Mechanism of dexamethasone-mediated interleukin-8 gene suppression in cultured airway epithelial cells

Abstract

The effects of dexamethasone, a glucocorticoid analog, on interleukin 8 (IL-8) gene expression were studied in cultures of primary human tracheobronchial epithelial cells and an immortalized human bronchial epithelial cell line, HBE1 cells. Dexamethasone inhibited IL-8 mRNA and protein expression in a concentration- and time-dependent manner. The inhibition did not occur at the transcriptional level since both nuclear run-on activity and IL-8 promoter-reporter gene expression assay revealed no significant effect. Instead, there was a change in IL-8 mRNA stability in dexamethasone-treated cultures. Under actinomycin D treatment, IL-8 mRNA was quite stable in dexamethasone-depleted cultures, while in dexamethasone-pretreated cultures, IL-8 message was rapidly degraded within the first hour, then leveled off. When dexamethasone and actinomycin D were added simultaneously to dexamethasone-depleted cultures, IL-8 mRNA remained rather stable. When cycloheximide was used to inhibit new protein synthesis, dexamethasone-dependent inhibition was not observed. These results suggest that a posttranscriptional mechanism, which requires dexamethasone-dependent new protein synthesis, is involved in the regulation of IL-8 mRNA by dexamethasone in airway epithelial cells.

Anti-inflammatory functions of glucocorticoid-induced genes

“…Consistent with this, the protein synthesis inhibitor cycloheximide (CHX) did not diminish the inhibitory effect of dex on expression of IL-6, intracellular adhesion molecule (ICAM)-1 or COX-2 mRNAs (De Bosscher et al, 1997;Wissink et al, 1998). In other studies, CHX blocked the GCmediated suppression of urokinase plasminogen activator, COX-2 and CXCL8 mRNA (Chang et al, 2001;Chaudhary and Avioli, 1996;Chivers et al, 2006;Henderson and Kefford, 1993;Newton et al, 1998;Tobler et al, 1992). This implies that GC-induced gene expression is involved, and that transrepression is insufficient for gene suppression in some circumstances.…”

“…In other cases data on the relative contributions of transcriptional and post-transcriptional mechanisms are contradictory. Dex did not strongly inhibit CXCL8 transcription in nuclear run-on experiments, but potently destabilized CXCL8 mRNA in bone marrow stromal cells, lung fibroblasts, epithelial cells or cell lines (Chang et al, 2001;Chaudhary and Avioli, 1996;Chivers et al, 2006;Tobler et al, 1992). In contrast, other…”

“…Targets of such regulation include COX-2 (Chivers et al, 2006;Lasa et al, 2001;Newton et al, 1998;Ristimaki et al, 1996), TNF (Han et al, 1990;Kontoyiannis et al, 1999;Swantek et al, 1997), vascular endothelial growth factor (Gille et al, 2001), interferon β (Peppel et al, 1991), colony stimulating factor 2 (CSF2) (Bergmann et al, 2004;Tobler et al, 1992;Tran et al, 2005), IL-5 (Staples et al, 2003), IL-6 (Amano et al, 1993;Tobler et al, 1992), IL-1α and -1β (Amano et al, 1993;Lee et al, 1988), inducible nitric oxide synthase (Korhonen et al, 2002) and several chemokines (Berkman et al, 1995;Brach et al, 1992;Chang et al, 2001;Chaudhary and Avioli, 1996;Chivers et al, 2006;Mukaida et al, 1991;Poon et al, 1999;Stellato et al, 1999;Tobler et al, 1992). The majority of these mRNAs have in common the presence of adenylate/uridylate-rich elements (AREs) in their 3' untranslated regions (UTRs).…”

Eotaxin/CCL11 Suppresses IL-8/CXCL8 Secretion from Human Dermal Microvascular Endothelial Cells

“…Previous reports have indicated that cellular control of IL-8/CXCL8 production is largely regulated at the posttranscriptional level (35,53,54). Experimental evidence suggests that binding of certain proteins to AU-rich regions in the 3Ј-untranslated regions of chemokine mRNAs modulates the stability of these mRNAs (55).…”

“…IL-8/CXCL8 production is controlled at the level of transcription and mRNA stability (34,35). Because eotaxin did not affect cytokine induction of adhesion molecules or other chemokines, we reasoned that the mechanism of eotaxin/CCL11 suppression of IL-8/CXCL8 release was likely to be posttranscriptional.…”