Jonathan W Lowery, Ph.D.
Medicine and Health Sciences
Osteoporosis is a disease characterized by low bone mineral density due to the rate of bone resorption exceeding that of bone formation. Substantial evidence indicates the Bone Morphogenetic Protein (BMP) pathway promotes bone formation through action of the effectors SMAD1/5/8 while the Activin pathway negatively influences bone mass through action of the effectors SMAD2/3. Recent studies from our lab suggest that BMP and Activin ligands regulate bone mass in a see-saw-like mechanism via competition for a shared pool of receptors, i.e. receptor-level competition. In the present study we seek to test this hypothesis in vitro via signaling responsiveness assays using pathway-specific western blot analyses in the osteogenic cell line W-20-17. We first confirmed that W-20-17 cells respond to exogenous stimulation by BMP2 and Activin-A. Then, we administered recombinant versions of naturally-occurring extracellular ligand traps for BMP2 or Activin ligands (Noggin and Follistatin, respectively) to examine basal antagonism between these pathways. This revealed that, under basal conditions, SMAD1/5/8 activation is repressed by Activin signaling; interestingly, the converse relationship was not observed. To determine the molecular mechanism allowing for this relationship, we treated W-20-17 cells with SB431542, which is an intracellular inhibitor of Activin signaling that functions downstream of receptor engagement, and found no effect on SMAD1/5/8 activation. Collectively, our results suggest Activin-mediated repression of BMP signaling is ligand-dependent but occurs upstream of SMAD2/3 activation. Current studies seek to identify the specific Activin ligand(s) responsible for this effect; gene expression analyses indicates that W-20-17 cells express multiple Activin subunits including Inhβa and Inhβb. Additionally, overpression studies are ongoing to determine if receptor-level competition is involved in mediating these effects. Collectively, our study seeks to elucidate the mechanism(s) that regulate antagonism BMP and Activin signaling pathways to identify novel opportunities for safer and more effective therapies for low bone mass in humans.
Copyright 2017 all authors
Chlebek, Sylvia; Newby, Jordan; Arthur, Jon; and Lowery, Jonathan W. Ph.D., "Elucidating the Antagonistic Relationship Between Bone Morphogenetic Protein and Activin Signaling Pathways in Osteoprogenitor Cells" (2017). MU-COM Research Day. 52.