Vitamin K2 likely exerts its protective effects during osteoporosis by promoting osteoblast differentiation and mineralization. However, the precise mechanism remains to be fully elucidated. Autophagy maintains cell homeostasis by breaking down and eliminating damaged proteins and organelles. Increasing evidence in recent years has implicated autophagy in the development of osteoporosis. The aim of the present study was to verify whether vitamin K2 (VK2) can induce autophagy during the differentiation and mineralization of osteoblasts. In the present study, MC3T3-E1 osteoblasts were treated with various doses of VK2 (10 −8 −10 −3 M) for 1–5 days. The results revealed no cytotoxicity at concentrations below 10 −5 M, but cell viability was reduced in a dose-dependent manner at concentrations above 10 −5 M. Furthermore, MC3T3-E1 osteoblasts were seeded in 6-well plates in complete medium supplemented with dexamethasone, β-glycerophosphate and vitamin C (VC) for osteogenic differentiation. MC3T3-E1 osteoblasts treated with different concentrations (10 −5 , 10 −6 and 10 −7 M) of VK2 for 24 h on days 1, 3, 5 and 7 of the differentiation protocol. It was confirmed that VK2 promoted osteoblast differentiation and mineralization by using alkaline phosphatase (ALP) and alizarin red staining. Using western blotting, immunofluorescence, monodansylcadaverine staining and reverse transcription-quantitative polymerase chain reaction, it was observed that VK2 induced autophagy in osteoblasts. The results revealed that VK2 (1 µM) significantly increased ALP activity and the conversion of microtubule associated protein 1 light chain 3-α (LC3)II to LC3I in MC3T3-E1 osteoblasts (P<0.05) at every time point. The number of fluorescent bodies and the intensity increased with VK2, and decreased following treatment with 3-MA+VK2. There was an increase in the mRNA expression levels of ALP, osteocalcin (OCN) and Runt-related transcription factor 2 in VK2-treated cells (P<0.01). The present study further confirmed the association between autophagy and osteoblast differentiation and mineralization through treatment with an autophagy inhibitor [3-methyladenine (3-MA)]. Osteoblasts treated with 3-MA exhibited significant inhibition of ALP activity and osteogenic differentiation (both P<0.05). In addition, ALP activity and osteogenesis in the VK2+3-MA group was lower compared with VK2-treated cells (P<0.05 for both). The present study confirmed that VK2 stimulated autophagy in MC3T3 cells to promote differentiation and mineralization, which may be a potential therapeutic target for osteoporosis.

Glucocorticoids (GCs) are closely associated with the progression of GC-induced osteoporosis (GIOP) by inhibiting osteoblast viability. However, endogenous GCs are important for bone development. In addition, previous studies have demonstrated that GCs could induce autophagy, a cytoprotective process that is protective against various stressors. In the present study, the aim is to explore whether osteoblasts exhibited dose-dependent viability in the presence of GCs due to autophagy. hFOB 1.19 osteoblasts were treated with various doses of dexamethasone (DEX; 10−8-10−4 M) for 0, 24, 48 and 72 h. The results revealed a biphasic effect of DEX on the viability of hFOB 1.19 cells; a high dose of DEX (≥10−6 M) accelerated cell apoptosis, while a low dose of DEX (10−8 M) increased cell viability. Furthermore, significantly increased autophagy was observed in the low dose DEX treatment group, as indicated by the expression of the autophagy-associated proteins beclin 1 and microtubule-associated protein light chain 3, and the detection of autophagosomes. Another finding was that DEX upregulated intracellular reactive oxygen species (ROS), which was decreased by the autophagy agonist rapamycin. The increase in autophagy and cell viability associated with low-dose DEX (10−8 M) was suppressed by the ROS scavenger catalase and the autophagy inhibitor 3-methyladenine. In conclusion, the results revealed that GCs affected osteoblast viability in a dose-dependent manner. A low dose of GCs increased osteoblast viability by inducing autophagy via intracellular ROS. The results indicate that autophagy may be a novel mechanism by which osteoblasts survive GC exposure and provide a potential therapeutic target for treating GIOP.

Environmental heat-to-electric energy conversion provides a promising solution to power sensors used for wearable and portable devices. Yet the near-room-temperature thermoelectric materials are extremely rare. The natural heterostructure [Bi2] m [Bi2Q3] n family provides an important platform to search and develop the cheaper and less toxic of such materials. However, the bottleneck problem in this family is how to enhance the interlayer electrical conductivity (σ). Herein, we uncover for the first time that the delocalized π-bond interaction between the stacking layers in the [Bi2] m [Bi2Se3] n family effectively increases the interlayer carrier mobility (μH) and σ. Moreover, we propose an empirical index, F = Dp x ,p y (Bi0)/Dp x ,p y (Bi3+) along the k z direction in the Brillouin zone to evaluate the strength of the interlayer delocalized π-bond. F is optimized at a value of 1, under which μH is maximized. Interestingly, Bi8Se7 possessing an optimal F = 1.06 is predicted to have the best μH in the [Bi2] m [Bi2Q3] n family. Our subsequent experiments confirm the as-synthesized Bi8Se7 exhibiting n-type behavior with a μH value (33.08 cm2/(V s) at 300 K) that is higher than that of BiSe (26.19 cm2/(V s) at 300 K) and an enhanced σ value. Furthermore, the Te/Sb codoping, via varying the top of the valence band, significantly increases the Seebeck coefficient and eventually enhances the ZT value to ∼0.7 in Bi5.6Sb2.4Se5Te2 at 425 K along the hot-pressing direction, which is comparable to the optimized value of BiSe. According to the single parabolic band model prediction, the ZT of Bi5.6Sb2.4Se5Te2 may reach ∼1.2 at 425 K, suggesting a novel and promising n-type thermoelectric material near room temperature.