Metabolic Reprogramming in Oral Squamous Cell Carcinoma

Lai WT^1,2, Wu TS^1,2 , Li YJ^1,2 and Cheng CC^1-6*

¹Graduate Institute of Clinical Dentistry, School of Dentistry, National Taiwan University, Taiwan

²Department of Dentistry, National Taiwan University Hospital. Taiwan

³Graduate Institute of Oral Biology, School of Dentistry, National Taiwan

⁴Angiogenesis Research Center, National Taiwan University, Taiwan

⁵Department of Medical Research, China Medical University, Taiwan

⁶Department of Biotechnology, Asia University, Taiwan

*Corresponding author: Cheng-Chi Chang, Graduate Institute of Oral Biology, School of Dentistry, National Taiwan University, Taiwan

Received: February 26, 2016; Accepted: March 28, 2016; Published: March 30, 2016

Abstract

Oral Squamous Cell Carcinoma (OSCC) is the sixth most common human malignancy worldwide. Metabolic reprogramming is one of the hallmarks of cancer, and metabolic change favors rapid energy production and biosynthetic capabilities. The energy adaptation pathways promote tumor cells to survive, proliferation, and metastasis, which may be induced by hypoxia, free radicals, and nutrient depletion from microenvironment. The stresses of microenvironment also cause cancer cell autophagy, which is the major way to escape from cell death. Thus, many metabolic enzymes have become potential targets for new cancer therapies. Uncovering the intrinsic and extrinsic mechanisms that control the maintenance of cancer metabolism and autophagy is critical for developing novel therapeutic strategies to target cancer progression and recurrence.

Keywords: Oral squamous cell carcinoma; Cancer metabolism; Mitochondrion; Autophagy

Abbreviations

OSCC is one of the 10 most frequent cancers worldwide [1-3], and five-year survival rate is less than 50% [1]. The reason of high mortality in OSCC is metastasis, a process that cancer cells spread from a primary site and form tumors at proximal lymph nodes or distant sites [4-8]. Metabolic reprogramming plays crucial roles in cancer progression, including metastasis. Tumor cells exhibit an altered metabolism that is characterized by elevated uptake of glucose and increased glycolytic rate; this observation was first reported by Otto Warburg [10]. Cancer cells generated the majority of ATP by glycolysis, even when grown in the presence of oxygen. However, recent studies have revealed the additional energy generation dependent on mitochondrial biogenesis [11].To addresses this issue; we aim to review the cancer metabolism and autophagy in OSCC progression. How does cancer cells adapt to microenvironment by using biogenetic reprogramming as a cell survival strategy? How does biogenetic reprogramming modulate a series of oncogenic and/or tumor suppressive signaling pathways? Understanding the metabolic pathways in tumors could contribute to the identification of novel therapeutic target and the development of more effective cancer therapeutic.

Cancer metabolism in microenvironment

Glycolysis and mitochondrial Oxidative Phosphorylation (OXPHOS) are the two main metabolic pathways to generate ATP. Glycolysis produces pyruvate which moving into the mitochondria converts to Acetyl-coenzyme A (acetyl-CoA), then enter the Tricarboxylic Acid (TCA) cycle and OXPHOS, generated up to 36 ATPs upon complete oxidative of one glucose molecule [12]. During OXPHOS, oxygen is reduced to water in mitochondrial Electron Transport Chain (ETC). Under the hypoxia conditions, pyruvate is converted to lactate which completes glycolysis cycle and triggers Warburg effect. As the early tumor expands, cancer cells are exposed to hypoxia condition. Consequently, tumor hypoxia is a poor prognostic factor in malignancy [13-15]. Hypoxia-inducible factor- 1a (HIF-1a) is a transcription factor that increases glycolytic capacity and decreases mitochondrial respiration in OSCC [16,17]. Decreased dependence on aerobic respiration becomes advantageous to tumor cells. It also stimulates angiogenesis by up regulating Vascular Endothelial Growth Factor (VEGF) [18]. Under normoxic conditions, the protein is tightly controlled by ubiquitin-dependent degradation; binding to the von Hippel-Lindau (pVHL) tumor suppressor protein [19]. Loss of VHL expression was closely associated with pathologic grading, lymph node metastasis, poor prognosis, and EMT in OSCC [20]. HIF-1a binds to pVHL only after it is hydroxylated by HIF Prolyl hydroxylase (PHD1) [21-23] and acetylated by Arrest-defective 1 protein (ARD1) acetyltransferase [24]. This posttranslational modification (i.e., hydroxylation and acetylation) of HIF-1a protein promotes its association with pVHL and subsequent degradation [25-27]. The Warburg effect that is, an uncoupling of glycolysis from oxygen levels, cannot be explained solely by upregulation of Glycolysis and mitochondrial Oxidative Phosphorylation (OXPHOS) are the two main metabolic pathways to generate ATP. Glycolysis produces pyruvate which moving into the mitochondria converts to Acetyl-coenzyme A (acetyl-CoA), then enter the Tricarboxylic Acid (TCA) cycle and OXPHOS, generated up to 36 ATPs upon complete oxidative of one glucose molecule [12]. During OXPHOS, oxygen is reduced to water in mitochondrial Electron Transport Chain (ETC). Under the hypoxia conditions, pyruvate is converted to lactate which completes glycolysis cycle and triggers Warburg effect. As the early tumor expands, cancer cells are exposed to hypoxia condition. Consequently, tumor hypoxia is a poor prognostic factor in malignancy [13-15]. Hypoxia-inducible factor- 1a (HIF-1a) is a transcription factor that increases glycolytic capacity and decreases mitochondrial respiration in OSCC [16,17]. Decreased dependence on aerobic respiration becomes advantageous to tumor cells. It also stimulates angiogenesis by up regulating Vascular Endothelial Growth Factor (VEGF) [18]. Under normoxic conditions, the protein is tightly controlled by ubiquitin-dependent degradation; binding to the von Hippel-Lindau (pVHL) tumor suppressor protein [19]. Loss of VHL expression was closely associated with pathologic grading, lymph node metastasis, poor prognosis, and EMT in OSCC [20]. HIF-1a binds to pVHL only after it is hydroxylated by HIF Prolyl hydroxylase (PHD1) [21-23] and acetylated by Arrest-defective 1 protein (ARD1) acetyltransferase [24]. This posttranslational modification (i.e., hydroxylation and acetylation) of HIF-1a protein promotes its association with pVHL and subsequent degradation [25-27]. The Warburg effect that is, an uncoupling of glycolysis from oxygen levels, cannot be explained solely by upregulation of 1a. These oxygen-independent mechanisms to activate oncogenesis tumor suppressor genes still need further investigations.

Important signaling pathways in OSCC metabolism

Cancer progression is dependent on the reprogramming of metabolism. Not only may the tumor microenvironment select for altered metabolic pathways, but also can oncogenes drive metabolic changes. The signaling molecule, Ras, a powerful oncogene when mutated, promotes glycolysis [28,29]. Ras was also confirmed as a direct target of miR-206, an important regulator in OSCC to reduce proliferation and invasion/migration [30]. The phosphoinositide 3-kinase (PI3K) is one of the most activated signaling pathways, which links oncogenesis and glucose metabolism in OSCC [31,32]. Mutations in PI3K could provide strong growth and survival signals to tumor cells, and contribute to oncogene activation of the AKT pathway [33,34]. AKT1 is the crucial driver of the aerobic glycolysis pathway to stimulate ATP production, which ensures cells that have bioenergetic capacity to respond to growth signals [35,36]. AKT1 stimulates glycolysis by increasing the membrane translocation of glucose transporters and phosphorylating key glycolytic enzymes, such as hexokinase II (HK II) and Phosphofructokinase 2 (PFK2) [37,38]. In addition to its well-described roles in controlling cell growth and proliferation, the downstream transcription factor, Myc, also has several important effects on OSCC metabolism [39,40], including glutaminolysis [41]. It may predict OSCC patients with poor prognosis [42]. Regulation of metabolism is involved in tumor suppressors, such as p53 [43]. p53 inhibits the glycolytic pathway by declined the transcription activity of the glucose transporter 1 (GLUT1) [44]. Loss of the tumor suppressor protein p53 resulted in Warburg effect. Clinically, numbers of OSCC patients have p53 mutation [45]. Taken together, tumor microenvironment may induce or interact oncogenes and tumor suppressor genes to drive metabolic shifts resulted in OSCC initiation, development, and progression.

Key enzymes of glucose metabolism in OSCC

Glucose is the major source of energy for cells, and GLUT1 is the most important transporter to facilitate the glucose transportation crossing the plasma membranes in humans [46]. GLUT1 is aberrantly expressed in several tumor types. Studies have implicated its expression as a prognostic and diagnostic marker in OSCC clinically [47]. Reduction of VHL [48] and miR-340 [49] playas the switches to contribute the glucose uptake in OSCC by regulating GLUT1 expression.HK are a family of enzymes that catalyze the first phosphorylation of glucose to glucose-6-phosphate.HK II binds to mitochondria is via the outer membrane protein known as the voltage-dependent anion channel VDAC [50]. It has been shown that high expression of HK II correlated with poor prognosis in OSCC, and the precise mechanism is still under investigation [51-52]. PKM2 is involved in OSCC initiation and progression by promoting cell proliferation and migration, and reducing apoptosis critically. Overexpression of PKM2 correlates with aggressive clinicopathological features and poor patients’ clinical outcome [53]. In cancer metabolism, lactate is also important in glucose pathway. Lactate is made from pyruvate by lactate dehydrogenase (LDH) enzyme. In tumor microenvironment, excess lactate is secreted, and contributes to an extracellular environment to promote OSCC progression [54]. The key enzyme, LDH, plays as a potential diagnostic marker and therapeutic index in OSCC [55,56]. Collectively, these studies support a model that metabolites, like lactate, facilitate malignant cancer development and metastasis, and could be the potential targets therapeutically in the future.

The role of mitochondria in cancer metabolism

The traditional view of cancer metabolism relying on glycolysis is due to mitochondrial dysfunction. However, the role of mitochondrial metabolism in modulating cancer progression is developed. Recent studies indicated that mitochondrial activity is essential for cancer cells. Mitochondrion plays important roles as energetic centers. Increased mitochondrial biogenesis promotes tumorigenesis, and loss of Mitochondrial DNA (mtDNA) copy number leads to decrease tumorigenesis due to OXPHOS impairment [57]. Many evidence showed that elevated OXPHOS and mitochondrial activity is associated with cancer aggressiveness. As we know, mitochondrial transcription factor A (mtTFA/TFAM) is necessary for mtDNA maintenance, mitochondrial function and morphology [58-60]. Recent studies indicated that mtTFA significantly correlates to cancer behavior [61,62]. Its expression also highly associated with tumor progression and poor prognosis of patients with endometrial adenocarcinoma [63], and colorectal adenocarcinoma [64]. Loss of mtTFA also inhibited Kras-mediated lung tumorigenesis [65]. Enhanced mitochondrial biogenesis via mtTFA results in aberrant cell proliferation in arsenical skin cancer [66-67]. These data indicate that increased mtTFA may provide more energy for cancer progression. In our preliminary study, we found that advanced invasive OSCC cells, such as SAS and CA922 cells, showed more elevated oxygen consumption rate than less motility cells, including TW2.6 and Cal27 (Figure 1).

Treatment	Advantage	Disadvantage	Cancer Type
Irradiation	Genotoxic stress	Autophagy inducer	Breast cancer and OSCC^88,89
Cisplatin	Genotoxic stress	Autophagy inducer	Esophageal cancer and OSCC^90,91
Doxorubicin	Genotoxic stress	Autophagy inducer	Breast cancer and OSCC^92,93
5-Fluorouracil	Thymidylate synthase inhibitor	Autophagy inducer	CRC and OSCC^91-94
Gefitinib	EGFR tyrosine kinase inhibitor	Autophagy inducer	NSCLC and OSCC^95,96
Erbitux	EGFR monoclonal antibody	Autophagy inducer	NSCLC and OSCC^97,98