Activation of cannabinoid receptor type 2-induced osteogenic differentiation involves autophagy induction and p62-mediated Nrf2 deactivation

BACKGROUND: Dysfunction in survival and differentiation of osteoblasts commonly occurs in patients with osteoporosis. Cannabinoid receptor type 2 (CNR2) is a major receptor of endocannabinoid system that is crucial for bone mass homeostasis. Our group prior demonstrated that activation of CNR2 signaling promoted osteogenic differentiation of bone marrow derived mesenchymal stem cells in vitro. Autophagy is reported to participate in osteoblastic differentiation. Whether autophagy is regulated by CNR2-mediated cannabinoid signaling is unknown, and how the autophagy-CNR2 interaction affects osteoblastic differentiation requires further elucidation. METHODS: hFOB 1.19 osteoblasts were treated with CNR2 agonists HU308 (5, 10, 25, 50 or 100 nM) and JWH133 (1, 2, 5, 10 or 20 μM) in presence or absence of autophagy inhibitor 3-Methyladenine (3-MA). The differentiation of hFOB 1.19 cells was determined via evaluating their alkaline phosphatase (ALP) activity and mineralization ability (Alizarin red staining). Alterations in autophagy-related molecules and osteogenic markers were analyzed via real-time PCR and/or immunoblotting assays. RESULTS: hFOB 1.19 cells spontaneously differentiated towards mature osteoblasts under 39 °C, during which CNR2 expression increased, and autophagy was activated. The strongest autophagy flux was observed at 192 h post differentiation─LC3I to LC3II conversion was enhanced and Beclin 1 expression was upregulated considerably, while p62 expression was downregulated. Treatment of HU308 and JWH133 promoted autophagy in a dose-dependent manner, and suppressed mTOR signaling pathway in hFOB 1.19 cells. In CNR2-silenced cells, HU308’s and JWH133’s effects on autophagy were weakened. HU308 and JWH133 enhanced the ALP activity and mineralization, and upregulated the expression of osteogenic markers, osteopontin and osteocalcin, in hFOB 1.19 cells. Intriguingly, such pro-osteogenic effects induced by CNR2 activation were markedly mitigated by 3-MA. In addition to provoking autophagy, CNR2 agonists also reduced nuclear Nrf2 accumulation and increased Keap1 expression. Further, re-expression of p62 inhibited CNR2 agonists-induced Nrf2 degradation. CONCLUSIONS: Osteogenic differentiation induced by CNR2 signaling activation involves autophagy induction and p62-mediated Nrf2 deactivation.