The biological significance of redox control in cultured cancer cells

Abstract

Glycogen, the highly-branched polymer of glucose, has been hypothesised to act as the major fuel reserve during energy crisis to provide readily available glucose. Subjecting to the metabolic demand, synthesis or breakdown of glycogen occurs in response to the level of glucose. Particularly, in oxygen- and nutrient-deficient microenvironment, glycogen serves as a vital energy source to sustain cell growth via glycogen breakdown to release free glucose, followed by glycolysis or pentose phosphate pathway to generate ATP.

In order to test the above hypothesis, glycogen metabolism was disrupted by abolishing the two major rate-limiting enzymes in glycogen metabolism, the glycogen phosphorylase (GP) and glycogen synthase (GYS) which correspond to their roles in glycogen breakdown and synthesis respectively. It is reasoned that if the above hypothesis is correct, disrupting glycogen metabolism will not affect the cells in anyway under conditions that there is no concern with energy deficiency.

Glycogen was almost absent in cells in GYS-knockout cells (in which GYS was not expressed). By contrast significant increase in glycogen accumulation over the wildtype cells was observed in the GP-knockout cells (in which GP was not expressed). The production of reactive oxygen species (ROS) was found to increase in all glycogen-metabolism-enzymes-knockout cells with GYS1-KO the highest, as compared to the wildtype cells, assessed by fluorescent staining and flow cytometry. Subsequent stress responses were further examined. The removal of GYS1 gene illustrated downregulation of G6PD and upregulation of HIF1α. However Western blot results of eIF2α-p, LDHA and LC3 presented uncertainty caused by inconsistency of results and may require further investigation.

Consequently, the results suggested that the role of glycogen is not only limited to fuel reserve during energy crisis. Without a functional glycogen metabolism, cultured cancer cells in the absence of an energy crisis produced more ROS, followed by upregulation of HIF1α and potentially downregulation of G6PD. As such, the observations in this study contradict the hypothesis and present a possible involvement of glycogen metabolism in cancer cells even during normal condition. Therefore, it would also be valuable to examine the impact of perturbed glycogen metabolism on cell viability and other regulatory proteins so as to further understand the utilisation of glycogen metabolism in cancer cells during tumour hypoxia.