Selective Prostacyclin Receptor Agonism Augments Glucocorticoid-induced Gene Expression in Human Bronchial Epithelial Cells

Pam Shen, Sylvia Wilson, Christopher Rider, Suzanne Traves, David Proud, Robert Newton, Giembycz Mark. University of Calgary, Calgary, Alberta, Canada.

Prostacyclin receptor (IP) agonists display anti-inflammatory and anti-viral activity in cell-based assays and in pre-clinical models of asthma and chronic obstructive pulmonary disease. We have extended these observations by demonstrating that IP-receptor activation also can enhance the ability of glucocorticoids to induce genes with anti-inflammatory activity. BEAS-2B bronchial epithelial cells stably transfected with a glucocorticoid response element (GRE) luciferase reporter were activated in a concentration-dependent manner by the glucocorticoid, dexamethasone (p[A]50 = 7.96 ± 0.11; Emax = 19.3 ± 3.8-fold). An IP-receptor agonist, taprostene, increased cAMP in these cells (p[A]50 = 8.01 ± 0.01) and augmented luciferase expression at all concentrations of dexamethasone examined. Analysis of the concentration-response relationship that described this effect showed that taprostene increased the magnitude of transcription (∼ 2-fold) without affecting the potency of dexamethasone (p[A]50 = 7.73 ± 0.2) and was, thus, steroid-sparing in this simple system. RO3244794, an IP-receptor antagonist, and oligonucleotides that selectively silenced the IP-receptor gene, PTGIR, abolished these effects of taprostene. Infection of BEAS-2B GRE reporter cells with an adenovirus vector (Ad5.CMV.PKIα) encoding a highly selective inhibitor of cAMP-dependent protein kinase (PKA) also prevented taprostene from enhancing GRE-dependent transcription. In BEAS-2B cells and primary cultures of human airway epithelial cells, taprostene and dexamethasone interacted in either an additive or positive cooperative manner in the expression of three glucocorticoid-inducible genes (glucocorticoid-induced leucine zipper [GILZ], mitogen-activated protein kinase phosphatase-1 [MKP-1] and kinase inhibitor protein 2 of 57 kDa [p57kip2]) that have anti-inflammatory potential. Collectively, these data show that IP-receptor agonists can augment the ability of glucocorticoids to induce anti-inflammatory genes in human airway epithelial cells by activating a cAMP/PKA-dependent mechanism. This observation may have clinical relevance in the treatment of airway inflammatory diseases that are either refractory, or respond sub-optimally, to glucocorticoids.