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A multifunctional cytokine with effects on lipid metabolism, coagulation, insulin resistance, and endothelial function

Structure of the TNF genes.

Structure of the TNF genes. The locations of MHC loci are shown. Hatched boxes indicate the relative positions of the indicated loci. Open boxes represent the untranslated portions of the TNF exons, and closed boxes represent translated portions. Lines indicate introns (areas of the gene that are spliced out of the mature RNA product). Arrows indicate the transcriptional orientation of the genes. Reprinted with permission by Webb et al.26 kb = kilobase. C2 = complement component 2; Bf = complement factor B; C4A = complement component 4A; C4B = complement component C4B; 21-OH = 21-hydroxylase. 

 

Map of the human TNF locus and HLA loci

sequence comparison tnf

RMP16, a recombinant TNF α-derived polypeptide comprising a specific human serum albumin (HSA)-binding 7-mer peptide identified by phage display screening (WQRPSSW), a cleavage peptide for Factor Xa (IEGR), and a 20-amino acid bioactive peptide P16 (TNF α segment including amino acid residues 75–94), was prepared by gene-engineering technology. RMP16 showed prolonged half-life, 13.11 hours in mice (half-lives of P16 and TNF α are 5.77 and 29.0?minutes, respectively), and obviously higher receptor selectivity for TNFRI than TNF ?. RMP16 had significant inhibition effects for multiple tumor cells, especially prostate cancer Du145 cells, and human vascular endothelial cells but not for human mammary non-tumorigenic epithelial cells. RMP16 can more effectively induce apoptosis and inhibit proliferation for DU145 cells than P16 and TNF α via the caspase-dependent apoptosis pathway and G0/G1 cell cycle arrest. In nude mice with transplanted tumor of DU145 cells, RMP16 significantly induced apoptosis and necrosis of tumor tissues but causing less side effects, and tumor inhibitory rate reached nearly 80%, furthermore, RMP16 can potently inhibit tumor angiogenesis and neovascularization. These findings suggest that RMP16 may represent a promising long-lasting antitumor therapeutic peptide with less TNF α-induced toxicity.

Ma Y, Zhao S, Shen S, et al. A novel recombinant slow-release TNF α-derived peptide effectively inhibits tumor growth and angiogensis. Sci Rep. 2015;5:13595.

Over the last decade, an abundance of evidence has emerged demonstrating a close link between metabolism and immunity. It is now clear that obesity is associated with a state of chronic low-level inflammation. In this article, we discuss the molecular and cellular underpinnings of obesity-induced inflammation and the signaling pathways at the intersection of metabolism and inflammation that contribute to diabetes. We also consider mechanisms through which the inflammatory response may be initiated and discuss the reasons for the inflammatory response in obesity. We put forth for consideration some hypotheses regarding important unanswered questions in the field and suggest a model for the integration of inflammatory and metabolic pathways in metabolic disease.

Wellen KE, Hotamisligil GS. Inflammation, stress, and diabetes. J Clin Invest. 2005;115(5):1111-9.

Over the last decade, an abundance of evidence has emerged demonstrating a close link between metabolism and immunity. It is now clear that obesity is associated with a state of chronic low-level inflammation. In this article, we discuss the molecular and cellular underpinnings of obesity-induced inflammation and the signaling pathways at the intersection of metabolism and inflammation that contribute to diabetes. We also consider mechanisms through which the inflammatory response may be initiated and discuss the reasons for the inflammatory response in obesity. We put forth for consideration some hypotheses regarding important unanswered questions in the field and suggest a model for the integration of inflammatory and metabolic pathways in metabolic disease.

Wellen KE, Hotamisligil GS. Inflammation, stress, and diabetes. Journal of Clinical Investigation. 2005;115(5):1111-1119. doi:10.1172/JCI200525102.

Genetic epidemiologic studies suggest a strong genetic influence on the outcome from sepsis, and genetics may explain the wide variation in the individual response to infection that has long puzzled clinicians. Several candidate genes have been identified as important in the inflammatory response and investigated in case-controlled studies, including the tumor necrosis factor (TNF)-alpha and TNF-beta genes, positioned next to each other within the cluster of human leukocyte antigen class III genes on chromosome 6. Other candidate genes for sepsis and septic shock include the interleukin (IL)-1 receptor antagonist gene, the heat shock protein gene, the IL-6 gene, the IL-10 gene, the CD-14 gene, the Toll-like receptor (TLR)-4 gene, and the TLR-2 gene, to name a few. In this review, we summarize the evidence for a genetic susceptibility to development of sepsis and death from sepsis, discuss design of clinical genetics studies relevant to the study of complex disorders, consider the candidate genes likely to be involved in the pathogenesis of sepsis, and discuss the potential for targeted therapy of sepsis and septic shock based on genetic variability.

Holmes CL, Russell JA, Walley KR. Genetic polymorphisms in sepsis and septic shock: role in prognosis and potential for therapy. Chest. 2003;124(3):1103-15.

Insulin resistance is increasingly recognized as a chronic, low-level, inflammatory state. Hyperinsulinemia and insulin action were initially proposed as the common preceding factors of hypertension, low high-density lipoprotein cholesterol, hypertriglyceridemia, abdominal obesity, and altered glucose tolerance, linking all these abnormalities to the development of coronary heart disease. The similarities of insulin resistance with another inflammatory state, atherosclerosis, have been described only in the last few decades. Atherosclerosis and insulin resistance share similar pathophysiological mechanisms, mainly due to the actions of the two major proinflammatory cytokines, TNF-alpha and IL-6. Genetic predisposition to increased transcription rates of these cytokines is associated with metabolic derangement and simultaneously with coronary heart disease. Dysregulation of the inflammatory axis predicts the development of insulin resistance and type 2 diabetes mellitus. The knowledge of how interactions between metabolic and inflammatory pathways occur will be useful in future therapeutic strategies. The effective administration of antiinflammatory agents in the treatment of insulin resistance and atherosclerosis is only the beginning of a promising approach in the management of these syndromes.

Fernández-real JM, Ricart W. Insulin resistance and chronic cardiovascular inflammatory syndrome. Endocr Rev. 2003;24(3):278-301.

A periurban outbreak of visceral leishmaniasis (VL) caused by the protozoan Leishmania chagasi is ongoing outside Natal, northeast Brazil. Manifestations range from asymptomatic infection to disseminated visceral disease. Literature reports suggest that both genetic and environmental factors influence the outcome of infection. Due to the association of the tumor necrosis factor (TNF) locus with other infectious diseases, we examined whether polymorphic alleles at this locus are associated with the outcome of L. chagasi infection. Neighborhoods with ongoing transmission were identified through patients admitted to local hospitals. Altogether, 1,024 individuals from 183 families were classified with the following disease phenotypes: (i) symptomatic VL, (ii) asymptomatic infection (positive delayed-type hypersensitivity [DTH+]), or (iii) no evidence of infection (DTH-). Genotypes were determined at a microsatellite marker (MSM) upstream of the TNFB gene encoding TNF-beta and at a restriction fragment length polymorphism (RFLP) at position -307 in the promoter of the TNFA gene encoding TNF-alpha. Analyses showed that the distribution of TNFA RFLP alleles (TNF1 and TNF2) and the TNF MSM alleles (TNFa1 to TNFa15) differed between individuals with VL and those with DTH+ phenotypes. TNF1 was transmitted more frequently than expected from heterozygous parents to DTH+ offspring (P = 0.0006), and haplotypes containing TNF2 were associated with symptomatic VL (P = 0.0265, transmission disequilibrium test). Resting serum TNF-alpha levels were higher in TNF1/2 heterozygotes than in TNF1/1 homozygotes (P < 0.05). These data led us to hypothesize that an individual's genotype at the TNF locus may be associated with whether he or she develops asymptomatic or symptomatic disease after L. chagasi infection. The results preliminarily suggest that this may be the case, and follow-up with larger populations is needed for verification.

Karplus TM, Jeronimo SM, Chang H, et al. Association between the tumor necrosis factor locus and the clinical outcome of Leishmania chagasi infection. Infect Immun. 2002;70(12):6919-25.

In recent years, much has been written about the importance of diabetes mellitus, both as a cause of widespread morbidity and mortality and in terms of the resultant overwhelming health care costs. In an analysis of worldwide diabetes in 1994, the World Health Organization reported that the age-standardized prevalence in European populations varied from 3% to 10%, while more restricted populations demonstrated prevalence of up to 50%. Over the same period, approximately 10.2 million people with diagnosed diabetes mellitus resided in the US, but another 5.4 million individuals with diabetes went undiagnosed. Perhaps even more impressive, these numbers have been estimated to represent as much as a 50% increase over the prevalence in equivalent populations during the previous decade. In 1997, health care expenditures attributable to diabetes in the US were estimated to be $98 billion. Undoubtedly, this is due not only to the widespread prevalence of the disease, but also to its chronic nature and disabling complications, affecting cardiovascular, renal, visual, and neurological function. Thus, perhaps the lack of an even more intense research effort into the pathophysiology of diabetes can only be attributed to the difficulty inherent in making progress in the study of this complex, multisystem disease. For these reasons, it is particularly important that in two recent articles — one appearing in the current issue of the journal Science and the other a recent issue of the JCI — Steven Shoelson, Gerald Shulman, and their colleagues present a new hypothesis, which not only purports to explain the insulin resistance of type 2 diabetes mellitus but also offers a clear basis for the development of novel therapeutics.

Over 90% of diabetes mellitus is accounted for by what is now called the type 2 variant. Unlike type 1 diabetes, for which there is a reasonable consensus that the disease results from autoimmune destruction of insulin-secreting pancreatic β cells, the etiology of type 2 diabetes remains a bit uncertain. Most investigators and clinicians agree that genetic and environmental factors contribute and that obesity is a frequent if not essential antecedent of the disease. Perhaps the most heated debate among diabetes researchers has concerned the nature of the primary inciting metabolic event, that is, whether it represents a disturbance in the normal pattern of insulin secretion or abnormalities in the action of insulin in peripheral tissues. Experiments in which defects in insulin secretion or action have been selectively introduced into mice by the modification of single or multiple genes have been surprisingly unhelpful at resolving this issue. Perhaps these genetic studies only serve to emphasize the multi–organ system nature of diabetes mellitus, in which several defects are required to elicit sufficient dysfunction to overwhelm physiological compensatory mechanisms and produce diabetes.

Nevertheless, investigators have postulated a reasonable series of events to explain the evolution of type 2 diabetes. According to this model, peripheral insulin resistance represents the earliest event, but this is initially compensated by enhanced insulin secretion. Later, the β cell no longer keeps pace with the increased needs, and a relative lack of insulin is followed by an absolute deficiency of the hormone. At about the same time, the liver develops insulin resistance, thus leading to accelerated production of glucose. Whatever the precise sequence of the events by which impaired glucose tolerance matures to diabetes, there is little doubt that insulin resistance represents an important component of the fulminant disease.

Birnbaum MJ. Turning down insulin signaling. Journal of Clinical Investigation. 2001;108(5):655-659.

Metabolism and immunity are closely linked. Both overnutrition and undernutrition have implications for immune function. Starvation and malnutrition can suppress immune function and increase susceptibility to infections. Obesity is associated with a state of aberrant immune activity and increasing risk for associated inflammatory diseases, including atherosclerosis, diabetes, airway inflammation, and fatty liver disease. Thus, optimal nutritional and metabolic homeostasis is an important part of appropriate immune function and good health.  Kathryn E. Wellen and Gökhan S. Hotamisligil. J. Clin. Invest. 115:1111-1119 (2005)
Metabolism and immunity are closely linked. Both overnutrition and undernutrition have implications for immune function. Starvation and malnutrition can suppress immune function and increase susceptibility to infections. Obesity is associated with a state of aberrant immune activity and increasing risk for associated inflammatory diseases, including atherosclerosis, diabetes, airway inflammation, and fatty liver disease. Thus, optimal nutritional and metabolic homeostasis is an important part of appropriate immune function and good health. 

Kathryn E. Wellen and Gökhan S. Hotamisligil. J. Clin. Invest. 115:1111-1119 (2005)

Lipids and inflammatory mediators
Lipids and inflammatory mediators: integration of metabolic and immune responses in adipocytes and macrophages through shared mechanisms. Under normal conditions, adipocytes store lipids and regulate metabolic homeostasis, and macrophages function in the inflammatory response, although each cell type has the capacity to perform both functions. In obesity, adipose tissue becomes inflamed, both via infiltration of adipose tissue by macrophages and as a result of adipocytes themselves becoming producers of inflammatory cytokines. Inflammation of adipose tissue is a crucial step in the development of peripheral insulin resistance. In addition, in proatherosclerotic conditions such as obesity and dyslipidemia, macrophages accumulate lipid to become foam cells. Adipocytes and macrophages share common features such as expression of cytokines, FABPs, nuclear hormone receptors, and many other factors. As evidenced by genetic loss-of-function models, adipocyte/macrophage FABPs modulate both lipid accumulation in adipocytes and cholesterol accumulation in macrophages, as well as the development of insulin resistance and atherosclerosis. PPAR and LXR pathways oppose inflammation and promote cholesterol efflux from macrophages and lipid storage in adipocytes.
 
Kathryn E. Wellen and Gökhan S. Hotamisligil. J. Clin. Invest. 115:1111-1119 (2005)

Model of overlapping metabolic and inflammatory signaling and sensing pathways in adipocytes or macrophages.
Model of overlapping metabolic and inflammatory signaling and sensing pathways in adipocytes or macrophages. Inflammatory pathways can be initiated by extracellular mediators such as cytokines and lipids or by intracellular stresses such as ER stress or excess ROS production by mitochondria. Signals from all of these mediators converge on inflammatory signaling pathways, including the kinases JNK and IKK. These pathways lead to the production of additional inflammatory mediators through transcriptional regulation as well as to the direct inhibition of insulin signaling. Other pathways such as those mediated through the SOCS proteins and iNOS are also involved in inflammation-mediated inhibition of insulin action. Opposing the inflammatory pathways are transcription factors from the PPAR and LXR families, which promote nutrient transport and metabolism and antagonize inflammatory activity. More proximal regulation is provided by FABPs, which likely sequester ligands of these transcription factors, thus promoting a more inflammatory environment. The absence of FABPs is antiinflammatory. The cell must strike a balance between metabolism and inflammation. In conditions of overnutrition, this becomes a particular challenge, as the very processes required for response to nutrients and nutrient utilization, such as mitochondrial oxidative metabolism and increasing protein synthesis in the ER, can induce the inflammatory response. IR, insulin receptor. 

Nutrient and pathogen sensing or response systems
Nutrient and pathogen sensing or response systems have important overlapping features, and their modulation by obesity or infection can lead to overlapping physiological outcomes. For example, the chronic inflammation of obesity leads to elevated plasma lipid levels and the development of insulin resistance, eventually resulting in fatty liver disease, atherosclerosis, and diabetes. Infection typically leads to a more transient and robust inflammatory response and short-term hyperlipidemia that aids in the resolution of the infection. In some circumstances of chronic infection, however, insulin resistance, diabetes, and atherosclerosis can result.
A simplified diagram of the innate immune response to infection
A simplified diagram of the innate immune response to infection and tissue injury involving the inflammatory cytokines and the coagulation cascade. –ve = negative; +ve = positive; PAI1 = platelet-activation inhibitor-1; ICAM = intracellular adhesion molecule. 
Effect of salicylates on insulin signaling during insulin resistance.
Effect of salicylates on insulin signaling during insulin resistance. (a) Normally, the occupied insulin receptor phosphorylates scaffold proteins, such as the IRS-1, on critical tyrosine residues. However, in insulin-resistant states, a number of agents, such as the cytokine TNF- or circulating FFAs, lead through intermediary signaling pathways to the activation of IKK, which in turn indirectly increases the number of phosphorylated serine and threonine residues (indicated by blue circles) on IRS-1. This modification blocks the tyrosine phosphorylation and converts IRS-1 into an insulin receptor inhibitory protein. (b) In the presence of salicylates, IKK activity is inhibited, reducing the IRS-1 serine/threonine phosphorylation and allowing IRS-1 to be phosphorylated on tyrosine. These phosphorylated tyrosine residues (black squares) serve as binding sites for a number of signaling molecules, most importantly PI 3'-kinase, which initiate signaling pathways regulating metabolism. Aspirin (ASA) also inhibits cyclooxygensases (COX) to prevent the generation of inflammatory prostaglandins (PGE) from arachidonic acid (AA) in a pathway unrelated to the effects of the drug on insulin action. 

Proposed pathophysiology of hypertension through inflammatory mechanisms
Proposed pathophysiology of hypertension through inflammatory mechanisms (see Section II). CNS, Central nervous system; SNS, sympathetic nervous system. José Manuel Fernández-Real and Wifredo Ricart . Endocrine Reviews 24 (3): 278-301


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