Redox-dependent AMPK inactivation disrupts metabolic adaptation to glucose starvation in xCT-overexpressing cancer cells

Significance 

A metabolic reprogramming is present in cancer cells to sustain their higher proliferative rate. The Warburg effect, which is also known as accelerated aerobic glycolysis, is a distinguishing metabolic characteristic of cancer cells. This changed metabolism makes cancer cells extremely dependent on glucose for survival, which creates special openings for the targeted killing of these glucose-dependent cancer cells. Varied metabolic reprogramming alterations are caused by tissue origin and oncogenic mutations, and these changes bestow different dependence on glycolysis. However, clinical investigations have shown that the therapeutic approaches that address this susceptibility are still ineffective and have unacceptably bad side effects. Therefore, improving the selective targeting of cancer cells will require the development of biomarkers to forecast therapeutic success.

Now, in a rigorous new study, Dr.¬† Younghwan Lee, Dr. Yoko Itahana, and Professor Koji Itahana from the Duke-NUS Medical School in Singapore have discovered that cancer cell lines responsive to glucose deprivation explicitly high levels of the cystine/glutamate antiporter xCT (xCT). By influencing intracellular NADPH levels during glucose deprivation, xCT’s expression increased sensitivity to glucose deprivation. The research team proposed that in xCT high cancers, the inactivation of AMPK (AMP-activated protein kinase) resulting from the collapse of the intracellular redox system due to rapid NADPH depletion serves as the metabolic driver for glucose deprivation-induced cell death. They also proposed that xCT may be employed as a diagnostic for cancer therapies that target glucose metabolism. This study, which uncovered the previously unknown link between xCT and AMPK in cancer cells responding to glucose deprivation, was published in the peer-reviewed Journal of Cell Science.

One of the pieces of evidence that researchers used is that glucose deprivation-sensitive cancer cell lines failed to activate AMPK due to redox-dependent inhibitory oxidation. AMPK is a key metabolic sensor that helps cells adjust to metabolic stress by inhibiting energy-consuming processes which are utilized by cancer cells to survive under nutrient starvation conditions, such as glucose starvation. However, in the cell lines studied, the failure of AMPK activation prevented them from being able to adjust to metabolic stress caused by glucose deprivation. Another piece of evidence researchers used is that xCT expression is an useful marker that specifies and predicts responsivity to glucose deprivation. xCT is a protein that is involved in the transport of the amino acid cystine into cells and the export of the amino acid glutamate out of cells. Researchers found that the expression levels of xCT were higher in sensitive cancer cell lines and that these levels were closely linked to the cell’s susceptibility to glucose deprivation. A third piece of evidence researchers used is NADPH depletion upon glucose deprivation-associated cell death. NADPH is a cofactor that plays a crucial role in maintaining the balance of reducing power in cells, and its depletion can lead to redox collapse and cell death. The authors showed that NADPH depletion was a major factor in cell death caused by glucose deprivation.

Despite the fact that the LKB1-AMPK axis (a signaling pathway that regulates AMPK activity) was intact in the cell lines that were sensitive to glucose deprivation, these cell lines showed the failure of AMPK activation during metabolic stress. This suggests that other factors, such as redox-dependent mechanisms, may be involved in the inhibition of AMPK activation during glucose deprivation. Although sensitive cancer cell lines showed highly sustained mTORC1 activity under glucose deprivation due to defective AMPK activation, glucose deprivation-induced cell death was not prevented by the mTORC1 inhibitor rapamycin. Researchers discovered that the FAO (fatty acid oxidation) activator and FAS (fatty acid synthase) inhibitor C75 prevented cell death brought on by a lack of glucose by increasing NADPH levels. FAO is a metabolic pathway that generates energy by breaking down fatty acids, while FAS is an enzyme that synthesizes fatty acids. C75 is a compound that activates FAO and inhibits FAS, which helps to prevent cell death caused by glucose deprivation.

The research team also discovered that AMPK-mediated adaptive response was inhibited by glucose restriction in sensitive cancer cells via a redox-dependent mechanism. In their investigation, glucose restriction caused an electrophoretic mobility change in the AMPK protein in nonreducing SDS-PAGE, which was reversed by the administration of an antioxidant or reducing agent. This suggests that the oxidation of AMPK is a key mechanism involved in the inhibition of AMPK activation during glucose deprivation. According to the authors, cell death caused by glucose deprivation and NADPH consumption are both regulated by xCT expression levels, which is a major metabolic factor in determining susceptibility to glucose deprivation. Glucose is a major source of NADPH through the pentose phosphate pathway (PPP). xCT exchanges intracellular glutamate for extracellular cystine which is rapidly converted to cysteine using NADPH and serves as the precursor for GSH generation. GSH is a powerful antioxidant that contributes to chemoresistance, tumor invasion, and poor survival. The research team discovered that cancer cells with high expression of xCT deplete intracellular NADPH under glucose deprivation where NADPH supply is limited. In contrast, cancer cells maintain high GSH generation that is beneficial for cancer cell survival when NADPH is in surplus by PPP in the presence of glucose. This highlights the complexity of the metabolic changes that occur during glucose deprivation and how different pathways and processes can interact to ultimately lead to cell death.

In conclusion, Professor Koji Itahana and colleagues showed that sensitive cancer cell lines with high levels of xCT expression had inactivated AMPK, and there was a unique relationship between xCT expression and AMPK dysregulation caused by its oxidation in response to glucose deprivation. For these sorts of malignancies, a technique that inhibits glucose uptake while also increasing tissue cystine concentration would be more therapeutically beneficial, and xCT expression might be taken into account as a biomarker for evaluating responsiveness to the treatment.

Reference

Lee Y, Itahana Y, Ong CC, Itahana K. Redox-dependent AMPK inactivation disrupts metabolic adaptation to glucose starvation in xCT-overexpressing cancer cells. Journal of Cell Science. 2022 ;135(15):jcs259090.

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