APLN and obesity due to melanocortin 4 receptor deficiency: Moreover, the most common isoform (apelin-36) is located in the lungs, uterus, and testicles.13 Studies have documented a significant role of apelin in the course of metabolic diseases, regulation of the appetite center function, adipose tissue growth, and pathogenesis of obesity.14,15 Apelin reduces blood glucose, increasing its uptake and absorption by muscle cells and adipose tissue.