SFRPs Are Biphasic Modulators of Wnt-Signaling-Elicited Cancer Stem Cell Properties beyond Extracellular Control

Area of Science:

• Molecular Biology
• Cancer Research
• Cell Signaling

Background:

• Secreted frizzled-related proteins (SFRPs) are known extracellular regulators that typically suppress Wnt signaling and act as tumor suppressors.
• The intracellular functions and mechanisms of SFRPs, particularly their interaction with β-catenin in the nucleus, remain less understood.

Purpose of the Study:

• To investigate the novel nuclear roles of SFRPs in modulating Wnt signaling.
• To elucidate the mechanisms by which SFRPs interact with β-catenin and influence TCF4 recruitment.
• To identify key structural features of SFRPs critical for their biphasic modulatory function in Wnt signaling and cancer stem cell (CSC) properties.

Main Methods:

• Utilized a CRISPR/Cas9 system to target SFRPs promoters and transcripts.
• Investigated the interaction of nuclear SFRPs with β-catenin using biochemical assays.
• Analyzed the effects of SFRPs on TCF4 recruitment and β-catenin transcriptional activity under varying Wnt signaling conditions.
• Identified critical structural elements, including disulfide bonds and phosphorylation sites, within SFRPs.

Main Results:

• Nuclear SFRPs were found to interact with β-catenin, influencing TCF4 recruitment in a dual manner (promoting or suppressing).
• SFRPs bind β-catenin at both N and C termini; N-terminus binding mediates repression, while C-terminus binding promotes activity.
• Under high Wnt activity, C-terminal binding dominates, upregulating β-catenin activity and cancer stem cell (CSC)-related genes.
• Disulfide bonds in the cysteine-rich domain (CRD) and threonine phosphorylation in the netrin-related motif (NTR) domain are essential for SFRPs' biphasic modulation.

Conclusions:

• SFRPs function as biphasic modulators of Wnt signaling, extending beyond their known extracellular roles.
• Nuclear SFRPs directly regulate β-catenin transcriptional activity and TCF4 recruitment, impacting CSC properties.
• Structural features of SFRPs, including CRD disulfide bonds and NTR phosphorylation, are critical for their dual regulatory capacity in Wnt signaling pathways.