The long-term goal of our work is to understand the mechanism of transcriptional regulation by steroid hormones and their dysregulation during disease. We seek to reveal how novel post-translational modifications govern the ability of steroid hormone receptors to translate hormonal signals into promoterdependent transcriptional outputs and how alterations in these mechanisms contribute both to disease and therapy. Using the glucocorticoid receptor as a paradigm, we have identified a novel class of regulatory sequences in steroid receptors and many other factors that operate by inhibiting synergistic transactivation in a promoter context dependent manner. We recently established that the function of these Synergy Control or SC motifs relies on their post-translational modification by members of the Small Ubiquitin-like MOdifier family of proteins (SUMOylation). Despite the widespread nature of SUMOylation, the underlying mechanisms responsible for the profound regulatory effects of this highly dynamic modification remain largely unknown. We propose the general hypothesis that conjugation of SUMO to the SC motifs of GR provides an additional functional surface that alters the nature, function and lifetime of GR-coregulator complexes nucleated at specific response elements on DNA. We have recently identified a key effector surface in SUMO responsible for its transcriptional inhibitory function and we propose that it serves as a binding site for novel receptor coregulators. By focusing on the regulation of endogenous genes responsible for the therapeutically relevant lymphocyte apoptosis and applying complementary genetic and biochemical approaches, we shall: 1) Define the factors that mediate the context-dependent effects of GR SUMOylation, 2) Investigate the molecular mechanisms by which SUMOylation limits GR transcriptional activation in a promoter-context manner; and 3) Probe how SUMOylation-dependent regulation of GR target genes influences the therapeutic glucocorticoid induced apoptosis of leukemic T cells. Defining the properties and function of GR SUMOylation will provide not only fundamental insights on the post-translational regulation of nuclear receptors and the mechanism of action of SUMO but also provide novel avenues to bias glucocorticoid responses to therapeutic advantage.
