APLN and pulmonary fibrosis: Only by binding to the APJ receptor could apelin exert their biologic function [19, 22, 23]; while apelin and the APJ receptor are widely distributed in different organs and tissues [22, 23], the apelin/APJ signal axis may play a role in diseases of different organs including pulmonary fibrosis, kidney inflammation [24], acute lung injury (ALI) [25], ischemic heart failure [26], and even osteoporosis [27, 28].