Intermittent Hypoxia Induces Expression of Epiregulin mRNA via Upregulation of Interleukin-6 in Human Coronary Artery Smooth Muscle Cells

Sleep apnea syndrome (SAS) is characterized by repetitive episodes of oxygen desaturation and re-saturation, such as intermittent hypoxia (IH), and well known as an independent risk factor for diabetes and atherosclerosis. We previously showed that IH directly induced the proliferation of cultured rat aorta smooth muscle cells through upregulations of epidermal growth factor family, especially epiregulin (EREG), and their erbB2 receptor. However, its underlying mechanism remains unclear. On the other hand, we also reported that IH increased interleukin (IL)-6 mRNA in pancreatic beta cells. In this study, we investigated the influence of IH on IL-6 production, and the impact of IL-6 on the expression of EREG, using human coronary artery smooth muscle cells (hCASMC) and in vitro IH system. Quantitative RT-PCR revealed that EREG mRNA was significantly increased by IH, but not by sustained hypoxia. IH also increased expression of IL-6 mRNA in IH cycle dependent manner (3∼72 cycles in 1∼24 hours). Mature IL-6 increased in IH-exposed cell conditioned medium as similar time-dependent manner as seen in mRNA. We next investigated whether IL-6 increases the expression of EREG mRNA in hCASMCs. The addition of IL-6 (100 ng/mL) induced a significant upregulation of EREG mRNA expression. Taking into account the IH-induced increase of IL-6 and IL-6-induced upregulation of EREG mRNA in hCASMCs, we hypothesized that elevated IL-6 production due to IH was responsible for the IH-induced increase in EREG mRNA. To determine the direct role of IL-6 in IH-induced increases in EREG mRNA expression, we applied siRNAs for IL-6 and the IL-6 receptor and obtained the result that both siRNAs for IL-6 and IL-6 receptor significantly suppressed the IH-induced increase of EREG mRNA. These results indicated that IL-6 plays a pivotal role in EREG upregulation by IH and consequently OSA-related atherosclerosis.