TGFβ signaling promotes cell cycle progression and resistance to the CDK4/6 inhibitor palbociclib through SOX4 transcriptional modulation in breast cancer cells.

Cancer signaling encompasses a wide array of entangled molecular cascades that promote oncogenic progression and counteract the effect of tumor suppressors. Transforming growth factor β (TGFβ) induces complex and stage-dependent effects throughout tumor progression. During pre-malignant hyperplastic growth, TGFβ restricts cell proliferation and inflammation, while on the other hand, TGFβ promotes migration and distal metastasis of cancer cells. To dissect the temporal chromatin-based transcriptional response to TGFβ, we employed 3D culture models of isogenic human breast epithelial cells, exemplified by non-oncogenic MCF-10A (MI) and their HRAS-transformed counterpart (MII). Genome-wide chromatin accessibility profiling revealed an extensive chromatin opening induced by TGFβ at transcription start sites and enhancer elements in both models, with a marked enrichment of SOX4 binding motifs in oncogenic cells. Transcriptomic analyses unexpectedly revealed the upregulation of DNA replication and DNA damage response pathways, following TGFβ stimulation of oncogenic MII 3D cultures. Canonical TGFβ-driven programs, including epithelial-mesenchymal transition and metabolic reprogramming, were activated in both models. Notably, single-cell RNA-seq of primary breast tumors confirmed co-expression of SOX4 and cell cycle regulators. Mechanistically, we show that TGFβ induces the interaction between the MH2 domain of SMAD3 and the intrinsically disordered regions of SOX4, co-activating downstream gene targets. Validating the genome-wide analyses, we found that resistance of breast cancer cells to the CDK4/6 inhibitor palbociclib conferred by TGFβ stimulation was functionally dependent on SOX4. Collectively, our findings reveal an apparent oncogenic function of TGFβ in promoting cell cycle progression and drug resistance through SOX4, highlighting the pro-tumorigenic role of TGFβ signaling in breast cancer progression.