骨骼内分泌功能研究进展

既往研究认为骨骼的主要功能包括:机械支持、保护重要器官、提供造血微环境、参与钙磷代谢。近年越来越多的证据表明,骨骼不仅是接受神经和体液调节的"惰性器官",而且是具有生物活性的"内分泌器官"。骨骼分泌的生物活性物质,如骨调节蛋白、生长因子、脂肪因子、细胞因子、活性多肽和激素等,能通过自分泌、旁分泌的方式调节骨的代谢,也能以远距分泌的方式作用于胰腺、肝脏、肾脏、骨骼肌、大脑、睾丸等靶器官或组织,发挥相应的生物学作用。     

骨骼的内分泌功能

既往认为骨骼是支持机体基本结构和参与运动及钙磷代谢的主要器官。近年发现组成骨骼的成骨细胞和破骨细胞能合成和分泌多种骨调节蛋白、生长因子、脂肪因子、炎症因子和心血管活性肽等多种生物活性物质,以旁/自分泌方式调节骨骼系统功能,并能通过血液循环远距分泌的方式,调节机体能量代谢、炎症反应和内分泌稳态等。     

骨源活性物质的中枢调控功能

骨是机体的主要支持结构,也是参与机体运动和钙磷代谢的主要器官。骨也是一种潜在的新型内分泌器官,其通过骨细胞和骨髓分泌的多种生物活性物质,参与心血管、消化、内分泌等多个系统的生理和病理生理过程。骨源活性物质还可直接作用于中枢神经系统,参与脑功能和个体行为的调节,骨-脑轴的双向调控也逐渐引起了神经科学研究领域的关注。本文综述了骨源活性物质和骨源细胞对个体脑功能和脑疾病的调节作用以及相关神经精神疾病的发生发展,简介了其中枢调控功能的相关机制,为基于骨-脑调控轴的神经精神疾病机制研究和防治策略的建立拓宽了思路。     

脂联素在骨骼肌中的新功能及其与运动的关系

既往研究认为脂联素（adiponectin,ADPN）是一种脂肪因子,可以促进骨骼肌的糖原合成来调节血糖。但是近期研究表明,骨骼肌同样是分泌ADPN的重要器官。此外,ADPN在骨骼肌中不仅可以调节糖代谢,还在改变肌肉类型、介导线粒体功能、改善胰岛素抵抗、提升肌肉收缩和钙调节、促进肌肉再生等均发挥重要的生物学作用。运动可以调节血清及骨骼肌中ADPN表达水平,但其结果还存在争议。因此,探索ADPN在骨骼肌中新的生物学功能以及运动对骨骼肌ADPN表达的确切效果可能为治疗骨骼肌相关疾病提供新思路。     

白细胞介素-6在骨骼肌质量调节中的作用

白细胞介素-6 (interleukin-6,IL-6)是一种多效性细胞因子参与机体免疫应答，并在不同器官、组织及细胞中发挥生物调节作用。IL-6具有抗炎和促炎的双重效应，在受到病原体感染发病的初期，机体内IL-6发挥抗炎作用，其水平在机体内适度升高，抵御机体炎症、维持机体内部稳态；但IL-6大量释放可造成过度炎症，引发机体的其他病理变化。而IL-6在调控骨骼肌质量方面亦有刺激骨骼肌蛋白质合成与降解的双重效应。骨骼肌作为机体重要的运动及代谢器官，也是IL-6的关键靶向之一。一方面，IL-6在应激骨骼肌的诱导和瞬时表达通过自分泌或旁分泌作用下，参与调节肌卫星细胞增殖、分化，介导骨骼肌生成与生长；另一方面，在衰老及病理条件下，机体IL-6水平显著提高，促使肌肉萎缩，因此，骨骼肌萎缩机制亦与IL-6相关。此外，骨骼肌也可作为内分泌器官，在运动应激下分泌并释放IL-6,后者作为“运动因子”实现骨骼肌与其他器官或组织间的“crosstalk”。鉴于IL-6在机体发挥的“多面手”作用，本文综述IL-6与骨骼肌质量调控机制相关研究进展，为揭示骨骼肌应激与适应分子机制提供理论参考。     

骨骼肌质量控制信号通路

骨骼肌质量约占健康成人体重的40%。骨骼肌不仅直接参与运动,还作为分泌器官分泌多种肌肉因子影响其它器官的功能,因此骨骼肌功能的维持对机体健康具有重要意义。骨骼肌质量作为骨骼肌功能的基础,常常受到运动、疾病等多种因素的影响。如抗阻运动可引起骨骼肌细胞蛋白质合成增加,诱发肌肉肥大;而肢体废用、慢性阻塞性肺疾病、心衰、慢性肾病、恶病质、杜氏肌营养不良等疾病可导致骨骼肌细胞蛋白质合成降低或降解增强,引起肌肉萎缩。骨骼肌肥大或萎缩的过程涉及多条信号通路的改变,如IGF-1/PI3K/Akt、肌肉生长抑制素、G蛋白等介导的信号通路参与了骨骼肌肥大的调控;而泛素-蛋白酶体途径、IGF-1/Akt/FoxO、自噬-溶酶体途径、NF-κB及糖皮质激素介导的信号通路则在调节肌肉萎缩中发挥重要作用。这些信号通路在不同的条件下被激活或抑制,共同调节骨骼肌质量。本文综述骨骼肌质量控制信号通路及其主要转导机制,以加深对骨骼肌质量调控的理解与认识。     

骨骼肌组织来源的外泌体促进骨骼肌干细胞增殖并抑制其分化

骨骼肌干细胞(又称为肌卫星细胞)位于肌纤维膜与基底膜之间。肌纤维分泌各种细胞因子到骨骼肌干细胞微环境中,进而调节肌卫星细胞的功能。最近的研究发现,骨骼肌来源的外泌体以内分泌方式影响其他组织的功能,但是,骨骼肌组织分泌的外泌体是否以旁分泌的方式调控肌卫星细胞的功能,目前并不清楚。该研究发现,骨骼肌来源的外泌体能够显著促进肌卫星细胞增殖、抑制其分化,这为揭示外泌体介导的骨骼肌组织微环境调控肌卫星细胞功能提供了实验证据。     

鸢尾素(Irisin)与能量代谢相关研究进展

Irisin是2012年被发现并报道的一种肌肉因子。Irisin在发现之初被确认具有促进白色脂肪组织的棕色化、加快机体能量消耗、调节能量代谢、并改善胰岛素抵抗的生物学作用。随后调查研究表明irisin在神经系统疾病,心血管疾病,非酒精性脂肪肝及肿瘤中也扮演了重要的角色。由于irisin的广泛活性,对其来源及其调控因素的探索可能会为疾病症状的改善提供途径。骨骼肌、白色脂肪组织等均能够合成并分泌irisin,而骨骼肌是其主要来源。骨骼肌分泌irisin活动受到运动、环境温度等因素的影响。其中运动对irisin分泌的影响是复杂的,如运动强度、持续时间、频率均与irisin血清浓度具有相关性,这也导致了不同运动过程使irisin血液水平展现不同变化。Irisin参与骨骼肌、脂肪、骨、胰腺的细胞增殖、分泌等活动。为了描述细胞中实现这些功能的物质基础,irrisin信号通路下游信号分子需要被探索,这类分子已报道的有AMPK、ERK、STAT3等。但是,目前irisin与PCG1α的关系,irisin受体的活性等问题还需要进一步实验的阐明。     

非突触信息传递方式

原始神经元在形成突触和传导性轴突之前,是把神经活性物质释放到含有淋巴血液的半开放循环系统中。通过这种类结缔组织,神经活性物质作用于邻近的效应器官和组织细胞,发挥调节作用。高等脊椎动物,神经元通过突触结构或/和神经内分泌方式进行调节。近年来发现,神经元可在无突触结构的情况下,以酷似上述第一种方式发挥调节作用,有人称之为“非突触信息传递方式”,它与经典的突触调节相并列,成为神经系统正常调节机能的重要组成部分。     

Musclin:新发现的骨骼肌分泌的生物活性因子

继心肌、血管和脂肪组织的内分泌功能发现后 ,传统的内分泌概念被打破 ,推测分泌活性物质的功能是各种组织细胞固有的生物学行为之一。最近 ,日本学者Nishizawa等在JBiolChem杂志上首次报道 :骨骼肌特异性葡萄糖转运蛋白 4 (GLUT4 )和过氧化物酶体增殖物激活受体γ(PPARγ)基因     

Myostatin regulates glucose metabolism via the AMP-activated protein kinase pathway in skeletal muscle cells

Myostatin (Mstn) is a secreted growth factor predominately expressed in skeletal muscle that negatively regulates skeletal muscle mass. Recent studies have indicated that loss function of myostatin not only increases muscle mass but also improves insulin sensitivity in vivo. In the present report, we demonstrated that myostatin regulates glucose metabolism by promoting glucose consumption and glucose uptake, increasing glycolysis, and inhibiting glycogen synthesis in skeletal muscle cells. Microarray analysis revealed that myostatin upregulates several genes involved in regulating glucose metabolism such as Glut1, Glut4, Hk2, and IL-6. Further investigation of the molecular basis of these phenomena revealed that AMP-activated protein kinase (AMPK), a key component for maintaining energy homeostasis, was activated by myostatin for promotion of glycolysis. Taken together, these findings provide the first experimental evidence that myostatin regulates glucose metabolism through the AMPK signal pathway in muscle cells. Importantly, our findings highlight that continued investigation of the metabolic function of myostatin is necessary for a comprehensive understanding of its active role in the regulation of skeletal muscle energy metabolism.     

Insulin-like growth factor-mediated muscle cell survival: central roles for Akt and cyclin-dependent kinase inhibitor p21

Polypeptide growth factors activate specific transmembrane receptors, leading to the induction of multiple intracellular signal transduction pathways which control cell function and fate. Recent studies have shown that growth factors promote cell survival by stimulating the serine-threonine protein kinase Akt, which appears to function primarily as an antiapoptotic agent by inactivating death-promoting molecules. We previously established C2 muscle cell lines lacking endogenous expression of insulin-like growth factor II (IGF-II). These cells underwent apoptotic death in low-serum differentiation medium but could be maintained as viable myoblasts by IGF analogues that activated the IGF-I receptor or by unrelated growth factors such as platelet-derived growth factor BB (PDGF-BB). Here we show that IGF-I promotes muscle cell survival through Akt-mediated induction of the cyclin-dependent kinase inhibitor p21. Treatment of myoblasts with IGF-I or transfection with an inducible Akt maintained muscle cell survival and enhanced production of p21, and ectopic expression of p21 was able to sustain viability in the absence of growth factors. Blocking of p21 protein accumulation through a specific p21 antisense cDNA prevented survival regulated by IGF-I or Akt but did not block muscle cell viability mediated by PDGF-BB. Our results define Akt as an intermediate and p21 as a critical effector of an IGF-controlled myoblast survival pathway that is active during early myogenic differentiation and show that growth factors are able to maintain cell viability by inducing expression of pro-survival molecules.     

Adaptations of the IGF system during malignancy: human skeletal muscle versus the systemic environment.

Presented in this study are data derived from a unique cohort of patients both with and without cancer, for whom we not only have serum samples, allowing us to investigate systemic factors impacting on skeletal muscle maintenance, but also primary skeletal muscle cultures giving us a model to mimic the in vivo muscle milieu. Possible local effects of autocrine/paracrine and endocrine IGF system components impacting on myoblast growth and differentiation could therefore be assessed. We report for the first time that the decrease in myoblast stem cell numbers seen with normal aging is lost in cancer patients. We further report that serum IGF-I, IGF-II and IGFBP-3 all show positive correlations with myoblast retrieval in control patients, but that with the exception of IGFBP-3 these correlations are lost in malignancy. Indeed IGF-II switches to a negative correlation with myotube formation in malignancy. Furthermore we provide initial evidence to suggest that there is an apparent altered regulation of local IGFBP-3 production during malignancy which may enable satellite cell proliferation, stem cell infiltration or both. Finally we show the importance of investigations not only monitoring the systemic impact of serum factors on skeletal muscle responses but also critically assessing the role that locally produced muscle IGFBP-3 may have on the systemic environment.     

Skeletal myocyte hypertrophy requires mTOR kinase activity and S6K1

The protein kinase mammalian target of rapamycin (mTOR) is a central regulator of cell proliferation and growth, with the ribosomal subunit S6 kinase 1 (S6K1) as one of the key downstream signaling effectors. A critical role of mTOR signaling in skeletal muscle differentiation has been identified recently, and an unusual regulatory mechanism independent of mTOR kinase activity and S6K1 is revealed. An mTOR pathway has also been reported to regulate skeletal muscle hypertrophy, but the regulatory mechanism is not completely understood. Here, we report the investigation of mTOR's function in insulin growth factor I (IGF-I)-induced C2C12 myotube hypertrophy. Added at a later stage when rapamycin no longer had any effect on normal myocyte differentiation, rapamycin completely blocked myocyte hypertrophy as measured by myotube diameter. Importantly, a concerted increase of average myonuclei per myotube was observed in IGF-I-stimulated myotubes, which was also inhibited by rapamycin added at a time when it no longer affected normal differentiation. The mTOR protein level, its catalytic activity, its phosphorylation on Ser2448, and the activity of S6K1 were all found increased in IGF-I-stimulated myotubes compared to unstimulated myotubes. Using C2C12 cells stably expressing rapamycin-resistant forms of mTOR and S6K1, we provide genetic evidence for the requirement of mTOR and its downstream effector S6K1 in the regulation of myotube hypertrophy. Our results suggest distinct mTOR signaling mechanisms in different stages of skeletal muscle development: While mTOR regulates the initial myoblast differentiation in a kinase-independent and S6K1-independent manner, the hypertrophic function of mTOR requires its kinase activity and employs S6K1 as a downstream effector.     

Regulation of pancreatic beta-cell growth and survival by the serine/threonine protein kinase Akt1/PKBalpha

The physiological performance of an organ depends on an interplay between changes in cellular function and organ size, determined by cell growth, proliferation and death. Nowhere is this more evident than in the endocrine pancreas, where disturbances in function or mass result in severe disease. Recently, the insulin signal-transduction pathway has been implicated in both the regulation of hormone secretion from beta cells in mammals as well as the determination of cell and organ size in Drosophila melanogaster. A prominent mediator of the actions of insulin and insulin-like growth factor 1 (IGF-1) is the 3'-phosphoinositide-dependent protein kinase Akt, also known as protein kinase B (PKB). Here we report that overexpression of active Akt1 in the mouse beta cell substantially affects compartment size and function. There was a significant increase in both beta-cell size and total islet mass, accompanied by improved glucose tolerance and complete resistance to experimental diabetes.     

Cytokine secretion by human adipocytes is differentially regulated by adiponectin, AICAR, and troglitazone

Adipose tissue is an active endocrine organ producing a variety of cytokines and chemokines, which may be involved in the deregulation of glucose and lipid homeostasis as well as in the inflammatory state observed in obesity. We have shown previously that differentiated human adipocytes secrete a variety of cytokines which are able to induce skeletal muscle insulin resistance. However, the regulation of these factors by anti-diabetic drugs has remained mainly undefined. Secretion of IL-6, IL-8, MIP-1alpha/beta, and MCP-1 by adipocytes was found to be downregulated by adiponectin. In parallel to adiponectin, the AMPK activator AICAR also decreased the secretion of most of the measured cytokines including IL-6 and MIP-1alpha/beta but not IL-8. In contrast, the thiazolidinedione troglitazone only slightly reduced cytokine secretion despite increasing the phosphorylation of AMPK. In conclusion, we show that adipocyte secretion is strongly inhibited by the anti-diabetic adipocyte hormone adiponectin, an effect that can also be mimicked by the AMPK activator AICAR. However, the PPARgamma agonist troglitazone is much less effective in reducing cytokine secretion.     

Protein kinase B/Akt: a nexus of growth factor and cytokine signaling in determining muscle mass.

Although the boundaries of skeletal muscle size are fundamentally determined by genetics, this dynamic tissue also demonstrates great plasticity in response to environmental and hormonal factors. Recent work indicates that contractile activity, nutrients, growth factors, and cytokines all contribute to determining muscle mass. Muscle responds not only to endocrine hormones but also to the autocrine production of growth factors and cytokines. Skeletal muscle synthesizes anabolic growth factors such as insulin-like growth factor (IGF)-I and potentially inhibitory cytokines such as interleukin (IL)-6, tumor necrosis factor (TNF)-alpha, and myostatin. These self-regulating inputs in turn influence muscle metabolism, including the use of nutrients such as glucose and amino acids. These changes are principally achieved by altering the activity of the protein kinase known as protein kinase B or Akt. Akt plays a central role in integrating anabolic and catabolic responses by transducing growth factor and cytokine signals via changes in the phosphorylation of its numerous substrates. Activation of Akt stimulates muscle hypertrophy and antagonizes the loss of muscle protein. Here we review the many signals that funnel through Akt to alter muscle mass.