Abstract | May 3, 2021
STK11 Loss in Lung Adenocarcinoma Drives Putrescine and GABA: A Potential Target to Reverse Immunosuppression and Inhibit Altered Metabolism
Learning Objectives
- Discuss the relevance and importance of identifying STK11 alterations in lung adenocarcinoma patients.
- Compare and contrast patient expression data to that of A549 cells (an STK11 mutant cell line).
Background/Knowledge Gap:
STK11 (LKB1; liver kinase B1) loss of function is common in lung adenocarcinomas (LUADs) representing approximately 15-30%. Patients harboring these mutations tend to respond poorly to immunotherapy and currently there is no targeted therapy available for their genetic profile.
Methods/Design:
Herein, we utilized two patient cohorts, The Cancer Genome Atlas’ TCGA LUAD study and the Moffitt Lung Adenocarcinoma, Overall Survival (MLOS) dataset, representing approximately 1,000 LUAD patients, with available mutation and gene expression data. A subset of MLOS tumor tissues were used for metabolite level analysis via untargeted liquid chomatography-mass spectrometry (LC-MS). These data sets were used to identify metabolic pathways strongly associated with STK11 loss in support of a hypothesis that STK11 mediated metabolic disruption may contribute to an immunosuppressive microenvironment. A549 cells overexpressing STK11 and Ornithine decarboxylase 1 (ODC1) were used for validation of patient data findings.
Results/Findings:
Multiple gene-set enrichment analyses reveal patient tumors with STK11 loss upregulate genes involved in amino acid catabolism and the urea cycle, and downregulate immune markers associated with immune infiltration and chemokine response. ODC1, a known immunosuppressant, is elevated in STK11 deficient tumors and is the rate-limiting enzyme for polyamine production. Additionally, monoamine oxidase A (MAOA) is responsible for the production of gamma-aminobutyric acid (GABA) and is also elevated in STK11 deficient tumors. Untargeted LC-MS revealed data supporting the influence of both elevated ODC1 and MAOA expression in that tumors with STK11 loss have increased production of the polyamine putrescine, GABA, and pyridoxal (vitamin B6) with subsequent decreases in ornithine, arginine, and histamine. Further, in vitro overexpression of STK11 in STK11-deficient A549 cells diminishes the expression of MAOA, supporting an STK11 dependent regulation. ODC1 overexpression in these cell lines lead to an increase in MAOA expression as predicted. Inhibition of ODC1 in STK11-deficient A549 cells resulted in an upregulation of PD-L1 mRNA and protein, suggesting a potential therapeutic window for anti-PD-L1 therapy in and otherwise unresponsive cohort.
Conclusions/Implications:
These data suggest a non-canonical and neuroendocrine-like utilization of putrescine for GABA production in STK11 loss LUADs with downstream effects on immune surveillance. We hypothesize that this metabolic pathway plays a role in the immune cold nature of these tumors and predict that inhibition of these pathways could serve as STK11-targeted therapeutic approaches.
