TUMOR NECROSIS FACTOR (TNF): STRUCTURE, MECHANISM OF ACTION, FUNCTION - BIOLOGY - 2025

The tumor necrosis factor (TNF), also known as cachectin, is a protein produced naturally in the phagocytes or macrophages of the human body and other mammalian animals. It is a very important cytokine that participates both in normal physiological processes and in a variety of body pathological processes.

Its discovery dates back to just over 100 years ago, when W. Coley used crude bacterial extracts to treat tumors in different patients and found that these extracts had the ability to induce necrosis of these tumors, at the same time that they triggered a systemic inflammatory reaction in patients.

Mouse tumor necrosis factor alpha (Source: TK Vallery / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

The main “pro-inflammatory” stimulator activated by the bacterial extracts used by Coley was identified in 1975, when it was demonstrated that a protein factor in the serum of treated patients caused tumor lysis, from which the name that identifies this group of proteins (TNF-α).

Approximately 10 years later, in 1984, the gene for “tumor necrosis factor” was isolated and characterized, and on the same date another similar protein was isolated and purified in T lymphocytes, which was called “T alpha lymphotoxin” (TLα), which was later renamed TNF-β factor.

At present, numerous proteins similar to TNF originally described have been defined, which make up the TNF family of proteins (tumor necrosis factor type) and which include the proteins TNF-α, TNF-β, the CD40 ligand (CD40L), the Fas ligand (FasL) and many others.

Structure

At the gene level

The gene coding for the TNF-α protein is found on chromosome 6 (chromosome 17 in rodents) and the one coding for the TNF-β protein precedes the previous one in both cases (humans and rodents). These two genes are found in a single copy and are approximately 3 kb in size.

In view of the fact that the sequence corresponding to the promoter region of the TNF-α gene has several binding sites for a transcription factor known as the “nuclear factor kappa B” (NF-κB), many authors consider that its expression depends on this factor.

The promoter region of the TNF-β gene, on the other hand, has a binding sequence for another protein known as "High Mobility Group 1" (HMG-1).

At the protein level

Two forms of tumor necrosis factor-alpha have been described, one that is membrane bound (mTNF-α) and the other that is always soluble (sTNF-α). Tumor necrosis factor beta, on the other hand, exists only in a soluble form (sTNF-β).

In humans, the membrane form of TNF-α consists of a polypeptide of just over 150 amino acid residues, which are linked to a "leader" sequence of 76 additional amino acids. It has an apparent molecular weight of about 26 kDa.

The translocation of this protein towards the membrane occurs during its synthesis and this form is “converted” into the soluble form (17 kDa) by an enzyme known as “the TNF-α converting enzyme”, capable of transforming mTNF-α into sTNF -α.

Mechanism of action

Proteins belonging to the group of tumor necrosis factor (TNF) exert their functions mainly thanks to their association with specific receptors in the cells of the human body and of other animals.

There are two types of receptors for TNF proteins on the plasma membranes of most cells in the body, with the exception of erythrocytes: type I receptors (TNFR-55) and type II receptors (TNFR-75).

Both types of receptors share a structural homology with respect to the extracellular binding site for TNF proteins and also bind to these with an equivalent affinity. They differ, then, in the intracellular signaling pathways that they activate once the ligand-receptor binding process has occurred.

Cell death or survival mediated by TNF proteins. The apoptotic pathway is shown on the left hand side of the graph and the "survival" path on the right (Source: Masmudur M. Rahman, Grant McFadden / CC BY (https://creativecommons.org/licenses/by/1.0) via Wikimedia Commons)

The TNF ligand-receptor interaction with any of its receptors promotes the binding of three receptors to a soluble TNF-α trimer (receptor trimerization), and this interaction triggers cellular responses even when only 10% of the receptors are occupied..

Junction in type I receivers

Ligand-receptor binding with type I receptors serves as a "recruitment platform" for other signaling proteins into the cytosolic domains of the receptors (the inner portion). Among these proteins, the first to "arrive" is the protein TRADD or protein with death domain associated with the TNFR-1 receptor (TNFR-1-associated death domain protein).

TNFR1 signaling pathway. The dashed gray lines represent multiple steps.

Subsequently, three additional mediators are recruited: the receptor-interacting protein 1 (RIP1), the Fas-associated death domain protein (FADD) and factor 2 associated with the TNF receptor (TRAF2, TNF-receptor-associated Factor 2).

Type II receptors

When TNF binds to type II receptors, this results in the direct recruitment of the TRAF2 protein, which in turn recruits the TRAF1 protein. These proteins activate the MAPK protein pathways (Mitogen-activated Protein Kinase), very important from the point of view of intracellular signaling in eukaryotes.

Many of the signaling pathways that are activated after the binding of TNF factors to their receptors are also related to the activation of specific transcription factors, which trigger common responses that have been described as biological "effects" of TNF proteins..

Function

The TNF-α protein is produced mainly by macrophages of the immune system, while the TNF-β protein is produced by T lymphocytes. However, it has been shown that other cells in the body also produce these factors, although to a lesser extent.

Tumor necrosis factor has been widely studied for its implications in normal physiological processes, as well as in acute and chronic inflammatory pathological processes, in autoimmune diseases, and in inflammatory processes related to various types of cancer.

These proteins are associated with rapid weight loss in patients with acute bacterial infections, cancer, and septic "shock."

Three different biological activities have been described for tumor necrosis factor:

- cytotoxicity against tumor cells

- suppression of adipocyte lipoprotein lipase (LPL) and

- reduction of the resting potential of the membrane of myocytes (muscle cells).

The membrane form of TNF-α promotes cytotoxicity and has been implicated in the paracrine activities of TNF in certain tissues.

When a stimulus such as a bacterial endotoxin is perceived, this form is proteolytically cleaved into a shorter polypeptide (17 kDa), which can associate non-covalently with three other equal polypeptides and form a bell-shaped trimer that corresponds to the shorter form. active TNF in serum and other body fluids.

Among their biological functions, TNF proteins can also contribute to the activation and migration of lymphocytes and leukocytes, as well as promoting cell proliferation, differentiation and apoptosis.

Inhibitors

Many treating physicians prescribe TNF protein inhibitors to patients with autoimmune diseases (anti-TNF therapy). These substances include: infliximab, etanercept, adalimumab, golimumab, and certolizumab pegol.

The most common form of use is by subcutaneous injections in the thighs or abdomen, and even direct venous infusion is practiced. Despite how much some of these substances can help certain patients, there are some adverse effects associated with their use, including an increased risk of contracting infections such as tuberculosis or other fungal infections.

Non-pharmacological "natural" inhibitors

Some "cut" fragments of the membrane receptors of TNF proteins (type I and type II) have also been known as TNF-binding proteins (TNF-BPs, Tumor Necrosis Factor Binding Proteins) have been detected in the urine of patients suffering from cancer, AIDS or sepsis.

In some cases, these fragments inhibit or neutralize the activity of TNF proteins, since they prevent ligand-receptor interaction.

Other “natural” inhibitors of TNF proteins have been detected in some plant products derived from turmeric and pomegranate, although studies are still being done on this.

References

1. Baud, V., & Karin, M. (2001). Signal transduction by tumor necrosis factor and its relatives. Trends in cell biology, 11 (9), 372-377.
2. Chu, WM (2013). Factor necrosis tumor. Cancer letters, 328 (2), 222-225.
3. Kalliolias, GD, & Ivashkiv, LB (2016). TNF biology, pathogenic mechanisms and emerging therapeutic strategies. Nature Reviews Rheumatology, 12 (1), 49.
4. Lis, K., Kuzawińska, O., & Bałkowiec-Iskra, E. (2014). Tumor necrosis factor inhibitors – state of knowledge. Archives of medical science: AMS, 10 (6), 1175.
5. Tracey, MD, KJ, & Cerami, Ph. D, A. (1994). Tumor necrosis factor: A pleiotropic cytokine and therapeutic target. Annual review of medicine, 45 (1), 491-503.
6. Wu, H., & Hymowitz, SG (2010). Structure and function of tumor necrosis factor (TNF) at the cell surface. In Handbook of cell signaling (pp. 265-275). Academic Press.