No abstract

SummaryAMER1 regulates the distribution of the tumor suppressor APC between microtubules and the plasma membrane

RAP46 is a eukaryotic cochaperone that associates with several proteins, including the heat shock protein hsp70/hsc70 and the glucocorticoid receptor (GR). Here we show a downregulation of GR-mediated transactivation by RAP46 via a mechanism independent of a cytoplasmic action of this cochaperone. We demonstrate a specific cytoplasmic–nuclear recruitment of RAP46 by the liganded GR that results in inhibition of the transactivation function of the receptor. A repeated sequence motif [EEX4]8 at the NH2 terminus of RAP46 or BAG-1L, a larger isoform of RAP46, is responsible for this downregulation of GR activity. BAG-1, a shorter isoform with only a duplication of the [EEX4] sequence, does not inhibit GR activity. The [EEX4]8 motif, when linked to an otherwise unrelated protein, abrogated the inhibitory action of endogenous RAP46 on GR-mediated transactivation. The nuclear effects of RAP46 and BAG-1L are specific since GR-mediated inhibition of AP-1 activity was not affected. These studies identify the [EEX4]8 sequence as a signature motif for inhibition of GR-mediated transactivation and demonstrate a specific nuclear action of a eukaryotic cochaperone in the regulation of GR activity.

Matrix metalloproteinases belong to a family of structurally related enzymes that plays important role in tissue morphogenesis, differentiation, and wound healing. Their expression is negatively regulated by several members of the steroid hormone receptor family. This is thought to occur through interaction of the steroid receptors with the transcription factor AP-1 that is otherwise required for positive regulation. Here, we demonstrate that AP-1 is not always a target for downregulation of expression of matrix metalloproteinases by steroid receptors. Androgen receptor negatively regulates matrix metalloproteinase-1 expression not through AP-1 but through a family of Ets-related transcription factors that are also required for positive regulation. This negative regulation is specific for the androgen receptor. It does not require the DNA binding activity but needs amino-terminal sequences of the receptor. These results identify a novel regulatory pathway for negative regulation utilized by a member of the steroid hormone receptor family for down-regulating the expression of matrix metalloproteinases.Matrix metalloproteinases (MMPs) 1 are composed of a family of metal-ion requiring enzymes that degrades components of the extracellular matrix. They can be subdivided into several classes on the basis of their substrate specificity. These classes include the interstitial collagenases, which degrade type I, II, and III collagens, the type IV collagenases, which degrade basement membrane collagens type IV and V, the stromelysins, which degrade proteoglycans, fibronectin, and laminin, and matrilysin, which has a wide range of substrates including proteoglycans, fibronectin, gelatin, and elastin (for reviews see Refs. 1 and 2).Regulated expression of MMPs accompanies changes in tissue organization in normal growth processes and pathological conditions. These are most evident in organogenesis, neovascularization, tissue repair, inflammation, arthritis, tumor invasion, and metastasis (1). In these processes, positive and negative regulation of expression of MMP genes by a variety of growth factors, cytokines, oncogenes, tumor promoters, and steroid hormones are required (2, 3). The steroid hormones in particular, negatively regulate the expression of the MMPs through the action of their corresponding receptors (4, 5).Glucocorticoid receptor (GR) exerts its negative regulation by interacting with the transcription factor AP-1, an important regulator of expression of several MMP genes (4 -8). Progesterone acting through its receptor in stromal cells of the endometrium down-regulates the levels of MMP mRNAs of the stromelysin family through a mechanism that is not yet known (9, 10). In the epithelial cells of the endometrium, progesterone down-regulates the expression of matrilysin through the release of transforming growth factor ␤1 from the stromal cells (10). Although androgen receptor (AR) represses the expression of several genes (11-17), it is not clear how this is effected nor is it clear whether MMPs belong to the...

RAP46 was first identified by its ability to bind the glucocorticoid receptor. It has since been reported to bind several cellular proteins, including the anti-apoptotic protein Bcl-2, but the biological significance of these interactions is unknown. Here we show that RAP46 binds the hinge region of the glucocorticoid receptor and inhibits DNA binding and transactivation by the receptor. We further show that overexpression of RAP46 in mouse thymoma S49.1 cells inhibits glucocorticoid-induced apoptosis. Conversely, glucocorticoid-induced apoptosis and transactivation were enhanced after treating S49.1 cells with the immunosuppressant rapamycin, which down-regulates cellular levels of BAG-1, the mouse homolog of RAP46. The effect of rapamycin can, however, be overcome by overexpression of RAP46. These results together identify RAP46 as a protein that controls glucocorticoid-induced apoptosis through its negative regulatory action on the transactivation property of the glucocorticoid receptor.