MicroRNA-181a/b-1 over-expression enhances osteogenesis by modulating PTEN/PI3K/AKT signaling and mitochondrial metabolism.

MicroRNAs are small non-coding RNAs that play important roles in many cellular processes including proliferation, metabolism and differentiation. They function by binding to specific regions within the 3'UTR of target mRNAs resulting in suppression of protein synthesis and modulation of potentially many cellular pathways. We previously showed that miRNA expression levels differed between cells from distinct regions of developing human embryonic long bones. Specifically, we found that miR-181a-1 was significantly more highly expressed in hypertrophic chondrocytes compared to proliferating differentiated or progenitor chondrocytes, suggesting a potential role in regulating chondrocyte hypertrophy and/or endochondral bone formation. The goal of this study was to determine how miR-181a-1 together with its clustered miRNA, miR-181b-1, regulates osteogenesis. We show that over-expression of the miR-181a/b-1 cluster enhanced osteogenesis and that cellular pathways associated with protein synthesis and mitochondrial metabolism were significantly up-regulated. Metabolic assays revealed that the oxygen consumption rate and ATP-linked respiration were increased by miR-181a/b-1. To further decipher a potential mechanism causing these metabolic changes, we showed that PTEN (phosphatase and tensin homolog) levels were suppressed following miR-181a/b-1 over-expression, and that PI3K/AKT signaling was subsequently increased. Over-expression of PTEN was found to attenuate the enhancing effects of miR-181a/b-1, providing further evidence that miR-181a/b-1 regulates the PTEN/PI3K/AKT axis to enhance osteogenic differentiation and mitochondrial metabolism. These findings have important implications for the design of miR-181a/b targeting strategies to treat bone conditions such as fractures or heterotopic ossification.