Functional domain analysis of interferon consensus sequence binding protein (ICSBP) and its association with interferon regulatory factors.

Interferon consensus sequence binding protein (ICSBP) is a member of the interferon regulatory factor (IRF) family of proteins that include IRF-1, IRF-2, and ISGF3gamma which share sequence similarity at the putative DNA binding domain (DBD). ICSBP is expressed exclusively in cells of the immune system and acts as a repressor of interferon consensus sequence (ICS) containing promoters that can be alleviated by interferons. In this communication, we have searched for functional domains of ICSBP by dissecting the DBD from the repression activity. The putative DBD of ICSBP (amino acids 1-121) when fused in frame to the transcriptional activation domain of the herpes simplex VP16 (ICSBP-VP16) is a very strong activator of ICS-containing promoters. In addition, ICSBP-VP16 fusion construct transfected into adenovirus (Ad) 12 transformed cells enabled cell surface expression of major histocompatibility complex class I antigens as did treatment with interferon. On the other hand, the DBD of the yeast transcriptional activator GAL4 was fused in frame to a truncated ICSBP in which the DBD was impaired resulting in a chimeric construct GAL4-ICSBP. This construct is capable of repressing promoters containing GAL4 binding sites. Thus, ICSBP contains at least two independent domains: a DBD and a transcriptional repressor domain. Furthermore, we have tested possible interactions between ICSBP and IRFs. The chimeric construct GAL4-ICSBP inhibited the stimulated effect of IRF-1 on a reporter gene, implying for a possible interaction between IRF-1 and ICSBP. Electromobility shift assays, demonstrated that ICSBP can associate with IRF-2 or IRF-1 in vitro as well as in vivo. Thus, ICSBP contains a third functional domain that enables the association with IRFs. These associations are probably important for the fine balance between positive and negative regulators involved in the interferon-mediated signal transduction pathways in cells of the immune system.

Mechanism of glucocorticoid induction of the rat plasminogen activator inhibitor-1 gene in HTC rat hepatoma cells: identification of cis-acting regulatory elements.

Type 1 plasminogen activator inhibitor (PAI-1) is the major physiological inhibitor of plasminogen activation, inhibiting both tissue- and urokinase-type plasminogen activators. In HTC rat hepatoma cells, glucocorticoids increase PAI-1 activity, antigen and mRNA accumulation 3- to 5-fold; this increase is due solely to an increase in the rate of PAI-1 gene transcription. We have identified the cis-acting sequences in the 5'-flanking sequence of the HTC PAI-1 gene that mediate this induction. Analysis of a series of hybrid genes containing various portions of the PAI-1 5'-flanking region fused to the chloramphenicol acetyltransferase reporter gene transfected into HTC cells localized the region involved in the transcriptional regulation by glucocorticoids to between -1237 and -764. Electrophoretic mobility shift assays and DNase-I protection assays showed that a glucocorticoid response element (GRE) 15-mer located at -1212 bound the glucocorticoid receptor DNA-binding domain protein in a concentration-dependent manner. Mutations created within this GRE eliminated its ability both to confer a glucocorticoid response and to bind the glucocorticoid receptor. When placed upstream of a heterologous promoter in either orientation, this GRE conferred glucocorticoid inducibility. We, therefore, conclude that the sole cis-acting sequence required for the glucocorticoid response of the PAI-1 gene in rat HTC hepatoma cells is the GRE at -1212.

Regulatory sequences and protein-binding sites involved in the expression of the rat plasminogen activator inhibitor-1 gene.

Plasminogen activator inhibitor-1 (PAI-1) is a serine protease inhibitor that inhibits both tissue-type and urokinase-type plasminogen activators. Expression of PAI-1 is regulated by growth factors, cytokines, and hormones. To determine the molecular mechanisms involved in the basal expression of the rat PAI-1 gene, we have analyzed the cis-acting sequences and the trans-acting factors involved in the transcription of this gene in the HTC rat hepatoma cell line. DNase I protection analyses revealed eight regions within the first 764 base pairs of 5'-flanking sequence that interact specifically with HTC cell nuclear proteins. The proteins that bind to five of the eight footprinted sites were identified as PEA3-, Sp1-, and CTF/NF-1-like proteins using competition electrophoretic mobility shift assays. The expression of fusion genes containing progressive 5' deletions of the rat PAI-1 promoter linked to the chloramphenicol acetyltransferase reporter gene were analyzed in transient transfection experiments in HTC cells. These studies demonstrated the Sp1 and CTF/NF-1 sites to be important for transcriptional activation. Two of the footprinted sites contain the sequence 5'-TTTGn(n)TCAAT-3' and were shown in competition electrophoretic mobility shift assays to bind the same or related protein(s). Sequences containing these sites, from -764 to -628 base pairs, and from -266 to -188 base pairs, were identified in functional studies as repressor elements of transcription.