The New Model of Snail Expression Regulation: The Role of MRTFs in Fast and Slow Endothelial–Mesenchymal Transition

Abstract

Endothelial–mesenchymal transition (EndMT) is a crucial phenomenon in regulating the development of diseases, including cancer metastasis and fibrotic disorders. The primary regulators of disease development are zinc-finger transcription factors belonging to the Snail family. In this study, we characterized the myocardin-related transcription factor (MRTF)-dependent mechanisms of a human snail promoter regulation in TGF-β-stimulated human endothelial cells. Although in silico analysis revealed that the snail promoter’s regulatory fragment contains one GCCG and two SP1 motifs that could be occupied by MRTFs, the genetic study confirmed that MRTF binds only to SP1 sites to promote snail expression. The more accurate studies revealed that MRTF-A binds to both SP1 elements, whereas MRTF-B to only one (SP1near). Although we found that each MRTF alone is capable of inducing snail expression, the direct cooperation of these proteins is required to reinforce snail expression and promote the late stages of EndMT within 48 hours. Furthermore, genetic and biochemical analysis revealed that MRTF-B alone could induce the late stage of EndMT. However, it requires a prolonged time. Therefore, we concluded that MRTFs might cause EndMT in a fast- and slow-dependent manner. Based on MRTF-dependent Snail upregulation, we recognized that TGF-β1, as an MRTF-B regulator, is involved in slow EndMT induction, whereas TGF-β2, which altered both MRTF-A and MRTF-B expression, promotes a fast EndMT process.

Regulation of Partial and Reversible Endothelial-to-Mesenchymal Transition in Angiogenesis

“…Moreover, Slug dominates during the early proliferative phase of angiogenesis, whereas in later stages of angiogenesis, its expression decreases as Snail expression increases (Welch-Reardon et al, 2014). Consistent with this finding, Snail upregulation is also delayed following TGF-β activation of complete EndoMT in cultured ECs (Sobierajska et al, 2020). Thus, the dynamic balance of Slug vs. Snail levels appears to modulate the EndoMT response to dictate the extent of (partial vs. complete) mesenchymal transition.…”

“…Whereas physiologic levels of Slug support early sprouting angiogenesis via induction of a partial EndoMT gene signature, high and prolonged overexpression of Slug -particularly in combination with Notch signaling inhibition -drives full dissociation of ECs from angiogenic sprouts, suggesting that Slug levels tightly regulate the extent of EC response to EndoMT signals (Hultgren et al, 2020). Meanwhile, delayed Snail upregulation during angiogenesis (Welch-Reardon et al, 2014) and in response to exogenous TGF-β-mediated EndoMT activation (Sobierajska et al, 2020) suggests that Snail functions later than Slug, perhaps to support a more robust EndoMT activation signal that pushes cells more aggressively down the EndoMT pathway. Consistent with this possibility, Snail has a higher affinity for target DNA binding sites than does Slug, suggesting that when expressed, Snail may exert a more potent effect to transactivate mesenchymal genes (Bolos et al, 2003).…”

Oxidative Stress Enhances the TGF-β2-RhoA-MRTF-A/B Axis in Cells Entering Endothelial-Mesenchymal Transition

“…As described above, the changes in cell morphology and behavior detected during mesenchymal transdifferentiation were associated with the modulation of contraction markers, which were enhanced by H 2 O 2 co-stimulation. Their expression is known to be regulated by MRTF translocation to the nucleus and activation of the MRTF-SRF loop [5,17]. Therefore, we analyzed whether the stronger EndMT induction observed in oxidative stress is related to changes in MRTF activation.…”

Cytoskeleton Reorganization in EndMT—The Role in Cancer and Fibrotic Diseases

“…A later study showed that MRTFs induce the expression of zinc-finger transcription factors. MRTF-A induces a TWIST1 upregulation in a STAT3-dependent manner [ 69 ], whereas both MRTFs isoforms are the direct inductors of Snail expression [ 70 ].…”