Histamine modulation of peripheral CRH receptor type 1alpha expression is dependent on Ca(2+) signalling and NF-kappaB/p65 transcriptional activity.

Histamine promotes immune complex-induced vascular leakage in vivo, a critical and early event that leads to joint-specific autoimmune damage. Initial assessment, using explanted human synovial tissue (ST), indicates that histamine can modulate local expression of type 1 alpha CRH receptors (CRH-R1alpha). The objective of this study was to elucidate the signalling events and transcriptional mechanism(s) controlling histamine-dependent regulation of CRH-R1alpha expression in human inflammatory arthritis. Histamine significantly promotes CRH-R1alpha mRNA and protein expression in a time- and concentration-dependent manner in human endothelial and synoviocyte cells. Transactivation of the human CRH-R1 promoter is significantly enhanced by histamine which can be mimicked by treatment with a Ca(2+) ionophore and completely diminished in the presence of a Ca(2+) chelator. Histamine-mediated responses involve enhanced activation and nuclear localisation of transcription factors including CREB, NF-kappaB and NR4A2. Functional consequences of enhanced CREB, NF-kappaB and NR4A2 activity confirm that NF-kappaB/p65 selectively controls CRH-R1 promoter activity. Co-transfection of NF-kappaB/p65 potently transactivates the CRH-R1 promoter while co-expression of a dominant negative IkappaBalpha kinase inhibits endogenous and histamine-induced promoter activity. Bioinformatic analysis identifies three putative kappaB consensus binding sites at proximal and distal positions and 5' deletional analysis identifies promoter region(s) required for activation by histamine and NF-kappaB/p65. We observe direct NF-kappaB/p65 interaction within the promoter region and site-directed mutagenesis reveals that all three kappaB sites are required to mediate histamine and NF-kappaB/p65 regulation of CRH-R1 promoter activity. These findings confirm that histamine, via enhanced Ca(2+) signalling and NF-kappaB/p65 activity, contributes to changes in ST inflammation by promoting CRH-R1alpha-mediated responses.