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K Hotta, Laboratory for Endocrinology and Metabolism, RIKEN, Yokohama, 230-0045, Japan
M Hosaka, Department of Molecular Medicine, Gunma University, Maebashi, Japan
A Tanabe, Laboratory for Endocrinology and Metabolism, RIKEN, Yokohama, Japan
T Takeuchi, Department of Molecular Medicine, Gunma University, Maebashi, Japan

Correspondence: Kikuko Hotta, Email: kikuko{at}src.riken.jp

Functional variations in the secretogranin III (SCG3) gene are associated with susceptibility to obesity. SCG3 forms secretory granules with orexin, melanin-concentrating hormone (MCH), neuropeptide Y (NPY) and proopiomelanocortin (POMC) in the hypothalamus. In this study, we screened proteins for SCG3-binding activity and identified secretogranin II (SCG2) using a yeast two-hybrid system. Immunoprecipitation revealed that SCG2 interacts with SCG3. <italic>In situ</italic> hybridization and immunohistochemistry indicated that SCG2 was highly expressed in the lateral hypothalamic area, paraventricular nucleus and arcuate nucleus of the hypothalamus. Double-labelling immunohistochemical analysis demonstrated that SCG2 was expressed in orexin-, MCH-, NPY- and POMC-expressing neurons. SCG2 was also co-expressed with SCG3. Upon introduction into neuroblastoma cells, SCG2 was expressed in the cytosol and formed granule-like structures with SCG3, orexin, NPY or POMC. SCG3 bound to POMC; however, it did not bind to orexin, MCH or NPY. In contrast, SCG2 formed aggregates with orexin, MCH, NPY and POMC. SCG2 may act as a hormone carrier for orexin, MCH, NPY and POMC by binding with SCG3, which targets proteins to the secretory granules. SCG2 mRNA levels increased along with those of SCG3, orexin, MCH and NPY after a 24-h fast, suggesting that the SCG2/SCG3 system may respond in an adaptive manner to acute body weight changes. However, this SCG2/SCG3 system appears to be unresponsive to chronic body weight changes, such as diet-induced obesity or obesity in ob/ob mice. We suggest that SCG2, as well as SCG3, may be a potential regulator of food intake based on its capacity to accumulate appetite-related hormones into secretory granules.