Recombinant Mouse Osteocalcin Secreted by Lactococcus lactis Promotes Glucagon-Like Peptide-1 Induction in STC-1 Cells

An osteoblastic protein, osteocalcin (OC), exists in vivo in two forms: carboxylated OC, and uncarboxylated or low-carboxylated OC (ucOC). ucOC acts as a hormone to regulate carbon and energy metabolism. Recent studies demonstrated that ucOC exerts insulinotropic effects, mainly through the glucagon-like peptide 1 (GLP-1) pathway. GLP-1 is an insulinotropic hormone secreted by enteroendocrine L cells in the small intestine. Thus, efficient delivery of ucOC to the small intestine may be a new therapeutic option for metabolic diseases such as diabetes and obesity. Here, we genetically engineered a lactic acid bacterium, Lactococcus lactis, to produce recombinant mouse ucOC. Western blotting showed that the engineered strain (designated NZ-OC) produces and secretes the designed peptide (rOC) in the presence of nisin, an inducer of the recombinant gene. Highly-purified rOC was obtained from the culture supernatants of NZ-OC using immobilized metal affinity chromatography. An in vitro assay showed that purified rOC promotes GLP-1 secretion in a mouse intestinal neuroendocrine cell line, STC-1, in a dose-dependent manner. These results clearly demonstrate that NZ-OC secretes rOC, and that rOC can promote GLP-1 secretion by STC-1 cells. Genetically modified lactic acid bacteria (gmLAB) have been proposed over the last two decades as an effective and low-cost mucosal delivery vehicle for biomedical proteins. NZ-OC may be an attractive tool for the delivery of rOC to trigger GLP-1 secretion in the small intestine to treat diabetes and obesity.