NESFATIN-1 AND NESFATIN-1-LIKE PEPTIDE: NUCLEOBINDIN-1/2-ENCODED INSULINOTROPIC AND ENTEROTROPIC PEPTIDES

Abstract

Nucleobindins are a class of secreted, multi-domain Ca2+ binding proteins that interact with nucleic acids. Two nucleobindins, nucleobindin-1 (NUCB1) and nucleobindin-2 (NUCB2) have been identified so far. In 2006, nesfatin-1, an 82 amino acid peptide encoded in NUCB2 was discovered. Nesfatin-1 is an anorexigenic and insulinotropic peptide found abundantly in hypothalamus, pancreas and stomach. Meal responsive insulin secretion is regulated by glucagon like peptide-1 (GLP-1), glucose dependent insulinotropic polypeptide (GIP), peptide YY (PYY) and cholecystokinin (CCK) secreted by intestinal mucosal cells. Since both nesfatin-1 and intestinal hormones modulate insulin secretion, nesfatin-1 could regulate intestinal hormones to elicit its insulinotropic action. Nucleobindin-1 primarily regulates Ca2+ homeostasis. Like NUCB2, NUCB1 is also present in the pancreas, stomach, intestine and pituitary. NUCB2 has a high similarity (62% in humans) to NUCB1. Both proteins also retain their prohormone convertase cleavage sites. However, no information exists on whether NUCB1 encodes bioactive peptides. The fact that NUCB1 is a secreted protein suggests an endocrine function for NUCB1 and/or its encoded peptide. This research hypothesizes that nesfatin-1 is enterotropic, and NUCB1 encodes an insulinotropic nesfatin-1-like peptide (NLP). Nesfatin-1 protein expression was found in STC-1 cells and it co-localized GLP-1, GIP, CCK and PYY in mouse enteroendocrine cells. Treatment of STC-1 cells with nesfatin-1 stimulated GLP-1, GIP, CCK mRNA expression and protein secretion, while opposite effects were found for PYY. In silico analysis of the NUCB1 amino acid sequence found a 77 amino acid NLP. Mouse pancreatic islets and MIN6 cells express NUCB1 mRNA and protein. NUCB1 was co-localized with insulin in mouse pancreatic islets. While treatment of cells with synthetic NLP increased preproinsulin mRNA expression and secretion, a scrambled peptide based on NLP was ineffective, indicating that the specific amino acid sequence is crucial for its insulinotropic action. Overall, the data presented supports the hypotheses. The studies reaffirm NUCB2 expression in intestine and provide the first set of evidence for nesfatin-1 regulation of enteric hormones. It also found a novel NUCB1 encoded insulinotropic NLP that could elicit other functions of nesfatin-1.