Glutamine Starvation Leads To The Formation Of GLS1 Superstructure Which Facilitates Tumor Cell Apoptosis

Glutamine,the most abundant free amino acid in human body,is an important provider of carbon and nitrogen sources for cells.In addition,glutamine metabolism plays a key role in maintaining intracellular redox homeostasis.It is well known that many tumor cells exhibit significantly increased consumption of glutamine compared to corresponding normal cells,and no longer survive in glutamine-deficient medium.Therefore,the addiction of tumor cell to glutamine metabolism has been considered as another important metabolic feature of tumor in addition to the Warburg effect.In recent years,great efforts have been made to reveal the mechanism underlying such addiction,however,evidence,especially from the aspect of glutamine deficiency-induced cell death,is still insufficient.Glutaminase 1(GLS1)is the key enzyme in glutamine metabolism,increased expression or excessive activation of which have been shown to contribute to tumorigenesis and tumor progression.Surprisingly,we found that knockout of GLS1 in various tumor cell lines including SF188,HT1080 and HEB significantly retarded glutamine starvation-induced apoptosis.Rescuing GLS1 expression in GLS1 knockout cells further recovered the sensitivity of these cell lines to glutamine starvation-induced apoptosis.These results demonstrate that GLS1 functions to facilitate apoptosis rather than maintain cell survival on the scenario that cell is extremely lacking glutamine.Immunofluorescence staining experiments showed that upon glutamine starvation GLS1 was assembled into a kind of rod-like superstructure,on the contrary,supplementation of glutamine rapidly disrupted such superstructure,indicating that intracellular glutamine concentration is the determinant for the formation of such GLS1 superstructure.To determine whether such superstructure formation plays a role in glutamine starvation-induced GLS 1-facilitated apoptosis,we constructed a GLS1 K507E mutant that fails to form superstructure in response to glutamine deprivation.Rescuing expression of this mutant failed to reconstitute the sensitivity of tumor cells to apoptosis induced by glutamine deprivation,revealing that superstructure formation is an essential event for GLS1 to accelerate glutamine-induced apoptosis.In future we will further determine how glutamine starvation causes the formation GLS1 superstructure and how such superstructure in turn facilitates glutamine starvationinduced apoptosis.In summary,we identified a novel function of GLS1 in strengthening apoptosis induced by glutamine deficiency,and shed a new light on the mechanism underlying tumor cell addiction to glutamine.This investigation will help us to deeply understand glutamine metabolism and stress response of tumor cells in response to glutamine deficiency.