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Caveolin-1/Caveolae are in Focus of Regulating EMT and MET

Anna L. Kiss

Department of Anatomy, Histology and Embryology, Semmelweis University, Budapest, Hungary

DOI: 10.15761/HCE.1000111

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During epithelial-mesenchymal transition (EMT) polarized epithelial cells undergo a complex proteomic remodelling, several biochemical and morphological changes converting them into mesenchymal-like cells. [1]. Besides of expressing mesenchymal markers (N-cadherin, vimentin, α-smooth muscle actin etc) they acquire the ability to produce extracellular matrix components, as well as metalloproteinases, inflammatory cytokines, fibrogenic and angiogenic factors [2]. Three types of EMTs have been distinguished so far: epithelial-mesenchymal transition during embryogenesis (type I), EMT associated with inflammation, wound healing, tissue regeneration and organ fibrosis (type II) and tumorigenesis (type III) [3]. EMT is triggered by extracellular signals including various cytokines, growth factors, extracellular matrix components. Transforming growth factorβ (TGFβ) family members are the most important regulators in this process. The TGFβ signalling pathways are generally divided into Smad-dependent and Smad–independent processes. When TGFβ binds to its primary serine/threonine kinase receptor (type II) it induces the formation of a heteroterameric receptor complex. Type II receptor then trans phosphorylates (activates) the signalling receptor (type I) that can initiate Smad2/3 signalling pathway. The receptor internalization is required for the initiation of downstream signalling [4]. In addition to promoting the signalling process by transporting these molecules to the signalling organelles, the receptor-ligand internalization can control the number of receptors present on the cell surface. Thus, receptor internalization regulates the signalling, receptor turnover, the magnitude and duration of the events. In EMT clathrin-mediated as well as caveolar endocytosis (internalization via caveolae) play an important role [5]. Caveolae are small omega- or flask-shaped plasma membrane invaginations, highly hydrophobic membrane domains (lipid rafts) containing a special protein, caveolin-1 [6,7]. It is generally accepted that endocytosis via caveolae directs the receptor-ligand complex to lysosomal and/or proteasomal degradation [8,9] which downregulates the receptors on the cell surface. Caveolin-1 downregulation together with caveolar internalization of TGFβ receptors turns off the TGFβ signalling pathway, thereby blocking EMT.

TGFβ can induce and/or promote EMT through a Smad-independent pathway as well. In response to TGFβ the type II TGFβ receptor can autophosphorylate tyrosine amino acids (Tyr259, 336 and 424) [10], and can undergo Src-mediated tyrosine phosphorylation as well [11,12]. The tyrosine phosphorylated TGFβ receptor can activate MAP kinase (MEK) and ERK1/2. Members of the Ras/Raf/MEK/ERK kinase cascades are present in caveolae and caveolin-1 promotes ERK activation. Kinase suppressor of Ras1 (KSR1) is a caveolin-1 interacting protein, and the KSR1-caveolin-1 interaction is required for the redistribution of MEK/ERK in caveolin-rich fractions, actually caveolin-1 facilitates the formation of KSR1/MEK/ERK complex. KSR1-caveolin-1 interaction regulates the phosphorylation of ERK, thus this interaction is essential for optimal activation of ERK [13]. The activated (phosphorylated) ERK triggers EMT by targeting the Egr1 transcription factor to the Snail-1 promoter, enhancing Snail-1 transcriptional activity [14]. Since Snail is the master factor of EMT directly inhibiting the E-cadherin expression [15], caveolin-1 through regulating the phosphorylation level of ERK1/2, has a pivotal role in the balance of epithelial versus mesenchymal state of cells. An inverse relationship between caveolin-1 expression and Raf/MEK/Erk activity has also been reported. Overexpression of caveolin-1 was found to attenuate ERK activity, whereas downregulation of caveolin-1 leads to hyper phosphorylation of ERK1/2 [2].

TGFβ can also activate p38, JNK, MAPK throughout activation of a TGFβ activated protein (TAK1). Caveolin-1 selectively augments the phosphorylation of p38 [16], the activity of which maintains E-cadherin expression, thereby inhibiting EMT, and maintaining the epithelial phenotype [17]. Recent date [18] show that caveolin-containing lipid raft (caveolae) serve as a platform for TGFβ induced JNK signalling. Caveolin-1 selectively inhibits JNK phosphorylation [16], inhibition of JNK leading again to maintain E-cadherin expression and blocking EMT [17].

Autophagy plays an important role in the regeneration phase following inflammation-induced EMT. During this process mesenchymal–like cells undergo a process opposite to EMT called mesenchymal-epithelial transition (MET). TGFβ has a regulatory effect on autophagy as well, by activating indirectly the PI3K-Akt-mTOR cascade [19,20]. The PI3K-Akt-mTOR pathway is crucial in regulating autophagy [21], since inhibition of mTOR increases autophagy, while its stimulation (phosphorylation) inhibits the process [22]. PI3K activated by TGFβ phosphorylates Akt, the p-Akt stimulates the downstream mTOR and autophagy is arrested, MET is blocked. PI3K-Akt pathway can directly promote EMT: cells producing constitutively the active form of Akt (pAkt) produce a transcription factor Snail, which represses the E-cadherin gene expression [23]. Thus PI3K-Akt pathway promotes EMT 1.) by supressing the E-cadherin gene expression [24] and 2.) arresting autophagy [25]. When TGFβ receptor is internalized via caveolae, the TGFβ signal is not transmitted to PI3K-Akt pathway, mTOR is not phosphorylated, it can accelerate autophagy, and E-cadherin can be synthesized. Accelerated autophagy helps to remove excess of cellular organelles, while E cadherin synthesis is necessary for reconstructing the cell-cell adhesion. Thus, caveolar internalization of TGFβ receptor strongly promotes mesenchymal-epithelial transition MET.

Bone Morphogenetic Proteins (BMP) are members of the TGFβ superfamily of cytokines. They form a large, evolutionarily conserved cytokine family, and are required for and involved in numerous developmental processes [26]. Amongst the six members of BMP superfamily BMP2 and BMP7 are known to counteract TGFβ and induce mesenchymal-epithelial transition-MET [27,28]. BMP signalling involves two types of transmembrane serine/threonine kinases, BMP type I and type II receptors (BMPR1 and BMPRII) [29]. The downstream of the receptor phosphorylation is the activation of Smads (1, 5 and 8). BMP receptors are present on the cell surface in hetero-oligomeric complexes. Although there are some data indicating that BMPRI and BMPRII are segregated and localized both in clathrin coated vesicles and caveolae [30,31] recent atomic force and combined confocal microscopic results showed, that BMP2 binds with high affinity to BMP receptors localized in caveolae, and the BMPRI receptor is phosphorylated in caveolae [32]. It is known that most of the signalling molecules (receptors, tyrosine kinases etc) are inhibited when bound to caveolin-1, stimulation with specific ligands releases caveolin-1 and the signalling is initiated [33]. In case of BMP signalling the binding of BMPRII to caveolin-1 inhibits the phosphorylation and the activation of BMPRI in the absence of the ligand [31]. Binding of BMP-2 to its receptors, releases caveolin-1 resulting in proceeding activation of Smad signalling pathway [31]. Moreover, if cavolae are disrupted BMP2 cannot initiate Smad signalling, indicating that caveolae are necessary for the initiation of Smad signalling [32].

In summary: many signalling pathway are regulated by caveolin-1, the principal protein of caveolae. Caveolae, these omega-shaped plasma membrane invagination have multiple functions in the signalling events: 1.) they provide assembly platforms for various signalling molecules; 2.) the interaction of signalling molecules with caveolin-1 (the major protein of caveolae) regulates their activity; 3.) the caveolar internalization of receptor-ligand complexes regulates the number of the specific receptors present on the cell surface, and determines the capability of the cell to respond the extracellular stimuli. Therefore, caveolar internalization of cytokine (TGFβ, BMPs, etc.) receptors regulates the intensity of both EMT, and MET; the balance between EMT and MET highly depends on the presence of caveolin-1/ caveolae on the plasma membrane [2,26]. Consequently, caveolin-1 is likely to be the major checkpoint in epithelial-mesenchymal transition [2].

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Editorial Information

Editor-in-Chief

Article Type

Short Communication

Publication History

Received: April 27, 2017
Accepted: June 17, 2017
Published: June 20, 2017

Copyright

©2017 Kiss AL. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Citation

Caveolin-1/Caveolae are in Focus of Regulating EMT and MET. Histol Cytol Embryol 1: doi: 10.15761/HCE.1000111

Corresponding author

Anna L. Kiss

Anna L. Kiss, Department of Anatomy, Histology and Embryology, Semmelweis University, Budapest, Hungary, Tel: +36 1 216-6920.

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