Focal adhesion kinase activation plays a critical role in the restoration of simulated microgravity-inhibited osteoblast differentiation by triggering transcriptional Wnt/B-Catenin-BMP2-COL1 and metabolic SIRT1-PGC1a-CPT1A pathways

BackgroundAerospace microgravity (AMG) poses a major threat during spaceflight, impairing osteoblast differentiation (OBD) and causing bone loss. To replicate AMG conditions for ground-based experimental procedures, simulated microgravity (SMG) has been implemented to explore the molecular mechanisms of AMG-induced alterations in osteoblast differentiation. However, SMG's metabolic implications contributing to defective osteoblast differentiation remain unexplored.MethodsTo investigate this, we investigated effects of SMG on pre-osteoblast MC3T3-E1 cells using Western-blotting to analyze expression of metabolic regulators and Seahorse assays to characterize cellular metabolism. The cytotoxic necrotizing factor-1 (CNF1), an activator of focal adhesion kinase (FAK), was applied to assess its ability to modulate SMG-inhibited OBD.ResultsOur results showed that, besides its established inhibition of FAK and the Wnt/β-catenin signaling cascade, SMG also induced a metabolic shift from fatty acid oxidation (FAO) to glycolysis by decreasing mitochondrial content and reducing expression of metabolic regulators [sirtuin-1 (SIRT1), peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), and carnitine palmitoyl transferase-1α (CPT1A)] that are critical for mitochondrial biogenesis and FAO capacity. CNF1 exposure was found to counteract the SMG's inhibitory influence by upregulating expression of above metabolic regulators and restoring mitochondrial content and FAO as the primary cellular metabolism and thereby rescuing OBD. Altogether, our findings emphasize that FAK activation plays a critical role in restoration of SMG-inhibited osteoblast differentiation by triggering transcriptional Wnt/B-Catenin-BMP2 (bone morphogenic protein-2)-COL1 (type-1 collagen) and metabolic SIRT1-PGC1a-CPT1A pathways.ConclusionOur data shed light on FAK as a potential therapeutic target to mitigate SMG-driven bone-loss for astronauts and attenuate bone defectiveness for clinical osteoporosis.