High concentrations of glucose suppress etoposide-induced cell death of B-cell lymphoma through BCL-6
Graphical abstract
Introduction
Glucose metabolism correlates with cellular proliferation in B-cell lymphoma and influences the response to therapy [1]. Hyperglycemia is known to be an important factor in the resistance to chemotherapy of breast cancer cells [2]. In lymphoma treatment, although etoposide can trigger a process of lymphoma cell death and suppress the energy production of lymphoma cells by impairing mitochondrial function [3], some lymphoma cells can remain resistant to this chemotherapy drug and obtain energy by using glucose. Thus, the possibility of glucose-mediated resistance to etoposide-induced B-cell lymphoma cell death has been proposed.
The Bcl-6 gene encodes a POZ/zinc finger protein that acts as a transcriptional repressor. The gene is involved in a chromosomal translocation in approximately 30% of diffuse large B-cell lymphomas (DLBCL). In gastric lymphomas, high levels of BCL-6 expression predict a favorable prognosis, independent of the Bcl-6 translocation status, translocation partner, or Bcl-6-deregulating mutation [4]. Glucose and serum starvation induce BCL-6 in pancreatic β-cells and decrease cyclin D2 activity and cell proliferation [5]. The level of etoposide controls the fate of the germinal center (GC) B cells by means of Bcl-6, suggesting that Bcl-6 may play a similar role in GC B-cell-like lymphomas. BCL-6 overexpression significantly inhibits the apoptosis caused by etoposide [6] and might link glucose metabolism and cell survival to impact chemotherapy efficacy.
The key tumor-suppressor gene p53 is mutated or lost in approximately 50% of all human cancer cases worldwide. The p53 protein can activate the transcription activity of the Bcl-6 promoter by binding to the Bcl-6 promoter p53RE in vivo [7]. By inducing various cell cycle checkpoints, apoptosis or cellular senescence, p53 can restrict proliferation in response to DNA damage or deregulation of mitogenic oncogenes. Consequently, p53 mutations increase cell proliferation and survival and in some settings promote genomic instability and resistance to certain anti-cancer drugs. Glucose restriction could increase cell death in tumors with p53 impairment. Another tumor-suppressor, vitamin D3 up-regulated protein 1 (VDUP1), can interact with BCL-6 to regulate GC B cells [8]. VDUP1 not only induces oxidative stress [9], [10], [11], [12] but also regulates cellular proliferation and the aging process [13]. VDUP1 can be induced by growth arrest stimuli, leading to cell cycle arrest at the G0/G1 phase [14]. In summary, p53 and VDUP1 might be involved in the Bcl-6 mediated cell signal pathway.
In this study, we attempted to determine whether glucose induced Bcl-6 gene expression and suppressed etoposide-induced B-cell lymphoma cell death. The expression of BCL-6 in B lymphoma cells was found to be up-regulated by high levels of glucose, whereas it was down-regulated by etoposide. The underlying mechanism of the glucose-induced BCL-6 role in B-cell lymphoma chemotherapy was explored. The induction of BCL-6 by glucose was synchronized with VDUP1 expression dependent on p53. Glucose prevented etoposide-induced cell death by inducing BCL-6 and suppressing etoposide-induced apoptosis via the caspase pathway.
Section snippets
Glucose starvation treatment and mRNA stability assays
For glucose starvation experiments, all cells were cultured in complete growth medium. Upon reaching 80% confluence, the cells were washed with pre-warmed PBS and cultured in serum-free medium for different times, depending on the survival of the cell lines. Cells were cultured in glucose-DMEM medium for various times with 20 M etoposide (Sigma–Aldrich) or different concentrations of glucose (5–25 mM). All experiments were repeated three times independently.
Luciferase reporter assay
The pBcl6-Luc plasmid was transfected
Glucose prevents etoposide-induced cell death
To explore whether glucose can prevent apoptosis of B-cell lymphoma cells, we first examined the effect of glucose on the apoptosis caused by etoposide. Ramos cells were cultured in glucose-free medium and treated with or without etoposide and glucose at different times. By observing live cells by microscopy (Fig. 1A), etoposide was found to induce apoptosis. After etoposide treatment (sample name: Eto4 h), the apoptotic cells became smaller and lost their luster. The nuclei of the apoptotic
Discussion
In the present study, we identified a novel, glucose-initiated signaling pathway that prevented etoposide-induced cell death in B-cell lymphoma. For the first time, we found that glucose rapidly induced up-regulation of Bcl-6 in B-cell lymphoma cells, and we studied the underlying mechanism. In addition, we found that glucose prevented etoposide-induced cell death, and this effect was BCL-6-dependent.
A growing body of evidence indicates that impaired glycolysis may limit the available
Acknowledgment
This article is dedicated to Dr Inpyo Choi, Cell Therapy Research Center, Korea Research Institute of Bioscience and Biotechnology, Republic of Korea. The authors thank Dr Inpyo Choi for supplying all the materials and for stimulating discussions.
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