Impairment of cold injury-induced muscle regeneration in mice receiving a combination of bone fracture and alendronate treatment

Effects of hyperoxia and hypoxia on the proliferation of C2C12 myoblasts

Hyperoxic conditions are known to accelerate skeletal muscle regeneration after injuries. In the early phase of regeneration, macrophages invade the injured area and subsequently secrete various growth factors, which regulate myoblast proliferation and differentiation. Although hyperoxic conditions accelerate muscle regeneration, it is unknown whether this effect is indirectly mediated by macrophages. Here, using C2C12 cells, we show that not only hyperoxia but also hypoxia enhance myoblast proliferation directly, without accelerating differentiation into myotubes. Under hyperoxic conditions (95% O₂ + 5% CO₂), the cell membrane was damaged because of lipid oxidization, and a disrupted cytoskeletal structure, resulting in suppressed cell proliferation. However, a culture medium containing vitamin C (VC), an antioxidant, prevented this lipid oxidization and cytoskeletal disruption, resulting in enhanced proliferation in response to hyperoxia exposure of ≤4 h/day. In contrast, exposure to hypoxic conditions (95% N₂ + 5% CO₂) for ≤8 h/day enhanced cell proliferation. Hyperoxia did not promote cell differentiation into myotubes, regardless of whether the culture medium contained VC. Similarly, hypoxia did not accelerate cell differentiation. These results suggest that regardless of hyperoxia or hypoxia, changes in oxygen tension can enhance cell proliferation directly, but do not influence differentiation efficiency in C2C12 cells. Moreover, excess oxidative stress abrogated the enhancement of myoblast proliferation induced by hyperoxia. This research will contribute to basic data for applying the effects of hyperoxia or hypoxia to muscle regeneration therapy.

MicroRNA-155 inhibition up-regulates LEPR to inhibit osteoclast activation and bone resorption via activation of AMPK in alendronate-treated osteoporotic mice

Osteoporosis is characterized by a progressive increase in bone fragility, leading to low bone mass and structural deterioration of bone tissue. MicroRNA-155 (miR-155) is highly expressed in osteoporosis. Thus, the current study aimed to investigate the effect of miR-155 on the inhibition of osteoclast activation and bone resorption by targeting leptin receptor (LEPR) through the adenosine monophosphate activated protein kinase (AMPK) pathway in alendronate-treated osteoporotic mice. An osteoporosis mouse model was established to examine the bone tension and bone density and the expression of miR-155 in osteoclasts. Binding sites between miR-155 and LEPR were verified. Osteoclasts in the treatment group were transfected with different mimic, inhibitor, vector, or siRNA for subsequent experiments. The expression of miR-155, LEPR, AMPK, p-AMPK, RANKL, OPG, M-CSF, RANK, TRAP, Bax, Bcl-2, and the contents of TNF-α and IL-1β were all examined. The proliferation and bone resorption of osteoclasts were also detected. Mice with osteoporosis exhibited decreased bone density and bone tension, along with elevated expression of miR-155. LEPR was verified as a target gene of miR-155. Down-regulated miR-155 was found to increase the expression of LEPR, AMPK, p-AMPK, OPG, Bax, decrease expression of TNF-α, IL-1β, RANKL, M-CSF, RANK, TRAP, Bcl-2, inhibit the cell proliferation and bone resorption of osteoclasts. Taken together, decreased miR-155 up-regulated LEPR via activation of AMPK, which ultimately repressed osteoclast activation and bone resorption of osteoclasts in alendronate-treated osteoporotic mice.