GLP1R and pancreatic insulinoma: GLP‐1 R not expressed in primary osteoblasts but in insulinoma cells and bone marrow stem cells; exenatide increased trabecular bone mass, bone strength, osteoblast number, bone formation markers (OC and P1NP) and suppressed adipocyte differentiation in bone marrow stem cells; exenatide treatment increased B‐catenin (Wnt pathway), Runx2, and osterix