Is FAM19A5 an adipokine? Peripheral FAM19A5 in wild-type, FAM19A5 knockout, and LacZ knockin mice.

FAM19A5 is a novel secretory protein expressed primarily in the brain. However, a recent study reported that FAM19A5 is an adipocyte-derived adipokine that regulates vascular smooth muscle function through sphingosine-1-phosphate receptor 2 (S1PR2). In our study, we investigated FAM19A5 transcript and protein levels in peripheral tissues, including adipose tissues, from wild-type, FAM19A5 knockout, and FAM19A5-LacZ knockin mice. We found that the FAM19A5 transcript levels in the central nervous system were much greater than those in any of the peripheral tissues, including adipose tissues. Furthermore, the FAM19A5 protein levels in adipose and reproductive tissues were below detectable limits for Western blot analysis and enzyme-linked immunosorbent assay (ELISA). Additionally, we found that the FAM19A5 protein did not interact with S1PR2 in terms of G-protein-mediated signal transduction, β-arrestin recruitment, or ligand-mediated internalization. Taken together, our findings revealed basal levels of FAM19A5 transcripts and proteins in peripheral tissues, confirming its primary expression in the central nervous system and lack of significant interaction with S1PR2.