Obestatin does not activate orphan G protein-coupled receptor GPR39

Recently, the ligand of the orphan G protein-coupled receptor GPR39 has been identified as obestatin, a 23-amino acid peptide derived from the ghrelin precursor protein. We used two methods to study the possible activation of GPR39 by obestatin: cAMP measurements based on a luminescent reporter gene and a fluorometric Ca(2+) flux method. The former was similar to that reported in the original publication of Zhang et al. [J.V. Zhang, P.G. Ren, O. Avsian-Kretchmer, C.W. Luo, R. Rauch, C. Klein, Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin's effects on food intake, Science 310 (2005) 996-999]. The latter method used promiscuous as well as chimaeric G-proteins commonly used to couple orphan G protein-coupled receptors to the phospholipase C pathway, that leads to intracellular Ca(2+) rise. We could, however, not demonstrate activation of the GPR39 receptor by obestatin via any of these signal transduction pathways. We could activate GPR39 by high concentrations of Zn(2+), demonstrating cell surface expression of a functional receptor that could elicit a Ca(2+) response. The Zn(2+) response was not affected by obestatin. The identity of the native ligand for GPR39 remains to be elucidated.