**4. Energy metabolism in oral cancer**

#### **4.1. Glucose metabolism in oral cancer**

apoptosis. Holistically, our results also suggest that there may be no correlation between the expression of TRAIL receptor and sensitivity to TRAIL-induced apoptosis in HOSCC cell lines and that TRAIL-resistant cells (HSC-2, HSC-3, HSC-4, and Ca9-22) may express cytoprotective proteins that block TRAIL-induced apoptosis or that the apoptotic effect of TRAIL is regulated by other mechanisms. It has also been described that TRAIL, in combination with an anticancer drug, acts cooperatively to induce apoptosis in various cancer cells that are resistant to TRAIL or chemotherapy [75, 82, 83]. This combination of TRAIL with chemotherapeutic reagents might be a useful therapeutic strategy against TRAIL-resistant cell lines such

In fact, we are currently investigating the synergistic effects of α-mangostin and TRAIL on induction of apoptosis via the mitochondrial pathway in squamous cell carcinoma of the oral cavity [84]. To summarize, mangosteen (*Garcinia mangostana*) is a tree discovered in Southeast Asia, and the pericarp of its fruit has been used in folk medicine for the treatment of many human diseases. The rinds of mangosteen fruit contain a high concentration of xanthone, a type of polyphenol. One form of xanthone, α-mangostin, has been described to exhibit chemopreventive effects against chemically induced colon cancer through a decrease of c-Myc expression, suppressing tumor growth in a mouse model of mammary cancer. A recent study has proved the inhibitory effect of α-mangostin on the growth of prostate cancer. However, it is still unclear whether α-mangostin induces cell death in oral cancer. Then, the present study examined the impact of α-mangostin on HOSCC. First, we analyzed the expression of c-Myc in five HOSCC cell lines (HSC-2, HSC-3, HSC-4, Ca9-22, and SAS). The highest level of c-Myc mRNA expression was found in SAS cells and the lowest in HSC-4 cells. Therefore, SAS cells were treated with α-mangostin, which was observed to exert a weak cytocidal effect. Since α-mangostin has been described to exert synergistic effects on cancers when combined with anticancer drugs, we tried to evaluate these synergistic effects of α-mangostin in combination with TRAIL. We found that this combination induced apoptosis in SAS cells through the mitochondrial pathway via activation of caspase-3/caspase-7 and caspase-9, following the release of cytochrome c. In addition, this apoptosis was induced by S/G2/M-phase arrest. Immunoreactivity for c-Myc was revealed in the cytoplasm of cancer cells in 16 (40%) of the 40 cases of HOSCC. These data showed that the combination of α-mangostin and TRAIL may

Further investigation of TRAIL-mediated cell death, including the interaction of TRAIL and its receptors in oral cancer cells under various conditions, will be required to establish a strat-

The tyrosine kinase receptor c-Met ordinarily binds with hepatocyte growth factor (HGF), which triggers its involvement in processes such as cell differentiation, cell growth, angiogenesis, and embryogenesis [85, 86]. However, c-Met activation is also associated with processes related to malignant transformation, such as invasion, tumor growth, angiogenesis, and metastasis [87–91]. In addition to autocrine or paracrine signaling via HGF, c-Met may also be activated via the mutation, protein overexpression or amplification, or transcriptional alteration (via

egy for TRAIL-based oral cancer therapy, which does not cause liver toxicity.

as HSC-2, HSC-3, HSC-4, and Ca9-22 used in our investigation.

92 Prevention, Detection and Management of Oral Cancer

have considerable potential for the treatment of oral cancer.

**3.2. Hepatocyte growth factor (HGF) and its receptor, c-Met**

In normal tissues, adenosine triphosphate (ATP) is mainly produced in mitochondria via complete oxidative phosphorylation (OXPHOS) of glucose. Conversely, only 10% of ATP is produced from glycolysis in which glucose is replaced to lactate [97]. Interestingly, cancer tissues possess high levels of glycolysis in the cytosol even under aerobic conditions, which is upregulated by PI3K/Akt signaling in the mitochondria, a phenomenon known as the "Warburg effect" or "aerobic glycolysis" [98–103]. More than 80 years ago, Otto Warburg

[109–113]. In addition to their role as structural components, lipids also act as energy resources and as signaling molecules to sustain cell growth [114–116]. Lipid metabolism is known to be largely altered in cancers [117–120], and worsening lipogenesis has been indicated to be a predominant characteristic of most tumors [114, 115, 121]. In oral cancer, cancer tissues include higher levels of unsaturated fatty acids than those in normal tissue (data not shown). Recent studies have revealed intrinsic molecular alterations in lipid metabolism. Especially, fatty acid synthase (FAS) is a key enzyme for synthesis of fatty acid from acetyl CoA, which is expressed at high levels in the adipose tissue and liver but at low levels in other tissues in humans [122]. FAS is overexpressed in several human cancers, including those of the ovary, bladder, stomach, breast, lung, prostate, oral cavity, and melanoma, and this overexpression is associated with poor prognosis [123, 124]. Furthermore, glutathione peroxidase 4 (GPX4) expression in tumors is positively correlated with tumor survival and linked to pathways that regulate cell proliferation, motility, and tissue remodeling [125]. Knockdown of GPX4 suppresses the formation and progression of cancer and leads to non-apoptotic cell death, ferroptosis. We have also examined the role of GPX4 in HOSCC (data not shown). Ferroptosis is a non-apoptotic form of cell death that can be triggered by conditions or small molecules that inhibit the glutathione-dependent antioxidant enzyme GPX4 or glutathione biosynthesis. This lethal process is defined by depletion of plasma membrane polyunsaturated fatty acids and the iron-dependent accumulation of lipid reactive oxygen species. It has also been reported that GPX4 is negatively regulated by the p53 gene [126]. These data suggest that GPX4 plays a significant role in proliferation and progression and may serve as a potential therapeutic target in HOSCC. Thus, GPX4 would be useful as a predictor of poor outcome in patients with oral cancer, and its antibody might be applicable as an inhibitor of oral cancer progression. Identification of the signaling pathways underlying these events might help to elucidate the mechanism of development of oral cancer. Further investigations into the role of GPX4 will be required to fully understand GPX4-mediated

The Mechanisms of Proliferation and Energy Metabolism in Oral Cancer

http://dx.doi.org/10.5772/intechopen.87091

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cancer proliferation and to establish a GPX4-based therapeutic strategy for oral cancer.

Unlike surgery, chemotherapy, and radiotherapy, which can have serious side effects on the human body, the use of agents that have no such side effects, such as TRAIL, cimetidine, mangostin, and antibodies, for cancer therapy mobilizes and regulates systemic functions, enhancing the body's ability to fight cancer. Therefore, these medicines may be more appropriate for patients with inoperable advanced cancer, those in periods of chemotherapy intermission, or those during postoperative recovery. There has been an increased emphasis on such agents for prevention of cancer and inhibition of cancer metastasis. There has been an impressive renaissance in the search for semi-synthetic drugs or derivatives from natural compounds. Progress in this regard not only adds to the chemical bank but also leads to a better comprehension of the chemical basis of treatments lacking side effects for the treatment of cancers using drugs obtained from natural sources. Further studies will be required to establish a strategy for basic molecular and clinical approaches for effective oral cancer therapy, which should be tailored to individual patients.

**5. Conclusions**

**Figure 6.** Schematic representation of the connecting glucose and lipid metabolism in cancer cells.

found that cancer cells expended much more glucose to generate lactic acid than normal cells even under conditions of O2 sufficiency. This finding was the first indication that cancer cells have an alteration of glucose metabolism. Warburg [99] considered that these defects of respiration caused a form of metabolic disturbance that was significant for carcinogenesis. After Warburg, many biologists attempted to clarify the molecular basis of aerobic glycolysis occurring in tumor cells. Accumulated evidence suggested that many cancer-related genes, such as p53, c-Myc, and Ras, are all associated with modulation of the Warburg effect [104]. As a master regulator of the cancer hypoxic response, hypoxia-inducible factor (HIF)-1 plays very important roles in modulating aerobic glycolysis to meet the biosynthetic demands of cancer cells and to protect them from damage due to hypoxic stress [105]. Warburg theorized that cancer cells shift from oxygen-dependent efficient ATP production via OXPHOS in mitochondria to the less efficient cytoplasmic glycolysis. As a result, cancer cells need to burn up more glucose to maintain their energy requirements for survival and growth. It has been reported that HIF-1α activates the expression of glucose transporter 1, 3 (Glut1, Glut3) under hypoxic conditions [106, 107], which acquires sufficient glucose uptake by tumor cells. We also examined HIF, and details can be found in Chapter 10 of the InTech book *Tumor Microenvironment and Myelomonocytic Cells* [108]. Although clarification of glucose metabolism is considered vital for understanding energy metabolism in oral cancer, lipid metabolism has also been receiving attention recently (**Figure 6**).

#### **4.2. Lipid metabolism in oral cancer**

Lipids are composed of phospholipids, triglycerides, cholesterol esters, cholesterol, fatty acids, sphingolipids, and other molecules, which are critical components of cellular membranes [109–113]. In addition to their role as structural components, lipids also act as energy resources and as signaling molecules to sustain cell growth [114–116]. Lipid metabolism is known to be largely altered in cancers [117–120], and worsening lipogenesis has been indicated to be a predominant characteristic of most tumors [114, 115, 121]. In oral cancer, cancer tissues include higher levels of unsaturated fatty acids than those in normal tissue (data not shown). Recent studies have revealed intrinsic molecular alterations in lipid metabolism. Especially, fatty acid synthase (FAS) is a key enzyme for synthesis of fatty acid from acetyl CoA, which is expressed at high levels in the adipose tissue and liver but at low levels in other tissues in humans [122]. FAS is overexpressed in several human cancers, including those of the ovary, bladder, stomach, breast, lung, prostate, oral cavity, and melanoma, and this overexpression is associated with poor prognosis [123, 124]. Furthermore, glutathione peroxidase 4 (GPX4) expression in tumors is positively correlated with tumor survival and linked to pathways that regulate cell proliferation, motility, and tissue remodeling [125]. Knockdown of GPX4 suppresses the formation and progression of cancer and leads to non-apoptotic cell death, ferroptosis. We have also examined the role of GPX4 in HOSCC (data not shown). Ferroptosis is a non-apoptotic form of cell death that can be triggered by conditions or small molecules that inhibit the glutathione-dependent antioxidant enzyme GPX4 or glutathione biosynthesis. This lethal process is defined by depletion of plasma membrane polyunsaturated fatty acids and the iron-dependent accumulation of lipid reactive oxygen species. It has also been reported that GPX4 is negatively regulated by the p53 gene [126]. These data suggest that GPX4 plays a significant role in proliferation and progression and may serve as a potential therapeutic target in HOSCC. Thus, GPX4 would be useful as a predictor of poor outcome in patients with oral cancer, and its antibody might be applicable as an inhibitor of oral cancer progression. Identification of the signaling pathways underlying these events might help to elucidate the mechanism of development of oral cancer. Further investigations into the role of GPX4 will be required to fully understand GPX4-mediated cancer proliferation and to establish a GPX4-based therapeutic strategy for oral cancer.
