Controlling Gene Expression With Synthetic Molecules
Aberrant gene expression patterns have been implicated in several pathological states. Synthetic molecules capable of functionally mimicking native transcription factors and regulating gene expression in a specific and predictable manner may represent a new paradigm in drug development. Native transcription factors are minimally composed of two domains, a DNA-binding domain (DBD) and an activation domain (AD). Several synthetic DBDs capable of recognizing DNA in a sequence specific manner have been reported in the literature. Furthermore, studies have demonstrated that coupling of these synthetic DBDs to peptides that are capable of acting as activation domains results in chimeric molecules that are capable of activating target gene expression. Since peptides and other biomolecules generally have poor cell-membrane permeability and are prone to rapid enzymatic inactivation inside cells, it is highly desirable to develop artificial molecules that are capable of mimicking native ADs. Towards this goal, a comprehensive methodology for the synthesis, screening and characterization of large peptoid libraries has been developed. Peptoids are a new class of peptidomimetic compounds that are resistant to proteolytic cleavage and are relatively simple and cheap to synthesize. One of the combinatorial libraries was screened against CBP (CREB-binding protein), an important transcriptional coactivator, and three novel, low micromolar affinity ligands were isolated. A cellbased reporter gene assay was employed to assess the cell permeability and transcription activation potential of the synthetic ligands in live mammalian cells. The assay consists of transfecting into HeLa cells a luciferase reporter gene harboring Gal4 binding sites and a construct in which the ligand binding domain of the Glucocorticoid receptor has been fused to Gal4 DBD. The cells are treated with the CBP-binding peptoids that have been chemically coupled to a dexamethasone derivative. Among the three peptoids tested, one of the molecules as a steroid conjugate, has been found to activate the transcription of a reporter gene nearly 1000-fold suggesting that it may be acting as an activation domain surrogate. The mechanistic aspects of the observed transcriptional activity of the peptoid-steroid conjugate remain to be elucidated.