Oxidative stress, inflammation and carcinogenesis are controlled through the pentose phosphate pathway by transaldolase

Abstract

Metabolism of glucose through the pentose phosphate pathway (PPP) influences the development of diverse pathologies. Hemolytic anemia due to deficiency of PPP enzyme glucose 6-phosphate dehydrogenase is the most common genetic disease in humans. Recently, inactivation of another PPP enzyme, transaldolase (TAL), has been implicated in male infertility and fatty liver progressing to steatohepatitis and cancer. Hepatocarcinogenesis was associated with activation of aldose reductase and redox-sensitive transcription factors and prevented by N-acetylcysteine. In this paper, we discuss how alternative formulations of the PPP with and without TAL reflect cell type-specific metabolic control of oxidative stress, a crucial source of inflammation and carcinogenesis. Ongoing studies of TAL deficiency will identify new molecular targets for diagnosis and treatment in clinical practice.

Figure 1

Conventional PPP. Conventional formulation of the pentose phosphate pathway (PPP) that allows the oxidative branch to produce two NADPH molecules per each molecule of glucose 6-phosphate (G6P). The non-oxidative branch recycles ribose 5-phosphate (R5P) back into G6P for the oxidative branch. R5P is required for the synthesis of nucleotides, RNA, and DNA in support of cell growth while NADPH is needed for biosynthetic reactions and protects against oxidative stress through neutralizing reactive oxygen intermediates (ROI) directly or indirectly via regeneration of reduced glutathione (GSH) from its oxidized form GSSG.

Figure 2

Alternative PPP. Alternative formulation of the PPP in the absence of transaldolase (TAL) leading to NADPH depletion, oxidative stress, and activation of β-catenin and c-jun. TAL, an enzyme of the non-oxidative branch, catalyzes the transfer of dihydroxyacetone from sedoheptulose 7-phosphate (S7P) and fructose 6-phosphate (F6P) to glyceraldehyde 3-phosphate (GA3P) and erythrose 4-phosphate (E4P), respectively. GA3P generated by TAL and TK connects the PPP to glycolysis. The forward reaction of TAL favors the generation of G6P; the reverse TAL reaction promotes the metabolism of G6P into R5P. Formulation of the PPP without TAL leads to the accumulation of S7P, R5P, X5P, RU5P, C5-polyols D-ribitol/D-arabitol/D-xylitol (red/upward arrows), and depletion of G6P and NADPH (blue/downward arrows).R5P, X5P, and RU5P inhibit 6PGD , while elevation of the total adenine nucleotide pool (AMP, ADP and ATP) inhibit both G6PD and 6PGD . Excess R5P (X5P and RU5P) can be converted to ribose by ribose-5-phosphatase , which is then reduced to ribitol by AR, thus further depleting NADPH . GSH reductase uses NADPH to regenerate GSH from GSSG. In the absence of TAL, the accumulation of S7P, C5 sugar phosphates R5P and X5P and the depletion of G6P indicate a failure to recycle R5P into G6P through the non-oxidative branch, thus reducing NADPH production by the oxidative branch ,,. Diminished production of NADPH leads to secondary depletion of GSH and oxidative stress marked by increased levels of lipid hydroperoxides (LPO), 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA). Redox-sensitive genetic changes, reduced β-catenin phosphorylation and increased c-jun N-terminal kinase (JNK) activity and c-jun expression occur in TAL-deficient livers, while expression of alpha-fetoprotein (AFP) and AR are increased in hepatomas . In addition to converting C5 sugars to C5 polyols, AR also neutralizes LPO at the expense of NADPH.

Figure 3

A) Schematic outline of mitochondrial and metabolic pathways connected to the PPP in hepatocytes. The PPP regulates the Δψ_m by providing i) NADPH that serves as a reducing equivalent for the activity of catalase and nitric oxide synthase (NOS) and for GSH regeneration from its oxidized form GSSG and ii) R5P for biosynthesis of nucleotides. Mitochondrial electron transport chain activity is reversibly inhibited by NO at complex IV/cytochrome c oxidase and irreversibly inhibited via S-nitrosylation of complex I in a state of GSH depletion . Blocked electron transport leads to the transfer of electron to molecular oxygen (O₂⁻) and the formation of reactive oxygen intermediates (ROI). Mitochondrial ROI production is neutralized through the activities of superoxide dismutase 2 (SOD2) and catalase at the expense of NADPH. Glucose is taken up by hepatocytes through the transporter GLUT1 and enters glycolysis or the PPP as G6P. The PPP and glycolysis are also connected via the common substrate GA3P. The glycolysis product pyruvate is converted to acetyl-CoA and thus enters the tricarboxylic acid cycle (TCA) in mitochondria. The mammalian target of rapamycin (mTOR) senses Δψ_m and ATP depletion and controls cellular metabolism via protein translation and authophagy . TAL-deficient hepatocytes have reduced mitochondrial mass. Mitochondrial homeostasis is maintained through a balance between de novo biogenesis elicited by NO and mitochondrial autophagy or mitophagy regulated by mTOR. B) Metabolic and genetic checkpoints of progressive liver disease leading from nonalcoholic fatty liver disease (NAFLD) to non-alcoholic steatohepatitis (NASH), cirrhosis, and hepatocellular carcinoma (HCC) in TAL deficiency. Hepatocytes of TAL-deficient mice exhibit increased susceptibility to APAP-induced necrosis and resistance to Fas apoptosis. Life-long supplementation of N-acetylcysteine (NAC) increases GSH levels, normalizes NADPH, blockes APAP susceptibility and restores Fas apoptosis, phosphorylation of β-catenin, and activation of c-jun and prevents the development of NASH, cirrhosis and HCC . However, NAC does not prevent NAFLD which may be attributed to stimulation of lipogenesis by X5P .