Dichloroacetate induces autophagy in colorectal cancer cells and tumours

Bypassing Mechanisms of Mitochondria-Mediated Cancer Stem Cells Resistance to Chemo- and Radiotherapy

Cancer stem cells (CSCs) are highly resistant to conventional chemo- and radiotherapeutic regimes. Therefore, the multiple drug resistance (MDR) of cancer is most likely due to the resistance of CSCs. Such resistance can be attributed to some bypassing pathways including detoxification mechanisms of reactive oxygen and nitrogen species (RO/NS) formation or enhanced autophagy. Unlike in normal cells, where RO/NS concentration is maintained at certain threshold required for signal transduction or immune response mechanisms, CSCs may develop alternative pathways to diminish RO/NS levels leading to cancer survival. In this minireview, we will focus on elaborated mechanisms developed by CSCs to attenuate high RO/NS levels. Gaining a better insight into the mechanisms of stem cell resistance to chemo- or radiotherapy may lead to new therapeutic targets thus serving for better anticancer strategies.

Metabolic interplay between glycolysis and mitochondrial oxidation: The reverse Warburg effect and its therapeutic implication

Aerobic glycolysis, i.e., the Warburg effect, may contribute to the aggressive phenotype of hepatocellular carcinoma. However, increasing evidence highlights the limitations of the Warburg effect, such as high mitochondrial respiration and low glycolysis rates in cancer cells. To explain such contradictory phenomena with regard to the Warburg effect, a metabolic interplay between glycolytic and oxidative cells was proposed, i.e., the "reverse Warburg effect". Aerobic glycolysis may also occur in the stromal compartment that surrounds the tumor; thus, the stromal cells feed the cancer cells with lactate and this interaction prevents the creation of an acidic condition in the tumor microenvironment. This concept provides great heterogeneity in tumors, which makes the disease difficult to cure using a single agent. Understanding metabolic flexibility by lactate shuttles offers new perspectives to develop treatments that target the hypoxic tumor microenvironment and overcome the limitations of glycolytic inhibitors.

Dichloroacetate, the Pyruvate Dehydrogenase Complex and the Modulation of mESC Pluripotency

Introduction

The pyruvate dehydrogenase (PDH) complex is localized in the mitochondrial matrix catalyzing the irreversible decarboxylation of pyruvate to acetyl-CoA and NADH. For proper complex regulation the E1-α subunit functions as an on/off switch regulated by phosphorylation/dephosphorylation. In different cell types one of the four-pyruvate dehydrogenase kinase isoforms (PDHK1-4) can phosphorylate this subunit leading to PDH inactivation. Our previous results with human Embryonic Stem Cells (hESC), suggested that PDHK could be a key regulator in the metabolic profile of pluripotent cells, as it is upregulated in pluripotent stem cells. Therefore, we wondered if metabolic modulation, via inexpensive pharmacological inhibition of PDHK, could impact metabolism and pluripotency.

Methods/Results

In order to assess the importance of the PDH cycle in mouse Embryonic Stem Cells (mESC), we incubated cells with the PDHK inhibitor dichloroacetate (DCA) and observed that in its presence ESC started to differentiate. Changes in mitochondrial function and proliferation potential were also found and protein levels for PDH (both phosphorylated and non-phosphorylated) and PDHK1 were monitored. Interestingly, we were also able to describe a possible pathway that involves Hif-1α and p53 during DCA-induced loss of pluripotency. Results with ESCs treated with DCA were comparable to those obtained for cells grown without Leukemia Inhibitor Factor (LIF), used in this case as a positive control for differentiation.

Conclusions

DCA negatively affects ESC pluripotency by changing cell metabolism and elements related to the PDH cycle, suggesting that PDHK could function as a possible metabolic gatekeeper in ESC, and may be a good target to modulate metabolism and differentiation. Although further molecular biology-based experiments are required, our data suggests that inactive PDH favors pluripotency and that ESC have similar strategies as cancer cells to maintain a glycolytic profile, by using some of the signaling pathways found in the latter cells.

Sensitization of Glioblastoma Cells to Irradiation by Modulating the Glucose Metabolism

Because radiotherapy significantly increases median survival in patients with glioblastoma, the modulation of radiation resistance is of significant interest. High glycolytic states of tumor cells are known to correlate strongly with radioresistance; thus, the concept of metabolic targeting needs to be investigated in combination with radiotherapy. Metabolically, the elevated glycolysis in glioblastoma cells was observed postradiotherapy together with upregulated hypoxia-inducible factor (HIF)-1α and its target pyruvate dehydrogenase kinase 1 (PDK1). Dichloroacetate, a PDK inhibitor currently being used to treat lactic acidosis, can modify tumor metabolism by activating mitochondrial activity to force glycolytic tumor cells into oxidative phosphorylation. Dichloroacetate alone demonstrated modest antitumor effects in both in vitro and in vivo models of glioblastoma and has the ability to reverse the radiotherapy-induced glycolytic shift when given in combination. In vitro, an enhanced inhibition of clonogenicity of a panel of glioblastoma cells was observed when dichloroacetate was combined with radiotherapy. Further mechanistic investigation revealed that dichloroacetate sensitized glioblastoma cells to radiotherapy by inducing the cell-cycle arrest at the G2-M phase, reducing mitochondrial reserve capacity, and increasing the oxidative stress as well as DNA damage in glioblastoma cells together with radiotherapy. In vivo, the combinatorial treatment of dichloroacetate and radiotherapy improved the survival of orthotopic glioblastoma-bearing mice. In conclusion, this study provides the proof of concept that dichloroacetate can effectively sensitize glioblastoma cells to radiotherapy by modulating the metabolic state of tumor cells. These findings warrant further evaluation of the combination of dichloroacetate and radiotherapy in clinical trials.

Dichloroacetate stimulates changes in the mitochondrial network morphology via partial mitophagy in human SH-SY5Y neuroblastoma cells

Dichloroacetate (DCA) is beneficial in cancer therapy because it induces apoptosis and decreases cancer growth in vitro and in vivo without affecting non-cancer cells. DCA stimulates the activity of the enzyme pyruvate dehydrogenase by inhibiting pyruvate dehydrogenase kinase. Consequently, DCA promotes oxidative phosphorylation after glycolysis. Therefore, DCA produces changes in energy metabolism that could affect the mitochondrial network and mitophagy. This investigation determined the effects of DCA treatment on mitophagy in human neuroblastoma SH-SY5Y cells. SH-SY5Y cells were cultured and distributed into 3 groups: control, NH4Cl and chloroquine. Each group was treated with DCA at 0, 5, 30 and 60 mM for 16 h. Samples were analyzed for cell viability, mtDNA copy number, mitochondrial network morphology and expression of key proteins involved in mitochondrial dynamics, such as LC3b, FIS1, OPA1, PARKIN and PINK1. In all groups, DCA caused a decrease in cell viability, an induction of autophagy in a dose-dependent manner and a decrease in Tim23, FIS1 and PARKIN protein expression, leading to profound morphological changes in the mitochondrial network resulting in shorter and more fragmented filaments. However, TFAM protein levels remained unchanged. Similarly, the mitochondrial copy number was not significantly different among the treatment groups. In conclusion, DCA induces mitophagy and remodeling of the mitochondrial network. In this remodeling, DCA induces a decrease in the expression of key proteins involved in protein degradation and mitochondrial dynamics but does not significantly affect the mtDNA density. Blocking late phase autophagy increases the effects of DCA, suggesting that autophagy protects the cell, at least partially, against DCA.

Restoration of mitochondria function as a target for cancer therapy

Defective oxidative phosphorylation has a crucial role in the attenuation of mitochondrial function, which confers therapy resistance in cancer. Various factors, including endogenous heat shock proteins (HSPs) and exogenous agents such as dichloroacetate, restore respiratory and other physiological functions of mitochondria in cancer cells. Functional mitochondria might ultimately lead to the restoration of apoptosis in cancer cells that are refractory to current anticancer agents. Here, we summarize the key reasons contributing to mitochondria dysfunction in cancer cells and how restoration of mitochondrial function could be exploited for cancer therapeutics.

Dual-targeting of aberrant glucose metabolism in glioblastoma

Background

Glioblastoma (GBM) is the most common and malignant primary brain tumor. In contrast to some other tumor types, aberrant glucose metabolism is an important component of GBM growth and chemoresistance. Recent studies of human orthotopic GBM in mice and in situ demonstrated GBM cells rely on both glycolysis and mitochondrial oxidation for glucose catabolism. These observations suggest that the homeostasis of energy metabolism of GBM cells might be further disturbed by dual-inhibition of glucose metabolism. The present study aimed to evaluate the efficacy and the mechanisms of dual-targeting therapy in GBM cells.

Methods

Representative GBM cells (immortalized GBM cell lines and patient-derived GBM cells) and non-cancerous cells were treated with 4-(N-(S-penicillaminylacetyl)amino) phenylarsonous acid (PENAO), an in-house designed novel arsenic-based mitochondrial toxin, in combination with dichloroacetate (DCA), a pyruvate dehydrogenase kinase inhibitor. The efficacy of this combinatorial therapy was evaluated by MTS assay, clonogenic surviving assay and apoptotic assays. The underlying mechanisms of this dual-targeting treatment were unraveled by using mitochondrial membrane potential measurements, cytosol/mitochondrial ROS detection, western blotting, extracellular flux assay and mass spectrometry.

Results

As monotherapies, both PENAO and DCA induced proliferation arrest in a panel of GBM cell lines and primary isolates. PENAO inhibited oxygen consumption, induced oxidative stress and depolarized mitochondrial membrane potential, which in turn activated mitochondria-mediated apoptosis. By combining DCA with PENAO, the two drugs worked synergistically to inhibit cell proliferation (but had no significant effect on non-cancerous cells), impair the clonogenicity, and induce mitochondria-mediated apoptosis. An oxidative stress of mitochondrial origin takes a prominent place in the mechanism by which the combination of PENAO and DCA induces cell death. Additionally, PENAO-induced oxidative damage was enhanced by DCA through glycolytic inhibition which in turn diminished acid production induced by PENAO. Moreover, DCA treatment also led to an alteration in the multidrug resistance (MDR) phenotype of GBM cells, thereby leading to an increased cytosolic accumulation of PENAO.

Conclusions

The findings of this study shed a new light with respect to the dual-targeting of glucose metabolism in GBM cells and the innovative combination of PENAO and DCA shows promise in expanding GBM therapies.

Dichloroacetate and cancer: new home for an orphan drug?

We reviewed the anti-cancer effects of DCA, an orphan drug long used as an investigational treatment for various acquired and congenital disorders of mitochondrial intermediary metabolism. Inhibition by DCA of mitochondrial pyruvate dehydrogenase kinases and subsequent reactivation of the pyruvate dehydrogenase complex and oxidative phosphorylation is the common mechanism accounting for the drug's anti-neoplastic effects. At least two fundamental changes in tumor metabolism are induced by DCA that antagonize tumor growth, metastases and survival: the first is the redirection of glucose metabolism from glycolysis to oxidation (reversal of the Warburg effect), leading to inhibition of proliferation and induction of caspase-mediated apoptosis. These effects have been replicated in both human cancer cell lines and in tumor implants of diverse germ line origin. The second fundamental change is the oxidative removal of lactate, via pyruvate, and the co-incident buffering of hydrogen ions by dehydrogenases located in the mitochondrial matrix. Preclinical studies demonstrate that DCA has additive or synergistic effects when used in combination with standard agents designed to modify tumor oxidative stress, vascular remodeling, DNA integrity or immunity. These findings and limited clinical results suggest that potentially fruitful areas for additional clinical trials include 1) adult and pediatric high grade astrocytomas; 2) BRAF-mutant cancers, such as melanoma, perhaps combined with other pro-oxidants; 3) tumors in which resistance to standard platinum-class drugs alone may be overcome with combination therapy; and 4) tumors of endodermal origin, in which extensive experimental research has demonstrated significant anti-proliferative, pro-apoptotic effects of DCA, leading to improved host survival.