Regulation of interleukin-8 gene expression by interleukin-1beta, osteotropic hormones, and protein kinase inhibitors in normal human bone marrow stromal cells

Abstract

Interleukin-8 (IL-8), a potent neutrophil chemotactic peptide that elicits pleiotropic biological effects is secreted in large amounts by normal human osteoblastic and bone marrow osteoprogenitor stromal (HBMS) cells in response to IL-1beta and tumor necrosis factor-alpha. In the present study we investigated the regulation of IL-8 gene expression by IL-1beta, osteotropic hormones, and protein kinase inhibitors in primary cultures of HBMS cells. The treatment of HBMS cells with IL-1beta increased the steady-state levels of IL-8 mRNA in a dose- and time-dependent fashion and was detectable within 1 h, reached maximal by 4 h, and remained elevated at 24 h, whereas parathyroid hormone (10(-7) and 10(-8) M) had no effect on IL-8 mRNA. Both synthetic and natural glucocorticoids dexamethasone (10(-7)-10(-10) M) and hydrocortisone (10(-6)-10(-8) M) inhibited IL-1beta-stimulated IL-8 mRNA expression. The suppressive effect of dexamethasone on IL-1beta-induced IL-8 mRNA was not observed in the presence of cycloheximide (5 microg/ml), indicating that the dexamethasone-mediated repression of IL-8 gene expression also depends on new protein synthesis. Experiments with actinomycin D demonstrated that IL-8 mRNA is long-lived and that glucocorticoids down-regulate IL-8 gene expression mainly by decreasing the mRNA stability in normal HBMS cells. Furthermore, as determined by nuclear run-on analysis, IL-1beta increased the rate of transcription of IL-8 gene and dexamethasone did not affect the IL-1beta-induced transcription of IL-8. 1-(5-Isoquinolinesulfonyl)-2-methylpiperazine, HCl (50 microM) and staurosporine (1 microM), potent inhibitors of protein kinase C, and genistein (100 microM), a specific protein tyrosine kinase inhibitor blocked IL-1beta-induced IL-8 gene expression. Because curcumin (20 microM), an inhibitor of c-jun/AP-1 and protein kinases, also blocked IL-1beta-stimulated IL-8 gene expression implicating c-JUN/AP-1 and protein phosphorylation in the induction of IL-8 gene expression by IL-1beta, we conclude that the regulation of IL-8 mRNA by IL-1beta is mediated via protein kinase-dependent signal transduction pathways. Our accumulated results have demonstrated that glucocorticoid suppression of IL-1beta-induced IL-8 mRNA occurs at the levels of post-transcription (mRNA stability) and protein synthesis.