ABSTRACT
In this study, we begin to investigate the underlying mechanism of leptin-induced vascular calcification. We found that treatment of cultured bovine aortic smooth muscle cells (BASMCs) with leptin (0.5-4 µg/ml) induced osteoblast differentiation in a dose-dependent manner. Furthermore, we found that leptin significantly increased the mRNA expression of osteopontin and bone sialoprotein, while down-regulating matrix gla protein (MGP) expression in BASMCs. Key factors implicated in osteoblast differentiation, including members of the Wnt signaling pathway, were examined. Exposure to leptin enhanced phosphorylation of GSK-3ß on serine-9 thereby inhibiting activity and promoting the nuclear accumulation of ß-catenin. Transfection of BASMCs with an adenovirus that expressed constitutively active GSK-3ß (Ad-GSK-3ß S9A) resulted in a >2-fold increase in GSK-3ß activity and a significant decrease in leptin-induced alkaline phosphatase (ALP) activity. In addition, qRT-PCR analysis showed that GSK-3ß activation resulted in a significant decrease in the expression of osteopontin and bone sialoprotein, but a marked increase in MGP mRNA expression. When taken together, our results suggest a mechanism by which leptin promotes osteoblast differentiation and vascular calcification in vivo.
Subject(s)
Calcification, Physiologic/drug effects , Cell Differentiation/drug effects , Glycogen Synthase Kinase 3/antagonists & inhibitors , Leptin/pharmacology , Muscle, Smooth, Vascular/drug effects , Myocytes, Smooth Muscle/drug effects , Osteoblasts/drug effects , Active Transport, Cell Nucleus/drug effects , Animals , Calcium-Binding Proteins/antagonists & inhibitors , Cattle , Cell Nucleus/metabolism , Cells, Cultured , Extracellular Matrix Proteins/antagonists & inhibitors , Glycogen Synthase Kinase 3 beta , Humans , Integrin-Binding Sialoprotein/biosynthesis , Muscle, Smooth, Vascular/cytology , Muscle, Smooth, Vascular/enzymology , Myocytes, Smooth Muscle/cytology , Myocytes, Smooth Muscle/enzymology , Osteoblasts/cytology , Osteopontin/biosynthesis , beta Catenin/metabolism , Matrix Gla ProteinABSTRACT
Recent clinical trials have shown that the risk of developing osteoporosis is substantially lower when low molecular weight heparins (LMWHs) are used in place of unfractionated heparin. While the reason(s) for this difference has not been fully elucidated, studies with animals have suggested that heparin causes bone loss by both decreasing bone formation and increasing bone resorption. In contrast, LMWHs appear to cause less bone loss because they only decrease bone formation. Whether all LMWHs decrease bone formation and therefore cause bone loss is unknown. For example, preliminary in vitro studies with the synthetic pentasaccaride, Fondaparinux, have suggested that it may not decrease bone formation and thus, may have no deleterious effects on bone. Further studies are required in order to determine if all LMWHs cause bone loss equally.
