L-Carnitine enhances human primary osteoblasts differentiation by acting on calcium homeostasis and mitochondrial function

Increasing evidence has demonstrated that inflammation and oxidative stress (OS) promote osteoblastic dysfunction, decrease bone mass and regenerative capacity inhibiting the expression of osteoblats-specific genes such as RUNX2, osterix (OSX) and osteopontin (OPN). Therefore, mitochondrial dysfunction responsible for oxidative stress plays a critical role in the pathophysiology of osteoporosis. Mitochondrial function and related calcium homeostasis are relevant in osteoblastic proliferation and differentiation and the maintenance of a pool of healthy mitochondria is essential for a proper osteoblasts metabolic activity and a proper osteogenesis. L-Carnitine (LC) is described as a skilled opponent of OS and a promoter of differentiation and hypertrophy through IGF-I pathway activation in C2C12 murine myoblasts. As myoblasts and osteoblasts derive from a common mesenchymal precursor, it is conceivable to hypothesize LC potential action on osteoblasts mitochondrial homeostasis, antioxidative response and differentiation process. uman osteoblasts (hObs) derived from trabecular bone samples have been treated with 5 mM LC at 5, 15, 60 min and 3, 6, 24, 4, 2 h. LC enhanced RUNX2 and mRNA level of downstream factor OSX (3h), promoting the expression of the organic matrix proteins OPN and BSP. Both IGF-1 and Calcium are known to promote RUNX2 expression, but LC increased IGF-1 mRNA after 6h and CaMKII activation after 5 min of treatment, suggesting a direct attivation of RUNX2 by CaMKII. To endorse the hypothesis, LC treatment increased Calcium influx from extracellular milieu and calcium release from intracellular stores. Calcium signaling is recognized to promote osteoblastogenesis AKT and ERK, crucial kinases involved in osteoblastic differentiation, were activated in hObs treated with LC (at 5 and 15 min). LC significantly increased the number of activated mitochondria (at 4 and 2 h), the expression of Superoxidative Dismutase 2 (SOD2), key enzyme involved in detox process (at 6 and 24 h) leading to a reduction of ROS production. These data indicate that LC can promote osteoblast differentiation by acting on calcium homeostasis and mitochondrial function and represent a new field of LC application in the treatment of bone complications associated with insulin resistance andor mitochondrial deficit. LC ability to positively modulate osteogenesis may represent a therapeutic approach aimed to counter the damage from insulin resistance on bone metabolism in diseases such as diabetes, obesity and osteoporosis.