Resistin also known as adipose tissue-specific secretory factor (ADSF) or C/EBP-epsilon-regulated myeloid-specific secreted cysteine-rich protein (XCP1) is a

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, some ...

-rich peptide hormone derived from

adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular ...

that in humans is encoded by the ''RETN''

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

. In

primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...

, pigs, and dogs, resistin is secreted by immune and

epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...

cells, while, in rodents, it is secreted by adipose tissue. The length of the resistin pre-peptide in human is 108

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

residues and in the mouse and rat it is 114 aa; the

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

is ~12.5 kDa. Resistin is an adipose-derived hormone (similar to a

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in au ...

) whose physiologic role has been the subject of much controversy regarding its involvement with

obesity Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's ...

and type II diabetes mellitus ( T2DM).

Discovery

Resistin was discovered in 2001 by the group of Dr Mitchell A. Lazar from the

University of Pennsylvania The University of Pennsylvania (also known as Penn or UPenn) is a Private university, private research university in Philadelphia. It is the fourth-oldest institution of higher education in the United States and is ranked among the highest- ...

School of Medicine. It was called "resistin" because of the observed

insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the ''INS'' gene. It is considered to be the main anabolic hormone of the body. It regulates the metabolism ...

resistance in mice injected with resistin. Resistin was found to be produced and released from adipose tissue to serve

endocrine The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...

functions likely involved in

insulin resistance Insulin resistance (IR) is a pathological condition in which cells fail to respond normally to the hormone insulin. Insulin is a hormone that facilitates the transport of glucose from blood into cells, thereby reducing blood glucose (blood sugar ...

. This idea primarily stems from studies demonstrating that

serum Serum may refer to: * Serum (blood), plasma from which the clotting proteins have been removed **Antiserum, blood serum with specific antibodies for passive immunity * Serous fluid, any clear bodily fluid *Truth serum, a drug that is likely to mak ...

resistin levels increase with obesity in several model systems (humans, rats, and

mice A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...

). Since these observations, further research has linked resistin to other physiological systems such as

inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...

and energy

homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...

. This article discusses the current research proposing to link resistin to inflammation and energy homeostasis, including its alleged role in insulin resistance in obese subjects, a subject reviewed by Vidal-Puig and O'Rahilly in 2001, and by M.A. Lazar in 2007.

Inflammation

Inflammation is the first

innate immune response The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the ...

infection An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable di ...

irritation Irritation, in biology and physiology, is a state of inflammation or painful reaction to allergy or cell-lining damage. A stimulus or agent which induces the state of irritation is an irritant. Irritants are typically thought of as chemical age ...

resulting from

leukocyte White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...

(

neutrophils Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying ...

mast cells A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a p ...

, etc.) accumulation and their secretion of inflammatory,

biogenic A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...

chemicals such as

histamine Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Since histamine was discover ...

prostaglandin The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are deriv ...

, and pro-inflammatory

cytokines Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...

. As cited, it has recently been discovered that resistin also participates in the inflammatory response. In further support of its inflammatory profile, resistin has been shown to increase transcriptional events, leading to an increased expression of several pro-inflammatory cytokines including (but not limited to) interleukin-1 (IL-1),

interleukin-6 Interleukin 6 (IL-6) is an interleukin that acts as both a pro-inflammatory cytokine and an anti-inflammatory myokine. In humans, it is encoded by the ''IL6'' gene. In addition, osteoblasts secrete IL-6 to stimulate osteoclast formation. Smoo ...

(IL-6), interleukin-12 (IL-12), and tumor necrosis factor-α ( TNF-α) in an

NF-κB Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a protein complex that controls transcription of DNA, cytokine production and cell survival. NF-κB is found in almost all animal cell types and is involved in cellular ...

-mediated (nuclear factor kappa-light-chain-enhancer of activated B cells-mediated) fashion. It has also been demonstrated that resistin upregulates intercellular

adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can ...

molecule-1 ( ICAM1)

vascular The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...

cell-adhesion molecule-1 ( VCAM1) and chemokine (C-C motif) ligand 2 ( CCL2), all of which are occupied in chemotactic pathways involved in

recruitment to sites of infection. Resistin itself can be upregulated by interleukins and also by

microbial A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

antigens such as

lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O- antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the out ...

, which are recognized by leukocytes. Taken together, because resistin is reputed to contribute to insulin resistance, results such as those mentioned suggest that resistin may be a link in the well-known association between inflammation and insulin resistance. In accordance, it is expected that, if resistin does serve as a link between obesity and T2DM while at the same time contributing to the inflammatory response, then proportional increases in chronic inflammation in association with obesity and insulin resistance should be observed. Recent data has shown that this is possible by demonstrating positive correlations between obesity, insulin resistance, and chronic inflammation, which is believed to be directed in part by resistin signaling. This idea has recently been challenged by a study showing that increased levels of resistin in people with chronic

kidney disease Kidney disease, or renal disease, technically referred to as nephropathy, is damage to or disease of a kidney. Nephritis is an inflammatory kidney disease and has several types according to the location of the inflammation. Inflammation can ...

are associated with lowered renal function and inflammation, but not with insulin resistance. Notwithstanding, regarding resistin and the inflammatory response, it can be concluded that resistin does bear features of a pro-inflammatory cytokine, and could act as a key node in inflammatory

diseases A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...

with or without associated insulin resistance. This adipokine is associated with markers of inflammation in seminal plasma and the concentrations of seminal resistin correlate positively with those of proinflammatory mediators such as interleukin-6 (IL-6), elastase and tumor necrosis factor-α (TNF-α). During inflammation, the concentrations of cytokines and ROS increase, and this may have a deleterious effect on the male reproductive function. One study showed that there was a negative correlation between the concentrations of seminal resistin and spermatic motility and vitality. (The seminal concentrations of resistin were significantly higher in cases of leukocyte spermia or if the patients were smokers.)

Obesity and insulin resistance

Arguments for

Much of what is hypothesized about a resistin role in energy

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...

and T2DM can be derived from studies showing strong correlations between resistin and obesity. The underlying belief among those in support of this theory is that

resistin levels will increase with increased adiposity. Conversely, serum resistin levels have been found to decline with decreased adiposity following medical treatment. Specifically, central obesity (waistline adipose tissue) seems to be the foremost region of adipose tissue contributing to rising levels of serum resistin. This fact takes on significant implications considering the well understood link between central obesity and insulin resistance, two marked peculiarities of T2DM. Although it seems that resistin levels increase with obesity, it is questioned whether such serum resistin increases are accountable for the

that appears to be associated with increased adiposity. Many researchers in their respective studies have shown that this is indeed the case by finding positive correlations between resistin levels and

. This discovery is further supported by studies that confirm a direct correlation between resistin levels and subjects with T2DM. If resistin does contribute to the pathogenesis of insulin resistance in T2DM, then designing drugs to promote decreased serum resistin in T2DM subjects might deliver immense therapeutic benefits. Resistin has been shown to cause high levels of 'bad'

cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...

via

low-density lipoprotein Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall densi ...

(LDL), increasing heart disease risk. Resistin increases LDL production in human

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...

cells and degrades LDL receptors in that organ, impairing the liver's ability to process 'bad' cholesterol. Resistin accelerates LDL accumulation in arteries, increasing heart disease risk, and has an adverse impact on the efficacy of

statins Statins, also known as HMG-CoA reductase inhibitors, are a class of lipid-lowering medications that reduce illness and mortality in those who are at high risk of cardiovascular disease. They are the most common cholesterol-lowering drugs. Low ...

, the primary drug used to reduce cholesterol in fighting of

cardiovascular disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, hea ...

Arguments against

The amount of evidence supporting the resistin link theory between obesity and T2DM is vast. Nevertheless, this theory lacks support from the entire

scientific community The scientific community is a diverse network of interacting scientists. It includes many " sub-communities" working on particular scientific fields, and within particular institutions; interdisciplinary and cross-institutional activities are als ...

, as the number of studies presenting evidence against it continues to expand. Such studies have found significantly decreased serum concentrations of resistin with increased adiposity, suggesting not only that resistin is downregulated in obese subjects, but also that decreased resistin levels may contribute to the links between

and T2DM. Data contradicting the idea that weight loss coincides with decreased serum resistin concentrations have also been presented; such studies instead report that weight loss is associated with marked increases in serum resistin. The idea that resistin links obesity to T2DM is now under even more scrutiny, as recent investigations have confirmed ubiquitous expression of resistin in many tissues, rather than those only characteristic of obesity, such as adipocytes. Although nearly as many scientists oppose the theory as those who support it, there is sufficient evidence to support the idea that resistin does have some incompletely defined role in

energy homeostasis In biology, energy homeostasis, or the homeostatic control of energy balance, is a biological process that involves the coordinated homeostatic regulation of food intake (energy inflow) and energy expenditure (energy outflow). The human brain, part ...

, while also demonstrating properties that help to incite inflammatory responses to sites of

Structure

Crystal structures of resistin reveal an unusual composition of several subunits that are held together by

non-covalent interaction In chemistry, a non-covalent interaction differs from a covalent bond in that it does not involve the sharing of electrons, but rather involves more dispersed variations of electromagnetic interactions between molecules or within a molecule. The c ...

s that make up its structure. The crystal structure shows a multimeric assembly consisting of hexamer-forming disulfide bonds. Each protein subunit comprises a carboxy-terminal disulfide-rich beta sandwich "head" domain and an amino-terminal alpha-helical "tail" segment. The

alpha-helical The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ea ...

segments associate to form three-stranded coils, and surface-exposed interchain disulfide linkages mediate the formation of tail-to-tail hexamers. The globular domain from resistin contains five disulfide bonds (Cys35-Cys88, Cys47-Cys87, Cys56-Cys73, Cys58-Cys75, and Cys62-Cys77). This suggests that the disulfide pattern will be conserved. The interchain disulfide bonds of resistin and resistin-like molecule β (RELMß) are novel in that they are highly solvent when exposed, ranging from 84.6% to 89.5%. An average solvent exposure for all disulfide bonds is 9.9%, and 16.7% for 1,209 interchain disulfide bonds. Therefore, the most highly uncovered disulfide bonds found for intact proteins are resistin's disulfides in high-resolution. A Cys6Ser resistin mutant was substantially more potent at the low concentration and had a greater effect than the wild-type resistin at the high concentration. This result suggests that processing of the intertrimer disulfide bonds may reflect a mandatory step toward activation. Other results also suggest that both the Cys6Ser-mutant and wild-type resistin target mainly the liver.

References

External links

* {{Hormones Tissues (biology) Endocrinology Obesity