Endocrine Abstracts

Adenosine 5′-monophosphate-activated protein kinase (AMPK), a regulator of energy homeostasis, has a central role in mediating the appetite-modulating and metabolic effects of many hormones and neuromodulators. We previously demonstrated similar neuroendocrine activation of AMPK in bone-forming osteoblasts. In this study, we tested whether stimulation of AMPK activity in osteoblasts plays a role in their function and whether deletion of the catalytic AMPKα1 subunit, the most abundant isoform in bone, affects bone mass in vivo. Primary osteoblasts were obtained from rat calvaria by trypsin/collagenase digestion and cultured for 14–17 days in the presence of two activators of AMPK, AICAR (a cell-permeable AMP analogue) and metformin. Formation of ‘trabecular-shaped’ bone nodules was evaluated following alizarin red staining. AMPKα1⁻/₋ mice were generated by Benoit Violet (INSERM U567, Paris). Tibia were harvested from 4 month old male wild-type (WT) and AMPKα1⁻/₋ KO mice (n=10/group) and bone characteristics determined using micro-CT. We demonstrated that both AICAR and metformin dose-dependently increase trabecular bone nodule formation. Quantitative analysis of alizarin red-stained cultures confirmed the potent dose-dependent stimulatory effects of AMPK activators on the number and size of bone nodules formed by osteoblasts. AMPK α1-deficient mice showed a bone loss in both cortical and trabecular bone compartments of the tibia. The knockout mice showed dramatic decreases in trabecular bone volume (−39.2%), trabecular number (−31.1%) and trabecular thickness (−12.2%) compared to WT mice. As expected, trabecular separation was significantly increased in KO mice. The cortical indexes were also decreased in mice lacking AMPKα1. Bone area (−28%) and cortical thickness (−17%) were significantly reduced in mice lacking AMPKα1 versus WT. Our data are consistent with AMPK playing an important role in osteoblast function. AMPK activators stimulate in vitro bone formation and ablation of AMPKα1 signaling decreases bone mass. Further studies will determine the role of AMPK in skeletal physiology.