The antitumorigenic potential of metabolic therapy

We have checked the possibility of activated gluconeogenesis in lung cancer xenograft

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tumors with citreoviridin treatment. This observation raised a question of how activation of gluconeogenesis inhibits the proliferation of lung cancer cells. The relationship between gluconeogenesis and tumorigenesis is quite ambiguous. There are only limited literatures describing the influence of gluconeogenesis on cancer and most of them were reported in 1970s. It was suggested that gluconeogenesis from alanine is increased in cancer patients with cachexia, a syndrome with significant loss of appetite resulting in weak and loss of weight⁶¹. It seems that gluconeogenesis is up-regulated in cancer. Nevertheless, the role of gluconeogenesis in cancer cell can vary depending on the gluconeogenic precursors. There are a variety of gluconeogenic precursors for gluconeogenesis, including lactate, pyruvate, amino acids and other metabolites. A recent report showed that gluconeogenesis was down-regulated in hepatocellular carcinoma and the reduced gluconeogenesis may facilitate tumorigenesis by accumulation of glucose 6-phosphate, the precursor for nucleotide synthesis⁶².

The expression profile of proteomes in control and citreoviridin-treated tumors has novel implications for understanding the antitumorigenic effect by activation of gluconeogenesis in cancer cells. First, the glucose synthesized could be converted into

myo-inositol, which has cancer inhibition activity. We observed the up-regulation of the

enzyme inositol-3-phosphate synthase 1 with treatment of citreoviridin (Table 11). This

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enzyme catalyzes the conversion of glucose 6-phosphate to 1-myo-inositol 3-phosphate, a key rate-limiting step in myo-inositol biosynthesis pathway. The higher level of

myo-inositol was found to be present in normal tissue compared to breast cancer tissue

myo-inositol was shown to have anti-cancer activity by inhibiting tumor formation of

colon, mammary, soft tissue and lung cancer. The phosphorylated myo-inositol, inositol hexaphosphate (IP6) was also recognized for its effectiveness in cancer prevention⁶⁵. IP6

is able to induce G1 cell cycle arrest by modulating cyclins, CDKs, p27^Kip1, p21^CIP1/WAF1 and pRb in prostate cancer and breast cancer⁶⁶.

The glucose synthesized from gluconeogenesis may also be converted to other compound and escapes from the utilization by glycolysis. The reduced in glycolysis flux results in decrease of glycolytic intermediates to sustain the continuous building blocks for macromolecular synthesis, such as generating nucleotides, lipids, amino acids³⁷. Therefore, the activation of gluconeogenesis may inhibit the proliferation of cancer cells.

For example, the surplus glucose synthesized from gluconeogenesis could be converted into sorbitol. We found that the expression level of aldose reductase that converts glucose to sorbitol was higher in citreoviridin-treated tumors (Table 11). The increased

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intracellular glucose results in its conversion to sorbitol. Although sorbitol entering the polyol pathway can be converted to fructose by sorbitol dehydrogenase, high glucose still favors the production of sorbitol.

Glucose synthesized from gluconeogenesis may also be polymerized into glycogen for storage. Thus, the decrease of glucose influx into glycolysis inhibits cell proliferation of cancer cells. The protein expression changes of enzymes involved in glycogen metabolism were listed in Table 11. In glycogen synthesis, the first step is conversion of glucose 6-phosphate to glucose 1-phosphate by phosphoglucomutase (PGM). Next, the key step of glycogen synthesis is the conversion of glucose 1-phoshpate to UDP-glucose by UTP--glucose-1-phosphate uridylyltransferase (UDP-glucose pyrophosphorylase, UDPGP). UDP-glucose is the immediate donor of glycogen synthesis by the reaction catalyzed by glycogen synthase. Glycogen branching enzyme (1,4-alpha-glucan-branching enzyme) catalyzes the reaction of branch synthesis in glycogen. A previous report showed that the expression level of UDPGP, activities of PGM and glycogen synthase were all decreased in tumor tissues, so the defective glycogen synthesis process is unable to competing with glycolysis⁶⁷. In our proteomic profiling data, we observed that the expression levels of PGM and UDPGP were higher with citreoviridin treatment in lung cancer. We also identified glycogen synthase muscle

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isozyme, but no alteration in expression level was observed. About glycogen breakdown, glycogen phosphorylase and glycogen debranching enzyme catalyze the reactions of sequentially removing the terminal glucose residues. Previous studies suggested that glycogen phosphorylase was expressed in tumor tissues and could be served as a target for anticancer therapy⁶⁸. In our proteomic profiling data, the expression level of glycogen debranching enzyme and glycogen phosphorylase brain form both remained unchanged with citreoviridin treatment. However, up-regulation of glycogen phosphorylase liver form by citreoviridin was also shown. Why there seems to be a contradiction between the expression levels of enzymes involved in glycogen synthesis and glycogen breakdown? Actually, glycogen synthase and glycogen phosphorylase are both allosterically regulated by kinases and phosphatases. Besides, the kinases and phosphatases that regulate the activity of these two enzymes are also post-translationally regulated. Therefore, the activities of glycogen synthase and glycogen phosphorylase in citreoviridin-treated tumors were remained unknown. Although we don’t have conclusive evidence for glycogen synthesis, it is possible that glucose from gluconeogenesis is converted into UDP-glucose in citreoviridin-treated lung cancer xenograft tumors.

We provided several implications about the relationship between gluconeogenesis

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and tumorigenesis. Nevertheless, the interplay between glycolysis and gluconeogenesis is very complicated. As we have known, several metabolites from processes of glycolysis, gluconeogenesis or other glucose metabolism pathways make feedback loops to regulate the activities of key enzymes in glycolysis and gluconeogenesis. There is no cause and effect relationship among these regulations. Besides, multiple kinases and phosphatases post-translationally regulate the activities of the enzymes.

Furthermore, genes encoding enzymes of glucose metabolism are also under a complex control of multiple transcription factors, not to mention that a variety of accessory regulators modulate the activities of transcription factors. The coordinated regulation of glycolysis and gluconeogenesis is important in maintaining the homeostasis in the cells.

The understanding of the complexity of metabolic regulations and the plasticity of cancer cells will shed light on the improvement of cancer therapy.

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