Exploring the Antifibrotic Mechanisms of Ghrelin: Modulating TGF-β Signalling in Organ Fibrosis

Abstract

Background: Fibrosis is a pathological condition that affects various organs by increasing fibrous connective tissue while reducing parenchymal cells. This imbalance can lead to compromised organ function and potential failure, posing significant health risks. The condition's complexity necessitates the exploration of effective treatments to mitigate its progression and adverse outcomes.

Aims: This study aims to investigate the role of ghrelin, a peptide hormone known for its anti-inflammatory and anti-fibrotic properties, in modulating fibrosis across different organs. By binding to the growth hormone secretagogue receptor type 1a (GHSR-1a), ghrelin has shown potential in attenuating the fibrotic process, particularly through its interaction with the TGF-β signalling pathway.

Methods: An extensive review of clinical and animal model studies focusing on liver, kidney, lung, and myocardial fibrosis was conducted. The primary focus was on examining how ghrelin influences the TGF-β signalling pathway, with an emphasis on the regulation of TGF-β expression and the suppression of Smad signalling molecules. The methodology involved analysing data from various studies to understand ghrelin's molecular mechanisms in combating fibrosis.

Results: The findings from the reviewed studies indicate that ghrelin exerts significant anti-fibrotic effects across multiple organ systems. Specifically, ghrelin was found to downregulate TGF-β expression and suppress Smad signalling molecules, leading to a marked reduction in fibrous tissue accumulation and preservation of organ function. In liver fibrosis models, ghrelin reduced TGF-β1 levels and Smad3 phosphorylation, while in kidney and cardiac fibrosis, similar protective effects were observed. The data also suggest that ghrelin's effects are mediated through both canonical and non-canonical TGF-β pathways.

Conclusions: Ghrelin presents a promising therapeutic agent in the management of fibrosis due to its potent anti-inflammatory and anti-fibrotic actions. Its ability to modulate the TGF-β signalling pathway underscores a vital molecular mechanism through which ghrelin can mitigate fibrotic progression in various organs. Future research should focus on further elucidating ghrelin's molecular interactions and potential clinical applications in fibrosis treatment, offering new avenues for developing effective anti-fibrotic therapies.

Keywords: TGF-β; antifibrotic mechanisms; fibrosis; ghrelin; organ.

Figure 1.

The acylation of Ghrelin. In X/A-like cells of the stomach, proghrelin is generated by specifically cleaving the signal peptide of preproghrelin, and then localized to the ER, where GOAT acylates proghrelin at Ser3 with n-octanoic acid. In the Golgi body, acylated proghrelin is transported so that PC 1/3 can cleave it and create a 28-amino acid AG, PC1/3 might also cleave non-acylated proghrelin to produce DAG. ER, endoplasmic reticulum; GOAT, ghrelin o-acyltransferase; PC1/3, prohormone convertase 1/3; AG, acylated ghrelin; DAG, desacyl ghrelin.

Figure 2.

The gene TGFB(A) and TGF-β activation. A Gene structure of TGF-β1, TGF-β2, and TGF-β3: The untranslated regions of 5′ and 3′ are marked in green and pink, respectively, while exons are represented in blue. B TGF-β is produced by various cell types. With a signal peptide in the large terminal portion called the latency-associated peptide (LAP) and a C-terminal fragment for mature TGF-β, pro-TGF-β is synthesized as a latent complex in the ECM. With the removal of the signal peptide, the precursor protein is dimerised. Following proteolytic cleavage, the TGF-β dimer binds non-covalently to mature TGF-β to form SLC. Then, SLC generally binds to LTBP, forming LLC. The LLC is then secreted into ECM. C The release of mature TGF-β from the latent form involves protease, integrin (ITG), ED-A Fn and matrix protein-mediated actions. These active TGF-β ligands bind to the TGFβRI/TGFβRII receptor complex on the cell surface and initiate intracellular TGF-β signalling. ECM, extracellular matrix; SLC, small latent complex; LTBP, latent TGF-β binding proteins; LLC, large latent complex.

Figure 3.

Targeting TGF-β signalling. Firstly, by inhibiting TGF-β from being translated and transcribed. Antisense oligodeoxynucleotides (ODN) (Trabedersen), siRNAs, or DNA enzymes can all inhibit the transcription or translation of TGF-β mRNA. Secondly, inhibition of latent TGF-β activation. Fibrosis can be treated with anti- αvβ6 integrin antibody while αvβ6 integrin activates TGF-β, and anti-αvβ6 antibody decreases collagen production and partially inhibits TGF-activity. Thirdly, Inhibition of activated TGF-β binding to its receptor by ligand trapping. TGFβRII/Fc, which consists of the extracellular domain of the TGFβ type II receptor combined with an immunoglobulin heavy-chain Fc fragment, efficiently hindered TGFβ1 to binding with the extracellular receptor. Fourthly, by blocking intracellular TGF-β signalling. Galunisertib is an oral inhibitor of TGF-β type I receptor kinase, and it inhibited the phosphorylation of Smad2/3, thereby blocking TGF-β signalling. TGF-β signalling is mediated via non-SMAD pathways, including PI3K/AKT, ERK and JNK, which can lead to ECM creation and myofibroblast activation. PI3K/AKT, phosphatidylinositol 3-kinase/protein kinase B; ERK(MAPK), mitogen-activated protein kinase; JNK, c-Jun N-terminal kinase. (The grey arrow means promotion and the T bar means inhibitory effect.).

Figure 4.

TGF-β signalling pathway and The roles of ghrelin in fibrosis. When TGF-β is activated, it phosphorylates TβR II, which recruits TβR I to phosphorylate the Smad protein receptor. Phosphorylated Smad 2/3 then combines with Smad4 to form a complex. Target gene transcription is regulated by the activated Smad complexes once they are transported to the nucleus. In renal fibrosis, exogenous ghrelin administration decreased TGF-β1, Smad, and p-Smad3 protein expression. Collagen I, collagen III, α-SMA, and fibronectin expression levels were all crease; however, after ghrelin treatment, they all decreased. Amelioration of fibrosis via the TGF-β1-Smad pathway. Ghrelin can also exert anti-fibrotic effects through non-TGF-β pathways, such as NF-kB, TNF-α, ROS, CTGF, and GDF-15. α-SMA, α-smooth muscle actin; NF-kB, Nuclear factor kB; TNF-α, Tumor necrosis factor-α; ROS, Reactive oxygen species; CTGF, Connective tissue growth factor; GDF-15, Growth differentiation factor 15. (The grey arrow means promotion and the T bar means inhibitory effect.).

Figure 5.

Ghrelin’s reward-seeking behavioural pathway. Ghrelin takes pre- and/or post-synaptic GHS-R1A signalling from VTA and LDTg to activate the cholinergic-dopaminergic reward link. This route increases DA release and activates VTA dopamine neurons that project to N.Acc, which stimulates reward-seeking behaviour. Ghrelin receptor antagonists reduce addictive drug-induced locomotor stimulation, dopamine release, and CPP. ACh, acetylcholine; DA, dopamine; LDTg, laterodorsal tegmental area; VTA, ventral tegmental area; N.Acc, nucleus accumbens, GHS-R1A, growth hormone secretagogue receptor; CPP, conditioned position preference.