Genome-wide CRISPR screen identifies noncanonical NF-kB signaling as a regulator of density-dependent proliferation

Epithelial cells typically stop dividing once they reach a homeostatic density—a process known as contact inhibition. Defects in this regulatory system are hallmarks of cancer initiation, often leading to hyperplasia. This study investigates the genes governing this density-dependent behavior in murine mammary epithelial cells by performing a genome-wide CRISPR/Cas9 knockout screen. The researchers utilized a specialized fluorescence-based cell cycle reporter (FUCCI) to identify cells that failed to exit the cell cycle and stop dividing even at high population densities.

A key discovery from the screen was the role of TRAF3, a protein classically understood as a negative regulator of NF-kB signaling. The study demonstrates that when TRAF3 is removed, cells escape density-dependent growth arrest. Unlike other known regulators of proliferation, such as the Hippo (YAP/TAZ) pathway, TRAF3 depletion specifically triggers the noncanonical NF-kB signaling cascade. This activation leads to the accumulation of NIK and the subsequent processing of p100 into p52, which, in complex with RELB, enters the nucleus to initiate an innate immune-like gene expression program.

Through further investigation, the researchers found that this aberrant signaling pathway functions independently of traditional cell-cycle checkpoints. Instead of relying on common mediators like cyclin-dependent kinase inhibitors or YAP/TAZ, the activated noncanonical NF-kB pathway forces the cell to remain in a proliferative state, preventing it from entering the G0 (quiescent) phase. This manifests as sustained expression of Cyclin D1 and continued activity of CDK2, overriding the signals that would normally instruct the cell to stop dividing upon contact with neighbors. These findings offer new insights into how dysregulated immune-related signaling can fundamentally rewire cell behavior to promote uncontrolled tissue growth.