Molecular mechanisms of PBX1 regulation of CD4 +T cell inflammatory phenotypes

Abstract

Abstract The pre-B cell leukemia homeobox1 (Pbx1) controls chromatin accessibility to a large number of genes in various cell types. Its dominant negative splice isoform, Pbx1-d that lacks the DNA and Hox-binding domains, is expressed more frequently in the CD4 +T cells from lupus-prone mice and SLE patients than healthy controls. We have shown that overexpression of Pbx1-d in CD4 +T cells impaired Treg cell homeostasis and expanded inflammatory CD4 +T cells. Here, we investigated the molecular mechanisms by which PBX1 regulates T cells. We showed that PBX1 is preferentially expressed in Treg cells over other CD4+ T cell subsets, and that it is negatively regulated by TCR activation. Furthermore, PBX1-D protein was less stable than the normal isoform, PBX1-B, in a pulse chase experiment. PBX1-B but not PBX1-D has two predictive ubiquitin binding sites, K292 and K293, located in the DNA binding domain. Mutation of these two sites reduced PBX1-B stability to a level similar to PBX1-D, suggesting that a loss of function in the lupus associated allele through the ubiquitin-proteasome dependent pathway. In addition, contrary to PBX1-B, PBX1-D is retained in the cytoplasm without the help of the MEIS or PREP1 co-factors, indicating different requirements for nuclear translocation. Overall, these findings suggest that multiple transcriptional and post-transcriptional mechanisms are responsible for PBX1-D loss of function and induction of CD4 +T cell inflammatory phenotypes in SLE. This work was supported by National Institutes of Health Grants AI045050