Secreted Frizzled-Related Protein 5 (SFRP5) in Patients with Obstructive Sleep Apnea

Objective Obstructive sleep apnea (OSA) is closely related to obesity, insulin resistance and inflammation. Secreted frizzled-related protein 5 (SFRP5) is a recently discovered adipokine. It is involved in insulin resistance and inflammation in obesity. This study aimed at evaluating the association between SFRP5 and sleeping characteristics as well as biochemical parameters of OSA patients. Methods This was a prospective case control study. Nondiabetic OSA patients and controls were consecutively recruited and divided into three groups: OSA group, apnea-hypopnea Index (AHI)≥5/h; healthy controls with normal body mass index (BMI); obese controls without OSA, and BMI > 24.0 kg/m². All participants underwent polysomnography (PSG). Plasma SFRP5 was examined using enzyme-linked immunosorbent assay (ELISA). Blood biochemical examinations, including fasting blood glucose (FBG), lipid profile, hypersensitive C-reactive protein (hsCRP), were performed early in the morning after PSG. Patients with severe OSA were treated with nasal continuous positive airway pressure (nCPAP), and plasma SFRP5 was repeatedly measured for comparison. Results Sixty-eight subjects were enrolled in the study, including 38 patients of OSA, whose medium AHI was 58.70 /h (36.63, 71.15), 20 obese controls, and 10 healthy controls. The plasma SFRP5 level of OSA patients was not significantly different from that of healthy controls or obese controls. In OSA patients, SFRP5 level correlated positively with triglyceride level (r=0.447, P=0.005) and negatively with LDL-cholesterol level and HDL- cholesterol level (r=-0.472 and P=0.003; r=-0.478 and P=0.002; respectively). SFRP5 level was not found correlating with FBG, AHI, or any of nocturnal hypoxia parameters. After overnight nCPAP treatment, plasma SFRP5 levels of OSA patients did not change significantly (t=1.557, P = 0.148) compared to that of pretreatment. Conclusions In nondiabetic OSA patients, plasma SFRP5 is associated with the lipid profile. However, no correlation was observed between SFRP5 and FBG or sleep parameters. The SFRP5 level of OSA patients did not differ from that of non-OSA individuals in our study.