Metabolic Functions of the Murine Lupus Susceptibility Gene Pbx1

Abstract Systemic Lupus Erythematosus (SLE) is an autoimmune disease that affects many organ systems. Poorly characterized genetic factors contribute to SLE, in part through the production of autoreactive or inflammatory T cells. Pre-B cell leukemia homeobox 1 (Pbx1) is a transcription factor whose Pbx1-d dominant negative splice isoform is overexpressed in CD4T cells of lupus patients as well as in the NZM2410 lupus-prone mouse as compared to the normal Pbx1-b isoform. Based on gene expression studies comparing murine CD4 T cells overexpressing Pbx1-d to controls, we hypothesize that Pbx1-d enhances cellular metabolism in T cells through the HIF1α and mTORc1 pathways. CD4 T cells expressing Pbx1-d present a higher cellular metabolism and show a higher mTORc1 activation than normal control T cells. Using mesenchymal stem cells, we showed that transfection of Pbx1-d was sufficient to increase glycolysis, a pathway linked to T cell activation. We found that Ddit4, an mTORc1 inhibitor, shows a lower expression in the Pbx1-d-expressing CD4 T cells than in normal T cells. We also discovered that Pbx1-d preferentially binds to the promoter of Ddit4, as well as Egln1 and Egln3, two HIF1a inhibitors. These results suggest that a mechanism by which the Pbx1-d allele contributes to lupus pathogenesis is to enhance CD4 T cell metabolism. Future work will define how Pbx1 controls the immune system and how the function of this transcription factor is linked to cellular metabolism.