TRIM21-mediated ubiquitination of SIX2 attenuates breast cancer stemness via LGSN suppression.

Breast cancer stem cells (BCSCs) are pivotal drivers of breast tumor initiation, metastasis, and therapy resistance. Our previous studies identified the transcription factor SIX2 as a key regulator in maintaining breast cancer stemness. Here, we demonstrate that TRIM21, as an E3 ubiquitin ligase downregulated in breast cancer tissues, binds to SIX2 via its PRY-SPRY domain and catalyzes K48-type ubiquitination at lysine residues K82, K89, and K97. This modification promotes the degradation of SIX2 via the ubiquitin-proteasome pathway, consequently attenuating the stemness and metastatic potential of breast cancer cells. Furthermore, SIX2 transcriptionally activates LGSN expression through direct binding to its promoter region, thereby promoting the stemness and metastatic capabilities of breast cancer cells. Clinically, elevated expression of both SIX2 and LGSN correlates with poor prognosis in breast cancer patients. These results establish that TRIM21-mediated degradation of SIX2 suppresses LGSN expression, ultimately inhibiting the stemness and metastatic abilities of breast cancer cells, underscoring the critical regulatory role of the TRIM21-SIX2-LGSN axis in breast cancer progression.