Decarboxylated osteocalcin improves human islet function and induces beta cells proliferation in-vitro and in-vivo

O. Sabek¹, D. Fraga¹, K. Nishimoto², A. Gaber^1
1 The Methodist Hospital Institute, Surgery, Houston, USA; ² University of Tennessee, Memphis, USA

Objective: The osteoblast-specific secreted molecule osteocalcin infusion in wild type mice was shown to increase the insulin genes Ins1 and Ins2 expression and to induce the expression of the genes necessary for in vivo β-cell proliferation such as cyclin D2 and Cyclin-dependent kinase 4 (Cdk4). In our laboratory, we found that non-function human islet show an up regulation of oxytocin gene expression which inhibits cell proliferation, and down regulation of cyclin D which is involved in beta cell proliferation. In this study, we investigate the impact of osteocalcin on human islets function and beta cells proliferation, in-vitro and in-vivo.

Methods: Aliquots of human islets from 7 donors were cultured in serum free media with 0.3, 1.0, 4.5, 15 ng/ml of decarboxylated osteocalcin. After 7, 14, and 28 days in culture, islets were analyzed for insulin content, insulin secretion and beta cells proliferation using western blot as well as static incubation assay. In-vivo function of human islets from 5 isolations with and without decarboxylated osteocalcin (4.5 ng/ml/hr) was tested using NOD-SCID mouse model (500IEQ/mice).

Results: Human islet co-cultured with decarboxylated osteocalcin showed a significant increase in insulin content as early as 7-post culture, insulin content was two fold higher than that of the control islet. Western blots of human islet showed an increase in protein expression for Cyclin D1&Cdk4, which are necessary for beta cells proliferation and SUR-1 which functions as a modulator of ATP-sensitive potassium channels and insulin release in islets cultured in 4.5ng/ml decarboxylated osteocalcin supplemented media. Moreover, supplementing the transplanted human islets in-vivo with decarboxylated osteocalcin resulted in significant increase in human insulin and c-peptide production p<0.05.

Conclusions: Culturing human islet with decarboxylated osteocalcin, results in an increase of Beta cells number, insulin content, insulin processing and significantly increase in-vivo production of c-peptide.