migration and their aggregation with MPM cells, and (iv) that the molecular mechanisms behind VPA modulation of monocytes involve a downregulation of the membrane receptors associated with the M2 phenotype, i.e., CD163, CD206, and CD209. These conclusions, thus, broaden our understanding about the molecular mechanisms involved in immunosurveillance of the tumor microenvironment and open new prospects for further improvement of still unsatisfactory MPM therapies.

In plain English

This was a laboratory study of cells grown in dishes. It was not a study of people or of asbestos exposure. The authors tested whether monocytes, a type of white blood cell, can kill malignant pleural mesothelioma cells by direct contact, and whether the drug valproate (VPA), an HDAC inhibitor, changes this. They mixed mesothelioma cell lines, mainly the epithelioid line M14K, with either a leukemia-derived monocyte cell line (THP-1) or monocytes purified from the blood of healthy donors. In time-lapse imaging, M14K cell death did not differ significantly with or without untreated THP-1 cells, but VPA significantly increased THP-1 killing of M14K cells. Blood-derived monocytes killed M14K cells after brief contact, particularly with a sub-toxic VPA dose (2 mM). VPA also increased how far and how fast the monocytes moved, and how much they clumped with tumor cells. In monocytes from 3 donors, those cocultured with VPA had lower expression of genes for three M2-linked receptors (CD163, CD206, CD209) than uncultured monocytes. The authors concluded that VPA enhances this cytotoxic activity. They said primary monocytes still need testing against other mesothelioma subtypes.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) cell-culture study: human monocytes cocultured with mesothelioma cell lines, with time-lapse imaging, flow cytometry, confocal microscopy and RNA sequencing
• Who was studied: Mesothelioma cell lines M14K (epithelioid subtype) and ZL34 (non-epithelioid subtype); the THP-1 monocytic cell line, which comes from a child with acute monocytic leukemia; and monocytes isolated from buffy coats of healthy blood donors (Red Cross of Belgium). RNA sequencing used monocytes from 3 different healthy donors.
• Exposure: Drug treatments, not asbestos. THP-1 cells were treated for 48 h with valproate (VPA, an HDAC inhibitor) at 2.5 mM, the EZH2 inhibitor EPZ005687 (EPZ) at 10 µM, or PMA at 16.2 µM. Primary monocytes were cocultured with M14K cells in the presence of VPA at 2 mM.
• Main results:
  • In time-lapse imaging, the percentage of apoptotic M14K cells did not differ significantly between no monocytes, untreated THP-1 and PMA-differentiated THP-1. Flow cytometry, run at a different cell ratio, did show an effect of PMA.
  • VPA significantly increased THP-1 killing of M14K cells. The EZH2 inhibitor EPZ had no statistically significant effect in eight independent experiments.
  • Blood-derived monocytes killed M14K cells after a transient contact, particularly in the presence of a sub-toxic dose of VPA (2 mM).
  • VPA increased the total migration and average speed of blood-derived monocytes and induced clusters between monocytes and M14K cells.
  • Coculture alone for 24 h (D1 vs. D0) produced no significantly differentially expressed genes in monocytes.
  • In monocytes cocultured with VPA compared with uncultured monocytes (D1+VPA vs. D0), genes for receptors associated with the M2 phenotype were downregulated: CD163 (Log2(Fold Change) −1.09; p-adj 0.05), CD206 (Log2(Fold Change) −1.13; p-adj 0.05) and CD209 (Log2(Fold Change) −1.64; p-adj 0.03).
• The authors' conclusion: The authors concluded that blood-derived human monocytes induce tumor cell death by direct cell-to-cell contact, and that VPA enhances this activity, increases monocyte migration and their aggregation with MPM cells, and downregulates M2-associated receptors (CD163, CD206, CD209). They said this opens new prospects for improving MPM therapies.
• Limitations the authors state:
  • The THP-1 cell line only imperfectly reproduces the characteristics of primary monocytes.
  • Primary monocytes were tested against epithelioid M14K cells. Experiments on other mesothelioma subtypes (sarcomatoid and biphasic) are still planned.
  • The experiments did not include human serum or antibodies against tumor antigens, which excluded antibody-dependent killing (ADCC).
  • Whether the genes changed by VPA are involved in monocyte cytotoxicity was not tested.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma, Tumor Microenvironment, Pleural Mesothelioma, Macrophage, Phagocytosis, Cytokine.

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