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

Technical Summary: PCNA-Pol κ-Polδ /USP18 Axes in Cisplatin Resistance

Problem Statement
Cisplatin and its analogues are potent anti-cancer therapeutics that function by targeting the genome, blocking DNA replication, and inducing apoptosis. However, cancer cells, particularly those in head and neck squamous cell carcinoma (HNSCC), activate counteractive mechanisms to maintain uninterrupted DNA replication and survive this stress. While these resistance mechanisms are known to exist, the specific molecular pathways and protein interactions that allow cells to bypass cisplatin-induced replication blocks remain largely unelucidated.

Methodology
The study utilized head and neck squamous carcinoma cells (HNSCC) to investigate the role of DNA polymerase Kappa (Pol{κ}), a trans-lesion synthesis (TLS) polymerase typically characterized as a mismatch extender. The research focused on analyzing the functional dynamics of Pol{κ} in cisplatin-resistant cells, specifically examining its catalytic activity versus its structural or scaffolding roles. The methodology involved dissecting protein-protein interactions, specifically tripartite complexes involving Pol{κ}, PCNA, and either DNA polymerase delta (Pol{δ}) or the deubiquitinase USP18. The study also assessed the efficiency of ubiquitin-mediated proteasomal degradation in the context of these interactions and their impact on the stability of replication forks and DNA repair pathways.

Key Contributions and Results
The study identifies two distinct, critical axes involving Pol{κ} that stabilize replication forks and facilitate their restart under cisplatin-induced stress:

1. The Pol{κ}-PCNA-Pol{δ} Axis: This interaction facilitates the efficient proliferation of cisplatin-resistant cells. Contrary to the traditional view of Pol{κ} as a primary bypass polymerase, the study found that its catalytic activity plays a minimal role in adduct bypass. Instead, its presence within this tripartite complex is essential for rewiring and stabilizing the stalled replication fork.
2. The Pol{κ}-PCNA-USP18 Axis: This axis is crucial for maintaining cellular homeostasis under stress. Pol{κ} recruits the deubiquitinase USP18, which acts to stabilize critical proteins within the ATM-ATR signaling, Homologous Recombination (HR), and Non-Homologous End Joining (NHEJ) pathways. By preventing the ubiquitin-mediated proteasomal degradation of these proteins, this axis protects the replication fork, repairs damage, and restarts DNA synthesis. The study notes that resistant cells already exhibit low efficiency in proteasomal degradation, a state further diminished by the recruitment of USP18 by Pol{κ}.

Significance and Claims
The paper claims to uncover, for the first time, two critical Pol{κ} axes that regulate cisplatin toxicity in HNSCC cells. A central finding is the redefinition of Pol{κ}'s role: rather than acting primarily as a catalytic bypass enzyme, it functions as a structural hub that stabilizes the replication machinery and repair pathways through protein-protein interactions.

The authors state that these findings provide a foundation for future drug discovery aimed at advanced HNSCC. Specifically, they propose that targeting this "non-essential" DNA polymerase could disrupt the stabilization of replication forks and the repair of cisplatin-induced damage, thereby overcoming resistance. The study emphasizes that the minimal catalytic role of Pol{κ} in adduct bypass, contrasted with its critical structural role in resistance, highlights it as a viable therapeutic target.