Presence of a functional receptor for GLP‐1 in osteoblastic cells,independent of the cAMP‐linked GLP‐1 receptor

Glucagon‐like peptide 1 (GLP‐1) controls glucose metabolism in extrapancreatic tissues through receptors other than the pancreatic cAMP‐linked GLP‐1 receptor; also, GLP‐1 induces an insulin‐ and PTH‐independent bone anabolic action in insulin‐resistant and type‐2 diabetic rats. Here we searched for the presence and characteristics of GLP‐1 receptors in osteoblastic MC3T3‐E1 cells. [¹²⁵I]‐GLP‐1 specific binding to MC3T3‐E1 cells was time‐ and temperature‐dependent, reaching maximal value at 30 min at 25°C; in these conditions, [¹²⁵I]‐GLP‐1 binding was dissociable, and displaced by GLP‐1, partially by GLP‐2, but not by exendin‐4 (Ex‐4), exendin‐9 (Ex‐9), glucagon or insulin; Scatchard analysis of the unlabeled GLP‐1 data showed high and low affinity binding sites; cross‐linking of GLP‐1 binding revealed an estimated 70 kDa band, almost undetectable in the presence of 10^?6 M GLP‐1. GLP‐1, Ex‐9, insulin or glucagon failed to modify cellular cAMP content, while GLP‐2 and Ex‐4 increased it. However, GLP‐1 induced an immediate hydrolysis of glycosylphosphatidylinositols (GPIs) generating short‐lived inositolphosphoglycans (IPGs), and an increase in phosphatidylinositol‐3 kinase (PI3K) and mitogen activated protein kinase (MAPK) activities; Ex‐4 also affected GPIs, but its action was delayed with respect to that of GLP‐1. This incretin was found to decrease Runx2 but increased osteocalcin gene expression, without affecting that of osteoprotegerin or the canonical Wnt pathway activity in MC3T3‐E1 cells which do not express the pancreatic GLP‐1 receptor. Our data demonstrate for the first time that GLP‐1 can directly and functionally interact with osteoblastic cells, possibly through a GPI/IPG‐coupled receptor. J. Cell. Physiol. 225: 585–592, 2010. © 2010 Wiley‐Liss, Inc.