Research progress on pathogenesis and related pathways of Cancer cachexia

Hao Wu ,Qin-You Ren ,Xing Tang ,Hui Zhang ,Can-Jun Zhao ,Jian-Gang Yang , Bi-Zhu Geng,Lan-Hui Zhang,Ao-Yu Shi,Jin Zheng*

1Department of traditional Chinese medicine,the Second Affiliated Hospital of Air Force Military Medical University,Xi'an 710038,China.2Xi'an brain disease hospital of traditional Chinese Medicine,Xi'an 710038,China.

Abstract Cancer cachexia is a complex syndrome of metabolic changes caused by many factors in tumor patients,which characterized by emaciation,weight loss,muscle atrophy,anemia and hypoproteinemia,it's seriously affects the quality of life,cycle and therapeutic effect of patients.At present,there is no curative effect in the treatment of cancer cachexia,because the pathogenesis is still unclear.So it is particularly important to study the pathogenesis,mechanism and related pathway of cancer cachexia.In this paper,the definition,stage and diagnosis of cachexia,traditional Chinese medicine theoretical research,related anorexia mechanism,protein decomposition mechanism,fat decomposition mechanism,sugar metabolism mechanism,inflammatory factor mechanism and other aspects were analyzed and summarized to provide theoretical basis for the study of cachexia pathogenesis and clinical application selection.

Keywords:Cancer cachexia,Pathogenesis,Pathways,Research progress

I ntroduction

Cancer cachexia (CC)is a multiple organ syndrome characterized by progressive decline in body mass,muscle atrophy and consumption of fatty tissue.Its main clinical manifestations are anorexia,progressive decline in body mass,sugar,fat,protein and other metabolic abnormalities and involving multiple organs.Among them,60% of the patients diagnosed the tumor for the first time with cachexia and 80% of the patients with advanced tumor accompanies with cachexia,which seriously affecting the quality of life of the tumor patients,shortening the survival time,affecting the treatment effect and prognosis[1-2].At present,the treatment of cancer cachexia is not clear and the existing unsatisfying nutrition and other symptomatic treatment effects are not widely recognized,so it is particularly important to study the pathogenesis of tumor cachexia and develop targeted therapeutic drugs and methods through the pathogenesis.This article reviews the regular and related research mechanism of tumor cachexia to provide further theoretical basis for the basic and clinical research of tumor cachexia.

1.Definition,stage and diagnosis of tumor cachexia

The term "cachexia" in cancer cachexia first appeared in Greek "kakos hexis",which means "abominable state",that is,chronic or end-stage disease is in a complex metabolic state.In 2011,Professor Fearon K gave the international consensus definition of tumor dystrophy:a multifactor syndrome characterized by persistent skeletal muscle loss (with or without adipose tissue loss),which cannot be completely relieved by conventional nutrition and gradually leads to organ function damage.There are three characteristics of cachexia:continuous loss of skeletal muscle,failure of routine nutrition to alleviate and organ function damage.Skeletal muscle loss is the main characteristic,and excessive protein (especially muscle protein)decomposition is an important pathological change.Fat tissue and thin tissue are decomposed in patients with tumor cachexia,in which the decomposition of thin tissue results in the decline of organ function activity,especially the decomposition of skeletal muscle resulted in physical weakness,weakness of activity,decreased respiratory function and even death.

Currently,the international consensus diagnosis standard of tumor cachexia in 2011 is adopted:unconscious weight loss ＞ 5% within 6 months;when BMI ＜ 20,weight loss ＞ 2% within 6 months or for patients with sarcopenia (the ratio of limb skeletal muscle mass to height square,male ＜ 7.26,female＜ 5.45)whose weight loss ＞ 2% within 6 months.According to any of the above three points,cancer cachexia diagnosis can be established [3].

According to the European Guidelines for the Classification of Tumor Cachexia,its can be divided into three stages:prophase of cachexia,cachexia stage and cachexia refractory stage.The early stage of cachexia is mainly manifested as anorexia,asthenia and metabolic changes.The weight loss of patients is usually lower than the diagnostic standard of cachexia;cachexia refractory stage same as the diagnostic criteria for the phase of cachexia;The refractory period of cachexia is that the patients with tumor progression or advanced stage of tumor fail to respond to relevant treatment,active metabolism and catabolism,continuous weight loss and can not be corrected as well as the survival period of patients is less than 3 months.Not all patients experienced three stages of cachexia.

2.Theoretical research of traditional Chinese medicine

The disease name of tumor cachexia has not appeared in ancient Chinese medicine books,but its symptoms such as emaciation,fatigue,lack of movement,laziness and weakness are similar to the syndrome of"asthenia and fatigue" in traditional Chinese medicine.Plain Question·On the true viscera of jade machine described :"the main skeleton of the body is withered or dried out,the main muscles are thin,the chest is full of air,the breathing is difficult,the body vibrates when breathing and it will die in six months…".The symptoms described in this paper are consistent with those of tumor cachexia.Cachexia may appear in the later stage of tumor,serious infection,AIDS and other diseases.The name of "asthenia fatigue disease"was first put forward in Synopsis of Prescriptions of the Golden Chamber.Throughout the discussion of "virtual labor" by doctors in the past dynasties,the syndrome differentiation is mainly based on the physiological activities and pathological changes of various visceral entities show various signs outside and the syndrome differentiation of viscera lies in the deficiency of Qi,blood,yin and Yang.Therefore,the"asthenia and fatigue" is mainly asthenia damage and the principle of treatment should benefit the deficiency of invigorating.This disease is mainly caused by deficiency of spleen and kidney,serious disease or long-term illness,improper or delayed treatment and inadequate diet,which leads to deficiency for a long time.You will get disease because of the deficiency and then lead to fatigue;or you will feel asthenia due to illness and if you can't recover for a long time,you will suffer from fatigue.In recent years,more and more TCM has been applied to the treatment and research of tumor cachexia and good results have been achieved.Whether it is traditional Chinese medicine prescription,traditional Chinese medicine extract or traditional Chinese medicine preparation,they through the aspects of Tonifying the healthy qi of human body and regulating the Yin and Yang of Qi and blood to improve the malignant liquid state of tumor,improve the tolerance of radiotherapy and chemotherapy,reduce adverse reactions and prolong the life of patients as much as possible.Traditional Chinese medicine,which has the functions of nourishing qi and spleen,warming spleen and kidney,nourishing yin and clearing heat,can obviously reduce the fatigue and anorexia of tumor patients,improve the quality of life and effectively inhibit the progress of tumor cachexia.

3.Pathogenesis of tumor cachexia

At present,the pathogenesis and causes of tumor cachexia are not completely clear.The pathogenesis of tumor cachexia is mainly divided into the metabolic disorders,including anorexia,sugar,fat,protein and metabolic factors,caused by the interaction of tumor factors and body factors.

3.1 Anorexia

50% of cancer patients showed anorexia at the early stage of diagnosis,while the main clinical manifestations of cancer patients with cachexia were anorexia,nausea,taste changes,etc.

Anorexia inevitably leads to weight loss in patients with tumor cachexia,among them long-term anorexia of tumor patients leads to the preferentially decomposition of ketone body energy generated by fat metabolism rather than glucose energy generated by muscle proteoglycan,which leads to the essential thin tissue of human body being decomposed in the late stage of starvation.Therefore,the energy consumption of patients with cachexia is mainly caused by metabolic abnormality or anabolic defect.At present,it is believed that the mechanism of anorexia[4]is mainly due to the dysfunction of brain feeding regulation center caused by peripheral and central factors.The mechanism may be that the growth of tumor promotes the concentration of tryptophan in plasma and increases the synthesis of 5-hydroxytryptamine,while the activity of 5-hydroxytryptamine is positively correlated with anorexia.Trayhurn[5]found that lactate and fatty acids in the blood can stimulate serotonin and catecholaminergic neurons,thus inhibiting the incoming to the hypothalamus to decrease appetite.However,modern studies have found that the release of inflammatory factors from tumor cells,such as interleukin-1 and tumor necrosis factor,or immune system lymphocytes and macrophages release inflammatory factors in the process of response,which can directly reduce appetite.

3.2 Abnormal Metabolic Mechanism of the body

The metabolism of malignant tumor patients is different from that of normal people because of the physiological changes.The toxic substances or inflammatory reactions produced by their own metabolism lead to the occurrence of tumor cachexia.At present,it is generally believed that cachexia is a metabolic syndrome caused by the disorder of carbohydrate,protein and fat metabolism caused by tumor factors,organism factors and the interaction between tumor and organism.

3.2.1 Abnormal Glucose Metabolism

The abnormal glucose metabolism in patients with tumor cachexia is mainly manifested in the decrease of glucose storage,the increase of glucose consumption,the enhancement of hepatic gluconeogenesis and insulin resistance.The main reason is that the consumption of glucose in tumor tissue is 7 times of that in normal tissue[6],which leads to the increase of glucose consumption in tumor tissue.Anaerobic glycolysis is the main metabolic mode of sugar consumed by tumors,which is considered to be one of the important characteristics of tumor cells,called "Warburg effect".On the one hand,anaerobic glycolysis is a low efficiency mode of production capacity,which produces a lot of lactic acid.The production of lactic acid not only causes acidosis,but also suppresses the appetite of patients.On the other hand,most of the lactic acid produced by glycolysis enters into the lactic acid cycle and re-synthesizes glucose in the liver,which is absorbed by tumor cells for glycolysis energy supply.This invalid cycle leads to a large amount of useless energy consumption of the body.Currently,Warburg effect is suggested that tumor cells mainly produce energy through glycolysis.The mechanism changes may be related to mitochondrial dysfunction and changes of enzymes related to glucose metabolism [7,11].

Modern studies have shown that the pathogenesis of tumor cachexia is related to insulin resistance and its mechanism mainly includes that tumor directly induces insulin resistance or tumor secretes inflammatory factors to promote the occurrence of insulin resistance.Insulin resistance leads to the decrease of glucose storage,the obvious decrease of glucose oxidation in peripheral tissues,and the conversion of oxidation substrates from glucose to fatty acids.Cancer patients also have an increased rate of glucose renewal,which can consume up to 40 percent of their sugar intake and can lead to weight loss.Thus,changes in carbohydrate metabolism can lead to cachexia syndrome [8-9].

3.2.2 Abnormal protein metabolism

The patients with tumor cachexia who has the abnormal protein metabolism mainly for abnormal metabolism of protein and amino acid.At present,it has been found that the proteolysis increase and synthesis decrease of skeletal muscle in cancer patients,the protein conversion rate increase,hypoproteinemia,the abnormal amino acid profile in plasma and the negative nitrogen balance in the body which leads to the decrease of protein synthesis in the whole body and the occurrence of cachexia [10].Skeletal muscle is the main site of endogenous nitrogen loss in cancer patients .Skeletal muscle,accounting for 40% of normal adult body weight, is the main component of thin tissue group.Less protein synthesis leads to longterm progressive consumption of skeletal muscle,then the consumption of skeletal muscle leads to loss of activity and the reduction of activity further inhibits protein synthesis in cancer patients.Therefore,the increase of skeletal muscle protein consumption is the main cause of cachexia in patients with malignant tumors.At present,there are three main mechanisms of protein degradation as follow:Autophagy lysosomal proteolytic pathway is mainly that lysosomal system reduces the hydrolysis of extracellular proteins and cell surface receptors;The Ca2+activated degradation system pathway is mainly that Ca2+activated protease participates in tissue damage,necrosis and cell autolysis;The ATP ubiquitin proteasome system is mainly involved in protein degradation in cells.At present,UPS is considered to be the main pathway of protein degradation in cachexia [12].

3.2.2.1 ATP-ubiquitin-proteasome pathway

The ATP-UPS pathway is mainly composed of ubiquitin molecules and proteasomes,among which ubiquitin molecules can control cell cycle,differentiation and apoptosis,regulate inflammatory response,repair DNA and antigen presentation,and have the function of cell signal transduction.Currently,it is believed that the pathway of ATP-UPS protein degradation mainly includes target protein ubiquitination and target protein degradation.The process of the target protein ubiquitination is mainly that ubiquitin molecules which are activated by ubiquitin activate enzyme (E1)firstly or ubiquitin chain combined with ubiquitin coupled enzyme (E2),and then forms the substrate covalent bond with its target protein under the action of the ubiquitin ligase (E3),so the ubiquitin protein can be identified and degraded by 26S proteasome.And the procedure of E3 with tissue specificity promoting ubiquitin transfer to target protein is pivotal steps,which has a rate-limiting specificity [13].It has been found that specific E3 in skeletal muscle tissues is highly expressed in patients with dyscrasia.Most studied for E3 in the skeletal muscle is about muscle ring finger -1(MuRF1)and muscle atrophy F– box(MAFbx),both of them have the effect of degrading myosin heavy chain of thick myofilament,troponinIof thin myofilament,actin and related components.The team of Bao [14]found that the synthesis of the zinc finger structure in MuRF1 increased in patients with dyscrasia.And MAFbx can trigger the eukaryotic translation initiation factor 3 F ubiquitin of protein translation pathway degradation,therefore,it is suggested that the structure of zinc finger in MuRF1 is related to the formation of MAFbx in cachexia.Kamp[15]took the quadriceps biopsy between I -Ⅲ lung cancer patients (loss of body mass ＜ 10%)and patients without lung cancer through the animal experiment.The results showed that the level of nuclear factor kappa B (NF -kB)has no obvious difference with the activity of UPS.Bossola [16]found that the transcriptional levels of UPS of loss of body mass more than 10% of patients with gastrointestinal cancer is increased compared with control group,but the difference is not evident when the body mass reduction is less than 10%.These data suggest that UPP may not be activated in the early stage of cachexia,which is likely to be other mechanisms,and the activity of UPP started to increase as the malignant tumour cachexia progressed to a certain degree.

3.2.2.2 Autophagy lysosome proteolytic pathway

Studies have found that autophagy mainly consists of three pathways:macroautophagy,microautophagy and molecular chaperone mediated autophagy.At present,studies on skeletal muscle atrophy autophagy mainly focus on macroautophagy,yet it is still unclear whether microautophagy and molecular chaperone mediated autophagy are involved in skeletal muscle atrophy with cachexia.The central factors of autophagy process are mammalian target of rapamycin (mTOR)and autophagy associated gene (Atg).The latter includes ulk1,LC3,Beclin1 and BNIP3,which are regulated by the former.Currently the role of autophagy pathway in skeletal muscle atrophy has not been emphasized.Under normal conditions,the basic level of human autophagy is very low.Lokireddy et al [17]found that the autophagy activity of mouse C12 muscle cells was significantly increased after treatment with C26 tumor cell culture medium.It is suggested that autophagy played an important role in the process of skeletal muscle atrophy in the state of cachexia of malignant tumor.Mcclung and others [18]found that the body can regulate ALP through p38MAPK in stress-induced muscular atrophy,suggesting that p38MAPK may play a vital part in skeletal muscle atrophy.At present,there are few studies on the mechanism of autophagy regulation in human malignant tumor cells in cachexia state,which should be worthy of a further investigation.

3.2.3 Aberrant fat metabolism

Aberrant fat metabolism of malignant tumor cachexia is most chiefly manifested as fat metabolism consumption,which is caused by decreased fat synthesis.The rapid steatolysis with the increase of fatty acid oxidation and the augment of fat conversion rate can result in Body fat storage capacity decrease and weight loss.Modern research discovered that the metabolism of glycerol and free fatty acid (FFA)is too fast in patients with cachexia,and those who found that the concentration of serum three acyl glycerin in the fasting patients with tumor is higher than cancer patients with stable weight.It's further proved that the effect of lipolysis is reinforced.When the fat decomposition increases,part of the fatty acids derived from the fat decomposition are reesterified into triglycerides,which are manifested as the enhancement of the cycle of triglycerides and fatty acids.This cycle process needs to consume energy,which trigger the increase of energy consumption of the body and may also be the cause of indirect tissue consumption of the body.Animal experimental studies [19]showed that fat mobilization factor (LMF)produced by mouse tumor cells could induce the degradation of white fat cells in rats,its main mechanism might be LMF promoting the synthesis of cAMP and increasing the sensitivity of fat cells to lipidolysis hormone,so as to enhance the body's fat mobilization.High affinity binding sites unlike those of other β3-agonists bind to β3-adrenergic receptors,promoting adipose-degrading sensitivity of adipose-degrading hormones.Therefore,β3 receptor antagonists have the effect of reducing fat decomposition and energy consumption.Vaitkus[20]conducted the animal experiments,founding that FFA and other substances released during the decomposition of adipose tissue could be obtained by muscle tissue,which further activated the expression of ubiquitin ligase (E3)MuRF1 and human muscular atrophy protein Fbox-1,induced the ATP-UPS pathway,promoted the consumption of muscle tissue,and thus led to weight gain and the formation of cachexia.

Study found that there are high levels of fat decomposition metabolites such as glycerol and free fatty acids in blood of patients with cancer cachexia.Therefore,it is proposed that fat metabolic changes of the tumor patient already exists in the early of the tumor.And clinical studies have found that the activity of free fatty acid has been increased in tumor patients before they loss weight,even if patients are supported by the exogenous nutrition,the phenomenon of continuous decomposition and oxidation of body fat can’t be inhibited.So,fat consumption becomes one of the main characteristics of tumor cachexia.In fact,fatty acids are the main energy resources utilized by the host in tumor-bearing state,and the utilization of lipids by both the host and the tumor increases.

3.3 Inflammatory factors

Inflammatory cytokines play an important role in tumor occurrence,development,metastasis and apoptosis.At present,studies have found that endogenous cytokines such as tumor necrosis factor (TNF-α),interleukin(IL-1,IL-6),γ-interferon (γ-IFN)and fat mobilization factor of tumor-derived metabolic factor (LMF),ect,playing a critical role in the development of cachexia.

3.3.1 TNF-α

Tumor necrosis factor is also called the cachectin,which is mainly secreted from immune cells(macrophages and lymphocytes)and tumor cells.Currently Langen [21]conducted the in vitro experiment,founding that TNF-α can activate the IRS -1 related JNK/stress activated protein kinase(SAPK)through MAPK pathway,which causing the IRS -1 serine phosphorylation and interfering with the normal amino acid phosphorylation.These hampered insulin signal transduction,reduced glucose uptake and metabolism ability,increased abnormal fat metabolism,which leads to the occurrence of the cachexia.Animal studies have shown that injecting TNF-α purified protein to animals would result in animals TNF-α factor rising or putting the TNF-α genes into animals high expression through other means,they found animal body fat and muscle decomposed vigorously,bringing about similar symptoms of cachexia such as continuous progressive emaciation,anorexia and so on.Therefore,the results showed that the TNF-α plays an important role in the formation of the cachexia.It has been found that TNF-α can increase the expression of ubiquitin ligase genes such as MAFbx in skeletal muscle by activating the NF-κB signaling pathway,increasing oxidative pressure and NOS production and so on,thereby the mediated ubiquitin-proteasome of muscle fibrin can be degraded.NF-κB activation that induced by TNF-α also degrades MyoD transcription factors for the purpose of inhibiting muscle production.In animal cachexia model [22],TNF-α also can induce mitochondrial respiratory chain dissociation and increase the production of metabolic energy.TWEAK,a structural homologous of TNF-α,can induce E3 ubiquitin ligase MuRF1 expression,and then trigger the degradation of myosin heavy chain of rough myosin filaments of myolemma to produce cachexia.

3.3.2 IL– 1 and IL -6

IL-1 that is produced primarily by activated macrophages can produce anorexia symptoms through different signaling pathways,which mainly includes decreasing food intake directly through the hypothalamic eating pivot or increasing release of corticotrophin releasing hormone (CRH)which can suppress appetite for inhibiting food.Animal studies have shown that injecting IL-1 into the brain of rats can significantly suppress appetite and induce muscle breakdown and gene related expression in rats.Recent clinical studies have found that IL-1 [23]may influence anorexia and depression in cachexia through common signaling pathways.Injecting IL-1 into the brain can induce the expression of genes associated with muscle breakdown and atrophy quickly.Studies in recent years have found that IL-6 [24]can shorten the half-life of many long-term proteins.By increasing the activity of 26S protease,cathepsin B and cathepsin L,muscle proteins can be degraded through non lysosomal(proteasome)and lysosomal (cathepsin)pathways.IL-6 secreted by activated macrophages in tumor patients can stimulate liver to produce acute response (APR)and regulate cachexia.

Regulatory factors produced by immune or tumor cells also play an important role in the development of cancerous cachexia,such as LMF,also known as zincα2-glycoprotein (ZAG)and proteolytic induction factor(PIF),which have the activity of degrading fat and skeletal muscle respectively.Tumor cells produce LMF,which can specifically act on adipose tissue through the cAMP pathway,stimulate GTP-dependent adenylate cyclase,activate HSL to induce adipo hydrolysis and increase the overall level of lipid oxidation.In addition,LMF can improve substrate utilization by increasing mitochondrial oxidation pathways in brown adipose tissue (BAT)and skeletal muscle.PIF[25]can induce increased expression of components of ubiquitin-proteasome signaling pathway,including 20S proteasome α-subunits such as MSS1,p42 and so on,which can reduce protein synthesis and increase protein degradation.Besides,PIF may also react on cachexia by affecting the production of cytokines and APP.Clinically,PIF can be detected in the urine of cancer patients with significant weight loss,but not in patients without weight loss.

Myostatin and activin [26]are recently particularly concerned cachexia regulators.Myostatin—a member of the family of TGFβ— is made up of skeletal muscle cell synthesis and secretion,which transmits signal by activating element typeⅡreceptors,recruiting Alk kinase family, and activating the transcription factor complex of smad2 and smad3.Overexpression of myostatin in mice leads to severe skeletal muscle atrophy,whereas expression of inhibition of myostatin in mice doubles muscle mass and muscle fiber size.The signaling pathways that myostatin promotes muscle loss include inhibition of Akt and its downstream TORC1 pathway,promotion of protein synthesis,and suppression of genes involved in muscle differentiation which is activated by smad2/3.In addition to myostatin,there are other TGFβ family members whose expression is induced by inflammatory factors.After activation of TNFα/TAK-1 signaling pathway,the level of Activin A in skeletal muscle cells is up-regulated.Activin A and Activin B are powerful growth and differentiation factors with many physiological functions,including regulating embryonic development,protecting nerves,regulating apoptosis and fibrosis,ect,which play an important role in cachexia.

4.Conclusion

To sum up,cancer cachexia is one of the clinical integrated manifestations associated with cancer,which often occurs in the terminal stage of cancer.It has been clear that disorder of material and energy metabolism is the main cause of cancer cachexia,and imbalance between pro-cachexia factor and anti-cachexia factor also acts crucially.Due to the imbalance in the expression of these cytokines,the normal functional state of the body is broken.With the enhancement of the cytokine produced by tumor cells and the extension of the action time,the imbalance is gradually aggravated,resulting in a vicious circle.Although there are various treatment methods for cancer cachexia,the overall therapeutic effect is not good [27].With the in-depth understanding of the pathogenesis of cancer cachexia,it is possible to develop more specific immune nutrients to block the development of cancer cachexia.