Hierarchy between the transmembrane and cytoplasmic domains in the regulation of syndecan-4 functions

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• 作者：Youngsil Choi^a ; Dongmin Kang^a ; Inn-Oc Han^b ; Eok-Soo Oh^a ; ^{OhES@ewha.ac.kr}
• 关键词：ECM ; extracellular matrix ; FN ; fibronectin ; GAG ; glycosaminoglycan ; HSPG ; heparan sulfate proteoglycan ; PIP2 ; phosphatidylinositol 4 ; 5-bisphosphate ; PKC ; protein kinase C ; PMA ; Phorbol 12-myristate 13-acetate ; REF ; rat embryo fibroblasts
• 刊名：Cellular Signalling
• 出版年：2012
• 期刊代码：42_08986568
• 类别：bio
• 出版时间：August, 2012
• 卷：24
• 期：8
• 页码：1522-1530
• 文件大小：1112 K

摘要

Syndecan-4, a transmembrane heparan sulfate proteoglycan, plays a critical role in cell adhesion. Both the transmembrane and cytoplasmic domains of syndecan-4 are known to contribute to its functions, but the regulatory mechanisms underlying the functional interplay between the two domains were previously unclear. Here, we examined the functional relationship between these two domains. Fluorescence resonance energy transfer (FRET)-based assays showed that syndecan-4 expression enhanced RhoA activation. Furthermore, rat embryonic fibroblasts (REFs) plated on fibronectin fragments lacking the heparin-binding domain that interacts with syndecan-4 showed much lower RhoA activation than that in cells plated on full-length fibronectin, indicating that RhoA is involved in syndecan-4-mediated cell adhesion signaling. Syndecan-4 mutants defective in transmembrane domain-induced oligomerization and syndecan-4 phosphorylation-mimicking cytoplasmic domain mutants showed decreases in RhoA activation and RhoA-related functions, such as adhesion, spreading and focal adhesion formation, and subsequent increase in cell migration, but the inhibitory effect was much higher in cells expressing the transmembrane domain mutants. The cytoplasmic domain mutants (but not the transmembrane domain mutants) retained the capacity to form SDS-resistant dimers, and the cytoplasmic mutants showed less inhibition of syndecan-4-mediated protein kinase C activation compared to the transmembrane domain mutants. Finally, cytoplasmic domain activation failed to overcome the inhibition conferred by mutation of the transmembrane domain. Taken together, these data suggest that the transmembrane domain plays a major role in regulating syndecan-4 functions, and further show that a domain hierarchy exists in the regulation of syndecan-4.