Interaction between the (-170) CRE and the (-150) CCAAT box is necessaryfor efficient activation of the fibronectin gene promoter by cAMP and ATF-2.

The fibronectin promoter contains an ATF/cyclic AMP (cAMP) response element (CRE) site two helical turns upstream of a CCAAT site with which it interacts. We investigated the effects of mutating these (-170) CRE and(-150) CCAAT elements on the promoter activity regulated by three different modulators previously known to act through CRE: ATF-2, cAMP and E1a. While the cooperation seems to play no role in E1a action, integrity of the (-150) CCAAT is necessary for ATF-2 and cAMP efficient activation in a cell-specific manner. These results show that the CRE and CCAAT elements function as a 'composite element' and establish a cell-specific function for CRE-CCAAT synergy.     

Modulation of CREB and NF-kappaB signal transduction by cannabinol in activated thymocytes

Cannabinoid compounds inhibit the cAMP signalling cascade in leukocytes. One of these compounds, cannabinol (CBN) has been shown to inhibit interleukin-2 (IL-2) expression and the activation of cAMP response element binding protein (CREB) and nuclear factor for immunoglobulin kappa chain in B cells (NF-kappaB) following phorbol-12-myristate-13 acetate (PMA) plus ionomycin (Io) treatment of thymocytes. Therefore, the objective of the present studies was to determine the role of cAMP and protein kinase A (PKA) in the CBN-mediated inhibition of IL-2, CREB, and NF-kappaB in PMA/Io-activated thymocytes. The inhibition of CREB/ATF-1 phosphorylation, or cAMP response element (CRE) or kappaB DNA binding activity produced by CBN in PMA/Io-activated thymocytes, could not be reversed by DBcAMP costimulation. Furthermore, DBcAMP failed to reverse the concentration-dependent inhibition of IL-2 protein secretion by CBN. Pretreatment of thymocytes with H89 produced a modest inhibition of PMA/Io-induced CREB/ATF-1 phosphorylation and CRE DNA binding activity but H89 had no effect on protein binding to a kappaB motif. Additionally, H89 modestly inhibited PMA/Io-induced IL-2 secretion. In light of the modest involvement of the cAMP pathway in CBN-mediated inhibition of CREB and IL-2 in PMA/Io-activated thymocytes, PD098059 (PD), the MEK inhibitor, was utilized to determine the role of ERK MAP kinases in thymocytes. ERKs play a critical role in IL-2 production but not for CREB phsophorylation. Collectively, these findings suggest that CBN may modulate several signalling pathways in activated T cells.     

A silencer element in the first intron of the glutamine synthetase gene represses induction by glucocorticoids

The enzyme glutamine synthetase (GS) ranks as one of the most remarkable glucocorticoid-inducible mammalian genes. In many tissues and cell lines, the synthetic glucocorticoid dexamethasone alone increases GS expression several fold. The direct response is mainly mediated by a cellular glucocorticoid receptor that, upon binding of the hormone, interacts with glucocorticoid responsive elements (GREs) of the gene. In cells of hepatocellular origin the response is mediated by a GRE located in the first intron of the gene. Surprisingly, hepatocytes do not respond to glucocorticoids with enhanced GS expression, despite the presence of an intact glucocorticoid receptor, which, in the same cells, stimulates expression of other genes such as tyrosine amino transferase. Reporter gene assays identified a sequence element downstream from the intronic GRE that inhibits the enhancement of expression by glucocorticoids. This silencer was designated GS silencer element of the rat. Gel mobility shift assays demonstrate the binding of a factor in hepatocyte nuclear extract. This yet unknown factor was designated GS silencer-binding protein. It is absent in FAO cells that respond to glucocorticoids with enhanced expression of GS and present in HepG2 cells that do not respond.