肌肉萎縮/肌少症之環境危險因子及治療策略探討:以三丁基錫及福善美為例

Abstract

肌少症是一種伴隨著老化所引起的肌肉質量減少、肌肉力量減弱和行動能力變差的疾病。肌少症可分為兩種，第一種是僅因年紀老化而造成的原發性肌少症，而第二種是因長期臥病、癌症、內分泌疾病或營養不良而造成的次發性肌少症。然而大多數的肌少症是由多重疾病或風險因子而造成的，因此它是一個涉及許多複雜因素的肌肉疾病。目前治療肌少症的策略主要以運動和飲食來延緩肌少症所造成的影響，然而這些治療策略對於因長期臥病在床無法行動或營養吸受不良而造成肌少症的病人是不具有顯著療效的。因此臨床上對於肌少症的治療仍需要一個確切性的治療藥物。先前研究指出雙磷酸鹽藥物福善美(Alendronate)除了能預防骨質疏鬆外，亦能改善肌肉功能，然而其確切機制尚不明瞭。此外，氯化三丁基錫(Tributyltin chloride)是一種內分泌干擾物質，被廣泛用來製造PVC塑膠的安定劑、殺蟲劑、油漆及抗汙劑等，先前研究指出暴露於低劑量的三丁基錫會影響小鼠的代謝系統及骨質密度，但三丁基錫於對骨骼肌方面的影響及其機制仍尚未明瞭。因此，本研究首先利用細胞與動物模式探討福善美是否具有預防肌肉萎縮的效果及其作用機轉。另外，本篇研究亦探討是否長期暴露於低劑量的三丁基錫會造成肌少症相關的症狀產生。首先，實驗結果顯示福善美能促進骨骼肌分化指標蛋白(Myosin heavy chain)表現，並能藉由減少泛素連接酶(Atrogin-1)及sirtuin-3的蛋白表現來抑制皮質類固醇(Dexamethasone)所誘發的肌肉萎縮。動物模式方面，將小鼠分成肌肉萎縮(去神經)及肌肉受損(甘油注射)兩種試驗模式，發現經由管餵小鼠福善美(0.5及1 mg/kg)四週，能顯著改善肌肉質量、強度及促進肌肉的再生能力。本篇研究亦發現暴露於低劑量的氯化三丁基錫(0.1-0.5 μM)會造成肌肉萎縮相關蛋白(Atrogin-1, Myostatin及Muscle RING-finger protein-1)表現並伴隨著單磷酸腺苷活化蛋白質激酶(5' Adenosine monophosphate-activated protein kinase)的大量表現，而管餵小鼠低劑量的氯化三丁基錫(5及25 μg/kg)四週後，造成小鼠體重及血糖上升並減少肌肉質量及肌肉強度。將小鼠肌肉組織切片進行免疫組織化學染色亦能發現肌肉萎縮相關蛋白表現。綜合上述，本論文研究成果證實福善美能藉由降低Atrogin-1及SIRT3訊號分子來改善肌肉萎縮之相關症狀;另一方面亦發現低劑量的氯化三丁基錫可經由AMPK訊號路徑來調控肌肉萎縮相關蛋白表現，進而導致肌肉萎縮與失能。未來仍需更進一步探討低劑量的氯化三丁基錫對於骨骼肌分化影響及機制，亦需進一步探討福善美使否也能改善氯化三丁基錫所造成的肌肉萎縮。

Sarcopenia is aging-related diseases accompanied with a progressive decline of muscle mass, strength and poor mobility. Sarcopenia can be divided into two types. The first is primary sarcopenia caused by aging, while the second is called secondary sarcopenia, which was caused by disuse atrophy, cancer, endocrine disease or malnutrition. However, sarcopenia can be induced via several factors, and thus it is a complex and numerous factor involved disease. Current therapeutic strategies for sarcopenia are mitigate the impact of sarcopenia by exercise and diet strategies. However, these treatment strategies are not effective for patients with severe physical disability, disuse atrophy and poor nutritional absorption. Therefore, alternative managements are needed to find out for the clinical efficacy of sarcopenia and disease-related muscle atrophy and dysfunction. Previous studies indicated that alendronate can improve bone loss and have a therapeutic potential on skeletal muscle function. However, the detail mechanism of alendronate on muscle function still remain unknown. In addition, tributyltin chloride is an endocrine disruptors chemical, and has a widespread application in PVC heat stabilizers, biocide and paints and antifouling paints. Previous studies indicated that low-dose tributyltin result in metabolism system disruption and lower bone density. The mechanism of low-dose TBTCL effect on skeletal muscle function is also still remain unknown. Thus, we investigate the therapeutic potential of alendronate in vivo and in vitro. Besides, the other aim of this study is to investigate the effect of low-dose tributyltin chloride on skeletal muscle. First, alendronate promotes the differentiation of C2C12 myoblasts and primary HSMPCs, and prevents dexamethasone-induce muscle atrophy via Atrogin-1 and SIRT3 down-regulation. Mice were treated with alendornate (0.5 and 1 mg/kg) for 4 weeks, has been shown an increase in muscle mass, muscle function and muscle regeneration. In the present study, we also found that low-dose tributyltin chloride (0.1-0.5 μM) causes muscle atrophy accompanied with the expression of muscle atrophic-related proteins (Atrogin-1, myostatin and MuRF1) via AMPK mediation, and reduce soleus muscle mass and muscle strength in mice. Immunohistochemistry was performed on fixed soleus muscle sections and low-dose tributyltin cause the muscle atrophic-related protein expressed. Taken together, the present study has been demonstrated that alendronate can improve muscle wasting via down-regulation of Atrogin-1 and SIRT3. On the other hand, these results suggest that low-dose tributyltin may through AMPK signal pathway to induce muscle atrophy and dysfunction. However, the mechanism of low-dose tributyltin chloride effect on skeletal muscle function needs to clarified in the future, and we also need to explore whether alendronate also have a therapeutic potential on tributyltin chloride-induced muscle wasting.