Abstract
Microgravity exposure can cause cardiovascular and immune disorders, muscle atrophy, osteoporosis, and loss of blood and plasma volume. A clinostat device is an effective ground-based tool for simulating microgravity. This study investigated how melatonin suppresses autophagy caused by simulated microgravity in preosteoblast MC3T3-E1 cells. In preosteoblast MC3T3-E1 cells, clinostat rotation induced a significant time-dependent increase in the levels of the autophagosomal marker microtubule-associated protein light chain (LC3), suggesting that autophagy is induced by clinostat rotation in these cells. Melatonin treatment (100, 200 nM) significantly attenuated the clinostat-induced increases in LC3 II protein, and immunofluorescence staining revealed decreased levels of both LC3 and lysosomal-associated membrane protein 2 (Lamp2), indicating a decrease in autophagosomes. The levels of phosphorylation of mammalian target of rapamycin (p-mTOR) (Ser2448), phosphorylation of extracellular signal-regulated kinase (p-ERK), and phosphorylation of serine-threonine protein kinase (p-Akt) (Ser473) were significantly reduced by clinostat rotation. However, their expression levels were significantly recovered by melatonin treatment. Also, expression of the Bcl-2, truncated Bid, Cu/Zn- superoxide dismutase (SOD), and Mn-SOD proteins were significantly increased by melatonin treatment, whereas levels of Bax and catalase were decreased. The endoplasmic reticulum (ER) stress marker GRP78/BiP, IRE1α, and p-PERK proteins were significantly reduced by melatonin treatment. Treatment with the competitive melatonin receptor antagonist luzindole blocked melatonin-induced decreases in LC3 II levels. These results demonstrate that melatonin suppresses clinostat-induced autophagy through increasing the phosphorylation of the ERK/Akt/mTOR proteins. Consequently, melatonin appears to be a potential therapeutic agent for regulating microgravity-related bone loss or osteoporosis.
Highlights
Microgravity induces physiological and environmental stresses on animals, including humans
In preosteoblast MC3T3-E1 cells, the expressions of the autophagosomal or autophagy marker protein microtubule-associated protein light chain (LC3) II significantly increased in a time-dependent manner by clinostat rotation (Figure 1A,B) and cell survival did not appeared significant (Figure 1C)
This suggests that the autophagy in preosteoblast MC3T3-E1 cells was induced by clinostat rotation without any decrease in cell viability
Summary
Microgravity induces physiological and environmental stresses on animals, including humans. Other studies have evaluated the molecular and cellular mechanisms of angiogenesis, cancer development, and stem cell biology. These studies can help in the development of new strategies for medical transplantation and regenerative medicine [5,12]. Despite the significant progress made in studies under microgravity and simulated conditions, the signal transduction mechanism of mechanotransduction in bone cells is still not well defined. A recent study reported that microgravity-induced autophagy plays an important role in enhanced osteoclast differentiation and may be a potential therapeutic target to prevent bone loss [27]. We provide the first demonstration that melatonin reduces autophagy induced by clinostat rotation in preosteoblast MC3T3-E1 cells
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