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LL-37 blocks an enzyme required by the original coronavirus and may eliminate damaged cells without inflammation.

“product By Nemo 1 month ago

LL-37 Activates cGAS-STING, TLR, PAMP, DAMP immune system detection pathways, and interferon immune response.

"This cationic peptide (with an a-helical structure) can bind the membranes of bacteria and enveloped viruses, polymerise on membrane surfaces and cause membrane disruption, killing invading organisms (10). In recent years, it has become evident LL-37 possesses numerous functions aside from its anti-microbial activity; many of which are immunomodulatory. Interestingly with regards to RNA aptamers, LL-37 has a high affinity for single and double stranded nucleic acids and is capable of enhancing inflammation through promoting toll-like receptor (TLR) activation. Furthermore, LL-37 has been shown to shuttle complexed nucleic acids across cell membranes, primarily via receptor-mediated endocytosis. However, in keratinocytes, uptake seems to occur by mechanisms that do not require activation of specific receptors (15, 16), promoting inflammatory and interferon responses via both TLR and cytosolic nucleic acid sensors such as the cGAS-STING and RIG-I Like Receptor (RLR) pathways (17, 18)." (9)

"The role of TLR s, RLR s, and NLR s in PAMP and DAMP recognition. Signaling pathways triggered by pathogen-associated molecular pattern (PAMPs) and damage-associated molecular pattern molecules (DAMPs)... The MyD88-dependent pathway is responsible for NF-kB and mitogen-activated protein kinase (MAPK) activation, which controls induction of proinflammatory cytokines. The TRIF-dependent pathway activates IRF3 by TANK-binding kinase 1 (TBK1)/IKKe, which is required for the induction of IFN-inducible genes. TLR1-TLR2 and TLR2-TLR6 recognize bacterial triacylated lipopeptide or diacyl lipopeptide, respectively, and recruit TIR adapter protein (TIRAP) and MyD88 at the plasma membrane to activate the MyD88-dependent pathway. TLR5 recognizes flagellin and activates the MyD88-dependent pathway. TLR3, TLR7, TLR8, and TLR9 reside in the endosome and recognize dsRNA, ssRNA, CpG DNA, or mitochondrial DNA (Mit DNA), respectively. They recruit TRIF or MyD88 to activate the IRF3 or IRF7 pathway." (13)

"PAMP and DAMP-mediated signaling and induction of an innate immune response usually results in resolution of infection, but may also cause chronic inflammation or autoimmunity by altering various cell death and survival mechanisms." (13)

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SARS-CoV inhibits type I interferon signaling by inhibiting STING.

SARS coronavirus papain-like protease inhibits the type I interferon signaling pathway through interaction with the STING-TRAF3-TBK1 complex.

"We found that expression of the membrane-anchored PLpro domain (PLpro-TM) from SARS-CoV inhibits STING/TBK1/IKKe-mediated activation of type I IFNs and disrupts the phosphorylation and dimerization of IRF3, which are activated by STING and TBK1. Meanwhile, we showed that PLpro-TM physically interacts with TRAF3, TBK1, IKKe, STING, and IRF3, the key components that assemble the STING-TRAF3-TBK1 complex for activation of IFN expression. However, the interaction between the components in STING-TRAF3-TBK1 complex is disrupted by PLpro-TM. Furthermore, SARS PLpro-TM reduces the levels of ubiquitinated forms of RIG-I, STING, TRAF3, TBK1, and IRF3 in the STING-TRAF3-TBK1 complex. These results collectively point to a new mechanism used by SARS-CoV through which PLpro negatively regulates IRF3 activation by interaction with STING-TRAF3-TBK1 complex, yielding a SARS-CoV countermeasure against host innate immunity." (4)

"In monocytes, LL37 promoted the uptake of self-DNA to activate type I IFN responses through cytosolic DNA sensor cGAS-STING (9). Overall, in different cell types LL37-DNA complexes are potent inducers of type-I IFN through cytosolic or endosomal sensing." (8)


LL-37 selectively inhibits the Cathepsin L protease involved in SARS-CoV activation while partially enhancing proteases Cathepsins S and K.

LL-37 inhibits cathepsin L at a concentration of 150 nM. (15) Cathepsin L primes (helps activate) the SARS-CoV RNA virus when it is entering the cell. (16)


Sensing infection, suppressing regeneration (cGAS activation worsening sepsis).

"Rehman said that the enzyme -- named cGAS -- acts as a DNA sensor that is being activated by the DNA released by damaged mitochondria of the blood vessel cells."

"By studying mice with sepsis -- a condition caused when the body's inflammatory response to a bloodstream bacterial infection spirals out of control -- they found that removal of the enzyme allows cells to fully regenerate."

"When cells are faced with an injury or an infection, it seems that they make a 'fight' or 'fix' choice," said UIC's Asrar Malik, senior author of the study and the Schweppe Family Distinguished Professor and head of pharmacology at the College of Medicine. "Inflammation is the 'fight' response, and the cells appear to delay regeneration while amplifying the inflammatory response."

"We think that over time cells have evolved to favor fighting an infection over repairing damaged cells, but in some cases, this preference to fight puts the body at further risk," said Dr. Jalees Rehman, co-senior author and UIC professor of medicine, pharmacology and bioengineering at the College of Medicine. "Especially when the immune response of the body to an infection is so excessive that it damages vital organs such as the lungs, it is absolutely vital that we learn how to help cells restore their ability to regenerate and resolve the inflammation."

"In an experimental model of bacterial sepsis, mice lacking this DNA sensor [cGAS] had much higher rates of survival and showed rapid regeneration of the blood vessels in the lung." (1)

The danger of cGAS activation worsening survival rates by increasing inflammatory cytokines is noted. Although, at low concentrations only, it is a primary part of the immune system's ability to sense foreign invaders in the cytoplasm to induce death of damaged cells through means independent of inducing hyper-inflammation.


LL-37 prevents DAMPs and PAMPs from destroying white bloodcells and protects against sepsis despite activating cGAS.

"Notably, human cathelicidin LL-37 exhibits the protective effect on the septic animal models. Thus, in this study, to elucidate the mechanism for the protective action of LL-37 on sepsis, we utilized LPS (lipopolysaccharide) and ATP (adenosine triphosphate) as a PAMP and a DAMP, respectively, and examined the effect of LL-37 on the LPS/ATP-induced pyroptosis of macrophage-like J774 cells. The data indicated that the stimulation of J774 cells with LPS and ATP induces the features of pyroptosis, including the expression of IL-1B mRNA and protein, activation of caspase-1, inflammasome formation and cell death. Moreover, LL-37 inhibits the LPS/ATP-induced IL-1B expression, caspase-1 activation, inflammasome formation, as well as cell death. Notably, LL-37 suppressed the LPS binding to target cells and ATP-induced/P2X7-mediated caspase-1 activation." (11)


High dose of LL-37 can evoke inflammatory cell death (necrosis) and worsen atherosclerosis but low dose inhibits death of white blood cells.

"Activation of caspase-3 and caspase-8 and induction of apoptosis in neutrophils are inhibited by LL-37 (31). Higher doses of this peptide induce necrotic cell death in neutrophils, possibly secondary to overcoming inhibitory effects of membrane cholesterol on LL-37 pore-forming abilities." (10)

"In contrast to necrosis, which is a form of traumatic cell death that results from acute cellular injury, apoptosis is a highly regulated and controlled process that confers advantages during an organism's life cycle. For example, the separation of fingers and toes in a developing human embryo occurs because cells between the digits undergo apoptosis. Unlike necrosis, apoptosis produces cell fragments called apoptotic bodies that phagocytic cells are able to engulf and remove before the contents of the cell can spill out onto surrounding cells and cause damage to them."

"Atherosclerosis is considered an inflammatory disease (75) in which innate immune pathways, including upregulation of type I IFNs, are activated in atherosclerotic plaques and contribute to disease development (76-79). A participation of LL-37 bound to immune complexes that promote type I IFN production and stimulate atherosclerosis development has been proposed (80). Transcription of LL-37 is elevated in human atherosclerotic aortas (81) and its association with neointima-associated macrophages has been demonstrated (82). Furthermore, proatherosclerotic Apoe-/- mice demonstrate elevated neutrophil-associated mCRAMP in the carotid arteries of mice fed high-fat chow. These mice lose mCRAMP staining and have substantial protection in plaque size and macrophage recruitment when crossed to mCRAMP-/- mice (10). This suggests that LL-37 plays an important role in recruitment of inflammatory cells to plaques as well as modulation of the plaque cytokine environment to promote atherosclerotic lesion development." (10)


"Chromatin-bound cGAS restrains DNA repair and accelerates genome destabilization, generation of micronuclei, and cell death independently of STING signaling."

Cyclic GMP-AMP synthase (cGAS) is best known as a cytosolic innate immune sensor critical for the outcome of infections, inflammatory diseases, and cancer. Here, we report that cGAS is primarily a chromatin-bound protein that inhibits DNA repair by HR, thereby accelerating genome destabilization, micronucleus generation, and cell death under conditions of genomic stress. (5)

When cGAS is activated, cells with DNA under high stress are programmed to die instead of attempting DNA repair. Once again, this highlights its danger at high concentrations but potential benefits under the right circumstances at low concentrations.



cGAS accumulates IRF3 phosphorylation slowly (at low and gradual concentrations) via STING pathway to kill damaged cells without triggering inflammation.

"...during mitotic arrest, low level cGAS-dependent IRF3 phosphorylation slowly accumulates without triggering inflammation. Phosphorylated IRF3, independently of its DNA-binding domain, stimulates apoptosis through alleviating Bcl-xL-dependent suppression of mitochondrial outer membrane permeabilization. We propose that slow accumulation of phosphorylated IRF3, normally not sufficient for inducing inflammation, can trigger transcription-independent induction of apoptosis upon mitotic aberrations. (Slow IRF3 accumlation destroys cells damaged from mitosis without inducing inflammation when in low enough concentrations). Accordingly, expression of cGAS and IRF3 in cancer cells makes mouse xenograft tumors responsive to the anti-mitotic agent Taxol." (3)

"Induction of mitotic cell death involves cGAMP synthesis by cGAS, as well as signal transduction to IRF3 by STING. We thus propose that cGAS plays a previously unappreciated role in guarding against mitotic errors, promoting cell death during prolonged mitotic arrest." (2)

Activating cGAS promotes apoptosis of cells damaged by errors during cell division by slowly accumulating IRF3 via STING activation and accelerating cell death under genomic stress. This highlights the importance of small amounts of cGAS activation to promote these two mechanisms without inducing the excessive immune response from a STING-induced cytokine release. The different mechanisms may be useful depending on the physiological circumstance and topic of research.


Cathelicidin LL-37 Induces Angiogenesis via PGE2-EP3 Signaling in Endothelial Cells, In Vivo Inhibition by Aspirin (6)

Aspirin appears to be able to inhibit the angiogenic (blood vessel growing) activities of LL-37, meaning it may be useful if the priority is to kill damaged cells during cell cycle arrest with cGAS activation. Angiogenesis in a hostile enviroment of viruses or excessive inflammation has been proven to increase risk in many instances. (6)


LL-37 Promotes Phagocytosis (phagocytosis clears senescent cells after being activated by resistance exercise.)

"We investigated the effect of LL-37 on bacterial phagocytosis by macrophages and demonstrate that LL-37 enhances phagocytosis of IgG-opsonized Gram-negative and Gram-positive bacteria in a dose- and time-dependent manner by dTHP-1 cells. In addition, LL-37 enhanced phagocytosis of nonopsonized Escherichia coli by human macrophages. Consistently, LL-37 elevated the expression of FcyRs on macrophages but not the complement receptors CD11b and -c. Further studies revealed that the expression of TLR4 and CD14 is also increased on LL-37-treated macrophages. Several lines of evidence indicated that the FPR2/ALX receptor mediated LL-37-induced phagocytosis. However, TLR4 signaling was also coupled to the phagocytic response, as a specific TLR4 antibody significantly suppressed phagocytosis of IgG-opsonized E. coli and nonopsonized E. coli by dTHP-1 cells." (17)

Phagocytosis activation by LL-37 is dependent on TLR activation.

"Senescent endothelial progenitor cells (p16Ink4a+/CD34+) in human skeletal muscle after resistance exercise. Senescent endothelial progenitor cells decreased in human skeletal muscle after a single bout of resistance exercise, and to a greater extends under low protein supplemented condition" (12)


cGAS is essential for senescent (zombie) cell elimination

"Cellular senescence is a natural barrier to tumorigenesis and it contributes to the antitumor effects of several therapies, including radiation and chemotherapeutic drugs. Senescence also plays an important role in aging, fibrosis, and tissue repair. The DNA damage response is a key event leading to senescence, which is character- ized by the senescence-associated secretory phenotype (SASP) that includes expression of inflammatory cytokines. Here we show that cGMP-AMP (cGAMP) synthase (cGAS), a cytosolic DNA sensor that activates innate immunity, is essential for senescence. Deletion of cGAS accelerated the spontaneous immortalization of mouse embryonic fibroblasts. cGAS deletion also abrogated SASP induced by spontaneous immortalization or DNA damaging agents, including radiation and etoposide. cGAS is localized in the cytoplasm of nondividing cells but enters the nucleus and associates with chromatin DNA during mitosis in proliferating cells. DNA damage leads to accumulation of damaged DNA in cytoplasmic foci that contain cGAS. In human lung adenocarcinoma patients, low expression of cGAS is correlated with poor survival." (7)

Loss of cGAS prevents senescence cells from being located by the immune system. Activation of cGAS may promote senescent cell recognition by immune cells, so that those zombie cells can be eliminated more easily, but excess cGAS activation can cause normal cells to become senescent via its strong immune system activation. Once again, it appears a small amount of cGAS activation can be incredibly useful under specific circumstances. Many SASPs (senescent cell detectors) are inflammatory cytokines. This is why excessive secretion of them can damage normal cells, but they are nevertheless needed to flag senescent cells so the immune system can assist in their clearance.


Sourced Studies:

(1) Huang, Long Shuang, et al. "MtDNA Activates CGAS Signaling and Suppresses the YAP-Mediated Endothelial Cell Proliferation Program to Promote Inflammatory Injury.". Immunity, vol. 52, no. 3, 17 Mar. 2020, pp. 475-486.e5, www.cell.com/immunity/pdf/S1074-7613(20)30073-X.pdf, 10.1016/j.immuni.2020.02.002.

(1a) "Sensing Infection, Suppressing Regeneration." Medicalxpress.Com, medicalxpress.com/news/2020-03-infection-suppressing-regeneration.html.

(2) Zierhut, Christian, et al. "The Cytoplasmic DNA Sensor CGAS Promotes Mitotic Cell Death." Cell, vol. 178, no. 2, 11 July 2019, pp. 302-315.e23, www.ncbi.nlm.nih.gov/pubmed/31299200, 10.1016/j.cell.2019.05.035.

(3) Zierhut, Christian, et al. "The Cytoplasmic DNA Sensor CGAS Promotes Mitotic Cell Death." Cell, vol. 178, no. 2, 11 July 2019, pp. 302-315.e23, www.ncbi.nlm.nih.gov/pubmed/31299200, 10.1016/j.cell.2019.05.035.

(4) Chen, Xiaojuan, et al. "SARS Coronavirus Papain-like Protease Inhibits the Type I Interferon Signaling Pathway through Interaction with the STING-TRAF3-TBK1 Complex." Protein & Cell, vol. 5, no. 5, 1 May 2014, pp. 369-381, www.ncbi.nlm.nih.gov/pmc/articles/PMC3996160/, 10.1007/s13238-014-0026-3.

(5) Jiang, Hui, et al. "Chromatin-bound CGAS Is an Inhibitor of DNA Repair and Hence Accelerates Genome Destabilization and Cell Death." The EMBO Journal, vol. 38, no. 21, 23 Sept. 2019, 10.15252/embj.2019102718.

(6) Salvado, M. Dolores, et al. "Cathelicidin LL-37 Induces Angiogenesis via PGE 2-EP3 Signaling in Endothelial Cells, In Vivo Inhibition by Aspirin." Arteriosclerosis, Thrombosis, and Vascular Biology, vol. 33, no. 8, Aug. 2013, pp. 1965-1972, 10.1161/atvbaha.113.301851.

(7) Yang, Hui, et al. "CGAS Is Essential for Cellular Senescence." Proceedings of the National Academy of Sciences, vol. 114, no. 23, 22 May 2017, pp. E4612-E4620, www.pnas.org/content/pnas/early/2017/05/17/1705499114.full.pdf, 10.1073/pnas.1705499114.

(8) Soni, Chetna, and Boris Reizis. "Self-DNA at the Epicenter of SLE: Immunogenic Forms, Regulation, and Effects." Frontiers in Immunology, vol. 10, 2019, p. 1601, www.ncbi.nlm.nih.gov/pubmed/31354738, 10.3389/fimmu.2019.01601.

(9) Macleod, Tom, et al. "Antimicrobial Peptide LL-37 Facilitates Intracellular Uptake of RNA Aptamer Apt 21-2 Without Inducing an Inflammatory or Interferon Response." Frontiers in Immunology, vol. 10, 2019, p. 857, www.ncbi.nlm.nih.gov/pubmed/31068939, 10.3389/fimmu.2019.00857.

(10) Kahlenberg, J. Michelle, and Mariana J. Kaplan. "Little Peptide, Big Effects: The Role of LL-37 in Inflammation and Autoimmune Disease." The Journal of Immunology, vol. 191, no. 10, 1 Nov. 2013, pp. 4895-4901, www.ncbi.nlm.nih.gov/pmc/articles/PMC3836506/, 10.4049/jimmunol.1302005.

(11) Z, Hu, et al. "Antimicrobial Cathelicidin Peptide LL-37 Inhibits the LPS/ATP-Induced Pyroptosis of Macrophages by Dual Mechanism." PloS One, 16 Jan. 2014, pubmed.ncbi.nlm.nih.gov/24454930/.

(12) Wan, Min, et al. "Antimicrobial Peptide LL-37 Promotes Bacterial Phagocytosis by Human Macrophages." Journal of Leukocyte Biology, vol. 95, no. 6, 1 June 2014, pp. 971-981, www.ncbi.nlm.nih.gov/pubmed/24550523, 10.1189/jlb.0513304.

(13) Tang, Daolin, et al. "PAMPs and DAMPs: Signal 0s That Spur Autophagy and Immunity." Immunological Reviews, vol. 249, no. 1, 14 Aug. 2012, pp. 158-175, 10.1111/j.1600-065x.2012.01146.x.

(14) Pm, Andrault, et al. "Antimicrobial Peptide LL-37 Is Both a Substrate of Cathepsins S and K and a Selective Inhibitor of Cathepsin L." Biochemistry, 5 May 2015, pubmed.ncbi.nlm.nih.gov/25884905/.

(15) Dana, Dibyendu, and Sanjai K. Pathak. "A Review of Small Molecule Inhibitors and Functional Probes of Human Cathepsin L." Molecules, vol. 25, no. 3, 6 Feb. 2020, p. 698, www.mdpi.com/1420-3049/25/3/698/pdf, 10.3390/molecules25030698.

(16) Simmons, Graham, et al. "Proteolytic Activation of the SARS-Coronavirus Spike Protein: Cutting Enzymes at the Cutting Edge of Antiviral Research." Antiviral Research, vol. 100, no. 3, Dec. 2013, pp. 605-614, 10.1016/j.antiviral.2013.09.028.

(17) Yang, Chi, et al. "Aged Cells in Human Skeletal Muscle after Resistance Exercise." Aging (Albany NY), vol. 10, no. 6, 27 June 2018, pp. 1356-1365, www.ncbi.nlm.nih.gov/pmc/articles/PMC6046228/, 10.18632/aging.101472.


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