Endothelin-1 induces interleukin-18 expression in human osteoblasts

Abstract

Objective: Both endothelin-1 (ET-1) and interleukin (IL)-18 induce osteoblast proliferation under normal and pathophysiological conditions. In the present study, we explored the interaction between the two proteins by examining the effect of ET-1 on IL-18 expression in cultured human osteoblasts.

Methods: Human osteoblasts were treated with ET-1 in different concentrations (1, 10, 20, 40, or 50 nM) for different length of time (1, 6, 12, 18, or 24 h) in the presence or absence of ET A receptor (ETAR) blocker BQ123, ET B receptor (ETBR) blocker BQ788, p38 mitogen-activated protein kinase (MAPK) siRNA, or different kinase inhibitors.

Results: ET-1 increased the IL-18 mRNA level in a statistically significant dose- and time-dependent manner within 18 h, which was reflected in dose-dependent induction of the human IL-18 gene promoter activity and IL-18 protein/secreted protein expression. BQ123 (1μ M) and p38 MAPK siRNA and inhibitor PD169316 (25 μM) completely abolished the promoting effect of ET-1 on IL-18 expression. [(3)H]thymidine incorporation assays showed that ET-1 lost a major part (57%) of its promoting effect on osteoblast proliferation when the endogenous IL-18 expression in osteoblasts was knocked down by 75%.

Conclusions: ET-1 induces IL-18 expression in human osteoblasts at the gene promoter/transcription level via ETAR by a p38 MAPK-dependent mechanism, and that IL-18 mediates a major part of ET-1 induced osteoblast proliferation. This study provides the first evidence of interaction between ET-1 and IL-18 in osteoblast and adds new insights into bone physiology and pathophysiology.

Keywords: Cell proliferation; Endothelin A receptor; Endothelin-1; Interleukin-18; Osteoblast; p38 mitogen-activated protein kinase.