Metrnl as a secreted protein: Discovery and cardiovascular research.

Secreted proteins have gained more and more attentions, since they can become therapeutic targets, drugs and biomarkers for prevention, diagnosis and treatment of disease and aging. In 2014, Metrnl (also named Meteorin-like, Cometin, Subfatin, Interleukin-39, Interleukin-41, Meteorin-β, and Metrn-β/Metrnβ), as a novel secreted protein released from a certain tissue, was reported by us and others. During the past decade, the number of articles on Metrnl has continued to increase. Different sources of Metrnl have been described with different functions, including Metrnl as an adipokine for insulin sensitization, a cardiokine against cardiac hypertrophy and dysfunction, an endothelium-derived factor against endothelial dysfunction and atherosclerosis, etc. Especially, we show that endothelial Metrnl is a major source for circulating Metrnl levels. Meanwhile, lots of clinical studies have investigated the relationship between blood Metrnl levels and metabolic, inflammatory and cardiovascular diseases. Metrnl appears a protective factor and a promising therapeutic target and/or drug against these diseases, given the relatively consistent conclusion from the preclinical studies. In addition to graphically demonstrating the role of Metrnl in various organs and diseases, this review will mainly describe the discovery of Metrnl, summarize the role of Metrnl in cardiovascular system that is a recently major progress in Metrnl research, and highlight several perspectives for future basic and translational research. Also, we suggest using one name Metrnl instead of other multiple names for the same protein.