Nuclear accumulation of symplekin promotes cellular proliferation and dedifferentiation in an ERK1/2-dependent manner

Abstract

Symplekin is a multifunctional protein that localizes to both tight junctions and the nucleus in polarized epithelial cells, with confirmed roles in mRNA maturation, transcriptional modulation and tight-junction assembly. However, the mechanisms governing its subcellular distribution and related functions remain unclear. In this study, we found that symplekin primarily localizes to the nuclei of cultured dedifferentiated colorectal cancer cells, and nuclear symplekin showed higher phosphorylation and binding affinity with YBX3 than its membrane fraction. Moreover, the accumulation of nuclear symplekin promoted cell proliferation and dedifferentiation as well as β-catenin transactivation in vitro. Nuclear symplekin acts as a transcriptional co-activator for the expression of many cell cycle-related genes. Furthermore, extracellular signal-regulated kinase (ERK) phosphorylated symplekin at T1257 to facilitate its nuclear accumulation upon epidermal growth factor (EGF) stimulation. Meanwhile, reduction of total symplekin also induced certain epithelial-mesenchymal transition features in HT-29 cells. Taken together, our results confirm the coordinated roles of symplekin in cell junctions and gene transcription, which are related to its subcellular localization. The significance of nuclear symplekin in tumorigenesis is also highlighted, and ERK-dependent phosphorylation represents a mechanism for its subcellular sorting.Symplekin is expressed in a wide range of cell types and participates in cytoplasmic mRNA polyadenylation 1 , cell proliferation 2 , differentiation 3 , mitosis 4 and tumorigenesis 5 . Previously, we confirmed the role of symplekin in cell tight-junction (TJ) assembly and polarity maintenance 6 . Among peripheral TJ proteins, Zonula Occludin-1 (ZO-1), Y-box transcriptional factor 3 (YBX3, also known as CSDA, DBPA or ZONAB) and symplekin have been reported to form functional protein complexes and to shuttle between the junctional plaques and the nucleus 2, 6, 7 . In addition to polarized epithelial cells, symplekin localizes exclusively to the nucleus of tight-junctionless cells, emphasizing its vital roles in the nucleus 8 . However, the underlying mechanism, including the translocation of symplekin, is poorly understood. By comparing nuclear and extra-nuclear symplekin, we observed that nuclear symplekin exhibited increased phosphorylation in the present study. Post-translational modifications such as phosphorylation, glycosylation, and ubiquitylation have emerged as dynamic and essential regulators for target proteins sorting and relocalization to participate in various cellular events 9 . The phosphorylation of several junctional components, such as ZO-1, occludin and β-catenin, has also been shown to determine their subcellular distrbutions [10][11][12] . We found that extracellular epidermal growth factor (EGF) signals induced the phosphorylation of symplekin on specific residues, followed by nuclear translocation, with nuclear symplekin serving as a trans-activator to promote cell proliferation ...

Interaction Between Sympk and Oct4 Promotes Mouse Embryonic Stem Cell Proliferation

“…Previous reports have shown that nuclear Sympk could interact with transcription factors such as ZONAB and HSF1 [33,34]. Moreover, the nuclear portion of Sympk is required for cell cycle-related gene regulation and β-catenin activation, and enhances proliferation and de-differentiation in colorectal cancer cells [17,18]. In addition, we found that Sympk interacted directly with core pluripotency transcription factors including Oct4, Nanog, and Sox2, in mESCs.…”

“…Post‐translational modifications (PTMs) including sumoylation and phosphorylation, are essential for the function of Sympk in the regulation of cell growth and viability in different tumor cells . Since PTMs are key regulatory process for protein stability, activation, interaction, and cellular localization, we speculate that the post‐translational modification of Sympk is also important for its proper functioning in mESCs.…”

“…Symplekin (Sympk) is a ubiquitously expressed protein that assembles in various protein complexes and displays dynamic functions in different biological process including cytoplasmic RNA polyadenylation , epithelial tight‐junction assembly , transcriptional modulation , kidney and intestinal cell differentiation , hematopoietic stem/progenitor cells , tumorigenesis , colon cancer cell growth and de‐differentiation , and mitosis in lung cancer cells . However, the role of Sympk in ESCs has not been reported to date.…”

Phosphoproteomics of cellular mechanosensing reveals NFATC4 as a regulator of myofibroblast activity

“…The protein is localized to tight junctions and the nucleus and is involved in transcriptional modulation and mRNA maturation as well as tight junction assembly. Interestingly, phosphorylation of SYMPK T1257 promotes nuclear localization of symplekin and interaction with YBX3, which enhances cell proliferation and differentiation (Zhang et al , 2017). Hyperphosphorylation of FOXO3 has been implicated with myofibroblasts in idiopathic pulmonary fibrosis (Al-Tamari et al , 2018).…”

The Pancreatic Cancer-Initiating Cell Marker CD44v6 Affects Transcription, Translation, and Signaling: Consequences for Exosome Composition and Delivery

“…We demonstrate that CD44v6 crosslinking, besides contributing to NOTCH1 transcription, promotes β -catenin-independent NOTCH and Nanog nuclear translocation. Activation of these transcription factors might be supported by CD44v6-associated MYH3 (Myosin-3) and YBX3 (Y-box-binding protein 3), which can contribute to EMT induction [14, 59–61]. Via its linkage to ERM proteins CD44 fosters cytosolic signaling molecule activation and cytoskeleton reorganization [62, 63].…”