Osteocalcin protects islet identity in LDL receptor knockout mice on high fat diet

Abstract

Metabolic syndrome (MetS) is an increasing global health threat and strong risk factor for type 2 diabetes (T2D). MetS causes both hyperinsulinemia and islet size overexpansion, and pancreatic beta (β)-cell failure impacts insulin and proinsulin secretion, mitochondrial density, and cellular identity loss. The low-density lipoprotein receptor knockout (LDLr-/-) model combined with high fat diet (HFD) has been used to study alterations in multiple organs, but little is known about changes to β-cell identity resulting from MetS. Osteocalcin (OC), an insulin-sensitizing protein secreted by bone, shows promising impact on β-cell identity and function. LDLr-/- mice at 12mo were fed chow or HFD for 3 months ± 4.5 ng/h osteocalcin. Islets were examined by immunofluorescence for alterations in nuclear Nkx6.1 and PDX1 presence, insulin-glucagon colocalization, islet size and %β-cell and islet area by insulin and synaptophysin, and mitochondria fluorescence intensity by Tomm20. Bone mineral density (BMD) and %fat changes were examined by Piximus Dexa scanning. HFD-fed mice showed fasting hyperglycemia by 15mo, increased weight gain, %fat, and fasting serum insulin and proinsulin; concurrent OC treatment mitigated weight increase and showed lower proinsulin/insulin ratio, and higher BMD. HFD increased %β and %islet area, while simultaneous osteocalcin-treatment with HFD was comparable to chow-fed mice. Significant reductions in nuclear PDX1 and Nkx6.1 expression, increased insulin-glucagon colocalization, and reduction in β-cell mitochondria fluorescence intensity were noted with HFD, but largely prevented with OC administration. Osteocalcin supplementation here suggests a benefit to β-cell identity in LDLr-/- mice and offers intriguing clinical implications for countering metabolic syndrome.