Cancer Anorexia-Cachexia Syndrome: Are Neuropeptides the Key?1

[CANCER RESEARCH 59, 4493–4501, September 15, 1999] Perspectives in Cancer Research

Akio Inui2 Second Department of Internal Medicine, Kobe University School of Medicine, Kobe 650-0017, Japan

Abstract moment of diagnosis, ϳ80% of patients with upper gastrointestinal cancers and 60% of patients with lung cancer have had substantial Progressive wasting is common in many types of cancer and is one of weight loss (2). Cachexia is more common in children and elderly the most important factors leading to early death in cancer patients. patients and becomes more pronounced as disease progresses (2). Weight loss is a potent stimulus to food intake in normal humans and animals. The persistence of anorexia in cancer patients, therefore, implies The anorexia-cachexia syndrome is a complex interplay of meta- a failure of this adaptive feeding response, although the weight loss in the bolic and behavioral variables that is correlated with poor outcomes patients differs from that found in simple starvation. Tremendous pro- and compromised quality of life (1–3). Although the etiology of gress has been made in the last 5 years with regard to the regulation of cachexia is not well defined, several hypotheses have been explored feeding and body weight. It has been demonstrated that leptin, a hormone including cytokines, circulating hormones, neuropeptides, neurotrans- secreted by adipose tissue, is an integral component of the homeostatic mitters, and tumor-derived factors (4–6). An emerging view is that loop of body weight regulation. Leptin acts to control food intake and the anorexia-cachexia syndrome is caused predominantly by cyto- energy expenditure via neuropeptidergic effector molecules within the kines either produced by cancer or released by the immune system as hypothalamus. Complex interactions among the nervous, endocrine, and a response to the presence of the cancer, as well as other tumor immune systems affect the loop and induce behavioral and metabolic products that induce profound lipolysis or protein degradation (1, 7). responses. A number of cytokines, including tumor necrosis factor-␣, interleukins 1 and 6, IFN-␥, leukemia inhibitory factor, and ciliary neu- The application of molecular and genetic techniques to the study of rotrophic factor have been proposed as mediators of the cachectic process. body weight regulation have produced exciting new insights into the Cytokines may play a pivotal role in long-term inhibition of feeding by physiological systems governing appetite, energy expenditure, and mimicking the hypothalamic effect of excessive negative feedback signal- metabolic signaling (8–11). Recent studies led to the hypothesis that ing from leptin. This could be done by persistent stimulation of anorexi- body weight is regulated by a feedback loop in which peripheral genic neuropeptides such as corticotropin-releasing factor, as well as by signals report nutritional information to an integratory center in the inhibition of the neuropeptide Y orexigenic network that consists of opioid hypothalamus of the brain, and neuropeptides are essential effector peptides and galanin, in addition to the newly identified melanin-concen- molecules within the hypothalamus (8–11). In this report, I review trating hormone, orexin, and agouti-related peptide. Information is being data that point to the hypothesis that dysregulation of the neuropep- gathered, although it is still insufficient, on such abnormalities in the tidergic circuitry controlling food intake and energy expenditure, and hypothalamic neuropeptide circuitry in tumor-bearing animals that coin- cide with the development of anorexia and cachexia. Characterization of thus energy homeostasis, is playing a major role in the cancer ano- the feeding-associated gene products have revealed new biochemical path- rexia-cachexia syndrome. ways and molecular targets for pharmacological intervention that will likely lead to new treatments. Although therapeutic intervention using Leptin and Body Weight Regulation neuropeptide agonists/antagonists is now directed at obesity treatment, it may also have an effect on treating cancer anorexia-cachexia, especially The concept that appetite-restraining signal(s) from adipocytes are when combined with other agents that have effects on muscle and protein integral components of the feedback loop between the periphery and breakdown. the brain for energy homeostasis gained firm ground with the cloning of the ob3 gene and its encoded protein, leptin, from adipocytes Introduction (10–13). Leptin is an afferent signal from the periphery to the brain that regulates adipose tissue mass (10, 13, 14). The level of leptin is Cachexia is a debilitating state of involuntary weight loss compli- positively correlated with body fat mass, and dynamic changes in cating malignant, infectious, and inflammatory diseases and contrib- plasma leptin concentrations in either direction activate the efferent uting significantly to mortality. Anorexia, also a frequent complica- energy regulation pathways (10, 11). Leptin reduces appetite and tion of these diseases, is a major contributor to the development of increases energy expenditure and evidently elicits these effects via the cachexia, although the pattern of weight loss in cachexia differs from central nervous system (10, 11). In the absence of leptin, such as in that seen with pure nutrient deprivation (1). ob/ob mice, animals fail to restrain their food intake and become The word “cachexia” is derived from the Greek “kakos” meaning massively obese. “bad” and “hexis” meaning “condition” (1). About half of all cancer Leptin acts via the leptin receptors with alternatively spliced forms patients show a syndrome of cachexia, characterized by anorexia and (10, 15). The short forms may function in the transport of leptin across loss of adipose tissue and skeletal muscle mass. In general, patients the BBB. The long (transmembrane) form of the receptor is expressed with solid tumors have a higher frequency of cachexia (2). At the at high levels in hypothalamic neurons of the ARC, DMH, PVN, ventromedial, and LH nuclei, each of which is important in regulating Received 2/1/99; accepted 6/22/99. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 3 The abbreviations used are: ob, obese; BBB, blood-brain barrier; ARC, arcuate 18 U.S.C. Section 1734 solely to indicate this fact. nucleus of the hypothalamus; DMH, dorsomedial nucleus of the hypothalamus; PVN, 1 The work was supported by grants from the Ministry of Education, Science, Sports paraventricular nucleus of the hypothalamus; LH, lateral hypothalamic area; NPY, neu- and Culture of Japan. ropeptide Y; MCH, melanin-concentrating hormone; AGRP, agouti-related peptide; CRF, 2 To whom requests for reprints should be addressed, at Second Department of Internal corticotropin-releasing factor; GLP-I, glucagon-like peptide I 7-36 amide; CART, co- Medicine, Kobe University School of Medicine, Kusunoki-cho, Chuo-ku, Kobe 650-0017, caine- and amphetamine-related transcript; UCP, uncoupling protein; TNF, tumor necrosis Japan. Phone: 81-78-382-5111, extension 5862; Fax: 81-78-382-2080; E-mail: inui@ factor; IL, interleukin; LIF, leukemia inhibitory factor; CNTF, ciliary neurotrophic factor; med.kobe-u.ac.jp. CCK, cholecystokinin; MC, melanocortin; MSH, melanocyte-stimulating hormone. 4493

Downloaded from cancerres.aacrjournals.org on September 28, 2021. © 1999 American Association for Cancer Research. NEUROPEPTIDES, CYTOKINES, AND CANCER ANOREXIA-CACHEXIA energy homeostasis (16). The association of mutations in leptin and its that induce or mimic the hypothalamic effect of excess negative receptor with massive obesity are observed in both rodents and feedback signaling from leptin, the expected outcome would be sus- humans (17–19). tained anorexia and weight loss that is not accompanied by the usual compensatory response. Neuropeptidergic Cascade Downstream of Leptin Signaling Cytokine Actions within the Regulatory Feedback Loop and Fig. 1 shows a simplified model for the interaction of leptin with Cancer Anorexia-Cachexia hypothalamic neuropeptidergic effector molecules within a regulatory feedback loop. The model emphasizes the feeding-drive systems that Numerous cytokines, including TNF-␣, IL-1, IL-6, and IFN-␥ have would underlie the hypothalamic response to starvation. NPY is the been postulated to play a role in the etiology of cancer anorexia- most potent orexigenic (feeding-stimulatory) peptide activated by the cachexia syndrome (Refs. 1, 4, 5, and 28–30; Fig. 1). Cytokines are fall of leptin and consists of an interconnected orexigenic network that protein molecules released by lymphocytes and/or monocyte macro- includes galanin, opioid peptides, MCH, orexin, and AGRP (8, 9, 11, phages. They may be released into the circulation and transported to 20–23). Most of these peptides are up-regulated in ob/ob mice, and the brain through the BBB and circumventricular organs (leaky areas their expressions are increased through fasting in wild-type mice and in the BBB), as is the case for IL-6 (29–34). Peripheral cytokines may are inhibited by leptin administration (11). Other effector molecules influence the brain via neural pathways or second messengers such as functioning in this homeostatic loop are the anorexigenic neuropep- NO and prostanoids. Vagotomy was reported to attenuate the acqui- tides such as CRF, melanocortin, GLP-I, neurotensin, and CART, sition of and facilitate the extinction of conditioned taste aversion expression of which is down-regulated in ob/ob mice and stimulated induced by i.p. IL-1␤ or TNF-␣ administration (35). Cytokines are by leptin (6, 8, 9, 11). The administration of the receptor antagonists also produced by neurons and glial cells within the brain, partly in of these peptides effectively blocks the reduction of food intake and response to peripheral cytokines (29–34). Although the site of syn- body weight by leptin (11). The orexigenic and anorexigenic neu- thesis of cytokines within the brain is dependent on the nature of the ropeptides decrease and increase sympathetic nervous activity, respec- stimulus, systemic disease seems to predominantly influence the ex- tively, which regulates energy expenditure by activating thermogen- pression in the hypothalamus, the area with highest densities of esis in brown adipose tissue and possibly in other sites such as white receptors for most cytokines being observed (33). adipose tissue and muscle, through induction of the mitochondrial High serum levels of TNF-␣, IL-6, and IL-1 have been found in uncoupling protein UCP-1 and the newly identified UCP-2 and some, but not all, cancer patients, and the levels of these cytokines UCP-3 (24–27). Thus, if a disease process were to produce factors seem to correlate with the progression of some tumors (36–38).

Fig. 1. A simplified model of the hypothalamic neuropeptide circuitry in response to starvation (A) and cancer anorexia-cachexia (B). Leptin acts as part of a feedback loop to maintain constant stores of fats. This is achieved by hypothalamic neuropeptides downstream of leptin that regulate food intake and energy expenditure. A loss of body fat (starvation) leads to a decrease in leptin, which in turn leads to a state of positive energy balance wherein food intake exceeds energy expenditure. This compensatory response is mediated by an increased production, release, and/or action of NPY and other orexigenic neuropeptides, as well as a decreased activity of anorexigenic neuropeptides such as CRF (A). In tumor-bearing states, cachectic factors such as cytokines elicit effects on energy homeostasis that mimic leptin in some respects, and the increased hypothalamic actions of these mediators induce anorexia and unopposed weight loss (B). This could be accomplished through persistent inhibition of the NPY orexigenic network and stimulation of CRF and other anorexigenic neuropeptides, although the exact nature and hypothalamic pathways participating in the response, especially relationships among newly identified peptides and cytokines, remain to be determined. Serotonin may have an important role in the development of cancer anorexia, partly through the inhibition of NPY signaling. Increased levels of plasma and brain tryptophan, the precursor of serotonin, and IL-1 may underlie the increased serotonergic activity. CNS, central nervous system. 4494

Chronic administration of these cytokines, either alone or in combi- neurons in the hypothalamus that project to PVN, DMH, LHA, and nation, is capable of reducing food intake and reproducing the differ- other hypothalamic sites. Although NPY has been demonstrated to ent features of the cancer anorexia-cachexia syndrome (1, 36–39). produce diverse effects on behavior and other functions, its most The problem with ascribing specific tissue responses to individual noticeable effect is the stimulation of feeding after central adminis- cytokines is that considerable overlap and redundancy exists in the tration (20, 21, 57–59). The feeding-stimulatory effect of NPY is cytokine network (28, 31–34). Administration of either TNF-␣ or IL-1 robust and ϳ500 times more potent on a molar basis than norepi- will induce the synthesis of a variety of other proinflammatory cyto- nephrine, an aminergic neurotransmitter (58). Multiple injections of kines such as IL-6. Thus, studies that use pharmacological adminis- NPY into the PVN or cerebral ventricle result in obesity, indicating tration of recombinant cytokines may not discriminate between bio- that NPY is capable of overriding powerful inhibitory signals on food logical responses induced directly by administered cytokine and those intake and body adiposity (60, 61). NPY produces a shift to positive induced secondarily by other cytokines stimulated. Systemic disease energy balance by increasing food intake, by decreasing energy ex- such as cancer and inflammation may elicit a cytokine cascade in penditure primarily with a reduction in thermogenesis in brown adi- which several cytokines are induced simultaneously (28). More direct pose tissue (62), and by facilitating fat deposition in white adipose evidence of cytokine involvement comes from the experiments in tissue partly through increased insulin activity (63). which specific neutralization of cytokines can relieve anorexia and NPY synthesis in the ARC and its release into the PVN, the most cachexia in experimental animal models (1, 37, 38, 40). Examples are abundant projection, are regulated by afferent signals such as leptin, the anti-TNF-␣, anti-IL-6, anti-IL-1, and anti-IFN-␥ antibodies, al- insulin (both inhibitory), and glucocorticoids (stimulatory; Refs. 14, though no single antibody has been proven to reverse all of the 20, 21, 58, 59, and 61). NPY synthesis and secretion are all up- features of wasting seen in cancer cachexia (37). These studies re- regulated in models of energy deficiency or increased metabolic vealed that cachexia can rarely be attributed to any one cytokine but demand such as starvation, insulin-dependent diabetes mellitus, lac- rather is associated with a set of cytokines and other cachectic factors tation, and physical exercise (20, 21, 59, 61, 64). The primary phys- that work in concert. iological role of the ARC NPY neurons may thus be to restore normal energy balance and body fat stores under conditions of energy deficit, Leptin the signals of which are falling leptin and/or insulin occurring in these conditions. The exact position of NPY in the hierarchy of neuropep- The crystal structure of leptin indicates that it is a member of the tidergic cascade, however, remains to be determined, and the rela- helical cytokine family (41), which includes the IL-6 family of IL-6, tively normal food intake and body weight of NPY-knockout mice IL-11, LIF, and CNTF. Most of these cytokines are able to cause body suggest that other systems cooperate to control energy homeostasis weight loss and/or anorexia (42). It was demonstrated that weight under various circumstances (65, 66). loss-inducing cytokines such as TNF-␣, IL-1 and LIF increase the Previous studies demonstrated that NPY feeding systems are dys- expression of mRNA for leptin in adipose tissue and plasma levels of functional in anorectic tumor-bearing rats. NPY injected intrahypo- leptin, despite the decrease in food intake that would normally sup- thalamically stimulated feeding less potently in rats bearing methyl- press leptin expression (43–45). The increased leptin could thus cholanthrene-induced sarcoma than in controls, which was observed contribute to anorexia by preventing the normal compensatory mech- prior to the onset of anorexia and became more severe as the rats anisms in the face of decreased food intake. However, these cytokines developed anorexia (67). The reduced affinity of hypothalamic NPY have also been shown to cause anorexia even in the absence of leptin receptors as well as refractory adenylate cyclase in response to NPY (46, 47), and failure to see elevated leptin levels was reported in suggests that the postsynaptic NPY-signaling systems are altered in tumor-bearing rats (48) and in patients with cancer cachexia (49–51). the hypothalamus of tumor-bearing rats (68, 69). The level or release CNTF is a pluripotent neurocytokine expressed by Schwann cells in of NPY in the PVN or the hypothalamus is also reduced in tumor- peripheral nerves and glial cells in the brain. CNTF was reported to bearing rats, whereas it is increased in fasting animals and in nutri- have more profound and longer lasting anorectic effects than other tional controls that have their food restricted to match their body cytokines and to reduce leptin gene expression (52, 53). Cytokines weight to the carcass weight of tumor-bearing rats (70, 71). IL-1␤ may elicit effects on energy homeostasis that mimic leptin in some administered intracerebroventricularly antagonizes NPY-induced regards, and the increased hypothalamic actions may contribute to feeding in rats at a dose that yields estimated pathophysiological anorexia and body weight loss. This appears to be a likely explanation concentrations in the cerebrospinal fluid such as those observed in because leptin receptor is homologous to the gp130 signal-transducing anorectic tumor-bearing rats (72–74). IL-1␤ decreases hypothalamic molecule associated with the IL-6-type cytokine receptor and shares NPY mRNA levels that are specific and not associated with a gener- the same postreceptor signaling pathway via activation of the signal alized reduction in the brain levels (73). NPY also blocks and reverses transducers and activators of the transcription family (54). Previous IL-1␤-induced anorexia, suggesting the importance of cytokine- studies demonstrated that leptin activates the Stat3 protein in the neuropeptide interactions. The hypothalamic NPY system is one of hypothalamus of normal mice but not in db/db mice that have a defect the key neural pathways disrupted in the anorexia induced by CNTF in the long form of leptin receptor (55). This action was shared by (75, 76), the synthesis of which is increased in the brain in response cytokines of the gp130 family (54), as well as endotoxin LPS, which to cancer and other clinical conditions characterized by loss of appe- causes anorexia and induces cytokines including IL-6 family mem- tite (77–79). Central as well as peripheral administration of CNTF bers. Therefore, if stat3 protein is essential for leptin-induced ano- produced anorexia and body weight loss; injection of NPY only rexia, the cytokines that share with leptin the same postreceptor marginally stimulated feeding in these rats. CNTF suppressed the signaling pathway could likewise induce anorexia and unopposed basal and fasting-induced NPY gene expression in the ARC of the weight loss. hypothalamus (75, 80), as well as the fasting-induced increase in NPY hypothalamic Y1 receptor expression (76), a supposed appetite receptor for NPY (21). The central CNTF actions in producing anorexia, NPY, a 36-amino acid peptide, is one of the most abundant and body weight loss, and suppression of gonadotropin secretion (80) widely distributed neurotransmitters in the mammalian brain (20, 21, involves suppression of NPYergic signaling in the hypothalamus in a 56–59). The ARC is the major site of expression for NPY within manner similar to that caused by leptin. Other metabolic alterations 4495

Downloaded from cancerres.aacrjournals.org on September 28, 2021. © 1999 American Association for Cancer Research. NEUROPEPTIDES, CYTOKINES, AND CANCER ANOREXIA-CACHEXIA seen in tumor-bearing rats such as hyperammonemia may also reduce of tumor-induced anorexia (96). This suggests that dynorphin, an NPY-induced feeding and hypothalamic NPY concentrations (68, 81). endogenous kappa opioid receptor agonist, may be involved in the However, no change or even increase in NPY mRNA levels were reduction of food intake, although the sensitivity or binding capacity reported in the hypothalamus of tumor-bearing rats (49, 82), suggest- of kappa opioid receptors does not appear to be altered in tumor- ing the involvement of other orexigenic and/or anorexigenic signals in bearing rats (96). Physiological anorexia of aging was reported to be the anorexia and cachexia. due, in part, to a decreased opioid (dynorphin) feeding drive (97). However, it is not apparent how opioid systems are involved in the MCH and Orexin development of cancer anorexia and cachexia. This is also the case for galanin, despite the fact that gonadal steroids show excitatory effects MCH is a cyclic 19-amino acid peptide originally isolated from on galanin expression in the PVN (90), and progestational drugs may salmon pituitaries that is expressed in the mammalian brain, specifi- stimulate appetite and body weight gain in cancer patients (98). cally the lateral hypothalamus and the zona incerta (8–11, 83, 84). The expression of this peptide was found to be increased in ob/ob Glucagon and Glucagon-like Peptide I mice and also in fasting wild-type mice (22). Intracerebroventricular administration of MCH produced a dose-dependent stimulation of A single proglucagon gene in mammals gives rise to an identical food intake in rats (22, 83). MCH-deficient mice have recently been proglucagon RNA transcript that is translated and processed differ- generated that have reduced body weight and leanness due to reduced entially in the brain, pancreatic islets, and intestine. Glucagon pro- feeding and an inappropriately increased metabolic rate (85). Another duced in the pancreas and GLP-I in the intestine and brain are potent family of neuropeptides in the LH was recently identified simulta- anorexigenic peptides (99, 100). A large body of evidence indicates neously and independently by two groups of investigators, who named that the peripheral administration of glucagon produces satiety and them orexins (23) or hypocretins (86). Orexins stimulate feeding when reduces food intake in experimental animals and in humans, and injected intracerebroventricularly, and the expression of orexin afferent fibers of the hepatic vagus are the key element supporting this mRNA is increased by food deprivation (85). Immunohistochemical behavioral effect (99). The administration of a highly specific anti- studies demonstrate projections from NPY neurons in the ARC to glucagon antibody increases food intake in rats, suggesting a physi- MCH and orexin cells in the LH, suggesting that the latter two ological role of glucagon in controlling food intake. GLP-I was shown peptides may function as downstream effector molecules of NPY in recently to elicit a potent suppression of food intake and to reduce the appetite-regulating pathways (87, 88). Although alterations in the body weight after intracerebroventricular administration (100). GLP- NPY system may affect MCH and orexins in the LH, it is not yet I-producing neurons are located in the brain, primarily in the nucleus known whether these newly identified peptides are involved in the of the solitary tract. Administration of exendin 9–39, a specific GLP-I anorexia and cachexia of cancer. receptor antagonist, increases food intake in several animal models, suggesting also that endogenous GLP-I in the brain is an inhibitor of Galanin and Opioid Peptides feeding (100). The tumor-bearing state is associated with a decrease in the host Other peptides that are known to consist of an interconnected insulin:glucagon ratio. Hyperglucagonemia in the tumor-bearing state orexigenic network with NPY are galanin and the opioids, ␤-endor- may be the most important hormonal alteration causing abnormal phin and dynorphin (89). Galanin is a 29–30 amino acid peptide that carbohydrate metabolism in cancer anorexia (101–104). Increased is widely distributed in the brain, including the PVN of the hypothal- glucagon levels can lead to anorexia, as well as increased hepatic amus (90–92). Central administration of galanin increases food intake gluconeogenesis and utilization of gluconeogenic amino acids at the in rats, and reduction of central galanin levels by antisense oligonu- expense of protein synthesis, leading to a negative energy balance. cleotide or central administration of galanin receptor antagonist de- Inhibiting glucagon secretion may increase carcass weight, preserve creases food intake. Although the NPY system is closely associated muscle protein, and even inhibit tumor growth (103, 104). Cytokines with carbohydrate ingestion and carbohydrate utilization, channeling such as IL-6 stimulate glucagon secretion in humans (105). Recently, nutrients toward the synthesis of fat, galanin appears to act preferen- a transplantable rat glucagonoma has been isolated, which shows tially in the ingestion of fat and may enhance fat deposition through highly increased circulating levels of glucagon and GLP-I, and is a reduction in energy expenditure (90). A high fat diet can enhance associated with severe anorexia, adipsia, and weight loss (49). A galanin production in the PVN, which was closely linked to body highly significant increase in hypothalamic NPY mRNA levels was adiposity (92). found in the anorectic rats. It thus appears that the glucagonoma Opioid peptides are also involved in the regulation of food intake, releases circulating factor(s) that override the hyperphagic action of although the increase in feeding after the administration of opioids or NPY. Given that the satiety effect of glucagon is vagally mediated opiates is not only short lasting in its effect but generally is mild (99), the lack of effects of vagotomy in this model may exclude relative to the increase in feeding after NPY or galanin administration glucagon as a causative agent. GLP-I is a likely candidate for the (89, 93). Opioids may alter taste and nutrient selection, eliciting a anorectic effects of the tumor because GLP-I can promote satiety and preference for high-fat diets. Opioid receptor antagonists decrease reduce food intake after continuous peripheral administration (106) feeding induced by fasting, as well as by galanin or NPY administra- and because GLP-I can inhibit NPY-induced feeding in the brain tion (89, 93, 94). The intimate connectivities among ␤-endorphin, (100). The glucagonoma syndrome in humans is also characterized by galanin, and NPY were indicated by the immunohistochemical obser- elevated levels of circulating proglucagon-derived peptides and is vation of synaptic contact among a population of these peptide- frequently associated with weight loss beyond what would be ex- producing neurons in the hypothalamus (89). pected for a relatively small tumor (107). It was reported that by using naloxone, a nonselective opioid ␣ ␤ receptor antagonist, IL-1 and IL-1 are able to stimulate the activity CCK of the hypothalamic endogenous opioid peptide system to inhibit luteinizing hormone-releasing hormone secretion (95). However, a Of several putative satiety signals, the small-intestinal hormone nocturnal depletion of hypothalamic dynorphin was reported in an- CCK has received the most attention (99). Systematic administration orexic Walker-256 carcinosarcoma rats that coincides with the phase of CCK reduces short-term food intake in many animal species and in 4496

Downloaded from cancerres.aacrjournals.org on September 28, 2021. © 1999 American Association for Cancer Research. NEUROPEPTIDES, CYTOKINES, AND CANCER ANOREXIA-CACHEXIA humans (99, 108, 109). CCK delays the rate of gastric emptying, receptor knockout mice (125). The output from the feeding inhibitory which may contribute to reduced food intake. Evidence for the im- MC-4 receptor may thus be determined by the ratio of agonist (MSH) portance of endogenous CCK for normal termination of feeding has and antagonist (AGRP) at MC-4 receptor neurons in the DMH and come from studies in which administration of highly potent and PVN of the hypothalamus (8, 127). selective CCK antagonists increased food intake or hunger sensations The MC-4 receptor may regulate the response to inflammatory in animals and humans. The CCK-evoked satiety signals are carried cytokines (127). Intracerebroventricular administration of ␣-MSH has by vagal afferent fibers and subsequently processed to the nucleus strong anti-inflammatory and antipyretic effects (127, 128). ␣-MSH is tractus solitarius and PVN of the hypothalamus, although further work the most potent endogenous antipyretic hormone and inhibits fever is required to identify the brain sites and brain pathways involved (99, induced by IL-1, IL-6, or TNF- ␣ (128). COOH-terminal tripeptide of 109). CCK is also localized within the brain, with particularly high ␣-MSH antagonizes IL-1-induced anorexia (129). Anticytokine ef- concentrations being found in the hypothalamus and cerebral cortex fects of ␣-MSH are not specific to a certain cytokine but rather they (109). CCK administered into the cerebral ventricle or PVN reduced include antagonism of all proinflammatory cytokines (128). The MC feeding, whereas CCK antagonists reliably increased feeding (108, system is thus involved in the central regulation of the immune 110, 111). The release of endogenous CCK octapeptide (CCK-8), a response, but the role of this system in causing the cachexia seen in predominant molecular form in the brain, at hypothalamic sites has cancer patients has not been examined. been demonstrated after intragastric administration of nutrients in animals (112, 113). Corticotropin-releasing Factor and Urocortin The initial studies demonstrated that plasma concentrations of immunoreactive CCK were not significantly altered in either Walker- CRF, a 41-amino acid peptide, is one of the key mediators involved 256 carcinosarcoma or the methylcholanthrene-induced sarcoma an- in stress-related endocrine, immune, visceral, and behavioral re- imal model of cancer anorexia (114). Furthermore, levels of immu- sponses (130–132). The CRF peptide family consists of CRF itself, noreactive CCK were significantly reduced in the hypothalamus and fish urotensin, frog sauvagine, and the recently characterized mam- cerebral cortex of animals bearing the methylcholanthrene sarcoma malian urocortin (133). Considerable evidence suggests a role for during mild and severe anorexia (114). However, the RIA used at that CRF systems in appetite regulation and energy balance (9–11, 132). time, unfortunately, did not recognize active CCK-8, leaving the Administration of CRF into the cerebral ventricle or the PVN of the possibility of CCK involvement in producing anorexia and cachexia. hypothalamus, its primary site of biosynthesis, decreases food intake It was demonstrated that the repeated (but not single) administration and increases sympathetic nervous activity, leading to increased ther- of CCK-8 facilitates anorexia and body weight loss in Walker-256 mogenesis, lipolysis, and blood glucose levels (132). Chronic admin- carcinosarcoma rats compared with controls (115, 116). This was istration of CRF causes sustained anorexia and progressive weight particularly evident during the early stages of tumor growth and loss. Recently, CRF1 and CRF2 receptors have been cloned and without the development of behavioral tolerance (116). CCK medi- characterized (130). The selective expression of CRF2 receptor ates, at least in part, IL-1␣-induced anorexia and gastric stasis (117). mRNA in the ventromedial nucleus and PVN of the hypothalamus Injection of IL-1␣ increased plasma CCK levels including CCK-8 and suggests that CRF2 receptor is involved in the regulation of food decreased food intake and emptying of gastric contents. Pretreatment intake and energy expenditure (134). Urocortin is a potent activator of with CCK-A (peripheral type) receptor antagonist partially blocked CRF2 receptor, and central administration of urocortin produces ap- the decrease in food intake and gastric emptying rate by IL-1␣ (117). petite-suppressing effects more potent than CRF, with very low anx- Because CCK rapidly mobilizes leptin from the stomach (118) and iogenic and aversive effects (133). interacts synergistically with leptin (119), increases in the release A well-studied endocrine effect of cytokine is activation of the and/or sensitivity to either factor might play a role in inducing hypothalamic-pituitary-adrenal axis, an action proposed to participate anorexia. Peripherally administered CCK was reported to decrease in the negative feedback control of the immune response (Fig. 1). NPY-induced food intake and hypothalamic NPY levels (59). Because glucocorticoids inhibit cytokine production, the ability of many cytokines such as IL-1, IL-2, IL-6, TNF-␣, and IFN-␥ to Melanocortin and AGRP stimulate hypothalamic CRF expression and/or release may be an important mechanism for limiting cytokine production (4, 132). How- The agouti mouse is similar to ob/ob, a monogenic model of obesity ever, it could induce anorexia and weight loss if a sustained increase that has an increased propensity to develop tumors (120). The gene in one or more of these cytokines occurring in the brain or periphery defect in this mouse model was found to involve a 133-amino acid caused persistent stimulation of CRF activity in the PVN. CRF blunts peptide that competes with MCs such as ␣-MSH for binding to their the adaptive hypothalamic response to weight loss through an inhibi- receptors (121). Agouti antagonized several members of MC recep- tion of the NPY system (11, 131, 132). It was reported that stimulation tors, including the MC-1 receptor in the hair follicle (expressing the of CRF by IL-1␤ is paralleled by a potent and sustained suppression yellow coat color) and the MC-4 receptor in the brain when expressed of food intake (135) that can be elicited by intracerebroventricular ectopically (developing obesity; Refs. 8–11). ␣-MSH, produced from administration at doses below those required to induce central effects proopiomelanocortin precursors, decreases feeding after central ad- such as fever (14). The immune neutralization of endogenous CRF by ministration, whereas MC-4 receptor antagonist increases feeding CRF antiserum inhibits IL-1-induced anorexia (136) as well as adre- (8–11, 122). Inactivation of the MC-4 receptor by gene targeting nocorticotropic hormone release, providing evidence for a role of produces a syndrome of obesity similar to that of the agouti mouse hypothalamic CRF in the response. Rats bearing the Yoshida sarcoma (123). Both the yellow mice and MC-4 receptor knockout mice have displayed anorexia that was associated with increased rectal temper- increased NPY levels in the DMH of the hypothalamus (124). Re- ature (and thus energy expenditure; Ref. 71). Although food-restricted cently, AGRP was discovered as a homologue of agouti, which is control rats showed a decrease in CRF levels in the PVN and ARC of downstream of leptin (125). AGRP acts as an antagonist of the MC-4 the hypothalamus, the tumor-bearing rats had elevated CRF and receptor and is expressed in the ARC of the hypothalamus in close reduced NPY levels in the PVN, suggesting increased CRF signaling association with NPY (126). The ectopic expression of AGRP in during weight loss (71). transgenic mice produces obesity closely resembling that of the MC-4 Substantial evidence shows that brain CRF plays a role in the 4497

Downloaded from cancerres.aacrjournals.org on September 28, 2021. © 1999 American Association for Cancer Research. NEUROPEPTIDES, CYTOKINES, AND CANCER ANOREXIA-CACHEXIA alterations of gastrointestinal motor function induced by stress (137). adiposity signal where insulin promotes a state of negative energy

Recently, it was demonstrated that CRF2 receptor is primarily in- balance (14). It is recognized that megestrol acetate, a synthetic volved in CRF-induced, long-lasting inhibition of gastric emptying of progesterone derivative, enhances appetite and causes body weight the meal (138). Gastric emptying plays an important role in regulating gain in a number of cachectic patients via stimulation of NPY and food intake, and a reduced rate of gastric emptying contributes to the other mechanisms (98, 149). There is a large effort to develop the inhibition of feeding (139). IL-1␤ inhibits gastric emptying that was agonists and/or antagonists of neuropeptide receptors that have been antagonized by the central administration of CRF receptor antagonist specifically associated with food intake and energy expenditure (150),

(140). It is likely that CRF is responsible, at least in part, for the including NPY Y1 and Y5 receptors (21), opioid receptor, MC-4 prolonged inhibition of gastric motor function seen in cancer, which receptor, CRF2 receptor, CCK-A receptor, and GLP-I receptor, some also leads to anorexia and cachexia. The role of endogenous urocortin of which are presently being evaluated in clinical trials. The augmen- in stress responses is presently unclear, but investigation of the effects tation of the NPY orexigenic network is apparently one of the key of CRF antagonists or antisera may not distinguish between a CRF or potential targets for the treatment of cancer cachexia. In this report, urocortin action (130). attention has been focused on neuropeptide and cytokine interactions within the regulatory feedback loop of body weight regulation. It is Concluding Remarks important, however, to keep in mind other hormones, neurotransmitters, enzymes, and substances that are likely to be involved in the In recent years, cachexia has been understood as the result of major cachectic process, notably lipid- and protein-mobilizing factors that metabolic abnormalities due to a combination of host cytokine release can directly mobilize fatty acids and amino acids from adipose tissue and tumor products rather than as a simple increase in energy con- and skeletal muscle, respectively (1, 7, 151–154). This is likely a sumption by the tumor and starvation by the patient (98). Hyperali- unique feature of tumor-bearing hosts, as might be the case for an mentation was thus unable to counteract wasting. Effective reversal of involvement of the brain serotonergic system in the development of cachexia appears to require pharmacological intervention, which in cancer anorexia-cachexia (Refs. 59, 155, and 156; Fig. 1). turn requires knowledge of the mediators and the processes. Since the discovery of leptin in 1994 (12), there has been an Acknowledgments explosion of information about the role of various genes and gene I am indebted to Dr. Susumu Nishimura, Banyu Research Institute, and to products, including neuropeptides, in the regulation of body weight Drs. Masato Kasuga and Shigeaki Baba, both of Kobe University, for many (8–11, 46). Although much of this knowledge has come from studies stimulating discussions. I sincerely thank Dr. Etsuro Itoh, Ube Research of obesity, there appear to be implications for clinical conditions Laboratory, for his cooperation. associated with weight loss (132). 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