Mechanism of action of STAMP - a new comodulator of glucocorticoid receptors

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The four aims of this project are 1) to determine whether the modulatory actions of STAMP are limited to GRs, 2) to define the interaction surfaces that are used in the interactions of GR, TIF2, and STAMP, 3) to identify the domains of each factor that are needed for the observed modulation of EC50 and partial agonist activity in addition to changes in total gene induction, or Vmax (He and Simons Jr., 2007, Mol. Cell. Biol., 27, 1467-1485), and 4) to search for other activities of STAMP. With regard to the first aim, androgen receptors (ARs) display the strongest interaction with STAMP in a mammalian two-hybrid assay (He and Simons Jr., 2007, Mol. Cell. Biol., 27, 1467-1485). Therefore, we are testing the ability of STAMP to modulate the EC50, partial agonist activity, and Vmax, of AR-regulated gene expression in transiently transfected CV-1 monkey kidney cells and LNCaP human prostate tumor cells. An ability of modify AR transcriptional properties would suggest a role for STAMP in the progression and treatment of prostate cancer. Aims 2 and 3 are being assessed by quantifying the ability of different deletions in GR, TIF2, and STAMP, both separately and in combination, to retain the modulatory activity for EC50, partial agonist activity, and Vmax. ChiP-reChIP (He and Simons Jr., 2007, Mol. Cell. Biol., 27, 1467-1485) and co-IP experiments support a model where GR, TIF2, and STAMP act via a complex containing all three factors. Preliminary data indicate that different regions of STAMP are required for the modulation of EC50 and Vmax. Finally, Aim 4 seeks to determine whether STAMP mediates any other actions. The only identifiable domain in STAMP is a tyrosine tubulin ligase-like (TTL) domain, which is not required for STAMP modulatory activity (He and Simons Jr., 2007, Mol. Cell. Biol., 27, 1467-1485). However, it is unlikely that STAMP, a protein of 1277 amino acids, does not have additional functions associated with the many regions not associated with TTL or modulatory activity. Preliminary experiments suggest that we may have discovered an additional activity, which is being pursued by identifying the region of STAMP required and the nature of the target molecules. In summary, we have identified a new cofactor (STAMP) that acts in an additive, and sometime synergistic, manner with the coactivator TIF2 to alter several properties in both major aspects of GR-regulated gene expression: induction and repression. As a result of our on-going studies, we have gained new molecular information about the modulation of the dose-response curve and total activity of agonists and the partial agonist activity of antisteroids by cofactors. These modulatory factors permit a continuum of responses and constitute new therapeutic targets for differential control of gene expression by steroid hormones during development, differentiation, homeostasis, and endocrine therapies. These combined findings contribute to our long-term goal of defining the action of steroid hormones at a molecular level and of understanding their role in human physiology.