The canonical Wnt signaling pathway regulates cellular protein levels of the transcriptional coactivator β-catenin, which controls critical developmental gene expression programs (Macdonald et al., 2009). Wnt binding to Frizzled receptors and LRP5/6 co-receptors induces the stabilization of cytoplasmic β-catenin and its translocation to the nucleus (Kikuchi et al., 2009, He et al., 2004, Macdonald and He, 2012). Inside the nucleus, it activates the Tcf/Lef transcription factors and regulates the transcription of target genes (Kikuchi et al., 2009, Behrens et al., 1996). Since Wnts play essential roles in diverse processes such as cell proliferation and migration, precise regulation of this signaling pathway is essential (Anastas and Moon, 2013). Aberrant activation of Wnt/β-catenin signaling is a hallmark of many human tumors, including breast cancer (Koval and Katanaev, 2018, Zhan et al., 2017). Elucidating the various mechanisms of regulation of Wnt signaling at upstream levels is of great interest as it may lead to the discovery of novel therapeutic targets for cancer (Malinauskas and Jones, 2014, Shaw et al., 2019b). Heparan sulfate proteoglycans (HSPGs) interact with Wnt ligands and are therefore likely to act as extracellular modulators of Wnt signaling pathways (Kikuchi et al., 2009, Yan and Lin, 2009). The effect of HSPGs on Wnt pathways can be positive or negative depending on the cell and tissue type (Kikuchi et al., 2009). Syndecans are the only family of transmembrane heparan sulfate proteoglycans (Multhaupt et al., 2009). The syndecan family has four members in vertebrates, which are differentially expressed in different cell types and developmental stages (Xian et al., 2010, Espinoza-Sánchez and Götte, 2020). In adult vertebrate tissues, syndecan-1 is predominantly expressed by epithelial and plasma cells (O’Connell et al., 2004). Syndecan-2 is mainly expressed by mesenchymal cells, whereas syndecan-3 is mainly expressed by neurons (Marynen et al., 1989, Carey et al., 1992). Syndecan-4 expression appears to be ubiquitous (David et al., 1992). Except for erythrocytes, almost all human cells express at least one syndecan protein, with some expressing all four (Motta et al., 2023). Syndecans have three distinct structural domains: The ectodomain with multiple glycosaminoglycan binding regions, a single transmembrane domain, and a short cytoplasmic domain (Bernfield et al., 1992). The ectodomain shows low sequence homology among family members (Choi et al., 2011). At the cell surface, the core protein and heparan sulfate chains of syndecans play several biological roles, including cell-cell and cell-ECM interactions involved in cell survival, adhesion, and migration (Couchman et al., 2001, Beauvais and Rapraeger, 2004, Woods et al., 1998, Afratis et al., 2017). Syndecans also bind through their heparan sulfate chains to numerous soluble molecules such as cytokines, chemokines, extracellular matrix glycoproteins, growth factors, and morphogens (Tkachenko et al., 2005, Perrimon and Bernfield, 2001). In addition, syndecans signal through their protein core, primarily through direct interactions with other signaling molecules (Lambaerts et al., 2009).

Syndecans also play an important role in cancer cell biology (Choi et al., 2010). Changes in the expression pattern or the number of glycosaminoglycan (GAG) side chains, as well as the shedding properties of syndecans, can influence tumorigenesis and cancer progression (Motta et al., 2023). The exact roles of syndecans in either enhancing or inhibiting cancer activity are unclear because of inconsistent research results concerning each syndecan’s specific functions in different cancers (Kim et al., 2024). The first evidence for the association of syndecans with Wnts dates back to 1997 (Haerry et al., 1997). However, the number of reports linking syndecans to Wnt/β-catenin signaling is small and there is much to be learned about this interaction. For example, our understanding of how cellular and tissue contexts influence syndecan crosstalk with Wnt signaling is rudimentary (Pataki et al., 2015). Exploring the interplay between syndecans and Wnt signaling in human cancers could be beneficial for identifying new therapeutic strategies, understanding tumor behavior, and improving patient outcomes. Among the syndecan family members, syndecan-2 (SDC2) appears to play a more important role in cancer progression (Afratis et al., 2017). There is sufficient evidence that the expression of this protein is typically higher in mesenchymal cells, and its overexpression in tumors of epithelial origin, including breast cancer, appears to induce aggressive behavior (Mytilinaiou et al., 2017).

In the current study, we sought to determine whether syndecan-2 plays a role in the regulation of Wnt/β-catenin signaling. We found that syndecan-2 overexpression enhances the response of BT-20 (a triple-negative breast cancer (TNBC) cell model) and HEK293T cells to Wnt3a-mediated activation of the Wnt/β-catenin pathway. Consistently, we show that CRISPR/Cas9-mediated deletion of SDC2 (the gene encoding syndecan-2) reduces β-catenin transcriptional activity upon Wnt3a stimulation in BT-20 cells.