Obestatin induction of early-response gene expression in gastrointestinal and adipose tissues and the mediatory role of G protein-coupled receptor, GPR39

Obestatin was identified as a brain/gut peptide hormone encoded by the ghrelin gene and found to interact with the G protein-coupled receptor, GPR39. We investigated target cells for obestatin based on induction of an early-response gene c-fos in different tissues. After ip injection of obestatin, c-fos staining was found in the nuclei of gastric mucosa, intestinal villi, white adipose tissues, hepatic cords, and kidney tubules. Immunohistochemical analyses using GPR39 antibodies further revealed cytoplasmic staining in these tissues. In cultured 3T3-L1 cells, treatment with obestatin, but not motilin, induced c-fos expression. In these preadipocytes, treatment with obestatin also stimulated ERK1/2 phosphorylation. Because phenotypes of GPR39 null mice are partially consistent with a role of GPR39 in mediating obestatin actions, we hypothesized that inconsistencies on the binding of iodinated obestatin to GPR39 are due to variations in the bioactivity of iodinated obestatin. We obtained monoiodoobestatin after HPLC purification and demonstrated its binding to jejunum, stomach, ileum, pituitary, and white adipose tissue. Furthermore, human embryonic kidney 293T cells transfected with plasmids encoding human or mouse GPR39 or a human GPR39 isoform, but not the ghrelin receptor, exhibited high-affinity binding to monoiodoobestatin. Binding studies using jejunum homogenates and recombinant GPR39 revealed obestatin-specific displacement curves. Furthermore, treatment with obestatin induced c-fos expression in gastric mucosa of wild-type, but not GPR39 null, mice, underscoring a mediating role of this receptor in obestatin actions. The present findings indicate that obestatin is a metabolic hormone capable of binding to GPR39 to regulate the functions of diverse gastrointestinal and adipose tissues.