PCNA-Polκ-Polδ/USP18 axes stabilize replication fork and restart to reduce cisplatin cytotoxicity

Cisplatin and its analogues are valuable anti-cancer drugs that target the genome, block DNA replication, and induce apoptosis. As a counteractive response, cancer cells activate several mechanisms to maintain uninterrupted DNA replication, and those are yet to be fully elucidated. This study using head and neck squamous carcinoma cells (HNSCC) demonstrated the involvement of DNA polymerase Kappa (Polκ), a trans-lesion DNA synthesis (TLS) polymerase that primarily functions as a mismatch extender, in cisplatin resistance. Interestingly, the catalytic activity of Polκ plays a minimal role in adduct bypass; rather, tripartite interactions involving it, rewire and stabilize the stalled replication fork. While the Polκ-PCNA-Polδ axis facilitates efficient proliferation of cisplatin-resistant cells, the Polκ-PCNA-USP18 axis stabilizes critical proteins of ATM-ATR, and HR and NHEJ pathways to protect replication fork, repair damage, and restart DNA synthesis under cisplatin-induced stress. In resistant cells, the efficiency of ubiquitin-mediated proteasomal degradation is low, which is further diminished by Polκ-recruited USP18 deubiquitinase, maintaining a cellular homeostasis. In conclusion, for the first time, we uncovered two critical Polκ axes crucial for regulating cisplatin toxicity in cells and provided foundation for future drug discovery against advance HNSCC by targeting this non-essential DNA polymerase.