Lipophagy in nonliver tissues and some related diseases: Pathogenic and therapeutic implications

Lipid autophagy (lipophagy) is defined as a selective autophagy process in which some intracellular lipid droplets are selectively degraded by autophagic lysosomes pathway. The occurrence of lipophagy was first discovered in liver tissues. Additionally, abundant evidence indicated that the occurrence of hepatic lipophagy has been implicated in many liver diseases including fatty liver diseases, nonalcoholic fatty liver diseases, liver fibrosis, and liver cirrhosis. However, recent studies suggested that hepatic lipophagy occurs not only in liver tissue but also in other nonliver tissues and cells. Furthermore, the occurrence of lipophagy plays a crucial role in nonliver tissues and some related diseases. For instance, lipophagy relieves insulin resistance in adipose tissue from obesity patient with type 2 diabetes. Additionally, lipophagy has the ability to remit neurodegenerative diseases by reducing activity-dependent neurodegeneration in nervous tissue. Lipophagy decreases muscle lipid accumulation and accordingly improves lipid storage myopathy in muscle tissue. Moreover, lipophagy alleviates the malignancy and metastasis of cancer in clear renal cell carcinoma tissue. Lipophagy is also involved in other processes, such as spermatogenesis, osteoblastogenesis, and mucosal ulceration. In conclusion, targeting lipophagy may be a critical regulator and a new therapeutic strategy for nonliver tissues and some related diseases.     

Telemetric control of peripheral lipophagy by hypothalamic autophagy

Autophagy maintains cellular quality control by degrading organelles, and cytosolic proteins and their aggregates in lysosomes. Autophagy also degrades lipid droplets (LD) through a process termed lipophagy. During lipophagy, LD are sequestered within autophagosomes and degraded by lysosomal acid lipases to generate free fatty acids that are β-oxidized for energy. Lipophagy was discovered in hepatocytes, and since then has been shown to function in diverse cell types. Whether lipophagy degrades LD in the major fat storing cell—the adipocyte—remained unclear. We have found that blocking autophagy in brown adipose tissues (BAT) by deleting the autophagy gene Atg7 in BAT MYF5 (myogenic factor 5)-positive progenitors increases basal lipid content in BAT and decreases lipid utilization during cold exposure—indicating that lipophagy contributes to lipohomeostasis in the adipose tissue. Surprisingly, knocking out Atg7 in hypothalamic proopiomelanocortin (POMC) neurons also blocks lipophagy in BAT and liver suggesting that specific neurons within the central nervous system (CNS) exert telemetric control over lipophagy in BAT and liver.     

Autophagy: Emerging roles in lipid homeostasis and metabolic control

Current evidence implicates autophagy in the regulation of lipid stores within the two main organs involved in maintaining lipid homeostasis, the liver and adipose tissue. Critical to this role in hepatocytes is the breakdown of cytoplasmic lipid droplets, a process referred to as lipophagy. Conversely, autophagy is required for adipocyte differentiation and the concurrent accumulation of lipid droplets. Autophagy also affects lipid metabolism through contributions to lipoprotein assembly. A number of reports have now implicated autophagy in the degradation of apolipoprotein B, the main structural protein of very-low-density-lipoprotein. Aberrant autophagy may also be involved in conditions of deregulated lipid homeostasis in metabolic disorders such as the metabolic syndrome. First, insulin signalling and autophagy activity appear to diverge in a mechanism of reciprocal regulation, suggesting a role for autophagy in insulin resistance. Secondly, upregulation of autophagy may lead to conversion of white adipose tissue into brown adipose tissue, thus regulating energy expenditure and obesity. Thirdly, upregulation of autophagy in hepatocytes could increase breakdown of lipid stores controlling triglyceride homeostasis and fatty liver. Taken together, autophagy appears to play a very complex role in lipid homeostasis, affecting lipid stores differently depending on the tissue, as well as contributing to pathways of lipoprotein metabolism.     

脂肪组织中自噬信号通路及其功能

细胞自噬是一种真核生物中高度保守的代谢过程,包括巨自噬、微自噬以及分子伴侣介导的自噬等。自噬过程可以清除受损的细胞器,降解糖原、脂类和蛋白质等生物大分子物质,供细胞重新利用,维持细胞内代谢平衡。自噬障碍与多种疾病的病理发生过程息息相关,包括肿瘤、2型糖尿病、肥胖、骨骼肌病以及神经退行性疾病等。脂肪组织是人体脂质储存的重要场所,广泛分布于全身各处,如内脏和皮下等。脂肪组织通过储存冗余脂肪并分泌脂肪因子,防止脂肪的异位堆积和脂毒性的发生,维持机体的脂质稳态。近期的许多研究表明,自噬进程深度参与脂肪细胞的细胞分化与能量代谢。因此,深入探究脂肪组织自噬过程与机体脂质稳态的调控关系,有利于揭示机体脂质平衡的内在机制,为新型药物靶点的开发提供扎实的理论依据和数据支持。本文就近年来关于自噬影响脂肪组织脂质代谢的最新研究进展作一综述。     

Autophagy,lipophagy and lysosomal lipid storage disorders

Autophagy is a catabolic process with an essential function in the maintenance of cellular and tissue homeostasis. It is primarily recognised for its role in the degradation of dysfunctional proteins and unwanted organelles, however in recent years the range of autophagy substrates has also been extended to lipids. Degradation of lipids via autophagy is termed lipophagy. The ability of autophagy to contribute to the maintenance of lipo-homeostasis becomes particularly relevant in the context of genetic lysosomal storage disorders where perturbations of autophagic flux have been suggested to contribute to the disease aetiology. Here we review recent discoveries of the molecular mechanisms mediating lipid turnover by the autophagy pathways. We further focus on the relevance of autophagy, and specifically lipophagy, to the disease mechanisms. Moreover, autophagy is also discussed as a potential therapeutic target in several key lysosomal storage disorders.     

Autophagy in obesity and atherosclerosis: Interrelationships between cholesterol homeostasis,lipoprotein metabolism and autophagy in macrophages and other systems

The incidence of diseases characterized by a dysregulation of lipid metabolism such as obesity, diabetes and atherosclerosis is rising at alarming rates, driving research to uncover new therapies to manage dyslipidemias and resolve the metabolic syndrome conundrum. Autophagy and lipid homeostasis – both ancient cellular pathways – have seemingly co-evolved to share common regulatory elements, and autophagy has emerged as a prominent mechanism involved in the regulation of lipid metabolism. This review highlights recent findings on the role of autophagy in the regulation of cellular cholesterol homeostasis and lipoprotein metabolism, with special emphasis on macrophages. From modulation of inflammation to regulation of cellular cholesterol levels, a protective role for autophagy in atherosclerosis is emerging. The manipulation of autophagic activity represents a new possible therapeutic approach for the treatment complex metabolic disorders such as obesity and the metabolic syndrome.     

Deacetylation of LAMP1 drives lipophagy-dependent generation of free fatty acids by Abrus agglutinin to promote senescence in prostate cancer

Therapy-induced senescence in cancer cells is an irreversible antiproliferative state, which inhibits tumor growth and is therefore a potent anti-neoplastic mechanism. In this study, low doses of Abrus agglutinin (AGG)-induced senescence through autophagy in prostate carcinoma cells (PC3) and inhibited proliferation. The inhibition of autophagy with 3-methyl adenine reversed AGG-induced senescence, thus confirming that AGG-triggered senescence required autophagy. AGG treatment also led to lipophagy-mediated accumulation of free fatty acids (FFAs), with a concomitant decrease in the number of lipid droplets. Lalistat, a lysosomal acid lipase inhibitor, abrogated AGG-induced lipophagy and senescence in PC3 cells, indicating that lipophagy is essential for AGG-induced senescence. The accumulation of FFAs increased reactive oxygen species generation, a known facilitator of senescence, which was also reduced in the presence of lalistat. Furthermore, AGG upregulated silent mating type information regulator 2 homolog 1 (SIRT1), while the presence of sirtinol reduced autophagy flux and the senescent phenotype in the AGG-treated cells. Mechanistically, AGG-induced cytoplasmic SIRT1 deacetylated a Lys residue on the cytoplasmic domain of lysosome-associated membrane protein 1 (LAMP1), an autolysosomal protein, resulting in lipophagy and senescence. Taken together, our findings demonstrate a novel SIRT1/LAMP1/lipophagy axis mediating AGG-induced senescence in prostate cancer cells.     

自噬在疾病中的双重作用

自噬是细胞的一种正常的生理活动,参与细胞内损伤的蛋白质和亚细胞器经溶酶体途径降解的过程。自噬可以抵御外界的不良环境,在多种疾病中起着重要作用。近年来,大量研究表明自噬在细胞新陈代谢和生理功能上有双重作用,在疾病发生的不同时期,自噬起到不同的作用。通常情况自噬可以及时的清除细胞内损伤的蛋白质,作为一种细胞的保护机制,但是自噬的持续活化,导致细胞内大量蛋白质的降解,使细胞无法维持其基本结构,最终将导致细胞坏死或凋亡。自噬、凋亡和坏死的转化,很有可能受到p53、Bcl-2、Beclin-1、ATG5、TG2及p62等信号分子调控。肝脏和心脏是维持人体生命活动的重要器官,自噬在脂肪肝、肝硬化、心肌梗塞及心脏衰竭等疾病中扮演着重要的角色。本文总结了自噬、凋亡及坏死的相互关系,自噬在疾病中的双重作用,并重点介绍自噬在肝脏和心脏疾病中的作用。     

Role of autophagy in breast cancer

Autophagy is an evolutionarily conserved process of cytoplasm and cellular organelle degradation in lysosomes. Autophagy is a survival pathway required for cellular viability during starvation; however, if it proceeds to completion, autophagy can lead to cell death. In neurons, constitutive autophagy limits accumulation of polyubiquitinated proteins and prevents neuronal degeneration. Therefore, autophagy has emerged as a homeostatic mechanism regulating the turnover of long-lived or damaged proteins and organelles, and buffering metabolic stress under conditions of nutrient deprivation by recycling intracellular constituents. Autophagy also plays a role in tumorigenesis, as the essential autophagy regulator beclin1 is monoallelically deleted in many human ovarian, breast, and prostate cancers, and beclin1(+/-) mice are tumor-prone. We found that allelic loss of beclin1 renders immortalized mouse mammary epithelial cells susceptible to metabolic stress and accelerates lumen formation in mammary acini. Autophagy defects also activate the DNA damage response in vitro and in mammary tumors in vivo, promote gene amplification, and synergize with defective apoptosis to accelerate mammary tumorigenesis. Thus, loss of the prosurvival role of autophagy likely contributes to breast cancer progression by promoting genome damage and instability. Exploring the yet unknown relationship between defective autophagy and other breast cancer promoting functions may provide valuable insight into the pathogenesis of breast cancer and may have significant prognostic and therapeutic implications for breast cancer patients.     

Dengue virus-induced autophagy regulates lipid metabolism

Autophagy influences numerous cellular processes, including innate and adaptive immunity against intracellular pathogens. However, some viruses, including dengue virus (DENV), usurp autophagy to enhance their replication. The mechanism for a positive role of autophagy in DENV infection is unclear. We present data that DENV induction of autophagy regulates cellular lipid metabolism. DENV infection leads to an autophagy-dependent processing of lipid droplets and triglycerides to release free fatty acids. This results in an increase in cellular β-oxidation, which generates ATP. These processes are required for efficient DENV replication. Importantly, exogenous fatty acids can supplant the requirement of autophagy in DENV replication. These results define a role for autophagy in DENV infection and provide a mechanism by which viruses can alter cellular lipid metabolism to promote their replication.     

Kaempferol alleviates palmitic acid-induced lipid stores,endoplasmic reticulum stress and pancreatic β-cell dysfunction through AMPK/mTOR-mediated lipophagy

Kaempferol, a natural flavonoid, has the beneficial effects of preserving pancreatic β-cell mass and function, but its action on β-cell lipid metabolism still remains elusive. Recently, autophagy has been reported to play a major role in lipid metabolism in various cell types, but its role in pancreatic β-cell's lipid metabolism is rarely reported. Here, we investigated the role of kaempferol-induced autophagy in inhibition of lipid stores, ER stress and β-cell dysfunction in palmitic acid-challenged RIN-5F cells and isolated pancreatic islets. The lipid-lowering effect of kaempferol was determined by Oil Red O staining, triglyceride assay, BODIPY labeling, RT-PCR and immunoblot analysis of PLIN2 (the lipid droplet coat protein) expression. Further, the involvement of AMPK/mTOR-mediated lipophagy was established by pharmacological and genetic inhibitors of autophagy and AMPK. The co-localization studies of lipid droplets with autophagosomes/lysosomes by BODIPY-MDC-LysoTracker co-staining, LC3/BODIPY labeling and LC3/PLIN2 double immunolabeling further strengthened the findings. Kaempferol treatment exhibited decreased lipid stores and increased co-localization of lipid droplets with autophagosomes and lysosomes in palmitic acid–challenged β-cells. Moreover, inhibition of autophagy led to decreased co-localization and increased lipid droplets accumulation. Kaempferol-induced alleviation of ER stress and β-cell dysfunctions was established by immunoblot analysis of CHOP-10 (a key mediator of cell death in response to ER stress) and insulin content/secretion analysis respectively. Together, these findings suggest that kaempferol prevents ectopic lipid accumulation and ER stress, thus restoring β-cell function through AMPK-mediated lipophagy. The current data implies that kaempferol may be a potential therapeutic candidate to prevent obesity-linked diabetic complications.     

Evaluation of autophagy using mouse models of brain injury

Autophagy is a homeostatic, carefully regulated, and dynamic process for intracellular recycling of bulk proteins, aging organelles, and lipids. Autophagy occurs in all tissues and cell types, including the brain and neurons. Alteration in the dynamics of autophagy has been observed in many diseases of the central nervous system. Disruption of autophagy for an extended period of time results in accumulation of unwanted proteins and neurodegeneration. However, the role of enhanced autophagy after acute brain injury remains undefined. Established mouse models of brain injury will be valuable in clarifying the role of autophagy after brain injury and are the topic of discussion in this review.     

Autophagy and inflammation: A special review issue

Macroautophagy/autophagy is a fundamental intracellular homeostatic process that is of interest both for its basic biology and for its effect on human physiology in a wide spectrum of conditions and diseases. Autophagy was first appreciated primarily as a metabolic and cytoplasmic quality control process, but in the past decade its role in immunity has been steadily growing. The connections between these aspects beckon explorations of the network and connections that exist between metabolism, quality control, and inflammation and immunity processes, which are so key to many human diseases including neurodegeneration, obesity and diabetes, chronic inflammatory conditions, cancer, infection, and aging. The purpose of this issue is to stimulate further the burgeoning studies of the intersections between autophagy and inflammation, and the inevitable overlaps with metabolic and quality control functions of autophagy.     

Regulation of autophagy by bile acids and in cholestasis - CholestoPHAGY or CholeSTOPagy

Autophagy is a lysosomal degradation pathway in which the cell self-digests its own components to provide nutrients in harsh environmental conditions. It also represents an opportunity to rid the cell of superfluous and damaged organelles, misfolded proteins or invaded microorganisms. Liver autophagy contributes to basic hepatic functions such as lipid, glycogen and protein turnover. Deregulated hepatic autophagy has been linked to many liver diseases including alpha-1-antitrypsin deficiency, alcoholic and non-alcoholic fatty liver diseases, hepatitis B and C infections, liver fibrosis as well as liver cancer. Recently, bile acids and the bile acid receptor FXR have been implicated in the regulation of hepatic autophagy, which implies a role of autophagy also for cholestatic liver diseases. This review summarizes the current evidence of bile acid mediated effects on autophagy and how this affects cholestatic liver diseases. Although detailed studies are lacking, we suggest a concept that the activity of autophagy in cholestasis depends on the disease stage, where autophagy may be induced at early stages (“cholestophagy”) but may be impaired in prolonged cholestatic states (“cholestopagy”).     

Autophagy regulation depends on ER homeostasis controlled by lipid droplets

Macroautophagy (hereafter autophagy) is a highly conserved homeostasis and quality control process critically linked to neurodegeneration, metabolic diseases, cancer, and aging. A key feature of autophagy is the de novo formation of autophagosomes, double-membrane vesicular structures encapsulating cytoplasmic cargo for vacuolar turnover and recycling. The membrane rearrangements underlying nucleation, expansion, closure, and vacuolar fusion of autophagosomes are driven by multicomponent core autophagy machinery in cooperation with numerous factors involved in a variety of cellular processes. Our current understanding of the origin and contribution of diverse membrane sources to autophagosome biogenesis and of cellular functions enabling stress-appropriate autophagy responses critical for cell health and survival remains limited. Here, we summarize and discuss our recent findings analyzing the role of lipid droplets (LDs), conserved intracellular storage compartments for neutral lipids, for autophagy regulation. Our data indicate that LDs are dispensable as membrane sources, but fulfill critical functions for maintaining endoplasmic reticulum (ER) homeostasis, including buffering of newly synthesized fatty acids and maintenance of phospholipid composition, required for intact autophagy regulation and cell survival during nutrient stress.     

Role of Autophagy in Breast Cancer

Autophagy is an evolutionarily conserved process of cytoplasm and cellular organelle degradation in lysosomes. Autophagy is a survival pathway required for cellular viability during starvation; however, if it proceeds to completion, autophagy can lead to cell death. In neurons, constitutive autophagy limits accumulation of polyubiquitinated proteins and prevents neuronal degeneration. Therefore, autophagy has emerged as a homeostatic mechanism regulating the turnover of long-lived or damaged proteins and organelles, and buffering metabolic stress under conditions of nutrient deprivation by recycling intracellular constituents. Autophagy also plays a role in tumorigenesis, as the essential autophagy regulator beclin1 is monoallelically deleted in many human ovarian, breast, and prostate cancers, and beclin1^+/- mice are tumor-prone. We found that allelic loss of beclin1 renders immortalized mouse mammary epithelial cells susceptible to metabolic stress and accelerates lumen formation in mammary acini. Autophagy defects also activate the DNA damage response in vitro and in mammary tumors in vivo, promote gene amplification, and synergize with defective apoptosis to accelerate mammary tumorigenesis. Thus, loss of the prosurvival role of autophagy likely contributes to breast cancer progression by promoting genome damage and instability. Exploring the yet unknown relationship between defective autophagy and other breast cancer-promoting functions may provide valuable insight into the pathogenesis of breast cancer and may have significant prognostic and therapeutic implications for breast cancer patients.

Addendum to:

Autophagy Mitigates Metabolic Stress and Genome Damage in Mammary Tumorigenesis

V. Karantza-Wadsworth, S. Patel, O. Kravchuk, G. Chen, R. Mathew, S. Jin and E. White

Genes Dev 2007; 21:1621-35     

Role of autophagy in aging: The good,the bad,and the ugly

Autophagy (self-eating) is a conserved catabolic homeostatic process required for cellular metabolic demands by removal of the damaged molecules and organelles and for alleviation of stress initiated by pathology and infection. By such actions, autophagy is essential for the prevention of aging, disease, and cancer. Genetic defects of autophagy genes lead to a host of developmental, metabolic, and pathological aberrations. Similarly, the age-induced decline in autophagy leads to the loss of cellular homeostatic control. Paradoxically, such a valuable mechanism is hijacked by diseases, during tumor progression and by senescence, presumably due to high levels of metabolic demand. Here, we review both the role of autophagy in preventing cellular decline in aging by fulfillment of cellular bioenergetic demands and its contribution to the maintenance of the senescent state and SASP by acting on energy and nutritional sensors and diverse signaling pathways.     

Lipophagy maintains energy homeostasis in the kidney proximal tubule during prolonged starvation

Macroautophagy/autophagy is a self-degradation process that combats starvation. Lipids are the main energy source in kidney proximal tubular cells (PTCs). During starvation, PTCs increase fatty acid (FA) uptake, form intracellular lipid droplets (LDs), and hydrolyze them for use. The involvement of autophagy in lipid metabolism in the kidney remains largely unknown. Here, we investigated the autophagy-mediated regulation of renal lipid metabolism during prolonged starvation using PTC-specific Atg5-deficient (atg5-TSKO) mice and an in vitro serum starvation model. Twenty-four h of starvation comparably induced LD formation in the PTCs of control and atg5-TSKO mice; however, additional 24 h of starvation reduced the number of LDs in control mice, whereas increases were observed in atg5-TSKO mice. Autophagic degradation of LDs (lipophagy) in PTCs was demonstrated by electron microscopic observation and biochemical analysis. In vitro pulse-chase assays demonstrated that lipophagy mobilizes FAs from LDs to mitochondria during starvation, whereas impaired LD degradation in autophagy-deficient PTCs led to decreased ATP production and subsequent cell death. In contrast to the in vitro assay, despite impaired LD degradation, kidney ATP content was preserved in 48-h starved atg5-TSKO mice, probably due to increased utilization of ketone bodies. This compensatory mechanism was accompanied by a higher plasma FGF21 (fibroblast growth factor 21) level and its expression in the PTCs; however, this was not essential for the production of ketone bodies in the liver during prolonged starvation. In conclusion, lipophagy combats prolonged starvation in PTCs to avoid cellular energy depletion.     

自噬在肝脏蛋白质和脂质代谢中的作用

自噬是一个通过降解细胞组分如细胞器和蛋白质等以维持细胞存活和功能的重要的溶酶体途径。肝脏作为新陈代谢的中枢器官,肝脏高度依赖于自噬以发挥正常功能并防止疾病发展。肝细胞自噬的改变参与肝损伤,脂肪肝等肝病的病理变化,以自噬为靶点寻求治疗各种肝病的方法已成为热点研究领域,但自噬在肝脏蛋白质和脂质代谢中的作用极其机制尚不清楚。本文对自噬在肝脏蛋白质和脂质代谢中的作用的最新进展进行综述。