Cancer Letters

Cancer Letters

Volume 299, Issue 1, 18 December 2010, Pages 72-79
Cancer Letters

Phospholipase D-mTOR requirement for the Warburg effect in human cancer cells

https://doi.org/10.1016/j.canlet.2010.08.006Get rights and content

Abstract

A characteristic of cancer cells is the generation of lactate from glucose in spite of adequate oxygen for oxidative phosphorylation. This property – known as the “Warburg effect” or aerobic glycolysis – contrasts with anaerobic glycolysis, which is triggered in hypoxic normal cells. The Warburg effect is thought to provide a means for cancer cells to survive under conditions where oxygen is limited and to generate metabolites necessary for cell growth. The shift from oxidative phosphorylation to glycolysis in response to hypoxia is mediated by the production of hypoxia-inducible factor (HIF) – a transcription factor family that stimulates the expression of proteins involved in glucose uptake and glycolysis. We reported previously that elevated phospholipase D (PLD) activity in renal and breast cancer cells is required for the expression of the α subunits of HIF1 and HIF2. We report here that the aerobic glycolysis observed in human breast and renal cancer cells is dependent on the elevated PLD activity. Intriguingly, the effect of PLD on the Warburg phenotype was dependent on the mammalian target of rapamycin complex 1 (mTORC1) in the breast cancer cells and on mTORC2 in the renal cancer cells. These data indicate that elevated PLD-mTOR signaling, which is common in human cancer cells, is critical for the metabolic shift to aerobic glycolysis.

Introduction

A hallmark of cancer cells is aerobic glycolysis whereby there is an increased utilization of glucose and glycolysis for energy and the raw materials needed for cell growth [1]. This effect is commonly referred to as the Warburg effect after its discoverer [2], [3]. Glycolysis generates the precursors needed for the synthesis of lipids and nucleotides for generating membranes and nucleic acids [4]. A shift away from mitochondrial respiration also occurs as a response to the stress of hypoxia where oxidative phosphorylation is not an option [5]. Much of the response to hypoxia is due to elevated expression of hypoxia-inducible factor-α (HIFα) – a family of transcription factors that stimulate the expression glycolytic and angiogenic genes [5]. HIFα expression is elevated in a significant percentage of human cancers [6].

The expression of the α subunits for both HIF1 and HIF2 is dependent upon phospholipase D (PLD) in human kidney and breast cancer cells [7], [8]. Elevated PLD activity in human cancer cells provides both survival and migration signals [8], [9]. The primary metabolite of PLD is phosphatidic acid (PA) and it is required for the activation of the mammalian target of rapamycin (mTOR) [10], [11], [12], which has also been implicated in survival signals and HIFα expression [13], [14], [15]. mTOR has been implicated as a sensor of nutritional sufficiency and elevated mTOR promotes cell cycle progression when there is sufficient nutrition for cells to double their mass and divide [16], [17]. Thus, there is a connection between PLD-mTOR survival signals and the Warburg effect in cancer cells. We have investigated whether the Warburg effect is dependent on PLD-mTOR signaling in human cancer cells.

Section snippets

Cells, cell culture conditions and transfection

The 786-O, MDA-MB-231, MCF-7, and HEK293 cells used in this study were obtained from the American Type Culture Collection. All cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM) with 10% fetal bovine serum. Transfections were performed using Lipofectamine LTX (Invitrogen) according to the vendor’s instructions.

Materials

Antibodies against mTOR, Rictor, Raptor, HIF2α, GLUT1, Actin, and hemagglutinin (HA) were obtained from Santa Cruz Biotechnology; antibodies against Akt1, Akt2, GLUT3 and

Elevated glucose uptake in human cancer cells is dependent on PLD activity

Glucose uptake was examined in four human cell lines – two breast cancer cell lines (MCF-7 and MDA-MB-231), a kidney cancer cell line (786-O), and HEK293 human embryonic kidney cells. These cells have been analyzed previously for their PLD activity with MDA-MB-231 cells having high levels of PLD activity relative to the MCF-7 cells, and 786-O cells having high levels of PLD activity relative to the HEK293 cells [7], [22], [23]. This is shown graphically in Fig. 1A. The level of glucose uptake

Discussion

The “metabolic transformation” [1] that takes place in most cancer cells has attracted renewed attention as it has become apparent that the altered metabolism is closely integrated with oncogenic transformation. The metabolic changes that occur in cancer cells confer several advantages that allow cells to survive in an emerging tumor mass where there is inconsistent vascularization. We reported previously that elevated PLD activity in renal cancer cells is required for the expression of both

Conflict of interest

There are no conflicts of interest regarding this paper.

Acknowledgments

We thank Michael Frohman (SUNY, Stony Brook) for the PLD genes used in this study. This work was supported by grant from the National Cancer Institute (CA46677) (DAF), and grants from the Canadian Cancer Society of the National Cancer Institute of Canada (MO). Research Centers in Minority Institutions (RCMI) award RR-03037 from the National Center for Research Resources of the National Institutes of Health, which supports infrastructure and instrumentation in the Biological Sciences Department

References (41)

Cited by (0)

1

Present address: New York University Cancer Institute, New York University School of Medicine, New York, NY 10016, United States.

2

Present address: Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel.

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