The challenge of restoring or increasing the elastic fiber content in aged patients or animals (or in pathological conditions) is of particular importance and consists in stimulating the production of the elastic fiber components at a time when the synthesis of several of them is very low.
Tropoelastin, the soluble precursor of elastin, and other elastic fiber components are synthesized in the skin by dermal fibroblasts and in vessels by medial VSMCs. The synthesis of tropoelastin is regulated by numerous chemokines and growth factors and also by potassium (K+) and calcium (Ca2+) ions. Indeed, K+ excess in the extracellular medium induces the depolarization of the plasma membrane, which causes the opening of voltage-dependent calcium channels (CaV) and leads to an increase in [Ca2+]i and a decrease in elastin synthesis. Such a decrease in elastin synthesis was also shown in VSMCs incubated with the calcium ionophore A23187, which triggers Ca2+ influx and [Ca2+]i elevation. On the contrary, K+ efflux from cells leads to an increase in elastin synthesis due to membrane hyperpolarization and CaV closing. This has been shown in experiments using the ATP-dependent K+ (KATP) channel opener minoxidil, a potent arterial vasodilator which has long been used to treat hypertension. Minoxidil-induced K+ efflux leads to enhancement of elastin synthesis in cultured VSMCs and skin fibroblasts. In vivo studies have also demonstrated that the elastin content and/or thickness of elastic lamellae are increased by minoxidil treatment in several animal models: Spontaneously Hypertensive Rats (SHR). Chronic treatment of mice with minoxidil also results in improvements of the arterial mechanics and cerebral perfusion and, at least in aged animals, in neosynthesis of elastic fibers and protection of pre-existing elastic fibers. Other KATP channel openers, like diazoxide or nicorandil, exert an effect on in vivo elastin synthesis similar to those induced by minoxidil, while pinacidil and some cromakalim or diazoxide derivatives have also been shown to induce elastin production by cultured VSMCs in rats. However, treatment of animals with other antihypertensive drugs, either angiotensin-II type 1 receptor blocker, beta blocker or calcium channel blocker, does not modify the elastin content in mouse arteries.
Numerous cytokines which inhibit tropoelastin transcription act through the activation of the Ras/MEK/ERK signaling pathway. The increase in [Ca2+]i in VSMCs also stimulates ERK1/2 phosphorylation. The inhibition of this signaling pathway could thus be a strategy to increase the elastin content in arteries. Actually, by performing in vitro and in vivo experiments in rats, it has been demonstrated that inhibition of the ERK1/2 phosphorylation leads to increases in elastin synthesis by VSMCs and elastin content in the aorta. The importance of this pathway in elastin metabolism has been confirmed by the demonstration that cortistatin reduces elastin degradation, MMP-2 and -9 expressions and aneurysm progression through the inhibition of the ERK1/2 signaling pathways. Of importance, it has also previously been shown that ERK1/2 signaling is involved in elastin peptide signaling through the elastin receptor complex (ERC). The general impact of the pharmacological treatments modulating the KATP channels-Ca2+ channels-ERK1/2 pathways described above is illustrated in Fig. 1.
Application of potassium channel openers to VSMCs leads to cell membrane hyperpolarization and calcium channel blockade. This induces a drop in intracellular Ca2+ concentration and inhibition of the ERK 1/2 pathway, resulting in the activation of the production of mRNAs involved in elastic fiber synthesis (tropoelastin, TE; fibulin-5, FBLN-5; lysyl-oxidase, LOX; fibrillin-1, FBN-1; ā¦) and secretion of the corresponding proteins. Treatment of the cells with an inhibitor of ERK 1/2 phosphorylation produces the same effect regarding elastin. Elastic fiber neosynthesis then precedes their integration/aggregation into pre-existing elastic fibers and elastic lamellae. Regarding the effect of minoxidil, a limitation of elastic fiber ruptures and AGE formation is also observed.
MEK inhibitors have also been used in order to stimulate elastin synthesis by cells isolated from patients with Costello syndrome, a developmental disorder characterized by hyper-activation of the mitogenic Ras-Raf-MEK/ERK pathway and inhibition of elastogenesis. The treatment of dermal fibroblasts derived from these patients with the MEK inhibitor PD98059 leads to the concomitant inhibition of cell proliferation and recovery of elastin production.
Completely different therapeutic strategies have also been tested to stimulate elastin synthesis using in vitro or in vivo models. Cenizo and colleagues have used a dill extract to induce LOXL gene expression in dermal fibroblasts, resulting in elastin content increase in dermal and skin equivalents as well as skin elasticity elevation in vivo in humans.
In addition, based on a previous work which has shown that microRNA (miR)-29 mimics downregulate the expression of elastin (ELN) as well as parts of collagen type I (COL1A1) and collagen type III (COL3A1) genes, the team of William C. Sessa has demonstrated that inhibition of miR-29a can increase ELN expression in human cells (skin fibroblasts, VSMCs, bioengineered vessels) and in cells from patients with ELN haploinsufficiencies (supravalvular aortic stenosis or Williams-Beuren syndrome). Using the same in vitro models, it was also demonstrated that engineered zinc-finger protein transcription factors that target the ELN gene can also be used to stimulate the expression of elastin. However, these potential therapeutic molecules have not been tested in vivo yet and need additional investigations for validation of their therapeutic potential.
https://sci-hub.se/downloads/2019-10-05/48/fhayli2019.pdf?download=true