Human monocytes undergo excessive apoptosis following temozolomide activating the ATM/ATR pathway while dendritic cells and macrophages are resistant.

Martina Bauer,Michael Goldstein,Daniel Heylmann,Bernd Kaina,Roland Jacobs

doi:10.1371/journal.pone.0039956

Martina Bauer, Michael Goldstein + Show 3 more

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https://doi.org/10.1371/journal.pone.0039956

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Abstract

Immunodeficiency is a severe therapy-limiting side effect of anticancer chemotherapy resulting from sensitivity of immunocompetent cells to DNA damaging agents. A central role in the immune system is played by monocytes that differentiate into macrophages and dendritic cells (DCs). In this study we compared human monocytes isolated from peripheral blood and cytokine matured macrophages and DCs derived from them and assessed the mechanism of toxicity of the DNA methylating anticancer drug temozolomide (TMZ) in these cell populations. We observed that monocytes, but not DCs and macrophages, were highly sensitive to the killing effect of TMZ. Studies on DNA damage and repair revealed that the initial DNA incision was efficient in monocytes while the re-ligation step of base excision repair (BER) can not be accomplished, resulting in an accumulation of DNA single-strand breaks (SSBs). Furthermore, monocytes accumulated DNA double-strand breaks (DSBs) following TMZ treatment, while DCs and macrophages were able to repair DSBs. Monocytes lack the DNA repair proteins XRCC1, ligase IIIα and PARP-1 whose expression is restored during differentiation into macrophages and DCs following treatment with GM-CSF and GM-CSF plus IL-4, respectively. These proteins play a key role both in BER and DSB repair by B-NHEJ, which explains the accumulation of DNA breaks in monocytes following TMZ treatment. Although TMZ provoked an upregulation of XRCC1 and ligase IIIα, BER was not enhanced likely because PARP-1 was not upregulated. Accordingly, inhibition of PARP-1 did not sensitize monocytes, but monocyte-derived DCs in which strong PARP activation was observed. TMZ induced in monocytes the DNA damage response pathways ATM-Chk2 and ATR-Chk1 resulting in p53 activation. Finally, upon activation of the Fas-receptor and the mitochondrial pathway apoptosis was executed in a caspase-dependent manner. The downregulation of DNA repair in monocytes, resulting in their selective killing by TMZ, might impact on the immune response during cancer chemotherapy.

Highlights

Immunosuppression is one of the most severe side effects of chemotherapy endangering lives of patients who undergo medical cancer treatment
In order to study the TMZ sensitivity and DNA damage response in human monocytes and their derivatives, macrophages and immature dendritic cells (DCs), monocytes were isolated from peripheral blood of healthy donors and either left untreated or treated with IL-4 and GM-CSF or GM-CSF alone in order to induce the differentiation of DCs or macrophages, respectively (Fig. 1A showing the typical shape and surface staining of monocytes and macrophages with CD14)
Treatment with the methylating anticancer drug TMZ resulted in a significant induction of apoptosis in monocytes, while DCs and macrophages were resistant to this agent (Fig. 1C)

Summary

Introduction

Immunosuppression is one of the most severe side effects of chemotherapy endangering lives of patients who undergo medical cancer treatment. The high proliferation rate of the immune response progenitor cells is considered accountable for their sensitivity to DNA damaging agents that are used for cancer treatment. Little attention has been paid yet to the toxicity of chemotherapeutic drugs in mature immune response cells. Originating from bone marrow precursor cells mature monocytes enter the blood stream where they circulate for several days [1]. After entering the tissue they differentiate into DCs and macrophages, both of which play an important role in the immune response. In the course of the current research we investigated the mechanism of cytotoxicity of the chemotherapeutic anticancer drug temozolomide (TMZ, Temodar) in human monocytes freshly isolated from peripheral blood.

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